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Understanding the causes of large fluctuations in lake water levels is important for adaptive resource management. The relatively simple water budgets of small seepage lakes make them potentially useful model systems, provided that key water balance components can be well constrained. Here, spatial variability in measured rates of evaporation (E) and precipitation (P) at the whole lake scale was investigated, and the effect on daily and seasonal water balance estimates was quantified. To estimate spatial variability, triplicate sensor platforms were deployed on and near an 18 ha seepage lake. Lake stage (S) was monitored at a single node in the lake. The water balance was closed by estimating net groundwater seepage (Gnet) analytically as Gnet = ΔS – (P – E). Instrumentation on a second seepage lake was maintained by citizen scientists to assess the potential for more widespread sensor deployments. Data were collected at 30-minute time steps for six months. The results indicate that low-cost sensor networks with single nodes to measure E, P, and ΔS provide well-constrained water budgets at daily and seasonal time scales.

Mercury (Hg) contamination in remote lakes stems from atmospheric Hg transport to surface waters and subsequent conversion by microbes to neurotoxic methylmercury (MeHg) that biomagnifies in pelagi...

Atmospheric deposition is the principal source of mercury (Hg) to remote northern landscapes, but its fate depends on multiple factors and internal feedbacks. Here we document long-term trends and cycles of Hg in the air, precipitation, surface water, and fish of northern Wisconsin that span the past three decades, and we investigate relationships to atmospheric processes and other variables, especially the regional water cycle. Consistent with declining emission inventories, there was evidence of declining trends in these time series, but the time series for Hg in some lakes and most fish were dominated by a near-decadal oscillation that tracked the regional oscillation of water levels. Concentrations of important solutes (SO4, dissolved organic carbon) and the acid–base status of lake water also tracked water levels in ways that cannot be attributed to simple dilution or concentration. The explanatory mechanism is analogous to the “reservoir effect” wherein littoral sediments are periodically exposed and reflooded, altering the internal cycles of sulfur, carbon, and mercury. These climatically driven, near-decadal oscillations confound short or sparse time series and complicate relationships among Hg emissions, deposition, and bioaccumulation.

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 (CH3Hg+) and Me2Hg ((CH3)2Hg) 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 Me2Hg is a significant Hg species in oceans but seemingly not in most freshwaters.

Global warming portends an accelerated water cycle as increased evaporation feeds atmospheric moisture and precipitation. To monitor effects on surface water levels, we describe a low-cost hydrologic observatory suitable for small to medium size lakes. The observatory comprises sensor platforms that were built in-house to compile continuous, sub-daily water budgets. The variables measured directly are lake stage (S), evaporation (E), and precipitation (P). A net inflow term (Qnet) is estimated as a residual in the continuity equation: ∆S = P - E + Qnet. We describe how to build in-lake stilling wells and floating evaporation pans using readily available materials. We assess their performance in laboratory tests and field trials. A 3-month deployment on a small Wisconsin lake (18 ha, 10 m deep) confirms that continuous estimates of ∆S, E, P, and Qnet can be made with good precision and accuracy at hourly time scales. During that deployment, daily estimates of E from the floating evaporation pans were comparable with estimates made using the more data-intensive Bowen ratio energy balance method and a mass transfer model. Since small lakes are numerically dominant and widely distributed across the globe, a network of hydrologic observatories would enable the calibration and validation of climate models and consumptive use policies at local and regional scales. And since the observatories are inexpensive and relatively simple to maintain, citizen scientists could facilitate the expansion of spatial coverage with minimal training.

Summary Evaporation (E) dominates the loss of water from many small lakes, and the balance between precipitation and evaporation (P–E) often governs water levels. In this study, evaporation rates were estimated for three small Wisconsin lakes over several years using 30-min data from floating evaporation pans (E-pans). Measured E was then compared to the output of mass transfer models driven by local conditions over daily time scales. The three lakes were chosen to span a range of dissolved organic carbon (DOC) concentrations (3–20 mg L −1 ), a solute that imparts a dark, tea-stain color which absorbs solar energy and limits light penetration. Since the lakes were otherwise similar, we hypothesized that a DOC-mediated increase in surface water temperature would translate directly to higher rates of evaporation thereby informing climate response models. Our results confirmed a DOC effect on surface water temperature, but that effect did not translate to enhanced evaporation. Instead the opposite was observed: evaporation rates decreased as DOC increased. Ancillary data and prior studies suggest two explanatory mechanisms: (1) disproportionately greater radiant energy outflux from high DOC lakes, and (2) the combined effect of wind speed ( W ) and the vapor pressure gradient ( e s − e z ), whose product [ W ( e s − e z )] was lowest on the high DOC lake, despite very low wind speeds ( −1 ) and steep forested uplands surrounding all three lakes. Agreement between measured (E-pan) and modeled evaporation rates was reasonably good, based on linear regression results ( r 2 : 0.6–0.7; slope: 0.5–0.7, for the best model). Rankings based on E were similar whether determined by measured or modeled criteria (high DOC −1 (C.V. 9%), but statistically significant differences between lakes resulted in substantial differences in cumulative E that were consistent from year to year. Daily water budgets for these lakes show that inputs were dominated by P and outputs by E; and our findings indicate that subtle changes in the variables that drive E can have measurable effects on water levels by shifting the balance between P and E.

The cycling of organic carbon is fundamental to aquatic ecosystems, reflecting processes that extend from terrestrial watersheds to fish. Here, we use embedded fluorescence sensors that sample at high frequency to investigate the daily dynamics of a proxy for the major pool of organic carbon (chromophoric dissolved organic matter, CDOM) in a clear-water Wisconsin lake (∼3 mg C·L−1). We compare the diel CDOM cycle in this lake with cycles observed previously in two dark-water lakes (10 to 20 mg C·L−1). Despite differences in DOM quality and quantity, diel fluorescence cycles were evident in the epilimnia and hypolimnia of all three lakes. The amplitude differed among lakes, but the timing of the diel cycles was similar, with increases in fluorescence during nighttime and decreases during daylight (except in the aphotic hypolimnion of the darkest lake). The amplitude of the diel cycle increased with increasing DOM concentration, and estimates of DOM turnover based on the magnitude of oscillation ranged from 0.28 mg C·L−1·day−1 in the darkest lake to 0.14 mg C·L−1·day−1 in the clear lake. Independent estimates of free water metabolism based on the daily dynamics of O2 or CO2 were in general agreement, ranging from 0.32 to 0.06 mg C·L−1·day−1. Although absolute rates of turnover varied directly with DOM concentration, relative rates were highest in clear waters (∼5%·day−1). We conclude that these daily oscillations may be a common property of lakes and that they may provide insights into internal DOM processing over short time scales.

