Deco-Sea fkearch I. Vol. 42. No. 11112. DD. 1965-1987. 1995 zyxwvutsrqp 0967-0637(95)00092-5 Copyright 0 1996 !&via Scmce Ltd Printed in Great Britain. All rights reserved C967%37/95 $9.5fl+lJ.o(1 Planktonic ciliate distribution relative to a deep chlorophyll maximum: Catalan Sea, N.W. Mediterranean, June 1993 JOHN R. DOLAN* and CELIA MARRASBt (Received 21 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB October 1994: in revised form 15 May 1995; accepted 3 July 1995) Abstract-Vertical distributions and relative contributions of distinct trophic guilds of ciliates were investigated in an oligotrophic system with a deep chlorophyll maximum (DCM) in early summer. Ciliates were classified as heterotrophic: micro and nano ciliates, tintinnids and predacious forms or photosynthetic: large mixotrophic oligotrichs (Laboea sfrobilia. Tontoniu spp.), and the auto- trophic M esodinium rubrum. Variability between vertical profiles (O-200 m) was relatively low with station to station differences (C.V.s of -30%) generally larger than temporal (1-4 day) differences (C.V.s of -lS%), for integrated concentrations. Total ciliate biomass, based on volume estimates integrated from O-SO m, averaged - 125 mg C mm’, compared to -35 mg m-’ for chlorophyll a (chl a), yielding a ciliate to chl ratio of 3.6, well within the range of 1 to 6 reported for the euphotic zones of most oceanic systems. Heterotrophic ciliate concentrations were correlated with chl ti concentration (r = 0.83 and 0.82, biomass and cells I-‘, respectively) and averaged -230 cells Il’ in near surface samples (chl a = 0.1 fig I-‘) to -850cells I-’ at 50 m depth, coinciding with the DCM (chl a = I-2pg I-‘). Tintinnid ciliates were diverse (36 species in 19 genera) but a minor part of heterotrophic ciliates. Nanociliates represented <l% of heterotrophic or total ciliate biomass, in contrast to reports on near-shore ciliate communities. Predacious ciliates were very rare. Large mixotrophic oligotrichs, while a minor portion of ciliate cells I-‘, were an important part of total ciliate biomass, representing 63% at 5 m and 21% integrated over O-80 m. M esodinium rubrum w as found throughout the water column, usually with a sub-surface peak (-100 cells II’ ), Concentrations of neither large mixotrophic oligotrichs, nor the autotrophic M . rubrum, were correlated with chl a. Estimates of the contribution of photosynthetic ciliate chl (mixotrophic and autotrophic) to total chl a (based on literature values of chl a cell-‘) ranged from -20% in some surface samples to ~0.5% in the DCM. INTRODUCTION In diverse marine systems, subsurface chlorophyll maximum layers are common, and a variety of governing mechanisms such as nutrient supply, differential grazing, and cell sinking have been proposed to account for them (e.g., Bienfang et al., 1983). In the oligotrophic Catalan Sea, a deep chlorophyll maximum (DCM) is present from early spring to autumn, between 40 and 60 m depth, coinciding with depth of both nitrate and phosphate nutriclines (Estrada et al., 1993). Production in the DCM ranges from 15 to *CNRS/INSU, Observatoire Oceanologique, URA 716, B.P. 28. F-06230 Villefranche-sur-mer. France. E-mail: dolan@ccrv.obs-vlfr.fr. tlnstitut de Citncies del Mar (CSIC), Passeig Joan de Borbo s/n, E-08039 Barcelona, Spain. E-mail: celia@masagran.icm.csic.es. 1965