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Abstract: We examined the impact of a cyclonic eddy and mode-water eddy on particle flux in the Sargasso Sea. The primary method used to quantify flux was based on measurements of the natural radionuclide, 234Th, and these flux estimates were compared to results from sediment traps in both eddies, and a 210Po/210Pb flux method in the mode-water eddy. Particulate organic carbon (POC) fluxes at 150m ranged 1–4mmolCm−2 d−1 and were comparable between methods, especially considering differences in integration times scales of each approach. Our main conclusion is that relative to summer mean conditions at the Bermuda Atlantic Time-series Study (BATS) site, eddy-driven changes in biogeochemistry did not enhance local POC fluxes during this later, more mature stage of the eddy life cycle (>6 months old). The absence of an enhancement in POC flux puts a constraint on the timing of higher POC flux events, which are thought to have caused the local O2 minima below each eddy, and must have taken place >2 months prior to our arrival. The mode-water eddy did enhance preferentially diatom biomass in its center, where we estimated a factor of three times higher biogenic Si flux than the BATS summer average. An unexpected finding in the highly depth-resolved 234Th data sets is narrow layers of particle export and remineralization within the eddy. In particular, a strong excess 234Th signal is seen below the deep chlorophyll maxima, which we attribute to remineralization of 234Th-bearing particles. At this depth below the euphotic zone, de novo particle production in the euphotic zone has stopped, yet particle remineralization continues via consumption of labile sinking material by bacteria and/or zooplankton. These data suggest that further study of processes in ocean layers is warranted not only within, but below the euphotic zone. [Copyright &y& Elsevier]

Aqueous production and water-air exchange of elemental mercury (Hg0) are important features of the environmental cycling of Hg. We investigated Hg0 cycling in ten Arctic Alaskan lakes that spanned a wide range in physicochemical characteristics. Dissolved gaseous Hg (DGM, dominated by Hg0) varied from 40 to 430 fM and averaged 200 fM. All surface waters were supersaturated relative to the atmosphere. DGM averaged 3 ± 2% of dissolved (i.e., filter passing) dissolved total mercury (DTM) and 15 ± 6% of dissolved labile Hg (DLM). In-lake DGM profiles generally followed the vertical distribution of light, indicating photoreduction of Hg(II) complexes as a source of Hg0. Additionally, DGM correlated linearly with DLM (r2 = 0.82, p < 0.0001) in the lake surface, signifying that Hg complexes (mostly organic Hg associations) in dissolved phase are photoreducible and contribute to production of DGM. Further, a positive relation between DGM/DTM and both Ka (light attenuation coefficient; r2 = 0.73, p < 0.02) and DOC (r2 = 0.60, p = 0.02) suggests that solar radiation and dissolved organic matter control DGM production and its cycling. An average rate of DGM formation (0.6 ± 0.2% of DTM d−1; range, 0.20.8) was estimated by assuming steady state with the evasional rate. In-lake DGM formation occurs at lower rates in waters with greater suspended particulate matter and dissolved organic carbon (DOC), pointing to the significant role of organic matter plays in controlling DGM formation in these aquatic systems. Estimated evasional fluxes of Hg0 (average, 140 ± 50 pmol m−2 d−1; range, 60–200) were comparable to those of temperate lakes (e.g., Wisconsin, Michigan). In arctic lakes, the rate of evasion during ice-free periods (7 ± 3 nmol m−2 yr−1) is similar to the atmospheric input of Hg (wet + dry) to the lakes based on levels in summertime precipitation but not including additional sources, e.g., springtime depletion. [Copyright &y& Elsevier]

• Hg is ubiquitous; could act as a foraging ecology tracer in marine predators' diet. • Well-documented adverse effects of Hg could also be an indicator of wildlife health. • Measured foraging ecology via stable isotopes; endocrine effect via prolactin hormone. • Two seabird species had different Hg, isotopes; reflected diverse foraging strategies. • No relationship between Hg, prolactin detected; Hg not indicator of breeding health. Marine predators are frequently exposed to contaminants through diet, and thus contaminants like mercury have the potential to be used as tracers of foraging ecology. Mercury's neurotoxic and endocrine-disrupting effects can have far-ranging consequences for both individuals and populations, and thus mercury concentrations could also be indicative of wildlife health. Because blood samples are relatively non-invasive and easy to obtain in seabird colonies, we investigated whether blood-based mercury concentrations were representative of foraging ecology and breeding hormone concentrations in seabirds. Blood-based mercury carbon and nitrogen stable isotopes, and the reproductive hormone, prolactin, were sampled from two seabird species that exhibit different foraging strategies in Western Australia: Great-winged Petrels (Pterodroma macroptera) are pelagic squid-specialists whose populations are under-studied; Flesh-footed Shearwaters (Ardenna carneipes) are coastal foragers that associate with fishing vessels, and are a species listed as Vulnerable in Western Australia. Mercury was six times higher in Great-winged Petrels (geometric mean ± SE: 3.360 ± 0.180 μg g−1 ww, n = 15) than Flesh-footed Shearwaters (0.554 ± 0.109 μg g−1 ww, n = 12). There was a significant difference in δ 15N between species, and within-species variation in δ 13C mirrored variation in mercury concentrations, supporting the view that foraging ecology plays a central role in mercury exposure. Furthermore, Great-winged Petrels' mercury concentrations are among the highest reported in seabirds. However, no relationship between mercury and prolactin concentrations was detected. Overall, these results demonstrate that mercury can be used as a foraging ecology tracer in these populations but may not be a good indicator of seabirds' breeding hormones like prolactin, though mercury may affect other aspects of reproduction that we did not measure. These results may aid in future assessment of population trends in these, and other, species. [ABSTRACT FROM AUTHOR]

Abstract: Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234Th activity distribution (in order to calculate 234Th fluxes) and an estimate of the C/ 234Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C/ 234Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C/ 234Th either increases or is relatively invariant with increasing particle size (size classes>1 to 100s μm). Observations also suggest that C/ 234Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2–3). We recommend sampling for C/ 234Th from a standard depth of 100 m, or at least one depth below the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234Th profiles and C/ 234Th samples during the course of longer observation periods to better sample temporal variations in both 234Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column. [Copyright &y& Elsevier]

As part of the ISCAT-2000 field study, aerosol particle samples were collected over ∼24 hr intervals at the South Pole (SP), and they were analyzed for trace elements and for selected ions important for Antarctic sulfur and nitrogen chemistry. This paper focuses on lead and mercury while a companion paper in this issue presents the results of the aerosol ion studies. The results showed that most trace metals were below their limits of detection. Two that were not were Pb and Hg. Lead was only quantified in 15 of 53 samples, and based on those data, the mean concentration of Pb would be <0.032 ng m-3. This is substantially lower than the aerosol Pb concentrations reported for the SP in the early to mid-1970s, suggesting that the decrease may be related to controls on pollution emissions; however, contamination of the earlier samples also would explain the difference. The arithmetic mean concentration of filterable Hg was 0.04 ng m-3; thus approaching levels reported for the Arctic. In contrast to the Arctic, however, filterable Hg at the SP was not associated with low ozone mixing ratios; rather the reactions driving the cycling of Hg at the SP appear to involve HOx radicals, reactive nitrogen, and more than likely other substances through complex and not fully understood pathways. [Copyright &y& Elsevier]