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The St. Johns River is one of the largest rivers in the Southeastern United States and serves as a major source of freshwater for agriculture and urban communities of the region. The role of meteorological, physical and chemical factors in guiding the structure and dynamics of phytoplankton in the river has received considerable attention, but little is known about the factors that control the zooplankton community. An existing 16-year historical data set for phytoplankton, zooplankton and physical-chemical factors (from September 1996 – March 2011) was used to examine the factors that may be driving zooplankton structure and dynamics, including succession patterns and the influences of stochastic events (i.e. hurricanes, prolonged drought and exceptional freezing conditions) and phytoplankton blooms, which are a common feature in the St. Johns River. The study focused on a sampling site in the northern reach of the lower St. Johns River in Lake George (Volusia County, Florida, USA), which is located about half way along the reach of the river. Zooplankton abundance (as carbon biomass) exhibited temporal trends and dynamics, either annually or among the years in different genera. Major zooplankton taxa (i.e. rotifers, cladocerans and copepods) showed different peaks in density (individual L-1) and carbon biomass. Peaks in zooplankton biomass were usually observed in Spring and Summer. Rotifers usually showed two peaks in abundance, first in early Spring and later in Summer. Copepod biomass increased as water temperature through the spring. Cladocerans biomass usually peaked between peaks in rotifers and copepods. Zooplankton community dynamics will be discussed in the context of correlations and time-series analyses between zooplankton biomass and physical, chemical and biological parameters. The patterns observed in the St. Johns River will be compared with other subtropical aquatic systems. Climate change is also considered as a potential factor that could influence zooplankton dynamics. |