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Topographically generated fronts,
very nearshore oceanography and
the distribution of larval invertebrates
and holoplankters
ALAN L. SHANKS, ANITA MCCULLOCH AND JESSICA MILLER
OREGON INSTITUTE OF MARINE BIOLOGY, UNIVERSITY OF OREGON, PO BOX
CORRESPONDING AUTHOR:
5389, CHARLESTON, OR 97420, USA
[email protected]
Foam lines oriented parallel to shore are common features of rocky shores. At times, the water
coloration is different on either side of the foam lines, suggesting they are associated with fronts. We
investigated the effect of shore-parallel foam lines and associated fronts on distributions of holo- and
meroplankton. We performed CTD transects to describe the fronts and carried out vertical
zooplankton tows to describe the distribution of zooplankton relative to the fronts. Fronts were
within tens of meters of shore and were apparently generated by the interaction of coastal currents
with local topography. We sampled four sites (three coves and one open coastal site), some of which
were separated by only a few hundred meters. At each site we found shore-parallel foam lines and
associated thermal fronts, but the characteristics of the fronts were different at three sites, suggesting
that three different mechanisms were generating the fronts. At two coves, the foam line and front
appeared to be due to the interaction of wind-driven currents from the north with coastal topography.
At the third cove, the front appeared to be due to the expansion of solar-heated surface waters out of
the cove. The foam line and front at the open coastal site appeared to be due to boundary mixing. At
the coves, the distributions of holoplankters, meroplankters and phytoplankton were clearly altered by
the presence of the fronts. At the open coastal site, the front had less effect on the distribution of
zooplankton. The coastal ocean is the source of new recruits to the intertidal zone and an important
source of food in the form of phytoplankton for filter feeders. We hypothesize that these very
nearshore fronts may play an important role in structuring intertidal communities with which they
are associated.
INTRODUCTION
Along a shoreline, the water bathing the shore is the first
encountered by larvae spawned in the intertidal zone,
and the last parcel of water they must cross before they
can settle at the shore. Nearshore currents could retain
larvae near their release site, causing high recruitment in
the natal population and generating relatively closed
populations. Alternately, larvae could be flushed from
the waters nearshore. The larvae of many intertidal and
shallow subtidal species go through development in
waters over the continental shelf. At the end of their
planktonic development, these larvae must migrate back
to shore to settle. Flow patterns immediately adjacent to
shore may prevent or aid this shoreward migration. Flow
patterns in the very nearshore waters (here defined as
waters 0–1 km from shore) may, by altering the supply
of larval settlers, play an important role in regulating
intertidal community structure.
The waters bathing the shore are also an important
source of food—in the form of phytoplankton and
zooplankton—for intertidal and shallow subtidal filter
feeders. The concentration, productivity and flux of these
particles to shore affect benthic secondary productivity.
Unfortunately for intertidal ecologists, the oceanography—and particularly the biological oceanography—of
waters immediately adjacent to shore has not been well
studied. Owing to a variety of engineering problems, the
physical oceanography of sandy beach surf zones has
been actively studied (Wright, 1995). The biological
doi: 10.1093/plankt/fbg090, available online at www.plankt.oupjournals.org
Journal of Plankton Research 25(10), # Oxford University Press; all rights reserved
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oceanography of these environments has also received
some attention, particularly with regard to surf zone
diatoms (McLachlan, 1983), but waters adjacent to
rocky shorelines have received less attention.
Researchers in Australia have studied the nearshore
circulation adjacent to shore in the Great Barrier Reef
and the effect of this circulation on zooplankton distributions (Hamner and Hauri, 1977, 1981; Alldredge and
Hamner, 1980; Wolanski et al., 1989; Willis and Oliver,
1990). They have identified a number of topographically
controlled circulation patterns that can concentrate zooplankton (Wolanski and Hamner, 1988). Concentrations
of zooplankton orders of magnitude higher than in surrounding waters were found in convergent fronts generated by the interaction of tidal currents with shoreline
topography. Fronts and associated zooplankton concentrations persisted as long as the flow regime persisted,
but these systems were primarily tidally driven and, with
change in the tide, topographically generated fronts
vanished and zooplankton concentrations dissipated.
Archambault and co-workers (Archambault et al.,
1998, 1999; Archambault and Bourget, 1999) investigated the role of shoreline configuration (in or out of
an embayment) and embayment size on the abundance
of zooplankton, larval settlement and growth of a filter
feeder (Mytilus edulis). These studies were carried out in
the St Lawrence Estuary, Canada, another tidally dominated system. They found consistently higher concentrations of zooplankton within embayments than in waters
outside, and attributed this to local retention and
production of meroplankton. Larval settlement and the
growth rate of mussels tended to be higher within
embayments than outside them. They attributed these
results to lower flow rates within bays. Flow rates outside
embayments were consistently high enough to inhibit
filtration rates, while, inside the bay, current speeds
were never so high. In this, another tidally dominated
system, topographically generated flow fields influenced
the distribution of zooplankton.
The settings that have been studied are dominated by
tides and topographically generated currents fluctuated
with the direction of tidal currents. Along many coasts,
the dominant currents, particularly alongshore currents,
are due to winds. Winds and associated wind-driven
currents can persist for days, sometimes weeks. These
more persistent currents may generate secondary circulation patterns (e.g. fronts, eddies, etc.) that cause relatively
long-term alterations in the distribution of zooplankton,
alterations that may, by affecting the supply of larvae,
food particles and nutrients, play an important role in
the ecology of intertidal and shallow subtidal populations.
The research reported here took place along the
Oregon coast. Here we have observed shore-parallel
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foam lines and associated changes in water coloration
to persist for days at the mouth of small coves and bays.
The accumulation of foam suggests that we are observing convergences, and the associated change in water
coloration suggests that the convergences delineate
fronts separating water masses, one in the cove or bay
and the other offshore. In several geographic settings we
collected physical oceanographic data across these foam
lines to test the hypothesis that they are fronts separating
water masses. We collected zooplankton samples to test
the hypothesis that the foam lines separate zooplankton
communities. In a companion paper (McCulloch and
Shanks, 2003) we tested the hypothesis that larval
settlement varied across the foam line at Sunset Bay,
Oregon.
METHOD
CTD data and zooplankton samples were collected
along transects through Sunset Bay, Cape Arago,
Oregon (43 20.130 N, 124 22.400 W; Figure 1A) during
the summers of 1997 (18 and 25 July, 1 August), 1999
(25 June, 20 July and 8 September) and 2000 (19 and
21 July, 16 and 24 August). In 2000, we also sampled
transects at Shore Acres and Miller’s Cove, Cape Arago,
Oregon (43 19.50 N, 124 23.50 W and 43 20.250 N,
124 22.50 W, respectively; Figure 1A) and Nellies Cove,
Port Orford, Oregon (42 44.750 N, 124 29.75W; Figure 1B).
Physical oceanographic data were collected with a
Seabird Model 19 CTD equipped with a flow-through
pump. Chlorophyll (Chl) fluorescence was measured
with a Wet Star in situ fluorometer on the CTD. The
fluorescence measurements were not calibrated against
extracted Chl samples; hence, values reported are relative Chl concentrations.
Transects were oriented along the long axis of each
cove sampled and perpendicular to shore at Shore Acres.
Sampling began as close to shore as possible, usually
<100 m, and extended offshore 1–2 km. Across foam
lines and close to shore, stations were 100–200 m apart.
In the ocean seaward of the foam line, station spacing
increased to 500 m. Station location and the location of
the adjacent shore were determined with a GPS.
Zooplankters were collected with vertical plankton
tows that extended from the bottom (or 20 m depth) to
the surface. Tows were repeated until 1–2 m3 of water
were filtered. During this study, two different sized 53 mm
mesh nets were used, 25 and 50 cm in diameter. A flow
meter would not fit in the smaller net; volume filtered
was equal to the length through which the net was pulled
times the mouth area of the net. The larger net was
equipped with a flow meter. Samples were preserved
with CaCO3-buffered formalin.
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Fig. 1. Study sites around Cape Arago (A) and Port Orford (B), Oregon. Sampled transects are indicated by the long lines with arrows at both ends.
In the laboratory, zooplankton, selected phytoplankton and detritus were enumerated. The phytoplankton
and detritus data will be reported in a separate paper
(McCulloch and Shanks, 2003). Larger zooplankters
were enumerated with the aid of a dissecting microscope. Samples were washed free of formalin on a
53 mm sieve, transferred to a 250 ml beaker and, with the
aid of an electronic balance, water was added to make
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the volume up to 200 ml (200 g). The sample was
homogenized by vigorous random stirring, and a 12 ml
subsample was removed with a Stempel pipette (Omori
and Ikeda, 1984). Aliquots (1 ml) of the smaller zooplankton were removed following this procedure, placed
in Sedgewick–Rafter slides and enumerated with the aid
of a compound microscope. Meroplankters were identified using identification keys in Shanks (Shanks, 2001).
Subsamples were counted until at least 100 individuals
of the more common organisms had been enumerated.
This yielded a sample standard deviation of 10% for
the more abundant organisms and of between 10 and
20% for the less common species (Venrick, 1978).
