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 Ph aeocystis globosa (Prymnesiophyceae) single cells were studied in the labor atory. The calanoid copepod Acartia tonsa fed on P. globosa at low clearanc e 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; incr ease 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 arr ested development relative to those feeding on R. salina. Among the three c iliates and two dinoflagellates tested, ingestion rates on P. globosa singl e cells increased with the protozoan cell size, and the growth yield in ter ms of biovolume ranged from 9 to 78%. We studied the trophic interactions a mong grazers in P. globosa-based food chains. Gryodinium dominans (dinoflag ellate) growing on P. globosa improved the nutritional quality of P. globos a by 7.8 times for A. tonsa, such that A. tonsa feeding on G. dominans as a n intermediate prey had an EPE of 35%. The ciliate Mesodinium pulex also ex ploited P. globosa indirectly by consuming the dinoflagellate Gymnodinoium sp. as an intermediate prey. Our study showed that the trophic efficiency o f a system dominated by P. globosa is dependent on the complex food-chain s tructures within the planktonic food web.