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Submerged Vegetation Responses to Climate Variation and Altered Hydrology in a Subtropical Estuary: Interpreting 33 Years of Change
Citation:
Douglass, J., R. Chamberlain, Y. Wan, AND P. Doering. Submerged Vegetation Responses to Climate Variation and Altered Hydrology in a Subtropical Estuary: Interpreting 33 Years of Change. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 43(6, Sep 2020):1406-1424, (2020). https://doi.org/10.1007/s12237-020-00721-4
Impact/Purpose:
Submerged aquatic vegetation (SAV), contributes to human well-being by stabilizing sediments, improving water quality, and enhancing commercial and recreational fisheries resources. However, SAV abundance has declined dramatically worldwide due to increased sediment and nutrient inputs and alteration of coastal and estuarine hydrology. We presented a comprehensive analysis of hydrologic, salinity, and SAV abundance data at nine sites in Florida’s Caloosahatchee River Estuary for the period from 1985-2015. In particular, we developed quantitative relationships between freshwater inflow, salinities, and SAV cover in the upper, middle, and lower estuary, respectively. Our results showed that SAV abundance can be predicted based on daily salinity/flow along with previous years’ SAV abundance. This predictive understanding explained the temporal variation in SAV abundance of the CRE in association with hydrologic alteration and climate variability over the past 30 years. We also identified the critical duration of healthy flow and salinity environment needed for SAV recovery after major SAV stress events. This observation is especially important in light of ongoing ecosystem restoration efforts in South Florida. This study provides the needed technical basis for resources managers to develop management strategies using water control infrastructure to restore and protect estuarine habitats under conditions of increased climate variability and degraded watershed functionalities.
Description:
Links between hydrologic modifications, flow and salinity regimes, and submerged aquatic vegetation (SAV) species composition and abundance were assessed with an empirical analysis of 33 years of monitoring data collected at nine sites in Florida’s Caloosahatchee River Estuary (CRE). Freshwater inflows to the estuary (30-day means) were often outside the previously recommended envelope of 12.74 to 79.29 m3 s−1. Discharges from Lake Okeechobee through a synthetic hydrologic link were responsible for 43% of the above-envelope flows, but reduced the incidence of below-envelope flows by 30%. A salinity model and salinity stress indices developed for each SAV species indicated that the observed flows generated variable salinity conditions likely to harm both seagrasses and freshwater SAV in the estuary. Regression modeling of SAVabundance generally confirmed the flow and salinity responses expected for each species: Halodule wrightii and Thalassia testudinum in the lower estuary were both harmed by high-flow, low-salinity conditions, while Vallisneria americana in the upper estuary was decimated by low-flow, high-salinity conditions. There was a species-specific effect of the seasonal timing of high flows—T. testudinum was more negatively correlated with high flows in the dry season; H. wrightii in the wet season. The regression analyses also highlighted strong, year-to-year autocorrelations in SAV abundance, indicating reduced resilience after severe losses, particularly for V. americana. Large residual variation in some regression models suggested that factors other than salinity (e.g., optical water quality or grazing impacts) may also influence the system dynamics and should be incorporated in continuing research. This analysis suggests that use of artificial water management infrastructure to reduce extreme high and low flows to the Caloosahatchee and other estuaries could help maintain SAV health in light of intensifying climate variability and degraded watershed flow regulation capacity.
URLs/Downloads:
DOI: Submerged Vegetation Responses to Climate Variation and Altered Hydrology in a Subtropical Estuary: Interpreting 33 Years of Change