Coupled long‐term limnological data and sedimentary records reveal new control on water quality in a eutrophic lake

Abstract

Human impacts on freshwater ecosystems are pervasive, but the short and discontinuous nature of most datasets limits our ability to understand the controls on water quality and effectively manage freshwater resources. We examine change in Lake Mendota (Madison, Wisconsin) over the last two centuries by pairing analyses of a sedimentary archive with the site's > 100 yr limnological record. We show that eutrophication of the lake, evident as an abrupt shift in sediment composition, began in the late 19 th century following the intensification of urban and agricultural land use in the watershed. Efforts to address deterioration of lake water quality, including the removal of point-source pollutants and biomanipulation, have had a measurable influence on sediment composition and water clarity. Since the early 1980s, quasi-seasonal cycles of phytoplankton blooms have induced calcite precipitation, leaving distinct laminations in the sedimentary record. These "whiting events" evidently did not accumulate in lake sediments until the late 20 th century, indicating that efforts to remediate water quality have shifted the lake to a new ecosystem state. Calcite whitings can improve water quality in eutrophic lakes by coprecipitation with phosphate, increasing phosphorus (P) burial in lake sediments. Using long-term limnological records, we report negative correlations between calcite saturation indices and P in lake surface waters and show that calcite whitings could partially explain recent P decline in Lake Mendota surface waters. Our study reveals a previously uncharacterized potential control on water quality in this eutrophic lake and demonstrates the benefit of coupling long-term limnological data with sedimentary records.

The aftermath of a trophic cascade: Increased anoxia following species invasion of a eutrophic lake

“…Phosphorus dynamics in the lake also shifted following the spiny water flea invasion (Walsh et al, 2019), with a decrease in SRP. While this pattern runs counter to known positive relationships between nutrient availability and phytoplankton biomass (Conley et al, 2009), biophysical processes may provide an explanation.…”

“…While this pattern runs counter to known positive relationships between nutrient availability and phytoplankton biomass (Conley et al, 2009), biophysical processes may provide an explanation. Whiting events can occur when phytoplankton blooms raise eplimnetic pH through the uptake of inorganic carbon, which triggers the precipitation of calcium carbonate and the co-precipitation of SRP (Walsh et al, 2019). Simultaneously, by reducing Daphnia , spiny water flea indirectly reduced grazing pressure on phytoplankton, causing spring diatom blooms to persist longer and at higher concentrations.…”

“…This suggests the model did not capture a potential ecosystem shift. Epilimnetic phosphate concentrations from 2010-2018 also decreased by 65% compared to 1995-2009, which could be explained by an increase in whiting events, whereby calcium carbonate precipitation adsorbs available phosphorus (Walsh et al, 2019). In addition, the reduction of grazers (i.e., Daphnia pulicaria ) and increased phytoplankton biomass may have led to increased phosphorus uptake that was exported to the hypolimnion via sinking algal cells.…”

Sediment phosphorus composition controls hot spots and hot moments of internal loading in a temperate reservoir

“…For example, eutrophication in lakes and reservoirs is often influenced by interacting slow and fast variables at the sediment–water interface. Sediments hold a pool of phosphorus (P), which is the legacy of past external loading and subsequent sedimentation (Søndergaard et al, 2003; Walsh et al, 2018). This P may be retained in sediments or mobilized and released into the overlying water (i.e., internal P loading; Orihel et al, 2017).…”

Phosphorus Inactivation in Lake Sediments Using Calcite Materials and Controlled Resuspension—Mechanism and Efficiency

“…Calcite naturally precipitates in surface waters mainly during spring and summer because of oversaturation in the productive zone caused by CO 2 assimilation during photosynthesis leading to a pH increase [18][19][20]. As orthophosphate is co-precipitated with newly forming calcite crystals and is removed from the water column during their settling, this process is an important self-purification mechanism [19,[21][22][23][24]. To mimic this phenomenon, lake marl-calcite rich sediment deposits-was excavated in two German lakes and distributed evenly on the lakes' surface to immobilize P [25,26].…”