INTERACTIONS AMONG PHYTOPLANKTON, PERIPHYTON, AND MACROPHYTES IN TEMPERATE FRESH-WATERS AND ESTUARIES

The physico-chemical environment and the resource needs of phytoplankton, periphyton and macrophytes are markedly different. In this paper we compare the characteristics of the different phototrophs with respect to water movements and diffusive boundary layers, nutrient demands, carbon and oxygen dynamics, and light climate and light requirements. We discuss how these characteristics affect growth dynamics, biomass limitation and biotic interactions of phototrophs in natural habitats, and, finally, we discuss how plant community dominance can be predicted from ecosystem size, depth and nutrient loading. Phytoplankters live in a stirred environment with thin diffusive boundary layers, low nutrient availability and a highly variable light climate. They grow fast and have high nutrient requirements but their biomass is often nutrient limited. Diffusive boundary layers of benthic microalgae and rooted macrophytes are thicker and reduce the exchange of gases and nutrients. Sessile organisms live in a more predictable light climate but may experience severe self-shading and generally grow more slowly than phytoplankters. The nutrient requirements of rooted macrophytes are lower than those of microalgae because of low growth rates, high internal C:N:P ratios and the existence of nutrient conserving mechanisms, and nutrient limitation is less important because the plants exploit the rich nutrient pools of the sediment. The phototrophs compete for light, nutrients and inorganic carbon, and the balance among phototrophs changes with size, depth and nutrient richness of the ecosystem. Phytoplankters dominate in deep lakes and oceanic waters and may also, together with periphyton, dominate in nutrient-rich shallow waters because of shading effects on macrophytes and benthic microalgae. However, shallow lakes and estuaries with low nutrient availability in the water column are dominated by benthic phototrophs because of their lower nutrient requirements and contact to sediment nutrient pools.