Background
The authors explain that among human impacts on aquatic biodiversity, eutrophication has caused a number of undesirable environmental effects worldwide, including "a general reduction in species richness (Smith and Schindler, 2009) and a loss of ecosystem resilience against further degradation (Folke et al., 2004)." As a result, they indicate that "lake restoration programs in many regions started at the peak of the eutrophication period (late 1970s, early 1980s)," but they write that the resulting reduction in nutrient loading "coincided with rising temperature from climate warming," citing Van Donk et al. (2003) and Jeppesen et al. (2005), so that we currently have "limited understanding of how combined climate change and reduction in nutrients affect species richness, ecosystem functioning or resilience."

What was done
In an effort to expand our understanding of these potential linkages, Pomati et al. analyzed a number of physical and biological parameters in Lake Zurich on the Swiss Plateau just north of the Swiss Alps at depths of 0, 1, 2.5, 5, 7.5, 10, 12.5, 15, 20, 30, 40, 80, 120 and 135 meters that had been collected monthly over a number of decades by the Zurich Water Supply Company, focusing on data from 1977 to 2008.

What was learned
The five scientists discovered that increases in water temperature (0.24°C/decade at the lake surface; 0.13°C/decade near the lake bottom) and spatial (depth) heterogeneity were the best predictors of phytoplankton richness, such that the number of phytoplankton species increased from about 40 to 100 and the number of phytoplankton families increased from approximately 25 to 45. And they add that phytoplankton richness and spatial heterogeneity had the strongest effects on zooplankton richness.

What it means
Quoting from the researchers' abstract, "our analysis highlights that climate warming and re-oligotrophication may favor an increase in spatial (depth) heterogeneity in the water column of deep lakes, enhancing the potential for phytoplankton species co-existence and an increase in plankton richness." And in the final sentence of their paper, they say that "although our study focuses on a single lake, the responses we have documented may be common at least throughout lakes within the European peri-alpine climatic region," citing the studies of Buergi and Stadelmann (2002), Buergi et al. (2003) and Anneville et al. (2005).

References
Anneville, O., Gammeter, S. and Straile, D. 2005. Phosphorus decrease and climate variability: mediators of synchrony in phytoplankton changes among European peri-alpine lakes. Freshwater Biology 50: 1731-1746.

Buergi, H.R. and Stadelmann, P. 2002. Change of phytoplankton composition and biodiversity in Lake Sempach before and during restoration. Hydrobiologia 469: 33-48.

Buergi, H.R., Bührer, H. and Keller B. 2003. Long-term changes in functional properties and biodiversity of plankton in Lake Greifensee (Switzerland) in response to phosphorus reduction. Aquatic Ecosystem Health and Management 6: 147-158.

Folke, C., Carpenter, S., Walker, B., Scheffer, M., Elmqvist, T., Gunderson, L. and Holling, C.S. 2004. Regime shifts, resilience and biodiversity in ecosystem management. Annual Review of Ecology, Evolution and Systematics 35: 557-581.

Jeppesen, E., Sondergaard, M., Jensen, J.P., Havens, K.E., Anneville, O., Carvalho, L., Coveney, M.F., Deneke, R., Dokulil, M.T., Foy, B., Gerdeaux, D., Hampton, S.E., Hilt, S., Kangur, K., Kohler, J., Lammens, E.H., Lauridsen, T.L., Manca, M., Miracle, M.R., Moss, B., Noges, P., Persson, G., Phillips, G., Portielje, R., Romo, S., Schelske, C.L., Straile, D., Tatrai, I., Willen, E. and Winder, M. 2005. Lake responses to reduced nutrient loading - an analysis of contemporary long-term data from 35 case studies. Freshwater Biology 50: 1747-1771.

Smith, V.H. and Schindler, D.W. 2009. Eutrophication science: where do we go from here? Trends in Ecology and Evolution 24: 201-207.

Van Donk, E., Santamaria, L. and Mooij, W.M. 2003. Climate warming causes regime shifts in lake food webs: a reassessment. Limnology and Oceanography 48: 1350-1353.