Dominant amphipods ofPosidonia oceanicaseagrass meadows display considerable trophic diversity

Abstract

Gut content examination and trophic markers (fatty acids, stable isotopes of C and N) were combined to delineate the diet of the dominant species of amphipods from Mediterranean Posidonia oceanica seagrass meadows and to highlight trophic diversity among this community. Our results indicate that, although all dominant species heavily relied on macroalgal epiphytes, considerable interspecific dietary differences existed. Carbon stable isotope ratios notably showed that some of the amphipod species favored grazing on epiphytes from leaves or litter fragments (Apherusa chiereghinii, Aora spinicornis, Gammarus aequicauda), while others such as Dexamine spiniventris preferred epiphytes from rhizomes. The remaining amphipods (Caprella acanthifera, Ampithoe helleri and Gammarella fucicola) readily consumed both groups. In addition, SIAR modeling suggested that most species had a mixed diet, and relied on several food items. Fatty acid analysis and gut contents revealed that contributions of microepiphytic diatoms and of benthic and suspended particulate organic matter to the diet of amphipods were anecdotal. None of the examined species seemed to graze on their seagrass host [low 18:2(n-6) and 18:3(n-3) fatty acids contents], but Gammarus aequicauda partly relied on seagrass leaf detritus, as demonstrated by the lesser 13 C-depletion of their tissues. Overall, our findings suggest that amphipods, because of their importance in the transfer of organic matter from primary producers and detritus to higher rank consumers, are key items in P. oceanica-associated food webs.

Seasonal dependence on seagrass detritus and trophic niche partitioning in four copepod eco-morphotypes

“…As with C. arcuicornis, this species may be found in two habitats, namely EMA and sediment under the EMAs. This may explain the particular δ 15 N value observed for this species, which is representative of sediment organic matter in the study area (Michel et al, 2015). Isotopic niches of the other two eco-morphotypes, which are more associated with substrate surface (sediment or phytal), were partly overlapping or were completely separated, depending on the season.…”

“…Both food sources could be valuable for copepods in terms of nutrient and fatty acid content, as they are not necromass in the litter. Epiphytes were already previously found as food source for copepods living in P. oceanica canopy (Mascart et al, 2013) and for many other crustaceans, such as amphipods (Michel et al, 2015). In Australian coastal waters, macroalgae consumption in seagrass-dominated litter has also been documented (Hyndes and Lavery, 2005).…”

“…FA profiles of live seagrass blades differ significantly from seagrass detritus, due to the loss of some FAs during decay and the presence of bacterial FAs in detritus (Kharlamenko et al, 2001;Leduc et al, 2009;Michel et al, 2015).…”

Impact of food type on respiration, fractionation and turnover of carbon and nitrogen stable isotopes in the marine amphipod Gammarus aequicauda (Martynov, 1931)

“…The stable isotope ratio, SIR ( consumer will differ slightly from that of its food source and a shift, referred to as trophic fractionation or trophic enrichment factor (TEF or Δ), occurs because isotopes of a given element, as a result of their different atomic mass, react slightly differently during all biochemical reactions (e.g., photosynthesis, respiration, organic matter incorporation) (Fry, 2006). The overall result of combined isotopic effects associated with trophic processes is generally a net enrichment of the consumer's tissue in TEF values for marine detritivorous invertebrates are rare in literature (Vanderklift and Ponsard, 2003;Kaufman et al, 2008;Mancinelli, 2012;Michel et al, 2015) and this might be an important issue for isotopic data interpretation (Bond and Diamond, 2011). The use of Bayesian mixing models is common nowadays to assess diets with many possible food sources and uncertainty on TEF values, isotopic values of the food sources and isotopic values of the consumers (Cherel, 2008;Browning et al, 2014;Michel et al, 2015), but these models all significantly depend on the number of replicates, on the number of food sources but also on the accuracy of the TEFs to give robust and reliable results (Bond and Diamond, 2011;Phillips et al, 2014).…”

Effects of fish predation on Posidonia oceanica amphipod assemblages

“…They are one of the most functionally diverse groups of crustaceans and are used for monitoring environmental effects on P. oceanica meadows (Sánchez-Jerez and Ramos-Esplá 1996;Sánchez-Jerez et al 2000). From an ecological perspective, they are an important trophic resource for fish (Bell and Harmelin-Vivien 1983; and play an essential role within communities associated with P. oceanica in terms of energy transfer from low to higher trophic levels within the food web (Scipione et al 1996;Michel et al 2015). Because amphipods play such an important trophic role, the possibility that the structure of amphipod assemblages could be influenced by the local level of fish predation should be investigated, particularly in the context of MPAs.…”