Trophic importance of diatoms in an intertidal Zostera noltii seagrass bed: Evidence from stable isotope and fatty acid analyses |
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Authors: | Benoit Lebreton,Pierre Richard,Robert Galois,Gilles Radenac,Camille Pflé ger,Gaë l Guillou,Franç oise Mornet,Gé rard F. Blanchard |
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Affiliation: | 1. Littoral Environnement et Sociétés (LIENSs), UMR 6250 CNRS-Université de La Rochelle, Institut du Littoral et de l’Environnement, 2 rue Olympe de Gouges, 17000 La Rochelle, France;2. Environnement Littoral Atlantique (ELA), UMS 3109 CNRS-IFREMER, Place Gaby Coll, BP 5, 17137 L’Houmeau, France |
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Abstract: | A current predominant paradigm emphasizes the role of epiphytic algae for invertebrates in most seagrass food webs. However, in some intertidal Zostera noltii beds, epiphyte biomass is very low compared to microphytobenthos and seagrass biomasses. We assessed the role of microphytobenthos in a temperate intertidal Z. noltii bed by combining stable isotope and fatty acid (FA) analyses on primary producers, composite sources — suspended particulate organic matter (SPOM) and sediment surface organic matter (SSOM) — and the main macrofaunal consumers. Z. noltii showed high δ13C (−9.9‰) and high 18:2(n-6) and 18:3(n-3) contents. Microphytobenthos was slightly more 13C-depleted (−15.4‰) and had high levels of diatom markers: 14:0, 16:1(n-7)c, 20:5(n-3). Low mean δ13C (−22.0‰) and large amounts of diatom and bacteria (18:1(n-7)c) markers indicated that SPOM was mainly composed of a mixture of fresh and decayed pelagic diatoms. Higher mean δ13C (−17.9‰) and high amounts of diatom FAs were found in SSOM, showing that microphytobenthic diatoms dominate. Very low percentages of 18:2(n-6) and 18:3(n-3) in consumers indicated a low contribution of Z. noltii material to their diets. Grazers, deposit and suspension-deposit feeders had δ13C close to microphytobenthos and high levels of diatom FAs, confirming that microphytobenthos represented the main part of their diet. Lower δ13C and higher amounts of flagellate FAs – 22:6(n-3) and 16:4(n-3) – in suspension feeders indicated that their diet resulted from a mixture of SPOM and microphytobenthos. These results demonstrate that invertebrates do not consume high amounts of seagrass and highlight the main role of benthic diatoms in this intertidal seagrass bed. |
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Keywords: | food web intertidal seagrass bed microphytobenthos Zostera noltii stable isotope ratio fatty acid profiles European Atlantic coast |
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