Field investigations of trace metal effects on lake erie phytoplankton productivity

Responses of phytoplankton to trace metal and phosphate enrichments were made in pelagic Lake Erie surface waters over the time period of 1999-2003. All experiments employed trace metal clean sampling protocols. Bioassays were incubated over a 0.75-4 d period. Response was evaluated by measures of biomass (chlorophyll-a; chl-a), photosynthesis (using the carbon-14 technique), and dilution assays used to measure chl-a specific growth and grazing rates. Metals assayed were Cd, Co, Zn (5-50 nM) and Fe over the range of (20-100 nM). Phosphorus was added singly (0.1-1 μM) or in addition with Zn or Fe. The principle finding from this study was that the frequency of observed trace metal limitation in pelagic Lake Erie phytoplankton was low. Picoplankton (0.2-2 μm) responded most frequently to the metal enrichment; metals were as frequently toxic as they were stimulatory. Nanoplankton (2-20 μm) were nearly insensitive to metal enrichment as were the microplankton (20-210 μm). An EDTA chelated mixture of Fe, Cu, Zn, Co, Mn, and Mo did stimulate picoplankton chl-a production over 3 days and the growth and grazing rate of this important size fraction. Toxicity of Zn at 50 nM was observed; the presence of phosphate reduced inhibition by Zn at this concentration. The results suggest that trace metals are not as important over the short term as the availability of phosphorus in controlling phytoplankton productivity; however, trace metal enrichment can periodically have a stimulatory effect, particularly on the picoplankton size class