Carbon and nitrogen sources for juvenile blue crabs Callinectes sapidus incoastal wetlands

We used a combination of field and laboratory techniques to examine the rel ative importance of food webs based on marsh detritus, benthic algae, or ph ytoplankton in supporting growth of the blue crab Callinectes sapidus. We c onducted a laboratory experiment to compare the growth of newly metamorphos ed juveniles fed natural diets from potential settlement habitats such as m arshes. The experimental diets consisted of zooplankton, Uca pugnax and Lit toraria irrorata tissue, a mixture of plant detritus and associated meiofau na and detritus only. Crabs fed the zooplankton diet showed the fastest gro wth and reached a mean dry weight of 32.4 mg, from an initial dry weight of 0.8 mg, during a 3 wk period. Based on the isotopic composition, juvenile crabs obtain carbon and nitrogen from various food sources. For example, cr abs fed zooplankton obtained their nutrition from phytoplankton derived org anic matter. consistent with zooplankton feeding on phytoplankton. The mean delta(13)C values for juveniles fed detritus and detritus-plus-meiofauna w ere considerably lighter (delta(13)C = 19 parts per thousand), than that of their respective diets (delta(13)C = -16 parts per thousand), suggesting t hat crabs were selectively ingesting prey items that obtain their nutrition from an isotopically Lighter carbon source like phytoplankton. Conversely, crabs fed U, pugnax or L, irrorata had isotopic ratios (delta(13)C = 16 to -14 parts per thousand) consistent with these species feeding on isotopica lly heavier marsh grass carbon. Isotopic ratios of crabs collected in the f ield appeared to corroborate the experiment and suggest that either Spartin a alterniflora detritus or benthic algae-based food webs supported juvenile crab growth in marsh environments, whereas phytoplankton-based food webs d ominate habitats more closely associated with the main estuary.