Drivers and Management Implications of Long-Term Cisco Oxythermal Habitat Decline in Lake Mendota, WI.

Cisco (Coregonus artedi) are an important indicator species for cold-water lake habitats in the Great Lakes region, and many populations have been extirpated at their southern range limit over the last century. Understanding the roles of climate and water quality in these extirpations should inform protection of cold-water fishes. Using the water temperature at the depth where dissolved oxygen falls to 3 mg L-1 (TDO3) as a metric, we investigated the roles of climate and water quality as drivers of habitat availability for cisco in Lake Mendota, WI, USA from 1976 to 2013. We find that summer (Jun-Aug) air temperatures, spring (Mar-May) phosphorus load, and spring inflow influence summer TDO3. Warm air temperatures lead to the greatest increases in TDO3, whereas reduced phosphorus loads can reduce TDO3, thus alleviating oxythermal stress. Under air temperatures expected under the A1B climate change scenario, a 25% reduction in phosphorus load would stabilize TDO3 at current levels, while a 75% reduction in phosphorus loading would be required to expand oxythermal habitat. Costs of these reductions are estimated to range from US$16.9 million (-25%) to US$155-167 million (-75%) over a 20-year period but may be feasible by expanding upon current watershed phosphorus reduction initiatives if sustained funding were available. Identifying targeted reductions will become increasingly important throughout the region as warmer temperatures and longer stratification reduces cool- and cold-water fish habitat in many Midwestern lakes under the expected future climate.

Phosphorus speciation in a eutrophic lake by ³¹P NMR spectroscopy.

For eutrophic lakes, patterns of phosphorus (P) measured by standard methods are well documented but provide little information about the components comprising standard operational definitions. Dissolved P (DP) and particulate P (PP) represents important but rarely characterized nutrient pools. Samples from Lake Mendota, Wisconsin, USA were characterized using 31-phosphorus nuclear magnetic resonance spectroscopy ((31)P NMR) during the open water season of 2011 in this unmatched temporal study of aquatic P dynamics. A suite of organic and inorganic P forms was detected in both dissolved and particulate fractions: orthophosphate, orthophosphate monoesters, orthophosphate diesters, pyrophosphate, polyphosphate, and phosphonates. Through time, phytoplankton biomass, temperature, dissolved oxygen, and water clarity were correlated with changes in the relative proportion of P fractions. Particulate P can be used as a proxy for phytoplankton-bound P, and in this study, a high proportion of polyphosphate within particulate samples suggested P should not be a limiting factor for the dominant primary producers, cyanobacteria. Hypolimnetic particulate P samples were more variable in composition than surface samples, potentially due to varying production and transport of sinking particles. Surface dissolved samples contained less P than particulate samples, and were typically dominated by orthophosphate, but also contained monoester, diester, polyphosphate, pyrophosphate, and phosphonate. Hydrologic inflows to the lake contained more orthophosphate and orthophosphate monoesters than in-lake samples, indicating transformation of P from inflowing waters. This time series explores trends of a highly regulated nutrient in the context of other water quality metrics (chlorophyll, mixing regime, and clarity), and gives insight on the variability of the structure and occurrence of P-containing compounds in light of the phosphorus-limited paradigm.