Water quality trends and eutrophication indicators in a large subtropical estuary: a case study of the greater Charlotte Harbor system in southwest Florida

Tracking symptoms of eutrophication over time with multiple lines of evidence provides critical information to support environmental management and restoration efforts. For this case study of the Greater Charlotte Harbor estuary system in southwest Florida (USA), we assembled and curated 22 years of monthly water quality data from a spatially stratified random sampling design; estimated trends in annual mean concentrations of nitrogen, phosphorus, and chlorophyll-a across 13 monitoring strata over a sliding 5-year window between 2000 and 2021; identified hot spots where annual mean concentrations were increasing or elevated between 2017 and 2021, relative to stratum-specific thresholds informed by regulatory criteria; and summarized concurrent data from long-term surveys of macroalgal abundance and seagrass acreage. The water quality trend analysis methodology, based on generalized additive models (GAMs), captured seasonality and nonlinear inter-annual tendencies while accounting for uncertainty. Throughout the system, concentrations of total nitrogen increased and exceeded stratum-specific thresholds during the 2010 decade, while concentrations of total phosphorus and chlorophyll-a typically decreased to levels near or below thresholds. Low concentrations of inorganic nitrogen fractions indicated rapid biological assimilation consistent with eutrophication, while low chlorophyll-a concentrations indicated that nitrogen enrichment did not translate into excessive phytoplankton production. Instead, macroalgal proliferation and substantial seagrass losses were observed following Hurricane Irma (September 2017). We speculate that nitrogen enrichment during the 2010s increased the system’s vulnerability to Irma’s effects and helped tip the system toward these profound ecological changes. This work provides a broadly applicable framework for documenting and evaluating symptoms of eutrophication in estuaries.

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