Biological Responses to Typhoon-induced Mixing in Two Morphologically Distinct Basins of Lake Biwa
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The collaborative. research program BITEX'93 (Biwako Transport Experiment) examined how physical, chemical and biological processes interact in controlling cyanobacteria, blooms in the South Basin of Lake Biwa. During the course of BITEX, we tested the hypotheses that: (1) the growth and/or population size of phytoplankton in the South Basin are limited by nitrogen and/or phosphorus, (2) addition of these nutrients by Kelvin Wave sloshing (KWS) or Kelvin Wave breaking stimulates primary production, (3) during the typhoon period, dissolved reactive nutrients enter the South Basin of Lake Biwa by KWS, (4) these nutrients stimulate the growth of bloom-forming cyanobacteria, which are then advected into the North Basin, and (5) thus, during the typhoon events, biological responses of plankton in the two basins are strongly interconnected by KWS. We present evidence, from the 3-week sampling program, that 1) phytoplankton in the South Basin of Lake Biwa were nutrient limited, 2) KWS did not occur, 3) an internal wave from the North Basin did not enrich nutrient concentrations in the South Basin during typhoon forcing, and 4) there was no strong interconnection between the North and the South basins during the typhoon events. Biological responses provided conclusive evidence for the lack of such a mechanism at three different time scales. At the bacterial-generation time (a few hours), we examined the exchange of dissolved organic matter between basins. Responses of bacteria (abundance and production) to major changes in mixing conditions and primary production greatly differed between the two basins and showed no interconnection. At the phytoplankton-time scale (days), we tested nutrient exchanges by sloshing. Biomass (chlorophyll a, particulate carbon and nitrogen) and photosynthetic rates of the <2 μm and >2μm phytoplankton remained higher in the South than in the North Basin during and after the typhoon events, indicating no intrusion from the North into the South Basin. At the zooplankton-time scale (weeks), we tested the advective exchange of water masses. Concentrations of crustacean zooplankton and one developmental stage of E. japonicus were lower in the South than in the North Basin and showed no interconnection between basins during and after the typhoon events. Results indicate that the two basins act as separate entities even during high energy events, so that management strategies should be fine-tuned for each basin.
- 日本陸水学会の論文
日本陸水学会 | 論文
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