Standing crop of planktonic ciliates in the East China Sea and their potential grazing impact and contribution to nutrient regeneration

The spatial distribution and standing crop of the ciliate community were investigated in the East China Sea during February to March 1993, September to October 1993, and July to August 1994. During these three periods, the ciliate community was dominated by aloricate forms, accounting for 50–93% of the total number of ciliates. All aloricate ciliates were assigned to three groups based on their trophic function: autotrophic ciliates (AC), mixotrophic ciliates (MC), and heterotrophic ciliates (HC). Sometimes AC constituted a sizable part of the ciliate community in abundance, but they were always less important in terms of biomass. MC, represented by oligotrichs of the genera Strombidium, Tontonia, and Laboea, were always a significant component in the upper water column and usually dominated during stratified periods in summer and autumn. HC were mainly comprised of oligotrichs of the genera Strombidium, Strobilidium, and Lohmaniella. They were common and most abundant in winter. Tintinnid ciliates (TC) were a minor component of the ciliate community, but they included of a large number of species, i.e. 74 species from 27 genera for all intact loricae in all seasons. The standing crop of total ciliates varied with depth and season from <10 to 4180 cells l−1 and <0.01 to 6.25 μg C l−1. Integrated values of total ciliates, ranging from 7.9 to 54.3×106 cells m−2 in abundance and from 8.5 to 136.3 mg C m−2 in biomass, were similar to those recorded from tropical and subtropical oligotrophic systems. Based on the results obtained by this first quantitative investigation on planktonic ciliates, their production, ingestion and excretion were calculated. Potential ingestion rates and their impact on reported primary production ranged, respectively, from 14% to 86%, except for the inner shelf where the impact was only 3% in the inner shelf waters in winter. NH4+ excretion rates of ciliates were estimated to be 0.1–63.8 mg N m−2 d−1, equivalent to 0.1–93.8% of the nitrogen requirement by primary producers. These values suggest that the nitrogen excretion by ciliates could be significant contribution to sustainable primary productivity during summer and autumn.