Abundance of autotrophic, mixotrophic and heterotrophic planktonic ciliates in shelf and slope waters

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Abstract
During early summer, the density of microplanktonic ciliates in the euphotic zone on Georges Bank (Northwest Atlantic) ranged from 600 to 13,000 cells l-1, in the slope waters to the southeast of the bank, densities ranged from 1900 to 2800 cells l-1. Myrionecta rubra, a photosynthetic autotroph ciliate with a reduced algal endosymbiont, numerically comprised an average of 30% of the microplanktonic ciliate fauna at stations in < 100 m depth, but 3% or less of the ciliate fauna at the deeper stations. Oligotrichous ciliates with chloroplasts are estimated to have contributed 34% of the ciliate fauna and were abundant at both shallow, unstratified stations on the bank and at deeper, stratified stations on the slope of the continental shelf and in the Gulf of Maine [USA]. Overall, about 50% of the ciliates in the euphotic zone contained chlorophyll. At the shallow water stations, 2 species, M. rubra and Laboea strobila (a mixotrophic oligotrich) accounted for over 50% of the biomass of ciliates with chlorophyll. At an irradiance of 100 .mu.E m-2 s-1, M. rubra and L. strobila has photosynthetic rates of 85 and 465 pg C fixed cell-1 h-1, respectively. During summer, when phytoplankton biomass is low, autotrophic and mixotrophic ciliates may make an important contribution to photosynthesis in the larger size fractions and be an important source of food for larger organisms that rely on high quality, .gtoreq. 15 to 20 .mu.m food particles.