A unique assemblage of photosynthetic picoplankton (Ppico) is present in Lake Erie as determined from samples collected between 2002-04.  Ppico contributes 25-31% to total chlorophyll, with the highest contribution in the eastern basin. Synechococcus sequences dominate the16S rDNA clone library (227 of 291 clones) for 7 Erie stations sampled in 2002 (Wilhelm, Eldridge and Bullerjahn, ASLO Winter Meeting 2005). The fact that Synechococcus sequences dominate our Lake Erie cyanobacterial clone libraries but are absent from reports spanning from 1969-1987 warrants more attention. Use of PCR primers specific to marine Prochlorococcus ITS regions suggest the presence of marine Prochlorococcus-like populations in samples collected in 2002 and several stations in 2004. These data suggest that abundant cyanobacteria are distributed throughout Lake Erie, and that populations may be atypical of those examined in other freshwater environments.  Overall, we will evaluate the ecology of Ppico, as defined by the physiology and population composition of dominant taxa present in Lake Erie during the summer.  The initial work involves phylogenic analyses to reveal and quantitate the Ppicos present in Lake Erie over the summer.    In sum, we pose the following question: What is the phylogenic diversity of dominant Ppico in the lake, and is it a stable or dynamic population through the summer? Ultimately, once we know the Ppicos present, we will culture representative species to determine their physiological characteristics.  

Reliable observations and predictions on pelagic food webs can only be drawn from sampling accurately reflecting key components of the food chain.  Our present knowledge of pelagic food webs in the Lake Erie is fragmented, because the abundance, taxonomic composition, and production of Ppico has not been adequately accounted for in present models. Our work will help provide estimates of primary production by Ppicos in Lake Erie across relevant ecological gradients.  Furthermore, our understanding of oxygen gradients in the Lake is incomplete and inadequate without considering the contribution of Ppico to total photosynthetic oxygen production. Overall, linking Ppico metabolism (contributing 30% of total chl) to environmental variables such as temperature, light, and nutrients provides information needed to manage long–term ecosystem alterations, such as climate change or the effect of invasive species.

The two year study will evaluate the phylogeny of the Ppico assemblage, with respect to relevant in-lake gradients that we know exist (light, temperature, oxygen). For example, we will analyze the taxonomic composition of the Ppico community before and after the onset of central basin anoxia using both direct counts and molecular tools described below.  Ppico populations will be studied by sampling primarily central basin stations. Samples will be assayed using both ecophysiological measurements (pigments, oxygen and nutrients), and molecular phylogenetics (PCR of 16S rDNA, 16S-23S ITS and rbcL sequences with quantification by qPCR).   PCR amplicons of sequences (16S, 16-23S ITS) from samples collected throughout the lake will be compared to identify the relatedness of these sequences to known sequences from marine and freshwater environments. Quantitative PCR (qPCR) with cyanobacterial specific and Prochlorococcus 16-23S ITS primers will reveal the relative abundance of the individual members of the Ppico assemblage, which in turn will be compared with water column estimates derived using direct counts.  Culturing of the abundant Ppicos from the hypoliminion prior and after anoxia will allow physiological studies aimed at determining their photosynthetic performance in response to temperature, light and nutrients.