Patterns and Trends in Secchi Disk Depth over Three Decades in the Chesapeake Bay Estuarine Complex
Water clarity is an important ecosystem indicator of eutrophication in Chesapeake Bay and other coastal and estuarine systems across the globe. Although a variety of measures are available to quantify light availability in water, Secchi disk depths have been the most consistent and frequent measure employed in water monitoring programs. Because light availability is influenced by multiple variables, such as phytoplankton biomass, non-living suspended particles, and colored dissolved organic matter (CDOM), understanding the factors driving long-term variability and trends in water clarity is critical for targeting watershed management actions related to eutrophication. Thus, we conducted a comprehensive statistical analysis of spatial and temporal variations in Secchi disk depth and the key internal and external variables that influence its variability in Chesapeake Bay and its tidal tributaries over the past 30 years. Our results indicate that although watershed nutrient, sediment, and freshwater inputs did not correlate with Secchi depth on a monthly timescale outside of low-salinity regions near river outflows, water-column variables that represent the consequences of those inputs (CDOM, chlorophyll-a, and total suspended solids [TSS]) were strongly associated with Secchi depth variability. The inconsistency of these two findings may be explained by controls on chlorophyll-a and TSS that are not directly related to watershed input, such as grazing and resuspension, and by lags of several months between watershed inputs and the associated water-column concentrations. While salinity (a proxy for CDOM) was a dominant spatial covariate with Secchi depth bay-wide, TSS concentrations were strongly associated with temporal changes in Secchi depths in low-salinity regions and indicators of phytoplankton biomass were more important in mesohaline and polyhaline regions. These findings related to spatially dependent controls on Secchi depth enhance our understanding of long-term changes in estuarine light availability and suggest a region-specific response of Secchi depth to variables (TSS and chlorophyll-a) targeted by watershed restoration actions designed to limit nutrient and sediment inputs to Chesapeake Bay.
KeywordsSecchi disk Time series Random forest Seasonality Clustering Water clarity
This study was motivated by the Chesapeake Bay Program Scientific and Technical Advisory Committee (STAC)’s 2017 workshop entitled “Understanding and Explaining 30+ Years of Water Clarity Trends in the Bay’s Tidal Waters.” We thank all the workshop participants for insightful discussions. This work has benefited from data collected through the EPA Chesapeake Bay Program Water Quality Monitoring Program, the Maryland Department of Natural Resources, and the United States Geological Survey River Input Monitoring Program. We thank Carl Friedrichs and Jennifer Keisman for helpful reviews of this manuscript. This is UMCES Contribution Number 5610.
This work was supported by the U.S. Environmental Protection Agency under grants “EPA/CBP Technical Support 2017” (No. 07-5-230480 & CB-96305401).
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