Utilization of Phytoplankton by Zooplankton during the Spring Bloom in a Nova Scotia Inlet

Abstract

Coulter counter and microscope counts were used to estimate filtration and ingestion rates experimentally for total zooplankton caught by a 233-μm mesh net from two depths during the spring bloom in Bedford Basin. Initially, most phytoplankton were <20 μm with larger, chain-forming diatoms becoming more important as the bloom developed. Peak concentrations were >14 μg chlorophyll∙L−1and >1 mg C∙L−1and particle volume was >10 mm3∙L−1. Maximum primary production was >30 mg C∙m−3∙h−1. Dominant phytoplankton included Chaetoceros septentrionale early in the bloom, followed by Skeletonema costatum and Thalassiosira, Rhizosolenia, and Chaetoceros spp. Barnacle nauplii and Acartia hudsonica were the most common zooplankton near surface and Pseudocalanus sp. in deeper water. Ingestion was usually greatest at or near peaks in the Coulter counter particle spectrum. Filtration rate was proportional to grazer size and amount ingested and also to ambient particle concentration for near-surface but not deeper zooplankton. Higher ingestion was measured with the Coulter counter but microscopic counts gave higher filtration rates. Smaller surface-layer zooplankton apparently destroyed cells or chains that were not completely ingested. Microscopic estimates of particle volume ingested were 9–60% of that measured with the Coulter counter, suggesting utilization of detritus. Despite discrepancies in what the methods apparently measured, neither suggested a saturating functional response at ambient concentration, but with further concentration, both methods showed reduced filtration activity. Neither method revealed evidence for size-selective feeding, even when particles were compared on the basis of their maximum dimensions, but optical counts showed that single cells or chains of cylindrical shape were preferred to needle-like or spiney cells. Selection for biomass peaks could not be demonstrated using cell volume calculated from microscopic measurement. The 233-μm zooplankton utilized about 10–30% of daily primary production during the bloom.