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MEPS prepress abstract   -  DOI:

Dissolved inorganic nutrient enrichment does not affect sponge growth or condition

Blake D. Ramsby*, Joshua Heishman, Mia O. Hoogenboom, Steve Whalan, Nicole S. Webster

*Corresponding author:

ABSTRACT: Changing land use and an increasing human population have led to increased terrestrial runoff, which delivers nutrients, pesticides, and heavy metals into aquatic ecosystems. Elevated nutrient levels can adversely affect nearshore corals by reducing the amount of light reaching the benthos, exacerbating coral disease and bleaching, as well as stimulating algal growth, but the effects on other reef taxa are poorly understood. We investigated the effects of dissolved inorganic nutrient enrichment and changes in irradiance on the growth and condition of 5 common Great Barrier Reef sponges: 4 sponges with photosynthetic symbionts and 1 lacking photosynthetic symbionts. Concentrations of up to 7 µM total dissolved inorganic nitrogen (DIN) did not significantly affect the growth, condition, or chlorophyll content of any sponge species after 10 wk exposure. However, 2 species lost >20% volume across all nutrient treatments, suggesting that aquarium conditions may have been suboptimal for these species. Irradiance (80 vs 160 µmol quanta m-2 s-1) did not affect 4 of the 5 sponge species; however, higher irradiance resulted in higher organic content and chlorophyll levels in the bioeroding sponge Cliona orientalis, the only studied species that associates with the photosynthetic dinoflagellate Symbiodinium, suggesting that sponge–Symbiodinium associations may be more sensitive to irradiance levels than sponge–Cyanobacteria associations. While elevated nutrient levels are exacerbating the decline of reef-building corals, exposure to the average DIN levels within flood plumes that reach inshore reefs appears to have negligible effects on reef sponges.