Phytoplankton ingestion by appendicularians in the North Water

We investigated the abundance, body-size distribution, diet, and ingestion rates of appendicularian tunicates at 8 stations in the North Water polynya, northern Baffin Bay, during late June and July 1998. Abundance of appendicularians in the chlorophyll-rich surface layer (40–125 m) ranged from 38 to 11248 m−2. Body size of individuals ranged from 0.21 to 4.8 mm. Gut chlorophyll content varied from 0 to 84 ng ind−1 and increased with increasing body size. Gut passage time varied from 42 to 104 min (mean±SD: 58±18 min). Phytoplankton ingestion rates ranged from 0.007 to 2.083 mg chlorophyll m−2 d−1, with a median of 0.49 mg chlorophyll m−2 d−1. The median daily grazing impact of the oikopleurid populations was 0.42% and 5.4% of total phytoplankton biomass and primary production, respectively. Since this represents ingested phytoplankton and does not include cells trapped within the mucous houses, the contribution of appendicularian populations to phytoplankton mortality could be 2-fold higher (i.e. ca. 10% of primary production). The faecal pellets of animals incubated onboard in water from the subsurface chlorophyll maximum contained primarily small diatoms at the northern stations, and a mixture of diatoms, dinoflagellates, flagellates, and ciliates at southern stations. The median, daily flow of biogenic carbon from phytoplankton to appendicularian faecal pellets was 8 mg C m−2, which represents 4% of biogenic carbon export during the month of July. Because the phytoplankton was dominated by small Chaetoceros cells, appendicularian ingestion rates were not inhibited by the clogging of the filtration mechanism at high Chl a concentrations. Thus, the impact of appendicularians on daily primary production during this season was a simple function of appendicularian abundance, population size structure, and gut throughput rate.