Journal of Coastal Research
SI
45
179–194
West Palm Beach, Florida
Fall 2004
The Role of Estuarine Hydrodynamics in
the Distribution of Kelp Forests in
Kachemak Bay, Alaska*
G. Carl Schoch*† and Héloı̈se Chenelot‡
†Kachemak Bay National
Estuarine Research Reserve
2181 Kachemak Drive
Homer, AK 99603, U.S.A.
‡245 O’Neill Building
School of Fisheries and Ocean
Sciences
University of Alaska
Fairbanks
Fairbanks, AK 99775, U.S.A.
ABSTRACT
SCHOCH, G.C. and CHENELOT, H., 2004. The Role of Estuarine Hydrodynamics in the Distribution of
Kelp Forests in Kachemak Bay, Alaska. Journal of Coastal Research, SI(45), 179–194. West Palm Beach
(Florida), ISSN 0749-0208.
Understanding the circulation and exchange rates of water masses in an estuary is critical to understanding
the movement and recruitment patterns of planktonic propagules and how these patterns relate to the
distribution and abundance of adults. Forests of bull kelp (Nereocystis luetkeana) are complex three-dimensional habitats that can support tightly linked trophic interactions between primary producers, herbivores,
and carnivores. Therefore, the spatial distribution of this important habitat can affect local food webs. Kelp
forests in Kachemak Bay, Alaska, were mapped from 2000 to 2002 using low-altitude aerial photography.
Over the 3 years, kelp forests were found only in the outer basin, and only a few attached sporophytes were
found in the turbid and lower-salinity inner basin. Many biotic and abiotic factors can affect the dispersal
and development of kelp. We tested the hypothesis that decreased salinity and light intensity in the inner
basin limit the spatial distribution of Nereocystis sporophytes in this system. Transplant experiments were
conducted between the marine and estuarine endpoints of kelp forest distribution. These experiments suggest that the growth of Nereocystis sporophytes transplanted from the marine to the estuarine endpoint
was impeded, but that they could survive under estuarine conditions. We tested a second hypothesis, that
estuarine circulation patterns control Nereocystis sporophyte distribution. Hydrographic measurements and
circulation studies suggest that a strong baroclinic jet develops in the late summer and fall and may prevent
Nereocystis spore dispersal into the inner basin. The cyclonic surface circulation in the outer basin may
contribute to the observed spatial distribution of kelp forests in this system.
ADDITIONAL INDEX WORDS: Nereocystis luetkeana, spore dispersal, distribution, baroclinic currents,
fjords, estuarine circulation, salinity gradient, light intensity.
INTRODUCTION
Kachemak Bay is located in the northern Gulf
of Alaska (GOA) at about 598359 N (Figure 1) and
is the only fjord in the National Estuarine Re* Corresponding author. E-mail: [email protected].
ak.us
* Present address for G. Carl Schoch: Prince William
Sound Science Center, P.O. Box 705, 300 Breakwater Avenue, Cordova, AK 99574, U.S.A.
* This research was made possible by a NERR Graduate Student Fellowship to H.C. Additional support was
provided to H.C. by grants from the Arctic Institute of
North America and Project AWARE (PADI) to purchase
field equipment. The University of Alaska Fairbanks provided laboratory space to H.C. at Kasitsna Bay. The CTD
equipment and profiles and the drift card project were
funded by grants to G.C.S. from the Exxon Valdez Oil
Spill Trustee Council and the Cook Inlet Regional Citizens’ Advisory Council.
search Reserve System (NERRS). The bay has an
area of 1,500 km2, a shoreline length of over 540
kilometers, and a maximum tidal range of 8 meters. The mountains on the south side of the bay
reach over 2,000 meters and are composed of rocky
terrains accreted onto the continental plate margin in association with marine plate subduction
processes. Seven glaciers discharge into Kachemak
Bay and deliver a large volume of freshwater and
sediments during the summer that decrease the
salinity and increase the turbidity along the axis
of the estuary. The bay is separated into two basins (inner bay and outer bay) by a 6-kilometerlong spit extending southeast from the village of
Homer (population 5,000). The bathymetry is characterized by deep trenches and holes extending to
depths of 200 meters. The benthic nearshore habitat of both inner and outer basins of Kachemak
180
Schoch and Chenelot
Figure 1. The location of Kachemak Bay in the Gulf of Alaska.
Bay consists of rocky headlands, seastacks, and
reefs along the south shore. The north shore of the
outer basin consists mostly of mixed boulder, cobble, and sand, but mud flats characterize the north
shore of the inner basin. Kelp forests dominate the
nearshore of the outer basin in depths less than
20 meters but have not been observed in the inner
basin.
The circulation and water mass properties of
Kachemak Bay reflect estuarine, oceanic, and atmospheric forcing. Atmospheric conditions are primarily established by the interaction of cyclonic
(counterclockwise) storms associated with the
Aleutian Low and the coastal mountains surrounding the GOA, leading to very high rates of
coastal precipitation (ROYER, 1998; WILSON and
OVERLAND, 1986). Much of this precipitation is
presumed to enter the ocean relatively rapidly because of the steep terrain, except in the winter season, when it is stored in mountain snowpacks.
Coastal freshwater discharge increases gradually
through spring and summer because of melting of
the snowpack, reaching a maximum in the fall
(ROYER, 1998; WEINGARTNER et al., 2002).
