Ronald G. Rada

Recent increases in atmospheric deposition of mercury to north-central Wisconsin lakes inferred from sediment analyses

Profiles of total mercury (Hg) concentrations in sediments were examined in 11 lakes in north-central Wisconsin having a broad range of pH (5.1 to 7.8) and alkalinity (−12 to 769 μeq/L). The sediments, which were hydrous and flocculent, were collected at or near the area of maximum depth in each lake with a diver-operated sampler that permittedin situ sectioning of a 1-m core. Mercury concentrations were greatest in the top 15 cm of the cores and were much lower in the deeper strata. The Hg content in the most enriched stratum of individual cores ranged from 0.09 to 0.24 μg/g dry weight, whereas concentrations in deep, precolonial strata ranged from 0.04 to 0.07 μg/g. Sediment enrichment factors varied from 0.8 to 2.8 and were not correlated with lake pH. The increase in the Hg content of recent sediments was attributed to increased atmospheric deposition of the metal. Eight of the 11 systems studied were low-alkalinity lakes that presumably received most (≥90%) of their hydrologic input from precipitation falling directly onto the lake surface. Thus, the sedimentary Hg in these lakes seems more likely linked to direct atmospheric deposition onto the lake surfaces than to influxes from the watershed. The data imply that a potentially significant fraction of the high Hg burdens measured in game fish in certain lakes in north-central Wisconsin originated from atmospheric sources.

Effects of disturbance on epiphytic community architecture

Microdistributional patterns of attached algal communities on Cladophora and glass slides were compared under varying disturbance regimes in the Upper Mississippi River, using light and scanning electron microscopy. Localized physical disturbance, induced by boat traffic and wind‐generated wave action in the main channel, inhibited development of complex attached algal communities and maintained an adnate flora with two‐dimensional architecture. In contrast, communities protected from disturbance developed a more complex, three‐dimensional architecture. A high degree of architectural similarity was observed between communities on Cladophora and glass substrates exposed to similar levels of disturbance. Communities exposed to severe disturbance resembled ones in early stages of colonization and development, whereas less disturbed communities were similar to ones in advanced stages of development. These results demonstrated that turbulence strongly influences the community structure of periphyton on both micro and macro scale levels.

Determination of total nitrogen in sediments and plants using persulfate digestion. An evaluation and comparison with the Kjeldahl procedure

Abstract Accuracy and precision of the persulfate and Kjeldahl determinations of total nitrogen in solid samples were assessed with certified standards, plant material and river sediments. Accuracy determined with the certified reference materials was within an acceptable range to assure quality control for routine laboratory analyses. In addition, results obtained by examining plant samples indicated that TPN and TKN were not significantly different. Precision for analyses of all samples was high; however a direct comparison of plant material indicated that TPN (RSD = 5.3%) was more precise than TKN (RSD = 8.4%). TPN is a reasonable alternative to the standard TKN procedure for analysis of sediment and plant materials. The TPN method would increase the feasibility of conducting replicate analyses and would result in increased quality control.

Environmental fate of mercury discharged into the upper Wisconsin river

We studied the distribution of Hg in sediments, fish, and crayfish in a 60 km reach of the Upper Wisconsin River that formerly received Hg in discharges from pulp and paper mills. The most heavily contaminated strata of sediments were deposited during the 1950s and early 1960s and buried under subsequent deposits; however, surficial sediments remained substantially enriched at certain sites in 1981. Median concentrations of Hg in surficial sediments, adjusted for grain size, were at least 10-fold greater at the main study area than at an upstream reference site. Total concentrations exceeded 1.0 μg g−1 wet weight in axial muscle tissue in only 2 of 173 fish analyzed from the study area; however, historical comparisons revealed that Hg contamination of fish (common carp Cyprinus carpio and walleye Stizostedion vitreum vitreum) and crayfish (Orconectes) in the river had not decreased since the early 1970s. The availability of Hg to biota in this system may be enhanced by rapid methylation of the metal in surficial sediments, despite burial of the most heavily contaminated sediments. Management practices for this river should be designed to prevent conditions favoring mobilization and methylation of Hg in sediments.

