Belinda Robson

Ecosystems comprise all the life forms (microbes, plants, animals) and the inorganic context (sediment types, water regimes, water quality, etc.) that they live in. Together, the life forms interact with each other. They influence, and are influenced by, their environment, carrying out ecosystem processes such as the transport of nitrogen and carbon. This chapter introduces the terminology used to describe lake and river ecosystems. It describes the function of freshwater ecosystems and the methods scientists use to quantify these functions, including concepts such as ecosystem productivity, metabolism, food webs, populations, and metapopulations. Furthermore, this chapter describes ecological communities, their diversity and assembly, and the concepts of metacommunities and metaecosystems.

Forested wetlands in temperate and subtropical climates undergo periodic flooding. One or two tree species dominate the canopy. Trees shape the flow of flood waters, the ground surface, and the understorey, as well as animal habitats. With flooding, complex aquatic food webs support turtles, frogs, fish and birds, but can produce microbial blooms with nutrients flushed from the floodplain. Many vertebrates use these wetlands as refuges during dry times.

This group includes high-productivity floodplain wetlands fed regularly by large inputs of allochthonous resources that drive strong bottom-up regulation, and smaller areas of disconnected oligotrophic wetlands. Functionally diverse autotrophs include phytoplankton, algal mats and epiphytes, floating and amphibious herbs and graminoids, and semi-terrestrial woody plants. Interactions of fine-scale spatial gradients in anoxia and desiccation are related to differential flooding. These gradients shape ecosystem assembly by enabling species with diverse life-history traits to exploit different niches, resulting in strong local zonation of vegetation and high patch-level diversity of habitats for consumers.

Highly episodic freshwater floodplains are distinct from, but associated with, adjacent river channels, which provide water and sediment during flooding. These are low-productivity systems during long, dry periods (maybe years), with periodic spikes of very high productivity when first inundated. These floodplains have a high diversity of aquatic and terrestrial biota in complex trophic networks, with ruderal life-history traits enabling the exploitation of transient water and nutrient availability.

Upland streams (orders 1-4) with highly seasonal flows generally have low to moderate productivity and a simpler trophic structure than lowland rivers. They tend to be shallow, hence benthic algae are major contributors to in-stream food webs and productivity, but riparian zones and catchments both contribute allochthonous energy and organic carbon through leaf fall, which may include an annual deciduous component. Primary production also varies with light availability and flow.

These large riverine systems (stream orders 5–9) can be highly productive with trophic structures and processes shaped by seasonal hydrology and linkages to floodplain wetlands. In combination with biophysical heterogeneity, this temporal variability promotes functional diversity in the biota. Although trophic networks are complex due to the diversity of food sources and the extent of omnivory amongst consumers, food chains tend to be short and large mobile predator,s such as otters, large piscivorous waterbirds, sharks, dolphins and crocodilians (in the tropics), can have a major impact on the food webs. Benthic algae are key contributors to primary productivity, although macrophytes become more important during the peak and late wet season when they also provide substrate for epiphytic algae.

Episodic rivers have high temporal variability in flows and resource availability, shaping a low diversity biota with periodically high abundance of some organisms. Productivity is episodically high and punctuated by longer periods of low productivity (i.e. boom-bust dynamics). The trophic structure can be complex and dominated by autochthonous primary production. Even though riparian vegetation is sparse, allochthonous inputs from connected floodplains may be important. Top-down control of ecosystem structure is evident in some desert streams. Episodic rivers are hotspots of biodiversity and ecological activity in arid landscapes, acting as both evolutionary and ecological refuges.

Small permanent freshwater lakes, pools or ponds are lentic environments with relatively high perimeter-to-surface area and surface-area-to-volume ratios. Most are <1 km2 in area, but this functional group includes lakes of transitional sizes up to 100 km2, while the largest lakes (>100 km2) are classified in F2.1. Niche diversity increases with lake size. Although less diverse than larger lakes, these lakes may support phytoplankton, zooplankton, shallowwater macrophytes, invertebrates, sedentary and migratory fish, reptiles, waterbirds, and mammals.