Kachemak Bay is strongly influenced by the seasonal and interannual patterns of the Alaska
Coastal Current (ACC). Farther offshore, the
mean wind field associated with the Aleutian Low
drives the cyclonic flow of the Alaskan Gyre. Along
the continental slopes of the GOA, this flow includes the broad and relatively sluggish Alaska
Current in the eastern and northern Gulf (Figure
1). Over the inner shelf, downwelling favorable
winds and freshwater discharge from the coastal
mountains drive and maintain the ACC. The ACC
originates on the British Columbian shelf and
flows counterclockwise around the GOA (R OYER,
1998). Seasonal variations in wind and coastal
buoyancy-forcing give rise to large changes in the
strength and density structure of the ACC (JOHNSON, ROYER, and LUICK, 1988). October is a transition month, during which time the ACC evolves
from its summer to its winter structure as winds
intensify and runoff increases. Maximum nearsurface currents are typically observed during the
fall as winds intensify and coastal runoff reaches
a maximum due to the strong baroclinic nature of
the current at this time (JOHNSON, ROYER, and
LUICK, 1988; STABENO, REED, and SCHUMACHER,
1995). Kachemak Bay communicates with the
GOA through a bathymetric trough extending
from Kennedy Entrance, through Lower Cook Inlet, and into the bay along the south shore. As the
westward-flowing ACC encounters Kennedy Entrance, a portion of this current turns northward
into Cook Inlet, and the remainder of the current
Journal of Coastal Research, Special Issue No. 45, 2004
The Role of Hydrodynamics in Kelp Distribution
continues west and then southwest through Shelikof Strait. Once in Kachemak Bay, the flow proceeds counterclockwise around the outer basin
(BURBANK, 1977).
The bull kelp (Nereocystis luetkeana) can form
extensive kelp forests along the eastern Pacific
coast from central California to the eastern Aleutians (MILLER and ESTES, 1989). Kelp forests are
very productive ecosystems in the coastal ocean
and are important shallow subtidal habitats. As
substantial primary producers, kelp forests support an extensive food web, including invertebrates, fish, seabirds, and marine mammals (for
review, see FOSTER and SCHIEL, 1985). Kelp forests are the prime feeding grounds for sea otters,
a keystone species in this ecosystem (e.g., DUGGINS, 1980; ESTES and PALMISANO, 1974; ESTES
et al., 1998). The structural complexity of kelp forests provides critical refuges and nursery grounds
for commercially and recreationally important fish
and shellfish species (e.g., BODKIN, 1988; CARR,
1994; EBELING and LAUR, 1985). Nereocystis is the
dominant canopy forming kelp in Kachemak Bay.
The presence of Nereocystis in Kachemak Bay was
documented in the late 1970s (LEES et al., 1980;
TRASKY, FLAGG, and BURBANK, 1977), and these
reports indicated that Nereocystis plants were not
found in the inner basin even though rocky habitat
and bladed understory kelps occur there, such as
Laminaria bongardiana, Laminaria saccharina,
Agarum clathratum, and Pleurophycus gardneri. In
this study, we address the overall question of:
what limits Nereocystis kelp forests to the outer
basin?
Factors that control Nereocystis kelp forest distribution are complex. Kelps, including Nereocystis, undergo heteromorphic alternation of generations (Figure 2). Conspicuous sporophytes produce
sori, reproductive structures located on the blades
that contain microscopic spores. Sori abscise from
the blade and sink to the sea floor, releasing
spores into the water column and on the bottom
(AMSLER and NEUSHUL, 1989). Spores germinate
into male and female gametophytes, and the zygotes produced by oogamous sexual reproduction
grow into adult sporophytes. The establishment of
kelp forests, therefore, requires the successful
completion of these different phases of the Nereocystis life cycle. The ultimate distribution of Nereocystis kelp forests depends on biotic and abiotic
factors that influence the annual dispersal of detached and drifting sporophytes, sori released by
attached or drifting sporophytes, and spores re-
181
Figure 2. Nereocystis undergoes a heteromorphic alternation of generations. Sexual zoospores are released from sori
located on the blades of mature sporophytes. The microscopic
spores germinate into male and female gametophytes. A zygote, produced by oogamous sexual reproduction, will grow
into an adult sporophyte to complete the life cycle (based on
SCAGEL et al., 1982).
leased from sori. Furthermore, physical, chemical,
and biological conditions at the settlement site can
have a critical impact on the survival, growth, and
successful reproduction of the different life stages.
The survival and successful development of
spores, gametophytes, and sporophytes can be affected by many intricate processes, including biotic
factors such as herbivory, competition for space,
shading, fouling, and diseases (e.g., DAYTON, 1985;
DAYTON et al., 1998; DUGGINS et al., 2001; FOSTER
and SCHIEL, 1985; REED and FOSTER, 1984) and
abiotic factors such as nutrient concentrations, salinity, light intensity, temperature, turbidity, and
sedimentation (e.g., DAYTON, 1985; DAYTON et al.,
1999; DEVINNY and VOLSE, 1978; DEYSHER and
DEAN, 1986; FOSTER and SCHIEL, 1985; KOPCZAK,
ZIMMERMAN, and KREMER, 1991). Kelps, including Nereocystis, are generally restricted to marine
habitats and are rarely found in areas of reduced
salinity and high turbidity, such as estuaries (e.g.,
DAYTON, 1985; DRUEHL, 1981). Salinity variation
can affect algal growth, respiration, and photosynthetic responses (e.g., BOLTON, 1979; GERARD, DUBOIS, and GREENE, 1987; HERBST and CASTENHOLZ, 1994). Few seaweeds are able to adjust their
water and mineral contents to survive at low salinities, resulting in a decrease in algal occurrence
toward brackish estuarine waters (DRUEHL, 1981).
Light quality and quantity are known to affect
photosynthetic responses and metabolic patterns.
Journal of Coastal Research, Special Issue No. 45, 2004
182
Schoch and Chenelot
The minimal light irradiance required for most
kelp is approximately 1% of the light reaching the
surface (LÜNING, 1981). High turbidity levels can
interfere with the penetration of sunlight through
the water column, thus limiting the amount of irradiance available to gametophytes and young
sporophytes (DREW and JUPP, 1976). The depth at
which Laminaria hyperborea is found throughout
Bantry Bay, Ireland, was found to decrease from
the open sea to the head of the bay because of increased turbidity; the density and standing stock
of the kelp were negatively correlated with the turbidity gradient (EDWARDS, 1980).
