BULLETIN OF MARINE SCIENCE, 70(1): 155–167, 2002
EVIDENCE THAT PHOSPHORUS LIMITS PHYTOPLANKTON
GROWTH IN A GULF OF MEXICO ESTUARY: PENSACOLA BAY,
FLORIDA, USA
Michael C. Murrell, Roman S. Stanley, Emile M. Lores,
Guy T. DiDonato, Lisa M. Smith and David A. Flemer
ABSTRACT
Nutrient limitation bioassays were conducted on six dates from November 1998 to
September 1999 at two sites, including oligohaline (Upper Bay) and mesohaline regions
(Lower Bay), in Pensacola Bay, Florida. Phytoplankton growth responses (measured as
changes in chlorophyll a concentration) to inorganic nitrogen (N) and phosphorus (P)
additions were monitored for three days. The results showed that, in eight of twelve
experiments, phytoplankton growth was stimulated by P additions in comparison with Namended and un-amended treatments. Nitrogen additions alone did not stimulate growth
over P additions in any experiment. The spatial patterns suggest that the potential for P
limitation was similar in Upper and Lower Bays. The four experiments with statistically
non-significant results were all conducted during winter and spring, suggesting a lower
potential for nutrient limitation during the cooler months when nutrient demand (i.e.,
productivity) is typically low and nutrient supply (i.e., freshwater runoff) is typically
high. This study adds to a small but growing literature suggesting that P limitation of
phytoplankton growth may be relatively common in warm temperate estuarine systems
such as those along the Gulf of Mexico coast.
The role of inorganic nutrients, particularly nitrogen and phosphorus, in limiting primary production in marine environments has been a topic of sustained interest (Ryther
and Dunstan, 1971; Boynton et al., 1982; Smith, 1984; D’Elia et al., 1986; Hecky and
Kilham, 1988; Howarth, 1988; Fisher et al., 1992; Fisher et al., 1995; Smith et al., 1999).
Human activities have dramatically changed the distribution of major nutrient elements
in the landscape and have increased nutrient loading to receiving waters (Smith et al.,
1999; NRC, 2000; Cloern, 2001). Nutrient pollution is the most common reason why
US water bodies fail to meet water quality standards set by the US Environmental Protection Agency (EPA). Estimates of the magnitude of nutrient-related problems are difficult to make, but approximately 20 to 40% of the nation’s fresh and estuarine water
bodies are adversely affected by nutrient pollution (US EPA, 1998). Because anthropogenic nutrient loading can have such severe consequences, resource management decisions need to be based upon a clear understanding of the cause-effect relations of increased nutrient loading.
Based on many years of active research, primarily in cool temperate ecosystems, a
generalization has emerged that freshwater systems tend to be phosphorus limited (Schelske
and Stoermer, 1971; Schelske et al., 1986; Smith et al., 1999), whereas estuarine and
marine systems tend to be nitrogen limited (Hecky and Kilham, 1988; Howarth, 1988).
Relatively little work has been done in warm temperate estuarine environments such as
those in the northern Gulf of Mexico region (Myers and Iverson, 1981; Flemer et al.,
1998). This study was designed to examine the potential of major inorganic nutrients,
nitrogen and phosphorus, in limiting phytoplankton growth in Pensacola Bay. It is part of
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a larger effort towards better understanding how changes in nutrient loadings affect primary productivity and the community composition of aquatic ecosystems.
STUDY LOCATION
Pensacola Bay is a moderately sized (370 km2) estuary in northwestern Florida (Fig.
1). It is characterized as a micro-tidal, partially stratified, drowned river valley estuarine
system (Schroeder and Wiseman, 1999). About 80% of the freshwater flow into the system comes from the Escambia River and empties into Escambia Bay (Olinger et al.,
1975), with an annual mean discharge of ca 200 m3 s–1. The other rivers include the
Blackwater, the Yellow and the East, which empty into the East Bay region. Exchange
with the Gulf of Mexico occurs through a narrow, deep pass at the western end of Pensacola
Bay and Santa Rosa Sound; the mean water residence time for the entire system is about
25 d (Solis and Powell, 1999). This study focused on two regions of the system; the
Upper Bay site, more influenced by Escambia River flow, and the Lower Bay, more
influenced by exchange with the Gulf of Mexico (Fig. 1). Mean depths for Escambia Bay
(Upper Bay) and Pensacola Bay proper (Lower Bay) are 2.4 and 5.9 m, respectively
(Olinger et al., 1975). There is little known about phytoplankton productivity or community composition in Pensacola Bay, and these are the subjects of ongoing studies. Median
chlorophyll concentrations are about 4 mg L–1 and generally ranges from 1 to 20 mg L–1
(US EPA unpubl. data).
