| 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 two forage fish species, Menidia menidia (Atlantic silverside) and Menidia beryllina (inland silverside), in the Upper Pettaquamscutt River Estuary (UPR) and the Upper Point Judith Pond (UPJP), Rhode Island, USA. 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 purpose of this research was to determine the effects of habitat quality, as expressed in the nutritional quality of the zooplankton, on the maternal provisioning for spawning and egg and larval quality.;I measured the proximate composition and essential fatty acids in the zooplankton, adult Menidia spp. eviscerated carcasses and their eggs to trace the biochemical compositions from prey to parent to offspring. I monitored weights of gonadal and somatic tissues and calculated the fraction of the caloric energy from the zooplankton that goes into the gonadal, liver and carcass tissues by measuring daily zooplankton caloric content consumption, and the caloric content of the tissues. Finally, I measured diameters of individual spawned eggs, percentage of eggs that hatched, total lengths of larvae at hatching and percentage of larvae that survived after 7 days.;This study was carried out in the springs of 2005, 2006 and 2007. In spring 2005 in UPJP, an unusually low abundance of Menidia spp. and high abundance of Cyanea capillata (lion's mane jellyfish) were recorded and appeared to be related, so only UPR data on Menidia spp. were available for that year. In 2006 and 2007, both UPR and UPJP had abundant Menidia spp., and in UPJP C. capillata abundance was low to non-existent. Zooplankton taxonomic composition in 2005, 2006 and 2007 in UPR was dominated by crustaceans and UPJP by polychaetes, as was previously seen in other studies. For proximate composition, significant differences were observed in lipid (but not protein or ash) content a) for zooplankton between estuaries, b) between Menidia spp., and c) between estuaries for each species. Significant differences were further observed in content of the total omega-3, omega-6 and omega-9 fatty acids for zooplankton and for M. menidia gonads between estuaries. Based on laboratory experiments, the absorption efficiency of M. menidia and M. beryllina from UPR and UPJP ranged from 90 to 93 % of the ingested energy. Retained energy for somatic and gonadal growth accounted for ∼40 % of absorbed energy for M. menidia and ~35% for M. beryllina in both estuaries. Menidia menidia devoted ∼ 25% of absorbed energy to gonadal growth and M. beryllina ∼ 20%. No significant differences were found in egg volume or % hatch between estuaries for either species; however, length at hatch for M. menidia at UPJP was significantly greater than that at UPR. I interpret these results to suggest that the eutrophied environment in UPJP leads to larger M. menidia larvae in most years, presumably beneficial for the species, offset by an occasional "bust" year, like 2005, when the species is effectively absent due to jellyfish. UPR results suggest greater environmental and reproductive consistency there from year to year. Environmental differences in zooplankton composition and biochemistry can influence forage fish reproduction. |