We monitored the dynamics of chromophoric dissolved organic matter (CDOM) fluorescence in two Wisconsin bog lakes over timescales ranging from hours to months. Peatland-derived dissolved organic matter (DOM) was the major solute in both bog lakes, and diel cycles were dominant features of the CDOM fluorescence time series. Two distinct diel cycles that differed in amplitude and timing were observed: one in oxic epilimnia and a second in anoxic, hypolimnetic waters. These cycles were not attributable to instrumental artifacts (i.e., daily oscillations of temperature, ambient light, or battery voltage), hydrologic forcing, or the effects of inner filtering, pH, or redox conditions. High light extinction coefficients, especially in the ultraviolet region (∼ 10 m−1 to 30 m−1), suggest that DOM photolysis was negligible at the depths of the CDOM fluorescence probes in these dark-water lakes (dissolved carbon concentration: 10 mg C L−1 to 20 mg C L−1). The diel cycles were apparently governed primarily by biological activities that mediate DOM production (release) and destruction (uptake). Rates of carbon turnover derived from properties of the epilimnetic CDOM fluorescence cycle (0.28 mg C L−1 d−1) were similar to rates of net ecosystem production based on daily CO2 dynamics (0.32 mg C L−1 d−1). It appears that a small, secondary pool of labile organic carbon turns over at relatively high rates in these bog lakes, consistent with the two-compartment view of DOM stability.

We report a unique hydrologic time series which indicates that water levels in lakes and aquifers across the upper Great Lakes region of North America have been dominated by a climatically driven, near-decadal oscillation for at least 70 years. The historical oscillation (~13 years) is remarkably consistent among small seepage lakes, groundwater tables, and the two largest Laurentian Great Lakes despite substantial differences in hydrology. Hydrologic analyses indicate that the oscillation has been governed primarily by changes in the net atmospheric flux of water (P − E) and stage-dependent outflow. The oscillation is hypothetically connected to large-scale atmospheric circulation patterns originating in the midlatitude North Pacific that support the flux of moisture into the region from the Gulf of Mexico. Recent data indicate an apparent change in the historical oscillation characterized by an ~12 years downward trend beginning in 1998. Record low water levels region wide may mark the onset of a new hydroclimatic regime.

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 m21 (shoreline) d21, WOC ranged from 0.72 to 3.00 g C m21 (shoreline) d21, and RDOC, normalized to 20uC, ranged from 0.00083 to 0.0015 d21. Total loads ranged from about 5 to 55 g C m22 yr21. 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.

We assessed the ecological risk of mercury (Hg) in aquatic systems by monitoring common loon (Gavia immer) population dynamics and blood Hg concentrations. We report temporal trends in blood Hg concentrations based on 334 samples collected from adults recaptured in subsequent years (resampled 2-9 times) and from 421 blood samples of chicks collected at lakes resampled 2-8 times 1992-2010. Temporal trends were identified with generalized additive mixed effects models and mixed effects models to account for the potential lack of independence among observations from the same loon or same lake. Trend analyses indicated that Hg concentrations in the blood of Wisconsin loons declined over the period 1992-2000, and increased during 2002-2010, but not to the level observed in the early 1990s. The best fitting linear mixed effects model included separate trends for the two time periods. The estimated trend in Hg concentration among the adult loon population during 1992-2000 was -2.6% per year, and the estimated trend during 2002-2010 was +1.8% per year; chick blood Hg concentrations decreased -6.5% per year during 1992-2000, but increased 1.8% per year during 2002-2010. This bi-phasic pattern is similar to trends observed for concentrations of methylmercury and SO(4) in lake water of an intensely studied seepage lake (Little Rock Lake, Vilas County) within our study area. A cause-effect relationship between these independent trends is hypothesized.

The effect of temperature on CDOM fluorescence was investigated in dystrophic freshwaters of Wisconsin and in aqueous standards. Laboratory experiments with two commercial in situ fluorometers showed that CDOM fluorescence intensity decreased as ambient water temperature increased. A temperature compensation equation was derived to standardize CDOM fluorescence measurements to a specific reference temperature. The form of the equation is: CDOM r = CDOM m /[1 + r(T m ‐ T r )], where T is temperature (°C), r is the temperaturespecific coefficient of fluorescence (°C ‐1 ), 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.

Total gaseous mercury (TGM) in the lower atmosphere of northern Wisconsin exhibits strong annual and diurnal cycles similar to those previously reported for other rural monitoring sites across mid-latitude North America. Annually, TGM was highest in late winter and then gradually declined until late summer. During 2002–04, the average TGM concentration was 1.4 ± 0.2 (SD) ng m−3, and the amplitude of the annual cycle was 0.4 ng m−3 (∼30% of the long-term mean). The diurnal cycle was characterized by increasing TGM concentrations during the morning followed by decreases during the afternoon and night. The diurnal amplitude was variable but it was largest in spring and summer, when daily TGM oscillations of 20–40% were not uncommon. Notably, we also observed a diurnal cycle for TGM indoors in a room ventilated through an open window. Even though TGM concentrations were an order of magnitude higher indoors, (presumably due to historical practices within the building: e.g. latex paint, fluorescent lamps, thermometers), the diurnal cycle was remarkably similar to that observed outdoors. The indoor cycle was not directly attributable to human activity, the metabolic activity of vegetation or diurnal atmospheric dynamics; but it was related to changes in temperature and oxidants in outdoor air that infiltrated the room. Although there was an obvious difference in the proximal source of indoor and outdoor TGM, similarities in behavior suggest that common TGM cycles may be driven largely by adsorption/desorption reactions involving solid surfaces, such as leaves, snow, dust and walls. Such behavior would imply a short residence time for Hg in the lower atmosphere and intense recycling – consistent with the “ping-pong ball” or “multi-hop” conceptual models proposed by others.

