David E. Armstrong

Factors affecting enhanced mercury bioaccumulation in inland lakes of Isle Royale National Park, USA

We investigated factors causing mercury (Hg) concentrations in northern pike to exceed the consumption advisory level (>500 ng/g) in some inland lakes of Isle Royale National Park. Using Hg-clean techniques, we collected water, zooplankton, macro invertebrates, and fishes in 1998 and 1999 from one advisory lake, Sargent Lake, for analysis of total mercury (Hg(T)) and methylmercury (MeHg). For comparison, samples were also collected from a non-advisory lake, Lake Richie. Concentrations of Hg(T) in northern pike were significantly higher in Sargent Lake (P<0.01). Counter to expectations, mean concentrations of both Hg(T) and MeHg in open water samples were slightly higher in Lake Richie. However, zooplankton in Sargent Lake contained higher average concentrations of Hg(T) and MeHg than in Lake Richie. Mercury concentrations in macro invertebrates were similar between lakes, but different between taxa. The two lakes exhibited similar Hg(T) concentrations in age-1 yellow perch and adult perch but concentrations in large adult perch (>160 mm) in Sargent Lake were twice the concentrations in Lake Richie. Analysis of stable isotopes (delta(13)C and delta(15)N) in biota showed that pike from the two lakes are positioned at the same trophic level (4.2 and 4.3), but that the food web is more pelagic-based in Sargent and benthic-based in Richie. Factors causing concentrations in large pike to be higher in Sargent Lake may include higher bioavailability of methylmercury and a food web that enhances bioaccumulation.

The influence of dissolved organic carbon, suspended particulates, and hydrology on the concentration, partitioning and variability of trace metals in two contrasting Wisconsin watersheds (U.S.A.)

Factors controlling the levels and forms of trace metals in streams, and transport from watersheds into streams, are poorly understood. We determined levels of filtrable ( Zn > Cd > Cu, whereas relationships to DOC indicated essentially the reverse order for metal binding to DOC. SPM was a good predictor of particulate metal levels, especially in the Milwaukee River (> 94% of variance in particulate Cd, Pb, and Zn was accounted for, vs. 44–69% in Wolf River). While DOC was able to account for a significant portion of the variation in certain filtrable metal levels (r2 = 0.52–0.65), other metals showed poorer correlations, probably due to variability in the composition of DOC and SPM and to variability in the forms of metal (aqueous complexes, colloid-bound) in the filtrable fraction.

Role of nutrient limitation and competition in controlling the populations of Asterionella formosa and Microcystis aeruginosa in semicontinuous culture

Ahstruct The nutrient uptake and growth kinetics of the freshwater diatom Asterionellu formosu and the blue-green alga Microcystis aeruginosa under P or Si limitation were determined in batch cultures at 20°C. Asterionellu had a higher maximum growth rate and a lower halfsaturation constant for growth than Microcystis under P limitation. The maximum uptake velocity for P was higher for Asterionellu than for Microcystis while the half-saturation constants for P uptake were similar for the two species. The Monad model of growth predicted a switch from Si limitation to P limitation for Asterionella at Si:P of about 93 (215). I n experiments with the two species grown together at various Si:P ratios, Asterionellu dominated steady state semicontinuous cultures when both species were P-limited, Si:P > 100. Microcystis became dominant when Asterionellu was Silimited, Si:P < 100. The competition experiments showed the importance of the conccntration of nutrients, as well as the ratio of the nutrients and rate of supply, in controlling the cell densities of the algal species.

Reply to comment on "Estimation of the atmospheric and nonatmospheric contributions and losses of polychlorinated biphenyls for Lake Michigan on the basis of sediment records of remote lakes.

Amounts of PCBs accumulated in sediments of Lake Michigan and four Wisconsin lakes, isolated from point sources, were measured. The flux of PCBs to Lake Michigan sediments was 4 times greater than the average loading of 1.9 +/- 1.1..mu..g/m/sup 2/ per year to the remote lakes and has been occurring longer. Comparisons of the remote lakes and Lake Michigan were used to estimate the net atmospheric and nonatmospheric contributions of PCBs to Lake Michigan. The net atmospheric input and the total atmospheric flux, reported previously, were used to calculate the net vapor-phase transfer. Total atmospheric (430 +/- 130 kg/year) and net nonatmospheric (330 +/- 140 kg/year) contributions to Lake Michigan were of similar magnitude. Volatilization losses (320 +/- 140 kg/year) were similar to sedimentation losses (440 +/- 130), indicating that volatilization may be a major removal mechanism of PCBs from lakes.

Influence of natural dissolved organic carbon on the bioavailability of mercury to a freshwater alga.

Bioavailability of mercury (Hg) to Selenastrum capricornutum was assessed in bioassays containing field-collected freshwater of varying dissolved organic carbon (DOC) concentrations. Bioconcentration factor (BCF) was measured using stable isotopes of methylmercury (MeHg) and inorganic Hg(II). BCFs for MeHg in low-DOC lake water were significantly larger than those in mixtures of lake water and high-DOC river water. The BCF for MeHg in rainwater (lowest DOC) was the largest of any treatment. Rainwater and lake water also had larger BCFs for Hg(II) than river water. Moreover, in freshwater collected from several US and Canadian field sites, BCFs for Hg(II) and MeHg were low when DOC concentrations were >5mg L(-1). These results suggest high concentrations of DOC inhibit bioavailability, while low concentrations may provide optimal conditions for algal uptake of Hg. However, variability of BCFs at low DOC indicates that DOC composition or other ligands may determine site-specific bioavailability of Hg.

