Planktonic Trophic Transfer in an Estuary: Seasonal, Diel, and Community Structure Effects

The high tertiary production of estuaries is largely supported by photoplankton primary production. An important question thus concerns how much phytoplankton production enters the food web through planktonic grazing and what physical, chemical, or biological factors influence this trophic transfer. We conducted a 2—yr, diel investigation of planktonic trophic transfer and the factors influencing in the Neuse River Estuary, North Carolina. Zooplankton community grazing rates were generally lowest in winter and highest spring through late summer, ranging from 0.1 to 310 mL°L — 1 °h — 1 . There were few significant diel differences in community grazing rates. The overall daytime mean (± 1 SE) rate was 3.30 ± 0.62 mL°L — 1 °h — 1 while the night mean rate was 3.07 ± 0.60 mL°L — 1 °h — 1 . Post—naupliar copepods were usually more abundant at night than day, but tintinnids were often more abundant by day, while total zooplankton, copepod nauplii, and rotifers displayed no significant diel abundance differences. Community grazing was positively correlated with primary productivity and the abundance of total phytoplankton, centric diatoms, dinoflagellates, and the small diatom Thalassiosira. Community grazing was also positively correlated with upstream river flow and negatively correlated with salinity. However, there were no significant correlations with water temperature, nutrient concentrations, or grazer abundance variables. On an annual basis, the zooplankton community grazed °38—45% of daily phytoplankton production. Planktonic trophic transfer was coincidentally greatest in late spring through summer, during the period when anadromous fish larvae migrating from the open ocean reach their estuarine primary nursery areas. The fish arrive when planktonic trophic coupling is strongest and depart in fall, when planktonic trophic transfer, zooplankton abundance, and phytoplankton productivity all decrease.