Owing to the limited resources that could be devoted to
this project, only a portion of the net tow samples could be
enumerated. Samples from three dates at Sunset Bay
(1 August 1997, 25 June 1999 and 20 July 1999) and one
sample set from Nellies Cove (17 August 2000) were
enumerated. Dates were haphazardly selected. All samples
from Shore Acres and Miller’s Cover were enumerated.
Observations were made at several sites around Cape
Arago to determine conditions under which shoreparallel foam lines formed and the amount of time they
were present. Observations were made roughly every
other day, starting in January 2000 and continuing
through September. Observations of Miller’s Cove and
Sunset Bay were made from an overlook just south of the
entrance to Sunset Bay State Park (Figure 2A, #1).
Observations of Shore Acres and Simpson Reef were
made from the Simpson Reef overlook parking area
(Figure 2A, #2). Observations at Cape Arago and
South Cove were made just to the north of the Cape
Arago overlook (Figure 2A, #3) and adjacent to the Sir
Francis Drake historical monument (Figure 2A, #4),
respectively. For each sample, the observer noted wind
speed and direction, estimated wave height and noted
the type of foam line present. Shore-parallel foam lines
were defined as continuous, uninterrupted foam lines
extending parallel to shore for hundreds of meters. If
they were located across the mouth of a bay or cove,
they needed to extend completely and continuously
across the bay mouth. See, for example, the foam lines
in the aerial photographs of Sunset Bay and Miller’s
Cove (Figure 2A) and Whale Cove, Depoe Bay and
Pirates Cove (Figure 2B). Foam lines that did not extend
completely across the mouth of a bay or cove (e.g. Rocky
Creek Cove; Figure 2B) were not counted as shoreparallel foam lines. Along exposed shorelines (e.g.
Shore Acres/Simpson Reef and Cape Arago) and during
periods of larger waves, foam lines associated with rip
currents were frequently observed. Rip current foam
lines are generally lobed with the foam line defining
the outer limit of the current (e.g. Figure 2D).
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To determine the spatial distribution of shore-parallel
foam lines along the coast, a large collection of aerial
survey photographs at the National Oceanographic and
Atmospheric Administration (NOAA) website (http://
www.mapfinder.nos.noaa.gov) was inspected. All coastal
photographs between the Straits of Juan de Fuca,
Washington, and San Francisco Bay, California, posted
(as of August 2001) at this NOAA website were inspected
for shore-parallel foam lines associated with headlands
and small bays and coves. Figure 2 presents examples of
these photographs. Categorization of foam lines in the
photographs was the same as that used in the observations made at Cape Arago (described above). In addition, we noted sea state (calm versus rough) and the
direction from which the seas were coming, from which
we inferred wind direction.
RESULTS
Sunset Bay
Site description
A crescent-shaped beach and stream are at the head of
Sunset Bay, a small bay on the southern Oregon coast
(Figures 1A and 2A). About 300 m from shore, the bay
becomes tightly constricted, then opens up to form the outer
portion of the bay. Away from the beach, the shoreline is
rocky; two reefs form the mouth of the bay. Depending on
the tide, the bay mouth is 0.7–0.85 km from the beach.
A shore-parallel foam line or, frequently, a pair of
parallel foam lines extends from the rock reef on the
north side of the bay to the reef to the south (Figure 1A).
The foam line is generally connected 25 m landward of
the most seaward rocks. Repeated observations of the
foam line over many days (see below) and during field
sampling indicated that the foam line does not change
position with the stage of the tide. The shape, position
and continuity of the foam line, however, changed with
wind direction and sea state (data presented below). At
times, we observed a clear change in water coloration
across the foam line. Dye released to either side of the
foam line moved steadily toward the foam, and then was
downwelled beneath the foam. The foam line invariably
contained large amounts of floating detritus. The foam
line(s) at the mouth of Sunset Bay appears to delineate a
convergence zone(s) associated with a front that separates
the waters in the bay from those in the coastal ocean. The
CTD sampling was designed to test this hypothesis.
Physical oceanography
Throughout the three summers of sampling, salinity varied
little along the transects; the total variation in all cases was
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Fig. 2. Examples of foam lines and rip currents in aerial photographs from the NOAA Mapfinder website. (A) Cape Arago, Oregon.
Oceanographic sampling presented in this paper took place at Miller’s Cove, Sunset Bay and Shore Acres. Numbers in white circles indicate
locations from which 9 months of observations were made of the presence or absence of foam lines around Cape Arago. (B) Coastline near Depoe
Bay, Oregon. Note foam lines extending across the mouths of Pirate Cove and Whale Cove, and the incomplete foam line across the mouth of
Rocky Creek Cove. (C) North of Depoe Bay, Oregon. Note foam lines across mouths of Boiler Bay and Pirate Cove. (D) Foam line generated by a
rip current.
<0.5. The lowest salinity was often adjacent to the beach,
undoubtedly due to Big Creek, which empties into Sunset
Bay. In summer, freshwater input is small (0.08 m3 s1 in
June and <0.02 m3 s1 in July, August and September;
Coos County Water Resources Department) and has little
effect on salinity in the Bay. In winter, however, larger
freshwater input (at times >5 m3 s1) may cause Sunset
Bay to behave as a small estuary.
Ten CTD transects were made across Sunset Bay
during three summers. On nine cruises, winds were
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Distance Offshore, Km
Fig. 3. Temperature ( C) isotherms from CTD transects sampled at Sunset Bay, Oregon. Diamonds above each figure indicate station locations.
The black area at the bottom left of each figure represents the bottom. Downward arrows indicate the location of the foam line and front at the
mouth of Sunset Bay.
upwelling favorable (i.e. NW) and on one (1 August 1997)
winds were downwelling favorable (i.e. SW). During
upwelling winds, a foam line extended completely across
the bay mouth. We observed a shallow (<5 m deep) lens of
warmer water landward of the foam line and in Sunset Bay
with water 0.2–0.5 C cooler seaward of the foam line
(Figure 3). Warmest waters were closest to the beach. On
1 August 1997, during downwelling winds, the foam line
did not extend across the mouth and there was no difference
in surface water temperatures.
During upwelling winds, the deeper isotherms (>5 m)
were bent downward just seaward of the bay mouth and
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and varied little along the transects (Figure 4). On
days with both higher Chl concentrations and upwelling
conditions, the Chl concentration was higher seaward of
the foam line and bay mouth than landward (Figure 4).
On 2 days with upwelling-favorable winds (21 July and
16 August 2000), the highest concentrations of Chl were
distributed vertically through the water column just seaward of the foam line (Figure 4). On 1 August 1997,
foam line (Figure 3). In addition, on several days (25
June 1999, 8 September 1999, 19 and 21 July 2000, 16
and 24 August 2000; Figure 3), isotherms appeared to
dome upward at or just seaward of the foam line. On 1
August 1997, the deeper isotherms were bent upward
and were roughly parallel to the bottom.
On three days (8 September 1999, 19 July 2000 and
24 August 2000), the Chl concentration was low (<1 mg l1)
0
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1.0 1.2 1.4
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Fig. 4. Plots of Chl (mg l1) from CTD transects sampled at Sunset Bay, Oregon. Diamonds above each figure indicate station locations. The
black area at the bottom left of each figure represents the bottom.
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during downwelling winds, the concentration of Chl was
highest inside the bay. Seaward of the bay mouth, Chl
concentrations were around 5–6 mg l1, while inside the
bay, adjacent to the beach, the concentration was up to
25 mg l1 (Figure 4).
During downwelling winds, surface waters within the
bay may not be separated from the coastal waters, and
high concentrations of Chl can be found in the bay. In
contrast, during upwelling conditions, the foam line at
the bay mouth delineated a convergent front separating
warmer waters, generally characterized by low Chl concentrations, within the bay from cooler offshore waters
with higher Chl concentrations and deeper isotherms
tilted downward seaward of this front.
Zooplankton distribution
Zooplankton data are presented for 1 August 1997, 25
June 1999 and 20 July 1999. On the first two dates,
three replicate samples were collected at two stations
landward of the front and one seaward. On the third
date, samples were collected at each station along the
CTD transect. Concentrations of zooplankton landward
of the foam line were compared with seaward concentrations using a Kruskal–Wallis one-way ANOVA by
ranks. With the 20 July 1999 data, stations landward
and seaward of the foam line were considered replicate
samples from their respective habitats. To assist in the
description of the distribution of zooplankton taxa, we
sorted organisms into groups using cluster analysis. Zooplankters were separated into groups based on their
spatial distribution. The concentrations of zooplankton
were standardized such that the mean equaled zero
and the standard deviation equaled one and, using the
Wards Method, zooplankton were grouped into clusters
by Euclidean distance (StatSoft, 1994).
On 25 June and 20 July 1999, winds were upwelling
favorable. On both dates, there were significantly higher
concentrations of nauplii and adult calanoid copepods
seaward of the front than landward (Figure 5; Table I).
Seaward of the foam line, concentrations of nauplii
and adult calanoid copepods were of the order of 104 m3
(Figures 5 and 6). Landward concentrations fell to <103 m3.
