Grazing on filamentous algae by herbivorous zooplankton

SUMMARY. 1. Feeding experiments were conducted to examine filtering rates and selectivity of a variety of zooplankton taxa (including cladocerans, copepods and a rotifer) for filamentous diatoms, green and blue-green algae.
2. Most herbivores were capable of consuming some filamentous algae at rates similar to or higher than those on unicellular algae. Only feeding of Diaphanosoma brachyurum Liéven and Moina micrura Kurz seemed to be primarily limited by the filamentous morphology.
3. Filtering rates and selectivities of most herbivores were much higher for the diatom Melosira granulata angustissima Müller than for similarly sized blue-green algal filaments, indicating that chemical factors strongly influence consumption of filamentous algae.
4. The toxic blue-green algal filament Anabaena flos-aquae De Brébisson NRC 44–1 had a much strong inhibitory effect on zooplankton feeding than other filaments. The only herbivores that were not inhibited by this strain have been shown to be resistant to blue-green algal toxins, or strongly avoided consuming the blue-green alga. These results indicate that the inhibitory effect of filamentous algae is due more to toxic or noxious chemicals than to the filamentous morphology.
5. Selectivities of zooplankton for filamentous algae were largely independent of herbivore body size. The small-bodied cladoceran Bosmina longirostris Müller had the highest selectivities for filamentous algae.