Award: OCE-1948739

Over the past few decades, the Great Lakes (containing ~ 20% of the freshwater on Earth) have been adversely impacted by anthropogenic stressors, and Lake Erie suffers from large scale, annually recurring cyanobacterial blooms in the summer months, while Lake Superior has experienced small cyanobacterial blooms in response to storm events. In addition to large blooms, picocyanobacteria contribute significantly to phytoplankton biomass (~50% of total chlorophyll) in the Great Lakes. Understanding how organic matter from different types of cyanobacterial biomass (e.g. Microcystis in Lake Erie vs picocyanobacteria in Lake Superior) influences carbon export and impacts the ability of heterotrophic bacteria in the water column and sediment to break down that organic matter and remove (anthropogenic) nitrogen from the system remains a major knowledge gap in limnology. This project provides a deeper understanding of the long-term state of the system upon which the human perturbation has been imposed by quantifying seasonal production, carbon/nitrogen cycling, and export dynamics. We apply a combination of novel (nitrogen isotope composition of chloropigments) and longstanding (¹⁵N tracer incubations for geochemical rates) techniques that have never been applied in the Great Lakes to address these knowledge gaps.

 This study examined nitrogen cycling, community composition, and the nitrogen isotopic composition of chloropigments to evaluate cyanobacterial productivity in modern and recent aquatic systems, specifically Lake Superior and Lake Erie. The nitrogen isotope composition of chloropigments provides a powerful proxy for understanding primary productivity and the relative importance of cyanobacteria to export production and nitrogen cycling. This proxy is valuable not only for management of modern systems but has significant implications for increasing our understanding of the role of cyanobacteria throughout Earth history. We tested this molecular isotopic proxy in contemporary aquatic ecosystems to assess the 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 investigated 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 compared 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 investigated 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. 

 Appropriate application of rigorously investigated proxies provides the best information about paleoenvironmental conditions, but the use of such proxies is only valid if we fully understand their limitations and apply them judiciously. The sediment trap, time-series remote access water samplers, time-series particulate matter filter samplers, and mooring flotation purchased as part of this project has been added to the R/V Blue Heron’s shared use equipment pool and therefore the UNOLS shared use equipment pool, and is available to the broader scientific community for future studies.   

Last Modified: 04/14/2025
Modified by: Josef P Werne