Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

The data in each year are grouped by Location, Filter size, and measured parameters for each size fraction, as well as the measured dissolved parameters:

Location: state, site name and sub-location (in many instances these are sites with high (HOC) and low (LOC) organic carbon sediments), sampling month and sampling time in terms of tide (HT = high tide, LT = low tide, FL = falling and RS = rising tide; if more than one sample at a tide was collected, this is indicated, e.g. HT1, HT2), latitude and longitude

Filter size: Filtering protocols were not exactly identical between years. In 2015, samples were sequentially filtered so particulate samples are >20 µm (large fraction) and 0.2-20 µm (small fraction). In 2016, water was sampled through filters independently so samples are >2 µm (bulk) and <20 µm (small fraction). For each filter, the following parameters were measured and are recorded: total suspended solids (TSS, mg/L), chlorophyll a (Chl a, µg/L) and phaeopigment (Pha, µg/L), particulate total Hg (part. HgT, pmol/g) and particulate methylmercury (part. MeHg, pmol/g).

Dissolved parameters: For each location, a filtered sample was analyzed for: dissolved total mercury (diss. HgT, pM), dissolved methylmercury (diss. MeHg, pM), and dissolved organic carbon (DOC, µM). Salinity and temperature (degrees C) were also recorded for each sample.

Coastal phytoplankton measurements were taken from 22 sites wtihin watersheds along the U.S. East Coast, from New Jersey to Maine, in 2015 and 2016.  Samples were collected from shore or small boat by hand using clean sampling techniques and processed within 12 hours of collection. Salinity and temperature were also measured, using a hand-held YSI sonde.

Water was filtered using either quartz filters combusted at 550 degrees C and 0.2 µm polycarbonate filters cleaned with 2% hydrochloric acid (2016) or sequentially filtered using 20 µm nylon filters and 0.2 µm polycarbonate filters cleaned with 2% hydrochloric acid (2015). The sequential filters in 2015 can be summed to calculate a bulk sample.  The filters were stored frozen until analysis for particulate HgT and MeHg, and ancillary parameters.

Dissolved samples were collected after the filter (filtrate) into acid cleaned iChem bottles and preserved with 0.2% HCl if intended for HgT or combined MeHg and HgT analyses. Samples for only dissolved MeHg were preserved with 0.5% H2SO4.  DOC samples were collected from the particulate Hg filtrate into muffled amber vials and nutrient samples into acid-cleaned plastic centrifuge tubes; both were preserved by freezing.  Samples were collected at various tidal stages (as explained above in Description section).

Particulate and dissolved MeHg samples were analyzed on a Tekran 2700 Automated Methylmercury Analysis System following standard techniques (Hammerschmidt and Fitzgerald, 2006; Gosnell et al., 2017; Munson et al., 2014; Seelen et al., 2021). Briefly, particulate samples were digested in 4.5N nitric acid overnight, neutralized with potassium hydroxide and acetate buffer, and ethylated using sodium tetraethylborate before separation by gas chromatography and cold vapor atomic fluorescence detection with calibration against a standard curve (Alfa Aesar CAS: 115–09–3, LOT: 1791821 spike recovery= 103±14%). Dissolved MeHg samples were analyzed similarly except the seawater was digested overnight in 1% H2SO4, and 2.5% L-ascorbic acid was added prior to ethylation (Munson et al., 2014) (Alfa Aesar CAS: 115–09–3, LOT: 1791821 spike recovery= 86±34%). The samples were not corrected for spike recovery. The methods are a modified version of EPA method 1630.

HgT was analyzed on a Tekran 2600 by cold vapor atomic fluorescence spectrometry following EPA method 1631, refined by Hammerschmidt and Fitzgerald (2006). The particulate HgT (pHgT) sample filters were digested in 4.5N nitric acid with bromine monochloride (BrCl) for 16 hrs, followed by hydroxylamine hydrochloride and stannous chloride additions before analysis. Calibration was against a standard curve and the results were spike corrected (J.T.Baker CAS: 7732–16–5, Batch No: 0000127949 spike recovery= 121±2%). For some samples, the same filter was used for both pMeHg and pHgT, in which case the BrCl addition was made after the MeHg analysis was complete. Dissolved HgT (dHgT) analysis was similar to the particulate analysis, except only BrCl was added >16 hr for digestion prior to reduction for analysis. The samples were blank and spike corrected (J.T.Baker CAS: 7732–16–5, Batch No: 0000127949 spike recovery= 76±5%).

Total suspended solids (TSS) was determined using the dried filters, as they were pre-weighed prior to use. Chlorophyll a (chl a) and phaeopigment (pha) concentrations were quantified from wet filters using fluorescence techniques after 90% acetone extraction and acidification for pha. DOC was analyzed on a Shimadzu TOC/TN analyzer. Duplicate average RSD = 6.6% (pigments) and 2.8% (DOC); Seelen et al., 2021; Taylor et al., 2019).

Details of filtering and other analytical methods can be found in Seelen et al. (2021) and other publications covering these studies (Taylor et al., 2019; Seelen et al., 2018). Blanks and replicate samples were included in the sampling and analysis, and spike recoveries and other QA/QC included in the analysis (see Table1 in Supplemental Files below).

BCO-DMO processing:
- Combined datasets from years 2015 and 2016
- Joined watershed, site location, and subsite information to dataset
- Added a conventional header with dataset name, PI names, version date
- Adjusted parameter names to comply with database requirements
- Units removed and added to Parameter Description metadata section
- Missing data replaced with 'nd' (BCO-DMO's default missing data identifier) 

NSF Award Abstract:
The accumulation of mercury (Hg) in seafood is a public health concern. The presence of Hg in seafood depends to a large degree on the air-sea exchange of Hg, with atmospheric deposition leading to accumulation of Hg in the ocean. The pathways to seafood start with the uptake of Hg by phytoplankton from seawater where is has always been assumed to accumulate to be eaten by grazers and passed on to larger organisms. This project challenges this assumption with preliminary data that suggests certain phytoplankton species can transform Hg to volatile forms (mercury vapor & dimethylmercury) that are lost to the atmosphere, a processes that removes Hg from the ocean rather than simply concentrating it into the ecosystem and seafood. This process, which has not been studied before, could dramatically alter our view of the Hg cycle in the ocean. The researchers funded by this project will look for the specific phytoplankton species that are capable of volatilizing Hg and quantify the rates at which they do so. They will also examine the suspected role of associated sulfur and selenium compounds in the process, as well as quantifying the changes in the Hg isotopic values for potential use as chemical tracers of the source of Hg in the ecosystem and food supply. These results should allow oceanographers to better quantify and refine our knowledge of Hg cycling in the ocean. The project will support participation of graduate students, a postdoctoral scientist, and incorporation of new information directly into courses taught by the researchers. Funding will also support continuing activities by the participants in activities that disseminate information on mercury and its effect on public and environmental health.

Biogeochemical cycling of mercury (Hg) in the ocean may be more complex than previously assumed. New evidence has challenged the idea that methylmercury (MeHg) merely accumulates in phytoplankton and undergoes little to no transformation before being passed into the food web. This project aims to more fully elucidate the mechanisms behind the intracellular transformation of MeHg to volatile Hg and dimethylmercury (Me2Hg) that can be lost to the atmosphere, as well as to evaluate the range of algal taxa that can perform this transformation using directed culture work. Additionally, the PIs will investigate evidence that thiols, organic selenium (Se) compounds, and sulfides are required to facilitate these reactions within the phytoplankton, and specific pathways will be investigated and quantified through this research. Stable Hg isotopic data has been used to track Hg sources and pathways in marine systems and its fractionation during these MeHg transformations will also be quantified for future field study of marine Hg. The investigators hypothesize that coccolithophorids and other haptophytes capable of these intracellular reactions may account for a significant portion of the production of volatile Hg in the ocean. If this turns out to be the case, understanding and quantifying these volatilization processes may significantly alter our current understanding of the overall biogeochemical cycling of Hg in the ocean.