Ciliated protozoan communities over Cobb Seamount : Increase in biomass and spatial patchiness

International audienceSpecies composition, abundance, biomass ans patchiness of ciliated protozoa were examined for evidence of inscresing standing stock in relation to productivity (chlorophyl a, primary production) over an eastern subartic Pacific seamount (Cobb, 46° 45' N, 130° 48' W). Ciliates were unsually duserve with 49 different taxa recorded ranging in size from 15 to 350 micrometers in length. Their abundance was relatively low while carbon biomass was very high compared to those of most marine systems. This implies that over Cobb Seamount, ciliates counterbalance their low numbers by insceasing in size, partly due to exlusion fo small planktonic common species (Strobilidium spp., scuticocilitates, hymenostome-type cells) by larger cells, particularly plastidic oligotrichs of the genus Laboa. Integrated biomass to 80 m depth at times peaked at 0.6 g C n-², which exceeds the standing stock of the entire microzooplankton in some oceanic areas. Statistical 2-dimensional analysis of the spatial structure showed the existence of patchiness. Meso-scale and circular patches (about 6 km or more un diameter, horizontally) were probably related to seamount-induced physical forcing which likely generates microhabitats favorable to the growth of opportunistic or physiologically adapted populations. Direct observations showed that ciliates were a major component of the microzooplankton over the seamount