The response of Oneirophanta mutabilis (Holothuroidea) to the seasonal deposition of phytopigments at the porcupine Abyssal Plain in the Northeast Atlantic

The impact of seasonal pulses of phytodetritus on the grazing behaviour of Oneirophanta mutabilis was assessed on the Porcupine Abyssal Plain (PAP) in the NE Atlantic, Sediment and sediment trap samples were analysed by HPLC to estimate the quantity and quality of the organic material in terms of ph ytopigments and nucleic acids. Food selection by Oneirophanta was estimated by analysing these constituents in the gut contents. The study area is characterised by large interannual variations: in the dep osition of fresh organic material. The mass fluxes at 10 m above bottom (ma b) varied from 0.25 g DW m(-2) d(-1) in September 1996 to <0.1 g DW m(-2) d (- 1) in March 1997. The material caught in the sediment trap in September 1996 had a relative fresh signature with a chlorophyll-a:phacophorbide rati o of 1.33. During the other seasons (March 1997, July 1997 and October 1997 ) the chlorophyll-a:phaeophorbide ratio remained low. In sediment cores thi s ratio showed a similar seasonal and inter-annual pattern, and again Septe mber 1996 was the period of maximum abundance of fresh organic material in the surficial sediment. The analyses of the gut contents of Oneirophanta mi rrored exactly the seasonal variation of the phytopigments in both the sedi ment and the sediment trap material. Concentrations of pigments in the fore -gut were 5 to 15 times higher than in the sediment and the nucleic acid co ncentrations were up to 80 times higher. This discrepancy between pigments and nucleic acids concentrations suggests that the latter are "indigenous" to the gut of Oneirophanta, either because the gut contains high numbers of actively-dividing bacteria or as a result of cell lysis of the gut epithel ium. The seasonal differences in the pigment concentration factor suggest t hat Oneirophanta does not actively search for hotspots where pigment concen trations are enriched. By using the degradation rate of chlorophyll-a in th e PAP sediments, the minimum residence time of chlorophyll in the sediment within the gut of Oneirophanta was calculated. In combination with gut volu me and density data it was estimated that each year the Oneirophanta popula tion skims a third of the sediment surface at the PAP site. (C) 2001 Elsevi er Science Ltd. All rights reserved.