Mireille Harmelin-Vivien

Rare species support vulnerable functions in high-diversity ecosystems

The most unusual, and thus irreplaceable, functions performed by species in three different species-rich ecosystems are fulfilled by only the rare species in these ecosystems.

Trophodynamic linkage between river runoff and coastal fishery yield elucidated by stable isotope data in the Gulf of Lions (NW Mediterranean)

The link between climate-driven river runoff and sole fishery yields observed in the Gulf of Lions (NW Mediterranean) was analysed using carbon- and nitrogen stable isotopes along the flatfish food webs. Off the Rhone River, the main terrestrial (river POM) and marine (seawater POM) sources of carbon differed in δ13C (–26.11‰ and −22.36‰, respectively). Surface sediment and suspended POM in plume water exhibited low δ13C (–24.38‰ and −24.70‰, respectively) that differed more from the seawater POM than from river POM, demonstrating the dominance of terrestrial material in those carbon pools. Benthic invertebrates showed a wide range in δ15N (mean 4.30‰ to 9.77‰) and δ13C (mean −23.81‰ to −18.47‰), suggesting different trophic levels, diets and organic sources. Among the macroinvertebrates, the surface (mean δ13C −23.71‰) and subsurface (mean δ13C −23.81‰) deposit-feeding polychaetes were particularly 13C depleted, indicating that their carbon was mainly derived from terrestrial material. In flatfish, δ15N (mean 9.42 to 10.93‰) and δ13C (mean −19.95‰ to −17.69‰) varied among species, indicating differences in food source and terrestrial POM use. A significant negative correlation was observed between the percentage by weight of polychaetes in the diet and the δ13C of flatfish white muscle. Solea solea (the main polychaete feeder) had the lowest mean δ13C, Arnoglossus laterna and Buglossidium luteum (crustacean, mollusc and polychaete feeders) had intermediate values, and Solea impar (mollusc feeder) and Citharus linguatula (crustacean and fish feeder) exhibited the highest δ13C. Two different benthic food webs were thus identified off the Rhone River, one based on marine planktonic carbon and the other on the terrestrial POM carried by the river. Deposit-feeding polychaetes were responsible for the main transfer of terrestrial POM to upper trophic levels, linking sole population dynamics to river runoff fluctuations.

Temporal variations in the diet of the damselfish Stegastes nigricans (Lacepède) on a Réunion fringing reef

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. The diet of the territorial damselfish Stegastes nigricans (Pomacentridae) was studied on a shallow coral reef flat of Reunion Island, S.W. Indian Ocean. Fishes were collected monthly from November 1989 to August 1990 in order to examine their stomach and gut contents. The temporal variability of the diet of S. nigricans was investigated on (1) a seasonal scale for the various size-classes and (2) a daily scale within two seasons. On average, algae accounted for 69.4% of the total weight of the stomach contents, all sizes combined. The total weight of all prey, and both the weight and the percentage of algae in stomach contents decreased in winter. This pattern was more pronounced for large (>8 cm total length) and medium (4-8 cm) than for small individuals (<4 cm). Small S. nigricans also fed on invertebrates such as polychaetes, ascidians and copepods, whereas medium and large individuals had a very low invertebrate consumption. Daily variations of diet were recorded, with two maxima of food intake in summer: between 0600 and 0930 and between 1430 and 1900. In winter, only one peak of consumption was observed between 1200 and 1700. An estimate of food intake of 2488 g.m −2 .year −1 (dry weight) for the S. nigricans population was determined from stomach and intestine contents. The seasonal variation in the diet of S. nigricans may be related to several interacting factors, such as availability of food resources, high rate of consumption of food in summer for energy storage to offset the lean winter period, and high energetic needs for summer reproductive activities. The ontogenic shift in diet appeared to be a response of small individuals to both a transition phase between larval and benthic phases, and a need for protein-rich prey in order to promote their growth. The daily variation in the diet is probably due to diurnal variations in the nutritional value and/or palatability of algal food.

