The role and quantitative importance of free-living nematodes in marine and estuarine soft sediments remain enigmatic for lack of empirical evidence on the feeding habits and trophic position of most nematode species. Here we use natural abundances of carbon and nitrogen stable isotopes of some abundant nematode species/genera from estuarine intertidal sediments to assess their trophic level and major food sources. In all stations, d15 N of di¡erent dominant nematode species/genera spanned a range of 3.6 to 6.3 ppt, indicating that at least two trophic levels were represented. The large nematodes Enoplus brevis, Enoploides longispiculosus and Adoncholaimus fuscus consistently had high d 15 N, in line with mouth-morphology based predictions and empirical evidence on their predacious feeding modes. Daptonema sp., Metachromadora remanei, Praeacanthonchus punctatus and 'Chromadoridae' (dominated by Ptycholaimellus ponticus) had comparatively lower d 15 N, and d 13 C suggesting that microphytobenthos (MPB) is their major carbon source, although freshly sedimented particulate organic matter may also contribute to their nutrition in silty sediments. The trophic position of Sphaerolaimus sp., a genus with documented predacious feeding mode, was ambiguous. Ascolaimus elongatus had d 15 N signatures indicating a predacious ecology, which is at variance with expectations from existing feeding type classi¢cations. Our study shows thatödespite limitations imposed by the biomass requirements for EA-IRMS (elemental analyseröisotope ratio mass spectrometry)önatural isotope abundances of carbon and nitrogen are powerful tools to unravel trophic structure within nematode communities. At the same time, the prominence of di¡erent trophic levels results in a large span of d 15 N, largely invalidating the use of nitrogen isotope abundances to assess food sources and trophic level of whole nematode communities.

An on-board experiment was performed during a research cruise to investigate the ability of deep-sea nematode species to actively colonise defaunated sediments. Small cylinders of 500 µm wire mesh filled with defaunated sediment were inserted into microcosms containing sediment with indigenous meiofauna collected from 1300 m depth in the Arctic Ocean. The defaunated sediments were either enriched with the diatom Thalassiosira weissflogii, or remained unenriched. Samples from the defaunated sediment were taken after 9 and 17 d. As controls, microcosms with sediment containing the indigenous meiofauna but without an internal cylinder were also sampled at each time interval. Nematodes colonised both enriched and unenriched sediments with abundances of up to 20% of the controls. Irrespective of the time of sampling, abundance and number of species were significantly higher in the enriched treatment, suggesting that the presence of food enhances colonisation and resilience. Nematode assemblages in the defaunated sediments were species-rich and differed from the controls. The majority of colonising species were rare or undetectable in the controls, suggesting that episodic disturbances may be necessary for their persistence in deep sea sediments. Colonisation was in part determined by species characteristics such as size and motility. At the same time, a large number of different species colonised the empty cylinders, resulting in poor similarity in community composition between replicates, particularly between samples enriched with diatoms. Our results indicate a poor predictability of community composition of recently disturbed sediments despite highly reproducible abundance and diversity patterns and lends experimental support to the existence of a spatio-temporal mosaic that emerges from highly localised colonisation patterns.

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The effects of 2 cold fronts on a shallow sublittoral system at the Island of Santa Catarina, southern Brazil, were investigated in June 2002. Atmospheric, pelagic and benthic data were taken daily covering the days before, during and after the passage of both fronts, for a total of 8 d of sampling. The passage of cold fronts, a short-term event, promotes changes in the entire shallow sublittoral ecosystem, i.e. both pelagic and benthic compartments. In the pelagic system, both fronts changed seston, nutrient and chl a concentrations. An increase in chl a just after the passage of the fronts probably resulted from phytoplankton retention and accumulation near the coast, from microphytobenthos resuspension and from an increase in nutrient availability. In the benthic system, sediment features and the 3 benthic components studied -microphytobenthos, meiofauna and macrofauna -were also affected by the passage of the cold fronts. The sediments at the site became more homogeneous. While mean grain size decreased, the fraction of fine sand gradually increased after the passage of the fronts. In contrast, sediment organic content was significantly higher immediately after the passage of the first cold front. Cold front effects in the benthos depended on the biological component, the vertical distribution of fauna and on the magnitude of the front. The sediment chl a concentration was lower exactly 1 d after the passage of the fronts, while phaeopigments were significantly higher. Both macrofauna and meiofauna, particularly nematodes, of the upper sediment layer exhibited a decrease in density and number of taxa during the passage of the first frontal system. The decrease in nematode abundance was mostly due to vertical migration to the deeper layers of the sediment, rather than being the result of erosion and transport during the passage of the cold fronts.KEY WORDS: Cold front · Short-term disturbance · Phytoplankton · Microphytobenthos · Meiofauna · Nematodes · Macrofauna Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 281: [79][80][81][82][83][84][85][86][87][88][89][90][91][92] 2004 blows from SW (in the cold sector) with a mean speed of 8 m.s -1 . Approximately 1 d after the passage of the front the wind returns to its predominant NE direction (see Fig. 1b).The surface wind field along the SBB is strongly influenced by the passage of frontal systems (Stech 1990). Considering that wind stress is one of the major low-frequency forcing mechanisms of the coastal circulation (Stech & Lorenzetti 1992), the frontal systems are expected to play a significant role in structuring the dynamics of shallow marine systems. However, effects of the passage of cold fronts on marine systems have thus far been inferred rather than documented (Odebrecht et al. 1995b, Sunyé 1999. This paper reports on the effects of the passage of cold fronts in a shallow sublittoral system of the Island of Santa Catarina, S Brazil. The study evaluates shortterm changes in abiotic and bio...