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- Title
Spatial and Temporal Variability of Dissolved Organic Matter Molecular Composition in a Stratified Eutrophic Lake.
- Authors
Berg, Stephanie M.; Peterson, Benjamin D.; McMahon, Katherine D.; Remucal, Christina K.
- Abstract
Dissolved organic matter (DOM) is an intermediate between organic carbon formed by primary producers and carbon dioxide (CO2) produced through respiration, making it a key component of the carbon cycle in aquatic ecosystems. Its composition influences the routes of mineralization. Here, we evaluate DOM composition as a function of time and depth in Lake Mendota, a highly productive eutrophic lake that stratifies in warm months and is located in Madison, Wisconsin, USA. Dissolved organic carbon concentrations and optical properties are presented for 73 samples collected at a single location at varying depths within the water column from June to November. A subset of samples is analyzed by Fourier transform‐ion cyclotron resonance mass spectrometry (FT‐ICR MS) to investigate DOM composition at the molecular level. Temporally, increases in more oxidized formulas are observed in both the epilimnion and hypolimnion. At the surface, correlations between DOM formulas and both chlorophyll concentrations and light intensity show that photochemical reactions contribute to DOM oxidation. In the hypolimnion, redox conditions and interactions with sediments likely influence temporal compositional change. Our results show DOM composition varies with depth with more highly oxidized formulas identified deeper in the water column. However, DOM composition varies more temporally than by location within the water column. This work has implications for climate change as DOM photooxidation in lakes represents an understudied flux of CO2 to the atmosphere. Additionally, lake eutrophication is increasing due to warming temperatures and this data set yields detailed molecular information about DOM composition and processing in such lakes. Plain Language Summary: Dissolved forms of carbon in lakes represent a significant portion of the global carbon pool. Excess phosphorus inputs to lakes cause severe algal blooms which alter other aspects of the lake such as mixing and redox conditions. Lake Mendota, located in Madison, Wisconsin, experiences severe algal blooms in the warm months, which cause oxygen depletion deep in the lake. In this study, we considered how changing redox conditions affect the composition of dissolved organic matter (DOM) within the lake. Samples were collected at the surface of the lake and at specific depths between June and November in 2017. Overall, we observed both spatial and temporal variabilities in DOM composition. Molecular formulas with higher numbers of oxygen atoms were enriched over our sampling period and with depth when the lake was stratified. No changes in DOM composition could be observed when the lake was mixed. Temporal variation was much greater than the observed spatial variability. At the surface, this variability is attributed to reactions occurring due to an input of sunlight. Key Points: O:C increases temporally in a eutrophic lake as a result of photochemical reactions in the epilimnionDOM composition varies as a function of depth only when the lake is stratified with higher O:C observed near the bottom of the lakeDOM composition varies more temporally than it does spatially within the water column
- Subjects
DISSOLVED organic matter; PHOTOCHEMISTRY; EUTROPHICATION; PHOTOOXIDATION; CHLOROPHYLL
- Publication
Journal of Geophysical Research. Biogeosciences, 2022, Vol 127, Issue 1, p1
- ISSN
2169-8953
- Publication type
Article
- DOI
10.1029/2021JG006550