Weather patterns, local wave dynamics, and
current velocities may play a significant role in the
ultimate spatial distribution of kelp forests, especially in high-latitude ecosystems. For example,
storms can increase the dispersal range of individual spores (REED, NEUSHUL, and EBELING, 1991),
and the export of detached sporophytes from kelp
beds on open coasts can also be significant. H ARROLD and LISIN (1989) estimated export rates of
Macrocystis pyrifera from Monterey Peninsula forests at 130,000 tons per year. Once detached from
the substrate, reproductive sporophytes can drift
with the wind and currents, potentially dispersing
large numbers of spores over relatively long distances. This mechanism may assist in maintaining
these species’ geographic ranges as well as increasing genetic exchange between isolated populations
(HARROLD and LISIN, 1989; LADAH, ZERTUCHEGONZALEZ, and HERNANDEZ-CARMONA, 1999;
VAN DEN HOEK, 1987). Although the dispersal of
kelp spores is thought to be limited to a short distance around the parent plants, spores of some
kelp species (e.g., Macrocystis spp.) have been observed to remain viable in the water column for
several days (BRZEZINSKI, REED, and AMSLER,
1993; REED, AMSLER, and EBELING, 1992) and
may be carried long distances by currents (e.g.,
NORTON, 1992; REED, LAUR, and EBELING, 1988;
VAN DEN HOEK, 1987).
The spatial and temporal dynamics of kelp forests in Kachemak Bay have not been studied, and
a better understanding of factors limiting the distribution of Nereocystis sporophytes to the outer
bay would benefit our understanding of how this
important habitat affects local food webs. In this
study, we quantified the spatial distribution pattern of Nereocystis kelp forests for a 3-year period
and focused on two hypotheses to help explain the
absence of kelp forests in the inner basin of Kachemak Bay:
H1: Decreased salinity and light intensity of estuarine conditions determine Nereocystis sporophyte distribution.
H2: Estuarine circulation patterns determine Nereocystis sporophyte distribution.
The first hypothesis was tested by performing a
transplant experiment of Nereocystis sporophytes
between two sites along the salinity and turbidity
gradient of the bay and by investigating the effects
of estuarine conditions on their growth. The second hypothesis was evaluated by studying the hydrography of Kachemak Bay with water column
profiles, the dispersal of drift cards, and the trajectory of satellite-tracked drifters.
METHODS
Determining Interannual Variability of Nereocystis
Sporophyte Distribution
To quantify the interannual spatial variability
of kelp forest distributions in Kachemak Bay, polygon maps were produced annually from 2000 to
2002. The coastline of the entire bay was flown,
and aerial photographs were obtained of all Nereocystis kelp beds in late summer when plant
growth was near maximum but before winter
storms could dislodge mature sporophytes. Images
were acquired with a Hasselblad 501CM camera
(Gothenburg, Sweden) using ISO 100 color print
film from an altitude of 200 meters during an extreme low-tide cycle and when the sun angle was
near zenith. These images were individually rectified to digital orthophoto quadrangles. Each kelp
bed was delineated by a polygon and incorporated
into a Geographic Information System (GIS) vector
data layer. Polygons were analyzed using analysis
of variance (ANOVA) for among-year differences in
size and density. Distributions from 2000 were
used to define the estuarine limit of Nereocystis
sporophytes.
H1: Decreased Salinity and Light Intensity of
Estuarine Conditions Determine Nereocystis
Sporophyte Distribution
Two study sites were chosen along the estuarine
gradient of Kachemak Bay (Figure 3A). The sites
were similar in terms of wave exposure (based on
fetch for the prevailing westerly winds), current
velocity, and substrate type. Port Graham (PGE:
59822.2709 N, 151853.7289 W) is located toward the
open sea and is characterized by clear oceanic water. The kelp forests in Port Graham were dense
Journal of Coastal Research, Special Issue No. 45, 2004
The Role of Hydrodynamics in Kelp Distribution
183
Figure 3. Generalized circulation pattern in Kachemak Bay, Alaska. (A) The location of experiment and sample sites. (B–D)
Polygons representing the annual spatial distribution and areal extent of kelp beds from 2000 to 2002.
and continuously distributed along a narrow band
within the 20-meter bathymetric contour. In early
July 2001, the blades and pneumatocysts of most
plants were visible at the surface in 5 meters of
water during low tide. Many juveniles about 1 meter long were distributed among the larger plants.
Halibut Cove (HCE: 59835.8669 N, 151816.1419 W)
is located in the inner basin, near the narrows
formed by the spit, and characterized by more
brackish water. The experiment site was located
adjacent to an exposed rocky headland. The Nereocystis plants at this site in July formed a small
(ca. 5 3 20 meters), patchy aggregation with no
plants visible at the surface in 5 meters of water
at low tide, and only a few scattered juveniles were
observed among the larger individuals. This site
Journal of Coastal Research, Special Issue No. 45, 2004
184
Schoch and Chenelot
Figure 4. (A) Transplant design at each site. Four 8-meter-long ground lines were placed 5 to 10 meters apart at a depth of
8 meters (from MLLW). Three transplants from each origin (Port Graham and Halibut Cove) were randomly placed 50 centimeters apart on each line. The schema in the insert represents the method used to attach each transplant on the ground
lines: (a) kelp stipe and holdfast, (b) cable ties, (c) soft nylon line, (d) small polystyrene float, (e) polypropylene line, (f) snap
hook, and (g) ground line. (B) The photo shows the deployed ground line with transplants.
was selected because it is at the distribution limit
of Nereocystis.
A transplant experiment was conducted in July
and August 2001 to determine the effects of lower
salinity and light intensity on the growth pattern
and survival of Nereocystis in estuarine conditions.
Four ground lines were placed end to end at a
depth of 8 meters from mean lower low water
(MLLW) at each site as shown in Figure 4. A total
of 24 Nereocystis juveniles were collected from a
100-meter-diameter area surrounding PGE, and
12 juvenile plants were collected near HCE. The
selected transplants had an average stipe length
of 47.8 centimeters (6 1.3 centimeters; n 5 36).
