THE ΕPOR PROXY IN MODERN LARGE LAKE SYSTEMS REPRESENTING TROPHIC EXTREMES

Chloropigments are light-harvesting pigments made by phototrophs. Diagenetic products of these pigments such as porphyrins and chlorins are phytoplankton biomarkers that constitute an important fossil material in the sedimentary record for understanding primary productivity in modern and ancient environments. Interest has recently resurfaced in the application of chloropigments as biomarkers, in particular the compound-specific N isotope composition of them as tracers of water column processes and sediment records of primary production. The nitrogen isotope composition of intact porphyrins and chlorins is not affected by diagenesis, hence, the nitrogen isotope offset between total particulate nitrogen (δ¹⁵N_TN) and chlorin/porphyrin N (δ¹⁵N_por), is a good approximation of the offset between biomass and chloropigments (termed ε_por). ε_por is independent of the N substrate utilized, which removes the otherwise confounding effect of variable isotopic composition of different N substrates (e.g. NO₃^-, NH₄⁺). A measurable and consistent isotopic offset has been observed in pure cultures of cyanobacteria, eukaryotic algae and anoxygenic photosynthetic bacteria that differs among phytoplanktonic groups, implying that ε_por can be used for assessing the proportional contribution of freshwater cyanobacterial vs eukaryotic (e.g. green algae) organic matter in the sedimentary record. Furthermore, the proportion of cyanobacteria relative to eukaryotic algae in modern Lake Erie during seasonal algal blooms was estimated based on a two-endmember mixing model, providing strong support to the use of ε_por as an indicator of the proportion of cyanobacterial biomass contributing to productivity. Here we present preliminary data on the molecular isotopic composition of porphyrins from suspended particulate matter in modern lake systems (Lakes Superior and Erie). We 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) to assess the fidelity of the ε_por proxy.