Stable isotope analyses are widely used to determine trophic levels in ecological studies. We have investigated the effects of carbonate removal via acidification on the stable carbon and nitrogen isotopic composition of 33 species of tropical benthic macrofauna, and we report guidelines for standardizing this procedure for higher taxa in tropical coral reef ecosystems. Many tropical benthic invertebrates are small in size, and therefore body tissue isolation (separating organic carbon from inorganic structures) is difficult and time-consuming. Literature reviews of invertebrate studies show a lack of consistent procedures and guidelines for preparation techniques, especially for carbonate removal via acidification of whole individuals. We find that acidification decreases the delta(13)C values of samples containing carbonate, with shifts ranging from 0.21 to 3.20 per thousand, which can be related to CaCO(3) content (assessed by a carbonate proxy), justifying acid pre-treatment. Carbonate-containing taxa benefiting from acidification included Amphinomida, Terebellida (Annelida), Anomura, Brachyura, Caridea, Amphipoda, Tanaidacea (Arthropoda) and Edwardsiida (Cnidaria). The delta(13)C shifts of samples containing no carbonate varied up to 0.02 +/- 0.20 per thousand. As this induced delta(13)C shift was lower than the range of an average trophic level shift (0.5 to 1 per thousand), we conclude that acid pre-treatment is unnecessary. Carbonate-free taxa consisted of Eunicida, Phyllodocida (Annelida) and Mollusca. We note minimal impact of acidification on delta(15)N values except for Brachyura, which showed a shift of 0.83 +/- 0.46 per thousand, which is still lower than a single trophic level shift (2.9-3.8 per thousand). We conclude that for trophic level studies, both the delta(13)C and the delta(15)N of carbonate-rich macrofauna can be determined from the same acidified sample.

Productivity, nutrient input, nutrient uptake, and release rates were determined for a coral-dominated reef flat at La Réunion, France, to assess the influence of groundwater nitrogen on carbon and nutrient budgets. Water samples were collected offshore in the ocean, at the reef crest and back reef for nutrients, picoplankton, pH, and total alkalinity. Volume transport of ocean water across the reef flat was measured using both current meters and drogues. Groundwater advected onto the reef flat and mixed with incoming ocean water. Metabolic rates for the reef community were determined to be: gross primary production = 1,000 mmol C m−2 d−1, community respiration = 960 mmol C m−2 d−1, and community calcification = 210 mmol C m−2 d−1. Across the reef flat, silicate behaved conservatively, there was net uptake of phosphate (0.06 mmol P m−2 d−1) and net release of nitrate, ammonia, dissolved and particulate organic nitrogen (total 7.0 mmol N m−2 d−1). Groundwater nitrate contributed 37% of the increase in nitrate plus ammonia. The first-order mass transfer coefficient of phosphate was 3.3 m d−1, and for nitrate plus ammonia, 5.9 m d−1. Gross N and P uptake from estimates of mass transfer and uptake of particles were 0.37 mmol P m−2 d−1 and 7.2 mmol N m−2 d−1, respectively giving an N:P uptake ratio of 20:1. Thus, the elevation of nitrogen across the reef flat maintains a high N:P flux, enhancing algal growth downstream of the transect. We conclude that net community production (40 mmol C m−2 d−1) was sustained by net uptake of phosphate from the ocean and net uptake of new nitrogen from groundwater

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The benthic ecosystem of the lagoon surrounding Tahiti, the most populated island of French Polynesia, was investigated to assess the impacts of terrestrial runoff on these benthlc cornmunities. Five lagoonal zones based on population densities around the coast of Tahti were identified, and within each zone a transect from the fringing reef to the barrier reef was sampled, a total of 18 stations.Only large macrofauna collected on a 2 mm sieve were considered in this study. Multivariate analysis uslng total biomass and environmental factors showed that the stations formed 3 main groups which were related to sediment characteristics, including percentage of silt, organic matter and phaeop~g-ment levels. The distribution of the major feeding groups was related to the amounts of terrestrial inputs and distance from the shore. The stations on the barrier reef and those in zones adjacent to low population areas were not impacted by these terrestrial inputs. Deposit-feeding communities of capitellid polychaetes were dominant in the channel parts of the lagoon, which acted as decanting ponds. Chaetopterid polychaetes played an important role in recycling sediments of terrigenous origin in the fringing ecosystem. The patterns of diversity, density and biomass of the benthos around the lagoon revealed that some areas were impacted by moderate terrigenous inputs. It appears that the intermediate disturbance hypothesis explains the functioning of the parts of the benthic lagoonal ecosystem which are subjected to human impact. The organic enrichment by terrestrial inputs contrasts with food limitation that appears to occur in the non-dsturbed areas. Despite high organic loads, the benthic communities present at the harbour station were not depauperate, as bioturbation by burrowing alpheids and callianassids prevented anoxic conditions from developing.

This review presents environmental and biological indicators of the impact of three major categories of inputs in coral reef lagoons i.e. particles, nutrients and metals. Information was synthesized to extract well established indicators together with some interesting new concepts currently under development, and to provide the reader with an assessment of their respective advantages and drawbacks. The paper has been organized according to the capacity of three categories of indicators to respond either in a specific or a non specific way to a given source of input. The first section focuses on abiotic indicators which main interest is to respond instantaneously and in a truly specific way to a given source of input. The second and third sections present informations on bioindicators either at the sub-individual level or at the individual to community level, indicator specificity generally decreasing as a direct function of biological or ecological complexity. This review showed that even though significant work has already been done on coral reef ecosystems, much more scientific studies are still needed to answer the growing local demands for simple and truly validated tools to be used in environmental surveys. It is further stressed that, due to the biological and environmental diversity of coral reef lagoons, a preliminary step of on-site validation must be considered as an absolute prerequisite when indicators are planned to be used in the frame of a local environmental monitoring programme.