LAKE DISSOLVED INORGANIC CARBON AND DISSOLVED OXYGEN: CHANGING DRIVERS FROM DAYS TO DECADES

Dissolved inorganic carbon (DIC) and dissolved oxygen (DO) are commonly measured to compute metabolism of aquatic ecosystems. However, concentrations of DIC and DO depend on many factors in addition to ecosystem metabolism, such as water temperature, gas exchange with the atmosphere, abiotic chemical reactions, and inputs in precipitation, groundwater, and surface water. We used 20-year time series from seven lakes to understand how DIC and DO concentrations are controlled as a function of time scale. Diel cycles of both solutes are controlled primarily by metabolism, exchange with the atmosphere, and temperature. At seasonal and annual scales, metabolism is important, but physical processes associated with spring and autumn mixing, as well as solute loading from the watershed, have comparably large effects. At decadal scales, effects of metabolism are negligible. Controls of the two solutes diverge, with variance in DIC explained largely by solute inputs and variance in DO explained largely by water temperature. Like other indicators in many ecosystems, variability of DIC and DO is strongly scale dependent and associated with different drivers depending on the time scale of the analysis.

Ground Water as a Silica Source for Diatom Production in a Precipitation-Dominated Lake

The short-term, seasonal input of ground water to a small, precipitation-dominated oligotrophic lake in northern Wisconsin amounts to less than 10 percent of the annual water budget of the lake but accounts for nearly all the external silica loading. Silica is a necessary nutrient for diatoms. A large spring diatom bloom occurs coincident with high silica inputs from ground water when other possible silica sources are low. The mass budgets of ground water and silica in the lake system demonstrate the importance of ground-water solute inputs to the lake.

Distribution and Characterization of PCBs in Lake Michigan Water

Abstract Polychlorinated biphenyls (PCBs) were measured in the water column of Lake Michigan at 28 stations. Distributions between dissolved and particulate phases were used to evaluate the partitioning behavior of PCBs. The average PCB concentration in Lake Michigan was 1.8 ng/L, corresponding to a total PCB burden of 8,900 kg. The average PCB concentration was significantly lower in open lake (1.2 ng/L) than in nearshore zone samples (3.2 ng/L). Concentrations in Green Bay were elevated compared to the rest of the lake. Variations in concentration with depth were small. The average log of the water-particle partition coefficient (Kp) was 5.9. Little dependence of Kp on particulate organic carbon was observed. A relationship was found between log Kp and log SPM (suspended particulate matter) concentration, perhaps reflecting association of a non-filterable fraction of the SPM with the aqueous phase. Selected PCB congener Kp and Koc (Kp × organic carbon weight-fraction) values were measured and related to congener Kow (octanol-water partition coefficient), yielding the predictive relationship log Koc = 4.2 + 0.34 log Kow. Congener log Kp values are predicted to range from 4.8 to 6.1, indicating that congener fates in the water column differ and are partly controlled by their physical chemical properties.

Role of nutrient limitation and competition in controlling the populations of Asterionella formosa and Microcystis aeruginosa in semicontinuous culture1

Ahstruct The nutrient uptake and growth kinetics of the freshwater diatom Asterionellu formosu and the blue-green alga Microcystis aeruginosa under P or Si limitation were determined in batch cultures at 20°C. Asterionellu had a higher maximum growth rate and a lower halfsaturation constant for growth than Microcystis under P limitation. The maximum uptake velocity for P was higher for Asterionellu than for Microcystis while the half-saturation constants for P uptake were similar for the two species. The Monad model of growth predicted a switch from Si limitation to P limitation for Asterionella at Si:P of about 93 (215). I n experiments with the two species grown together at various Si:P ratios, Asterionellu dominated steady state semicontinuous cultures when both species were P-limited, Si:P > 100. Microcystis became dominant when Asterionellu was Silimited, Si:P < 100. The competition experiments showed the importance of the conccntration of nutrients, as well as the ratio of the nutrients and rate of supply, in controlling the cell densities of the algal species.

Estimation of vapor pressures for polychlorinated biphenyls: a comparison of eleven predictive methods.

Eleven methods were used to predict vapor pressures at 25.0/sup 0/C for 15 polychlorinated biphenyls with experimental values. These results permitted an assessment of the predictive ability of these methods for compounds with low vapor pressures (<1.0 Pa) and one or less experimental determinations. The error for theoretically based methods was high and increased with decreasing vapor pressure. The correlative methods, based on a set of compounds with known vapor pressures, had much better predictive power. The best correlative method was based on a relationship between ..delta..G/sub v/ and gas-liquid chromatographic retention indexes. The predictive error for this method was estimated to be a factor of 1.75. Vapor pressures obtained by using the three best correlative methods for all polychlorinated biphenyls are reported.