Results from field observations and in situ 13C feeding experiments in the Nazaré Canyon conducted during RRS James Cook 10 cruise Leg 2

Submarine canyon systems provide a heterogeneous habitat for deep-sea benthos in terms of topography, hydrography, and the quality and quantity of organic matter present. Enhanced meiofauna densities as found in organically enriched canyon sediments suggest that nematodes, as the dominant metazoan meiobenthic taxon, may play an important role in the benthic food web of these sediments. Very little is known about the natural diets and trophic biology of deep-sea nematodes, but enrichment experiments can shed light on nematode feeding selectivity and trophic position. An in-situ pulse-chase experiment (Feedex) was performed in the Nazaré Canyon on the Portuguese margin in summer 2007 to study nematode feeding behaviour. 13C-labelled diatoms and bacteria were added to sediment cores which were then sampled over a 14-day period. There was differential uptake by the nematode community of the food sources provided, indicating selective feeding processes. 13C isotope results revealed that selective feeding was less pronounced at the surface, compared to the sediment subsurface. This was supported by a higher trophic diversity in surface sediments compared to the subsurface, implying that more food items may be used by the nematode community at the sediment surface. Predatory and scavenging nematodes contributed relatively more to biomass than other feeding types and can be seen as key contributors to the nematode food web at the canyon site. Non-selective deposit feeding nematodes were the dominant trophic group in terms of abundance and contributed substantially to total nematode biomass. The high levels of 'fresh' (bioavailable) organic matter input and moderate hydrodynamic disturbance of the canyon environment lead to a more complex trophic structure in canyon nematode communities than that found on the open continental slope, and favours predator/scavengers and non-selective deposit feeders.

Feeding of Sagitta setosa in the Black Sea

Gut dissection of fixed individuals from samples collected during Cruise 6 of R/V Vityaz-2 in April-May 1984 was used to study feeding of Sagitta setosa in the layers of daytime plankton accumulation at the lower boundary of the oxycline. The principal food was copepodite stage V of Calanus and females of Calanus and Pseudocalanus. Analysis of daytime and night data with reference to length of migratory alterations of Sagitta populations and gut passage time indicates that they feed actively in the layers of day¬time plankton accumulations. Total food consumption during time spent in the subsurface layers ranged from 0.025-0.097 cal/indiv. in 12 h, equivalent to 37-143% of their metabolic energy expenditure. Over the course of 12 h Sagitta population consumes 0.3-5% and 0.5-6% of population of stage V copepodites and females of Calanus and Pseudocalanus females, respectively.