Blue Crab Trophic Dynamics: Stable Isotope Analyses in Two North Carolina Estuaries
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Date
2003-11-25
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Abstract
Eutrophication is increasing in estuaries as a result of anthropogenic activity along the land-sea margin. Human activities contribute large amounts of nitrogen and carbon compounds to watersheds, resulting in changes in resource availability through alteration of biogeochemical cycles. Although the effects of poor water quality on lower trophic level biota is well understood, the impact of nutrient waste on upper trophic levels, such as blue crabs (Callinectes sapidus), has not been well studied. Stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope ratios can provide time and space integrated information about feeding relationships and energy flow through food webs. An isotopic comparison of the trophic structure of two North Carolina estuaries was undertaken to understand the impacts of anthropogenic runoff on blue crab tissue enrichment and feeding interactions. This study examined isotopic signatures of primary producers, as well as blue crab and their bivalve prey (Rangia cuneata & Corbicula fluminea) as indicators of potential changes in food web relationships in response to eutrophication. The Neuse River Estuary is an "impacted" system that experiences high nitrogen loading and drains areas of urban development, row crop agriculture, and concentrated animal operations. The Alligator River Estuary by comparison, is designated as a "less-impacted" system in this study. The Alligator River Estuary is classified as having "Outstanding Resource Waters" and low nutrient loading. In each estuary, samples were collected in the upper, middle and lower regions of the river. Bivalves collected from the Neuse River Estuary yielded a significant difference (p<0.0001) in mean nitrogen isotopic composition of tissue (10.4% ± 0.82; N=66) compared to the bivalves collected from the Alligator River Estuary (6.4% ± 0.63; N=45). Similarly, the Neuse River Estuary blue crabs had a mean nitrogen isotopic composition of 11.41% (± 1.3, N=77), which was significantly different (p<0.001) than the less-impacted Alligator River blue crabs (9.655 ± 0.6; N=77). The mean nitrogen isotopic ratios between blue crabs and bivalves were significantly different (p<0.0001) in the Neuse (1.01% ± 0.13) compared to the Alligator River Estuaries (3.2% ± 0.1). Linear regression analyses showed a significant inverse relationship between δ¹⁵N values of blue crab tissue and water quality for the Neuse River Estuary (R2 = 0.7; p= 0.01). A generalized linear model was conducted using blue crab tissue δ¹⁵N as the dependent variable and river estuary as the independent variable of interest. This analysis showed a significant difference (p<0.0001) between rivers controlling for size, site, and the river by site interaction. The results of this study indicate that 1) a relationship exists between the uptake of anthropogenic nutrients by primary producers and the subsequent energy transfer to estuarine consumers, represented by δ¹⁵N and δ¹³C ratios, in two North Carolina estuaries; and 2) an inverse relationship exists between blue crab tissue enrichment and water quality in an impacted estuary.
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Keywords
stable isotopes, food webs, blue crab, bivalve, nutrients, estuary
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences