Richard C. Back

Mercury content and speciation in the plankton and benthos of Lake Superior.

As part of a study is to assess the importance of watersheds in controlling sources, transport, fate, and bioavailability of monomethyl mercury (MeHg) in Lake Superior, biotic samples were collected and analyzed to determine total mercury (HgT) and MeHg content, and to examine size, species, trophic and geographic trends. Plankton was collected in two ways: vertical tows of non-metallic, 153 microm mesh net (bulk zooplankton), and by passively filtering near-surface water through stacked Nitex sieves, generating size-fractionated seston (<35, 35-63, 63-112, and >112 microm). Benthos was sampled using a Ponar grab to collect sediment, and a non-metallic sieve to separate biota from substrate. Samples were processed to quantify dry weights, HgT and MeHg. Results for bulk zooplankton sampled offshore showed a range of approximately from 35 to 50 ng MeHg/gram dry weight (gdw) and from 80 to 130 ng HgT/gdw during April, and from 15 to 25 ng MeHg/gdw and from 20 to 70 ng HgT/gdw during August. Results from sieved, near-surface water from offshore sites in April showed a dominance by the <35 microm size fraction both in total mass and mass of MeHg compared to other size fractions. On a dry weight basis, however, we found little difference between the size fractions in April (MeHg ranges from 2 to 10 ng/gdw). During the summer cruise, we found similar concentrations in the <35 microm fraction, but higher in the 112-243 microm size fraction (MeHg 14-16 ng/gdw). The MeHg concentration in Mysis relicta ranged from 33 to 54 ng/gdw throughout the lake. Chironomid larvae were 8 ng MeHg/gdw and amphipods were 32 ng MeHg/gdw.

Early Observations on an Emerging Great Lakes Invader Hemimysis anomala in Lake Ontario

ABSTRACT Hemimysis anomala, a Ponto-Caspian littoral mysid, is an emerging Great Lakes invader that was discovered in Lakes Michigan and Ontario in 2006. Similar to the native mysid Mysis diluviana, Hemimysis exhibits a diel vertical migration pattern but generally inhabits shallower and warmer waters than M. diluviana. Because basic information on the distribution, habitat use, and biology of Hemimysis in the Great Lakes is scarce, the potential for food web disruption by Hemimysis cannot easily be predicted. Preliminary observations indicate widespread invasion of Hemimysis in Lake Ontario. In this study, we confirm the presence of Hemimysis at sites spanning the northern and southern shores of Lake Ontario and the presence of the individuals during winter months. In one horizontal tow in November 2007, over 26,000 individuals were collected with a length range of 4.4 to 9.0 mm and an average caloric density of 611 cal/g wet weight. The most effective methods for sampling Hemimysis were horizontal tows with either a Zooplankton net in the water column or a benthic sled near the lake bottom. Although more quantitative data on the life history and distribution of this species is necessary, our preliminary observations support the prediction that the potential for Hemimysis to impact the nearshore food web in Lake Ontario appears high.

Seasonal and Size-specific Distribution of Methylmercury in Seston and Zooplankton of Two Contrasting Great Lakes Embayments

Abstract The use of a large volume sieve system is described which allows quantification of mass, pigments, and methylmercury (MeHg) in five distinct size classes of suspended material: > 243 μm, 112–243 μm, 63–112 μm, 35–63 μm, and

Population Characteristics and Spawning Migration Dynamics of Pink Salmon in U.S. Waters of the St. Marys River

Population attributes and migratory dynamics of spawning pink salmon Oncorhynchus gorbuscha were examined in U.S. waters of the St. Marys River from 1998 through 2002. Spawning migrations were monitored twice each week from late August through early October of each sampling year using a single gill net set immediately below their spawning grounds. Pink salmon were captured between 23 August and 11 October, with the peak migration event in all years occurring between 10 and 22 September. Catch-per-unit-effort was greater in even years (57 fish/night) than in odd years (30 fish/night). Water temperature during spawning migrations ranged from 11.4 to 21.4°C, with nearly 90% of fish captured between 15.0 and 19.7°C. The proportion of females captured (mean = 0.25; range, 0.09 to 0.35) declined after the peak-migration event, with few females caught during October. Total length and wet weight of male and female fish displayed much variability within and among years. Relative condition of male pink salmon declined over the spawning migration, with a sharp decline observed after peak migration events. Pink salmon representing ages 2 through 4 were captured during the study period, with a large percentage (range, 14.6 to 50.6%) of these fish deviating from their usual two-year life cycle. These are the first reported age-3 pink salmon from a Lake Huron tributary and first age-4 fish observed in any freshwater or marine system. Our results suggest that the naturalization of pink salmon to the upper Great Lakes has resulted in system-specific modifications to their potamodromous life history.

Watershed influences on mercury transport to Lake Superior

Mercury concentrations in river waters represent the result of various geochemical and anthropogenic processes in a watershed. In rivers remote from direct point source inputs of Hg (e.g. effluent discharge, contaminated landfills), Hg sources are usually limited to chemical weathering and atmospheric deposition. Transport through the watershed may occur either by association with particulate or filtered phases. Several recent studies have shown that land use and land cover patterns exert a strong influence on Hg and methyl Hg (MeHg) release from watersheds (ST. Lours et al. 1994, HURLEY et al. 1995, BABIARZ et al. 1998). In particular, forested and wetland watersheds appear to release more total Hg (HgT) in the filtered phase relative to other land use/ land cover patterns. This is most likely due to the association of Hg with dissolved organic carbon (DOC). Secondly, and importantly from a bioaccumulation standpoint, wetland systems have been shown to be important sites for conversion of inorganic H g to MeHg (ST. Lours et al. 1994, HuRLEY et al. 1995, KRABBENHOFT et al. 1995, 1998). Microbial sulfate reduction, a process that has been shown to mediate rhe conversion of Hg to MeHg (CoMPEAU & BARTHA 1985) is important in most wetland systems (ZILLIOUX et al. 1993, GrLMOUR er al. 1998).