Martin T. Dokulil

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

Long-term compositional response of phytoplankton in a shallow, turbid environment, Neusiedlersee (Austria/Hungary)

Data on phytoplankton biomass and on nutrient concentrations from Neusiedler See (mean depth 1.3 m) covering more than two decades are presented. The lake underwent strong eutrophication during this period. The response of annual average phytoplankton biomass and chlorophyll-a to the increase of phosphorus concentration from 10 to > 100 µg l-1 was moderate (7-fold increase). This is caused by light limitation of the system because of the high inorganic turbidity of the lake. Analyses of the spring, summer and autumn seasons at the generic and higher taxonomic levels show significant changes in composition of the phytoplankton community. Diatoms were more important during the pre-eutrophication phase while Chlorophyceae became most prominent during the peak of the eutrophication process. Blue-green algae, including Microcystis, became more apparent after this period. The abundance of some groups or genera, e.g. Euglena, was linked to the decline and re-appearance of submerged macrophytes in the lake. Abiotic and biotic interactions as causes for the observed changes are discussed.

Successful Restoration of a Shallow Lake: A Case Study Based on Bistable Theory

Eutrophication of shallow lakes provides an excellent example of alternative stable states. Clear water, macrophyte-dominated stages can alternate with turbid conditions characterized by high algal concentrations. Stable states can switch from one to the other domination through alterations of natural factors such as changes in water level and reduction in throughflow. Forward switches are often associated with anthropogenic pressure. In such cases, backward shift to the original, macrophyte-dominated stage may be difficult. Return times are often prolonged due to hysteresis as a result of resilience. The theory is exemplified with results from a shallow, urban, seepage lake ‘Old Danube,’ which is within the city limits of Vienna. Causes and consequences of switches between stable states including resilience and hysteresis are discussed. The remediation measures are explained and the success of the restoration is explained in detail.