Coral Reers (1994) 13: 199-201

Perspective

Reef metabolism and endo-upwelling in perspectiveG. W. Tribble I , M. J. Atkinson2, F. J. Sansone3 and S. V. Smith2 •3

1 US Geological Survey, 677 Ala Moana, Suite 415, Honolulu, HI 96813, USA2 Hawaii Institute of Marine Biology, PO Box 1346, Kaneohe, HI 96744, USAJ Department or Oceanography, University or Hawaii, Honolulu, HI 96822. USA

Accepted 24 February 1994

Coral Reefs© Springer-Verlag 1994

Rougerie and Wauthy (1993) have recently advanced theconcept of endo-upwelling to explain high rates of meta­bolism for oceanic coral reefs and atolls. The principalhypothesis advanced in this and previous papers (Rougerieand Wauthy 1986, 1988; Rougerie et al. 1992) is that geo­thermal heat, deep within the basalt and limestone under­lying Pacific island reefs, stimulates nutrient-rich AntarcticIntermediate Water at depths of several hundred metersto flow upward through the reef structure. The emergenceof this nutrient-rich water in the shallow surface water ofreefs is used to explain high rates of carbon fixation,commonly measured on reef flats. There are two problemswith this hypothesis, one of a general nature, and one thatis specific to the data presented: (I) productivity on reefsdoes not require a large supply of "exotic" nutrients, and(2) the data on nutrient concentration of interstitial waterare easily explained by the oxidation of organic matterwithin the reef structure and sediments.

Rougerie and Wauthy state that gross photosyntheticproduction of oceanic reefs (4-12 g C m - 2d - 1 or 330­1000 mmol C m - 2d - I) is paradoxical in light of open­ocean primary production rates of 0.1-1 g C m - 2d - I.They argue that the nutrients required to resolve thisapparent paradox come from deep water that is geo­thermally convected through the atoll framework to thesurface. Several recent reviews (Kinsey 1985; Smith 1988;Crossland et al. 1991) demonstrate that high photosyntheticproduction of reefs is nearly completely offset by high ratesof respiration, and that net production of coral reefs iscomparable to that found in oligotrophic plankton com­munities. Net production of coral reefs systems is very low,implying that a new source of nutrients is not required tosupport production.

Other studies have demonstrated that because reefautotrophs typically have high C: N: P ratios, reef commu­nities require a smaller quantity of nutrients than previouslysupposed to support the measured rates of production(Atkinson 1981, 1987, 1988, 1992; Atkinson and Bilger

Correspondence 10: G. W. Tribble

1992). Nutrient uptake is also positively correlated withwater velocity and few field measurements have been madeat high water velocities. Reef communities can take upmuch larger quantities of nutrients than previously mea­sured. Thus, we believe that high rates of gross photo­synthetic production are not a paradox, but are maintainedby nutrient removal from surface seawater, by nitrogenfixation, and by nutrients released by the high rates ofrespiration.

A comparison of the phosphate flux delivered byendo­upwelling with the flux delivered by horizontal flow ofsurface seawater puts these processes in perspective. Usingdata from Rougerie and Wauthy (1993), a fluid with a Pconcentration of I mmol m - 3 moving upward at a velocityof I cm d - 1 through a framework with an effective porosityof0.3 (assumed), will deliver 31lmol Pm - 2d - I. In contrast.Atkinson (1992) calculates an uptake of P from surfaceseawater moving horizontally across a coral reef commu­nity as I mmol m - 2d - I, and notes that this is only 4-5%of the total P flux delivered to the reef community. Thus,the total flux delivered by surface seawater is about20 mmol Pm - 2d - I, or nearly 4 orders of magnitudehigher than the endo-upwelling flux.