MATERIALS AND METHODS
Nutrient bioassay experiments (sensu Ryther and Dunstan, 1971) were conducted on six dates
from November 1998 to September 1999 with water collected from two sites in Pensacola Bay
designated Upper Bay and Lower Bay (Fig. 1). The choice of sampling site was based on salinity
regime rather than exact geographic location, with targeted salinities of five for Upper Bay and 20
for Lower Bay. At each site, hydrographic profiles (temperature, salinity, pH and dissolved oxygen) were collected using a Hydrolab® Surveyer II, and water clarity was measured with a Secchi
disk. Nutrient samples were not taken during these experiments; however, we present nutrient data
from a related quarterly monitoring study of Pensacola Bay. Using a probabalistic sampling design
(Summers et al. 1992), the US EPA Monitoring and Assessment Team collected water from multiple sites in Escambia and Pensacola Bays, broadly corresponding to our Upper Bay and Lower
Bay regions, respectively. Nutrient analyses (NH4, NOx, PO4, Si) were performed using standard
methods (APHA, 1989); the DIN reported is the sum of NH4 + NOx.
The bioassay experiments were conducted by varying the nutrient regime among treatments and
comparing phytoplankton growth against unamended controls. Water was collected with a polyethylene bucket, screened through an 80 mm mesh nylon screen and transported in darkened polyethylene carboys. This treatment likely removed the larger chain-forming diatoms, which tend to be a
more significant component of the phytoplankton community during winter and spring, but much
less so during summer and fall (Murrell, unpubl. data). At the laboratory, a portion of the water was
filtered through a 0.22 mm pore size filter to make cell-free water. This filtrate was recombined with
unfiltered water in 9:1 proportion, and distributed into triplicate 1 L polycarbonate bottles. The
water was diluted to minimize the effect of microzooplankton grazing on phytoplankton growth.
Treatments included: (1) no addition (Control), (2) nitrogen only (15 mM N, half as NH4+ and half
as NO3–), (3) phosphorus only (1 mM P as PO4) and (4) nitrogen + phosphorus (15 mM N and 1 mM
P as above). The bottles were placed in a running seawater incubator to maintain ambient temperature. To prevent photo-inhibition of phytoplankton productivity (Mallin and Paerl, 1992), the bottles
MURRELL ET AL.: PHOSPHORUS LIMITATION IN PENSACOLA BAY, FLORIDA
157
Figure 1. Map of the Pensacola Bay system in northwestern Florida (USA) showing the Upper Bay
and Lower Bay sampling sites. Station locations were keyed to salinity, so the exact geographic
location varied from date to date, however the ovals encompass the sampling regions.
were screened to ca 50% ambient irradiance with neutral density screening during the summer and
fall experiments. Samples of 100 ml were removed from each bottle at 24 h intervals for 72 h,
filtered onto Whatman GF/F filters and frozen at –70 ∞C until fluorometric analysis. The chlorophyll was extracted from the filters using buffered methanol and sonication by a Vibra Cell® probe
(Jeffrey et al., 1997). The fluorescence was measured with a Turner Designs® Model 10 AU fluorometer calibrated to read chlorophyll a in the presence of chlorophyll b and pheophytin
(Welschmeyer, 1994).
Phytoplankton growth rates in each bottle were calculated assuming exponential growth, as the
slope of the natural log of phytoplankton biomass vs. time, and reported in d–1 units. Treatment
effects on phytoplankton growth were tested with an analysis of variance (ANOVA) and post-hoc
BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 1, 2002
158
comparisons of treatment means were conducted with a Tukey test. In one case (November, Upper
Bay) the loss of one replicate produced an unbalanced design and a Scheffe post-hoc test was used
instead. All statistical analyses were performed using SAS (SAS Institute, Cary, North Carolina).
