Abstract

In the aquatic ecosystem, mesozooplankton are the major secondary producers, which graze on phytoplankton and in turn are preyed on by carnivorous invertebrates such as jellyfish and planktivorous fish. Zooplankton are the main prey of forage fish in estuaries. In the Upper Pettaquamscutt River Estuary (UPR) and the Upper Point Judith Pond (UPJP) the main forage fish are the Menidia menidia (Atlantic silverside) and Menidia beryllina (inland silverside). The zooplankton communities in the UPR and UPJP were previously shown to be quite different, especially in spring when these two species ripen for spawning. The zooplankton community at UPR is dominated by calanoid copepods, suggesting a rather clean environment, whereas that at UPJP is dominated by polychaete larvae, suggesting a somewhat degraded environment. The presence of particular zooplankton in the guts of these forage fish has previously been shown to represent the most abundant zooplankton prey in the estuaries. I study the effect of zooplankton composition, i.e., food quality, on the provisioning of the two species for reproduction and on reproductive success, examine whether the forage fish M. menidia and M. beryllina select particular prey based on their caloric values, and trace the flow of energy from prey to predator and to their offspring. During spring 2005, an unusually low abundance of Menidia spp. and a high unusual abundance of Cyanea capillata (lion’s mane jellyfish) were recorded at UPJP. The invasion of a gelatinous zooplanktivore has been shown to upset trophic levels leading to environmental degradation, for example the invasion of the Black Sea, Caspian Sea and Sea of Azov by Mnemiopsis leidyi have led to the demise of many of their forage fish species. In 2006, the UPJP system returned to normal with reduced numbers of C. capillata and more abundant Menidia spp

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