Critical points for lake assemblages

Posted on March 4, 2011

The species composition of mountain lake ecosystems varies considerably across broad environmental gradients related to altitude, geology and lake size.  Within the range of each gradient, it’s possible that ecological thresholds exist, which means that ecosystem organisation may change relatively quickly above and below a particular point on the given gradient.  The search for ecological thresholds has occupied a large international team of scientists who surveyed 235 alpine lakes across Europe, from thePyreneesto the Balkans.  For each lake they collected data on physical features, water chemistry, sediment composition and the occurrence of selected groups of algae and invertebrates.  They used multivariate analyses to identify clusters of lakes with similar species composition and to relate these clusters to different environmental conditions.  The analysis revealed that species patterns were affected by four main environmental factors: lake size, productivity, acid-base balance and ice-cover duration.  These factors were quite independent.  The authors suggest that: (a) acid-base balance has its strongest influence on the physiology of primary producers; (b) lake size and productivity affect ecosystem structure and function respectively, and (c) ice-cover duration represents a climatic constraint.  While lake size had a more pervasive impact than the other factors and had a significant effect on rotifer species, acid-base balance was especially important for diatoms, productivity was more important for chironomids, and ice-cover duration was important for microcrustaceans.  For each of the four factors there was at least one ecological threshold, namely 3 ha for lake size, 0.6 mg / litre for dissolved organic carbon, 190 days / year for ice-cover duration, and 200 microequivalents / litre for acid neutralising capacity.  These outcomes will assist in developing systems of lake classification that can help in assessing environmental quality, conserving biodiversity and predicting the impacts of climate change. 

Reference:  Catalan, J., et al. 2009.  Ecological thresholds in European alpine lakes.  Freshwater Biology 54, 2494–2517.