Feeding rates and abundance of marine invertebrate planktonic larvae under harmful algal bloom conditions off Vancouver Island

The interactions between toxic phytoplankton and their potential grazers are poorly understood aspects of the ecology of harmful algal blooms. In this study, we determined the feeding rates, prey selection and trophic impact of different marine invertebrate planktonic larvae on the natural bloom of Heterosigma akashiwo and Prorocentrum triestinum which occurred on the west coast of Vancouver Island in July of 2006. Additionally, we estimated the abundance, biomass and composition of zooplankton before and during the harmful algal bloom. Feeding experiments were performed with polychaete (Serpula columbiana), echinoderm (Stronglyocentratus purpuratus) and cirripede (Balanus crenatus) larvae obtained from laboratory cultures, and bivalve and gastropod larvae collected from the study site by plankton tows. All larvae fed on H. akashiwo whereas only cirripede nauplii and echinoderm larvae fed on P. triestinum. H. akashiwo was the main component of all larval diets (>64%). We observed a positive relationship between prey availability in the food assemblages and their contribution to all larval diets. The potential trophic impact of meroplanktonic larvae on bloom forming phytoplankton species was low (<1.5%). The ingestion of bloom forming phytoplankton did not appear to have any adverse effects on the studied grazers after 48 h of incubation. In contrast, field abundance of planktonic larvae and other zooplankton continuously decreased throughout the progression of the bloom, with losses approaching 75% in comparison to their pre-bloom abundance. The presence of H. akashiwo negatively affected the abundance of meroplanktonic larvae, despite efficient grazing of these larvae. Therefore, grazing pressure was reduced which likely contributed to the growth and persistence of the bloom. The reduction in meroplanktonic larvae and other zooplankton abundance associated with the H. akashiwo bloom may have potential impacts on benthic recruitment and energy transfers to higher trophic levels in marine food webs. (C) 2010 Elsevier B.V. All rights reserved.