Microbial Distributions and the Impact of Phosphorus on Bacterial Activity in Lake Erie

In light of recent suggestions concerning the relaxation of controls on phosphorus loading in Lake Erie, and in consideration of our current lack of baseline information on microbial communities in this system, a two-part analysis of the microbial ecology of the lake has been conducted. A comprehensive survey of the surface waters collected data on phytoplankton abundance, bacterial productivity and abundance, and viral abundance that were previously not available for Lake Erie. In parallel, phosphorus amendment experiments were conducted in each of the three hydrologically distinct basins of the lake to determine the effect of increased phosphorus loading on bacterial abundance and productivity. After 72 hours incubation, the addition of phosphorus had a significant impact on the microbial food web. A proliferation of different size classes of phytoplankton was accompanied by increases in bacterial production, but not necessarily bacterial abundance. Observations confirmed previous studies suggesting that the microbial communities in each of the three basins responded differently to the addition of phosphorus. Accompanying nutrient data suggest that the responses were due to limitation by other factors in the presence of excess phosphorus; namely that phytoplankton became nitrogen limited whereas bacteria became carbon limited. These results confirm the importance of the microbial food web in understanding the impact of phosphorus loading on the biotic carbon flow in the Lake Erie ecosystem. INDEX WORDS: Bacteria, viruses, bacterial production, phosphorus, chlorophyll. J. Great Lakes Res. 30(1):166–183 Internat. Assoc. Great Lakes Res., 2004