Riverine Discharge Fuels the Production of Methylmercury in a Large Temperate Estuary.

Estuaries are an important food source for the world's growing population, yet human health is at risk from elevated exposure to methylmercury (MeHg) via the consumption of estuarine fish. Moreover, the sources and cycling of MeHg in temperate estuarine ecosystems are poorly understood. Here, we investigated the seasonal and tidal patterns of mercury (Hg) forms in Long Island Sound (LIS), in a location where North Atlantic Ocean waters mix with the Connecticut River. We found that seasonal variations in Hg and MeHg in LIS followed the extent of riverine Hg delivery, while tides further exacerbated the remobilization of earlier deposited riverine Hg. The net production of MeHg near the river plume was significant compared to that in other locations and enhanced during high tide, possibly resulting from the enhanced microbial activity and organic carbon remineralization in the river plume. Statistical models, driven by our novel data, further support the hypothesis that the river-delivered organic matter and inorganic Hg drive net MeHg production in the estuarine water column. Our study sheds light on the significance of water column biogeochemical processes in temperate tidal estuaries in regulating MeHg levels and inspires new questions in our quest to understand MeHg sources and dynamics in coastal oceans.

Reducing Hypoxia in an Urban Estuary Despite Climate Warming.

Seasonal hypoxia is a serious threat to coastal ecosystems. This study on hypoxia in Long Island Sound (LIS), a large urbanized estuary, focuses on responses to managed nitrogen load reductions and climate change. At the analyzed station in western LIS, warming in bottom waters (0.8 °C per decade) favors hypoxia. Total nitrogen concentrations have decreased (0.06 mg L-1 per decade) with load reductions, but no linear temporal trend in chlorophyll is discernible. Bottom dissolved oxygen has increased (0.48 mg L-1 per decade), despite warming-induced solubility decreases (0.13 mg L-1 per decade). Decreasing trends in hypoxic area and volume (100 km2 and 1 km3 per decade) reflect improved conditions and are coincident with reducing loads. Regressions link hypoxic extent to nitrogen loads, chlorophyll, salinity, and winds. Though mitigation has reduced hypoxia, these improvements will not be sustained in the warming climate without continued intervention. The warming-induced oxygen solubility decrease forecasted for 2099 (0.4 mg L-1) would erode 35% of the observed oxygen gains. Implementing a nitrogen load reduction of 1.2 × 106 kg year-1 before the century's end would offset the oxygen solubility decline. This overall approach is applicable to areas experiencing warming and continued development that complicate efforts to reign in hypoxia.

Citizen science observations reveal rapid, multi-decadal ecosystem changes in eastern Long Island Sound.

Long-term environmental records are among the most valuable assets for understanding the trajectory and consequences of climate change. Here we report on a newly recovered time-series from Project Oceanology, a non-profit ocean science organization serving New England schools (USA) since 1972. As part of its educational mission, Project Oceanology has routinely and consistently recorded water temperature, pH, and oxygen as well as invertebrate and fish abundance in nearshore waters of the Thames River estuary in eastern Long Island Sound (LIS). We digitized these long-term records to test for decadal trends in abiotic and biotic variables including shifts in species abundance, richness, and diversity. Consistent with previous studies, the data revealed an above-average warming rate of eastern LIS waters over the past four decades (+0.45 °C decade-1), a non-linear acidification trend twice the global average (-0.04 pH units decade-1), and a notable decline in whole water-column dissolved oxygen concentrations (-0.29 mg L-1 decade-1). Trawl catches between 1997 and 2016 suggested a significant decrease in overall species diversity and richness, declines in cold-water adapted species such as American lobster (Homarus americanus), rock crab (Cancer irroratus), and winter flounder (Pseudopleuronectes americanus), but concurrent increases in the warm-water decapod Libinia emarginata (spider crab). Our study confirmed that Long Island Sound is a rapidly changing urban estuary, while demonstrating the value of long-term observations made by citizen-scientists, educators, and other stakeholders.