Ammonia and phosphate excretion by zooplankton from the inshore waters of the Great Barrier Reef. II. Their in situ contributions to nutrient regeneration

Abstract

Zooplankton excretion was estimated by combining biomass data with experimental laboratory data on excretion rates at three stations, characterized by low nutrient levels, in the inshore waters of the Great Barrier Reef. Hourly ammonia excretion (as nitrogen) by net zooplankton (>205 µ) was calculated to range from 11.9 to 22.6 µ m^-3 or from 1.6 to 5.0 g m^-2 per year, and hourly phosphate excretion (as phosphorus) from 1.4 to 2.8 µ m^-3 or from 0.16 to 0.63 g m^-2 per year. Hourly excretion of ammonia by microzooplankton ( < 205 µ) was calculated to be 2.6 µ m^-3 (as nitrogen) and of phosphate 0.55 µ m^-3 (as phosphorus), values that were 15 and 27% of the excretion by net zooplankton, respectively. Combined excretion rates by net zooplankton and microzooplankton could supply only 9.0-29.2% of the nitrogen and 6.6-25.6% of the phosphorus for an assumed yearly primary production of carbon of 100 g m^-2 (= 17.6 g m^-2 of nitrogen and 2.5 g m^-2 of phosphorus). Calculations from an empirical equation relating temperature to oxygen consumption by a bottom community indicated a high potential for benthic nutrient regeneration in reef inshore waters (27.1 g m^-2 per year, as nitrogen). The bottom community therefore appears to be the most important source of nutrient regeneration within the area studied.