Over a one-year study period (2003), we monitored total Hg (HgT) and methyl Hg (MeHg) at two sites in a Swedish forest stream located above (Site(ref)) and below a stretch of Hg-contaminated sediments (Site(Hg)). We also monitored HgT, MeHg, and ancillary water chemistry in peat water close to the stream and HgT in open field wet deposition. Despite the presence of historical Hg contaminants, direct atmospheric Hg deposition and transfer of Hg from the catchment explained more than half of the annual HgT load at Site(Hg). The concentrations of both HgT and MeHg were sensitive to changes in water discharge (Q) and water temperature (T) at both sites, suggesting that the stream HgT and MeHg load can change dramatically in response to changing weather conditions. The 2003 data together with data from 1996 disclosed intersite differences and temporal variation in the relationships between HgT, MeHg, and TOC (total organic carbon), reflecting variable sources of HgT, MeHg, and TOC and temporal changes in factors affecting Hg speciation. (Less)

Atmospheric deposition is the ultimate source of Hg to many remote lakes, but the response to depositional change remains uncertain because of potential buffering by historical pools of Hg in sediments and soils. We investi- gated the responsiveness of two lakes over time scales ranging from weeks to decades. For waterborne Hg, results indicated that a precipitation-dominated seepage lake and a wetland-dominated drainage lake responded similarly to depositional change, despite high loadings of terrigeneous matter to the drainage lake. For methylmercury (meHg), the response was more complicated, reflecting the influence of multiple factors on in-lake methylation. In the seepage lake, waterborne meHg was correlated with the atmospheric deposition of Hg (II) and SO4, and there was an indirect depend- ence on the water cycle. In the drainage lake, although wetland export was always the dominant external source of meHg, mass balance indicated that net in-lake methylation was four- to seven-fold greater than loading from the wet- land. However, in-lake meHg production was related to the export of Hg (II) ,S O4, dissolved organic carbon, and P from the wetland to the lake. The results suggest that atmospheric Hg deposition, weather, and microbial activity interac- tively effect the aquatic mercury cycle in ways that can be independent of historical pools of Hg in catchments. Resume : Les depots atmospheriques constituent la source ultime de Hg dans plusieurs lacs retires, mais la reaction de ces lacs aux changements dans les depots atmospheriques reste incertaine a cause d'un tamponnage potentiel par les pools historiques de Hg dans les sediments et les sols. Nous avons etudie la reactivite de deux lacs a diverses echelles temporelles allant de semaines a des decennies. Nos resultats de Hg en solution aqueuse indiquent qu'un lac alimente par l'infiltration et domine par les precipitations et un lac alimente par le drainage et domine par les terres humides reagissent de facon semblable aux changements dans les depots atmospheriques, malgre l'apport considerable de ma- tiere terrigene au lac de drainage. Dans le cas du methyl-mercure (MeHg), la reaction est plus compliquee, ce qui re- flete l'influence de facteurs multiples sur la methylation au sein du lac. Dans le lac d'infiltration, la quantite de MeHg en solution aqueuse est reliee aux depots atmospheriques de Hg (II) et de SO4 et elle depend indirectement du cycle de l'eau. Dans le lac de drainage, meme si l'apport provenant des terres humides est toujours la source externe dominante de MeHg, le bilan massique indique que la methylation nette au sein du lac represent e4a6f oisl'apport des terres humides. Cependant, la production de MeHg dans le lac est reliee a l'exportation de Hg (II) ,d e SO4, de DOC et de P des terres humides vers le lac. Nos resultats laissent croire que les interactions des depots atmospheriques de Hg, des conditions climatiques et de l'activite microbienne affectent le cycle aquatique du mercure d'une facon qui peut etre independante des pools historiques de Hg dans les bassins versants. (Traduit par la Redaction) Watras and Morrison 116

The cycle of waterborne methylmercury (meHg) in Little Rock Lake is characterized by a period of accumulation during summertime (when the lake is warm and open to the atmosphere) and a period of decline during winter (when the lake is sealed by ice). We followed this cycle for 16 yr, during which time the lake was acidified with H2SO4 and then allowed to recover naturally as part of a long-term field experiment on acidic rain. Mass balance was used to quantify meHg sources and sinks during acidification and recovery. Although year-to-year variability in the summertime accumulation of meHg was high during both acidified and de-acidified years (C.V. 0.7 and 0.5, respectively), on average 65% more meHg accumulated in the water column during acidification. Most of the meHg mass accumulated in the anoxic hypolimnion (.70%), even though the hypolimnion constituted ,5% of the lake volume. In hypolimnetic waters, we observed a direct correlation between the maximum meHg concentration and the sulfate deficit for each year (r 2 5 0.5‐0.9) and a direct correlation between meHg and sulfide concentrations (r 2 5 0.7). Sulfide was directly related to dissolved organic carbon at concentrations between 300 and 600 mmol L 21 carbon (C). Seasonal changes in waterborne Hg (II) , meHg, and sulfate reduction covaried with the atmospheric deposition of Hg (II) and SO . Across all years, the interaction term [SO 3 Hg (II) ] explained 70% of the variation 22 22 44 in the meHg accumulation rate during summer. These results indicate that meHg production was co-mediated by several simultaneously occurring processes that affect the supply of Hg (II) substrate to the anoxic hypolimnion and the activity of methylating bacteria that are present there. They imply that meHg levels in lakes may respond to future changes in atmospheric Hg deposition in a rapid but complex way, modulated by environmental variables that can interact synergistically with Hg (II) supply. Such variables include sulfate in acid rain, organic carbon in terrestrial runoff, and temperature.

Hg and SO4 are priority pollutants in fresh waters throughout the northern hemisphere; both have atmo - spheric sources associated with anthropogenic emissions to the lower troposphere. Although depositional change has had a demonstrable impact on lakes over decadal time scales, effects over shorter time scales remain uncertain. To better understand the responsiveness of lakes to changing pollutant inputs, we examined the coupling of water chemis- try to atmospheric deposition over time scales of weeks to years in Little Rock Lake. The results indicate that the lake is tightly coupled to its air shed with respect to Hg. The annual cycle of Hg in surface waters closely tracks the annual cycle of Hg in precipitation. For SO4, the annual cycle in lake water is damped and not in phase with the cycle of at- mospheric deposition because of more intense internal recycling. These observations are in keeping with the residence times of Hg and S in lake water (Hg in months; S in years). Nevertheless, because atmospheric deposition is the domi- nant source of both pollutants, trends in lake water track trends in precipitation over a decadal time scale. Thus, remote lakes may be highly responsive to short term changes in Hg deposition, whereas responses to SO4 deposition may be more gradual. Resume :H g et SO 4 constituent des polluants des eaux douces de toute premiere importance dans l'hemisphere nord; tous les deux proviennent de sources atmospheriques associees aux emissions anthropiques dans la troposphere infe- rieure. Bien que les changements dans ces precipitations aient eu des effets quantifiables sur les lacs a l'echelle tempo- relles de decennies, leurs effets sur des echelles temporelles plus courtes restent mal connus. Pour mieux comprendre la reaction des lacs aux apports changeants de polluants, nous avons etudie conjointement la chimie des eaux et les precipitations atmospheriques a l'echelle de semaines et d'annees au lac Little Rock. Le Hg du lac est couple de facon tres serree a l'atmosphere environnant et le cycle annuel du Hg dans les eaux de surface suit de tres pres le cycle an- nuel du Hg dans les precipitations. Le cycle annuel du SO4 dans l'eau est tamponne et n'est pas en phase avec le cycle des precipitations atmospheriques a cause d'un recyclage interne plus important. Ces observations sont compatibles avec les durees de sejour du Hg (en mois) et du S (en annees) dans l'eau de lac. Neanmoins, puisque les precipitations atmospheriques sont la source principale des deux polluants, les tendances dans l'eau du lac suivent celles des precipi - tations a l'echelle des decennies. Ainsi, les lacs isoles peuvent reagir fortement a des changements a court terme dans les precipitations de Hg, alors que leur reaction aux precipitations de SO 4 peuvent etre plus graduelles.