Several types of meroplankters had similar distributions.
On June 25 1999, barnacle nauplii stage 3, barnacle
nauplii stages 4, 5 and 6 combined, polychaete larvae with
more than four setigers, mussel larvae, total decapod zoeae
and echinoplutei had distributions similar to those of the
calanoid copepods; concentrations of these meroplankton
decreased landward of the foam line by almost an order of
magnitude (Figure 5). Cluster analysis grouped these taxa
with calanoid copepods (Figure 7). On 20 July 1999, barnacle
nauplii stages 4, 5 and 6 combined, polychaete larvae with
more than three setigers, mussel larvae and total decapod
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zoeae exhibited distributions similar to those of the calanoid
copepods (Figure 5). In most cases, statistical analysis
indicated that significantly more larvae were present seaward
of the front; concentrations fell by around an order of magnitude landward of the front (Figure 5; Table I). Cluster
analysis grouped these meroplankters with calanoid
copepods and their nauplii (Figure 7).
A set of organisms tended to be more concentrated
landward of the front. On 25 June 1999, harpacticoid
copepods, barnacle cyprids, barnacle nauplii stages 1
and 2, and Littorina egg cases were grouped in the same
cluster (Figure 7). However, only Littorina egg cases were
significantly more concentrated landward of the front
(Figure 6). Barnacle cyprids and harpacticoid copepods
were more abundant landward of the front, but not
significantly (Figure 6), and there was little variation in
the concentration of barnacle nauplii stages 1 and 2. On
20 July 1999, the nearshore cluster was composed of
harpacticoid copepods, polychaete larvae with three or
fewer setigers, barnacle nauplii stages 1 and 2, barnacle
nauplii stage 3, barnacle cyprids, Littorina egg cases and
veligers, and phoronid larvae (Figure 7). Harpacticoid
copepods, polychaete larvae with three or fewer setigers,
barnacle nauplii stages 1 and 2, and Littorina egg cases
and veligers were at significantly higher concentrations
landward of the front (Table I). Low concentrations of
barnacle nauplii stage 3, barnacle cyprids and phoronid
larvae were found seaward of the front and close to
shore; highest concentrations were at the stations 0.4
and 0.7 km from shore (Figure 5).
On these 2 days with upwelling winds, the Sunset Bay
front separated two zooplankton communities: one
community was in the bay landward of the front; the
other was seaward of the front. Differences in concentration across the front between stations separated by only
a few hundred meters or less were, in many cases, as large
as an order of magnitude.
On 1 August 1997, winds were downwelling favorable
and zooplankton distributions were quite different than
on days with upwelling winds. Calanoid copepods and
their nauplii were far less abundant, and their concentration did not change significantly across the bay mouth
(Figure 8); highest concentrations were actually found
inside the bay. There were no significant differences in
the concentrations of gastropod veligers, echinoplutei,
total decapod zoeae, mussel larvae, barnacle nauplii
stages 4, 5 and 6, starfish brachiolaria and polychaete
larvae with more than three setigers across the bay
mouth; the highest concentrations were, as with copepods, usually found at the station just inside the bay
mouth (Figure 8). No significant differences were found
in the distributions of Littorina egg cases or veligers
(Figure 8); there was little variation in their concentrations
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A
20000
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1000
26.10
800
Calanoid
15000
Adults
10000
Nauplii
26.1
Nauplii 1,2
Nauplii 3
600
25.60400
5000
Nauplii 4,5
25.6
200
0
25.10 0
0
0.5
1
1.5
C
200
25.1
0
2
600
26
0.5
1
D
Harpacticoids
1.5
2
26.1
Cyprids
Mussels
400
Zoeae
3
B
26
100
25.6
200
25
0
0
0.5
1
E
1000
1.5
0
2
0.5
1
F
26.1
Polychaete Larvae
>4 Setigers
25.6
1.5
2
26.1
Littorina
100
<4 Setigers
500
25.1
0
Eggs
Veligers
25.6
50
25.1 0
0
0
0.5
1
1.5
60
25.1
0
2
G
0.5
1
1.5
2
26.10
Phoronid Larvae
40
25.60
20
0
25.10
0
0.5
1
1.5
2
Distance Offshore, km
Fig. 5. Distribution of zooplankton on 20 July 1999 during upwelling winds in relation to the foam line and front at the mouth of Sunset Bay,
Oregon. Density is plotted as the fine dotted line and the vertical dashed lines indicate the location of the foam line.
across the bay mouth. As on the upwelling sample dates,
no significant variations in the distributions of barnacle
nauplii stages 1 and 2, and barnacle cyprids were
observed; highest concentrations were found at the
station just inside the bay mouth (Figure 8). Significant
variation was only found in the distribution of harpacticoid copepods (Figure 8); significantly higher concentrations were caught at stations inside the bay. Cluster
analysis produced two groups (Figure 7): one composed
of organisms that had their highest concentrations at the
station just inside Sunset Bay; the second composed of
organisms with either highest concentrations at the most
seaward station or little variation in concentration across
the transect. On this date, with downwelling winds and
oceanographic conditions indicative of a relaxation
event, we found little variation in zooplankton distributions across the mouth of Sunset Bay.
Miller’s Cove
Site description
Miller’s Cove is located just to the north of Sunset Bay
(Figure 1). The mouth of the cove has roughly the same
orientation as Sunset Bay. The north side of Miller’s
Cove is composed of Gregory Point (an island). Situated
within the mouth of Miller’s Cove are two narrow
islands oriented parallel to Gregory Point. Deep channels separate the islands. At the landward end of Miller’s
Cove is a shallow opening (nearly closed at low tide)
that separates Gregory Point from the mainland and
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Table I: Results of Kruskal–Wallis one-way ANOVA by ranks comparing stations sampled on
20 July 1999 at Sunset Bay that were seaward ( 1 km from shore, n = 4) and landward of the
foam line (< 1 km from shore, n = 4)
Organism
Distributiona
H
P
Calanoid copepods
+
5.33
0.0209
Copepod nauplii
+
5.33
0.0209
Harpacticoid copepods
–
5.33
0.0209
Barnacle nauplii stages 1, 2
–
4.08
0.0433
0.00
1
5.40
0.0202
1.33
0.2480
+
5.33
0.0209
Polychaete larvae with >3 setigers
_
4.08
0.0433
Mussel larvae
+
4.08
0.0433
Littorina egg cases
+
3.94
0.0472
Littorina veligers
+
4.08
0.0433
Phoronid larvae
0.77
0.3808
Total zoeae
1.71
0.1913
Barnacle nauplii stage 3
Barnacle nauplii stages 4, 5, 6
+
Barnacle cyprids
Polychaete larvae with
3 setigers
Data are plotted in Figure 7.
a
Larvae that were significantly more abundant landward and seaward of the front and foam line are indicated by – and + signs, respectively.
connects Miller’s Cove to Lighthouse Beach. Two foam
lines were often present across the mouth of Miller’s
Cove. The foam lines extended from the landward
end of the two islands in the cove, south to the rock
reef separating Miller’s Cove from Sunset Bay (Figure 2).
We have one complete data set of physical and biological data collected in Miller’s Cove on 24 August 2000.
These data were collected during upwelling-favorable
winds.
Physical oceanography
Surface temperature fronts associated with foam lines
were observed at 0.2 and 0.5 km from shore (Figure 9).
Surface water landward of the foam lines was 0.5 C
warmer than seaward. Like Sunset Bay, the warmest
waters were found within a couple of hundred meters
of shore (Figure 9). Seaward of the foam lines, at
0.6 km from shore, subsurface isotherms were domed
upward, and landward of this dome they bent steeply
downward so that the thermocline (centered around the
9.0 C isotherm) contacted the bottom at 0.1 km from
shore.
Surface Chl concentrations were higher seaward of
the foam line than landward, with lowest values found
immediately adjacent to shore (Figure 10). High concentrations of Chl were found just below the thermocline
where it was bent downward toward the bottom. The
highest concentrations were found beyond 0.8 km from
shore and below 15 m depth.
The oceanographic data, though limited, suggest that,
like Sunset Bay, foam lines at Miller’s Cove delineate
fronts separating a warmer Chl-poor water mass from a
cooler offshore water mass with higher Chl concentrations.
Zooplankton distribution
Zooplankton samples were collected at each CTD
station. The cluster analysis suggested that there were
two patterns of distribution (Figure 11): organisms with
highest concentrations seaward of the inner foam line
and front (i.e. seaward of 0.2 km); and those with their
highest concentrations landward of the inner foam line
(i.e. landward of 0.2 km). The former group of organisms, adult and nauplii calanoid copepods, barnacle
nauplii stages 4, 5 and 6, zoeae, polychaete larvae with
three or fewer setigers and polychaete larvae with more
than three setigers, were significantly more concentrated
seaward of the front (Table II). Barnacle cyprids, phoronid larvae, gastropod veligers, mussel larvae, and
planula, were found at higher, but not significantly
higher, concentrations seaward of this front (Figure 10;
Table II). In contrast, harpacticoid copepods and
Littorina egg cases were most abundant at the stations
landward of the inner foam line. However, only Littorina
egg cases were significantly more concentrated landward
of this front (Table II). These data suggest that associated
with the foam lines in Miller’s Cove are abrupt changes
in the community structure and abundance of a variety
of zooplankters.