The holdfast of each transplant was carefully woven through a soft nylon line and secured with cable ties on each side. This technique minimized
abrasion on the stipes and holdfasts. The nylon
line was secured to a small, labeled float equipped
with a snap hook. This design had several advantages, because it standardized the substrate on
which the holdfasts were secured (the small float).
The flotation lifted the ground lines off the seafloor, thus preventing sea urchin grazing and potential effects of sedimentation. This design also
minimized the handling of transplants underwater, because each float could easily be clipped onto
the ground lines using the attached snap hook. On
each ground line, plants were attached at intervals
of 50 centimeters to mimic natural densities (LEES
et al., 1980), with 12 from Port Graham and 12
from Halibut Cove at HCE, and 12 plants from
Port Graham at PGE.
Water column profiles of salinity and downwelling photosynthetically active radiation (PAR) were
conducted monthly (n 5 3) near the Port Graham
and Halibut Cove experiment sites using a caged
Seabird SBE 19plus conductivity-temperaturedepth (CTD) profiler (Bellevue, Washington) and
a Licor Model LI-192SA quantum sensor (Licor,
Lincoln, Nebraska). The salinity data were compiled into 1-meter depth bins and plotted. Salinity
and PAR attenuation with depth relative to surface values were compared between sites using a
t test.
Water samples were collected in July and August near each experiment site for analyses of nitrate (NO32), a major nutrient required for kelp
growth. Considering the stratified summer water
column in Kachemak Bay, on each occasion, three
samples were collected from the surface water (1
meter below the surface) and three from the bottom water (1 meter above the bottom, at a depth
of 8 meters at MLLW). Samples were filtered
through a 0.45-micrometer disk filter and stored
at 2208C until analyzed using standard methods
(ARMSTRONG, STEARNS, and STRICKLAND, 1967).
A time series of light intensity and water tem-
Journal of Coastal Research, Special Issue No. 45, 2004
The Role of Hydrodynamics in Kelp Distribution
perature was measured at each site during the experiment using data loggers attached to the
ground lines (8 meters MLLW). Light intensity
(Onset StowAway, Falmouth, Massachusetts) was
measured hourly, and temperature (Onset Hobo,
Falmouth, Massachusetts) every 15 minutes. The
mean and maxima daily light values and the mean
daily temperature values were compared between
sites using a t test for paired samples. An additional time series of salinity was provided by the
NERRS System-Wide Monitoring Program with
instruments deployed in the outer basin near the
village of Seldovia and in the inner basin near the
end of the Homer spit (Sanger et al., 2002). YSI
6600 data loggers (Yellow Springs Instrument
Company, Yellow Springs, Ohio), deployed 1 meter
above the bottom in 8 meters of water, measured
conductivity and recorded salinity at 30-minute intervals. The variation in daily mean salinity during the field experiment was compared between
sites using a t test for paired samples.
Underwater stipe measurements were conducted weekly for 6 weeks using SCUBA. On each sampling event, the total stipe length was measured
as the distance between the holdfast and the pneumatocyst. This technique of measuring the stipe
length and elongation rates of kelp as a way of
evaluating its development has been used in several other studies (DUNCAN, 1973; MAXELL and
MILLER, 1996; MILLER and DORR, 1994; NICHOLSON, 1970). In this paper, only total stipe length
for the 6-week transplant experiment will be reported. Other results will be detailed in another
manuscript. A two-way ANOVA for repeated measures, followed by a Tukey test, was used to determine differences in stipe lengths among sites
and origins (SAS INSTITUTE INC., 1999).
H2: Estuarine Circulation Patterns Determine
Nereocystis Sporophyte Distribution
Hydrographic data were collected along a northsouth transect in the outer basin during the spring
and fall to compare seasonal differences in vertical
and horizontal structure of the water column
(OKKONEN and HOWELL, 2003). A 20-kilometer
transect line was sampled every 2 kilometers using a caged Seabird SBE 19plus conductivity-temperature-depth (CTD) profiler. The temperature
and salinity data were compiled into 1-meter
depth bins and then plotted using Matlab (MATHWORKS, INC., 2001).
To investigate the trajectory of dominant surface
185
currents, we first used drift cards as a low-cost
means to study and infer regional surface currents. Over 1,000 7- 3 10-centimeter plywood drift
cards were deployed near Port Graham along a 5kilometer transect, perpendicular to shore, on
three separate excursions in May, September, and
March (3,000 cards total). We relied on the public
and community volunteers to collect and report
stranded cards, and in October 2001, a bay-wide
beach walk by community volunteers provided a
spatially comprehensive survey of stranded cards
prior to the onset of winter storms.
We deployed satellite-tracked drifter buoys
(Technocean, Cape Coral, Florida) near floating
rafts of detached kelp on three separate occasions
at approximately 2-week intervals during August
and September to better understand the path and
duration of successive trajectories. These buoys
were released from the same approximate location
in the outer basin near Port Graham. The drifters
had a surface float with a global positioning system (GPS) receiver for positioning and telemetry
electronics for communicating with the Argos satellite (Service Argos, Largo, Maryland). At regular
intervals, the drifters communicated with the Argos satellite and uploaded hourly GPS positions,
which were plotted on a map.
