CoNwry, H. L., J' I. penrrn, E. M. yecucHr, eNo D. L. MrlLrNcrn. 1977. Biological utilization and regeneration of silicon in Lake Michigan. J. Fish. Res. Board Can-. 34: 537_544.Depth profiles of dissolved reactive silicon and amorphous pafticulate silicon were made at monthly intervals (April-November 1975) at stations along a iouthwest transect tiom Grand Haven, Michigan, to a point approximately in the center of the southern basin of Lake Michigan. Biological utilization of reactive silicon occurred prior to stratification in Iate May or early Jine. A shift from a 100vo diatom-dominated phytoplaniton community in rpiing to -12%diatoms in August was attributed to the low dissolved iilicon values obser-ved ln tf,e surfuce water (<2 pmolll) during summer. The total amount of biologically active silicon (TBAS) for the lake was found to be approximately 19 p.mol l(. Winter values were -.-2 pmol/l amorphous silicon and -l7 pmoll( reactive silicon. During the period June-August 807o of TBAS had been utilized by the diato-m community, with only 20%o rimaining as ,euciiue silicon. Greater than S|Voof TBAS was lost from the water column during spring and early summer, which was attributed to settling of diatom fru stules and the.sinking of zooplankton fecal pellets containing fil stu les. Th is silicon was subsequently retumed, in.a soluble foim, to the deepwater during thJfall. The amount of TBAS that was recycled was estimated to be g0_100%.

Structural and functional responses of plankton communities to cadmium stress were studied during 1977 in Lake Michigan using small-volume (8 L) completely sealed enclosures, and in Canada's Experimental Lakes Area (ELA) Lake 223 using large-volume (1.5 × 105 L) open-surface enclosures. In Lake Michigan, reductions of the average abundance of micro-crustaceans by cadmium were significantly greater in "light" or shallow epilimnetic incubations than they were in "dark" or deep epilimnetic incubations. Measurements of dissolved oxygen indicated that this interaction with light (depth) was an indirect effect due to a reduction of photosynthesis and primary production. Zooplankton density and species diversity were not significantly affected within 21 d by cadmium concentrations [Formula: see text] and [Formula: see text] Cd/L, respectively, whereas final dissolved oxygen concentration and percentage similarity (PS) of the crustacean zooplankton community were significantly reduced by [Formula: see text] Cd/L. In the ELA Lake 223 experiment, the reducing effect of cadmium on zooplankton density increased up to 31 d after Cd enrichment and then decreased, probably due to decreasing Cd concentrations in the water. Values of PS on day 24 for the ELA enclosures enriched with 1 and 3 μg Cd/L were within the 95% confidence limits for individual values predicted from a regression of PS on cadmium for the 21-d Lake Michigan experiments.Key words: plankton communities, zooplankton, phytoplankton, cadmium stress, Lake Michigan, Canadian Shield lakes

We conducted four in situ experiments in the southern basin of Lake Michigan to determine the responses of the plankton community to additions of cadmium (3 μg/L) and zinc (15, 30, 60, and 90 μg/L) as well as the bioaccumulation of zinc by different species of crustacean zooplankton. Total cadmium and zinc concentrations in treated enclosures (accounting for pretreatment lakewater concentrations of ~ 20 ng Cd and ~ 1 μg Zn per litre) were calculated from measurements of 109Cd and 65Zn with known specific activities.Zinc significantly reduced zooplankton populations and several other properties of community structure and function at concentrations much lower than those previously reported to be toxic to zooplankton and well below the levels established for protection of freshwater life. Zinc additions as low as 15 μg/L significantly reduced chlorophyll a, primary productivity, dissolved oxygen, specific zooplankton populations, zooplankton species diversity, and community similarity within 2 wk. Populations of several zooplankton species were severely reduced by 15 μg/L; for example, the population of the rotifer Conochilus unicornis was reduced to less than 1% of that in controls. Secondary, indirect effects included significant increases of a few populations, including Bosmina longirostris and Keratella cochlearis, for zinc additions of 15 and 30 μg/L. Most of the populations and community properties that we measured showed somewhat different percentage reductions in response to a given zinc addition at different times, but their relative (rank) values at different times were similar. Specific zooplankton populations were more sensitive to zinc than community similarity indices, and species diversity indices were relatively insensitive. Bioaccumulation of zinc by different species of crustaceans increased with added zinc but showed few significant differences among species. The small differences among species could, nevertheless, partially account for some of the observed changes in zooplankton community structure.

Four in situ experiments with small-volume (28-L) enclosures were conducted in Experimental Lakes Area Lake 382 to assess the effects of inorganic mercury and cadmium on the plankton community. Additions of 0.9 × 10−8 mol Cd/L caused significant reductions of crustacean zooplankton density, zooplankton community similarity, zooplankton species diversity, and dissolved oxygen concentration over 3-wk periods. Additions of 1.0 × 10−8 mol Hg/L caused reductions similar to those of 0.9 × 10−8 mol Cd/L. Phytoplankton primary productivity was reduced by additions of 2.0 × 10−8 mol Hg/L but not by 1.0 × 10−8 mol Hg/L. On a molar concentration basis, the toxicities of inorganic mercury and cadmium were quite similar. Significant changes in lake plankton communities may occur at molar concentrations above 0.2 × 10−8 mol/L. Potentially toxic concentrations of cadmium may occur in many contaminated lakes, but toxic concentrations of mercury probably occur in few lakes.Key words: Precambrian Shield lakes, plankton communities, phytoplankton, zooplankton, cadmium, mercury toxicity

During 1976 and 1977, the authors developed and tested a new in situ method for toxicological studies on natural plankton communities using small-volume (8- and 25-litre) enclosures. The effects of enclosure for 4 to 21 days on the plankton in northern Green Bay, Lake Michigan, were studied in connection with cadmium addition experiments. In situ incubations in opaque enclosures or in translucent ones at depths of &gt;8 m caused large changes in the zooplankton community structure, as measured by two similarity indexes, the coefficient of community (CC) and the percentage similarity (PS), whereas incubations in translucent enclosures at 3 to 8 m caused relatively small changes. The effect of cadmium additions (0 to 5 µg cadmium per litre) on total zooplankton abundance in translucent enclosures at 3 to 5 m was significantly greater than in those at 6 to 8 m. This interaction between cadmium and light (depth) was probably due to a reduction of phytoplankton photosynthesis, as indicated by changes in dissolved oxygen (DO) concentrations. Zooplankton abundance was not significantly affected by &lt;1.6 µg cadmium per litre, whereas DO and PS were significantly reduced by ≥0.2 µg cadmium per litre. Values of PS for zooplankton communities in large-volume (1.5 × 105 litre) enclosures in a small Canadian lake in northwestern Ontario 3 weeks after the addition of cadmium were within the 95 percent confidence limits predicted from the results of 3-week experiments in Lake Michigan.