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15000
1000
750
10000
500
5000
250
0
Copepod
Nauplii
Calanoid
Copepods
4000
Stages 1,2
Stage 3
Barnacle Nauplii
o
Stages 4,5,6
300
3000
200
2000
100
1000
0
< 3 Setigers
> 4 Setigers
Polychaete Larvae
Cyprids
150
300
Landward of Front
Near Beach
200
Landward of Front
100
Seaward of Front
100
50
Mussel Larvae
0
Littorina Egg
Cases
Harpacticoid
Copepods
Decapod Zoea
500
6000
400
4000
300
200
2000
100
Pseudo-Nitzschia
spp.
Nocticluca
scintillans
Terrestrial Detritus
Fig. 6. Distribution of zooplankton on 25 June 1999 during upwelling winds in relation to the foam line and front at the mouth of Sunset Bay,
Oregon. Plotted are the mean and standard error of three replicate tows.
Shore Acres
Site description
The Shore Acres transect was located 0.5 km south of
Sunset Bay and 300 m north of the Shore Acres State
Park wave viewing kiosk (Figure 1). The shoreline from
Sunset Bay south for 3 km is oriented roughly toward the
NW and composed of rocky reefs and cliffs (Figure 2).
Transects were sampled on 19 July and 16 August 2000,
during upwelling winds. The transect on 19 July was only
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Fig. 7. Cluster analysis of the distribution of zooplankton at Sunset Bay, Oregon, on 25 June and 20 July 1999 during upwelling winds, and on
1 August 1997 during downwelling winds.
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A. L. SHANKS ETAL.
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H=0.7
p=0.08
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VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION
H=0.7
p=0.80
Landward of Front Near Beach
500
Landward of Front
250
Seaward of Front
Copepod
Nauplii
150
H=5.4
p=0.02
Calanoid
Copepods
H=0.6
p=0.44
H=0.3
p=0.61
H=0.1
p=0.80
H=3.3.
p=0.07
Number Per Meter Cubed
100
50
Harpacticoid
Copepods
H=0.0
p=1
Gastropod
Veligers
H=2.4
p=0.12
Echinoplutei
Total Crab
Zoeae
H=2.4
p=0.12
Mussel Larvae
500
H=8.3
p=0.07
20
250
10
Stages 1,2
8
H=2.5
p=0.11
Stage 3
Barnacle Nauplii
H=0.8
p=0.36
Stage 4,5,6
H=0.4
p=0.52
Barnacle Cyprids
H=0.65
p=0.42
6
4
2
Littorina Egg
Cases
Littorina
Veligers
Brachiolaria
Polychaete Larvae
>3 Setigers
Fig. 8. Distribution of zooplankton on 1 August 1997 during downwelling winds in relation to the foam line and front at the mouth of Sunset Bay,
Oregon. Plotted are the mean and standard error of three replicate tows. Values above graphs are results of Kruskal–Wallis one-way ANOVA by ranks.
400 m long, while that on 16 August was 1.4 km long. On
both dates, data were also collected at Sunset Bay.
Physical oceanography
On both days, the distribution of temperature was similar. Approaching the shore, the thermocline isotherms
tilted upward toward the surface. On 19 July, this
occurred within 100 m of shore, and the thermocline
did not contact the surface (Figure 12). A foam line was
located 200 m from shore. On 16 August, the upward
tilt was stronger and began 0.6 km from shore. A
thermal front and associated foam line were located
0.4 km from shore, where the thermocline (9.9 C isotherm) contacted the surface. Landward of this front,
surface waters were cooler and denser than on the seaward side, and isotherms cooler than 9.8 C were bent
downward (Figures 12 and 13).
The lowest concentrations of Chl were found in the waters
within several hundred meters of shore and extended
throughout the water column (Figure 12). Higher Chl
concentrations were within or below the thermocline,
and just seaward of these waters with the lowest Chl
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9.0
0.5
5
1.0
10
10
15
0.8
5
8.6
1.0
15
8.2
1.5
20
20
Temperature
Chl a
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
Distance Offshore, Km
Fig. 9. Plots of temperature ( C) and Chl (mg l1) from CTD transects at Miller’s Cove, Oregon, on 24 August 2000 during upwelling winds.
Diamonds and downward arrows above each figure indicate station and foam line locations, respectively. The black area at the bottom left of each
figure represents the bottom.
6000
10.1
Calanoid Adult Nauplii
9.9
4000
2000
0
0.0
A
0.5
200
3
9.3
9.9
Zoeae
C
9.5
B
0.5
9.5
Polychaete Larvae <3 >3 Setigers
Cyprids
D
9.9
9.7
9.7
0
0.0
9.5
9.3
0.5
1.0
10.1
Mussels
200
9.9
Littorina Eggs
9.7
100
9.5
9.3
0.5
10.1
9.9
10.1
E
9.3
1.0
200
1.0
500
40
9.7
400
9.3
0.5
1000
0
0.0
0
0.0
9.9
9.7
50
1500
9.5
250
10.1
Barnacle Nauplii 1,2 3 4,5,6
10.1 600
100
0
0.0
9.7
500
1.0
Harpacticoids
150
750
0
0.0
1.0
F
0.5
9.5
9.3
1.0
10.1
Larval Gastropods Phoronids
9.9
9.7
20
0
0.0
G
0.5
9.5
9.3
1.0
Distance Offshore, km
Fig. 10. Cluster analysis of the distribution of zooplankton at Miller’s Cove, Oregon, on 24 August 2000.
concentrations. On 16 August, the highest Chl concentrations were beneath the front where the thermocline
intercepted the surface (Figure 12).
Distributions of oceanographic parameters at Sunset
Bay on 19 July and 16 August (Figures 3 and 4) were
quite different from those observed at Shore Acres
(Figure 12). At Sunset Bay on 19 July, there was a lens
of warm water adjacent to shore, and at the bay mouth
isotherms were bent downward. No lens of warm water
was observed off Shore Acres and isotherms tilted
upward as shore was approached (Figure 12). At Sunset
Bay on 16 August, there was also a lens of warm water
adjacent to shore and isotherms within the thermocline
(9.9–9.0 C) were horizontal. At Shore Acres, waters
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Copepod Nauplii
Barnacle Nauplii 1,2
Gastropod Larvae
Calanoid Copepods
Total Zoeae
Barnacle Nauplii 4,5,6
Barnacle Nauplii 3
Mussel Larvae
Cyprids
Polychaetes-Early
Polychaetes-Late
Phoronid Larvae
Harpacticoid Copepds
Littorina Egg Cases
j
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VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION
20
30
40
50
60
70
80
90
100
Cluster #1 - Highest Concentrations
Found Seaward of the Inner Front
Cluster #2 - Highest Concentrations
Found Landward of the Inner Front
Fig. 11. Distribution of zooplankton on 24 August 2000 during upwelling winds in relation to the foam line and front at the mouth of Miller’s
Cove, Oregon.
Table II: Results of Kruskal–Wallis one-way ANOVA by ranks comparing stations sampled on
24 August 2000 at Miller’s Cove that were seaward (0.2 km from shore, n = 5) and landward of
the foam line (< 0.2 km from shore, n = 3)
Organism
Distributiona
H
P
Calanoid copepods
+
5.06
0.025
Copepod nauplii
+
5.06
0.025
Harpacticoid copepods
1.14
0.285
Barnacle nauplii stages 1, 2
2.36
0.124
Barnacle nauplii stage 3
2.69
0.101
5.00
0.025
3.24
0.072
+
4.46
0.035
+
5.00
0.025
1.12
0.291
Barnacle nauplii stages 4, 5, 6
+
Barnacle cyprids
Polychaete larvae with
3 setigers
Polychaete larvae with >3 setigers
Mussel larvae
Littorina egg cases
–
6.56
0.010
Phoronid larvae
+
5.38
0.020
5.24
0.022
Total zoeae
Data are plotted in Figure 12.
Larvae that were significantly more abundant landward and seaward of the front and foam line are indicated by – and + signs, respectively.
a
close to shore were cooler than offshore and the thermocline was tilted upward, contacting the surface. Although
Sunset Bay and Shore Acres were sampled only an hour
apart and are separated by <500 m, the distribution of
water masses was dramatically different. Foam lines and
fronts were present at both locations, but very different
distributions of oceanographic properties at the sites
suggest that different mechanisms must be generating
the foam lines and fronts.
