Too Much Freshwater, Not Enough, or Just Right? Long-Term Trawl Monitoring Demonstrates the Impact of Canals that Altered Freshwater Flow to Three Bays in SW Florida

Abstract

Understanding the impact of anthropogenically altering freshwater flow to estuaries is a growing information need for coastal managers. Due to differences in watershed development, drainage canals, and water control structures, the Ten Thousand Islands area of southwest Florida provides an ecosystem-scale opportunity to investigate the influence of both more, and less, freshwater flow to coastal bays compared to locations with more natural hydrology. Bottom trawl and water quality data spanning 20 years were used to investigate how environmental and hydrological differences among three bays affect community structure of small estuarine fishes. Relationships between fish community structure and salinity and temperature variables were evaluated over timescales from 1 day to 3 months prior to each trawl. Longer-term aspects of temperature (i.e., 2–3 months) exhibited the highest correlations in all bays, suggesting that spawning cycles are the main cause of seasonal changes in fish communities, rather than differences in freshwater flow. Despite major contrasts in watershed manipulation and the seasonal salinity of one bay being much less than the others, the bays differed primarily based on relative abundances of more common species rather than due to unique suites of species being present. Truly freshwater conditions were never detected, and high salinity conditions were experienced in all bays during dry seasons. This likely prevents a community shift to freshwater species. The range in flow characteristics among bays and general similarity in fish communities suggest that conditions will remain within the tolerance of most fishes in all three bays following restoration to more saline conditions.