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Abstract Temperate reservoirs and lakes worldwide are experiencing decreases in ice cover, which will likely alter the net balance of gross primary production (GPP) and respiration (R) in these ecosystems. However, most metabolism studies to date have focused on summer dynamics, thereby excluding winter dynamics from annual metabolism budgets. To address this gap, we analyzed 6 years of year‐round high‐frequency dissolved oxygen data to estimate daily rates of net ecosystem production (NEP), GPP, and R in a eutrophic, dimictic reservoir that has intermittent ice cover. Over 6 years, the reservoir exhibited slight heterotrophy during both summer and winter. We found winter and summer metabolism rates to be similar: summer NEP had a median rate of −0.06 mg O2L−1 day−1(range: −15.86 to 3.20 mg O2L−1 day−1), while median winter NEP was −0.02 mg O2L−1 day−1(range: −8.19 to 0.53 mg O2L−1 day−1). Despite large differences in the duration of ice cover among years, there were minimal differences in NEP among winters. Overall, the inclusion of winter data had a limited effect on annual metabolism estimates in a eutrophic reservoir, likely due to short winter periods in this reservoir (ice durations 0–35 days), relative to higher‐latitude lakes. Our work reveals a smaller difference between winter and summer NEP than in lakes with continuous ice cover. Ultimately, our work underscores the importance of studying full‐year metabolism dynamics in a range of aquatic ecosystems to help anticipate the effects of declining ice cover across lakes worldwide.
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Productivity and photophysiology in a large, oligotrophic lake
We used a combination of approaches to measure primary production and plankton photophysiology in oligotrophic Flathead Lake, Montana (USA). Estimates of net ecosystem production (NEP) based on measurements of O2 to Ar ratios, together with radiocarbon (14C) assimilation incubations, revealed seasonal patterns in NEP and 14C-primary production. NEP was elevated during the summer, becoming negative during the winter. Rates of 14C-primary production were similarly seasonal, with peak rates in the summer and lower rates in the winter. Photosynthesis-irradiance curves indicated that plankton productivity in the subsurface chlorophyll maximum was light-limited year-round, while plankton production in the near-surface waters was light-saturated during the summer. We found that, despite physiological evidence of photoinhibition during the summer, this process appears to play a minor role in constraining primary production in Flathead Lake. Finally, use of metagenomic sequencing provided insight into photophysiological potential among the abundant cyanobacteria in the lake. Cyanobacteria belonging to Synechococcus/Cyanobium were well represented, some of which demonstrated seasonality while others appeared to be present year-round. Analyses of the metagenomic assembled genomes (MAGs) from these cyanobacteria revealed genes involved in phycoerythrin and phycoerythrobilin syntheses, with one MAG also possessing genes that encode phycourobilin. Such results point to flexibility in pigmentation as central to the physiology and competitive success of cyanobacteria in this lake.
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- Award ID(s):
- 1951002
- PAR ID:
- 10493467
- Publisher / Repository:
- Association for the Sciences of Limnology and Oceanography
- Date Published:
- Journal Name:
- Association for the Sciences of Limnology and Oceanography
- Format(s):
- Medium: X
- Location:
- Palma de Mallorca, Spain
- Sponsoring Org:
- National Science Foundation
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