Zooplankton distribution
The zooplankton data from 19 July and 16 August 2000
are presented in Figures 13 and 14. Data from the longer
transect sampled on 17 August were analyzed statistic-
ally and this analysis suggests there were two general
patterns of zooplankton distribution (Figure 15). The
first pattern was typified by the distribution of calanoid
copepods and their nauplii. Concentrations were relatively high at the most seaward stations, peaked at
the station on the seaward side of the thermal front
(Figure 13A, 0.7 km), and then fell to low concentrations
at stations landward of the thermal front, with lowest
concentrations at the station adjacent to shore. Note,
however, that concentrations varied by only about a
factor of four over the entire transect. Organisms with
similar distributions to calanoid copepods included
gastropod veligers, mussel larvae, nudibranch veligers,
zoeae and Littorina veligers (Figure 16). Only zoeae were
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1.5
10
0.4
0.6
0.8
1.0
2.5
1.2
0.2
1.0
1.5
10
1.5
2.0
1.5
Chl a
0.4
1.4
1.5
2.0
15
Chl a
9.6
1.0
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2.5
0.0
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1.0
1.5
2.0
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9.2
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8.8
Temperature
Temp.
5
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8.4
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1.0
9.8
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9.2
0.0
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10.0
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16 August
0
5
j
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Distance Offshore, Km
Fig. 12. Plots of temperature ( C) and Chl (mg l1) from CTD transects at Shore Acres, Oregon, on 19 July and 16 August 2000, days with
upwelling winds. Diamonds and downward arrows above each figure indicate station and foam line locations, respectively. The black area at the
bottom left of each figure represents the bottom.
significantly more concentrated seaward of the front
(Table III).
The second pattern of zooplankton distribution can be
typified by the distribution of polychaete larvae with
three or fewer setigers (Figure 13E). Seaward of the
thermal front, concentrations were very low, increased
rapidly landward of the front and then fell sharply (by a
factor of 10) within 200 m of shore. Organisms with
similar distributions included harpacticoid copepods, all
stages of barnacle nauplii, barnacle cyprids, polychaete
larvae with more than three setigers and planula. Of
this list, however, only barnacle stages 1 and 2, and
polychaete larvae with three or fewer setigers were
significantly more concentrated landward of the front
(Table III).
Distributions of zooplankton along the shorter transect sampled on 19 July 2000 appear similar to those
within 500 m of shore on 16 August (Figures 13 and 15).
On both dates, relatively high concentrations of copepods and copepod nauplii were found within 100 m of
shore. As on 16 August, relatively high concentrations of
all stages of barnacle nauplii, bivalve larvae, gastropod
veligers and polychaete larvae were found within 200 m
of shore. As on 16 August, cyprids and harpacticoids
were most abundant close to shore.
The difference in zooplankton distributions at Sunset
Bay (Figures 5 and 7) and Shore Acres (Figures 13 and
14) are most clearly demonstrated with the distribution
of calanoid copepods. At Sunset Bay, calanoid copepods
were significantly more abundant seaward of the front,
and their numbers fell by at least an order of magnitude
across the front. In contrast, at Shore Acres, while the
highest concentration of calanoid copepods was on the
seaward side of the front, numbers remained high to
within 200 m of shore, and even at the most landward
station their numbers were reduced by only a little more
than half. Unlike at Sunset Bay, ‘offshore’ zooplankton
taxa were found at relatively high concentrations within
200 m of shore at Shore Acres.
Nellies Cove
Site description
Nellies Cove is located 0.5 km from the breakwater at
Port Orford, Oregon (Figure 1) and 12 km south of
Cape Blanco, a major upwelling center. At Port Orford,
a headland (altitude 100 m) juts out into the ocean,
forming a bight to the south that is sheltered from the
north winds (Figure 1). Nellies Cove is composed of two
narrow (<100 m wide) coves surrounded by high cliffs
(Figure 1). Transects began <30 m from the cobble
beach at the head of the western arm of Nellies Cove
and ran along the long axis of the cove, and out to sea.
Physical oceanography
On 17 and 31 August 2000, a slender foam line (width
1 m) extended across the mouth of Nellies Cove.
The foam line did not extend straight across the
mouth, but rather bulged out from the bay mouth
50 m. On both dates, we observed a change in water
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26.0 3000
A
6000
B
Barnacle Nauplii 1,2 3 4,5,6
26.0
2000
4000
25.9
25.9
1000
2000
Calanoid Nauplii Adults
0
0.0
100
0.5
25.8
0
25.8
0.0
1.5
Harpacticoids
C
3
1.0
26.0 600
0.5
D
1.5
26.0
Cyprids
Zoeae
80
1.0
Mussels
400
60
25.9
25.9
40
200
20
0.0
2000
0.5
1.0
1.5
Polychaete Larvae
E
25.8
0
25.8
0
0.0
26.0100
0.5
1.5
26.0
Zoeae
F
Nudibranch Larvae
<3 Setigers
1500
1.0
>3 Setigers
25.9 50
1000
25.9
500
25.8 0
0
0.0
0.5
1.0
1.5
25.8
0.0
0.5
1.0
1.5
Distance Offshore, km
Fig. 13. Distribution of zooplankton on 16 August 2000 during upwelling winds in relation to the foam line and front at Shore Acres, Oregon.
Density is plotted as the fine dotted line and vertical dashed lines indicate the location of the foam line.
coloration associated with this foam line: bay waters were
milky in appearance, while those offshore were clear.
Warmest waters were located inside Nellies Cove and
landward of the foam line (Figure 16). The fluorometer
failed on 17 August. On 31 August, Chl concentrations
were highest below the foam line and front at the cove
mouth (Figure 16); concentrations were up to seven times
higher than to either side of the front. The foam line
delineated a front in water coloration and temperature.
At 0.5 (17 August) and 1.1 km (31 August) from shore
we observed a second surface thermal front (Figure 16).
At 1–1.5 km from shore, seaward of the shelter of the
Port Orford Bight, the mixed layer deepened and isotherms below 15 or 20 m tilted downward (Figure 16).
Zooplankton distribution
On 17 August 2000, there were two patterns of zooplankton distribution (Figure 17). Most organisms were
more abundant seaward of the foam line at the cove
mouth (Figure 18). For example, calanoid copepods
were at concentrations around 104 m3 seaward of the
foam line, but concentrations fell to 102 m3 inside the
cove. Significantly higher concentrations of copepod
nauplii, calanoid copepods, barnacle nauplii stage 3,
barnacle nauplii stages 4, 5 and 6, phoronid larvae,
toredo larvae and total zoeae were found seaward of
the front (Table IV).
The cluster analysis identified a second group of
organisms most concentrated around the foam line at
the cove mouth (Figure 17). The distributions of harpacticoid copepods and their nauplii were particularly
striking (Figure 18). At the two stations centered on the
foam line, harpacticoid nauplii were at concentrations
between 104 and nearly 105 m3, but just tens of meters
away, at adjacent stations, their numbers fell to near
zero.
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Fig. 14. Distribution of zooplankton on 19 July 2000 during upwelling winds in relation to the foam line and front at Shore Acres, Oregon.
Density is plotted as the fine dotted line and vertical dashed lines indicate the location of the foam line.
Copepod Nauplii
Hermit Crab Zoeae
Gastropod Veligers
Calanoid Copepods
Mussel Larvae
Nudibranch Veligers
Harpacticoid Copepds
Barnacle Nauplii 1,2
Cyprids
Barnacle Nauplii 3
Barnacle Nauplii 4,5,6
Polychaetes-Late
Polychaetes-Early
Cluster #1 - Highest Concentrations Found
Seaward of the Front
Cluster #2 - Highest Concentrations Found
Landward of the Front
Fig. 15. Cluster analysis of the distribution of zooplankton at Shore Acres, Oregon, on 16 August 2000.
Conditions favoring shore-parallel foam
lines
During periods of large waves (nearshore amplitude
>2 m), shore-parallel foam lines were seldom present
(Table V). None was present at Sunset Bay, Miller’s
Cove, Cape Arago or South Cove during these conditions. At Simpson Reef/Shore Acres, out of 21 largewave days, there were shore-parallel foam lines on four.
On days with large waves, if foam lines were present,
they were nearly always lobed shaped (Figure 2D), indicative of foam lines generated by rip currents. Shoreparallel foam lines were much more common when
waves were smaller (nearshore amplitude <2 m; Table V).
On several winter days we observed Sunset Bay during large wave events. Larger waves arrived in sets,
causing the sea level to rise 0.3 m. With diminishing
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Fig. 16. Plots of temperature ( C) and Chl (mg l1) from CTD transects at Nellies Cove, Port Orford, Oregon, on 17 and 31 August 2000 during
upwelling winds. Chlorophyll data for 17 August were lost due to mechanical problems. Diamonds and downward arrows above each figure
indicate station and foam line locations, respectively. The black area at the bottom left of each figure represents the bottom.
wave size, excess water pushed into Sunset Bay flowed
out as a narrow fast current that generated a rip current
foam line extending hundreds of meters seaward from
the bay mouth. When waves were large, water was
rapidly flushed from the bay and exchanged with coastal
water.
The mouths of Sunset Bay and Miller’s Cove have
roughly the same north–south orientation and shoreparallel foam lines were generated by the same conditions. At Sunset Bay, when waves were relatively small
and winds were from the NE, SE or SW (51 days), a
shore-parallel foam line was observed once (Table V).
