Importance of ecological boundaries in aquatic ecosystem processes

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During recent past decades, harmful algal blooms have increased world wide in aquatic ecosystems, and not only in eutrophic ecosystems. In continental waters these blooms are mainly due to potentially toxic blue-green algae (cyanobacteria). The relationship between blue-green blooms and lake fertilisation by nutrients, particularly phosphorus (P), is known since the early 70s. The question today is why the blooms also occur in lakes with low nutrient concentrations in their surface waters. In some of these lakes external P-loading has been decreased in the last 20-30 years, in others external anthropogenic sources are unknown. In these low surface nutrient lakes the occurrence and intensity of the blooms varies between years. Recently, the increase of noxious blue-greens has been attributed to global warming, and the author of this hypothesis advocates for more detailed studies of climate change and population dynamics of bloom-forming blue-greens. In our 5-yr study we wish to verify some of the mechanisms related to climate that could control algal community structure and blue-green blooms in small lakes. In particular, we want to measure the importance of wind-induced physical forcing at the bottom of lakes (benthic boundary layer) in the fluxes of nutrients and organisms from littoral and deep habitats to the water column (pelagic habitats); and between physically constrained adjacent areas (e.g. thermocline in stratified lakes). Our hypotheses are that: i) it is the P-accumulated in the sediments that allows the growth of the bottom resting stages of algae in lakes with low external P loads and, ii) the intensity of the physical forcing of nutrients and organisms through boundaries links the different habitats (from the bottom to the water column and through the water column) and creates a positive or negative feedback in algae recruitment. Our study constitutes a novel approach to explaining the inter-annual variability in algae community structure in stratified lakes and may explain cyanobacteria blooms. Our understanding of the importance of the physical forcing on ecological processes in stratified small lakes should produce better tools for noxious bloom management and control.