Seasonal transformations of dissolved organic matter and organic phosphorus in a polymictic basin: Implications for redox-driven eutrophication

Redox-driven internal phosphorus (P) loading from lake sediments is a key process for propagating and sustaining cyanobacterial blooms in freshwater lakes. Missisquoi Bay in Lake Champlain, VT regularly experiences cyanobacterial blooms driven by internal P loading as well as from seasonal transitions, but the response of dissolved organic matter (DOM) to these changing conditions has previously not been investigated in this system and seldom in other redox-dynamic freshwaters. In this study, Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) has been employed to explore the seasonal transformation and distribution of DOM, including organic P (DOP), from spring 2017 to winter 2018 along with high-frequency geochemical monitoring. DOM was largely composed of allochthonous compounds with unique molecules from seasonal autochthonous sources and greater proportions of N,S-containing formulae after the early summer bloom season and during the winter. DOM compositions from spring to late summer was more aliphatic during the early bloom season, while DOM between late summer to winter was more aromatic with greater similarities between the two seasons. In contrast, DOP compositions from all seasons were highly dissimilar and suggested a compartmentalized water column, where riverine DOP compounds that were degraded during transport and in surface waters were largely absent in the benthic DOP. Additional sediment core incubation experiments were implemented to study the effects of short-term oxygen limitation on DOM and DOP composition to better constrain potential redox-based drivers of their mobility between sediment and the water column. Short-term incubations suggested an increase in aliphatic DOM and new DOP compounds, with little change in aromatic DOM, suggesting selective mobilization driven by microbial iron(III) and manganese(IV) reduction. Together, these detailed FT-ICRMS data show, for the first time, how DOM and DOP from various sources respond to changing physical and geochemical conditions in redox-dynamic freshwaters, demonstrating how these compounds cycle in freshwater settings and likely impact nutrient bioavailability.