Long-term monitoring of a precipitation-dominated lake in northern Wisconsin indicates decreasing mercury concentrations over the past decade. In the surface waters of Little Rock Lake, aqueous HgT has decreased at an average rate of 0.04 ng/L/yor roughly 40% from 1988 to 1999. The concentration of aqueous meHgT in Little Rock Lake has decreased tooat an average rate of 0.004 ng/L/y (roughly 50% since 1988). Atmospheric monitoring also indicates decreased mercury in bulk precipitation. For those years when we have a continuous data record (1994 through 1999), the annual Hg cycle in bulk precipitation has been damped and the average rate of bulk atmospheric Hg deposition decreased by roughly 50% (from 11.2 ug/m2/y to 5.5 ug/m2/y). These trends suggest that the lake waters have responded to decreased atmospheric Hg depositionperhaps driven by reduced anthropogenic emissions in the region. Ancillary data and mass balance modeling are used to more fully investigate recent changes in the aquatic mercury cycle.

A single stress, acidification with sulfuric acid, was applied to Little Rock Lake in a whole-ecosystem manipulation. We documented a wide range of responses to the acidification, including increases in the concentrations of various chemicals, shifts in microbial processes and a major increase in water clarity to UV-B radiation. Each of these changes could in itself be considered as a separate ecosystem stress that is distinct from the intended manipulation. Acidification in Little Rock Lake was accompanied by a number of substantial changes in the occurrence of organisms. A series of detailed investigations indicates that the mechanisms underlying these organismal changes are varied but cannot usually be tied to the direct effects of acidification. Overall, our results demonstrate how multiple stresses can arise from a single agent operating on an ecosystem and suggest that singly operating stresses may actually be quite rare.

Current paradigms regarding the bioaccumulation of mercury are rooted in observations that monomethyl mercury (meHg) biomagnifies along pelagic food chains. However, mechanisms regulating the formation of meHg, its initial incorporation at the base of pelagic food chains, and its subsequent trophic transfer remain controversial. Here we use field data from 15 northern Wisconsin lakes, equilibrium aqueous speciation modeling, and statistical modeling to revisit several hypotheses about the uptake, distribution, and fate of inorganic Hg (HgII) and meHg in aquatic biota. Our field data comprise determinations of total Hg (HgT) and meHg in surface waters, sediments, microseston, zooplankton, and small fish in each of the study lakes. For these lake waters, strong positive correlations between DOC and aqueous concentrations of mercury along with negative correlations between DOC and the seston-water partitioning of mercury indicate that organic ligands bind HgII and meHg strongly enough to dominate their apparent aqueous speciation. In the microseston, zooplankton and fish, meHg concentrations and bioaccumulation factors (BAFs) increased with increasing trophic level while biotic concentrations of HgII decreased--indicating that meHg was indeed the biomagnified species of mercury. For all trophic levels, meHg concentrations varied positively with the calculated aqueous concentration of meHg+ (free ion), especially when coupled with pH, or meHgOH (hydroxide) species but not with meHgCl0, the neutral chloride complex. These findings suggest that: (1) the passive uptake of meHg does not control bioaccumulation at the base of aquatic food webs in nature (i.e. phyto- and bacterioplankton); (2) correlation with pH and DOC largely reflect the supply and bioavailability of meHg to lower trophic levels; and (3) meHg concentrations at higher trophic levels reflect uptake at low trophic levels and other factors, such as diet and growth. Low concentrations of meHg in surficial sediments indicate that the fates of biotic HgII and meHg are different. Most biotic meHg is demethylated rather than buried in lake sediments.

Here, we describe and evaluate two low-power wireless sensor networks (WSNs) designed to remotely monitor wetland hydrochemical dynamics over time scales ranging from minutes to decades. Each WSN (one student-built and one commercial) has multiple nodes to monitor water level, precipitation, evapotranspiration, temperature, and major solutes at user-defined time intervals. Both WSNs can be configured to report data in near real time via the internet. Based on deployments in two isolated wetlands, we report highly resolved water budgets, transient reversals of flow path, rates of transpiration from peatlands and the dynamics of chromophoric-dissolved organic matter and bulk ionic solutes (specific conductivity)—all on daily or subdaily time scales. Initial results indicate that direct precipitation and evapotranspiration dominate the hydrologic budget of both study wetlands, despite their relatively flat geomorphology and proximity to elevated uplands. Rates of transpiration from peatland sites were typically greater than evaporation from open waters but were more challenging to integrate spatially. Due to the high specific yield of peat, the hydrologic gradient between peatland and open water varied with precipitation events and intervening periods of dry out. The resultant flow path reversals implied that the flux of solutes across the riparian boundary varied over daily time scales. We conclude that WSNs can be deployed in remote wetland-dominated ecosystems at relatively low cost to assess the hydrochemical impacts of weather, climate, and other perturbations.

A new method for processing zooplankton improves the accuracy and precision of mercury (Hg) and methylmercury (MeHg) determinations and permits the analysis of Hg species and ancillary variables, such as dry weight and C, N, and protein contents on subsamples of the same tissue homogenate basing clean technique. In this study, 10-50 individual zooplankton were sorted from live samples, homogenized in 500 pL of low-Hg water, subsampled, and then digested for analysis without contamination. Detection limits ranged from LC to 12 pg for Hg and from 0.8 to 4.3 pg for MeHg. Procedural blanks averaged 68.7 pg Hg and 5.3 pg MeHg. Yield from certified reference materials was consistently high on small masses of tissue homogenate (96% for Hg and 89% for MeHg), and recovery from spiked samples was good (99% for Hg and 92% for MeHg). We also present results for Daphnia and Chaoborcfs from Mud Lake, Wisconsin, processed in this manner. The new method improves our ability to determine the distribution of Hg species at lower levels of aquatic food webs and to investigate factors potentially regulating the bioaccumulation of Hg and MeHg in specific taxa. RCsumC : Une nouvelle mCthode de traitement du zooplancton arnCZiore l'exactitude et la prCcision des dosages du mercure (Hg) et du mCthylmercure (MeHg), et permet l'analyse des espkces de Hg et des variables secondaires, comrne le poids sec, le carbone, l'azote et les protCines dans des

