What lives in forest streams?

Brook lamprey (Lampetra planeri) spend most of their lives as larvae, so-called “ammocoetes”, buried in the sediment, where they filter food particles from the water. In late summer to autumn, the ammocoetes that have metamorphosed into adults leave the sediment. They then swim upstream and no longer feed. In spring, the adults can finally be observed laying their eggs in so-called spawning pits in the fine sand.

The endangered and strictly protected thick-shelled river mussel (Unio crassus) mainly colonises clean, oxygen-rich watercourses up to the upper reaches, with a moderate to strong current. The mussels are dependent on a healthy fish population in their habitat. The larval stages of the mussels live for several weeks as parasites on the gills of their host fish (e.g. minnows, bullheads, chubs). The mussels are very sensitive to disturbances in their habitat and sediment inputs.

Most organisms in the stream can only be seen when you look under the stones in the stream bed. Numerous small invertebrates, so-called macroinvertebrates, live in the gap system of pore spaces between substrate particles on the stream bed through which the water flows, the so-called interstitial. They include many water-bound insects (e.g. dragonflies, stoneflies, mayflies, caddisflies, among others). They spend their larval stage here in the protected gap system. The larval stage usually lasts much longer than the adult stage. Mayflies, for example, spend several months to years as larvae. As adults they usually die after 1-4 days. Many of these insect larvae are important indicator organisms for good water quality (e.g. stonefly larvae, Figure 8). In addition to insect larvae, the macroinvertebrates of forest streams include the freshwater amphipods Gammarus fossarum, annelids, flatworms and diving beetles.

Three native crayfish species are found in Baden-Württemberg: the noble crayfish (Astacus astacus), also known as European crayfish; the stone crayfish (Austropotamobius torrentium); and the white-clawed crayfish (Austropotamobius pallipes), also known as the Atlantic stream crayfish. All three are on the Red List as critically endangered (noble crayfish and stone crayfish) or threatened with extinction (white-clawed crayfish) (as of 2014). The main threat is the spread of invasive North American crayfish species through direct competition and transmission of the so-called crayfish plague. Crayfish plague is caused by a pseudo-fungus (Aphanomyces astaci) and leads to the death of European crayfish species within a short period of time.