RESULTS
Determining Interannual Variability of Nereocystis
Sporophyte Distribution
The distribution of kelp forests from 2000 to
2002 is shown in Figure 3B–D. Polygons representing the annual spatial distribution and areal
extent of canopy kelp forests were analyzed for
among-year differences. For the purposes of this
analysis, a kelp forest was considered to be an aggregation of six or more Nereocystis sporophytes
with a separation distance of no more than 5 meters. The number of aggregations varied from year
to year (2000: 168; 2001: 138; 2002: 140). The areas of the smallest and largest aggregations were
(respectively) in 2000: 139 m2 and 64,008 m2; 2001:
294 m2 and 41,261 m2; and in 2002: 181 m2 and
40,819 m2. Among-year density differences of individual kelp forests fluctuated significantly
(2000: 3.86 plants/m2; 2001: 2.32 plants/m2; 2002:
3.77 plants/m2;; p # 0.001). Kelp forests were more
dense on the south shore than the north shore of
the outer basin (south shore: 3.51 plants/m2; north
shore: 1.22 plants/m2; p # 0.001). Over the entire
bay, kelp forests were found consistently in the
Journal of Coastal Research, Special Issue No. 45, 2004
186
Schoch and Chenelot
same location during the 3-year study, but the total areas among years were significantly different
(2000: 177,903 m2; 2001: 146,073 m2; 2002:
197,858 m2; p # 0.001). The one exception where
a major kelp forest was entirely absent over the 3year study occurred off the end of the Homer Spit
in 2001 (Figure 3C). This was the primary reason
for the significant decrease in total area that year
and is explained by the onshore migration of a
very large sandbar that temporarily buried the
rocky habitat (Adams et al., 2004). Although individual Nereocystis sporophytes and small patches
of a few plants were observed on rocky reefs near
Halibut Cove, kelp forests were not observed in the
inner basin during this study.
H1: Decreased Salinity and Light Intensity of
Estuarine Conditions Determine Nereocystis
Sporophyte Distribution
The water column profiles showed that PAR
measured at the surface varied from 2,020 to 297
mmol/s/m2 depending on cloud cover and time of
day, and PAR measured at 10 meters varied between 314 and 40 mmol/s/m2 and 59 and 12 mmol/
s/m2 at PGE and HCE, respectively. In terms of
relative attenuation, only 8.7% 6 0.8% of surface
PAR reached a depth of 10 meters at HCE, whereas 19.9% 6 1.5% of surface PAR was available at
10 meters at PGE (Figure 5A). The difference in
PAR attenuation between the two sites was significant (t value 5 6.69; p , 0.001; n 5 11). The
mean summer salinity profile (Figure 5B) shows
that surface salinities at Port Graham (30.1 6 1.0
psu) were considerably higher than at Halibut
Cove (24.9 6 1.0 psu), but at 10 meters the salinities were very similar (Port Graham: 31.38 6 1.0
psu; Halibut Cove: 31.30 6 1.0 psu). The difference
in salinities between the two sites was significant
(t value 5 2.56; p 5 0.015; n 5 11). Furthermore,
the halocline was at 2 meters near Port Graham
and at 4 meters near Halibut Cove, suggesting
that the plants at Halibut Cove were subjected to
lower salinities at shorter stipe lengths than the
plants at Port Graham.
The water samples collected in July and August
at the surface (1 meter depth) and bottom (8 meters depth) were analyzed for nitrates (NO32), and
the mean values (6 1 standard error [SE]) are
shown on Figure 5C. The difference in nitrate concentration between the two sites was significant (t
value 5 6.75; p , 0.001; n 5 12). Nitrate concentrations in the outer basin bottom water near Sel-
Figure 5. Profiles of PAR and salinity were measured to
compare the vertical structure of the water column at the
two experiment sites. (A) Photosynthetically active radiation
wavelengths were more rapidly attenuated at Halibut Cove
than at Port Graham. (B) At both sites, salinity was lower
at the surface than at the bottom, but at Halibut Cove the
surface salinity was much lower than at Port Graham. Furthermore, the halocline was much shallower at Port Graham
(2 meters) than at Halibut Cove (4 meters). (C) Triplicate
samples from surface and bottom water were analyzed for
nutrient concentrations on two occasions. Nitrate concentrations in the outer basin were higher compared to the inner
basin, and at both sample sites the surface nitrate concentrations were more depleted than at depth.
dovia (5.10 6 1.2 mM) were about twice the concentrations in the inner basin near Homer (2.16 6
0.5 mM), and for the surface water, the nitrate concentrations in the outer basin near Seldovia (3.78
6 0.5 mM) were about five times the concentration
Journal of Coastal Research, Special Issue No. 45, 2004
The Role of Hydrodynamics in Kelp Distribution
near Homer (0.66 6 0.5 mM). These data suggest
that the entire water column of the inner basin is
nutrient diminished relative to the outer basin,
but in particular the surface water in the inner
basin is nearly depleted of nitrates.
The time-series measurements of water temperature and total light at each experiment site and
salinity at the NERRS monitoring site showed consistently lower light and salinity in the inner basin, but little difference in mean (6 1 SE) water
temperature between the inner and outer basins.
The daily mean bottom water temperatures (Figure 6A) at PGE (9.8 6 0.18C) and HCE (9.7 6
0.18C) were not significantly different (t value 5
1.92; p 5 0.062; n 5 38). However, HCE had significantly lower mean daily minima (8.4 6 0.18C)
and higher daily maxima (11.7 6 0.18C) than PGE
(9.5 6 0.18C and 10.0 6 0.18C, respectively). As a
result, the daily variation was greater at HCE
than at PGE (t value 5 14.45; p , 0.001; n 5 38).
Synchronous fluctuations in daily mean levels of
total light between PGE and HCE suggest the
overall pattern was dictated by similar variations
in cloud cover and sun angle (Figure 6B). HCE received less than 74% of the light intensity measured at PGE. The summer daily mean light levels
were 0.81 6 0.03 log lumen/m2 in Port Graham
versus 0.68 6 0.3 log lumen/m2 at HCE (t value 5
9.71; p , 0.001; n 5 40). The same pattern was
observed when comparing averaged daily light
maxima (t value 5 6.24; p , 0.001; n 5 40). Salinity differences were found to be significant between basins (Figure 6C), with the mean salinity
in the inner basin (31.39 6 0.05 psu) lower than
the outer basin (31.89 6 0.02 psu; t value 5 11.02;
p , 0.001).
