Salinity

Coastal streams can have quite different algal communities to those found further inland. The estuarine reaches of streams and rivers that carry salty water from the sea are unsuitable to many freshwater algae, but they are ideal for certain salt-tolerant filamentous green algae, rhodophytes (red algae) and some diatoms. Note that if you’re sampling in an estuarine environment, any algae you collect that was not attached to the bed may not be well suited to that habitat – they may be freshwater taxa that have drifted downstream, or they may be sea water taxa that have been carried upstream by the rising tide. Non-estuarine algae that become trapped in estuarine habitats often die and decompose, causing unpleasant-smelling deposits that are sometimes the subject of complaints from the public.

Colonial or filamentous green algae belonging to the genus Ulva (particularly species formerly named Enteromorpha) may form prolific bright green growths attached to the beds or banks of tidally affected streams/rivers. These growths can help to define the zone of seawater influence. Examples of algae that can grow quite happily in the brackish reaches of coastal streams are shown below: