Planktonic ciliate distribution relative to a deep chlorophyll maximum: Catalan sea, NW Mediterranean, June 1993

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 helerotrophic: micro and nano ciliates, tintinnids and predacious forms or photosynthetic: large mixotrophic oligotrichs (Laboea strobilia, Tontonia spp.), and the autotrophic Mesodinium rubrum. Variability between vertical profiles (0-200 m) was relatively low with station to station differences (C.V.s of similar to 30%) generally larger than temporal (1-4 day) differences (C.V.s of similar to 15%), for integrated concentrations. Total ciliate biomass, based on volume estimates integrated from 0-80 m, averaged similar to 125 mg C m(-2), compared to similar to 35 mg m(-2) for chlorophyll a (chi a), yielding a ciliate to chi 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 chi a concentration (r = 0.83 and 0.82, biomass and cells l(-1), respectively) and averaged similar to 230 cells l(-1) in near surface samples (chi a = 0.1 mu g l(-1)) to similar to 850 cells l(-1) at 50 m depth, coinciding with the DCM (chi a = 1-2 mu g l(-1)). Tintinnid ciliates were diverse (36 species in 19 general but a minor part of heterotrophic ciliates. Nanociliates represented <1% 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 l(-1), were an important part of total ciliate biomass, representing 63% at 5 m and 21% integrated over 0-80 m. Mesodinium rubrum was found throughout the water column, usually with a sub-surface peak (similar to 100 cells l(-1)). Concentrations of neither large mixotrophic oligotrichs, nor the autotrophic M. rubrum, were correlated with chi a. Estimates of the contribution of photosynthetic ciliate chi (mixotrophic and autotrophic) to total chi a (based on literature values of chi a cell(-1)) ranged from similar to 20% in some surface samples to <0.5% in the DCM.