inorganic P concentrations during upstream-downstream experiments (Pilson and Betzer, 

 1973). In fact, DIN and dissolved organic N (DON) concentrations generally increase as 

 oceanic waters cross the reef, and a net export of N has been found for some reefs 

 (Johannes, et al., 1972; Webb, et al., 1975) . This implies a source of fixed N from within 

 the reef community, which can be attributed to N2 fixation by benthic blue-green algae and 

 N2 -fixing bacteria (Mague and Holm-Hansen, 1975; Burris, 1976; Capone, et al., 1977; 

 Wiebe, et al., 1975). Rates of N2 fixation on coral reefs have been found to exceed those of 

 alfalfa fields, the terrestrial community with the highest reported rates of N2 fixation. 

 These high rates have led many investigators to conclude that fixed N is plentiful on reefs, 

 and that concentrations of P must be controlling reef productivity. However, measurements 

 of N2 fixation generally have been restricted to shallow reef flats or back reef areas and 

 denitrification rates have not been adequately measured in any reef environment. Until 

 they are, any conclusions about N availability are premature. 

 Regenerated Nutrients ; 



It generally is believed that the main evolutionary adaptation to low nutrient conditions 

 in reef environments has been the evolution of relationships that lead to efficient recycling 

 of nutrients. The foremost example of this type of relationship is the endosymbiosis 

 between algae and invertebrates. Present day coral reefs are physically dominated by a 

 variety of orders and classes of coelenterates and virtually all of them have symbiotic 

 dinoflagellates (zooxanthellae) in their tissues. It has been repeatedly demonstrated that 

 these animals do not excrete waste products as do other nonsymbiotic animals (Kawaguti, 

 1953; Pomeroy and Kuenzler, 1969) and that there is even a measurable uptake of dissolved 

 nutrients by them attributable to the presence of the algae (see earlier references). Other 

 invertebrate groups, including sponges, molluscs and ascidians, also have some species with 

 algal symbionts. This form of recycling is the most efficient possible (often 100%) as the 

 nutrients are available to the algae in concentrated form. It should cost the algae much less 

 energy to take up the nutrients they need from a concentrated source than to take them up 

 once they had been excreted and diluted. 



As important as this type of relationship may be, there are still many groups of algae and 

 many more of vertebrates and invertebrates living on the reef that are not involved in 

 endosymbiotic relationships. These plants and animals need nutrients and excrete nutrients, 

 respectively, and depending on their biomass these fluxes may be quite large. There are few 

 estimates of the absolute and/or relative biomasses of reef organisms. Odum and Odum 

 (1955) found that zooxanthellae make up roughly 15% of the biomass of primary production 

 on a Enewetak reef flat and that coral polyps make up about 50% of the biomass of 

 consumers. However, they had very poor estimates for the biomass of reef fishes and their 

 samples were from the reef flat, which is topographically and biologically less diverse than 

 most forereef areas. Measurements of biomass on a Caribbean fore reef (Szmant-Froelich, 

 1972) show a greater percentage of the algal biomass to be made up of zooxanthellae and a 

 smaller percentage of the consumer biomass to be made up of symbiotic coelenterates. In 

 order for the reef as a whole to be efficient in recycling nutrients, there must be 

 mechanisms for recycling nutrients among these free-living plants and animals. The main 

 problem arises when one considers that the same high water flow over the reef that assures 

 a large source of low-nutrient oceanic water also assures that any nutrients excreted into 

 the water by animals will be rapidly diluted and carried away. Therefore, what is needed is 

 a mechanism to prevent dilution and loss. 



I would like to bring attention here to a little studied mechanism, that of particle 

 entrapment and nutrient regeneration within the reef framework. 



REGENERATIVE SPACES 



Coral reefs are riddled with holes and tunnels of all sizes. From 50 to 75% of the reef 

 volume can be made up of these voids (Ginsburg, 1983). These holes contain varying 

 amounts of sediment which comes from a variety of sources, including carbonate sediments 



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