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Abstract. River deltas are particularly important in the marine carbon cycle as they represent the transition between terrestrial and marine carbon: linked to major burial zones, they are reprocessing zones where large carbon fluxes can be mineralized. In order to estimate this mineralization, sediment oxygen uptake rates were measured in continental shelf sediments and river prodelta over different seasons near the outlet of the Rhône River in the Mediterranean Sea. On a selected set of 10 stations in the river prodelta and nearby continental shelf, in situ diffusive oxygen uptake (DOU) and laboratory total oxygen uptake (TOU) measurements were performed in early spring and summer 2007 and late spring and winter 2008. In and ex situ DOU did not show any significant differences except for shallowest organic rich stations. Sediment DOU rates show highest values concentrated close to the river mouth (approx. 20 mmol O 2 m −2 d −1 ) and decrease offshore to values around 4.5 mmol O 2 m −2 d −1 with lowest gradients in a south west direction linked to the preferential transport of the finest riverine material. Core incubation TOU showed the same spatial pattern with an averaged TOU/DOU ratio of 1.2±0.4. Temporal variations of sediment DOU over different sampling periods, spring summer and late fall, were limited and benthic mineralization rates presented a stable spatial pattern.Correspondence to: C. Cathalot (cecile.cathalot@lsce.ipsl.fr) A flood of the Rhône River occurred in June 2008 and delivered up to 30 cm of new soft muddy deposit. Immediately after this flood, sediment DOU rates close to the river mouth dropped from around 15-20 mmol O 2 m −2 d −1 to values close to 10 mmol O 2 m −2 d −1 , in response to the deposition near the river outlet of low reactivity organic matter associated to fine material. Six months later, the oxygen distribution had relaxed back to its initial stage: the initial spatial distribution was found again underlining the active microbial degradation rates involved and the role of further deposits. These results highlight the immediate response of the sediment oxygen system to flood deposit and the rapid relaxation of this system towards its initial state (6 months or less) potentially linked to further deposits of reactive material.

Fauna have been found to regulate important biogeochemical properties and ecosystem functions in benthic environments. In this study, we focused on how functional biodiversity and species-specific traits of benthic macrofauna affect key ecosystem functions related to organic matter mineralization and cycling of nutrients in surface sediments. Dominant benthic invertebrates from the Baltic Sea and the Skagerrak were classified into functional groups in accordance with their behaviour, feeding and sediment reworking activities. Macrofauna species were added in different combinations to defaunated Baltic sediments in 2 parallel microcosm systems fuelled with brackish and marine water. In total, there were 12 treatments that differed in terms of functional diversity of benthic fauna. The experiments demonstrated that faunal activities directly affected benthic oxygen and nutrient fluxes, sediment reactivity and pore-water distribution under both Baltic and Skagerrak conditions. Benthic fluxes, sediment reactivity and pore-water distribution were similar in Baltic and Skagerrak treatments, in which the same functional biodiversity and species-specific traits of benthic macrofauna were observed. Although no significant effects of functional biodiversity could be detected under Baltic or Skagerrak conditions, treatments with bioturbating fauna from the Skagerrak enhanced oxygen consumption and nutrient fluxes compared to treatments with Baltic fauna and Skagerrak fauna with functional groups similar (parallel) to the Baltic fauna. Moreover, speciesspecific traits related to the Skagerrak fauna (e.g. the thalassinid shrimp Calocaris macandreae) exceeded the effects of all other faunal treatments. This suggests that species-specific traits of macrofauna may override species richness and functional biodiversity of macrofauna when regulating important ecosystem properties and functions in benthic environments.

