Abstract
The varying magnitude and seasonality of freshwater inflow complicates its impact on biological processes in estuaries. Estuarine zooplankton respond directly and sensitively to abiotic and biotic dynamics, making them an ideal indicator of estuarine conditions. To establish linkages between variable freshwater inflow and zooplankton responses, daytime and nighttime surveys were conducted over 47 km of the Caloosahatchee River and Estuary in Southwest Florida. This investigation also sought to determine the spatial and temporal distribution and density of ichthyoplankton and copepods in relation to the estuarine turbidity maximum (ETM), which is regularly associated with the location within the estuary where the average near-bottom salinity is 2 psu (X2). Inflow varied significantly between dry and wet months transforming the system from a well-mixed to a salt-wedge estuary, often leading to a truncated salinity gradient. Changes in physical dynamics were reflected in biological shifts: during dry months, overall abundance was greater and copepods showed a stronger affinity to the classical ETM. Spatial shifts along the estuarine axis were also notable between wet and dry months. During the wet season, species such as Acartia tonsa, Labidocera aestiva, and Pseudodiaptomus pelagicus and larval Anchoa spp. were relocated substantially (>22 km) downstream compared to their dry season centers of abundance, altering their spatial relationship with the ETM between seasons. These seasonal and spatial differences in zooplankton assemblages can be used to assess ecosystem change, predict future environmental impact based on changes in precipitation and water use, and develop targets for restoration and enhancement.