Aquatic ecosystems in hot water : effects of climate on the functioning of shallow lakes

Het warmer worden van de aarde heeft mogelijk vergaande gevolgen voor meren. Om de mogelijke klimaatseffecten te onderzoeken hebben we 83 meren bemonsterd in verschillende klimaatzones in Zuid Amerika. De geselecteerde meren leken zoveel mogelijk op elkaar qua grootte en diepte, maar varieerden per klimaatzone zoveel mogelijk qua vervuiling met voedingsstoffen zoals fosfaat. De onderzoeksresultaten van Sarian Kosten geven aan dat de negatieve effecten van de vervuiling met voedingsstoffen waarschijnlijk sterker zijn in een warm klimaat dan in een kouder klimaat. "In gebieden met weinig vorstdagen veranderen meren waarschijnlijk al bij lagere concentraties fosfaat in een groene algensoep waarin onderwatervegetatie geen kans krijgt om te groeien dan in gebieden met langere vorstperioden. Dit geeft aan dat, wanneer we de ondergedoken vegetatie willen behouden, de fosfaat concentraties mogelijk verlaagd zullen moeten worden in regio’s waar klimaatverandering zal leiden tot minder vorstdagen. Tevens blijkt dat warmere meren een grotere kans hebben om gedomineerd te worden door blauwalgen ten opzichte van koude meren met hetzelfde gehalte aan voedingsstoffen. Tenslotte suggereren mijn onderzoeksresultaten dat koude meren mogelijk meer koolstof uit gaan stoten als ze opwarmen, wat de klimaatverandering verder kan versterken". cum laude graduation (with distinction) There is concern that a warmer climate may boost carbon emissions from lakes and promote the chance that they lose their vegetation and become dominated by phytoplankton or cyanobacteria. However, these hypotheses have been difficult to evaluate due to the scarcity of relevant field data. To explore potential climate effects we sampled 83 lakes along a latitudinal gradient of more than 6000 km ranging from Rio Grande do Norte in Brazil to the South of Argentina (5-55 oS). The lakes were selected so as to be as similar as possible in morphology and altitude while varying as much as possible in trophic state within regions. All lakes were sampled once during summer (subtropical, temperate and tundra lakes) or during the dry season (tropical lakes) between November 2004 and March 2006 by the same team. In the first chapters I address the question how climate might affect the chances for shallow lakes to be dominated by submerged plants. It has been shown that temperate lakes tend to have two contrasting states over a range of conditions: a clear state dominated by aquatic vegetation or a turbid state. The turbid state is typically dominated by phytoplankton and often characterized by poorer water quality than the clear state. The backbone of the theory explaining this pattern is a supposed positive feedback of submerged vegetation on water clarity: vegetation enhances water clarity and clearer water, in turn, promotes vegetation growth. The theory furthermore asserts that submerged vegetation coverage diminishes when nutrient concentrations increase until a critical point at which the entire vegetation disappears due to light limitation. Both aspects of the alternative state theory have been well studied in temperate shallow lakes, but the validity of the theory for warmer lakes has been questioned. In chapter 2 a graphical model is used to show how climate effects on different mechanisms assumed in the theory may affect the general predictions. An analysis of our data presented in chapter 4 reveals that submerged vegetation has similar overall effects on water clarity across our climatic gradient. Nonetheless, the results hint at differences in the underlying mechanisms between climate zones. For example, the data suggest that the positive effect of vegetation on top-down control of phytoplankton by zooplankton is lost at high densities of fish that are often found in warmer regions. The main factor explaining differences in the water clearing effect of vegetation among lakes in our data set was the concentration of humic substances. In lakes with a high concentration of humic substances vegetation did not enhance the water clarity.