RESULTS
Hydrographic data reflect expected seasonal and spatial variation in temperature and
salinity (Table 1). In Upper Bay, chlorophyll a concentrations averaged 8.2 mg L–1 (range
2.5 to 14 mg L–1) and 5.4 mg L–1 (range 4.3 to 8.7 mg L–1) in Lower Bay. Chlorophyll
concentrations tended to be higher during spring and summer months. Secchi depths
averaged 1.2 m and 3.1 m in Upper and Lower Bays, respectively reflecting the higher
light attenuation in the Upper Bay.
Nutrient concentrations (Table 2) were similar in Upper and Lower Bays based on the
means (the mean of all the medians-by-date), but were slightly higher in Upper Bay. In
Upper and Lower Bays, respectively, DIN concentrations averaged 4.9 and 3.3 mM, while
DIP concentrations averaged 0.6 and 0.4 mM. On a given date, however, the Upper Bay/
Lower Bay gradient in nutrient concentrations was more pronounced (see Table 2). Within
date median DIN concentrations ranged from 0.9 to 13.3 mM in Upper Bay and from 0.9
to 6.8 mM in Lower Bay, while DIP ranged from 0.1 to 1.9 mM. in Upper Bay and from
n.d. to 1.0 mM in Lower Bay. The DIN:DIP ratios ranged from 2 to 120 in Upper Bay and
from 3 to 77 in Lower Bay. The high site-to-site variability in the range of DIN:DIP
values (Table 2) within a given date and region obscures any consistent seasonal patterns.
On each date, the potential for P limitation was represented by the percentage of sites
with DIN:DIP ratios above Redfield. This percentage varied from 0 to 100%, but averaged 35% in Upper Bay and 40% in Lower Bay. The period when potential P limitation
was lowest occurred during the period from February to July 1999 at both sites.
Table 1. Hydrographic conditions at Upper and Lower Bay sites during this study. Variables
include temperature (° C), salinity (PSU), chlorophyll a (µ g L-1), and Secchi disk depth, Zd (m).
Upper Bay
Date
18 Nov 98
26 Jan 99
1 Apr 99
4 May 99
27 Jul 99
28 Sep 99
Average
Lower Bay
Date
18 Nov 98
26 Jan 99
1 Apr 99
4 May 99
27 Jul 99
28 Sep 99
Average
Temp
20.1
15.6
18.4
22.9
28.3
25.3
Sal
4.9
3.7
1.5
6.1
5.3
7.5
Chl
2.5
10.2
5.0
14.1
10.2
7.2
Zd
1.5
1.5
0.5
1.0
1.0
1.9
21.8
4.8
8.2
1.2
Temp
20.6
15.6
18.5
23.2
28.6
25.8
Sal
18.9
17.5
14.2
22.4
32.7
27.5
Chl
5.1
4.3
4.6
4.6
8.7
5.5
Zd
3.0
3.0
3.0
2.5
3.5
3.5
22.1
22.2
5.4
3.1
0.6
4.9
Feb-00
Average
0.6 (0.4-0.8)
0.2 (nd-1.0)
0.1 (nd-0.2)
0.4
2.0 (1.5-4.8)
2.6 (1.4-4.4)
1.3 (0.2-31.8)
3.3
Nov-99
Feb-00
Average
May-99
Jul-99
1.0 (0.5-1.5)
0.6 (0.4-0.8)
6.2 (2.8-14.4)
Nov-98
2.1 (1.3-3.5)
0.6 (nd-0.9)
6.8 (5.1-10.9)
Aug-98
Feb-99
0.0 (nd-0.1)
0.0 (nd-0.2)
0.9 (0.5-1.5)
1.7 (0.9-8.0)
Feb-98
May-98
DIP
0.1 (nd-0.7)
DIN
6.1 (3.3-8.8)
Date
Pensacola Bay (Lower Bay)
0.1 (nd-0.4)
Jul-99
0.1 (nd-0.3)
3.9 (1.7-6.6)
May-99
2.4 (1.8-4.6)
0.5 (0.3-0.7)
1.5 (0.8-10.0)
4.1 (0.4-15.1)
1.9 (1.1-3.3)
0.9 (0.5-1.0)
13.3 (8.1-18.4)
Nov-98
Feb-99
Nov-99
0.1 (0.1-0.2)
1.7 (0.5-3.3)
1.1 (1.0-8.2)
7.5 (6.0-12.9)
0.1
0.9
Feb-98
May-98
Aug-98
DIP
0.1 (nd-0.2)
DIN
9.4 (2.8-10.9)
Date
Escambia Bay (Upper Bay)
37.
