Microcystin contamination of shellfish along the freshwater-to-marine continuum within US mid-Atlantic and Northeast estuaries

Estuaries are dynamic ecosystems that are an important habitat for bivalves. The freshwater bodies that discharge into estuaries can introduce cyanobacteria and cyanotoxins that may accumulate within food webs. Microcystin is a hepatotoxin that causes adverse health effects in humans and can be harmful to terrestrial and aquatic organisms. Microcystin has been detected in marine bivalves and the rate of microcystin accumulation and depuration differs between bivalve species. No study has explored the presence or dynamics of microcystins in bivalves in the Northeast US, where they represent a major fishery. This study quantified levels of microcystins in wild and cultured bivalves as a time series from 2017 to 2021 in three of the largest US East Coast estuaries (Chesapeake Bay, the Hudson River Estuary, and Long Island Sound) that have hosted microcystin-producing cyanobacterial harmful algal blooms (CHABs) within their watersheds. During this study, microcystins were rarely detected in bivalves across Chesapeake Bay but were commonly quantified in multiple bivalve species in the Hudson River estuary and within two harbors of Long Island Sound, Stony Brook Harbor and Conscience Bay. Microcystins were detected in clams (Mercenaria mercenaria and Corbicula fluminea), Eastern oysters (Crassostrea virginica), and mussels (Mytilus edulis and Geukensia demissa). Eastern oysters (C. virginica) had significantly higher levels of microcystin than other bivalve species (p < 0.05) and often contained microcystin even when other bivalves sampled concurrently did not, suggesting oysters may be a vector for hepatotoxic shellfish poisoning in estuaries. Microcystins were detected in oysters even in fall months after water column cyanobacterial biomass and microcystins had decreased to low levels, suggesting toxin depuration slows during colder months. Collectively, this study demonstrates that microcystin accumulation in estuarine bivalves, particularly Eastern oysters (C. virginica), occurs within several of the larger US East Coast estuaries and could represent a public health risk.