The influence of freshwater inflow on zooplankton distribution and density in Estero Bay, FL

To investigate the influence of freshwater inflow on zooplankton distribution and density in Estero Bay, 500-micron mesh plankton nets were towed monthly through the water column on a flood tide. Sampling was conducted at 16 stations over a period of two years. Decapods dominated the catch, comprising 87% of the total by number: the xanthids Eurypanopeus depressus and Rhithropanopeus harrisii as well as the porcellanid Petrolisthes armatus were most abundant. The commercially important Florida stone crab Menippe mercenaria was also commonly encountered as were mud crabs of the genus Panopeus. Ichthyoplankton represented only 4% of the total catch by number but were more diverse than either decapods or gelatinous zooplankton, with 48 species being present. Ichthyoplankton was dominated by the eggs of anchovies, drums and herrings as well as by the larvae of anchovies, gobies, skilletfish, blennies, and herrings. Anchovies were dominated by the bay anchovy Anchoa mitchilli, gobies of the genus Gobiosoma were found in greater numbers than those of the genus Microgobius, and the Florida blenny Chasmodes saburrae was the most abundant, identifiable blenny. Gelatinous zooplankton were the least diverse of zooplankton groups but comprised 9% of the total catch. Hydromedusae and chaetognaths dominated, representing 99% of the gelatinous zooplankton catch. Among the hydromedusae, Bouganvillia sp., Clytia sp., Eutima sp. and Obelia sp. were frequently collected; among the chaetognaths, Sagitta tenuis and Ferosagitta hispida were collected frequently and, in the case of S. tenuis, in high numbers.

The estuarine zooplankton of Estero Bay are strongly influenced by two sources of freshwater inflow: Mullock Creek/Tenmile Canal to the north, and the Imperial River to the south. These influences were apparent in zooplankton distribution and density, which demonstrated an association with one or more tributaries during reduced inflow and became dispersed into downstream open waters of the bay during higher inflow. Multivariate analysis demonstrated that samples from sites associated with these tidal bributaries-Hendry and Mullock Creek/ Tenmile Canal to the north, and Imperial River and Fish Trap Bay to the south-exhibited community structure that was distinct from that present at other sites, especially during the wet season when estuarine gradients were more fully established. Various regressions relating organism abundance to freshwater inflow were developed and these can be used to predict ecological impacts of inflow reduction, enabling resource managers to manage the bay-wide impacts of changes to inflow from tributaries. Such relationships were typically nonlinear, meaning that inflows at the low end of the inflow spectrum generally had stronger effects on abundance and distribution than inflows at the high end of the spectrum. One management objective should be to keep gelatinous predators (ctenophores, hydromedusae, jellyfishes) out of estuarine tributaries that serve as nursery habitat for estuarine-dependent fishes. In these confined tributary waters, gelatinous predators compete with young fish for prey and may even consume the young fish directly. It appears that relatively small amounts of inflow are effective in dispersing these predators into downstream areas.

Additional Readings:
Tolley, SG, EB Peebles, SE Burghart, B Bachelor, J Evans & J Nelson. 2007. Freshwater inflow and utilization of the estuarine tributaries of Estero Bay. Report to the South Florida Water Management District, 227 pp

This project was a collaboration between FGCU and the University of South Florida and was funded by the South Florida Water Management District.