Spatial and temporal variability of water transparency, its primary drivers, and other optical properties in the Hudson River Estuary

Water transparency, measured as the diffuse attenuation coefficient of photosynthetically-active (400-700 nm) radiation, KdPAR, was measured monthly from May-Oct 2020 at 73 locations along 250 km of the Hudson River estuary (HRE) between New York City, NY and Waterford, NY. Three primary optical drivers of KdPAR variability: chlorophyll concentration (CHL), turbidity, and light absorption by colored dissolved organic matter (aCDOM), were measured concurrently to assess their relative importance to spatial and temporal KdPAR patterns. KdPAR was higher and more variable in the brackish HRE relative to limnetic regions further north. KdPAR was also significantly higher nearshore and near sewage treatment plant outfalls relative to midchannel sites. Temporal variability at individual stations was generally lower than cross-channel or cross-region spatial variability. Turbidity was the most important factor in KdPAR variability in all regions of the HRE, followed by CHL, with aCDOM playing a lesser role. Although many water quality variables have changed in the HRE over recent decades, KdPAR measurements in the brackish regions overlapped with published observations from the 1970s-1990s. KdPAR in the limnetic regions was lower in 2020 than measurements made prior to the Zebra mussel invasion in the early 1990s, but similar to measurements in the limnetic regions made since the invasion. Results of this study can be used to optimize satellite-based remote sensing algorithms for synoptic water quality observations of the HRE. Proposed future alterations in the HRE, including dredging and the construction of storm-surge barriers, would likely change existing KdPAR patterns. This study therefore provides an updated baseline for optical parameters in the HRE and highlights the importance of understanding the impacts of any proposed alterations to upper-water-column turbidity in this system. Results also demonstrate that KdPAR, euphotic depths, and Secchi depth measurements were positively correlated throughout the HRE. Secchi depth measurements may therefore be useful for additional retrospective studies, and future monitoring (including by satellite-derived KdPAR) of water transparency in the HRE.