Differential biomagnification of PCB, PBDE, Hg and Radiocesium in the food web of the European hake from the NW Mediterranean

Consumption of marine organisms represents one of the main exposure sources of contaminants for human populations. To obtain a global view of the contamination in commercial fish in the NW Mediterranean Sea, we analysed four types of priority contaminants (PCBs, PBDEs, Hg and (137)Cs) in the European hake, Merluccius merluccius, from the Gulf of Lions in relation with organisms trophic level (δ(15)N). All contaminants presented a significant increase in concentration in hake muscle with trophic level. However, obvious differences between contaminants were evidenced. Biomagnification factors (BMF and FWMF) along the hake food web were higher for Hg and CB-153 than for BDE-47 and (137)Cs, and increase in contaminant concentration with trophic level occurred at different rates depending on contaminants. Such differences of biomagnification patterns can be related to physico-chemical properties of the different contaminants.

Impact of hurricane Firinga on fish community structure on fringing reefs of Reunion Island, S.W. Indian Ocean

SynopsisThe reef flats of Saint-Pierre and Saint-Leu (Reunion Island, Indian Ocean) suffered badly from hurricane Firinga on 29 January 1989. The high degree of silting due to increased run-off killed the coral colonies. Fish communities were surveyed at four periods following the hurricane (March and September 1989, March and September 1990). An increase in both species richness (31 to 47 spp. per census) and abundance (169 to 265 individuals per census) of fishes was observed with time, along with changes in their trophic structure. This positive succession may be linked to the disappearance of the silt layer from reef flats since September 1989. Nevertheless, there were differences in fluctuations and trophic structure of the fish community between back-, inner- and outer-reef flats. Finally, differences in recovery between the two reefs are related to the overall environmental degradation of the Island, chiefly by human perturbation, prior to the hurricane.

An ecosystem-based approach to assess the status of a Mediterranean ecosystem, the Posidonia oceanica seagrass meadow.

Biotic indices, which reflect the quality of the environment, are widely used in the marine realm. Sometimes, key species or ecosystem engineers are selected for this purpose. This is the case of the Mediterranean seagrass Posidonia oceanica, widely used as a biological quality element in the context of the European Union Water Framework Directive (WFD). The good quality of a water body and the apparent health of a species, whether or not an ecosystem engineer such as P. oceanica, is not always indicative of the good structure and functioning of the whole ecosystem. A key point of the recent Marine Strategy Framework Directive (MSFD) is the ecosystem-based approach. Here, on the basis of a simplified conceptual model of the P. oceanica ecosystem, we have proposed an ecosystem-based index of the quality of its functioning, compliant with the MSFD requirements. This index (EBQI) is based upon a set of representative functional compartments, the weighting of these compartments and the assessment of the quality of each compartment by comparison of a supposed baseline. The index well discriminated 17 sites in the north-western Mediterranean (French Riviera, Provence, Corsica, Catalonia and Balearic Islands) covering a wide range of human pressure levels. The strong points of the EBQI are that it is easy to implement, non-destructive, relatively robust, according to the selection of the compartments and to their weighting, and associated with confidence indices that indicate possible weakness and biases and therefore the need for further field data acquisition.

Spatio-temporal variation of suspended and sedimentary organic matter quality in the Bay of Marseilles (NW Mediterranean) assessed by biochemical and isotopic analyses.

Isotopic and biochemical features of suspended particulate organic matter (POM) in the water column and of sedimentary organic matter (SOM) were investigated seasonally in the Bay of Marseilles. Biochemical compounds (carbohydrates, lipids and proteins) were consistently more concentrated in POM than in SOM, with SOM mainly composed of insoluble carbohydrates. POM displayed lower δ(13)C and higher δ(15)N values than SOM. Phytoplanktonic production represented the major contributor of POM year-round with spatial and seasonal variations. Climatic parameters and wind-induced currents created differences in POM contributions, with more important inputs of terrestrial OM at one sampling site. Spatial and seasonal variations were lower for SOM. The composition of this pool appeared to be linked with the permanent inputs of phytoplankton and Posidonia oceanica detritus. The combined use of biochemical and isotopic analyses was a useful tool to characterize OM pools and would help understanding the trophic functioning of this coastal environment.

