Combined Effects of Experimental Acidification and Eutrophication on Reef Sponge Bioerosion Rates

Abstract

Health of tropical coral reefs depends largely on the balance between constructive (calcification and cementation) and destructive forces (mechanical-chemical degradation). Gradual increase in dissolved CO 2 and the resulting decrease in carbonate ion concentration ("ocean acidification") in ocean surface water may tip the balance toward net mass loss for many reefs. Enhanced nutrients and organic loading in surface waters ("eutrophication"), may increase the susceptibility of coral reef and near shore environments to ocean acidification. The impacts of these processes on coral calcification have been repeatedly reported, however the synergetic effects on bioerosion rates by sponges are poorly studied. Erosion by excavating sponges is achieved by a combination of chemical dissolution and mechanical chip removal. In this study, Cliona caribbaea, a photosymbiont-bearing excavating sponge widely distributed in Caribbean reef habitats, was exposed to a range of CO 2 concentrations, as well as different eutrophication levels. Total bioerosion rates, estimated from changes in buoyant weights over 1 week, increased significantly with pCO 2 but not with eutrophication. Observed chemical bioerosion rates were positively affected by both pCO 2 and eutrophication but no interaction was revealed. Net photosynthetic activity was enhanced with rising pCO 2 but not with increasing eutrophication levels. These results indicate that an increase in organic matter and nutrient renders sponge bioerosion less dependent on autotrophic products. At low and ambient pCO 2 , day-time chemical rates were ∼50% higher than those observed at night-time. A switch was observed in bioerosion under higher pCO 2 levels, with night-time chemical bioerosion rates becoming comparable or even higher than day-time rates. We suggest that the difference in rates between day and night at low and ambient pCO 2 indicates that the benefit of acquired energy from photosynthetic activity surpasses the positive effect of increased pCO 2 levels at night due to holobiont respiration. This implies that excavation must cost cellular energy, by processes, such as ATP usage for active Ca 2+ and/or active proton pumping. Additionally, competition for dissolved inorganic carbon species may occur between bioerosion and photosynthetic activity by the symbionts. Either way, the observed changing role of symbionts in bioerosion can be attributed to enhanced photosynthetic activity at high pCO 2 levels.

Nutrient pollution disrupts key ecosystem functions on coral reefs

“…To date, only one controlled laboratory study, focused on the Caribbean excavating sponge Cliona caribbae, has experimentally tested the impacts of nutrient pollution on bioerosion. The authors manipulated labile DOC for one week which subsequently drove increases in NH þ 4 and PO 32 4 , but kept NO 2 3 þ NO 2 2 stable: chemical bioerosion rates, but not total bioerosion (quantified as a change in buoyant weight), increased in their eutrophic treatments relative to controls [62]. Our study used different methods to manipulate nutrients, had a longer exposure time and also focused on a multi-species assemblage (coral rubble).…”

Quantification of chemical and mechanical bioerosion rates of six Caribbean excavating sponge species found on the coral reefs of Curaçao

“…Subsequently, all particulate material produced during the entire duration of the incubation (~1 h) was collected as a measure for mechanically removed substratum (mechanical erosion). Chemical and mechanical erosion of sponge infested and control cores was determined at day and night in the FT incubation because erosion rates of sponges, particularly symbiont-bearing species, were found to be affected by light availability (diurnal variation) [ 34 , 64 , 65 ]. Flow through incubations were run for all six sponge species.…”

“…This technique has now been widely adopted to quantify effects of environmental change and future climate scenarios on rates of bioerosion (e.g. [ 34 , 35 , 47 , 48 , 49 ]). A major disadvantage of this method, however, is that sponges are kept in a limited body of water for a relatively long time, which could strongly affect the metabolism of the sponge following food and oxygen depletion and the accumulation of waste products [ 50 ].…”

“…Indeed, periodic expelling of chips and/or contamination of filters by other material may induce error in the quantification of mechanical bioerosion (e.g. [ 34 , 35 ]). Evidently, the considerable variation in published sponge bioerosion rates stresses the need for consistency in applied procedures.…”

Peer Review #1 of "In-situ incubation of a coral patch for community-scale assessment of metabolic and chemical processes on a reef slope (v0.1)"

“…Furthermore, the integrated effects due to organismal interactions cannot be assessed with such an approach. For example, ocean acidification may reduce coral NCC (Andersson et al, 2012) and at the same time increase erosion rates by sponges (Fang et al, 2013;Webb et al 2017). Census approach has yet to include the role of several bioeroders (excavating sponges) (Murphy et al, 2016), which have been observed to become increasingly dominant on Caribbean reefs (Chaves-Fonnegra, Zea and Gómez, 2007).…”