A mathematical expression is proposed to describe the relationship between the photosynthetic capacity (P,,,,) of natural assemblages of motile benthic diatoms and temperature. Experiments were performed in 2 different seasons to document the response of P,,,, to a rapid increase in temperature (3°C h-'). In both cases, there was a progressive increase in P,,,,, up to an optimum temperature (Top,). beyond which P,,,,, declined rapidly. Top, did not change from September to December 1995, but the maximum photosynthetic capacity (PMAx, i.e. P,,,,, at Top,) was twice as high in September as in December

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 δ 13 C (−9.9‰) and high 18:2(n-6) and 18:3(n-3) contents. Microphytobenthos was slightly more 13 C-depleted (−15.4‰) and had high levels of diatom markers: 14:0, 16:1(n-7)c, 20:5(n-3). Low mean δ 13 C (−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 δ 13 C (−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 δ 13 C close to microphytobenthos and high levels of diatom FAs, confirming that microphytobenthos represented the main part of their diet. Lower δ 13 C 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.

International audienceThe role of epiphytes in an intertidal Zostera noltii seagrass bed in Marennes-Oléron Bay was assessed in comparison with the other main benthic primary producers (Z. noltii, microphytobenthos) at two bathymetric levels and on a seasonal basis. Assemblage and biomass of epiphytes were studied using Scanning Electron Microscopy (SEM). Zostera noltii and its detrital matter followed a typical seasonal pattern: microphytobenthos was present in large quantities throughout the year representing 21 % of the total biomass while detrital matter, above-ground parts and below-ground parts accounted for 65, 9 and 5% respectively. Only two species of epiphytic diatoms, Cocconeis scutellum and Cocconeis placentula, were 2 observed on seagrass leaves. Epiphyte biomass was very low, representing on average less than 0.001 % of that of microphytobenthos or leaves. This low epiphyte biomass is linked with the absence of macroalgae and also with the low biovolume of Cocconeis, which formed a monolayer of cells on leaves. This can be explained by the severe conditions of the intertidal and the high leaf turn-over of Zostera noltii leaves