OCE-PRF Track 1: Biological Mercury Hotspots: The Role of Phytoplankton Blooms in Nearshore Marine Mercury Bioaccumulation

Phytoplankton are the entry point for mercury in marine food webs, and subsequent mercury accumulation in fish is the primary route of human exposure. Yet despite the fact that phytoplankton form this critical link, their role in marine mercury dynamics is not clear, especially with respect to seasonal and geographic variability. This study is designed to evaluate trends in mercury cycling in highly productive coastal marine environments, with a focus on the seasonal effects of phytoplankton blooms in a range of coastal lagoon systems. Lagoons provide an accessible natural laboratory and are potential hotspots for mercury accumulation in animals. Because the processes at play in coastal lagoons are prevalent in nearshore marine habitats (e.g., the occurrence of algal blooms, the dynamics of freshwater-seawater interaction), results from this study will be applicable to coastal marine systems in general and will close major gaps in our understanding of nearshore mercury-food web dynamics. This topic is relevant to environmental regulators who are tasked with selecting viable remediation alternatives and evaluating strategies for contaminant reduction. Furthermore, because this project is lab and field intensive, it will provide hands on training for 20-30 undergraduate volunteers, many of whom will be recruited from underrepresented groups.<br/><br/>Coastlines support a range of productive marine ecosystems that sustain complex food webs. Such environments are particularly susceptible to biologically persistent contaminants, such as mercury, that accumulate exponentially along trophic levels. Phytoplankton represent the first and largest step in this process by concentrating mercury in their cells up to 100,000x above ambient levels. Yet little is known about phytoplankton mercury uptake mechanisms or how phytoplankton growth rate and/or biomass affects uptake rates in diverse ecosystems. To evaluate how effectively mercury enters the food web during algal blooms, and to assess the long-term effects of blooms on seasonal mercury cycling in nearshore systems, three coastal lagoon systems will be studied. Lagoons provide a semi-enclosed water body with a captive food web, and are therefore ideal locations to sample phytoplankton over the entire duration of a bloom cycle. To constrain the drivers of mercury accumulation, field sampling will be augmented with lab-based experiments to (1) quantify how rapidly different species of phytoplankton assimilate mercury, (2) assess the mechanism by which phytoplankton take up mercury (internal assimilation vs. external sorbtion) and (3) evaluate whether groundwater-surface water interaction enhances mercury bioavailability at the coastal margin. This investigation will identify fundamental biogeochemical processes that lead to the toxicity, accumulation, and transport of mercury in coastal marine systems and enable a more accurate account of coastal mercury sources and sinks. These issues are of interest to both research and environmental resource organizations.