Dynamic changes in dissolved organic matter composition in a Mountain Lake under ice cover and relationships to changes in nutrient cycling and phytoplankton community composition

Abstract

We studied Bear Lake, a subalpine oligotrophic lake in Rocky Mountain National Park, Colorado USA, during the winter ice cover period of 2018. Our goal was to understand relationships between biogeochemical and biotic processes and the chemical quality of the dissolved organic matter (DOM), a major pool of organic carbon in all lakes. The lake was stratified with limited light availability and had a consistent oxycline at 7 m. Nutrient concentrations were low and the depletion of nitrogen species was related to increased phytoplankton abundance. The phytoplankton community became progressively dominated by a potentially mixotrophic dinoflagellate species, Gymnodinium sp., and a deep chlorophyll maximum developed above the hypolimnion. Changes in the chemical quality of DOM, as measured through high-resolution mass spectrometry, were associated with these shifts in nutrient and phytoplankton composition. The spectrometry data revealed a pool of organic compounds many of which contained phosphorus, nitrogen, and sulfur. This pool evolved through the winter, with phosphorus-containing compounds becoming more predominant in the upper oxic zone and sulfur-containing compounds becoming more predominant in the anoxic bottom waters. Reduced chemo diversity of the DOM pool in the surface waters developed through the winter. In the bottom waters, more unique compounds were produced through the winter, and the favorability of the DOM to support microbial respiration also increased. These observed differences in DOM molecular composition were not reflected in the spectroscopic signature of the DOM pool. Overall, our results show chemical changes in the DOM pool correlate to dynamic redox conditions, biogeochemical cycling, and biotic response to ice cover.