Ammonium excretion in two benthic cnidarians: Alcyonium digitatum (Linnaeus, 1758) and Urticina felina (Linnaeus, 1767)

As part of an evaluation of nitrogen fluxes between the water column and a macrobenthic community of the Strait of Dover (eastern English Channel), ammonium excretion was measured throughout the year in two common cnidarians (the octocoral Alcyonium digitatum and the sea anemone Urticina felina) which together accounted for 37% of the biomass. Mean ammonium excretion rates were 6.48 (±2.44 s.d.) μg N g−1 h−1 (ash-free dry weight) for A. digitatum and 4.12 (±l.38 s.d.) μg N g−1 h−1 for U. felina. Seasonal variations were observed in the excretion of A. digitatum, whereas such variations were less pronounced in U. felina. These trends are discussed in relation to temperature, food availability, nutritional state and nutritional habits of the two species. The results of this and a previous study indicate a nitrogen production of 18.9 g N m−2 y−1 for the entire benthic community.