Effects of Variable Flows on Water Chemistry Gradients and Fish Communities at the Hillsborough River, Florida

Abstract

We evaluated the effects of variable flows on water chemistry and fish communities to recommend biologically based minimum flows that protect low-salinity zones for freshwater–oligohaline (FO) fishes below the Hillsborough River Dam, Tampa, Florida. We plotted the distributions of hypoxic (dissolved oxygen (DO) ≤ 4 mg/L) and meso- and polyhaline (salinity > 5‰) habitats and evaluated changes in species richness and spatial distributions of FO fishes on five sampling dates from October 2002 to July 2004. During low flows (0.12 m³/s; <25th percentile) in May 2004, oligohaline habitat was reduced as a salt wedge (>5‰) migrated upstream to within 1,300 m of the dam and DO decreased to less than 4 mg/L in the lower part of the reach. However, intermediate (1.19–2.18 m³/s; 25th–50th percentiles) and high flows (5.49 m³/s; 50th–75th percentiles) on four of the five sampling dates maintained oligohaline salinities (i.e., ≤5‰) and DO greater than 4 mg/L throughout most of the reach. Using fyke net data, we found no flow-related trends in FO fish species richness. However, spatial occurrence data indicated that the distribution of FO fishes shifted upstream to within 1,660 m of the dam during a low flow in May 2004. Thus, FO fishes may have congregated in the upper part of the reach or moved out of the lower reach because of unsuitable water quality during low flows. Our results suggest that the best minimum flow to preserve obligate freshwater and oligohaline fishes in the lower Hillsborough River would be between 0.12 m³/s, which eliminated freshwater and reduced oligohaline habitat, and 1.19 m³/s, which provided freshwater and oligohaline habitat throughout most of the study reach. Freshwater flows below the Hillsborough Dam influenced salinity gradients and the spatial occurrence of FO fishes in this system. Low-salinity habitats and FO fish taxa should be considered when setting minimum flows for estuaries.