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Using trace‐metal‐clean sampling and handling techniques along with ultrasensitive analytical procedures, it is possible to measure both total Hg and monomethylmercury (methyl‐Hg) in natural planktonic communities with the same level of taxonomic, ontogenic, and trophic resolution that is currently possible in fish communities. In an experimentally manipulated lake, both acidification and trophic position enhanced the bioaccumulation of methyl‐Hg in the plankton. A consistant pattern of methyl‐Hg enrichment (2−4 ×) in water, bulk phytoplankton, and individual zooplankton was associated with a 1.5 unit pH decrease in Little Rock Lake. Regardless of pH, bioconcentration factors [Bf = log(Cb/Cw), where Cb and Cw are Hg concentrations in biota and water] were substantially higher for methyl‐Hg than those for total Hg or nonmethyl‐Hg at three pelagic trophic levels (~10−100×). Between each trophic level, the Bf(methyl‐Hg) increased by ~0.5 log units, clearly indicating biomagnification. Although somewhat higher in the acidified basin, Bf(methyl‐Hg) was more strongly influenced by trophic position than by pH. This suggests that methyl‐Hg was bioaccumulated largely in proportion to supply and that acidification may have directly increased supply to the base of the food chain.

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296As interest in the aquatic cycle of organic carbon (OC) has increased, the deployment of in situ optical sensors to measure CDOM fluorescence (chromophoric dissolved organic matter) as a proxy for OC concentration has become more common (e.g., Downing et al. 2009;Sandford et al. 2010). CDOM sensors typically use UV light (~350 nm) to excite the emission of blue light (~450 nm) from certain organic fluorophores, allowing investigators to distinguish CDOM from more commonly measured phytoplankton pigments. Given that CDOM may be more labile than previously thought and given that rates of OC mineralization may vary with fluctuating environmental factors, such as temperature and light, these inexpensive sensors could afford a substantial advantage over traditional wet chemistry methods-provided that the artifactual effects of environmental factors on fluorescence efficiency are well constrained (Graneli et al. 1996;Bertilsson and Tranvik 2000;Bastviken et al. 2004;Hanson et al. 2003;Vahatalo 2009).Here, we quantify the effect of temperature on the fluorescence of CDOM from two dystrophic Wisconsin lakes and an aquatic NOM reference material. Based on laboratory experiments over a wide range of OC concentrations, we derive a function that can be used to standardize CDOM measurements to any reference temperature (and, thereby, remove the effect of temperature variation on CDOM fluorescence). Using a reference temperature of 20°C, we then apply the function to field data and show how temperature compensation affects temporal changes in CDOM fluorescence under natural conditions. Methods and proceduresTwo commercial CDOM fluorometers were used: 1) the C3 Submersible Fluorometer from TurnerDesigns, Inc.; and 2) the ), and the subscripts r and m stand for the reference and measured values. (We note that an analogous function is used widely to calculate temperature-specific conductance from the measured conductivity of natural waters.) For the two sensors we tested, the temperature-specific fluorescence coefficients (r) were -0.015 ± 0.001 and -0.008 ± 0.0008 for Wisconsin bog waters at 20°C. When applied to field data, temperature compensation removed the effect of multi-day trends in water temperature, and it also damped the diel CDOM cycle. We conclude that temperature compensation is a necessary and important aspect of CDOM monitoring using in situ fluorescence sensors.

