As atmospheric CO concentrations rise, associated ocean warming (OW) and ocean acidification (OA) are predicted to cause declines in reef-building corals globally, shifting reefs from coral-dominated systems to those dominated by less sensitive species. Sponges are important structural and functional components of coral reef ecosystems, but despite increasing field-based evidence that sponges may be 'winners' in response to environmental degradation, our understanding of how they respond to the combined effects of OW and OA is limited. To determine the tolerance of adult sponges to climate change, four abundant Great Barrier Reef species were experimentally exposed to OW and OA levels predicted for 2100, under two CO Representative Concentration Pathways (RCPs). The impact of OW and OA on early life-history stages was also assessed for one of these species to provide a more holistic view of species impacts. All species were generally unaffected by conditions predicted under RCP6.0, although environmental conditions projected under RCP8.5 caused significant adverse effects: with elevated temperature decreasing the survival of all species, increasing levels of tissue necrosis and bleaching, elevating respiration rates and decreasing photosynthetic rates. OA alone had little adverse effect, even under RCP8.5 concentrations. Importantly, the interactive effect of OW and OA varied between species with different nutritional modes, with elevated pCO exacerbating temperature stress in heterotrophic species but mitigating temperature stress in phototrophic species. This antagonistic interaction was reflected by reduced mortality, necrosis and bleaching of phototrophic species in the highest OW/OA treatment. Survival and settlement success of Carteriospongia foliascens larvae were unaffected by experimental treatments, and juvenile sponges exhibited greater tolerance to OW than their adult counterparts. With elevated pCO providing phototrophic species with protection from elevated temperature, across different life stages, climate change may ultimately drive a shift in the composition of sponge assemblages towards a dominance of phototrophic species.

In situ growth of two abundant diatom-bearing benthic foraminiferal species (Amphistegina radiata, Heterostegina depressa) was investigated to elucidate whether differences in abundance between inshore and offshore reefs are influenced by water quality. Five experiments were conducted over 2 yr, comparing growth rates of foraminifera at inshore coral reefs, which are exposed to terrestrial runoff, with those at reefs further offshore. Concentrations of most dissolved and particulate water quality variables were significantly higher inshore and during periods of high runoff (summer wet season). Foraminiferal growth was generally significantly lower on inshore reefs than offshore reefs, and growth of both species was reduced during the wet season. Depth transplantations confirmed that light was not an important factor in growth regulation. In contrast, multiple regression analyses of the effects of water quality variables on foraminiferal growth explained 69% of the variance in growth for A. radiata and 78% for H. depressa. Increased concentrations of dissolved nitrogen were associated with reduced growth. Intracellular carbon-to-nitrogen ratios in the foraminifera also reflected patterns in water quality, with generally lower values in foraminifera from inshore or during periods of high runoff, driven by higher intracellular N contents during these periods. It is suggested that increased nutrient availability releases foraminiferal symbionts from nutrient limitation. This may lead to reduced translocation of organic carbon to the host and resulting reduced host growth.