Effects of climate, nutrients and vegetation on the trophic cascades in lakes; a study along a latitudinal gradient in South-America
Freshwater of good quality is an increasingly limiting resource in the world. The majority of the water quality problems are related to high nutrient loading, which promotes excessive phytoplankton development. However, the risk of algal problems depends also on the potential of algal control through the food chain, the so-called 'trophic cascade'. Phytoplankton biomass can be kept low by zooplankton. However, when fish predation is strong, large-bodied zooplankters disappear and the zooplankton community structure shifts towards smaller-bodied species that are much less effective at controlling phytoplankton biomass. This insight is important as in temperate lakes it led to a novel way of water quality management called biomanipulation. It implies that fish density is reduced in order to promote an increase in the abundance of water fleas. Although it has been suggested that these insights may be used to fight excessive algal growth also in (sub)tropical lakes, there are indications that things work quite differently at lower latitudes. Our aim in this project is to find out how climate, nutrient load, and the presence of vegetation interact to affect the functioning of the trophic cascade at lower latitudes. We hypothesise that changes in the fish community structure occurring at warmer climates and at higher nutrient levels, reduce the average size of the zooplankton community implying a loss of top-down control of algae, which may be ameliorated by vegetation. To address our hypothesis we will sample 100 shallow lakes from Brazil to Argentina, and take sediment cores to reconstruct the history of a subset of them. We will use these data to analyse how climate (i.e. latitude) alters the way in which zooplankton community structure is related to the fish predation pressure and nutrient load. The results will help to understand the potential for biomanipulation as a way to control water quality changes with latitude. In addition, our envisioned results will provide an indication of the way in which climatic change may affect water quality and biodiversity of shallow lakes. Such lakes are the dominant lake type in most flat areas of the world including The Netherlands and large parts of the South American Continent. They are abundant in riverine and coastal zones where most of the population is concentrated, and intensively used for water supply, fish farming and recreation activities.
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