101 Ecosystem Processes

Biological production in inland waters is limited by flows of energy and nutrients across landscapes that undergo complex spatial and temporal dynamics driven by hydrological processes. Primary production (by photosynthesis or chemosynthesis), nutrient uptake and cycling, and decomposition of organic matter are all interconnected by the movement of water and organisms among interconnected ecosystems. Hydrology influences rates of transport, deposition, and recycling of inorganic materials, which, in turn, influence the various species and their associated ecosystem processes. Ratios of nutrients limit productivity and influence foraging behavior of consumer species. The ratio of carbon to nitrogen to phosphorus (C:N:P) available to plants often limits rates of photosynthesis. In many inland aquatic ecosystems, the scarcity of phosphorus limits plant productivity, although nitrogen or light are often limiting factors in lakes and shaded streams. Suspended sediment transported by runoff carries adsorbed nutrients and influences light penetration that, in different combinations, can limit rates of photosynthesis. Dispersal of individuals to high-quality habitats and assembly of natural communities sustain functional groups of species and ecosystem processes following disturbances (such as severe floods and droughts or accidental spills of toxins). This dispersal is often limited by hydrologic connections among different habitats. Some “fugitive species” are especially well adapted for dispersal, and may include other nonaquatic modes of movement. These highly mobile species rapidly colonize new habitats or disturbed habitats and are widespread, generalist species in terms of their roles in ecosystem functioning. Other species have more specialized ecosystem roles in particular habitats and are less adapted for dispersal. These species have restricted distributions and different adaptations for persistence, in relatively isolated ecosystems with limited habitat connectivity. If population densities increase beyond carrying capacity in optimal habitats, the movement of species from their high-quality habitats to low-quality habitats creates sink populations where individuals survive but do not reproduce and cannot sustain ecosystem services. Source populations are those that continue to reproduce in high-quality habitats and to produce individuals who disperse to locations with lower or variable habitat quality. The occasional flow of genes among these usually isolated subpopulations (metapopulations) influences the long-term evolution of species and aggregations of partially isolated communities (metacommunities). These biotic processes and their associated ecological services depend on sustainable flows of water throughout habitats in entire drainage networks. However, high degrees of connectivity also allow introduced, nonnative species to spread and possibly displace native species. Over time, the loss of native species which are well adapted for long-term variability in environmental conditions can, in some catchments, lead to instability and unreliable provisioning of ecosystem services by nonnative species. Keywords: catchment complexity; drainage network; dispersal; ecosystem functioning; energy flow; food webs; fugitive species; hydrologic connectivity; hydroperiod; metacommunities; nutrient cycling; nutrient ratios; productivity; reservoirs; spatial coupling; stream density; topographic controls; trophic cascades; vulnerability to invasive species