Kelp transplants were selected for size similarity and condition (47.6 6 1.1 centimeters; F 5
3.19; p 5 0.082). Six weeks after being transplanted, the longest individual stipe measured 456 centimeters for a Halibut Cove transplant to HCE
(the length was 48 centimeters when transplanted), and the shortest stipe was 43 centimeters for
a Port Graham transplant to HCE (the length was
35 centimeters when transplanted). After 6 weeks,
the total stipe lengths of Halibut Cove plants
transplanted to HCE were the longest (228.2 6
30.1 centimeters; n 5 12), and Port Graham plants
transplanted to HCE were the shortest (91.2 6
16.6 centimeters; n 5 10) (Figure 7). At HCE, Port
Graham transplants grew relatively little compared to Halibut Cove transplants, and the difference between the two sets of plants was visible
187
Figure 6. Data loggers were deployed on the ground lines
(8 meters MLLW) at Port Graham and Halibut Cove to continuously measure (A) temperature in 8C, and (B) light intensity in log lumens/m2. (C) The NERRS monitoring program provided continuous measurements of salinity in psu
near the Port Graham (Seldovia) and Halibut Cove (Homer)
experiment sites. The vertical dotted line represents the conclusion of the 6-week transplant experiment.
after only 2 weeks (46.6 6 2.9 centimeters for Port
Graham plants and 78.1 6 4.5 centimeters for
Halibut Cove plants on week 2; ANOVA; df 5 1; p
, 0.001 for weeks 2, 3, 4). After 6 weeks, plants
collected from Port Graham and transplanted to
HCE (91.2 6 16.6 centimeters) were significantly
shorter than Port Graham plants transplanted to
PGE (170.1 6 22.4 centimeters) (ANOVA; df 5 1;
F 5 8.01; p 5 0.011). Some Port Graham transplants died at both PGE and HCE over the course
of the experiment, resulting in a sample size decrease from 12 to 10.
Journal of Coastal Research, Special Issue No. 45, 2004
188
Schoch and Chenelot
Figure 7. Stipe length (cm) measured after 6 weeks for
transplants from Port Graham to the Port Graham and Halibut Cove experiment sites and for transplants from Halibut
Cove to the Halibut Cove experiment site. The values represent the mean 6 1 SE of stipe length.
H2: Estuarine Circulation Patterns Determine
Nereocystis Sporophyte Distribution
Hydrographic transects were conducted in the
spring and fall along the line shown in Figure 3A.
The spring CTD profiles across the axis of the outer basin show the water column to be relatively
well mixed (Figure 8A), with the north side of the
bay being slightly more stratified than the south
side. The fall profiles show the development of a
strong gradient across the axis of the outer basin
caused by less saline water stratified at about 15–
20 meters within about 5 kilometers from the
north shore (Figure 8B). This south-to-north density decrease will drive a westward-flowing geostrophic baroclinic current along the north shore
of the bay. These data suggest that estuarine flow
is maximized in the late summer and fall coinciding with peak freshwater discharges from the surrounding rivers, glaciers, and snowfields, and resulting in a westward-flowing baroclinic jet
trapped against the north shore by Coriolis deflection.
The drift cards released from the outer basin
were consistently not transported (i.e., recovered)
into the inner basin during spring, fall, or winter.
Figure 9A shows the release and recovery locations for all deployments. About 38% of the cards
were recovered from the spring and summer deployments, but only 1% were recovered from the
winter deployment. About 3% of the recovered
cards were found on islands near the south shore,
and 90% of the recovered cards were stranded on
the north shore of the outer basin. About 5% of the
Figure 8. Hydrographic transects in the (A) spring and (B)
fall across the axis of the outer basin identified seasonal
changes in the vertical and horizontal salinity (density)
structure of the water column (from OKKONEN and HOWELL,
2003). Higher-salinity oceanic water flows into the bay along
the south shore (left side of plot) and is gradually diluted by
rain, snowmelt, and glacial runoff. The total freshwater discharge from the inner basin peaks in the late summer and
fall. This results in a strongly stratified water column and a
baroclinic (density-driven) current that intensifies through
the narrows separating the inner and outer basin and is
forced along the north shore (right side of plot) by Coriolis
deflection.
recovered cards were found north in Cook Inlet,
and less than 2% were found far to the south in
Shelikof Strait.
Satellite-tracked drifter buoys were deployed on
three successive occasions in August and September. None of the buoys entered the inner basin.
All of the buoys followed a trajectory similar to
Drifter #39,994 released on August 23 for a 2-week
Journal of Coastal Research, Special Issue No. 45, 2004
The Role of Hydrodynamics in Kelp Distribution
189
deployment as shown on Figure 9B. The drifter
was advected into Kachemak Bay with the prevailing current flowing from the GOA along the
south shore. After 3 days, the drifter encountered
the strongly stratified estuarine flow discharging
from the inner bay and over several tidal excursions was repeatedly rejected from entering the inner basin by the baroclinic jet. After 5 days, the
drifter entered the stream of water flowing out of
Kachemak Bay along the north shore. Two days
later the drifter was caught in a strong tide rip in
Cook Inlet off of Anchor Point and remained there
for 4 days before being advected out of Cook Inlet
and south through Shelikof Strait (not shown).
DISCUSSION
Figure 9. (A) Recovery locations of drift cards released from
a transect in the outer basin on three separate occasions in
May, September, and March. Most of the cards stranded on
the north shore of the outer basin, and some were advected
out of the bay. No cards were transported into the inner
basin. (B) Trajectory of one of three satellite-tracked drifters
(Buoy 39,994) deployed in August. The results of these studies suggest that the circulation of Kachemak Bay may limit
the exchange of surface water between the outer and inner
basins. A density-driven surface current flows out of the inner basin, strengthening during the late summer and fall
when freshwater runoff reaches a maximum. This provides
a likely mechanism preventing drifting rafts of dislodged
sporophytes from entering the inner basin. Deep-water (.20
meters) exchange still occurs at tidal frequencies, and kelp
spores may be advected into the inner basin at depth.
