Great Lakes Cyanos

This study will examine nitrogen cycling, community composition, and the nitrogen isotopic composition of chloropigments to evaluate cyanobacterial productivity in modern and recent aquatic systems. The nitrogen isotope composition of chloropigments may provide a powerful proxy for understanding primary productivity and the relative importance of cyanobacteria to export production and nitrogen cycling. Such a proxy would be valuable not only for management of modern systems but has significant implications for increasing our understanding of the role of cyanobacteria throughout Earth history. Here we will test these molecular isotopic proxies in contemporary aquatic ecosystems to assess their efficacy for: 1) determining the relative contributions of cyanobacteria vs eukaryotic algae (e.g. diatoms) to primary production, 2) evaluating export production of cyanobacterial productivity (including blooms), and 3) constraining historical cyanobacteria productivity in the sedimentary record. We propose to investigate the impact of export production on nitrogen cycling (N recycling vs. removal rates) using a combination of water column and sediment incubations and the molecular isotopic composition of porphyrins from both suspended particulate matter and sinking particulate material in modern lake systems (Lakes Superior and Erie). We will compare a system characterized by eutrophication and seasonal cyanobacterial blooms (Lake Erie) with one characterized by significant picocyanobacteria productivity but the near-absence of cyanobacterial blooms (Lake Superior). Finally, we will investigate the paleorecord of cyanobacterial activity using sediment cores spanning the past several hundred years to evaluate the system prior to human impact relative to the present condition.