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Boston University Marine Program, Marine Biological Laboratory, Woods Hole, Massachusetts 02543
1 Iowa State University, Ames, IA.
2 Universidad Nacional de Mar del Plata, Mar del Plata, Argentina.
Land-derived nitrogen impacts are a major agent of change affecting estuarine populations. Some changes include macroalgae and phytoplankton blooms, which alter food webs and benthic habitats (1). N loads may influence the abundance, species composition, and growth rates of the shrimp species that are common in estuaries of Cape Cod, such as those in the genera Palaemonetes and Crangon (2). Estuaries of the Waquoit Bay estuarine system offer the opportunity to examine how shrimp of different species respond to different land-derived N loads, because different sub-estuaries are subject to different land-derived loads. For example, Sage Lot Pond, Quashnet River, and Childs River receive N loads of 15.9, 310.3, and 360 kg N ha-1 y-1, respectively (3). The estuaries have similar water residence times, about 1–2 days, and range from 0–32 ppt (1).
In this study we assessed the effects of differences in land-derived N loads on shrimp abundance, shrimp species composition, growth rate, and reproduction in estuaries of Waquoit Bay, Massachusetts.
To estimate the abundance and size of shrimp of the different species, we walked a 5-m seine for 10 m in each of five arbitrary locations along the shore, beginning with the most fresh to the most saline of each estuary, during high tide. Shrimp were identified, counted, and measured from the tip of the rostrum to the end of the carapace. To estimate growth rates in Palaemonetes pugio, we first identified the modal carapace length of each cohort present, using the software program Mix 3.1.3, and calculated the increment in size per month. In addition, we recorded percent of ovigerous females in each estuary. We used ANOVA to compare species abundance and percent ovigerous females among the estuaries. A log10 transformation was necessary to meet the ANOVA assumptions to compare abundances. An a posteriori Tukey test was used to test differences among estuaries.
We found four species of shrimp in the estuaries: Palaemonetes pugio, P. vulgaris, Crangon septemspinosa, and Hippolyte zostericola (Fig. 1, first panel). There was no detectable effect of salinity on the abundance of shrimp, so in this paper we pooled data from all stations. Abundance of P. pugio was significantly higher in Quashnet River and Childs River than in Sage Lot Pond (F = 7.83, df = 2, P < 0.01); abundance of P. vulgaris and C. septemspinosa did not change significantly (F = 0.073, df = 2, P = 0.93; F = 0.016, df = 2, P = 0.9). H. zostericola was found only in Sage Lot Pond, the estuary with the lowest N load (Fig. 1, first panel).
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We determined growth and reproductive effort for P. pugio, the most abundant shrimp species found (Fig. 1, second panel), as also reported in an earlier study (2). Size frequency histograms clearly revealed two cohorts of P. pugio in all estuaries. Individuals in cohort 1 had a carapace length of 8–13 mm, and individuals in cohort 2 had lengths of 14–18 mm. Growth rate of cohort 1 was higher than those of adults and was unaffected by N loads (Fig. 1, third panel). Growth rate of adults decreased slightly with increased N loads (Fig. 1, third panel). Egg-bearing females were only found in cohort 2 individuals. There was no effect of level of N loading on the percent of ovigerous females in cohort 2 (F = 1.7, df = 2, P = 0.24). This suggests that food supply in the estuaries was sufficient to support reproductive needs (4).
Higher N loads lead to higher populations of plants and algae, and the protein content of these are higher. P. pugio may be better at exploiting these resources then the other shrimp species (5, 6), although we lack an explanation for the mechanism underlying the different responses by these four shrimp species. We can say, however, that our results suggest that P. pugio may be a useful indicator species for increased N loads, and H. zostericola may indicate more pristine conditions.
A grant (OCE-0097498) from the National Science Foundation-Research Experience for Undergraduates supported this research.
Literature Cited
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  J. A. Morgan, A. B. Aguiar, S. Fox, M. Teichberg, and I. Valiela Relative Influence of Grazing and Nutrient Supply on Growth of the Green Macroalga Ulva lactuca in Estuaries of Waquoit Bay, Massachusetts Biol. Bull., October 1, 2003; 205(2): 252 - 253. [Full Text] [PDF]
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) and adult cohorts (
) (third panel), and percent ± SE of ovigerous females of P. pugio (fourth panel) in relation to land-derived N loads for the watersheds. Pp = P. pugio; Pv = P. vulgaris; Cs = Crangon septemspinosa; Hz = Hippolyte zostericola. CR = Childs River; QR = Quashnet River; SLP = Sage Lot Pond.