New Nutrients : 



New nutrients can enter reef systems from both terrestrial and oceanic sources and, in 

 the case of nitrogen, by in situ N 2 fixation. Nutrient input to reefs from terrestrial sources 

 remain poorly studied. However, we can make several general statements. Coral reefs 

 located off the coasts of high volcanic islands or on continental shelves may receive a 

 considerable supply of nutrients via terrestrial runoff (Marsh, 1977). However, coastal 

 areas with high runoff are usually devoid of coral reefs unless estuarine systems, such as 

 mangrove forests, trap the nutrients near shore. It is not clear at this time whether the 

 negative effect of runoff on reef development is due to siltation stress, salinity stress or 

 nutrient stress (in the latter case, high nutrient concentrations can promote high 

 phytoplankton and macroalgal growth rates, which in turn can result in unfavorable 

 environmental conditions for reef-building corals). Most likely, all three stresses contribute 

 to the effect. In any case, the on-shore current patterns that generally dominate coral reef 

 circulation should limit the amount of terrestrial runoff that reaches most offshore reefs. 

 One interesting phenomenon that has been reported recently is the submarine seepage of 

 NO.j enriched fresh water inshore of a fringing reef (D'Elia, et al., 1981; Johannes, 1980). It 

 is unlikely that this kind of seepage will reach offshore reefs^ and there is little or no 

 information on how frequently this phenomenon occurs. Reefs located offshore of low 

 carbonate islands should receive little input of terrestrial nutrients because runoff from 

 these terrains are usually low in nutrients unless fertilizer has been applied for agricultural 

 purposes. Atoll reefs generally have no significant land masses nearby from which to 

 receive terrestrial runoff. 



Oceanic sources of nutrients will be dependent on the concentration of nutrients in the 

 source water, the rate of flow of water over the reef, and the ability of primary producers 

 to take up the nutrients at the given concentration. Nutrient concentrations in tropical 

 oceanic waters are generally near limits of detection, but there are reports that upwelling 

 may occasionally result in higher concentrations (Thompson and Golding, 1981; Andrews and 

 Gentien, 1982). The rates of water flow over the reef are high, and several species of reef 

 coral have been found to be able to take up nutrients from these low concentrations 

 (Franzisket, 1974; D'Elia, 1977; D'Elia and Webb, 1977; Webb and Wiebe, 1978). Atkinson 

 (1981) has estimated that there was a sufficient uptake of dissolved phosphorus from water 

 crossing the reef flat to account for the measured primary production (&C>2) if a Redfield 

 ratio (C:P) of 490 was assumed (the oceanic C:P ratio is 106). He also found that marine 

 algae from a variety of sources had C:N:P ratios much higher than the Redfield ratio 

 (Atkinson and Smith, 1983), a possible indication of an evolutionary adaptation to low 

 nutrient conditions. An alternative explanation is that macroalgae need a greater amount of 

 C-rich structural material than phytoplankton, which results in higher C:N:P ratios for the 

 former. It should be instructive to measure the C:N:P ratio of other reef organisms, 

 especially the microcrustaceans, which have body structures similar to planktonic 

 organisms. 



In any case, a quick calculation shows that for the following average conditions: 

 dissolved inorganic nitrogen (DIN) concentrations in the range of 0.5 to 1.0 /jM, currrent 

 velocities in the order of 0.06 to 0.50 cm/s, annual gross productivity of 3,220 gC • m -2 . y- 

 1, P/R ratio of 1.4 and net productivity of 950 gC • m-2 . y-1, the first m2 of reef substrate 

 to make contact with oceanic water would need to strip out 0.08 to 1.3% of the DIN to 

 support net production and maintain a C:N of 6.6. This assumes a mixed layer 1 m deep; 

 twice that percentage would have to be taken up if the organisms can only strip nutrients 

 from the bottom-most 0.5 m of water. Uptake of nutrients to support all of gross 

 production (i.e., if no recycling occurred) would require stripping somewhere between 0.25 

 to 8.5% of DIN. This means that for many reef flats, where the water column is usually less 

 than a meter deep, waters flowing over downstream areas may be significantly depleted in 

 nutrients. In fore-reef zones, vertical mixing should prevent this type of depletion from 

 being as important, but it is still apparent that at low concentrations and at slow current 

 velocities it is unlikely that uptake of new oceanic nutrients could account for much more 

 than net productivity. Otherwise, we should see a much greater drop in DIN and dissolved 



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