Ecology of a marineRivularia population

An account is given of the environmental chemistry and physiological ecology of a population ofRivularia atra growing in the upper eulittoral of Tyne Sands, a sheltered bay in S-E. Scotland. Large masses of detached seaweed tend to be deposited in the supralittoral of this bay and their decay leads to elevated levels of dissolved phosphate (typically 50–150 μg 1⁻¹ P) in the water of shallow pools of the upper eulittoral. Much (usually 50%) of this phosphate is organic, as opposed to phosphate in the open sea just outside the bay, where it is almost entirely inorganic. This organic phosphate is presumably available to theRivularia, as colonies show marked alkaline phosphatase activity. The colonies are small (mostly <1 mm diameter), but with a high nitrogenase activity (expressed per unit chlorophyll) in the light are high (sometimes approaching 0.2 nM C₂H₄ μg chl a⁻¹ min⁻¹ × 10⁻³) but there is a rapid and very marked drop on transfer to the dark. It is suggested that this latter feature may be of adaptive significance for this population, as colonies in many pools are covered intermittently by sand.