The impact of appendicularian grazing on natural food concentrations in situ1

Abstract

The filtering rates of Oikopleura dioica and Stegasoma magnum (Tunicata: Appendicularia) were measured in situ by a SCUBA diver in the Gulf of California. Simultaneous measurement of the age class structure of each appendicularian population, natural food densities, and the rate of increase of food particles allowed calculation of the grazing impact of each population on natural food assemblages in the field. Filtering rates increased exponentially with animal size and reached maxima of 12.5 ml·animal −1 ·h −1 for O. dioica and 627 for S. magnum. The filtering rate of S. magnum was an order of magnitude higher than that of copepods of similar ash‐free dry weight. At densities ranging from 205 to 4,587 animals·m −3 , O . dioica populations filtered from 1.3 to 37.7% of each cubic meter in 24 h. At maximum population densities clearance exceeded the replacement rate of the particulate food. Stegasoma magnum ranged from 11 to 63 animals·m −3 and filtered from 5.4 to 13.4% of each cubic meter in 24 h. Direct field evidence indicates that appendicularian grazers exert significant grazing pressure on natural food assemblages in situ.