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Anomalies in Coral Reef Community Metabolism and Their Potential Importance in the Reef CO2 Source-Sink Debate

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Anomalies in Coral Reef Community Metabolism and Their Potential Importance in the Reef CO2 Source-Sink Debate Proc. Natl. Acad. Sci. USA Vol. 95, pp. 6566–6569, May 1998 Population Biology Anomalies in coral reef community metabolism and their potential importance in the reef CO2 source-sink debate JOHN R. M. CHISHOLM* AND DAVID J. BARNES Australian Institute of Marine Science, Private Mail Bag No. 3, Mail Centre, Townsville, Queensland 4810, Australia Communicated by Andrew A. Benson, University of California, San Diego, CA, March 20, 1998 (received for review September 23, 1997) ABSTRACT It is not certain whether coral reefs are ratio of photosynthesis to respiration on unperturbed reefs sources of or sinks for atmospheric CO2. Air–sea exchange of over 24 h is considered to be close to unity (10). CO2 over reefs has been measured directly and inferred from In March 1996, we made an expedition to Lizard Island, changes in the seawater carbonate equilibrium. Such mea- northern Great Barrier Reef, Australia (Fig. 1), to measure surements have provided conflicting results. We provide changes in the O2 concentration and pH of seawater flowing community metabolic data that indicate that large changes in across a 300-m section of the reef flat by using a floating CO2 concentration can occur in coral reef waters via biogeo- instrument package (11–14). Measurements were made in chemical processes not directly associated with photosynthe- March when tides permitted the instrument package to float sis, respiration, calcification, and CaCO3 dissolution. These freely over the reef flat, a short distance above the benthos. processes can significantly distort estimates of reef calcifica- On arriving at Lizard Island, we encountered environmental tion and net productivity and obscure the contribution of coral conditions that we had not anticipated. Almost twice as much reefs to global air–sea exchange of CO2. They may, nonethe- rain had fallen at Lizard Island between January 1, 1996, and less, explain apparent anomalies in the metabolic perfor- March 14, 1996 (1,097 mm), as had fallen, on average, in each mance of reefs close to land and reconcile the differing of the 10 previous summers (January 1 to 31 March; mean rainfall 5 599 mm). Moreover, 42% of this rain fell in the 12 experimental findings that have given rise to the CO2 debate. days that preceded our measurements (Fig. 2A). The salinity of seawater around Lizard Island was 3‰ below normal on the There is debate as to whether coral reefs are sources of or sinks day before measurements began (i.e., 32‰), rising to 1‰ for atmospheric CO2 (1–4). They may take up about 2% of the below normal by the time measurements were completed (Fig. annual anthropogenic production of CO2 if they are sinks (5) 2B). Concurrence of unusually low tides and heavy rains had or they may release up to 8% if they are sources (6). Gross caused reef flat organisms to be directly exposed to freshwater productivity on coral reefs is among the highest for natural or to greatly reduced salinities. In addition, seawater temper- ecosystems (7) and photosynthesis by reef benthos encourages ature had been high, close to 30°C, over the previous 3 months invasion of CO2 from the atmosphere by reducing its concen- (Fig. 2B) and reef flat coral communities had been recently tration in overlying seawater. Respiration and formation of attacked by crown of thorns starfish. In consequence, live coral reef rock (calcification) have the reverse effect. Although cover on the reef flat was very low (0–10%) and there was a calcification decreases the overall concentration of inorganic considerable amount of dead coral overgrown by filamentous carbon in seawater, associated acidification, algae. 21 1 1 3 1 1 We began floating the instrument package across the reef Ca CO2 H2O CaCO3 2H , flat with the incoming tide shortly after the heavy rain ceased (Fig. 2A). The package was floated across the reef at various increases the amount present as dissolved (gaseous) CO . 