Under the same conditions at Miller’s Cove (35 days),
there were 4 days with shore-parallel foam lines (Table
V). When waves were small and winds were other than
upwelling favorable, either no foam lines were present at
the bay mouths or foam lines did not extend completely
across the mouth of the bays but were attached to rocks
on the south side of the bays and curved sharply into the
bays. When waves were small and winds were from the
NW (upwelling favorable), shore-parallel foam lines
extending completely across the mouths of the bays
were common: 46 out of 46 days at Sunset Bay and 18
out of 21 days at Miller’s Cove (Table V).
South Cove opens to the south (Figure 2); shoreparallel foam lines were seldom observed. Out of 16 days
with large waves, shore-parallel foam lines were present
only once (Table V). Even when waves were small,
shore-parallel foam lines were not common; during
small waves and winds from the NE, SE or SW (28 days),
shore-parallel foam lines were present on three dates.
In contrast to Sunset Bay and Miller’s Cove, out of
16 days with small waves and NW winds, shore-parallel
foam lines were never present.
The observations made at Simpson Reef overlook and
Cape Arago were of more exposed sections of shoreline
(Figure 2A). Both shorelines have a roughly north–south
orientation. At Simpson Reef overlook, the observations
were made to the north of Simpson Reef itself, toward
Shore Acres State Park. This is exposed coast with no
offshore rocks and deeper water (>10 m depth) within
tens of meters of shore. At Cape Arago, the shore is equally
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Table III: Results of Kruskal–Wallis one-way ANOVA by ranks comparing stations sampled on
16 August 2000 at Shore Acres that were seaward (0.4 km from shore, n = 4) and landward
of the shore-parallel foam line (< 0.4 km from shore, n = 3)
Organism
Distributiona
Calanoid copepods
3.13
H
P
0.077
Copepod nauplii
0.00
1.00
Harpacticoid copepods
0.951
0.330
Barnacle nauplii stages 1, 2
–
Barnacle nauplii stage 3
4.50
0.034
0.80
0.373
Barnacle nauplii stages 4, 5, 6
0.80
0.373
Barnacle cyprids
2.00
0.157
3 setigers
3.92
0.048
Polychaete larvae with >3 setigers
0.80
0.373
Polychaete larvae with
–
Mussel larvae
0.292
0.589
Nudibranch larvae
3.08
0.079
Gastropod veligers
1.13
0.289
4.67
0.031
+
Total zoeae
Data are plotted in Figure 14.
a
Larvae that were significantly more abundant landward and seaward of the front and foam line are indicated by – and + signs, respectively.
1
Copepod Nauplii
Mussel Larvae
Total Zoeae
Barnacle Nauplii 1,2
Polychaetes-Late
Calanoid Copepods
Barnacle Nauplii 3
Barnacle Nauplii 4,5,6
Toredo
Cyprids
Polychaetes-Early
Planula
Gastropod Larvae
Harpacticoid Copepds
Phoronid Larvae
10
20
30
40
50
60
70
80
90
100
Cluster #1 - Highest Concentrations
Seaward of the Front at the Mouth of
Nellie’s Cove
Cluster #2 - Highest Concentrations Landward of the
Front at the Mouth of Nellie’s Cove
Fig. 17. Cluster analysis of the distribution of zooplankton at Nellies Cove, Oregon, on 17 August 2000.
exposed to large waves, but offshore rocks extend from
Simpson Reef past the Cape Arago site. When large waves
were present, shore-parallel foam lines were observed on
four out of 21 days at Simpson Reef and never at Cape
Arago (Table V). Shore-parallel foam lines were more
common when waves were small at Simpson Reef (10
out of 62 days), but they were not preferentially associated
with any wind direction (Table V). At Cape Arago, when
waves were small and winds were from the NE, SE or SW,
shore-parallel foam lines were present on only two out of
37 days, but, when winds were from the NW, they were
present on 11 out of 21 days (Table V).
Spatial distribution of shore-parallel foam lines
Several hundred aerial photographs archived at the
NOAA Mapfinder website were inspected for the
presence of shore-parallel foam lines or changes in water
coloration associated with coves and headlands. The
photographs are standard color aerial survey photographs taken of the shoreline between the Straits of
Juan de Fuca, Washington, and San Francisco Bay, California. Resolution is generally excellent and foam lines and
changes in water coloration are readily apparent (Figure 2).
The survey flights were made during clear weather and
often during light winds (i.e. calm seas without white caps
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Fig. 18. Distribution of zooplankton on 17 August 2000 during upwelling winds in relation to the foam line and front at the mouth of Nellies
Cove, Oregon. Temperature is plotted as the fine dotted line and vertical dashed lines indicate the location of the foam line.
Table IV: Results of Kruskal–Wallis one-way ANOVA by ranks comparing stations sampled on
17 August 2000 at Nellies Cove that were seaward (0.3 km from shore, n = 4) and landward
of the shore-parallel foam line (<0.3 km from shore, n = 4)
Distributiona
Organism
H
P
Calanoid copepods
+
5.333
0.021
Copepod nauplii
+
5.333
0.021
Harpacticoid copepods
2.86
0.907
Harpacticoid nauplii
2.29
0.131
Barnacle nauplii stages 1, 2
3.04
0.081
Barnacle nauplii stage 3
+
3.94
0.047
Barnacle nauplii stages 4, 5, 6
+
6.05
0.014
0.75
0.387
2.11
0.147
Barnacle cyprids
Polychaete larvae with
3 setigers
Polychaete larvae with >3 setigers
2.99
0.083
Mussel larvae
2.99
0.083
Toredo larvae
+
6.05
0.014
Phoronid larvae
+
3.94
0.047
3.15
0.076
6.14
0.013
Gastropod veligers
Total zoeae
+
a
Larvae that were significantly more abundant landward and seaward of the front and foam line are indicated by – and + signs, respectively.
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Table V: Observations of the presence or absence of shore-parallel foam lines at locations
around Cape Arago, Oregon
Location
Wave size
Shore-parallel foam line present
Sunset Bay
Large, >2 m
Yes
0
0
0
0
No
0
20
13
6
Small, <2 m
Miller’s Cove
Large, >2 m
Small, <2 m
Shore Acres/Simpson Reef
Large, >2 m
Small, <2 m
Cape Arago
Large, >2 m
Small, <2 m
South Cove
Large, >2 m
Small, <2 m
NE winds
SE winds
SW winds
NW winds
Yes
0
0
1
46
No
8
18
24
0
Yes
0
0
0
1
No
0
3
11
5
Yes
2
0
2
18
No
4
10
17
3
Yes
0
0
3
1
No
0
3
9
5
Yes
1
2
3
4
No
6
11
16
19
Yes
0
0
0
0
No
0
3
11
5
Yes
1
0
1
11
No
6
12
17
10
Yes
0
0
1
0
No
0
3
8
4
Yes
1
1
1
0
No
5
12
8
16
Wind direction is the direction from which the wind was blowing (meteorological standard). Wind data are from the NOAA Cape Arago weather station. Wave
size was estimated by the observer.
and with slicks present in the offshore waters), conditions
not conducive to the formation of shore-parallel foam lines.
Sets of photographs were taken on different dates and
multiple sets of photographs were available of the shore.
There were large variations in the presence of shoreparallel foam lines between photographs of the same
shore on different days. When seas indicated that
winds were light or white caps were from the south,
shore-parallel foam lines were seldom observed. However, when the white caps were from the north, shoreparallel foam lines and nearshore water color changes
were more common (Figure 2). The presence of a foam
line at a cove mouth appeared to depend on the orientation of the cove mouth relative to wind direction.
Generally, when the mouth of a cove was oriented
roughly parallel to the direction of white caps, a foam
line was present when the white caps were from the
north. When the bay mouth was perpendicular to the
direction of white caps, shore-parallel foam lines were
much less common (e.g. South Cove; Figure 2A).
Between the Straits of Juan de Fuca and San Francisco
Bay, we found 88 photographs that captured shoreparallel foam lines or changes in water coloration at
coves and nine photographs with shore-parallel foam
lines extending from promontories (Table VI). Under
the right conditions, shore-parallel foam lines are relatively common.
DISCUSSION
During research extending over three summers, we
extensively sampled Sunset Bay and collected data sets
at two additional coves (Miller’s Cove and Nellies Cove)
and a section of exposed open coast (Shore Acres). At
each site, we found shore-parallel foam lines associated
with fronts and large (order of magnitude) changes in
abundances of zooplankton across the foam lines and
fronts. The physical and biological oceanography of
Sunset Bay and Miller’s Cove appears similar, but that
of Nellies Cove and Shore Acres appears quite different.
At the four sites there may be at least three different
mechanisms for forming shore-parallel foam lines and
very nearshore fronts. If we had sampled more sites, we
may have found fronts generated in other ways. The
shore-parallel foam lines sampled for this study are
common features of all rocky shoreline; the physical
and biological oceanography described here is probably
what one will find along shorelines around the world.
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Table VI: Results of a search for
shore-parallel foam lines or water color
changes associated with the mouths of coves,
bays or points in the aerial photographs
available on the NOAA mapfinder website
Location
Number of photographs
with shore-parallel foam lines
or water color changes
Washington
Bays/coves
Points
36
2
Oregon
Bays/coves
Points
32
5
Northern California
Bays/coves
Points
20
2
All photographs available at the website (http://www.mapfinder.nos.noaa.gov) as of August 2000 between the Straits of Juan de Fuca, Washington,
and San Francisco Bay, California, were inspected.