50.(12-93)
5.(4-5)
55.(33-72)
31.(10-37)
37.(19-50)
17.(5-29)
35.(21-46)
28.(14-35)
Si
71.(32-120)
63.
148.(92-234)
6.(5-12)
90.(63-97)
66.(39-91)
50.(41-57)
57.(20-73)
56.(42-74)
25.
DSi
67.(19-74)
22.
13.(3-348)
10.(2-59)
3.(2-7)
3.(2-6)
6.(5-16)
12.(8-270)
45.(4-266)
29.(16-43)
DIN:DIP
77.(10-252)
27.
37.(10-133)
46.(6-92)
8.(3-21)
2.(2-10)
6.(5-8)
4.(3-24)
11.(5-104)
7
DIN:DIP
120. (36-248)
18.
45. (0-250)
2. (1-150)
32. (15-37)
14. (3-27)
6. (2-7)
3. (1-4)
17. (5-52)
30. (9-60)
DSi:DIN
11. (8-20)
21.
37.(16-252)
3.(2-5)
25.(12-52)
35.(9-57)
4.(3-5)
7.(2-9)
40.(9-61)
28.
DSi:DIN
7.(2-26)
40%
40%
36%
0%
0%
0%
27%
64%
100%
N:P >16
91%
35%
67%
89%
22%
0%
0%
11%
22%
0%
N:P >16
100%
10
11
11
9
11
11
11
11
n
11
6
9
9
8
9
9
9
1
n
9
Table 2. Median (range) nitrogen, phosphorus, and silicate concentrations (µ M) from quarterly sampling in Pensacola Bay over a two year period overlapping
the time frame of this study. Multiple sites (as denoted by n) were occupied in Escambia Bay and Pensacola Bay, which generally correspond to Upper and
Lower Bay respectively. DIN is the sum of NH4+ and NO3-+NO2- analyses, while DIP is orthophosphate. DIN:DIP and DSi:DIN ratios were calculated at
each site before calculating medians. When non-detectable (i.e., DIP £0.03 µ M), the detection limit was used to calculate the DIN:DIP ratio. The N:P > 16
column is the percentage of stations with N:P greater than the Redfield ratio.
MURRELL ET AL.: PHOSPHORUS LIMITATION IN PENSACOLA BAY, FLORIDA
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Phytoplankton growth appeared to be exponential for most time courses in Upper Bay
(Fig.2) and Lower Bay (Fig. 3). In most experiments, the addition of P (P, N + P) stimulated growth compared to treatments with no added P (Control and N). There were no
cases when N-only additions (N) resulted in higher mean growth than P additions (P and
N + P). In two cases (Fig. 2B,C), no differences in growth were evident among all treatments. In one case (Fig. 3E), the three nutrient amended treatments (N, P, N + P) grew
faster than the unamended control treatment. Growth rates for all experiments and treatments ranged from 0.04 to 1.46 d–1 (Table 3). Results from ANOVA post-hoc tests revealed that in 8 of the 12 experiments, phytoplankton growth was stimulated by P additions compared to N addition or un-amended treatments. Even when not statistically significant, the growth rates of P amended treatments were always numerically higher than
N-amended and unamended treatments (Table 3).
