Planktonic ciliate distribution relative to a deep chlorophyll maximum: Catalan Sea, N.W. 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 heterotrophic: 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 ∼30%) generally larger than temporal (1–4 day) differences (C.V.s of ∼15%), for integrated concentrations. Total ciliate biomass, based on volume estimates integrated from 0–80 m, averaged ∼ 125 mg C m−2, compared to ∼35 mg m−2 for chlorophyll a (chl 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 chl a concentration (r = 0.83and0.82, biomass and cells l−1, respectively) and averaged ∼230 cells 1−1 in near surface samples (chl a = 0.1 μg 1−1) to ∼850 cells 1−1 at 50 m depth, coinciding with the DCM (chl a = 1–2 μg 1−1). Tintinnid ciliates were diverse (36 species in 19 genera) 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 (∼100 cells 1−1). Concentrations of neither large mixotrophic oligotrichs, nor the autotrophic M. rubrum, were correlated with chl a. Estimates of the contribution of photosynthetic ciliate chi (mixotrophic and autotrophic) to total chl a (based on literature values of chla cell−1) ranged from ∼20% in some surface samples to <0.5% in the DCM.