Nutrient regeneration by abundant coral reef holothurians

To investigate quantity and quality of nutrients regenerated by sediment feeding holothurians excretion experiments were conducted with the common species Holothuria atra and Stichopus chloronotus on the Great Barrier Reef (GBR). These experiments were supplemented with measurements of ammonium concentrations behind holothurians in situ, and in the respiratory trees of the animals. For both H. atra and S. chloronotus, ammonium is the only important N-excretion product. Ammonium excretion rates were higher in the latter species and they were higher in summer than in winter for both species. Small amounts of phosphate were also released by both holothurian species, Area-specific nitrogen (0.52-5.35 mg m(-2) day(-1)) and phosphor-us (0.01-0.47 ing m(-2) day(-1)) regeneration rates of the holothurians were in a similar range as previously reported nutrient fluxes on coral reefs. Ammonium excretion rates were reduced in products of asexual reproduction (transverse fission) of S. chloronotus, but not in those of H atra. The fact that anterior sections of both species (which do not have respiratory trees) have normal (H. atra) or only slightly reduced (S. chloronotus) excretion rates, indicates that intact organisms also excrete ammonium through the bodywall. Compared to ambient levels in the water column, ammonium concentrations were higher in the respiratory trees (8-15 muM elevation) and directly behind (1.3-2.4 muM elevation) holothurians when water expulsions through the anus occur. Ammonium excretion through the bodywall in direct contact with benthic microalgae and elevated levels in the respiratory water ejected in close proximity to the sediment are suggested as important mechanisms to retain nutrients in the benthic recycling system. (C) 2001 Elsevier Science B.V. All rights reserved.