Recent influxes of metals into Lake Pepin, a natural lake on the upper Mississippi River

The recent chronology of metal deposition was examined in Lake Pepin, a large natural lake on the Upper Mississippi River about 75 km downstream from the Minneapolis-St. Paul, Minnesota, metropolitan area. The lake, which has a high trapping efficiency for suspended sediments, serves as a sink for metals from industrial and domestic effluents discharged into the river. Sediment cores collected in 1981 from seven locations in the lake were analyzed for Cd, Cr, Cu, Ni, Pb, Zn, and Al (a reference element). Dating of a core with137Cs indicated that inputs of these metals increased markedly in the late 1950s, peaked in the early 1970s, and decreased in the late 1970s to 1981. Sediment enrichment factors decreased in the order Pb>Cd>Cu⩾Cr>Zn>Ni. The depth of the most highly contaminated strata in the core profiles decreased longitudinally from upstream to downstream—paralleling the upstreamto-downstream decrease in sedimentation rates. Changes in metal inputs to the lake would be most readily detected by analysis of sediments from the uppermost reaches, but the potential for release of sediment-associated metals to the water column is probably greatest in downstream reaches.

Cadmium and mercury in emergent mayflies (Hexagenia bilineata) from the upper Mississippi River.

Burrowing mayflies (Hexagenia bilineata) were analyzed to assess longitudinal patterns in their cadmium and mercury content along the upper Mississippi River. Emergent mayflies (subimagoes and imagoes) were collected in 1988 at 34 sites (25 navigation pools), extending 1260 km from Little Falls, Minnesota, downstream to St. Louis, Missouri. Cadmium concentrations in composite samples of whole mayflies ranged from 7 to 219 ng/g dry weight in females and from <7 to 265 ng/g in males. Cadmium concentrations were highest (⩾150 ng/g) in samples from Pools 2 and 3 (downstream from the Twin Cities metropolitan area), 15 (near the Quad Cities metropolitan area), and 27 (near the St. Louis metropolitan area). Cadmium concentrations in female mayflies decreased significantly with distance downstream from Pool 2 at river mile 825 to Pool 9 at river mile 648, paralleling spatial trends in the cadmium contamination of sediments in the reach downstream from the Twin Cities metropolitan area, reported in earlier studies; cadmium burdens in mayflies followed a similar spatial trend. Concentrations of mercury were much less variable, ranging from 44 to 102 ng/g dry weight in female mayflies and from 60 to 177 ng/g in males; concentrations and burdens were highest in mayflies collected at Pools 2, 20, 22, 25, and 27. Mercury concentrations in females decreased significantly with distance downstream from Pool 2 at river mile 825 to Pool 5A at river mile 728.5. Concentrations of mercury and cadmium in composite samples of female mayflies were not correlated, indicating dissimilar longitudinal patterns in concentrations of the two metals. Concentrations and burdens of both metals varied significantly between males and females; consequently, we recommend that programs involving analyses of mayflies to survey or monitor metals in aquatic systems analyze separately males and females.

Effects of cadmium-spiked sediment on cadmium accumulation and bioturbation by nymphs of the burrowing mayfly Hexagenia bilineata

We assessed accumulation of cadmium (Cd) and bioturbation by nymphs of the burrowing mayfly Hexagenia bilineata as indicators of exposure to Cd-spiked sediment in a 21-d test. Surficial sediments (top 5 cm) from Pool 7 of the Upper Mississippi River were spiked with Cd to concentrations of 3, 7, and 15 μg Cd g-1 dry weight. The experimental design was completely randomized, with three Cd-spiked sediment treatments plus an unspiked sediment control (1 μg Cd g-1 dry weight), and 10 nymphs in each of six replicates per treatment. Nymphs accumulated Cd during the 21-d exposure; mean concentrations varied from 0.22 to 6.24 μg g-1 dry weight, and tissue concentrations were correlated with Cd concentration in unfiltered test water (r = 0.93, P <0.01) and test sediment (r = 0.93, P <0.01). The effect of Cd on bioturbation by nymphs, as indicated by turbidity, differed significantly among treatments (P = 0.045) and over time within treatments (P = 0.01). Turbidity progressively decreased as Cd concentration in the sediment increased, up to 7 μg g-1; however, turbidity in the 15 μg g-1 treatment (our greatest exposure concentration) did not differ significantly from the control. Concentrations of Cd in unfiltered, overlying test water increased significantly within treatments during the test, indicating that nymphs mobilized sediment-associated Cd into the overlying water, presumably through burrowing and respiratory activities.

