In shallow polymictic lakes where the resuspension of sediments frequently occurs, tripton (non-living particulate materials) can affect physical, chemical and biological processes. Over the last four decades, substantial changes in tripton concentrations and therefore limnological parameters have been observed in Lake Kasumigaura, a shallow eutrophic lake.To build a conceptual model of the various relationships between these, we analyzed data on monthly water and yearly sediment quality at several stations together with meteorological information. Large short-term and decadal variations were characteristics for tripton. Annual averages in tripton concentration changed substantially (e.g., from 3 to 25 mg l −1 at the lake center), peaking in the early 2000s, when phosphorus and silicon in water were high, and we observed a large proportion of diatoms in phytoplankton and low light penetration. Tripton concentration showed a positive correlation with sediment water content (WC) and a negative one with sediment ignition loss (IL) with a few years' delay, confirming the results of previous hydraulic experiments. Using these parameters, multiple regression models for estimating tripton concentrations were constructed and showed good performances (e.g., adjusted r 2 = 0.61 at the center). Furthermore, changes in inorganic ions in water, probably resulting from meteorological variations, were inferred to bring about changes in sediment WC. On the other hand, the amount of Microcystis sp. in water was suspected to govern the sediment IL. We reveal many processes (physical, chemical, biological and meteorological phenomena) related to tripton, and thus, their relationships could be considered a conceptual model for a shallow lake.
During the last four decades, substantial changes have been observed not only in tripton concentrations but also in other limnological parameters in Lake Kasumigaura, a shallow eutrophic lake. A previous study reported that tripton concentration showed a positive correlation with sediment water content (WC) and a negative one with sediment ignition loss. To explain the changes in sediment WC, we conducted an analysis of the changes in mineral concentrations and sediment WC during 1979-2016 to determine their relationship. High mineral concentrations in water throughout the lake were observed in the latter half of the 1990s and higher WC in sediments followed with a time lag of a few years, resulting in significant correlations. The relationship between them was confirmed in laboratory experiments that indicated swelling of sediments (slowdown of sediment compaction) by the addition of NaCl into the water-sediment system. In contrast, significantly lower SS concentrations in overlying waters observed in higher mineral conditions were similar to previous studies. The significantly low annual precipitation during 1994-1997 probably caused high Na + and Clconcentrations in the lake waters in the latter half of the 1990s. The combined influence of sea-spray aerosol and evaporation was suspected to be the cause of high salt concentrations. The intrusion of salty water through the downstream gate and/or surrounding construction structures was another possibility.
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