Gross sedimentation rates (GSR) were monitored together with the main characteristics of the collected matenal (i.e. organic content, C, N, total proteins, lipids, carbohydrates, available proteins and amino acids) over a 2 yr cycle at a shallow station (18 m) of the bay of Banyuls, France. In addition, sediment pigment concentrations and meiofaunal densit~es were recorded monthly during a 1 yr period. GSR ranged between 0.6 and 317.8 g DW m ' cl'' Spring and summer were characterized by relatively low and constant GSR whereas fall and wlnter were characterized by relat~vely high and hlghly variable GSR. There was a negatlve relationship between GSR and the organic content of material collected within the sediment traps, suggesting the importance of resuspension in controlling GSR. Growth rates of the deposit-feeding bivalve Abra ovata fed sediment trap material collected on 6 sampling dates were also measured. Significant changes in important nutrient components (e.g. available proteins) of sedimenting materials from these different sampling dates correlated with growth differences and confirmed the existence of temporal changes in the quality of the material collected in the sediment traps. The highest growth rate was obtained for the material collected during May 1993, which coincided with maximal meiofauna densities and maximal pigment concentrations at the surface of the sediment. The best descript.ion of growth was obtained when using available proteins and total lipids as the independent variables of simple linear regression models. This supports the use of these parameters as descriptors of food quality. The consequences of our results on the parameterization of changes in food quality within models of continental shelf food webs are discussed. It is suggested that such changes should be simulated through control functions based on available protein contents.

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We used a new experimental approach involving thin aquaria, luminophores, time lapse photography and image analysis to assess sediment reworking in 2 closely related bivalves, Abra ovata and A. nitida. The method proved efficient based on the highly significant correlation between the concentrations of luminophores assessed using image analysis and direct counting of sediment slices. A. ovata and A. nitida exhibited different sediment reworking behaviours. A. ovata remained immobile within the sediment and transferred luminophores within the sediment through its siphonal activity, which resulted in the creation of typical inverse conical structures. A. nitida moved within the sediment and reworked a thinner sediment layer. Both A. ovata and A. nitida were characterised as biodiffusers. Biodiffusion coefficients (D b ) were maximal at intermediate food concentration in A. ovata and at high food concentration in A. nitida. This new approach allowed assessment of the effects of spatial scale and vertical grid size on the computation of D b . In both species D b decreased with spatial scale up to 3.750 cm and then remained constant. It is suggested that this pattern partly resulted from heterogeneity linked to: (1) the mode of sediment reworking (A. ovata) and (2) the relative proportion of reworked sediment surface (A. nitida). Vertical grid size > 0.250 cm resulted in a significant overestimation of D b in A. nitida, due to the low thickness of the sediment layer reworked by this species. The implications of these results on the main characteristics (duration, spatial scale, vertical grid size) of classical luminophore experiments are discussed. KEY WORDS: Sediment reworking · Image analysis · Food availability · Scaling · Luminophores · Abra ovata · Abra nitida Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 319: [135][136][137][138][139][140][141][142][143][144][145][146][147][148][149][150][151][152][153] 2006 metal-doped sediment, chlorophyll a, luminophores) have been used to estimate sediment reworking by benthic fauna (Robbins 1982, Krezoski et al. 1984, Smith et al. 1986, White et al. 1987, Sharma et al. 1987, Sun et al. 1991, Wheatcroft 1992, Gérino et al. 1998. These tracers are indicative of sediment reworking occurring at different timescales. Luminophores (i.e. fluorescent particles) have been widely used in laboratory (Mahaut & Graf 1987, Gérino et al. 1998, Ouellette et al. 2004, Mermillod-Blondin et al. 2005 and in situ experiments (Mahaut & Graf 1987, Gérino 1990, Gérino et al. 1994, 1998, Biles et al. 2002, Mugnai et al. 2003, Solan et al. 2004b. Experiments classically consist of spreading luminophores at the surface of experimental sediment cores containing test organisms at time 0 and then incubating those cores for a known amount of time. At the end of the experiments, the cores are sliced horizontally and the vertical profiles of luminophore concentration assessed. The profiles are fitted to mathematical models that allow computat...