Total and dissolved concentrations of Hg and methyl-Hg (MeHg) were determined in the surface waters of 19 northern Wisconsin lakes under ice-cover when differences due to temperature, hydrology, productivity, and atmospheric exchange were minimal. Measured concentrations ranged from 0.3 to 5.3 ng/L for HgT and from 0.01 to 2.8 ng/L for MeHgT. Dissolved species comprised 30% to 95% of the HgT and MeHgT. MeHg was strongly correlated with Hg for both total and dissolved fractions. Thirteen ancillary constituents were measured in conjunction with the Hg determinations (pH, DOC, DIC, DO2, conductivity, suspended particulate matter (SPM), Ca, Mg, Mn, Fe, Na, SO4, Cl). Simple linear regressions indicated that DOC explained 87% of the variability in HgT and 79% of the variability in MeHgT. Of the other measured variables, pH, DO2, Fe and Mn showed weak but significant simple correlations with Hg and MeHg (@ p < 0.05). Multiple regression models containing two independent variables, (DOC and pH), explained 92% of the variability in HgT and 83% of the variability in MeHgT. Models containing DOC alone fit the dissolved Hg data well. We conclude that organic carbon concentrations have a strong effect on the concentrations of Hg and MeHg in these lakewaters.

We determined concentrations of mercury (Hg) and methylmercury (MeHg) in 12 pristine lakes of Glacier National Park (GNP) and compared our observations with data from published studies of remote lakes in north-central Wisconsin and the Adirondack region of New York. Despite marked differences in water chemistry, biology, and hydrogeology, concentrations of Hg and MeHg in all regions were strongly correlated with dissolved organic carbon (DOC). Variables related to the acid–base status of lake waters had secondary effects on the concentration of waterborne mercury species. Although Hg and MeHg were strongly correlated with DOC in all three regions, MeHg concentrations were lower and increased less per unit organic carbon in GNP lakes than in either Wisconsin or New York. In GNP lakes, MeHg averaged only 4 ± 2% (mean ± SD) of the Hg, while in Wisconsin and New York lakes MeHg averaged 10–14% of the Hg. Based on simple regression modelling, we estimate that the maximum MeHg fraction (MeHg/Hg) in dystrophic GNP lakes would be three- to six-fold lower than in Wisconsin or New York waters of similarly high DOC. Our results suggest that this regional difference involves factors regulating the net production of MeHg in lakes or watersheds.

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 air- water 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 disproportion- ately more Hg in the dissolved phase. These observations are consistent with previous data on bioaccu- mulation factors for zooplankton and fish.

Three methods for estimating atmospheric Hg deposition were compared at a site in north central Wisconsin. Mean volume-weighted Hg concentrations varied 2-fold among methods : 8.6 ng/L for manual, event sampling (wet only) ; 12.3 ng/L for automated, wet-only collections ; and 15.5 ng/L for triplicate bulk collections. Mean Hg loading estimates ranged from 0.24 to 0.44 μg m -2 week -1 . Agreement among the three bulk collectors was good : 0.44 ± 0.03 μg m -2 week -1 (mean ± SD). Potassium dichromate preservative produced high blanks in the automated, wet-only sampler (2-55% of the measured Hg). Data from two of the collectors showed a strong negative correlation between Hg concentration and precipitation volume. This correlation became weaker when the data were lumped into multi-week intervals and disappeared altogether at a collection interval of 1 month. A negative power function fit the weekly data well, and simple modeling demonstrated that missing early storm stages and small rain events can strongly bias estimates of Hg concentration and loading.

Twenty-seven laboratories from around the world agreed to participate in an intercomparison exercise for total Hg (Hgt) and methyl Hg (MMHg) in pristine lake water. Unfiltered samples from a remote brown water lake in northern Wisconsin (USA) were collected into acid cleaned Teflon® bottles using ultra-clean sample handling techniques. The samples were acidified in the field with 0.4% by volume of pre-analyzed HQ (12N

Experimental results and field data indicated that methyl-Hg was produced within a layer of bacterioplankton near the top of the anoxic hypolimnion of Pallette Lake. In situ incubations at ambient Hg concentrations indicated that the net flux of methyl-Hg from the layer was between 50 and 100 pmol/m2*d. This input was sufficient to account for the summer accumulation of methyl-Hg in the entire hypolimnion and it exceeded atmospheric inputs by 2 orders of magnitude. Maximum rates of net methylation occurred in the same region of the water column where we observed maximum rates of sulfate reduction. The measured rates were: 100 fmol rriethyl-Hg/L*d and 90 nmol SO4/L*d. Sulfate reducing enrichment cultures isolated from the hypolimnion were also able to methylate Hg in the laboratory. Sulfate reduction did not occur in anoxic profundal sediments during summer and we infer from ancillary data that methylation in profundal sediments was also low. Whole-lake rates of sulfate reduction in the hypolimnetic layer and shallow sediments were roughly equivalent, but we cannot yet compare methylation rates at these sites due to large uncertainties in the littoral flux of methyl-Hg. We propose that zones of Hg methylation and SO4 reduction follow the oxic/anoxic boundary in both the watercolumn and sediments. The relative importance of watercolumn and sediment processes will depend on the physical and chemical structure of a given lake.

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.

Acidification can affect aquatic organisms directly through hydrogen ion toxicity, and indirectly through disrupted food web dynamics and altered abiotic conditions. Field populations from selected taxa were studied during the Little Rock Lake whole-basin acidification experiment to illustrate patterns whose timing suggests direct (i.e. immediate) or indirect (i.e. delayed or non-uniform) responses to pH change. As the treatment basin was acidified to pH 5·6, 5·2 and 4·7, immediate changes consistent with a direct pH response were observed for species representing several trophic levels. For other taxa (e.g. littoral invertebrates associated with filamentous algal mats, several species of pelagic zooplankton), indirect mechanisms induced by food web changes were more likely explanations for abundance patterns. The results presented here suggest that the responses of aquatic ecosystems to acidification involve a complex interplay between direct pH effects and subsequent indirect interactions.

