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Cornell College, Mount Vernon, Iowa 52314
1 Bowdoin College, Brunswick, ME 04011.
2 Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543.
3 Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853.
4 Boston University Marine Program, Marine Biological Laboratory, Woods Hole, MA 02543.
5 Woods Hole Research Center, Woods Hole, MA 02543.
Phytoplankton growth requires nitrogen (N) and phosphorus (P) in an approximate molar ratio of 16:1 (the Redfield ratio; 1). N or P limitation in an aquatic system is considered to occur when the availability of N relative to P is well below or above this ratio, respectively (2, 3). Past studies have shown that marine systems of moderate to high productivity are typically N limited, while similarly productive freshwater systems are most often P limited (2, 3). However, relatively little is known about low-salinity estuaries. The Baltic Sea is perhaps the best-studied estuary of this type; there, productivity has been shown to be limited by P at salinities lower than 3 to 4
and by N at higher salinities (4).
Here, we report the results of a comparative set of nutrient limitation experiments in two coastal systems in Falmouth, Massachusetts, of very different salinities: Vineyard Sound and Oyster Pond (32 and 2.3, respectively). Previous studies have reported N limitation in Vineyard Sound (5, 6) as would be expected for a high-salinity coastal ecosystem (2, 3). In an October 1986 study, phytoplankton in Oyster Pond did not respond to N or P enrichments (5); Boston University Marine Program students obtained the same result from a similar experiment performed on Oyster Pond in October 2001. However, these experiments were not done during the peak growing season. Oyster Pond is currently considered to be mesotrophic to eutrophic (7), and with the watershed nearing buildout, effective management of nutrient inputs may be important in controlling eutrophication and algal blooms of concern.
We conducted two sets of bottle enrichment experiments, from June 30 to July 5, 2002, and from July 22 to July 26, 2002. For both experiments, we sieved water through a 150-µm mesh to remove large zooplankton. In the first experiment, 12 replicate, 2-l polycarbonate bottles from each system received enrichments of NaNO3 or NaH2PO4 that increased ambient concentrations of nitrate by about 50 µM (N treatment) or phosphate by about 10 µM (P treatment); 12 control bottles from each system received no nutrient additions (C treatment). As a safeguard against short-term CO2 depletion in the bottles, we added NaHCO3 (2.0 mM) to the Oyster Pond samples. At the beginning of the experiment, nine bottles were sampled immediately (three each of controls and three each of the PO4 and NO3 additions) to determine initial chlorophyll a concentrations and confirm the effectiveness of the nutrient enrichments. The remaining bottles containing Oyster Pond or Vineyard Sound water were incubated 0.5 m to 1 m below the surface of Oyster Pond on a floating rack, at a light intensity of about 330–560 µE m-2s-1 (peak daylight hours). We collected three replicate bottles of each treatment on days 2, 3 and 4. Subsamples were filtered (GF/F) and chlorophyll a concentrations were determined fluorometrically (8).
We started our second set of experiments on July 22, 2002. The nutrient treatments were identical to the first experiment, except a treatment was added for Oyster Pond water in which both NO3 and PO4 were added to increase ambient concentrations to 50 µM and 3 µM, respectively, to parallel another concurrent set of experiments done in Oyster Pond (7). We repeatedly removed 100-ml samples from each of twelve 2-l bottles over time for chlorophyll analysis, rather than having replicate bottles for each time point. We incubated the bottles in a growth chamber on a 15:9 h light: dark cycle at a light intensity of 280–350 µE m-2 s-1 and a temperature of 24 to 29 °C. All treatments were sampled initially, and on days 1, 2, and 4.
In the first experiment with Vineyard Sound water, chlorophyll a concentrations increased in the N-enriched treatment by day 2, and rapidly declined thereafter (Fig. 1); Concentrations were significantly higher than those of the controls and P-enriched treatment. In the second experiment, chlorophyll concentrations in the N-enriched bottles peaked on day 1 and were always significantly higher than the controls. In contrast, P-enriched treatments were never significantly different from the controls in either experiment (Fig. 1). Both experiments indicate that phytoplankton growth in Vineyard Sound was N limited, as previously reported (5, 6).
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) which concluded that P was limiting (4). Our results contribute to the large body of experimental evidence that finds N limitation in temperate coastal marine ecosystems of moderately high salinity, such as Vineyard Sound. For low-salinity estuaries, there are fewer studies on nutrient limitation, but our finding of N limitation is unusual. The difference between earlier studies in Oyster Pond and our study may reflect seasonal changes in nutrient limitation. Nitrogen may be limiting during the summer (our study) while neither N nor P is limiting in mid-fall (previous studies), either because there is less overall demand for nutrient late in the season or because N fixation over the summer and early fall has helped alleviate N limitation. Further research is needed to better understand nutrient limitation in low-salinity ecosystems, and to evaluate the relative importance of the many biogeochemical processes including N fixation that may regulate limitation in these systems. Nonetheless, our study suggests that N availability, rather than P, currently regulates phytoplankton growth in Oyster Pond during the summer.
We thank the Valiela Laboratory, Ecosystems Center, and the Oyster Pond Environmental Trust. This project was funded by NSF-Research Experience for Undergraduates site grant OCE-0097498.
Literature Cited
This article has been cited by other articles:
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  S. Barron, C. Weber, R. Marino, E. Davidson, G. Tomasky, and R. Howarth Effects of Varying Salinity on Phytoplankton Growth in a Low-Salinity Coastal Pond Under Two Nutrient Conditions Biol. Bull., October 1, 2002; 203(2): 260 - 261. [Full Text] [PDF]
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