Julian wt al. 2023. Dark Waters: Evaluating Seagrass Community Response to Optical Water Quality and Freshwater Discharges in a Highly Managed Subtropical Estuary

Julian II, Paul, Mark Thompson, and Eric Milbrandt. “Dark Waters: Evaluating Seagrass Community Response to Optical Water Quality and Freshwater Discharges in a Highly Managed Subtropical Estuary.” Regional Studies in Marine Science 69 (November 1, 2023): 103302. Redirecting.

Seagrass communities are vitally important ecosystems that support and foster organisms across trophic levels. Understanding the drivers of seagrass distribution and extent in estuary systems is valuable for conservation and restoration efforts. A critical driver of seagrass distribution is how light moves through the water column (i.e. light attenuation) and the availability of light to reach the estuary bottom. Light attenuation combined with the depth of seagrass colonization is used to estimate the percent surface irradiance and is an indicator of seagrass light requirements The objective of this study was to evaluate optical water quality parameters within the Caloosahatchee River Estuary (CRE) relative to changes in freshwater discharge conditions. During the study period, freshwater discharge significantly increased resulting in an increase of annual stressful and damaging discharge events to marine seagrass species. Concurrent with changes in freshwater discharge conditions, changes to optical water quality parameters including color, chlorophyll-a , total suspended solids, and light attenuation were detected along the estuary. Using photo-interpreted seagrass coverage data combined with bathymetric data, the depth of colonization was estimated for three survey years (2008, 2014, and 2020). This information combined with a spatial and temporal light attenuation generalized additive model percent surface irradiance (%SI) was estimated. Over the study period, %SI significantly increased across the lower CRE indicating an improvement in light attenuation. The current freshwater management plan has increased stressful and damaging discharge events, however recent changes in operations has minimized these damaging events which could be linked to the precipitous increase in %SI across the lower CRE. Seagrass density data collected from transects within the estuary suggest that prolonged stress and damaging (>5.13 hm3d−1) has significantly reduce species density. Future water management strategies are expected to reduce the frequency and volume of discharges to the CRE, effectively reducing stress and damaging freshwater discharge event.

Fig. 7. Boxplots comparing Halodule wrightii (left) and Thalassia testudinum (right) density between annual cumulative days of stress and damaging events (14-day moving average discharge > 5.13 hm−3 d−1) by estuary region. Red astriks between boxplots indicate a significant difference between disturbance categories.