DISCUSSION
This study reports a strong phytoplankton growth response to P additions for both Upper and Lower Pensacola Bay water samples. The phytoplankton growth response to
nutrient additions was usually not apparent until day 2 (Figs. 2,3) of our incubations. This
is in contrast to studies that have shown rapid (within 1 d) phytoplankton growth responses to nutrient additions (D’Elia et al., 1986; Fisher et al., 1992; Holmboe et al.,
1999). Our approach of diluting the phytoplankton with filtered seawater may have delayed the onset of nutrient stress by reducing the demand on ambient pools, but would
likely not alter response mode (N or P stimulation). Furthermore, if phytoplankton were
nutrient-starved at the outset of the experiment, the growth response should be immediate, regardless of dilution. Therefore, our results suggest a potential for P limitation in
Pensacola Bay, but do not necessarily imply that the phytoplankton were experiencing
nutrient stress, in situ.
The nutrient data for Pensacola Bay system provide a broad indicator of nutrient conditions in this system, and support the bioassay results, showing low P concentrations and
high DIN:DIP ratios (Table 2). Phosphorus concentrations were frequently 0.1 mM (below detection at certain sites as seen in the range data, see Table 2) in Upper and Lower
Bays. This concentration is at or below the half saturation values of 0.1 to 0.2 mM typical
for phytoplankton populations (Fisher et al., 1995). Median DIN:DIP ratios varied widely
from 2 to 120, suggesting possible shifts from N limiting to P limiting conditions. DIN:DIP
ratios were similar for Upper and Lower Bays revealing no consistent spatial patterns.
The large variability among samples making up within-date medians (as seen with the
range data) implies spatial heterogeneity. This high station-to-station variability may be
partly due to the sampling design that placed some stations in very shallow waters fringing the shoreline or near localized nutrient sources. The potential for P limitation, as
expressed by the percentage of stations with N:P ratios greater than Redfield (Table 2),
was fairly coherent between Upper and Lower Bays. Surprisingly, the period of lowest
potential P limitation (Feb–July 1999), overlapped with several bioassay experiments
showing P limitation (Table 3), such as May and July in Upper Bay and April in Lower
Bay. This mismatch between nutrient conditions and bioassay results may simply reflect
the spatial and temporal heterogeneity in this system, but may also suggest that DIN and
DIP concentrations do not accurately reflect nutrient availability (Smith, 1984; Hecky
and Kilham, 1988; Howarth, 1988).
MURRELL ET AL.: PHOSPHORUS LIMITATION IN PENSACOLA BAY, FLORIDA
161
Figure 2. Phytoplankton biomass vs. time during bioassay experiments from Upper Bay. a) 18
November 1998, b) 26 January 1999, c) 1 April 1999, d) 4 May 1999, e) 27 July 1999, f) 28
September 1999. Each symbol represents a mean of three replicate bottles.
The potential for persistent P limitation in Pensacola Bay is supported by U.S. Geological Survey nutrient data that was collected from the Escambia River at monthly to
quarterly intervals from 1973 to 1994 (Alexander, 1996). A review of these data suggest
that, historically, the delivery of P has been low relative to N. Total P concentrations
averaged 1.2 ± 0.6 mM and TN:TP ratio averaged 40 ± 26 (mean ± SD, n = 147). These
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Figure 3. Phytoplankton biomass vs. time during bioassay experiments from Lower Bay. a) 18
November 1998, b) 26 January 1999, c) 1 April 1999, d) 4 May 1999, e) 27 July 1999, f) 28
September 1999. Each symbol represents a mean of three replicate bottles.
ratios would favor P limitation in Pensacola Bay given that the Escambia River is the
major freshwater source and, therefore, a major source of N and P to the Bay. A study is
currently underway to better resolve the spatial and temporal distribution of inorganic
nutrients along the axis of the system (Murrell et al., in prep).
MURRELL ET AL.: PHOSPHORUS LIMITATION IN PENSACOLA BAY, FLORIDA
163
Table 3. Bioassay results from Upper Bay and Lower Bay sites. Phytoplankton growth rates (±SD)
were calculated assuming exponential growth and units are day-1. Potential limitation is indicated
in the "Lim?" column, when growth rates in experimental treatments were significantly higher
than controls according to Tukey or Scheffe post-hoc tests (P < 0.05).
