Determining causes of lake eutrophication requires investigation of bottom up (nutrient loading) and top down forces (food web structure). Square Lake is the clearest of the more than 950 lakes in the Minneapolis-St. Paul (Minnesota, USA) metropolitan area, but has experienced a trend of declining water clarity (due to increased algal biomass) over the past 30 years. Cause(s) for this trend have yet to be conclusively determined. Investigations of the lake’s watershed have not identified significant sources of nutrient loading that could account for the eutrophication trend. However, previous research has shown that the abundance of large-bodied Daphnia (D. pulicaria) 1) is significantly negatively correlated with the concentration of algal biomass, and 2) has declined over the past decade. Predation by rainbow trout (annually stocked since the early 1980s) is one potential cause for the decline in D. pulicaria. The objectives of this study were to 1) identify sources of mortality to D. pulicaria by characterizing the diets of potential Daphnia predators (including rainbow trout, other species of fish, and phantom midge larvae), and 2) explore recent and historical data to evaluate whether climate warming has altered patterns of temperature and oxygen stratification and deep-water habitat availability for D. pulicaria.
Results/Conclusions
Rainbow trout consumed significantly more D. pulicaria per capita than any of the fish species examined. Bluegill sunfish also consumed D. pulicaria, but only the relatively large (> 15 cm) individuals did so. Smaller-bodied bluegills (<15 cm) consumed prey from the littoral (larval insects) and shallow pelagic (the smaller-bodied D. mendotae) zones. Chaoborus (the phantom midge) crops analyzed did not contain D. pulicaria. Historical temperature/oxygen data show that several times in the last decade Square Lake has not become fully aerated prior to the onset of summer stratification. We hypothesize that incomplete springtime mixing (due to climate warming) and predation on D. pulicaria by trout over winter and during the spring are interacting to cause the eutrophication trend in Square Lake. Predation over winter and in spring may be inhibiting the ‘spring clear-water phase’ in which grazers (e.g., D. pulicaria) reach high densities after springtime diatom blooms. Increased sediment deposition of unconsumed algae would then promote higher rates of hypolimnetic oxygen depletion during summer, restricting the habitat space for D. pulicaria in deep water and limiting the size of the population. Earlier onset of stratification in spring stratification is expected to exacerbate the restriction of D. pulicaria habitat.