Concentrations of monomethylmercury [CH3Hg] were measured in the water and seston of five nearly pristine Wisconsin lakes, which span a range of pH from about 4.6 to 7.2. Previous studies had established a clear inverse relationship between [CH3Hg] in fish and the pH of lakes in this region. Here, we examined the pH dependency of [CH3Hg] in lake water and explored the partitioning of CH3Hg between water, seston, and fish as a function of pH. Results indicate that [CH3Hg] in lake water tends to increase as pH decreases, but that seasonal and spatial variability of [CH3Hg] in individual lakes confounds a simple analysis of the relationship. The partitioning of CH3Hg was related only weakly, if at all, to pH. Average partitioning coefficients (log kd=log (Cp/Cw)) were higher for yearling yellow perch (6.0 to 6.5) than for seston (5.5 to 6.0) but did not vary significantly between lakes. This suggests that acidification has a stronger effect on the supply of CH3Hg to the ecosystem than on specific rates of uptake by the biota.

During summer stratification, total mercury (Hgτ) reached maximum concentrations in the O2 :depleted, hypolimnion of Little Rock Lake, Wl. Initially, the hypolimnetic increase was attributed solely to redox-controlled release of Hg from bottom sediments. However, subsequent depth profiles of Hg indicated that hypolimnetic Hg enrichment could also result from the downward transport and recycling of particulate Hg prior to incorporation in the sediments. Contrasts between Fe and Hg cycles in this lake reinforce this notion. Increases in hypolimnetic Fe were observed during both summer and winter O2 decreases. In contrast, hypolimnetic Hg concentrations declined during winter. In the ice-free season, the distribution of particulate mercury (Hgp) correlated with the distribution of chlorophyllous particulates in this lake, re-emphasizing the importance of biotic processes in controlling Hg cycling in the hypolimnion.

Reverse-phase HPLC has been used successfully to quantify phytoplankton pigments in lakes and oceans. Here we extend the technique to photosynthetic bacterioplankton and demonstrate the identification of five bacteriochlorophylls (BChl a, b, c, d, e) extracted from pure cultures with 90% acetone. The technique was then applied to natural plankton samples from two oligotrophic-mesotrophic lakes in northern Wisconsin. In both lakes, previously undetected layers of phototrophic bacteria were identified based on their pigment compositions. In one of the lakes, the bacterial layer previously had been misidentified as a deep zone of abundant pheopigment. These observations re-emphasize concerns that traditional protocols (i.e. measuring the absorbance of mixed acetone extracts at 665 nm before and after acidification) have serious limitations in natural systems and can lead to misinterpretations of planktonic distributions and processes. The potential importance of phototrophic bacteria in dimictic temperate lakes is demonstrated. Without significantly modifying standard reverse-phase HPLC protocols, unamibiguous determinations of eucaryotic and procaryotic chlorophylls and degradation products can be made simultaneously.

We studied the partitioning of mercury (Hg) among air, water, sediments and fish at Little Rock Lake, a clear water seepage lake in north-central Wisconsin. The lake was divided with a sea curtain into two basins, one acidified with sulfuric acid to pH 5.6 for two years and the other an untreated reference site (mean pH 6.1), to document the effects of acidification. Trace-metal-free protocols were used to measure Hg at the picomolar level in air and water. Total gaseous Hg in air samples averaged 2.0 ng/m3. Total Hg in unfiltered water samples collected in 1986 after the fall overturn averaged about 1 ng/L in the acidified and reference basins. Mercury in surficial sediments was strongly correlated with volatile matter content and ranged from 10 to about 170 ng/g (dry weight) in both basins. Total Hg concentrations in whole, calendar age-1 yellow perch (Perca flavescens), sampled after one year of residence in the lake, averaged 114 ng/g (fresh weight) in the reference basin and 135 ng/g in the acidified basin – a highly significant (p < 0.01) difference. The mean whole-body burden (quantity) of Hg in age-1 perch did not differ between basins after the first year, but was significantly greater in the treatment basin than in the reference basin after the second year of acidification. Differences between the two basins in the bioaccumulation of Hg were attributed to internal (within-lake) processes that influence the bioavailability of the metal. An initial Hg budget for the treatment basin of Little Rock Lake showed that atmospheric deposition and sedimentary remobilization of Hg are potentially important processes influencing its biogeochemical cycling and uptake by fish.

SynopsisIn experiments lakes 223 (L223) and 302 South (L302S) in the Experimental Lakes Area in north-western Ontario, and Little Rock Lake (LRL) in northern Wisconsin, were progressively acidified with sulphuric acid from original pH values of 6.1–6.8 to 4.7–5.1. Although the lakes were at different locations with different physical settings and assemblages of plants and animals including fish, there were remarkable similarities in their responses, particularly in regard to biogeochemical processes and effects on biota at lower trophic levels.All three lakes generated an important part of their buffering capacity internally b\ the reduction of sulphate, and to a lesser extent by the reduction of nitrate. Alkalinity production increased as concentrations of biologically-active strong acid anions increased. Models relating the residence times of sulphate and nitrate to water renewal, or first-order kinetics, effectively predicted events.Acidification disrupted nitrogen cycling in all three lakes. Nitrification was inhibited in L223 and L302S, while in LRL, nitrogen fixation was greatly decreased at low pH.The phytoplankton communities of all three lakes were originally dominated by chrysophyceans and cryptophyceans. However acidification changed the dominant species and decreased diversity. Acidification tended to increase phytoplankton production and standing crop slightly, probably because light penetration was increased.Littoral zones of all three lakes became increasingly dominated by a few species of filamentous green algae, which created nuisance blooms by pH 5.6. Mats or clouds of algae changed the entire character of the littoral zone.Acidification of L223 and L302S caused the loss of several species of large benthic crustaceans as pH changed from 6 to 5.6. Large, acid-sensitive littoral crustaceans were absent from LRL before acidification, probably because the lake was already too acidic.As acidity increased, the dominance of cladocerans within zooplankton communities increased.Daphnia catawbaappeared at pH values near 5.6 and became more abundant at lower pHs as the lakes were acidified. Its appearance coincided with a decline in otherDaphniaspecies: another cladoceran,Bosmina longirostris, increased in the experimentally-acidified lakes as didKeratella taurocephala: they became the dominant rotifers. Several sensitive zooplankton species declined or disappeared as the lakes were acidified, most notablyDaphnia galeata mendotae, Epischura lacustris, Diaptomus sicilisandKeratella cochlearis.The responses of different fish varied; they appeared to depend on the sensitivity of key organisms in the food chain. The ability of key fish species to reproduce was impaired as early as pH 5.8; their reproduction, except for yellow perch in LRL, had ceased at pH 5.0 in all the three lakes.Acidification consistently reduced the diversity and richness of species in taxonomic groups studied, these effects resulting from losses of species and the increased dominance of a few acidophilic taxa.Responses of experimentally-acidified lakes in north-western Ontario and atmospherically-acidified lakes in eastern Ontario were similar in most respects where records allowed comparisons to be made, notably in relation to biogeochemical processes and the disappearance of acid-sensitive biota.When the acidification of L223 was reversed, several biotic components recovered quickly. Fish resumed reproduction at pHs similar to those at which it ceased when the lake was being acidified. The condition of lake trout improved as a result of greatly increased populations of small fish, their prey. Many species of insects and crustaceans that had been extirpated by acidification returned. Assemblages of phytoplankton and chironomids have retained an acidophilic character, although their diversity during recovery is similar to that at comparable pHs during progressive acidification. As their chemistry recovered, atmospherically-acidified lakes in the Sudbury area were able to sustain recruitment by species offish, including lake trout and white sucker, with rapid increases in the diversity of invertebrate taxa. Results from both L223 and lakes near Sudbury suggest a rapid partial recovery of lacustrine communities when acidification is reversed.It is concluded that the experimental lakes responded similarly to acidification, and that experimental acidification can reliably indicate the effects of acidification attributable to acidic precipitation.

