Preliminary observations on Gibbula umbilicalis (da Costa, 1778) on the Portuguese coast

Physiological indicators of nitrogen and phosphorus deficiency were used to determine the role of these nutrients in the development and proliferation of Cladophora in the Manawatu River, New Zealand. Nitrogen indicator tests showed that surplus nitrogen was available at all times. During a sustained low flow period when the dissolved reactive phosphorus was between 4–5 mg m^–3, phosphorus indicator tests revealed that the Cladophora proliferations were phosphorus deficient. A downstream trend of reduced nitrogen and phosphorus availability was also observed during this period. Sudden high river flows were frequent and had a major influence on the Cladophora biomass.