Environmental factors driving microcystin contamination of estuarine bivalve populations downstream of freshwater cyanobacterial blooms

Estuaries are dynamic environments that provide vital habitat to ecologically and commercially important bivalves. In some cases, freshwater tributaries can introduce cyanobacteria and associated cyanotoxins into estuaries that may subsequently accumulate in estuarine bivalves. Temporarily open/closed estuaries (TOCEs), which only experience tidal input for limited periods of time, may be particularly vulnerable to the accumulation of cyanotoxins in bivalves as they can be subject to freshwater input without tidal flushing and may experience lower salinities and cyanobacterial blooms. This study quantified levels of microcystin in bivalves collected as a time series over a five-year period (2017-2021) from Mecox Bay, a TOCE on Long Island, NY, USA, that hosts a productive oyster fishery and is downstream of a freshwater body that hosts microcystin-producing cyanobacterial blooms. During the study, microcystin was detected in all bivalves monitored including Eastern oysters (Crassostrea virginica), blue mussels (Mytilus edulis), and soft-shell clams (Mya arenaria), with levels in oysters exceeding those in other species and frequently exceeding 10 ng g^-1, the California regulatory action level for microcystin in tissue. While oysters were capable of depurating 60-90 % of microcystin after four-to-six weeks during summer, microcystin loads in bivalves often peaked in cooler months after water column cyanobacteria and microcystin levels had seasonally declined, suggesting toxin depuration slowed at colder temperatures. Multiple linear regression models established that time-integrated measurements of pelagic microcystin concentrations in freshwater and estuarine locations, water temperature (inverse correlation), and salinity had highly significant (r² = 0.71; p < 0.001) predictive power of the microcystin content in oysters. These findings demonstrate that bivalves, particularly oysters, in TOCEs located downstream of microcystin-producing cyanobacterial blooms are vulnerable to microcystin contamination, especially during fall months when temperature-dependent toxin depuration rates are likely slow.