Ecosystems in Flow: Plankton Between Rivers, Ponds, and Lakes

Along the course of a river, the water is particularly rich in oxygen in the valley-bottom stretch: here the riverbed widens and becomes deeper, the water discharge increases, and the current slows down, flowing with reduced turbulence—compared with the cold, turbulent mountainous stretch, rich in gorges and waterfalls—over a bed made up of pebbles. In the plains, on the other hand, the river slows its course, takes in the waters of its tributaries, and warms up: here nutrients are more abundant, but the amount of oxygen decreases. Every section of a watercourse, in any case, depends on what lies upstream. The health not only of the fish that swim following the flow (and sometimes against it), but also of algae, insects, and crustaceans depends on the condition of the entire river network. The river, rather than being an ecosystem in its own right, consists of a succession of interconnected ecosystems.

The clearer the water, the lower the concentration of plankton. Greenish or brownish waters correspond to a higher percentage of small floating organisms. Ponds, marshes, and swamps, where the water remains still and motionless except for the strokes of fish fins and the faint traces left by animals, algae, and wandering cells: here planktonic biomass flourishes (almost) undisturbed and continues to grow over time. Where rivers and streams flow rapidly, plankton populations move quickly, are diluted within the volume of their world, and reduce their concentrations.

Phytoplankton multiplies under conditions of particularly high nutrient availability, thanks to sufficient amounts of light and heat. In these phases the water is typically green, with limited visibility. Toward summer, when water bodies are in balance and healthy, the time of maximum zooplankton production arrives: it feeds on phytoplankton and brings it back to a more limited concentration. The water becomes clear again and visibility increases. As nutrients decline, zooplankton also decreases until autumn, when diatoms and the microbes that feed on them proliferate.

In lakes, in particular, aquatic life is richer and more abundant in the littoral zones, the areas where light can reach the bottom and stimulate plant growth, and from which most of the organic material entering the system originates. In these stretches, in association with these plants, very species-rich communities can be found, from gastropods to protozoa, with fish such as pike and sunfish. It is in the limnetic zone, or open waters, however, that the smallest organisms live—those that are part of the plankton—and those that are able to swim freely and belong to the category of nekton. This part of the lake is still illuminated by sunlight. Where this light cannot reach lies the deep zone, in which conditions of hypoxia (oxygen deficiency) may also occur, so that among the organisms able to survive here only a few are aerobic, while most are anaerobic, independent of respiration and capable of proliferating even in seemingly hostile environments. Near the bottom, where debris from the upper layers accumulates, lies the benthos.