Our study of interannual variability of Nereocystis kelp forests in Kachemak Bay showed that the
total area of kelp forests varied significantly and
that individual kelp beds varied in size and density. However, with the exception of one major
kelp forest habitat that was overwhelmed by migrating sand, the kelp forests were consistent in
terms of where they occurred from year to year.
Kelp forests occurred along the south shore in narrow dense bands that reflect the steep bathymetry.
The shallower bathymetry along the north shore
of the outer basin provides more habitat, resulting
in very large, but less dense, kelp forests. No kelp
forests were observed in the inner bay during the
3-year study, although a small and sparse kelp aggregation was observed every year of the study at
Halibut Cove. The kelp forests in Port Graham
were continuous and considerably denser than the
aggregations in Halibut Cove. We first tested the
hypothesis that decreased salinity and light intensity of estuarine conditions determines Nereocystis
sporophyte distribution.
The results of our Nereocystis sporophyte transplant experiments from Port Graham (the oceanic
endpoint) to Halibut Cove (the estuarine endpoint)
suggest a negative effect on growth. The transplants from Port Graham to the Halibut Cove experiment site grew less than the transplants from
Port Graham to the experiment site in Port Graham. However, Port Graham transplants in Halibut Cove appeared healthy and robust and showed
no signs of herbivory. The light levels (PAR in
mmol/s/m2 and total light in log lumens/m2)
measured at the study sites showed that less light
penetrates through the water column in Halibut
Cove than in Port Graham, suggesting that the
Journal of Coastal Research, Special Issue No. 45, 2004
190
Schoch and Chenelot
slower elongation of oceanic plants in Halibut
Cove compared to plants in Port Graham may be
due in part to light attenuation (e.g., DEAN and
JACOBSEN, 1984; HAN and KAIN, 1996; LÜNING,
1981). Lower salinity and lower nitrate levels may
also have affected the performance of Nereocystis
sporophytes in the inner bay. Although most
transplants in Halibut Cove survived throughout
the experiment, it is possible that those plants and
the surrounding natural population were stressed
by the estuarine environment and were more susceptible to other negative factors. By the end of the
summer, sori were appearing on some transplants
at PGE, although the transplants were behind
compared to the natural population in Port Graham. Halibut Cove plants were still not reproductive in October during our final visit to the experiment site.
Conversely, higher salinity and light intensity in
Port Graham were anticipated to promote kelp
growth. But interestingly, the Halibut Cove transplants to the experiment sites at Halibut Cove
grew faster than the Port Graham transplants at
either site. These results suggest that plants from
different origins may be adapted to grow best under the conditions at their original site. Several
studies suggest that salinity and light levels different from where the individual grew have a negative impact on algal growth (BOLTON, 1979; DUNCAN, 1973; GERARD, DUBOIS, and GREENE, 1987),
but it is difficult to disentangle the cause of phenotypic variability, especially in a wild population
of unknown genotype, because both polygenic and
environmental factors can be responsible.
Nereocystis stipe elongation is complex and is affected by many different factors, including light
quality and quantity and nutrient availability
(DUNCAN, 1973; DUNCAN and FOREMAN, 1980).
Monitoring stipe length is an easy way of evaluating the influence of environmental factors, but it
may not be directly related to productivity or reproduction success of kelp. The results of this
study suggest that plants from different origins respond differently to estuarine conditions. Although
Halibut Cove plants grew tall, they were not observed to develop to sexual maturity. More research is needed to determine the fitness level of
estuarine plants. It is possible that Halibut Cove
plants have adapted to grow faster to maximize
exposure to the higher-quality light available only
at the surface. Although this would be an intriguing direction for further research, for the purposes
of this study, the estuarine conditions encountered
in Halibut Cove were not sufficiently detrimental
to prevent Nereocystis sporophytes from growing.
We also tested the hypothesis that estuarine circulation patterns determine Nereocystis sporophyte distribution. The hydrographic data from
the CTD profiles confirmed the intensification of a
baroclinic jet along the north shore of the outer
basin in the late summer and fall. This current
starts in the inner basin where most of the freshwater accumulates from glacial runoff. The jet intensifies in the narrows formed by the Homer Spit,
and then Coriolis deflection forces the jet north
against the north shore of the outer basin. The
drift card recovery locations concur with these observations on the regional surface circulation. All
drift cards deployed in the outer basin were recovered in the outer basin or were advected out of the
system. None of the released cards were found in
the inner basin. This was consistent with the Argos drifter trajectory, which followed the south
shore until it was repeatedly rejected from the inner basin, and then advected with the baroclinic
jet along the north shore.
These results suggest that the strong estuarine
flow from the inner basin is a mechanism restricting surface water exchange between the outer and
inner basins of Kachemak Bay. This mechanism
may prevent rafts of dislodged Nereocystis sporophytes from dispersing into the inner basin. At
this latitude, Nereocystis sporophytes mature in
late summer or early fall, coinciding with the intensification of winter storm systems over the
GOA. Many of the large sporophytes cannot withstand the drag imposed by higher waves and break
free from their holdfasts. Important causes of detachment for large kelp sporophytes include actual
removal by storm waves; weakening of aging holdfasts, which increases the potential for detachment
during storms (HARROLD and LISIN, 1989; MCPEAK, GLANTZ, and SHAW, 1988); and grazing on
the stipe by snails (Lacuna vincta; DUGGINS et al.,
2001). Spores can be dispersed over great distances when rafts of free-floating sporophytes are carried by the prevailing circulation pattern (HARROLD and LISIN, 1989; LADAH, ZERTUCHE-GONZALEZ, and HERNANDEZ-CARMONA, 1999; VAN
DEN HOEK, 1987). However, drifting rafts of reproductive sporophytes carried with the prevailing
currents in Kachemak Bay may not be able to penetrate the barrier imposed by the strong estuarine
flow conditions, and instead they are swept along
the north shore by the baroclinic jet, similar to the
trajectory of the Argos drifter. Yet this is not con-
Journal of Coastal Research, Special Issue No. 45, 2004
The Role of Hydrodynamics in Kelp Distribution
sistent with the observed kelp aggregation at Halibut Cove. A possible explanation for this persistent patch is that the baroclinic jet may occasionally weaken under conditions of low freshwater input to the inner basin, and this may allow an
episodic supply of reproductive drifting sporophytes to be advected into the inner basin. Spores
may also be advected into the inner basin below
the surface jet in the more tidally dominated subsurface flow. However, if this were a dominant
process of connecting outer and inner basin kelp
populations, then we would expect to see more
kelp forests occupying the rocky reefs of the inner
basin. Because the Halibut Cove transplants were
not observed to reach the reproductive stage of development, it is possible that the local reproductive
output is not sufficient to maintain and expand a
large bed. It is likely that this population relies on
an outside source for local recruitment. A sporadic
and limited source of propagules for the inner bay
may increase the chances of genetic isolation of the
Halibut Cove aggregation. Further research into
the genetic differentiation of Nereocystis populations between the inner and outer basins may reinforce this explanation.
