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Coastal Watershed Institute
Florida Gulf Coast University
10501 FGCU Blvd S.
Fort Myers, FL 33965-6565

Phone: 239-590-7526

Mike Parsons


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

Bethany Bachelor monitoring water qualityTo investigate the influence of freshwater inflow on zooplankton distribution and density in Estero Bay, 500-um mesh plankton nets were towed monthly through the water column 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. Larval fishes represented only 4% of the total catch by number but were more diverse (48 species) than either decapods or gelatinous zooplankton. Most abundant among the fish larvae were 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 blenny. Gelatinous zooplankton James Evans deploying plankton netwere the least diverse of zooplankton groups but comprised 9% of the total catch. Hydromedusae and chaetognaths dominated, representing 99% of the gelatinous 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 high inflow. Samples from sites associated with these tidal tributaries--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 salinity gradients were more fully established. Regressions relating organism abundance to freshwater inflow 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, with inflows at the low end of the flow spectrum generally Jennifer Nelson navigating to the next stationhaving stronger effects on abundance and distribution than high inflows. 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.

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