Seasonal changes in environmental nutrient availability and biomass composition in a coral reef sponge

Sponges are crucial ecosystem engineers in most marine habitats, playing a critical role in cycling elements between the water column and the sea floor. Despite this, it is unclear how the nutritional status of the seawater surrounding a sponge influences its biochemical composition. Here, we investigate seasonal availability of the major nutrients in the water surrounding Amphimedon queenslandica , a coral reef demosponge inhabiting a low-energy reef flat environment adjacent to Heron Island on the southern Great Barrier Reef. Specifically, we investigate how nutrient availability might influence the biomass composition of this sponge species by analysing five replicated water and sponge samples collected over 13 months. Eleven environmental parameters had significant differences between at least two seasons. Some of these changes in nutrient availability were consistent with known ecological processes occurring on Heron Island and its surrounding waters. For instance, the availability and sources of carbon and nitrogen changed throughout the year, with both strongly influenced by nutrients emanating from the summer seabird rookery on the island. Several environmental parameters were strongly correlated, such as total and dissolved organic carbon with each other and with nitrate and sulfate, respectively. Amongst biomass components, skeletal content was significantly correlated with temperature, DNA with total organic carbon, and skeleton and lipid biomass with orthophosphate concentration, which was significantly higher in lower water temperatures. Nonetheless, we observed few compelling correlations between biomass composition in A. queenslandica and nutritional status of the surrounding seawater on a seasonal time scale.