Sponges as important sources of nitrate on an oligotrophic continental shelf

Abstract

In situ field measurements of dissolved inorganic nitrogen (DIN) release by sponges in southwestern Australia revealed nitrate release rates of 0.022 to 0.743 µmol g dry weight−1 (dry wt) h−1 and ammonium release rates of between 0.002 and 1.366 µmol g dry wt−1 h−1. The highest and most consistent rates of nitrate release were among the Thorectidae (0.324 to 0.725 µmol g dry wt−1 h−1), while mycalid and verongid sponges were highly variable (0.024 to 0.743 µmol g dry wt−1 h−1). The ratio of nitrate to ammonium in released nitrogen ranged from 0.1 to 197.0, indicating a wide range of nitrogen release modes by sponges, from predominantly ammonium to very efficient nitrate producers. The study more than doubles the number of temperate sponge species recorded to release nitrate. Nutrient concentrations near the seabed sponge assemblages were higher at low wind speed (a proxy for turbulent mixing). These observations and our measurements of nitrogen release rates from sponges are consistent with the hypothesis that primary production in the region depends on wave‐induced mixing at the seabed for resupply of remineralized nitrogen to a nutrient‐impoverished water column and that sponges make an important contribution to these fluxes. Based on known biomass of sponges in southwestern Australia, we calculate that sponges may contribute DIN of 1.8 to 3.2 g N m2 yr−1. Taking into account the distribution of sponge habitat across the continental shelf to 100 m depth, this constitutes a contribution of 10% to 18% of the total recycled nitrogen flux required from the benthos to balance a regional nitrogen budget.