Rougerie and Wauthy (1993) argue that the highnutrient concentrations in reef interstitial waters providedirect evidence of endo-upwelling. This is not necessarilytrue, and the claim of Rougerie and Wauthy that "endo­genic production of nutrients as important as thosemeasured in these interstitial waters would have totallyexhausted the available oxygen" is not supported byexamination of their own data. Consider the data in theirTable I. By assuming that the loss of I mole of O 2corresponds to an oxidation of I mole of organic C, thedifferences in O 2 and nutrient concentrations betweenaverage interstitial water and surface ocean water can beused to estimate the CNP ratio of organic materialundergoing remineralization. These differences in Table Iare about 4 L m - 3 of 02' or 179 mmol O 2m - 3 (molarvolume of O 2 = 22.4 L mol-I); 4.4 mmol N m- 3 for N03plus NH 4 ; and 0.82 mmol Pm - 3 for P04 . Assuming nodifferential loss of N or P during diagenesis, these differ-

200

ences in concentrations represent organic material with aCNP ratio of 179:4.4:0.82, or dividing by the value for P,218:5.4: 1. This estimate falls between the range of C: N: Pratios for plankton (106: 16: I; Richards 1965) and tropicalmarine plants (550: 30: I; Atkinson and Smith 1983). Basedon the C: P ratio, the data also indicate that the organicmaterial may be mostly phytoplankton or zooxanthellae.The low N:P ratio and relatively high C:N ratio indicatesome loss of N during diagenesis via denitrification. Themain point of this discussion is that the observed increasesin nutrient concentration of interstitial water can easily beexplained by oxidation of organic maUer, which is consis­tent with the observed decrease in O 2 concentrations.

A similar analysis can be made using Fig. 4 of Rougerieand Wauthy (1993), which shows increases in both in­organic Nand P with depth into the reef framework atMururoa Atoll. An increase in N of 15 mmol m - 3 and Pof 0.6 mmol m - 3 can easily be explained by the oxidationof 125 mmol m- 3 of organic matter having a C:N:P ratioof 208: 17: 1. This amount of oxidation consumes onlyabout half of the O 2 dissolved in surface seawater, and isconsistent with levels measured at several other sites (seelater). Again, the point is that the high nutrient concen­trations can be explained by a process that is simpler andbetter documented than endo-upwelling. As such, thenutrient data presented by Rougerie and Wauthy do notnecessarily provide evidence for the endo-upwellinghypothesis.

In contrast with the hypothesis of endo-upwelling, theoxidation of organic matter within reefs is well establishedin a variety of environments. These include Davies Reef,Great Barrier Reef (Andrews and Muller 1983; Sansone1985; Buddemeier and Oberdorfer 1986; Oberdorfer andBuddemeier 1986), Tague Bay reef, St. Croix, USVI (Zankland Multer 1977; Roberts et al. 1988; Carter et al. 1989),and Checker Reef, Oahu, Hawaii (Sansone et al. 1988a, b;Tribble et al. 1988, 1990, 1992, 1993). Organic matteroxidation has also been shown to be prevalent in carbonateplatforms of both Florida and Hawaii (Sansone et al. 1990;Chanton et al. 1991; Martens et al. 1991), in the carbonatesediments of many reef environments (for exampleDiSalvo 1971; Hines and Lyons 1982; Entsch et al. 1983;Skyring 1985; Pigott and Land 1986; Sarazin et al. 1988;Walter and Burton 1990) and inside coral heads (Risk andMuller 1983). Thus, the importance of organic matteroxidation in interstitial marine waters of carbonatesystems is well documented. However, this process seemsto have been ignored by Rougerie and Wauthy.

We find the concept of endo-upwelling both interestingand insightful. Profiles of 3He (Rougerie et al. 199\),salinity, and chlorofluorocarbons (Andrie et al. in press)strongly support the validity of endo-upwelling as ageophysical process. It is possible that endo-upwelling isa significant factor in the long-term diagenesis of oldercarbonates (Buddemeier and Oberdorfer 1988). However,we remain unconvinced that (1) reefs require an exoticsource of nutrients to maintain themselves, and (2) highnutrient concentrations in interstitial water provide evi­dence that endo-upwelling is occurring at a rate that issignificant relative to reef metabolism. It may be thatendo-upwelling is important for reefs on the atolls studied

by Rougerie and Wauthy, but the data they present areinsufficient to prove its importance. Suggestions thatendo-upwelling is a general process that controls reefproductivity appear to be unjustified at this time.

Acknowledgements. We thank B. Buddemeier and S. Dollar forconstructive criticism of the manuscript. Aspects of this work weresupported by NOAA Sea Grant Program grant NA36RG0507 toUniversity of Hawaii Project No. R/ME-2 (to MJA) and US NationalScience Foundation grants OCE-8911150 and OCE-9116288 (toFJS). This is contribution No. 3594 of the School of Earth andOcean Sciences and Technology of the University of Hawaii.

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