Abstract
Current research on coral reefs seeks to link the responses to anthropogenic stressors (such as global warming and ocean acidification [OA]) among differing functional levels of biological organization. While experimental studies have identified ex situ taxon-specific responses to OA and global warming, isolating and connecting these effects in situ at the community-level has proved difficult. The difficulties arise from the large number of naturally varying parameters affecting corals reefs, such as light intensity and seawater residence time that affect net community production and calcification. To control variation in seawater residence time and allow light intensity to vary naturally, experimental outer reef (17-m depth) benthic communities composed of calcified algae, corals, and reef pavement were constructed in large outdoor flumes in Mo’orea, French Polynesia. Net community production (P), net community calcification (G), the ratio of P/G (P/Gratio), and slope of P regressed on G (P/Gslope) were calculated for the communities, and concurrently for the constituent members under the same temperature, light, and flow conditions. P and G, for both the communities and constituent members, were correlated positively with light intensity, whereas P/Gratio and P/Gslope were unaffected by light intensity. P/Gratios and P/Gslopes exhibited values that were specific to each community member. These results suggest that the P/Gratio and P/Gslope may be unaffected by natural variability in light intensity and could serve as useful metrics to relate responses at the taxon and community level, which is an important step in assessing the effects of environmental changes on coral reefs.
Highlights
The excess dissolution of anthropogenic CO2 emissions into the global surface oceans, described as Ocean Acidification (OA), is decreasing surface seawater pH at 0.0017 units y−1 (Clarke et al, 2014), and threatens to impair physiological processes of marine organisms (Gattuso et al, 2015)
Constituent members in the flume communities exhibited a G of 3.2 ± 2.7 mmol CaCO3 m−2 h−1, which was highest for P. verrucosa (6.9 ± 0.5 mmol CaCO3 m−2 h−1), and lowest for P. onkodes (0.7 ± 0.1 mmol CaCO3 m−2 h−1)
Correlations between the P/Gratio and PAR revealed that P and G were positively correlated with PAR, which has been shown in most previous studies of these relationships (Chalker and Taylor, 1978; Barnes and Chalker, 1990; Gattuso et al, 1993)
Summary
The excess dissolution of anthropogenic CO2 emissions into the global surface oceans, described as Ocean Acidification (OA), is decreasing surface seawater pH at 0.0017 units y−1 (Clarke et al, 2014), and threatens to impair physiological processes (e.g., calcification) of marine organisms (Gattuso et al, 2015). Anthropogenic CO2 is warming the planet, a process referred to as Organisms Exhibit Taxon-Specific P:G Relationships global warming, which is predicted to increase sea surface temperature (SST) 1.2–3.2◦C by the end of the century (Clarke et al, 2014) and may decrease calcification in marine organisms (Hoegh-Guldberg et al, 2007). Seawater warming studies performed ex situ at the organismal scale (e.g., Reynaud et al, 2003; Comeau et al, 2016a) show that G and P in tropical marine calcifiers is enhanced until a threshold temperature (∼28◦C in many organisms, but highly site specific; Pratchett et al, 2015) and dramatically decreases thereafter due, in part, to bleaching or mortality (Hughes et al, 2017). Considering that many tropical reefs currently experience temperatures >28◦C during the summer (Atkinson, 2011), there are concerns that even the minimum warming-driven increase in SST of 1.2◦C will decrease G and P in tropical marine calcifiers by the end of this century (Hughes et al, 2017)
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