FUNCTIONAL ASPECTS OF NUTRIENT CYCLING ON CORAL REEFS 



Alina Szmant Froelich 



Rosenstiel School of Marine and Atmospheric Science 



University of Miami 



4600 Rickenbacker Causeway 



Miami, Florida 33149 



ABSTRACT 



Coral reef waters are generally very low in nutrients, yet benthic productivity is among 

 the highest in the world. A general concern of reef ecologists has been to identify the 

 sources of nutrients for reef productivity and the processes responsible for nutrient 

 recycling on reefs. One potentially important process that has been poorly studied is the 

 recycling of regenerated nutrients from sediments and feces trapped in reef holes and 

 crevices. These nutrients, if available to the benthos in concentrated form, could have a 

 major influence on reef productivity. 



INTRODUCTION 



Coral reef communities are extremely diverse and variable, and we should not expect 

 that the relationships between reef organisms or the dynamics of the system to be simple to 

 understand. On the other hand, by analyzing these complex systems in terms of functional 

 groups, we may gain a better understanding of important processes, and identify research 

 areas that need our attention. In this paper, I will try to identify processes and functional 

 groups involved in nutrient dynamics on coral reefs. 



Understanding nutrient dynamics on coral reefs is important because, as in any other 

 ecosystem, primary production is controlled by the availability of nutrients, and primary 

 production is one of the main factors that determines community biomass and secondary 

 production. 



Coral reefs occur in tropical areas, where oceanic waters are generally very low in 

 nutrients, yet these complex ecosystems have some of the highest biomasses and 

 productivity that have been reported (Lewis, 1977). This dilemma can been termed the 

 'paradox of the coral reef. There has been a great interest in identifying mechanisms that 

 could explain this paradox. Two important ones that have been well studied are the 

 recycling of nutrients between algal/invertebrate symbionts and the fixation of N 2 by blue- 

 green algae and bacteria. Another approach has been to identify additional sources of 

 dissolved nutrients, such as from ground water seepage or upwelling of nutrient enriched 

 deepwater. A final approach has been to examine the reef ecosystem for structural and 

 functional properties that lead to enhanced conservation of nutrients. This is the approach 

 that will be emphasized here. 



SOURCES OF NUTRIENTS 



There are two general sources of nutrients for primary production: New nutrients and 

 regenerated (recycled) nutrients. If only regenerated nutrients were available, gross 

 photosynthesis could not excede respiration (i.e., P/R ratios could not excede 1.0) unless 

 there was a change in elemental ratios (e.g., an increase in the C/N and C/P ratios). Net 

 production (growth) requires the input of new nutrients into the system, as does net export 

 from the system. It is important to point out that nutrient recycling mechanisms, even 

 when 100% efficient, cannot supply nutrients for a positive net production. Furthermore, if 

 recycling mechanisms are inefficient it will take an input of new nutrients to maintain a 

 steady state biomass. 



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