At Sunset Bay and Miller’s Cove, shore-parallel foam
lines extending across the mouth of the bays were nearly
always present when waves were small (<2 m amplitude)
and winds were upwelling favorable (i.e. NW; Table V).
When waves were large (>2 m in amplitude) or winds were
other than upwelling favorable, shore-parallel foam lines
were rare. During winter, waves on the Oregon coast are
frequently large and winds are often downwelling favorable; we seldom observed shore-parallel foam lines. In
summer, waves are generally smaller and winds are often
upwelling favorable for days; foam lines were common and
persistent. Thus, there is a seasonal component to the
presence of foam lines at the mouth of these bays.
Given the consistency with which foam lines formed
during periods of relatively small waves and upwelling
winds (Table II), we can estimate the amount of time
during summer that foam lines occupy the mouths of
these coves. During summer 2000, small waves and NW
winds were observed on 73 days; foam lines should have
been present 80% of the time. The average duration of
these conditions was 6 days (range 1–21 days) and there
were four periods that lasted a week or longer.
Foam lines and associated fronts appear to act as
barriers to shoreward migration of larvae that developed
over the shelf (e.g. mussel larvae, zoeae, echinoplutei and
polychaete larvae) and, for much of summer, settlement
of these larvae may be limited. During summer, settlement within these bays of larvae developing offshore
may only occur during downwelling winds, when the
front is not present. Settlement of larvae with offshore
development may be higher during winter when the
front is not present to act as a barrier.
The front may also act as a barrier to the seaward
movement of larvae spawned in the bay. For example,
during upwelling winds, Littorina egg cases were only
caught on the landward side of the front (Figures 5 and
6), but they were distributed throughout the bay and out
into coastal waters on 1 August 1997 during downwelling winds when the front was not present. Larvae
spawned within these bays may remain there as long as
the front prevents their dispersal into coastal waters. We
found a large seasonal variation in the presence of the
front: it was seldom present in winter and often present
in summer. Perhaps species dependent on larval retention for successful recruitment spawn during summer.
During summer, the front comes and goes with changes
in wind direction. Larvae spawned during upwelling
conditions, when the front is present, may be flushed
from the bay upon a wind reversal to downwelling
winds. Within a summer, recruitment of larvae with
short planktonic durations may vary depending on the
stability of the front; relatively long periods of upwelling
winds will produce a stable front that may lead to high
recruitment, while frequent wind reversals may lead to
the reverse.
We find the distributions of barnacle larvae in Sunset
Bay particularly interesting. The abundances of naupliar
stages 1–3 and cyprids either did not vary significantly
across the front or were higher in Sunset Bay than offshore. In contrast, late-stage barnacle nauplii (stages 4, 5
and 6) were, on days with upwelling winds, more abundant outside the bay (Figures 5 and 6), but when the
wind was downwelling favorable there was not a significant difference in their abundance across the front
(Figure 8). The front appears to act as a barrier to
dispersal of late-stage barnacle nauplii, but not to
early-stage nauplii or cyprids. These differences may be
due to ontogenetic changes in vertical distributions of
larval stages or their behavior upon contacting the convergent front. Because cyprid abundance varied little
across the front, barnacle settlement may not be different
on either side of the front.
What causes the foam line and front at the mouth of
Sunset Bay and Miller’s Cove? What we know at this
point is that the foam lines extend across the mouth of
coves only during upwelling winds (in the southern
hemisphere, upwelling winds from the south generate
shore-parallel foam lines at the mouth of bays; Shanks
and Manrı́guez, unpublished data), that the foam line
delineates a convergent front and that frequently there is
a pair of parallel foam lines. The foam line across the
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mouth of Sunset Bay extended across the entire width of
the bay mouth, but foam lines across the mouth of larger
bays tend to degrade in the downwind direction (As
Shanks and Manrı́guez, personal observation). At times,
isotherms were found to dome upward just seaward of
the foam line at the mouth of Sunset Bay (Figure 3),
suggesting a possible divergence in the flow just seaward of
the convergent front. Without additional oceanographic
data, particularly data on water flow, we can only speculate
as to the mechanism forming the fronts and foam lines.
In both data sets from Shore Acres, we found isotherms tilted upward close to shore. We suggest that
the observed temperature distribution was due to boundary mixing (Wolanski and Hamner, 1988). Enhanced
nearshore mixing could be due to breaking waves in
the surf zone or to turbulence generated by flow over
rough and sloping bottom. Waters bathing the shore
appeared to be a mixture of surface waters and waters
from within and just below the thermocline. A convergent front was observed where the thermocline contacted the surface. Oceanographic conditions at Shore
Acres were quite different from those observed several
hundred meters away at Sunset Bay and Miller’s Cove.
The distribution of zooplankton at Shore Acres was
quite different from that observed at Sunset Bay. Holoand meroplankters that were most abundant offshore of
the front at the mouth of Sunset Bay were found at
relatively high concentrations close to the coast at Shore
Acres and landward of the front. For example, during
upwelling winds, calanoid copepod numbers fell by at
least an order of magnitude across the front at Sunset
Bay. In contrast, at Shore Acres, the concentration of
copepods within 100 m offshore, well landward of the
foam line, was about half the average concentration
along the transect. Mussel larvae, late-stage barnacle nauplii, gastropod veligers, cyprids and late-stage polychaete
larvae were all at relatively high concentrations shoreward
of the foam line and close to shore. The front off Shore
Acres may not act as a barrier to shoreward movement of
larvae preparing to settle in the intertidal zone; settlement
rates of some larval types (e.g. mussels) may be higher in
the intertidal zone at Shore Acres than in Sunset Bay.
Unlike the Sunset Bay front, that at Shore Acres does
not appear to be a barrier to the shoreward dispersal of
zooplankton. During boundary mixing, intermediate
density water is formed by mixing surface and deeper
waters. Water of intermediate density flows away from
the mixing zone (e.g. seaward) at depth in the water
column (Wolanski and Hamner, 1988); water must be
continually added to the boundary-mixing zone from
both surface and deeper waters. Organisms in the mixed
layer should move across the front as new water is added to
the boundary-mixing zone. Organisms below the mixed
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layer may be carried to the surface by boundary mixing,
leading to high numbers next to shore; in a sense, they
disperse under, rather than across, the front. Testing this
hypothesis awaits collection of vertically stratified plankton
samples across boundary-mixing fronts.
Nutrient concentrations are usually higher below
the thermocline than within or above it (Mann and
Lazier, 1991). In boundary mixing, nutrient-rich subthermocline waters are brought to the surface and
these waters mixed with surface waters bathe the shore.
Productivity of intertidal and subtidal algae may be
higher in areas of boundary mixing than settings like
Sunset Bay or Nellies Cove where waters landward of
the front may become nutrient depleted.
Nellies Cove, situated on the south side of a high
promontory (Figure 1), is completely sheltered from
north winds. During both visits to Nellies Cove, the
winds were from the NW, yet within the cove the air
was still. The mouth of the cove faces south. The front at
Nellies Cove cannot be generated by the same mechanism that produces the foam lines and fronts at Sunset
Bay and Miller’s Cove. Waters within Nellies Cove were
warmer than those seaward of the foam line, and the
foam line bulged out from the cove mouth. Nellies Cove
appears to be a small thermal estuary; that is, because
waters within the cove are sheltered from winds, they
are fairly stagnant and solar warming occurs (Boden,
1952; Wolanski and Hamner, 1988; Kingsford, 1990;
Kingsford et al., 1991; Bakun, 1996). Like an estuary,
these lower density waters expand out of the cove and
the foam line delineates the front separating these buoyant
waters from the surrounding ocean. We were unable to
sample South Cove (Figure 2) at Cape Arago, but given
the orientation of this cove and the high headland to the
north, it is possible that South Cove is oceanographically
similar to Nellies Cove.
Boden described a similar phenomenon in Bermuda:
lagoon waters, heated by the sun, expand out into the
ocean and zooplankton concentration changed across
the thermal front separating lagoon from ocean waters
(Boden, 1952). Our observations were similar: zooplankton concentrations changed across the Nellies Cove
thermal front. Most zooplankters were more abundant
seaward of this front. The exception was harpacticoid
copepods and their nauplii, both were abundant within
the front and rare or absent to either side. The front at
Nellies Cove appears to act as a barrier to the shoreward
distribution of a variety of holo- and meroplankters.
Settlement inside Nellies Cove by meroplankters that,
due to the front, are prevented from entering the cove
may be low. Given the position of offshore rocks (The
Heads and Tichenor Rock; Figure 1), Nellies Cove may
also be sheltered from SW winds as well. The waters
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within Nellies Cove may not be disturbed until they are
flushed by swells from the first winter storms; cove
waters may be little disturbed for months.
