Submerged macrophyte decline in shallow lakes: What have we learnt in the last forty years?

Abstract

Over the last 40 years there has been substantial evidence that high biomasses of submerged aquaticplants and phytoplankton rarely occur together in shallow lakes, but it is clear that when present, plantshave a competitive advantage over algae. Aquatic plants provide habitat structure, which influences the fish community such that zooplanktonand other macroinvertebrates maintain a top-down control on algal growth, and this control is largelyindependent of the nutrient supply to the lake. Nonetheless it is clear that many, but not all, lakes losetheir vegetation as nutrient loading increases. However, in eutrophic lakes, the subsequent dominanceby phytoplankton is more likely to be a result of the loss of vegetation rather than the cause. At higher nutrient levels, grazing or mechanical damage can reduce plant cover allowing rapid devel-opment of algae. Changes to fish community structure or the influence of toxic chemicals can reduceinvertebrate algal grazers, overcoming the positive feedback loops that stabilise the plant dominance. The longer-term stability of macrophyte dominance is also reduced if there are few surviving plantspecies. Such loss of species richness is associated with increased nitrogen loading. Submerged plantsalso depend on a spring clear-water phase to become established, and local weather conditions duringwinter and spring may determine the relative success of phytoplankton and plant growth, leading to aprogressively longer period of algal dominance and fewer surviving plant species. The loss of submerged vegetation from lakes, although often perceived as a rapid change, is more likelyto be the final conclusion of a process in which the competitive advantage of a diverse plant communityis eroded by many pressures that are collectively interpreted as eutrophication. In attempts to manage our environment we hope to find simple, closed stable systems that will respondto measures designed to meet our perceptions of improved ecological quality. What we increasingly findare more complex open systems, which do not necessarily respond as expected. We look for simple andwidely applicable explanations where none are likely to exist.