Several lines of evidence suggest that wetlands may be a major source of methylmercury (MeHg) to receiving waters, perhaps explaining the strong correlation between concentrations of waterborne MeHg and dissolved organic carbon (DOC) in regions such as northern Wisconsin. We evaluated the relative importance of wetland export in the MeHg budget of a wetland-dominated lake in northern Wisconsin using mass balance. Channelized runoff from a large headwater wetland was the major source of water and total mercury (HgT) to the lake during the study period. The wetland also exported MeHg in high concentrations (0.2-0.8 ng L(-1)), resulting in an export rate similar to those reported for other northern wetlands (ca. 0.3 microg MeHg m(-2) y(-1)). Yet, based on intensive sampling during 2002, the mass of MeHg that accumulated in the lake during summer was an order of magnitude greater than the export of MeHg from the wetland to the lake. Hence, a large in-lake source of MeHg is inferred from the mass balance. Most of the accumulated MeHg built-up in anoxic hypolimnetic waters; and the build-up was roughly balanced by losses of inorganic Hg (Hg(II)) implying a chemical transformation within the anoxic water column. An abundance of sulfate-reducing bacteria (SRB) in hypolimnetic waters, established by DNA analysis of the pelagic microbial community, along with a previous report documenting high methylation rates in the hypolimnion of this lake (ca. 10% d(-1)), suggest that this transformation was microbially mediated. These findings indicate that the direct effect of wetland runoff may be outweighed by indirect effects on the lacustrine MeHg cycle, enhancing the load of Hg(II), the activity of SRB, and the retention of MeHg, especially in northern lakes with flushing times longer than six months.

Rates of Hg methylation and demethylation were measured in anoxic hypolimnetic waters of two pristine Wisconsin lakes using stable isotopes of Hg as tracers. One of the lakes is a clear-water seepage lake situated in sandy terrain with minimal wetland influence. The other is a dark-water lake receiving channelized inputs from a relatively large terrestrial wetland. Methyl mercury (MeHg) accumulated in the anoxic hypolimnia of both lakes during summer stratification, reaching concentrations of 0.8 ng·L–1 in the clear-water lake and 5 ng·L–1 in the dark-water lake. The stable isotopic assays indicated that rate constants of Hg(II) methylation (Km) ranged from 0.01 to 0.04·day–1 in the clear-water lake and from 0.01 to 0.09·day–1 in the dark-water lake, depending on the depth stratum. On average, Km was threefold greater in the dark-water lake. Hypolimnetic demethylation rate constants (Kdm) averaged 0.03·day–1 in the clear-water lake and 0.05·day–1 in the dark-water lake. These methylation rates were sufficient to account for the observed accumulation of MeHg in hypolimnetic water during summer in both lakes. Despite substantial export of MeHg from the wetland to the dark-water lake, our study indicates that in-lake production and decomposition of MeHg dominated the MeHg cycle in both lakes.

The atmospheric deposition of H+, SO4, and Hg to Little Rock Lake in northern Wisconsin has declined substantially during the past decade. Parallel decreases have been observed in the surface waters of the lake. Here we extend the observations to the fish community and we present evidence of a contemporaneous decline in levels of Hg in fish tissue. By comparing data from two separated basins of the lake, we then make an initial effort to isolate and quantify the relative importance of de-acidification and reduced Hg deposition on mercury contamination in fish. Statistical modeling indicates that fish Hg in both basins decreased by roughly 30% between 1994 and 2000 (-5%/y) due to decreased atmospheric Hg loading. De-acidification could account for an additional 5% decrease in one basin (-0.8%/y) and a further 30% decrease in the other basin (-5%/y), since the basins de-acidified at very different rates. These results are consistent with the hypothesis that depositional inputs of SO4 and Hg(II) co-mediate the biosynthesis of methyl mercury and thereby co-limit bioaccumulation. And they suggest that modest changes in acid rain or mercury deposition can significantly affect mercury bioaccumulation over short-time scales.

Comparative tests of net mercury methylation potentials, with cultivated and macrophyte-associated periphyton and using stable ((200)HgCl(2) and CH(3)(199)HgCl) and labeled ((203)HgCl(2)) mercury, have been conducted in the Everglades nutrient removal area (Florida, USA) and in a tropical coastal Brazilian lake (RJ, Brazil). More methylmercury was formed by macrophyte-associated (up to 17% of added (203)Hg(II)) than cultivated (up to 1.6%) periphyton and methylmercury formation was lower in periphyton exposed to light (0.2%). High methylation was also observed for samples incubated with stable mercury isotopes (1.5-7.7% of added (200)Hg(II)), confirming the results obtained with labeled mercury. Simultaneous addition of (200)HgCl(2) and CH(3)(199)HgCl indicated that CH(3)(199)HgCl had no inhibitory effect on Hg methylation. The elevated methylation potentials observed in macrophytes, because of their root-associated periphyton, might contribute significantly to the high levels of methylmercury observed in Everglades biota. Comparative mercury methylation tests were also conducted in the water of a stratified temperate lake (Wisconsin, USA). Similar trends were observed for both stable and radioisotopes, with increasing mercury methylation along the depth profile. The highest levels (0.9% (203)Hg(II) and 0.8% (200)Hg(II)) were obtained below the oxic/anoxic boundary, where sulfide starts to increase, probably as a result of the intense activity of sulfate-reducing bacteria in the anoxic layer.

