A study of composition, abundance, and fatty acid profiles of zooplankton in Albemarle Sound and Chowan River, North Carolina during spring and early summer

In the Albemarle Sound and Chowan River, North Carolina, river herring (alewife and blueback herring) once comprised a commercially important fishery\; however, this fishery has since collapsed and a moratorium on river herring harvest is currently in place. River herring stocks have not recovered despite this moratorium. These rivers are critical nursery habitat for larval river herring and one potential reason for the lack of river herring recovery may be related to poor water quality that could affect the zooplankton nutrition and therefore larval river herring nutrition. The goal of this thesis was to examine the species composition, abundance, and nutritional quality of zooplankton, measured using fatty acid profiles, to determine if the zooplankton prey available to larval river herring are of sufficient quality to support the nutritional needs of larval fish. In the western Albemarle Sound and Chowan River, the zooplankton fatty acid profiles and community structure changed over time and space. In April, the zooplankton composition for 200 µm mesh size was comprised of freshwater species, mainly Cyclopoids and Bosmina spp. The most noticeable change in the zooplankton species composition occurred during the month of May when precipitation was very low. This resulted in a salt intrusion that reached midway up on the Chowan River. The salt intrusion caused a decline in the freshwater species, and an increase in brackish water species in the middle to lower estuary. The upper river sites were dominated by Leptodora spp., a freshwater, predatory zooplankter. This was followed by a wet June, which led to an influx of freshwater, returning the salinity to zero. The zooplankton species composition then returned to one dominated by freshwater species as an increase in water flow moved this community down river, resulting in higher overall abundances. The results demonstrated that there are two distinct size classes of prey for larval river herring, as evidenced by the distinct communities represented by the two mesh sizes. The rotifers, a small bodied zooplankton that have high reproductive rates, were abundant in the 60 µm mesh size samples. In contrast, the 200 µm mesh size samples showed variability in the dominant species, suggesting that a wide range of potential prey for larger herring larvae exists. The May saltwater intrusion also changed the fatty acid profiles of the zooplankton. The amount of DHA in the system increased due to the higher abundance of a dominant brackish water copepod species, Acartia spp. Overall, zooplankton fatty acid profiles during the salinity increase in May were higher in PUFAs, DHA and EPA. Salinity played the most important role in structuring the zooplankton community which, in turn, explained the fatty acid profiles seen. This change in the overall fatty acid composition over the spring period suggests that larval river herring may experience a range of prey items that vary considerably in fatty acid composition. Therefore, the fatty acid profiles of the zooplankton prey field likely have considerable influence over the growth and development of larval river herring. At first feeding, larval river herring consume rotifers and smaller bodied cladoceran which have lower PUFAs compared to larger bodied zooplankton. This study suggests that adequate prey abundance and prey types exist for larval river herring\; however, more work is needed to determine the influence of the fatty acid profiles of the zooplankton community on larval herring growth and survival