Spatial variability of surface water chemistry in a eutrophic lake during fall turnover

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Assessments of spatial variability in lakes are often limited and extrapolations from a small number of sampling points increase uncertainty with respect to elemental budgets, and the underlying drivers of important processes such as primary production. We mapped surface water chlorophyll and CO₂ on Lake Mendota, WI in Oct-Nov 2014 during fall turnover using a high-resolution sensor platform (> 35,000 spatially-distributed measurements). Surface water concentrations of chlorophyll and CO₂ increased during lake turnover suggesting that the hypolimnion released stored nutrients and dissolved gas. Turnover increased lake-wide primary production and carbon dioxide efflux. Spatial patterns of chlorophyll and CO₂ revealed strong contrasts across the lake’s surface. Littoral zones and river mouths had higher concentrations of chlorophyll and lower CO₂ than pelagic zones. Spatial variability in these parameters was greatest in the nearshore areas, as complex sediment-surface water interactions lead to large variations in surface water conditions. While using single point observations in the middle of a lake may accurately represent metabolic and efflux rates across the pelagic zone, such datasets are not representative of processes in littoral and riverine transition zones.