To investigate relationshi.Qs between mercury speciation and site-specific factors in temperate freshwaters, we measured the concentration of seven Hg species along with 18 environmental variables in the surface waters of 23 northern Wisconsin lakes during spring and fall. The lakes spanned relatively wide gradients in Hg (0.15-4.8 ng liter ') and methylmercury (MeHg: 0.04-2.2 ng liter-I). Over the range ofmeasured variables, Hg and MeHg were most strongly correlated with each other (r2 = 0.83-0.88) and with dissolved organic C (DOC) (r2 = 0.64-0.92). Multiple regression models containing DOC and a (DOC x pH) interaction term accounted for 85-90% of the variability in Hg and MeHg between lakes.Observed differences between lakes reflected internal cycling processes and external transport pathways. Internally, high DOC and low pH favored Hg methylation and retention over Hg evasion across the airwater interface. Externally, watershed mapping suggested that the cotransport of DOC, Hg, and MeHg from riparian wetland was also a potentially important process. Observed seasonal differences indicated a 30% increase in MeHg across lakes during summer due to internal or external processes.The effects of DOC on bioaccumulation may be twofold and antagonistic. Although waterborne Hg and MeHg increased with DOC, seston-water partition coefficients tended to decrease, indicating disproportionately more Hg in the dissolved phase. These observations are consistent with previous data on bioaccumulation factors for zooplankton and fish.

Methylmercury (MeHg) is an environmental contaminant of concern because it biomagnifies in aquatic food webs and poses a health hazard to aquatic biota, piscivorous wildlife and humans. The dominant source of MeHg to freshwater systems is the methylation of inorganic Hg (IHg) by anaerobic microorganisms; and it is widely agreed that in situ rates of Hg methylation depend on two general factors: the activity of Hg methylators and their uptake of IHg. A large body of research has focused on the biogeochemical processes that regulate these two factors in nature; and studies conducted within the past ten years have made substantial progress in identifying the genetic basis for intracellular methylation and defining the processes that govern the cellular uptake of IHg. Current evidence indicates that all Hg methylating anaerobes possess the gene pair hgcAB that encodes proteins essential for Hg methylation. These genes are found in a large variety of anaerobes, including iron reducers and methanogens; but sulfate reduction is the metabolic process most often reported to show strong links to MeHg production. The uptake of Hg substrate prior to methylation may occur by passive or active transport, or by a combination of both. Competitive inhibition of Hg uptake by Zn speaks in favor of active transport and suggests that essential metal transporters are involved. Shortly after its formation, MeHg is typically released from cells, but the efflux mechanisms are unknown. Although methylation facilitates Hg depuration from the cell, evidence suggests that the hgcAB genes are not induced or favored by Hg contamination. Instead, high MeHg production can be linked to high Hg bioavailability as a result of the formation of Hg(SH) 2 , HgS nanoparticles, and Hg−thiol complexes. It is also possible that sulfidic conditions require strong essential metal uptake systems that inadvertently bring Hg into the cytoplasm of Hg methylating microbes. In comparison with freshwaters, Hg methylation in open ocean waters appears less restricted to anoxic environments. It does seem to occur mainly in oxygen deficient zones (ODZs), and possibly within anaerobic microzones of settling organic matter, but MeHg (CH 3 Hg + ) and Me 2 Hg ((CH 3 ) 2 Hg) have been shown to form also in surface water samples from the euphotic zone. Future studies may disclose whether several different pathways lead to Hg methylation in marine waters and explain why Me 2 Hg is a significant Hg species in oceans but seemingly not in most freshwaters.

We quantify the allochthonous organic carbon (OC) budgets for seven north temperate lakes, using diverse information about their land cover, hydrology, and limnological characteristics. We develop a simple equilibrium model within a Bayesian framework that exploits the differences among the lakes to estimate three key rates: aerial loading (AOC) and wetland loading (WOC) from adjacent ecosystems and whole‐lake mineralization of OC (RDOC). Combined with observational data, these rates allow for estimates of the total OC loads, mineralization, and sedimentation within lakes and export to downstream ecosystems. AOC was 1.15 g C m−1 (shoreline) d−1, WOC ranged from 0.72 to 3.00 g C m−1 (shoreline) d−1, and RDOC, normalized to 20°C, ranged from 0.00083 to 0.0015 d−1. Total loads ranged from about 5 to 55 g C m−2 yr−1. Ecosystems immediately adjacent to lakes accounted for one‐half or more of total OC loads for some lakes. Whether a lake processed and stored more allochthonous OC than it exported depended primarily on hydrologic residence time. Our equilibrium model provides a parsimonious approach to quantifying allochthonous OC budgets in lakes with relatively minimal baseline data.

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