Patrick Dauby

Tuna and Dolphin associations in the North-east Atlantic : Evidence of different ecological niches from stable isotope and heavy metal measurements

Associations of tunas and dolphins in the wild are quite frequent events and the question arises how predators requiring similar diet in the same habitat share their environmental resources. As isotopic composition of an animal is related to that of its preys, stable isotope ( 13 C/ 12 Ca nd 15 N/ 14 N) analyses were performed in three predator species from the North-east Atlantic: the striped dolphin, Stenella coeruleoalba, the common dolphin Delphinus delphis and the albacore tuna, Thunnus alalunga, and compared to their previously described stomach content. Heavy metals (Cd, Zn, Cu and Fe) are mainly transferred through the diet and so, have been determined in the tissues of the animals. Tuna muscles display higher d 15 N than in common and striped dolphins (mean: 11.4 vs. 10.3‰ and 10.4‰, respectively) which reflects their higher trophic level nutrition. Higher d 13 C are found in common ()18.4‰) and striped dolphin ()18.1‰) muscles than in albacore tuna ()19.3‰) probably in relation with its migratory pattern. The most striking feature is the presence of two levels of cadmium concentrations in the livers of the tunas (32 mg kg ˇ1 dry weight (DW) vs. 5 mg kg ˇ1 DW). These two groups also diAer by their iron concentrations and their d 15 N and d 13 C liver values. These results suggest that in the Biscay Bay, tunas occupy two diAerent ecological niches probably based on diAerent squid inputs in their diet. ” 2000 Elsevier Science Ltd. All rights reserved.

Trophic diversity within the eastern Weddell Sea amphipod community

Amphipod crustaceans form one of the most diversified animal groups within the Antarctic macrozoobenthos, both from the taxonomic point of view (more than 800 species have been recorded in the Southern Ocean) as by niche occupation and at the community level. Thus, amphipods are likely to play an important role in the organic matter fluxes that occur on the Antarctic sea floor. The dietary behaviour of these peracarids is still poorly known, and only few species have been analysed. This paper describes the trophic preferences of some dominant amphipod species of the Eastern Weddell Sea benthos, deduced from stomach content analyses and behavioural observations in aquaria. More than 1000 specimens, belonging to 40 species (representing 27 genera and 15 families) were dissected; and several thousands of individuals were kept in aquaria for 6–9 weeks and presented with various potential foods. These two approaches revealed at least eight different feeding types: suspension-feeding, deposit-feeding, deposit-feeding coupled with predation, opportunistic predation, micropredatory browsing, macropredation coupled with scavenging, opportunistic necrophagy and true necrophagy. These different behaviours cover almost all the possible feeding types with the exception of macroherbivorous browsing. Among the eight described feeding types, no particular one is dominant. In the same way, types involving microphagy and macrophagy are equally represented. Predatory types (opportunistic or exclusive) account for 64% of the species analysed, while scavenging types (facultative or obligate) account for 60%. The overlap suggests that many amphipod species have a wide dietary spectrum and are able to take advantage of different food resources.

A stable isotope approach to the eastern Weddell Sea trophic web: focus on benthic amphipods

Abstract. Stable isotope (13C/12C and 15N/14N) analyses were performed on 90 species belonging to different benthic communities sampled in the eastern Weddell Sea. The study focused on eight amphipod species whose isotopic composition was compared to their previously described respective gut contents. Amphipod stable isotope ratios correspond fairly accurately to the trophic classification based on gut contents and attest to their wide spectrum of feeding types. Since the fundamental difference between the isotope and the gut content approaches to diet studies is the time scale each method addresses, this coincidence indicates that there would be no significant changes in feeding strategies over time. Three levels of the food web are covered by the eight species and, instead of belonging strictly to one trophic category, amphipods display a continuum of values from the suspension-feeder to scavengers.

Methods for removing epiphytes from seagrasses: SEM observations on treated leaves

Abstract Several techniques commonly used to remove epiphytes from seagrasses were tested on the same Posidonia oceanica (L.) Delile material: shaking, scraping, sonicating and soaking in acid. The effects of each treatment on the epiphyte removal rate and on the seagrass itself were assessed by scanning electron microscopy. The advantages and disadvantages of each technique are discussed. It appears that the combination of an acid treatment with moderate scraping provides the best results as nearly all epiphytes are removed without seriously damaging the leaf substratum.

Food sources of two detritivore amphipods associated with the seagrass Posidonia oceanica leaf litter

Abstract This study focused on the ingestion and assimilation of Posidonia oceanica (L.) Delile litter by Gammarella fucicola Leach and Gammarus aequicauda Martynov, two dominant detritivore amphipods of the P. oceanica leaf litter. Scanning electron microscope observations indicated that leaf litter is highly colonized by diverse diatoms, bacteria and fungi, which may constitute a potential food source for the litter fauna. Gut content observations demonstrated that these species eat P. oceanica litter, and that this item is an important part of their ingested diet. Stable isotope analyses showed that the species do not experience the same gains from the ingested Posidonia. Gammarella fucicola displayed isotopic values, suggesting a major contribution of algal material (micro- and macro-epiphytes or drift macro-algae). On the other hand, the observed isotopic values of G. aequicauda indicated a more important contribution of P. oceanica carbon. The mixing model used agreed with this view, with a mean contribution of P. oceanica to approximately 50% (range 40–55%) of the assimilated biomass of G. aequicauda. This demonstrated that the two species, suspected to be detritus feeders, display in reality relatively different diets, showing that a certain degree of trophic diversity may exist among the detritivore community of the seagrass litter.