Upper Bay
Date
18 Nov 98
26 Jan 99
1 Apr 99
4 M a y 99
27 J ul 99
28 Sep 99
Unamended
0.32 (0.04)
0.52 (0.07)
1.09 (0.04)
0.46 (0.06)
0.10 (0.12)
0.67 (0.13)
N
0.21 (0.03)
0.60 (0.03)
1.19 (0.03)
0.39 (0.03)
0.04 (0.12)
0.78 (0.16)
P
0.87 (0.08)
0.66 (0.06)
1.21 (0.08)
0.90 (0.08)
0.74 (0.04)
1.46 (0.05)
N+P
0.85 (0.12)
0.62 (0.09)
1.21 (0.05)
0.75 (0.12)
0.78 (0.09)
1.46 (0.09)
Lim?
P
P
P
P
Unamended
0.25 (0.03)
1.04 (0.17)
0.44 (0.09)
0.36 (0.17)
0.44 (0.08)
0.42 (0.04)
N
0.20 (0.05)
1.06 (0.31)
0.37 (0.09)
0.23 (0.24)
0.91 (0.07)
0.41 (0.10)
P
0.93 (0.16)
1.35 (0.03)
0.75 (0.05)
0.53 (0.14)
0.94 (0.07)
1.12 (0.05)
N+P
0.90 (0.04)
1.43 (0.05)
0.75 (0.05)
0.47 (0.02)
0.97 (0.09)
1.40 (0.08)
Lim?
P
P
N, P
P
Lower Bay
Date
18 N ov 98
26 Jan 99
1 Apr 99
4 May 99
27 Jul 99
28 Sep 99
In our bioassay experiments, we diluted phytoplankton to 10% of ambient levels in
order to minimize microzooplankton grazing effects, and may have increased the nutrient
limitation effects. Microzooplankton grazers should act to reduce nutrient limitation by
simultaneously increasing supply and decreasing demand. Grazing increases nutrient supply by remineralizing nutrients and decreases nutrient demand by lowering the net growth
of phytoplankton. Microzooplankton grazers appear to exert a strong grazing pressure on
phytoplankton productivity in Pensacola Bay. In a parallel study, Murrell et al. (in press)
conducted a set of microzooplankton experiments that found microzooplankton grazed
an average of 62% of the potential primary production. Larger macrozooplankton, in
turn, may constrain microzooplankton abundance, hence affecting their role in relieving
nutrient stress. This appears plausible based on a set of mesocosm experiments in Pensacola
Bay, which found evidence that zooplankton significantly affected microzooplankton
abundance (Lores et al., in review). Because of complex food web dynamics, we interpret
these bioassay results as revealing the limiting nutrient during periods of low grazing
pressure on phytoplankton.
In Pensacola Bay, P limitation of phytoplankton may occur when top-down controls
are relaxed (i.e., grazing), but may also be promoted by weak bottom-up controls. For
example, light limitation of phytoplankton productivity is common in many estuarine
systems, including the Chesapeake Bay (Harding et al. 1986), San Francisco Bay (Cole
and Cloern, 1984; Cloern, 1987, 1999), Delaware Bay (Pennock and Sharp, 1994, and the
Cape Fear Estuary (Mallin et al., 1999). Pensacola Bay has relatively high light availability as evidenced by Secchi depth data that suggests the euphotic zone extends to the
bottom over much of the Bay (Table 1). This implies a minor role of light in limiting
phytoplankton productivity in this shallow, relatively non-turbid system, and may only
become significant during high runoff periods when turbidity increases.
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Another consideration is whether periodic silica limitation may shift phytoplankton
from a diatom-dominated community to one dominated by other algae, and thereby potentially alter relative demand for N and P. Silica depletion in the Great Lakes is well
documented (Schelske and Stoermer, 1971; Schelske et al., 1986), but it also occurs late
in the bloom cycle in estuaries like the Chesapeake Bay (Conley and Malone, 1992), the
San Francisco Bay (Cloern, 1996), and the Mississippi River plume (Dortch and Whitledge,
1992; Justic et al., 1995). Silica concentrations in Pensacola Bay (Table 2) range from 5–
150 mM, similar to other estuaries, but do not appear to become depleted (Si:N ratios
always >1), suggesting it plays a minor role in controlling phytoplankton productivity in
Pensacola Bay.
