Phaeocystis globosa (Prymnesiophyceae) and the planktonic food web:: Feeding, growth, and trophic interactions among grazers

The feeding and growth of copepods and heterotrophic protozoans when fed Phaeocystis globosa (Prymnesiophyceae) single cells were studied in the laboratory. The calanoid copepod Acartia tonsa fed on P. globosa at low clearance rates (< 20 ml ind(-1) d(-1)). P. globosa appeared to be a poor diet for A. tonsa in terms of fatty acid composition, yielding a low egg production rate (0-4.5 eggs ind(-1) d(-1)) and an egg production efficiency (EPE; increase in egg production/increase in ingestion) close to zero, in contrast to an EPE of 35% when Rhodomonas salina (Cryptophyceae) was offered as food. Nauplii of A. tonsa feeding on P. globosa suffered higher mortality and arrested development relative to those feeding on R. salina. Among the three ciliates and two dinoflagellates tested, ingestion rates on P. globosa single cells increased with the protozoan cell size, and the growth yield in terms of biovolume ranged from 9 to 78%. We studied the trophic interactions among grazers in P. globosa-based food chains. Gryodinium dominans (dinoflagellate) growing on P. globosa improved the nutritional quality of P. globosa by 7.8 times for A. tonsa, such that A. tonsa feeding on G. dominans as an intermediate prey had an EPE of 35%. The ciliate Mesodinium pulex also exploited P. globosa indirectly by consuming the dinoflagellate Gymnodinoium sp. as an intermediate prey. Our study showed that the trophic efficiency of a system dominated by P. globosa is dependent on the complex food-chain structures within the planktonic food web.