Response of Rocky Reef Top Predators (Serranidae: Epinephelinae) in and Around Marine Protected Areas in the Western Mediterranean Sea

Groupers species are extremely vulnerable to overfishing and many species are threatened worldwide. In recent decades, Mediterranean groupers experienced dramatic population declines. Marine protected areas (MPAs) can protect populations inside their boundaries and provide individuals to adjacent fishing areas through the process of spillover and larval export. This study aims to evaluate the effectiveness of six marine reserves in the Western Mediterranean Sea to protect the populations of three species of grouper, Epinephelus marginatus, Epinephelus costae and Mycteroperca rubra, and to understand in which circumstances MPAs are able to export biomass to neighbouring areas. All the studied MPAs, except one where no grouper was observed, were able to maintain high abundance, biomass and mean weight of groupers. Size classes were more evenly distributed inside than outside MPAs. In two reserves, biomass gradients could be detected through the boundaries of the reserve as an indication of spillover. In some cases, habitat structure appeared to exert a great influence on grouper abundance, biomass and mean individual weight, influencing the gradient shape. Because groupers are generally sedentary animals with a small home range, we suggest that biomass gradients could only occur where groupers attain sufficient abundance inside MPA limits, indicating a strongly density-dependent process.

Identifying carbon sources and trophic position of coral reef fishes using diet and stable isotope (δ15N and δ13C) analyses in two contrasted bays in Moorea, French Polynesia

Stable isotope ratios (δ15N and δ13C) and diet of three fish species, Stegastes nigricans, Chaetodon citrinellus and Epinephelus merra, were analyzed on the fringing coral reefs of two bays that are differentially exposed to river runoff on Moorea Island, French Polynesia. S. nigricans and C. citrinellus relied mostly on turf algae and presented similar trophic levels and δ15N values, whereas E. merra fed on large invertebrates (crabs and shrimps) and had higher trophic levels and δ15N values. Discrepancies existed between stomach content and stable isotope analyses for the relative importance of food items. Bayesian mixing models indicated that sedimented organic matter was also an important additional food for S. nigricans and C. citrinellus, and fishes for E. merra. The main sources of organic matter involved in the food webs ending with these species were algal turfs and surface sediments, while water particulate organic matter was barely used. Significant spatial differences in C and N isotopic ratios for sources and fishes were found within and between bays. Lower 13C and higher 15N values were observed for various compartments of the studied trophic network at the end of each bay than at the entrance. Differences were observed between bays, with organic sources and consumers being, on average, slightly more 13C-depleted and 15N-enriched in Cook’s Bay than in Opunohu Bay, linked with a higher mean annual flow of the river at Cook’s Bay. Our results suggest that rivers bring continental material into these two bays, which is partly incorporated into the food webs of fringing coral reefs at least close to river mouths. Thus, continental inputs can influence the transfer of organic matter within coral reef food webs depending on the diet of organisms.

The Great Melting Pot. Common Sole Population Connectivity Assessed by Otolith and Water Fingerprints

Quantifying the scale and importance of individual dispersion between populations and life stages is a key challenge in marine ecology. The common sole (Solea solea), an important commercial flatfish in the North Sea, Atlantic Ocean and the Mediterranean Sea, has a marine pelagic larval stage, a benthic juvenile stage in coastal nurseries (lagoons, estuaries or shallow marine areas) and a benthic adult stage in deeper marine waters on the continental shelf. To date, the ecological connectivity among these life stages has been little assessed in the Mediterranean. Here, such an assessment is provided for the first time for the Gulf of Lions, NW Mediterranean, based on a dataset on otolith microchemistry and stable isotopic composition as indicators of the water masses inhabited by individual fish. Specifically, otolith Ba/Ca and Sr/Ca profiles, and δ13C and δ18O values of adults collected in four areas of the Gulf of Lions were compared with those of young-of-the-year collected in different coastal nurseries. Results showed that a high proportion of adults (>46%) were influenced by river inputs during their larval stage. Furthermore Sr/Ca ratios and the otolith length at one year of age revealed that most adults (∼70%) spent their juvenile stage in nurseries with high salinity, whereas the remainder used brackish environments. In total, data were consistent with the use of six nursery types, three with high salinity (marine areas and two types of highly saline lagoons) and three brackish (coastal areas near river mouths, and two types of brackish environments), all of which contributed to the replenishment of adult populations. These finding implicated panmixia in sole population in the Gulf of Lions and claimed for a habitat integrated management of fisheries.