Watershed export of fine sediment, organic carbon, and chlorophyll-a to Chesapeake Bay: Spatial and temporal patterns in 1984–2016

Abstract

Chesapeake Bay has long experienced nutrient enrichment and water clarity deterioration. This study provides new quantification of loads and yields for sediment (fine and coarse grained), organic carbon (total, dissolved, and particulate), and chlorophyll-a from the monitored nontidal Chesapeake Bay watershed (MNTCBW), all of which are expected to drive estuarine water clarity. We conducted an integrated analysis of nine major tributaries to the Bay to understand spatial and temporal export patterns over the last thirty years (1984–2016). In terms of spatial pattern, export of these constituents from the MNTCBW was strongly dominated (~ 90%) by the three largest tributaries (i.e., Susquehanna, Potomac, and James). Among the nine tributaries, the ranking of constituent export generally follows the order of their watershed sizes, with other factors such as land use and reservoir playing important roles in some exceptions. In terms of partitioning, suspended sediment (SS) export was dominated by fine-grained sediment (SSfine) in all nine tributaries; overall, ~ 90% of the MNTCBW SS is SSfine. Total organic carbon (TOC) export was dominated by dissolved organic carbon (DOC) in all tributaries except Potomac River; overall, ~ 60% of the MNTCBW TOC is DOC. A comparison with literature shows that the MNTCBW SS and TOC yields were ~ 80% and ~ 60% of the respective medians of worldwide watersheds. In terms of temporal pattern, flow-normalized yields from the MNTCBW show overall increases in SS (both long-term [1984–2016] and short-term [2004–2016]), SSfine (long-term and short-term), TOC (long-term), and chlorophyll-a (short-term). The rises in SS, SSfine, and TOC were largely driven by Susquehanna River where Conowingo Reservoir's trapping efficiency has greatly diminished in the last twenty years. Overall, these new results on the status and trends of sediment, organic carbon, and chlorophyll-a provide the foundation for building potential linkages between riverine inputs and estuarine water clarity patterns.