Mass balance studies in Wisconsin, Canada, and Sweden indicate that atmospheric deposition is the principal source of Hg to many lakes, whether delivered directly or indirectly via terrestrial runoff. The reported depositional flux varies 10-fold among these northern regions (3 to 30g Hg/km2/y). Watershed export rates vary from about 1 to 6 g Hg/km2/y, indicating that a substantial fraction of the atmospheric load is retained by terrestrial catchments. Hg residence times vary from 150 to 300 days in small Wisconsin lakes. Losses occur through sedimentation, outflow, and gaseous evasion. Environmental factors such as pH and dissolved humic matter (DHM) effect the budgets, concentrations, and speciation of Hg in lakewaters. Sedimentation is favored by low pH and low DHM, gaseous evasion is favored by high pH, and high MeHg concentrations are favored by high DHM and low pH. Multiple regression models containing DHM and pH explained 85% to 90% of the variability in waterborne Hg (0.2 to 4.8 ng/L) and methyl mercury (MeHg: 0.04 to 2.2 ng/L) among WI lakes ranging in pH from 4.5 to 8 and in organic carbon from 1 to 22 mg/L. It is unclear whether DHM affects MeHg production in lakes or simply reflects co-transport with organic C from riparian wetland. Northern wetlands export 0.1 to 1 g MeHg/km2/y along with 10 to 100 kg C/ha/y (versus

Mercury content and speciation were determined in freshwater Zooplankton from twelve northern Wisconsin (USA) lakes that spanned gradients of dissolved organic carbon (DOC, 1.6 to 20.9 mg/L) and pH (4.6 to 7.2). MeHg in crustacean taxa ranged from 1 to 479 ng/g diy weight, and from 2 to 45 ng/g in the invertebrate predators. Total Hg in the predators ranged from 20 to 153 ng/g. Although the highest MeHg values were found in the herbivores from high DOC lakes (and the experimentally acidified basin of Little Rock Lake), we observed considerable variation in the relationship between MeHg content of Zooplankton and lake water DOC. Bioconcentration factors (BCF) for both MeHg (3.5 to 7.1 log units) and Hg (3.7 to 5.4 log units) decreased with increasing lake DOC, while pH effects were not as apparent. Bioconcentration of MeHg was higher than Hg indicating that MeHg increases while non-methyl Hg declines in progressively higher trophic levels. Biomagnification factors (BMF) for Hg and MeHg were low relative to BCF. The BMF for crustaceans averaged 0.4 log units for MeHg and −0.5 log units for Hg, indicating that MeHg increased 2.5-fold from seston to crustacean herbivores, while non-MeHg concentrations declined. Unlike BCF, BMF were not related to DOC or pH. In contrast to studies of vertebrate predators, both BCF and BMF in the invertebrate predator Chaoborus, were lower than those in presumed prey. These observations point toward several complexities in the transport of Hg species in the lower levels of aquatic foodwebs.

The controlled acidification of a two-basin lake is described. The lake was divided by a vinyl curtain in 1984; acidification of one basin began in 1985. Target pH values of 5.5, 5.0 and 4.5 are planned for 2-yr increments. Biotic and chemical responses and internal alkalinity generation are being studied. Baseline studies, initial results at pH 5.5, and predictions of lake responses to acidification are described.

A recently described technique for measuring chlorophyll biomass in small water samples (Phinney and Yentsch, 1985) was evaluated on a set of lakes in northcentral Wisconsin. Since the new technique requires only 1.5 mL of unconcentrated lake water, it eliminates several steps and sources of error in the traditional protocol and it permits a very high degree of spatial precision for point estimates of chlorophyll concentration. For the midsummer plankton of six lakes (oligotrophic to mesotrophic), agreement between the new whole-water micro-extraction and the traditional filtered-sample techniques was quite good (r2 = 0.82 to 0.88) but recovery by the whole-water method was generally higher, perhaps because some plankton passed through the filters. Using unmodified, generally available instrumentation, limits of detection were about 0.3 µg Chl L−1in situ; and the literature suggests that these limits can be improved easily by a factor of three. Our results indicate that whole-water micro-extraction of chlorophyll is an acceptable technique to use for many lakes.

Ecosystem-level experiments are essential in assessing the effects of environmental perturbations like acidification. To date in North America, such experiments have been rare and geographically limited to drainage lakes in south-western Ontario and to streams in the northeastern US. Seepage lakes, which are the dominant hydrologic type in large regions of the US, have received limited attention from many perspectives, including whole-system manipulation. The Little Rock Lake Acidification Project was initiated to expand insights from previous acidification experiments with whole drainage lakes to a seepage lake system. It involves the gradual acidification of a small (18 ha), seepage lake in northcentral Wisconsin. The lake has been divided into a treatment and reference basin using a flexible, inert barrier; and the treatment basin is being acidified in steps of 0.5 pH units/2 yr period from a starting pH of 6.1 to a final pH of 4.6 (roughly the average pH of rain in this region). The goals are to document the biological and chemical changes which occur, to identify the direct and indirect mechanisms which regulate responses, and to expand insights to a class of lakes previously understudied. In this paper, we describe the history and rationale of the project and we discuss in general terms the utility and constraints of whole-ecosystem manipulations.

Periodic changes in the oviducts of Diaptomus characterize a reproductive cycle in which females oscillate between gravid and nongravid conditions. The duration of the nongravid condition sets an upper limit for rates of clutch production. In laboratory studies of Diaptomus leptopus, D. pygmaeus, D. pallidus, and D. dorsalis, the cycle was unaffected by photoperiod but showed a strong temperature dependency. At a given temperature, the four species had a common cycle period but spent different amounts of time in the two phases of the cycle. Interspecific differences in the duration of the nongravid phase produced substantial variation in reproductive potential.

As part of an ongoing program to investigate mechanisms regulating the aquatic biogeochemistry of Hg, unfiltered surface waters of eight rural lakes in northcentral Wisconsin were collected and analyzed for reactive and total Hg. Samples were collected during autumn mixis using ultra-clean, trace-metal-free protocols which have been applied successfully in the marine environment. Results indicate that Hg concentrations are considerably lower than previously reported, and approach levels observed in remote ocean waters. Concentrations of reactive Hg ranged from 0.7 to 2.9 pM in the eight lakes studied. Total Hg, determined following strong oxidation using BrCl, ranged from 4.7 to 9.7 p M in four of the lakes. Although the data set is limited, these concentrations are 20–100 times lower than previous estimates for lakes in the region. The results reemphasize the importance of uncompromised clean laboratory protocols in the collection and analysis of trace constituents in natural waters. They also imply a reevaluation of ideas regarding the sources and distribution of Hg in lakes. A simple mass balance for Little Rock Lake clearly shows the potential importance of both atmospheric Hg deposition and sedimentary remobilization to the geochemical cycling and bioaccumulation of Hg.