Phycoperiphyton in selected reaches of the Upper Mississippi River: community composition, architecture, and productivity

Attached algal communities were studied during ice-free periods along the borders of the main channel and in backwaters of the Upper Mississippi River. Diatoms strongly dominated the phycoperiphyton except during late June through August when the green alga Stigeoclonium was abundant. Two distinct assemblages were apparent: a diverse, complex assemblage during spring and late fall annd a less complex, adnate, two-dimensional summer assemblage dominated by Cocconeis placentula var. euglypta and Navicula tripunctata var. schizonemoides. Simultaneous studies revealed two-dimensional periphytic colonization in the main channel and more complex communities in backwaters. Greater physical turbulence (e.g., surface waves) in the main channel may have favored adnate taxa and two-dimensional architecture, whereas lesser turbulence (e.g. protected backwaters) favored the more diverse, complex community. Community architecture and species composition were similar among communities on artificial substrates within each navigation pool (median SIMI≥0.87), but communities on artificial substrates were not taxonomically very similar (median SIMI=0.44) to epiphytic communities on Cladophora.Accrual of algal cells, chlorophyll a, and aufwuchs ash-free dry weight was usually greater in Pool 5 than in Pool 9. This may have been due to differences in discharges and/or sampler placement. The greatest accrual of cells and chlorophyll a occurred during summer and early fall. Chironomid and tricopteran larvae were common at that time and are known to affect algal accumulation by grazing.

Accumulation of mercury by aufwuchs in Wisconsin seepage lakes: Implications for monitoring

We examined temporal variation in the total Hg content of aufwuchs collected from artificial substrates in 11 seepage lakes in north-central Wisconsin and its relation to the Hg content of resident yellow perch Perca flavescens from the lakes. Dry weight concentrations of Hg in aufwuchs varied temporally, as follows: summer/fall 1985 > summer 1985 > spring/summer 1986. Areal concentrations of Hg during the 1985 sampling periods were greater than concentrations during the spring/summer 1986 period, but did not differ from each other. Although chlorophyll a accrual was correlated with the areal concentration of Hg in aufwuchs during each period of study, partial correlation coefficients between Hg in aufwuchs and the chlorophyll a, organic (ash-free dry weight), and inorganic (non ash-free dry weight) content of aufwuchs during spring 1986 were not significant. Thus, we were unable to quantify the partitioning of Hg among the organic (photosynthetic, heterotrophic, and detrital) and inorganic components of the aufwuchs. Concentrations of Hg in aufwuchs were not correlated with lake pH during any of the three periods, even though the Hg content of fish in these and other lakes in the region are strongly correlated with pH. Moreover, the only significant correlation between Hg in aufwuchs and Hg in age-2 yellow perch from the lakes were weak negative correlations for areal Hg concentrations in aufwuchs during summer 1985. The analyses of aufwuchs do not indicate potential accumulation of Hg in fish in these lakes.

A comparison of solids collected in sediment traps and automated water samplers

Sediment traps are being used in some pollution monitoring programs in the USA to sample suspended solids for contaminant analyses. This monitoring approach assumes that the characteristics of solids obtained in sediment traps are the same as those collected in whole-water sampling devices. We tested this assumption in the upper Mississippi River, based on the inorganic particle-size distribution (determined with a laser particle-analyzer) and volatile matter content of solids (a Surrogate for organic matter). Cylindrical sediment traps (aspect ratio 3) were attached to a rigid mooring device and deployed in a flowing side channel in Navigation Pool 7 of the upper Mississippi River. On each side of the mooring device, a trap was situated adjacent to a port of an autosampler that collected raw water samples hourly to form 2-d composite samples. Paired samples (one trap and one raw water, composite sample) were removed from each end of the mooring device at 2-d intervals during the 30-d study period and compared. The relative particle collection efficiency of paired samples did not vary temporally. Particle-size distributions of inorganic solids from sediment traps and water samples were not significantly different. The volatile matter content of solids was lesser in sediment traps (mean, 9.5%) than in corresponding water samples (mean, 22.7%). This bias may have been partly due to under-collection of phytoplankton (mainly cyanobacteria), which were abundant in the water column during the study. The poisoning of water samplers and sediment traps in the mooring device did not influence the particle-size distribution or total solids of samples. We observed a small difference in the amount of organic matter collected by water samplers situated at opposite ends of the mooring device.