MEPS 210:125-138 (2001)  -  doi:10.3354/meps210125

ABSTRACT: The close coupling between producers and consumers of inorganic nutrients is generally assumed to be important to maintaining high productivity on coral reefs. I examined whether a tight cycling of nutrients exists between benthic microalgae and sediment-feeding holothurians (Stichopus chloronotus and Holothuria atra). Benthic microalgae had an increased production (measured as O₂ evolution) after exposure for more than 3 h to effluent water from tanks containing holothurians. Direct addition of phosphate and ammonium suggested that this increase was mainly caused by the excretion of ammonium. In experiments in which a part of the sediment area was inaccessible to the holothurians, microalgae production significantly increased in aquaria containing holothurians, irrespective of whether microalgae were grazed or protected from grazing. Thus, it could be inferred that production enhancement is solely a nutrient effect and not due to other effects described for other grazers (e.g. removal of senescent cells or reduction of self-shading). The effects on the microalgal biomass (measured as chlorophyll a and phaeophytin) depended on the actual grazing intensity. When a high grazing pressure was simulated (e.g. 2 H. atra on 0.116 m²), microalgal biomass was reduced significantly after 7 d compared to control aquaria. At lower grazing intensities (e.g. 1 S. chloronotus on 0.232 m²), the microalgal biomass increased significantly. A significant negative correlation was found between the amount of sediment consumed in each single aquarium (measured as the weight of the faeces produced) and the increase in chlorophyll a and phaeophytin concentrations in the sediment. A comparison with in situ sediment-consumption rates suggested that holothurians in natural densities have an overall beneficial effect on the benthic microalgal community. I propose that holothurians and other sediment-feeders are important components of a benthic recycling system that may have some similarity to the planktonic microbial loop.

KEY WORDS: Productivity · Sediment-feeders · Microalgae · Holothurians · Grazing · Recycling · Close coupling