Richard C. Lathrop

Epilimnetic temperatures from early 1980s through 2017 were analyzed for 12 Wisconsin (USA), German, and Finnish lakes. Seasonal temperature metrics exhibited large interannual variability with trends differing among regions. In the Wisconsin lakes, only late summer and fall temperatures increased significantly. In the 2 northeastern German lakes, temperatures increased in all seasons (not all significantly). The 3 Finnish lakes exhibited large spring temperature increases influenced by earlier ice-out; summer and fall temperatures also increased except for Konnevesi (central Finland) in early summer and polar Kevojärvi in midsummer. Kevojärvi also exhibited no increase in maximum recorded temperatures. Earlier records from 4 lakes were analyzed to elucidate longer-term epilimnetic temperature patterns. For Lake Mendota (southern Wisconsin), spring and late fall temperatures have increased modestly but significantly since 1894; summer temperatures have also increased modestly because of higher frequency of warm temperatures in recent summers and not from new record high temperatures. Trout Lake (northern Wisconsin) exhibited warm temperatures in some summers during the 1930s–1940s, similar to warm temperatures in some recent summers. Air–water temperature relationships coupled with long-term regional air temperature data also indicated summer epilimnetic temperatures in the study lakes were likely as warm in the 1930s–1940s as in recent years. Lake data confirmed cooler epilimnetic temperatures occurred in many summers during the 1950s–1980s coincident with cooler air temperatures. Because epilimnetic temperatures have not increased monotonically since 1900, our study supports continued temperature monitoring in lakes with extensive historical data to better understand and predict future effects of climate change on lake ecosystems.

Trophic responses to phosphorus (P) loads spanning 29–33 years were assessed for the eutrophic Yahara chain of lakes: Mendota (area = 39.6 km2, mean depth = 12.7 m, flushing rate = 0.23 yr-1); Monona (13.7 km2, 8.3 m, 1.3 yr-1); Waubesa (8.5 km2, 4.7 m, 4.3 yr-1); and Kegonsa (13.0 km2, 5.1 m, 3.0 yr-1). During extended drought periods with low P loads, summer (Jul–Aug) total P (TP) concentrations declined substantially in all 4 lakes, with Mendota achieving mesotrophic conditions (<0.024 mg L-1). In years when P loads were high due to major runoff events, summer TP in the lakes was high (especially in shallower Waubesa and Kegonsa); in some summers dissolved inorganic P was elevated, indicating algae growth was not P limited. Summer TP returned to normal levels following both low and high load years, signifying the lakes were responsive to P load changes. The proportion of P input loads passed via a lake's outlet to the next lake downstream increased as flushing rates increased. Because Monona, W...

Eutrophication of the four Yahara lakes—Mendota, Monona, Waubesa, and Kegonsa—near Madison, Wisconsin, has been dramatic since the mid-1800s. For Lake Mendota, the erosion of sediments from higher water levels established by the damming of the lake's outlet, plus the agricultural expansion of its watershed, resulted in blue-green algal growths. These impacts, however, were dwarfed by water quality problems stemming from Madison's wastewater inputs that directly entered Lake Monona from the late 1800s through 1936, and then Lake Waubesa until 1958. Blue-green algal blooms were so bad in the lower Yahara lakes that the Madison Public Health Department conducted major copper sulfate treatments during 1925–1954. During the wastewater input years, inorganic nitrogen (N) and especially dissolved reactive phosphorus (DRP) concentrations in the surface waters were very high (particularly in Waubesa and Kegonsa), indicating neither nutrient was limiting algal growth. No P legacy from the wastewater inputs was found in Waubesa and Kegonsa's sediments; minimal P-binding potential due to low iron (Fe) availability is the hypothesized reason. Mendota's algal blooms were not a problem until the mid-1940s when wastewater inputs from upstream communities increased as well as the agricultural use of N and P fertilizers. This increase in eutrophication symptoms coincided with an increase in indices of DRP and inorganic N concentrations in the lake. After wastewater diversion in 1971, blue-green algal blooms persisted in Lake Mendota, and the onus of the problem shifted to agricultural and urban nonpoint source pollution. While much progress has been made in recent years to control these pollution sources to Mendota, manure runoff during late winter continues as a management problem. As evidence, P loadings during January to March constituted 48% of total loadings measured for 1990–2006 in the Yahara River subwatershed. Much of this runoff P was dissolved and not associated with high sediment loads, whereas during other months, more of the runoff P was bound to sediments that could settle out in lower stream reaches prior to entering the lake. However, low P-binding potential of recently deposited sediments in Mendota along with signs of water quality improvements following periods of drought indicate the lake could respond rapidly to nutrient input reductions. Finally, DRP and inorganic N concentrations since 1980 have indicated that algal growth in the Yahara lakes during July–August may have been limited by not only P, but N (especially in the lower Yahara lakes). Aggressive programs to reduce inputs from both nutrients will be important to prevent scum-forming blue-green algal blooms and filamentous algal growths that could become problematic once zebra mussels become established in the Yahara lakes.

(2005). Restoration of a Wisconsin (USA) seepage lake by hypolimnetic withdrawal. SIL Proceedings, 1922-2010: Vol. 29, No. 1, pp. 482-487.

(2000). Interacting factors causing exceptional summer water clarity in Lakes Mendota and Monona. SIL Proceedings, 1922-2010: Vol. 27, No. 4, pp. 1776-1779.

Lake Mendota, which is plagued by periodic cyanobacterial blooms, is typical of many lakes in the Laurentian Great Lakes region that are vulnerable to zebra mussels (Dreissena polymorpha) but have yet to be invaded. We coupled removal estimates with chlorophyll production estimates in a dynamic model to predict the likely impact of mussel-mediated removal of phytoplankton on water clarity across a range of hypothetical zebra mussel densities. Models were fit to chlorophyll and temperature data collected biweekly from Lake Mendota during 1977-1993. When we assumed daily epilimnetic circulation, the percentage of days when the chlorophyll concentration exceeded 50 µg·L^-1 was decreased threefold at mussel densities as low as 1000 mussels·m^-2. When we assumed less frequent epilimnetic circulation, the density of mussels required to substantially improve water clarity increased dramatically. We predict that zebra mussel invasion would lead to increased water clarity in Lake Mendota. Cyanobacterial blooms would be reduced but not eliminated. Negative impacts on other lake processes following zebra mussel invasion could outweigh the benefits of lower phytoplankton concentrations.