The statement that N limits phytoplankton productivity in marine and estuarine systems (review in Howarth, 1988) has become somewhat dogmatic; even in well-studied
cool temperate estuaries there is considerable system-to-system and within-system seasonal variability. For example, the Chesapeake Bay and tributaries appears to switch
between N and P limitation in a fairly predictable seasonal pattern, with spring-time P
limitation and summer-time N and Si limitation (D’Elia, 1986; Fisher et al., 1992; Malone
et al., 1996). Other systems, such as the Delaware estuary, appear to shift between nutrient and light limitation (Pennock and Sharp, 1994). In others, such as Laholm Bay (an
embayment of the Baltic Sea), N limitation appears to persist year round (Graneli et al.,
1986).
Estuaries of the northern Gulf of Mexico are less well studied than estuaries of the
Atlantic. However, one characteristic of Gulf estuaries is that the lack of strong seasonal
forcings found in cool temperate systems. Temperatures do not approach freezing, and
the timing and magnitude of freshwater flow, while largely seasonal (maximal in spring,
minimal in summer), can vary considerably from year to year due to large rain events.
While there are often spring and summer peaks in productivity, large storms can disrupt
this pattern. Therefore, Gulf estuaries may be more event-driven than seasonally-driven.
It is not clear whether this distinction between cool- and warm-temperate systems affects
patterns of nutrient limitation, but it is interesting to note that two previous studies (Myers
and Iverson, 1981; Flemer et al., 1998) also conducted in Gulf of Mexico estuaries revealed evidence that P limited phytoplankton growth. Myers and Iverson (1981), in a
widely cited study, found that P stimulated phytoplankton C fixation and P uptake rates in
several embayments and coastal sites in Northern Florida. Flemer et al. (1998) found that
P stimulated phytoplankton growth in mesocosm experiments in Perdido Bay, Alabama/
Florida, but also observed a seasonal shift from N to P limitation. The present study in
Pensacola Bay adds to the small but growing literature suggesting that P limitation of
phytoplankton growth may be relatively common in these warm temperate estuarine systems.
These results are important for management of nutrient related problems in estuaries.
Due to the primary focus on nitrogen limitation of primary production in estuarine waters
(Hinga et al., 1995), little attention has been directed towards managing P inputs to estuaries. This work and the evidence of P limitation in tropical systems (Smith, 1984) suggest that P limitation in estuaries may be more common than previously thought. It is
possible that many of the estuaries of the Gulf of Mexico may be P limited and therefore
may require a different strategy for the management of nutrient problems.
MURRELL ET AL.: PHOSPHORUS LIMITATION IN PENSACOLA BAY, FLORIDA
165
CONCLUSIONS
1. Nutrient bioassay experiments showed that addition of P stimulated phytoplankton
growth in 8 of 12 total experiments. N additions alone never stimulated growth over P
additions.
2. Spatial patterns suggest that the potential for P limitation occurs equally in both the
Upper and the Lower reaches of Pensacola Bay. Seasonal patterns suggest that nutrient
replete conditions prevailed at both sites during winter and spring months.
3. Ambient nutrient concentrations in Pensacola Bay system show that DIP concentrations were frequently near detection limits and that N:P ratios often exceeded Redfield
ratios. These data support the bioassay results implicating P as the potentially limiting
nutrient.
ACKNOWLEDGMENTS
We thank G. Craven, J. Patrick, and B. Quarles for field help in collecting water. This manuscript
benefited from discussions with J. M. Caffrey. Contribution No. 1131, US EPA, Gulf Breeze, Florida.
Mention of trade names or commercial products in this manuscript does not constitute endorsement
by the US EPA.
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DATE SUBMITTED: June 22, 2001.
DATE ACCEPTED: December 21, 2001.
ADDRESSES: US EPA, NHEERL, Gulf Ecology Division, One Sabine Island Dr., Gulf Breeze, Florida
32561. CURRENT ADDRESS: (D.A.F.) US EPA, Office of Water, 401 M St., SW, Washington, D.C. 20460.
CORRESPONDING AUTHOR: (M.C.M.) E-mail: <[email protected]>, Tel. (850) 934-2433, Fax: 2403.