Trophic importance of diatoms in an intertidal Zostera noltii seagrass bed: Evidence from stable isotope and fatty acid analyses

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 delta C-13 (-9.9 parts per thousand) and high 18:2(n-6) and 18:3(n-3) contents. Microphytobenthos was slightly more C-13-depleted (-15.4 parts per thousand) and had high levels of diatom markers: 14:0, 16:1(n-7)c, 20:5(n-3). Low mean delta C-13 (-22.0 parts per thousand) 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 delta C-13 (-17.9 parts per thousand) 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 delta C-13 close to microphytobenthos and high levels of diatom FAs, confirming that microphytobenthos represented the main part of their diet. Lower delta C-13 and higher amounts of flagellate FM - 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. (C) 2010 Elsevier Ltd. All rights reserved.