Zooplankton feeding ecology: Grazing by marine copepods and cladocerans upon phytoplankton and cyanobacteria from Kingston Harbour, Jamaica

Grazing by the copepods Temora turbinata and Acartia lilljeborgii and the m arine cladoceran Penilia avirostris on natural phytoplankton and cyanobacte ria assemblages from Kingston Harbour, Jamaica, was examined in summer 1992 . Food assemblages were often dominated by an unidentified filamentous cyan obacterium, with abundances of up to 1.16 x 10(4) filaments.ml(-1). Other a bundant phytoplankters included microflagellates and diatoms of the genus N itzschia. Mean clearance rates for the entire food assemblage ranged from 0 .10 to 2.41 ml.animal(-1).h(-1), although most mean clearance rates were be tween 0.5 and 1.5 ml.animal(-1).h(-1). Patterns of selection of different f ood items by various grazers on different dates were variable and inconsist ent. Microscopic measurements revealed that cyanobacterial filament lengths were shorter in grazed than initial aliquots during a cyanobacterium bloom in August. This suggests that grazers reduced lengths of many filaments by biting off portions, in addition to consuming entire filaments. Such 'fila ment clipping' of cyanobacteria filaments has been recorded previously in f reshwater studies and suggests that grazers impact a larger portion of the phytoplankton assemblage than is accounted for by ingestion and clearance r ates based only upon removal of filaments. Grazers exhibited no adverse eff ects from consuming cyanobacteria. Cyanobacteria are often considered to be toxic or of poor food quality for planktonic grazers, but due to the high abundance and dominance of filamentous cyanobacteria in Kingston Harbour, i f these cyanobacteria were ungrazed, there would at times be little alterna tive phytoplankton food for copepods and cladocerans. Our results suggest t hat the trophic role of filamentous cyanobacteria in pelagic food webs of t he tropical ocean should be further investigated and possibly reconsidered.