Organisms Composing an Experimental Coral Reef Community from Mo'orea, French Polynesia, Exhibit Taxon-Specific Net Production: Net Calcification Ratios

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/G ratio ), and slope of P regressed on G (P/G slope ) 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/G ratio and P/G slope were unaffected by light intensity. P/G ratios and P/G slopes exhibited values that were specific to each community member. These results suggest that the P/G ratio and P/G slope 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.

Taking the metabolic pulse of the world’s coral reefs

“…This adds insight into the lack of correlation between average ΔTA and TA-DIC slopes from the in situ data ( S2 Fig ), and indicates that reef sites with similar TA-DIC slopes may have different benthic community composition with different calcification capacities. Alternatively, recent work from Mo’orea has also shown TA-DIC slopes to be taxa-specific, suggesting that a community’s TA-DIC slope may not only be dependent on the overall percent cover of calcifiers, but also the relative abundance and diversity of these calcifiers [ 35 ]. On an ecosystem scale, TA-DIC slopes reveal the relative balance between NCP and NCC, and observations over time at a single reef location will be useful for detecting local changes in the percent cover of calcifiers and shifts in biogeochemistry due to environmental perturbations.…”

“…Another potential metric of reef biogeochemistry derived from TA and DIC measurements is the slope of TA-DIC regressions, which reflects the balance of NCP and NCC, such that in a system dominated by NCP the slope approaches 0 while in a system dominated by NCC the slope approaches 2. Because of these properties, graphical vector analysis of seawater TA and DIC (e.g., the TA-DIC slope or ΔTA:ΔDIC) quantifies the relative balance of CaCO 3 to organic carbon fluxes [ 29 , 32 , 33 ], which can directly relate to properties such as community composition (e.g., calcifying vs. non-calcifying organisms) and net reef metabolic status ( Fig 1 ) [ 34 , 35 ]. The balance of NCP and NCC also reflects how a reef modifies seawater pH, and thus, whether net reef metabolism elevates or suppresses reef pH relative to the open ocean.…”

Coral Reef Carbonate Chemistry Variability at Different Functional Scales

“…Higher slopes in TA vs. DIC indicate a higher NCC:NCP , and can be calculated using a simple equation (Koweek et al, 2015). Distinct TA-DIC relationships have been observed for well-defined communities in mesocosms with larger slopes for calcifying (e.g., corals, sediments, and calcifying algae) relative to non-calcifying communities (e.g., fleshy algae), and intermediate slopes for mixed communities of calcifiers and noncalcifiers Page et al, 2016;Lantz et al, 2017). However, it is not clear at what spatiotemporal scales TA and DIC samples should be collected in order to obtain a robust estimate of NCC:NCP in natural environments.…”

“…Hog Reef has a higher abundance of calcifying organisms relative to Bailey's Bay, implying there should be a higher NCC:NCP at Hog Reef (Page et al, 2016;Lantz et al, 2017). As expected, the TA-DIC slope was higher at Hog reef (0.35; NCC:NCP = 0.21) relative to Bailey's Bay (0.20; NCC:NCP = 0.10), most likely explained by the different benthic communities between the two sites (Figure 6).…”

Sea surface carbonate dynamics at reefs of Bolinao, Philippines: Seasonal variation and fish mariculture-induced forcing

“…The magnitude of diel fluctuations in reef carbonate chemistry is dependent on the relative balance of community metabolism (NCP and NCC), metabolic rates, depth, and residence time (Takeshita et al, 2018). The extent to which community metabolism can influence carbonate chemistry is dependent on benthic community composition, organismal abundances, health of organisms, and individual metabolic rates, especially in shallow coral reefs where smaller water volumes lead to more pronounced carbonate chemistry alterations (Koweek et al, 2014;Page et al, 2016;Lantz et al, 2017). In Bolinao, benthic composition among the three reefs could potentially contribute to the observed differences in community metabolism, as implied in the diel variability of carbonate chemistry and further supported here by nTA-nDIC relationships.…”