Sponge physiology and function in a changing ocean: responses to ocean acidification and increased sea surface temperature

Abstract

As a result of anthropogenic impacts, the concentration of atmospheric carbon dioxide (CO₂) is currently 400 μatm, and is rising at a rate 100 times faster than any change during the last 650,000 years. Oceans have taken up approximately one-third of CO₂ produced in the last two hundred years, significantly altering the buffering ability of the ocean. As CO₂ enters seawater it causes a reduction in pH and carbonate saturation, and an increase in dissolved inorganic carbon. As a result, ocean surface pH is 0.1 units lower than preindustrial values, and has been predicted to decrease a further 0.4 pH units and increase in temperature by 1-3 °C by the end of 2100.

The effects of warmer, more acidic waters on marine sponges are largely unknown, and the majority of studies that have been conducted have mostly focused on tropical species. Sponges are important spatial competitors in the marine environment, forming associations with microbial organisms and performing many vital functional roles, including bioerosion and bentho-pelagic coupling. Environmental degradation can alter sponge diversity and abundance, potentially influencing the distribution of other benthic marine organisms. The aim of this thesis was to investigate how temperate sponges might be impacted by rise in pCO₂, and consequent decline in pH by up to 0.4 pH units and an increase in temperature, by up to 4 °C. These changes were based on IPCC (2014) scenarios. Tethya bergquistae and Crella incrustans are both demosponges, and are two of the most common species inhabiting shallow-water rocky reefs in New Zealand (Berman and Bell, 2010; Berman, 2012). Both sponge species are important structural components of intertidal reef systems around New Zealand, yet relatively little is known about their physiological responses to environmental factors. In light of this, both Tethya bergquistae and Crella incrustans, were subjected to 24 day experiments looking at experimentally increased temperature (ambient 13.5, 18, 20, 22 ° C) and seawater pH (ambient pH 8.1 and pH 7.6). Respiration rates were measured weekly, along with mortality rates and disease prevalence, to investigate how these species may respond physiologically to climate change induced stressors.

Temperature had a significant impact on sponge respiration and survival rates for both species. Disease prevalence was significantly higher in both species subjected to thermal stress, relative to controls, with Crella incrustans exhibiting greater mortality and diseased or bleached tissue (> 80% of surface area) than Tethya bergquistae (> 60% surface area). Survival rates declined with increased temperature for both species, and mortality rates were highest in the 20 and 22 ° C treatments. Most mortality of C. incrustans (70%) occured in the first four days of the experiment, with 100% mortality occurring by days 6 and 7 in the two highest temperature treatments. Similarly, T. bergquistae exhibited significant mortality in the two highest temperature treatments, although this species did have slightly higher survival rates with median longevities of 9 and 5 days. Temperature also had a significant effect on respiration rates of both species, with increases from baseline rates of 0.32 and 1.82 mg O₂ g⁻¹ DWh����⁻¹, to 8.46 and 47.63 mg O₂ g⁻¹ DWh⁻¹ in the most extreme cases of T. bergquistae and C. incrustans, respectively. In addition, thermal stress appeared to have an effect on the proportion of inorganic matter in T. bergquistae, with a significant increase being reported at higher seawater temperatures.

In contrast, seawater pH had very little effect on any response variables measured, despite specimens being subjected to greater than the worst case IPCC (2014) prediction of a reduction in pH by 0.4 units. Crella incrustans showed no detectable responses to changes in seawater pH, with survival curves showing no difference between controls and overall, mortality was extremely low (only one case per treatment). Tethya bergquistae showed a small, but significant decline in survival when exposed to seawater with lowered pH (7.6), and several individuals showed signs of physical degradation.

These results suggest that some sponge species may be able to tolerate decreasing pH with the onset of ocean acidification, although synergistic interactions with other environmental stressors may cause negative effects. Stress associated with increased seawater temperatures, however, may result in declining sponge populations in temperate rocky reef ecosystems.