Bioaccumulation of methylmercury within the marine food web of the outer Bay of Fundy, Gulf of Maine

Abstract

Mercury and methylmercury were measured in seawater and biota collected from the outer Bay of Fundy to better document mercury bioaccumulation in a temperate marine food web. The size of an organism, together with δ13 C and δ15 N isotopes, were measured to interpret mercury levels in biota ranging in size from microplankton (25μm) to swordfish, dolphins and whales. Levels of mercury in seawater were no different with depth and not elevated relative to upstream sources. The δ13 C values of primary producers were found to be inadequate to specify the original energy source of various faunas, however, there was no reason to separate the food web into benthic, demersal and pelagic food chains because phytoplankton has been documented to almost exclusively fuel the ecosystem. The apparent abrupt increase in mercury content from "seawater" to phytoplankton, on a wet weight basis, can be explained from an environmental volume basis by the exponential increase in surface area of smaller particles included in "seawater" determinations. This physical sorption process may be important up to the macroplankton size category dominated by copepods according to the calculated biomagnification factors (BMF). The rapid increase in methylmercury concentration, relative to the total mercury, between the predominantly phytoplankton (<125μm) and the zooplankton categories is likely augmented by gut microbe methylation. Further up the food chain, trophic transfer of methylmercury dominates resulting in biomagnification factors greater than 10 in swordfish, Atlantic bluefin tuna, harbour porpoise, Atlantic white-sided dolphin and common thresher shark. The biomagnification power of the northern Gulf of Maine ecosystem is remarkably similar to that measured in tropical, subtropical, other temperate and arctic oceanic ecozones.

Fig 1. Locations in the outer Bay of Fundy where seawater, plankton and nekton collections were made, on consecutive years between 2000 and 2002, from the research vessel CCMV Navicula.

Fig 2. The relationship between MeHg and THg concentrations on both an organism (ng size category/g wet weight) and a volume (pg size category/m³ seawater) basis plotted against organism size (ESD).

Also illustrated on a volume basis is biomass (mg wet weight/m³ seawater) versus ESD.

Fig 3. The relationship between stable isotope values of δ¹³ C versus δ¹⁵ N for organisms from the components of the marine ecosystem at the outer Bay of Fundy, Gulf of Maine.

Fig 4. The relationship between the estimated spherical diameter (ESD) of an organism and its calculated trophic level from δ¹⁵ N values, for that portion of the food web from the outer Bay of Fundy where stable isotopes were measured.

Y = 0.003X^6.7, r² = 0.69, n = 202.

Fig 5. Potential surface area available for mercury sorption over the marine size spectrum calculated from the nekton/plankton categories reported here (16mm—25μm), together with the particulate (1μm ~ 450nm) and colloidal (~ 450nm- 5nm) material from the literature [113, 114].

This particulate fraction includes nanoplankton, bacteria, cyanobacteria, picoeukaryotes, viruses and inert particulates [102].

Fig 6. The relationship between ng THg/g wet weight and trophic level (a) and equivalent spherical diameter (b), and between ng MeHg/g wet weight and trophic level (c) and equivalent spherical diameter (d).