The oceanography of Port Orford Bight appears to be
complex. We observed a front at the mouth of Nellies
Cove and additional fronts at 0.5 and 1–1.2 km from
shore (Figure 18). The geographic setting of Port Orford
Bay is quite similar to the north side of Monterey Bay,
California, where Graham and co-workers also found a
lens of warm water on the lee side of a bay for an
upwelling-favorable wind (Graham et al., 1992; Graham,
1993; Graham and Largier, 1997). They called this an
‘upwelling shadow’. Port Orford bight also appears to be
the site of an upwelling shadow. It is not clear, however,
which offshore thermal front represents the upwelling
shadow front. Graham and co-workers also described
the zooplankton community across the front. It is tempting to compare data sets collected in such similar geographic settings, but large differences in zooplankton
sampling techniques prevent such a comparison.
Most studies of coastal plankton have been made from
large research vessels and these vessels seldom sample
close to shore; samples collected even a kilometer offshore are rare. The zooplankton distributions reported
here are fairly unique; there is little reported data from
within 1 km of shore and hardly any from within
hundreds of meters of shore (Gaines et al., 1985). Roughgarden and co-workers have published a series of papers
in which they have presented increasingly sophisticated
models that attempt to predict barnacle population
dynamics from pre- and post-settlement processes
(Roughgarden et al., 1988; Possingham and Roughgarden,
1990; Alexander and Roughgarden, 1996). In these
models, larval barnacles are released at the shore, development occurs in coastal waters and, at the end of the
planktonic phase, the distance they have dispersed from
shore depends on the strength of upwelling. During strong
upwelling, they are carried well offshore and settlement
is low. During weak or no upwelling, the reverse is true.
The idea that upwelling affects the distance to which
barnacle larvae are dispersed offshore was developed
from inspection of CalCOFI (California Cooperative
Oceanic Fisheries Investigation) samples collected off
central California (Roughgarden et al., 1988). In these
samples, the stations closest to shore were 9 km from
land. Owing to the mesh size (505 mm) of the nets used,
they only caught stage 6 barnacle nauplii and they found
low numbers in these samples, generally <8 m3. They
present data suggesting that the offshore limit of the
distribution of stage 6 barnacle nauplii varied positively
with the strength of upwelling. These results are consistent
with their hypothesized effect of upwelling on the dispersal
of barnacle larvae, but 9 km is a goodly distance from
shore and quite a bit might be happening closer to shore
that was missed by the CalCOFI sampling grid.
Our samples were collected from tens of meters to
<2 km from shore during the upwelling season. Peak
concentrations of stage 6 barnacle nauplii along the
transects ranged from 1.6 to 98 m3 with an average
of 52 m3. Stage 6 barnacle nauplii are not competent to
settle, cyprids are the settlement stage. The strength of
settlement into the intertidal zone should vary with the
abundance of settlement stage larvae in waters adjacent
to shore (Gaines and Roughgarden, 1985; Gaines et al.,
1985). In our samples, cyprid concentrations within
1 km of shore ranged from 25 to 1400 m3, with an
average peak concentration of 430 m3. At Shore
Acres, peak cyprid concentrations were within 200 m
of shore.
Perhaps barnacle larvae are not regularly carried far
offshore by upwelling. This could be accomplished by
larvae avoiding the surface Ekman layer that is advected
offshore by the winds. The use of depth regulation by
zooplankton to control their horizontal dispersal is an
old idea originally proposed by Hardy (Hardy, 1956). In
a classic paper, Peterson et al. demonstrated that copepods could maintain their cross-shelf position despite
upwelling and downwelling (Peterson et al., 1979). The
cross-shelf distribution of a species was maintained by
ontogenetic changes in depth. If ontogenetic changes in
the depth of copepod nauplii, organisms with a swimming capacity similar to that of barnacle nauplii, can
affect the subsequent distribution of the adult population, then, perhaps, changes in the vertical position of
barnacle nauplii can affect their transport and distribution as well. Data in a paper by Peterson and Miller
suggest that 90% of the barnacle larvae remain within
3 km of shore (Peterson and Miller, 1976). Shanks and
co-workers found that bivalves can remain within 5 km of
shore during both upwelling and downwelling conditions
(Shanks et al., 2003). Modeling by Austin and Lentz, in
fact, suggests that larvae released from shore are not
carried out to sea during upwelling, but remain close to
shore (Austin and Lentz, 2002). Perhaps most larval
barnacles, or at least those that successfully return to
shore, are found relatively close to the coast and the
stage 6 nauplii that Roughgarden et al. enumerated in
CalCOFI samples were unlucky ones swept far from
shore (Roughgarden et al., 1988).
In a series of papers, Menge and co-workers describe
consistent long-term differences in community structure
and function at two sites, Boiler’s Bay and Strawberry
Hill, on the Oregon coast (Dahlhoff and Menge, 1996;
Menge et al., 1997a,b; Menge, 2000; Sanford and
Menge, 2001). They attribute these differences to bottomup effects: Strawberry Hill is different from Boiler’s Bay
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because it receives more settling larvae and higher concentrations of phytoplankton (Menge et al., 1997b). They
hypothesize that these differences are due to consistent
differences in the offshore oceanography at the sites: the
pattern of upwelling off Boiler’s Bay consistently transports
recently upwelled waters rapidly southward, while off
Strawberry Hill the pattern of upwelling favors the concentration and onshore transport of both phytoplankton
and larvae [see Menge et al. (Menge et al., 1997b) for a
more detailed description of their hypothesis].
We would like to present an alternative hypothesis to
explain differences between Boiler’s Bay and Strawberry
Hill: differences between these sites are due to local
topographically generated flow patterns. We hypothesize
that the local flow pattern at Boiler’s Bay is similar to that
at Sunset Bay and Miller’s Cove. At the mouth of Boiler’s
Bay, a front is generated during upwelling-favorable winds
(A. Shanks, personal observation). Such a front can be seen
in aerial photographs of Boiler’s Bay (e.g. see Figure 2C). If
the front behaves similarly to those at Sunset Bay and
Miller’s Cove, then it may act as a barrier to the onshore
movement of larvae and phytoplankton. We hypothesize
that this front will cause lower larval settlement and
reduced growth of filter feeders within the bay. The front
may also reduce the offshore movement of larvae spawned
within the bay, leading to higher settlement rates of organisms with short planktonic larval durations. This could
generate the observed higher abundances of macroalgae
in Boiler’s Bay. We hypothesize that Strawberry Hill is a
site with nearshore oceanography similar to that at Shore
Acre; waters adjacent to Strawberry Hill are dominated by
surf zone flow and boundary mixing, leading to a more
open system. The concentration of larvae with offshore
development will be higher adjacent to the shore and this
may lead to higher settlement rates in the intertidal zone.
The more open system would tend to flush larvae with
short planktonic periods away from shore, leading to lower
intertidal abundances of adults of these species. Phytoplankton concentrations will be higher close to shore, perhaps, especially during periods when phytoplankters are
concentrated on the thermocline. This may cause higher
growth rates of filter feeders. We hypothesize that differences between Boiler’s Bay and Strawberry Hill are due to
differences in oceanography, but it is the flow patterns
generated by interactions with local topography that are
the critical difference between the sites.
Differentiating between these alternate hypotheses
should be relatively simple. If Menge et al.’s hypothesis
(differences are due to mesoscale differences in oceanography) is correct, then all sites near Boiler’s Bay should
have similar characteristics. If our hypothesis is correct,
then bay sites near Boiler’s Bay (e.g. Whale Cove, Pirate
Cove, Rocky Creek Cove, Depoe Bay; Figure 2B and C)
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should be similar to Boiler’s Bay, but exposed open
coastal sites between these coves should appear more
similar to Strawberry Hill.
There is a growing interest by intertidal ecologists in
attempting to ascribe alongshore variations in the structure of intertidal communities to variations in coastal
oceanography (Menge, 2000). The coastal ocean, by
supplying larvae and phytoplankton, may influence the
composition of intertidal communities and the types of
interactions that take place. Results of this and companion studies on the effect of nearshore fronts on larval
settlement (McCulloch and Shanks, 2003) and the distribution of phytoplankton (Shanks and McCulloch,
2003) directly and importantly impact this area
of study. Our results suggest that the coastal ocean is
not an open bath that washes the shore, unhindered and
unmodified, but that between the coastal ocean and
shore there may be topographically generated fronts
that are more or less open to the passage of continental
shelf waters, and more or less modify these waters before
they contact shore. The oceanography of waters immediately adjacent to shore is a mosaic of processes generated by the interaction of coastal currents with shore
topography. This mosaic, by altering the supply of
larvae, phytoplankton and, perhaps, nutrients, may in
part be responsible for alongshore variations in the
structure of intertidal communities.
ACKNOWLEDGEMENTS
J. April, D. Williams and R. Hooff provided field assistance. This research was funded in part by a National
Science Foundation Small Grant for Exploratory Research
(OCE-0002891). The manuscript was written during
a Fulbright Scholarship to the Pontificia Universidad
Catolica de Chile, Estación Costera de Investigaciones
Marinas in Las Cruces, Chile, and was much improved
by comments from Drs S. Navarrete, P. Manrı́quez and
J. C. Castilla.
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Received on August 28, 2002; accepted on July 15, 2003
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