CONCLUSIONS
There are many biotic and abiotic factors that
act individually or in complex combinations to ultimately affect the spatial distribution of kelp forests. This study provides evidence that the distribution of Nereocystis kelp forests in Kachemak Bay
is not solely limited by the negative effect of the
estuarine environment on the growth of Nereocystis sporophytes. Furthermore, the distribution
may be influenced, at least in part, by the circulation patterns causing surface water to be deflected from the inner basin by a strong surface current
(i.e., baroclinic jet) leaving the inner basin during
periods of high freshwater runoff. This surface current is sufficiently strong and consistent to prevent drifting debris, such as rafts of dislodged sporophytes, from entering the inner basin. However,
spore dispersal by drifting rafts of sporophytes is
only one of many mechanisms for spore dispersal.
Recent work on the causes of spatial variation on
the recruitment of benthic algae (Macrocystis pyrifera) suggest that spore dispersal rather than
drifting fertile sporophytes may allow kelp recruitment from distant populations (REED, SCHROETER, and RAIMONDI, 2004). If sinking sori or
spores can penetrate the halocline in Kachemak
191
Bay, thereby escaping the surface current, the tidally dominated subsurface currents could readily
provide a transport mechanism into the inner basin.
Although the lower light, salinity, and nitrate
concentration of the inner basin did not affect Nereocystis sporophytes sufficiently to prevent
growth, the experiment did not evaluate these effects on productivity and reproductive output of
sporophytes or on other stages of the kelp life cycle. Microscopic stages of kelp are known to suffer
high mortality rates and are commonly considered
the bottleneck phase of the kelp life cycle (FLETCHER and CALLOW, 1992; SANTELICES, 1990). The
germination of Nereocystis spores collected from
Port Graham was observed to be impeded by low
salinity, suggesting that spores may be more sensitive to estuarine conditions than sporophytes.
These findings will be documented in another
manuscript. Further work on the effects of salinity, light, and temperature on gametophyte development, gametogenesis, and sporophyte production would help determine other critical factors
that ultimately dictate the distribution of Nereocystis kelp forests in Kachemak Bay.
The significance of our finding that the development of a seasonal baroclinic jet may impede the
exchange of surface water relates to the importance of nearshore coastal habitats to juvenile and
early life stages of many species of fish, invertebrates, and marine mammals. It is coastal and estuarine environments, such as Kachemak Bay and
Lower Cook Inlet, that have been most affected by
natural and human-induced sedimentation, contaminant delivery, and infrastructure development. Kachemak Bay has experienced the collapse
and closure of several fisheries over the last 100
years, and interestingly, several of these exploited
fisheries have been closed for over a decade but
show little evidence of recovery. These include
King crab (Paralithodes camtschaticus), Pacific
herring (Clupea pallasii), shrimp (Pandalus spp.),
urchins (Stronglyocentrotus spp.), Dungeness crab
(Cancer magister), and Tanner crab (Chionoecetes
bairdi). Elders in the native village of Seldovia
have noted the long-term decrease in density and
size of Nereocystis kelp forests. Therefore, understanding the circulation and exchange rates of water masses is critical to understanding the movement and recruitment patterns of planktonic propagules and how these patterns relate to the distribution and abundance of adults. For example, the
cyclonic circulation observed in the outer basin of
Journal of Coastal Research, Special Issue No. 45, 2004
192
Schoch and Chenelot
Kachemak Bay could be extremely important for
localized movement of nutrients, drift debris, and
planktonic organisms. This study provides evidence suggesting that the strongly stratified estuarine circulation patterns in Kachemak Bay result in a surface circulation pattern that favors
planktonic propagule export over retention. Our
findings in this study lead to a new hypothesis
that weak retention of planktonic propagules in
Kachemak Bay may explain the slow rates of recovery for adult populations, which are vulnerable
to rapid depletion following a disturbance. The interacting physical and biological processes controlling species abundances and distribution in Kachemak Bay warrant further study to improve our
understanding of variability in populations and
community structure and our ability to manage
economically important species.
ACKNOWLEDGMENTS
Steve Baird and Jenny Cope rectified the aerial
photos and digitized the kelp beds. Steve Okkonen
provided the CTD data with funding from the
Coastal Marine Institute of the University of Alaska Fairbanks and Cook Inlet Regional Citizens Advisory Council. The satellite-tracked drifters were
provided by Mark Johnson, who also graciously
provided the trajectory plot, with funding from the
Coastal Marine Institute of the University of Alaska Fairbanks and Cook Inlet Regional Citizens’
Advisory Council. We are very grateful for the support provided by these organizations and individuals. We thank the anonymous reviewers for comments that greatly improved an early version of
this manuscript, and we sincerely appreciate the
efforts of Michael Kennish for editing this special
issue of JCR.
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