Carbon flux on coral reefs:effects of large shifts in community structure

Abstract

The effect of replacement of live coral cover by epilithic algae on patterns and magnitudes of carbon flux is examined for the shallow front slope of a midshelf reef in the Great Barrier Reef (GBR) complex of Australia. A steady-state network of carbon exchange among 19 trophic compartments is constructed for the coral-dominated state. From this, 2 scenarios for patterns of carbon flux when algae dominate are derived, viz.(1) the increase in algal production is channeled to detrital pathways (grazers do not respond), and (2) grazers utilise the increase in production of algal carbon so that transfers to detritus and grazers are in the same proportion as occurs when coral cover is high. The 3 models summarise current knowledge of carbon flux on GBR reef fronts and are compared using network analysis. Because fluxes in the reef front zone are dominated by exogenous imports and exports as a result of the high volume of water passing around and over the reef, the analyses ignore advective fluxes across the zone that are not internalised.The shift in structure to an algae-dominated system realises lower rates of benthic primary production, and thus system slze and activity (i.e. total system throughput, internal throughput, development capacity and ascendancy) are reduced, suggesting a disturbed system. With loss of coral cover, the proportion of the total flow that is recycled and transferred to the detritus pool increases (although the structure of recycling is not affected), and the balance of pathways in the network is changed: average path length increases, while the average trophic level of most of the second order consumers, and trophic efiiciencies of most trophic categories, decreases. Also, there are marked changes in dependencies of particular trophic groups on others. The analysis shows that, in the coral-dominated state, carbon fixed by zooxanthellae is used indirectly by most organisms in the system, even those seemingly remotely connected. Differences between the coral-and algae-dominated systems were much greater than differences between the 2 scenarios for the algae-dominated state. However, the exact fate of additional algae-derived carbon In the system is an important consideration since the 2 scenarios for the algae-dominated state yielded dissimilar values for some parameters (e.g. flow diversity, trophic dependencies and effective trophic levels of some compartments, relative importance of recycling, trophic efficiency of some trophic categories).

Determining Natural Scales of Ecological Systems

“…In our systems where different length scales arise for different species, differential connectivity among species arises through spatial selforganizing. We anticipate that other factors which influence the connectivity among species, such as the topology of food webs in which some groups of species are tightly coupled trophically while others are poorly connected (e.g., O'Neill et al 1986, Johnson et al 1995, will have a similar effect. Because the approach is based on reconstruction of deterministic dynamics, it should not be surprising that species that are weakly linked dynamically can indicate different length scales for their different behaviors.…”

A boom–bust phylum? Ecological and evolutionary consequences of density variations in echinoderms

“…Once the tissue is removed by the sea star, coral skeletons are rapidly (in less than 24 hours [Belk and Belk 1975]) colonized by filamentous cyanobacteria and algae resulting in shifts in trophodynamic fluxes (Johnson et al 1995). Densities of several fish species feeding on corals decline after A. planci outbreaks (Williams 1986).…”

“…For marine ecologists, the most familiar examples include the changes from coral-to algal-dominated systems associated with the die-off of the tropical sea urchin Diadema in the Caribbean (Lessios 1988, Hughes 1994) and the transitions between temperate kelp forests and ''urchin barrens'' associated with extreme abundance fluctuations of herbivorous sea urchins (Elner andVadas 1990, Steneck et al 2004). Variations in predatory sea star populations are also associated with marked change in ecosystem structure, such as changes in carbon flow through the system and local extinction of prey species (Paine 1966, Johnson et al 1995, Witman et al 2003, Ross et al 2004.…”

Interactions between sediment-feeders and microalgae on coral reefs: grazing losses versus production enhancement

“…At lower grazing intensities (e.g. 1 S. chloronotus on 0.232 m 1984, Johnson et al 1995, Arias-Gonzáles et al 1997. The level of herbivory is high on coral reefs compared to other benthic systems and a large proportion of benthic primary productivity may be directly consumed by herbivores (Carpenter 1986, Klumpp & Polunin 1990.…”