Lake Ecosystems

Abstract

Abstract Lakes are bodies of nonmarine standing water connected by water flow and aerial inputs to their surrounding landscapes (watersheds). As relatively discrete ecosystems, the interplay between physical, biogeochemical and organismal processes in them is especially clear, and can be studied, understood and put to use in effective management. Sunlight penetrating from the lake surface provides energy that warms the surface water, energy for photosynthesis and an environment suitable for predators that hunt by sight. The depth to which light penetrates is determined by the amount of suspended particles (phytoplankton, organic and inorganic sediments) and coloured organic chemical compounds dissolved in the water. Important chemicals entering from the watershed include essential nutrients (chiefly phosphorus and nitrogen) and pollutants that are taken up and passed through the food chain from primary producers (phytoplankton and rooted plants) to consumers (animals that eat plants and other animals). All organisms in lakes have adaptations that affect the strengths of their interactions with their physical and biogeochemical environments and with other species in the food web. Introduced species, pollutants, and other changes in the environment result in rapid evolution of the adaptations that determine interaction strengths. These processes are particularly obvious in discrete lake ecosystems. Key Concepts: Lakes are relatively discrete ecosystems; the interplay between physical, biogeochemical and organismal processes in them can be easily studied. Lakes take up a small proportion of the Earth's surface but their ecological importance is disproportionately high. Lake ecosystems are influenced by their watersheds; a lake and its watershed are often considered to be a single ecosystem. Thermal stratification in lakes generates vertical structure and compartments with different physical, chemical and biological properties. The shallow‐water littoral and the open‐water pelagic are the two major horizontal zones in lakes; each zone has its characteristic food chain based on macrophytes and benthic algae or phytoplankton. Carbon, nitrogen and phosphorus are the major nutrients affecting lakes and their watersheds as part of their biogeochemical cycles. Production is limited by phosphorus in most, but not all, lakes. The lake sediment plays an important role as habitat for rooted plants and animals, as nutrient storage (particularly phosphorus), and as a repository of decayed material and dormant stages of lake organisms. Both bottom‐up and top‐down processes determine the trophic structure and dynamics in lake food chains and webs. Lake ecosystems are shaped by both ecological and evolutionary processes that occur on the same time scale.