2 times of the day and night to determine diel rates of benthic Measurements of organic and inorganic carbon metabolism photosynthesis, respiration, calcification, and solution of reef on different reefs have yielded conflicting data on the direc- rock. These parameters were estimated from equations for the tion of the net CO flux (1, 8). Metabolism on the Tiahura 2 seawater carbonate equilibrium (17) wherein DpH describes fringing barrier reef, Moorea, apparently released CO2 to the the total change in all CO2 species resulting from both organic atmosphere (8), whereas metabolism on Shiraho Reef, Ishi- and inorganic carbon metabolism and DO multiplied by the gaki Island, Japan, had apparently the reverse effect (1). Most 2 appropriate metabolic quotient describes the change in CO2 researchers consider that reefs are a source of CO2 (2, 3, 6, 8, due to organic metabolism (12). The advantage of this pH–O2 9). Critics of the Shiraho Reef study argue (i) that the reef must technique is that all necessary measurements can be made with have been dominated by noncalcareous algae that increased electrodes. Its disadvantage is that the photosynthetic and the ratio of organic to inorganic carbon metabolism, (ii) that respiratory quotients (PQ and RQ) must be used to convert the measurements were not representative of the whole reef, D D measurements of O2 to CO2. These quotients are estimated and (iii) that erroneous conclusions were drawn from inade- by adjusting their values until estimates of calcification at quate data (2, 3). Critics also suggest that insufficient mea- saturating light intensities and at night accord with total surements were taken to differentiate changes in the concen- alkalinity-based (TA) measurements (calcification 5 0.5DTA) tration of CO2 caused by benthic metabolism from natural (13). Accordingly, water samples for determination of TA were variability in the CO2 concentration of seawater flowing onto taken at 100-m intervals as the instrument package crossed the the reef (2). Overall, these criticisms follow from the general reef on two transects around noon and on two transects shortly view that coral reefs are sources of CO2. The net air–sea flux after dusk. of CO2 is thought to be controlled by calcification because the Subsequent analysis of data revealed that the changes in TA of seawater crossing the reef flat at Lizard Island were so large The publication costs of this article were defrayed in part by page charge that they provided unrealistic values for the metabolic quo- payment. This article must therefore be hereby marked ‘‘advertisement’’ in tients. We thus estimated organic and inorganic carbon me- accordance with 18 U.S.C. §1734 solely to indicate this fact. © 1998 by The National Academy of Sciences 0027-8424y98y956566-4$2.00y0 *To whom reprint requests should be addressed. e-mail: j.chisholm@ PNAS is available online at http:yywww.pnas.org. aims.gov.au. 6566 Downloaded by guest on September 25, 2021 Population Biology: Chisholm and Barnes Proc. Natl. Acad. Sci. USA 95 (1998) 6567 nities (10). Data given herein are for PQ 5 RQ 5 1.0. Use of PQ 5 1.1 and RQ 5 0.9 decreased hourly estimates of photosynthesis and respiration by #10% and peak rates of calcification by ,20%. The global standards for coral reef metabolism are gross productivity (P) 5 6–8gofCperm2 per ' 2 day and calcification (G) 3–5 kg of CaCO3 per m per year 5 2 (10). Our pH–O2 data gave P 9.3gofCperm per day and ' 2 G 9.3 kg of CaCO3 per m per year. Thus, our measurement of productivity was marginally high while that of calcification was about 2–3 times the rate expected on a healthy reef flat. Measurements of calcification from DTA gave a rate about 3–5 ' 2 times higher (G 15.1 kg of CaCO3 per m per year) than the suggested standard. Total calcification during daylight, estimated from pH–O2 2 data (41.0 g of CaCO3 per m ), was similar to that estimated D 2 from TA (36.8 g of CaCO3 per m ). However, the two techniques gave very different rates of nighttime calcification 2 (Fig. 3): pH–O2 data indicated solution of reef rock ( 15.5 g 2 of CaCO3 per m ), whereas TA data indicated net precipitation FIG. 1. Map of Lizard Island, adjacent land formations, and (4.7 g of CaCO per m2). surrounding reefs showing the location of the experimental transect. 3 (Inset) Location of Lizard Island relative to the Australian mainland Both techniques thus indicated that the impoverished (minimum distance 5 15 nautical miles); a detailed description of the heavily stressed reef flat at Lizard Island had a much higher reef system at Lizard island and of the benthic communities present on rate of calcification than pristine reefs growing in clear ocean the transect is provided elsewhere (11). water (10). Moreover, these estimates were made by using the same techniques that have fired the coral reef CO2 source-sink D D tabolism from measurements of O2 and pH by assuming debate. Measurements of coral reef CO2 dynamics are founded PQ 5 1.0 or 1.1 and RQ 5 0.9 or 1.0. These values encompass upon the assumption that changes in the inorganic carbon most previous estimates of PQ and RQ for coral reef commu- content of seawater over reefs are almost entirely due to photosynthesis, respiration, calcification, and solution of reef rock (18–21). Our results indicate that other biogeochemical processes can significantly alter the seawater carbonate equi- librium by affecting total alkalinity. The most likely of these is organic matter decomposition (22–24) that leads to the formation of phosphoric acid, ~ ! ~ ! ~ ! 1 3 CH2O x NH3 y H3PO4 z xO2 1 1 1 xCO2 xH2O yNH3 zH3PO4, FIG.
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