Distribution of surface seawater partial C02 pressure in the English Channel and in the Southern Bight of the North Sea

Abstract The concentrations of dissolved CO 2 dissolved oxygen and chlorophyll have been determined in the surface seawater of the English Channel and of the Southern Bight of the North Sea in June 1992, April–May 1993 and September–October 1993. The partial pressure of CO 2 (PCO 2 ) displays large spatial and temporal variations ranging from 100 ppm up to 670 ppm. The relationship between seawater pCO 2 and chlorophyll distribution is discussed and compared with data recently obtained in the North Atlantic by A. J. Watson, C. Robinson, J. E. Robertson, P. J. le B. Williams and J. R. Fasham (1991) Nature , 350, 50–53. Air-sea CO 2 fluxes have been calculated from wind speed and pCO 2 data and, due to the combined effect of both of those parameters, the flux can be as high as about 150 mmol m −2 d −1 . This implies that shelves could play an appreciable role in the global C budget. Further extended C0 2 level measurements should be made to improve flux estimations for the coastal zone on a global basis.

Experimental evidence for N recycling in the leaves of the seagrass Posidonia oceanica

Abstract A one-year in situ experiment using 15 N as a tracer was designed to assess the N recycling in the leaves of the seagrass Posidonia oceanica (L.) Delile. P. oceanica was shown to partly recycle the internal nitrogen pool of its leaves in order to contribute to new leaf growth. The leaves sampled in June 1999 contained 20% of the quantity of 15 N found in June 1998. N recycling caused a difference between N and biomass turnover rate (0.8 vs 1.3 y −1 ) of Posidonia leaves. This 40% difference should correspond to the contribution of recycled N to the annual N requirement of Posidonia leaves. The N recycling appears to be insufficient to significantly reduce the quantitative impact of N loss due to autumnal leaf fall. However, new leaf growth between June and October is mainly sustained by this recycling because the tracer concentration in new leaves was the same as in the other leaves. By contrast, tracer concentration decreased drastically between October 1998 and June 1999, showing the more important contribution of N uptake during winter and spring. Nevertheless, recycling occurs throughout the year as demonstrated by the presence of tracer in the youngest leaves of shoots sampled one year after the tracer addition.

Distribution of inorganic carbon and related parameters in surface seawater of the English Channel during spring 1994

Abstract The complete CO 2 system, dissolved oxygen, nutrients and chlorophyll concentrations, fluorescence, salinity and temperature of surface seawater have been determined in the English Channel and the Southern Bight of the North Sea during Spring 1994. As already mentioned in a previous study, this area is characterized by important spatial and temporal variations of the considered set of parameters. This paper focuses on the distribution of the homogeneous buffer factor [ β = dln(pCO 2 ) dln(TCO 2 ) ]. It is shown that this parameter is a useful tool for identifying processes that mainly affect the CO 2 distribution, such as primary production or water mass mixing, even in such a complex area.

Carbon stable isotope analysis as a tool for tracing temperature during the El Tremedal underground coal gasification at great depth

Abstract The new opportunity given by the underground gasifier developed at Alcorisa in Spain (Province of Teruel) in the framework of an European experiment has promoted a better understanding of gasification in a natural reactor. The thermodynamical equilibria of gasification reactions and the repartition of the stable isotopes of carbon (13C/12C) in the produced gases have been used to monitor the process. An estimation of the temperatures inside the gasifier and at the exhaust has been performed. As shown by the isotopic balances, the tar formation is negligible or null and the pyrolysis zone spreads continuously. The study has confirmed the reality of the 13C isotopic abundance measurements for the system CO/CO2 as an indicator of the temperature inside the gasifier. During the gasifier expansion, the temperature at the exhaust decreases whereas the temperature inside the gasifier is practically constant showing a slight increasing trend. As pointed out by the data, the oxygen enrichment of the gasifying agent plays an important role on the estimated temperatures.

A Gas Chromatographic Separation for the H and C Stable Isotope Ratio Determination of Coal Compounds

A new, completely automated gas chromatography technique has been developed to separate the different gaseous compounds produced during underground coal gasification for their (13)C/(12)C and D/H isotope ratio measurements. The technique was designed for separation and collection of H(2), CO, CO(2), H(2)O, H(2)S, CH(4), and heavier hydrocarbons. These gaseous compounds are perfectly separated by the gas-phase chromatograph and quantitatively sent to seven combustion and collection lines. H(2), CO, CH(4), and heavier hydrocarbons are quantitatively oxidized to CO(2) and/or H(2)O. The isotopic analyses are performed by the sealed-tube method. The zinc method is used for reduction of both water and H(2)S to hydrogen for D/H analysis. Including all preparation steps, the reproducibility of isotope abundance values, for a quantity higher than or equal to 0.1 mL of individual components in a mixture (5 mL of gases being initially injected in the gas chromatograph), is ±0.1‰ for δ(13)C(PDB) and ±6‰ for δD(SMOW).