In situ feeding physiology and grazing impact of the appendicularian community in temperate waters

Abstract

The physical and biological factors affecting the abundance and ingestion rates of different appendicularian species were investigated from April 1999 to May 2000 at a coastal station in the English Channel and on a transect of 3 stations across the shelf in the central Cantabrian Sea. Individual gut chlorophyll and gut food volume contents were used to determine the ingestion rates on autotrophic prey and on total particulate material. Body size was the variable explaining most of the variability in gut contents. For most species, over 60% of the ingested material came from nonchlorophyll-containing prey. Appendicularian community grazing impact was higher at the oceanic stations during early spring and autumn, with maximum values close to 10% of the total phytoplankton biomass removed daily. Oikopleura longicauda and O. fusiformis were the species with the highest grazing impact. During the study period, appendicularians removed an average of 8% of the primary production (PP) measured at the only station for which we have concurrent measurements. Our results suggest that the bulk of appendicularian populations resides in the surface mixed layer and that their grazing rates increase with increasing PP. However, the percentage of the PP removed by the appendicularian community decreases with increasing productivity, indicating that their grazing impact is relatively more important under oligotrophic conditions. Appendicularians could account for close to 40% of the total mesozooplankton grazing.KEY WORDS: Appendicularian · Gut content · Feeding · Grazing · Tunicate · Oikopleura · Fritillaria Resale or republication not permitted without written consent of the publisher

Grazing of two common appendicularians on the natural prey assemblage of a tropical coastal ecosystem

“…Feeding inhibition was Ingestion rates were determined indirectly using measured clearance rates and assuming that 30 ± 5% of the particles cleared were retained on filtering apparatus and not ingested. Error bars represent upper 95% confidence intervals of the mean also observed by López-Urrutia et al (2003) for Oikopleura fusiformis feeding in temperate waters during periods of increased large cell abundance within the chlorophyll a range of our experiments. Moreover, O. fusiformis was the only species in the study of López-urrutia et al (2003) for which the abundance of > 30 µm cells had a strong, statistically significant effect when total ingestion was considered (their Fig.…”

“…Perhaps not surprisingly, given the relative importance of picoplankton in tropical oceans, Oikopleura longicauda and O. fusiformis can clear naturally occurring prey in this size range at rates substantially higher than those of comparably sized appendicularians studied to date (King et al 1980, Alldredge 1981, Bedo et al 1993, Acuña & Kiefer 2000, Broms & Tiselius 2003, López-Urrutia et al 2003, Fernandez et al 2004, and vastly exceeding the rates of co-occurring copepods (Calbet et al 2000). In fact, grazing by O. longicauda can lead to mean picophytoplankton mortality rates comparable to those resulting from protozoan grazing (specifically 2 to 5 µm heterotrophic nanoflagellates) in Kaneohe Bay (Landry et al 1984, Scheinberg 2004.…”

“…The few studies that have investigated the feeding capabilities of both Oikopleura longicauda and O. fusiformis varied both in methodology and results (Alldredge 1977, López-Urrutia et al 2003 (Table 3). López-Urrutia et al (2003) found that O. longicauda and O. fusiformis ingested phytoplankton at comparable rates in temperate waters with chlorophyll a concentrations similar to those in Kaneohe Bay.…”

“…l -1 (Owen 1966, Alldredge 1982. In addition to their numerical dominance, López-Urrutia et al (2003) recently found that these 2 species together had the greatest grazing impact among appendicularians on phytoplankton populations in the temperate open ocean. However, based on the relative temporal and spatial distributions of O. longicauda and O. fusiformis, it appears that they may be maximally successful under different environmental conditions (Acuña & Anadón 1992, Acuña 1994, Fenaux 1998, Scheinberg 2004.…”

“…Although numerous studies have investigated the rates at which appendicularians can clear and/or ingest particles from temperate, subarctic and arctic waters (Acuña et al 1999, López-Urrutia et al 2003, Fernández et al 2004, only a handful have been conducted with warmwater species (Alldredge 1977, 1981, Sato et al 2004 and almost all utilized proxies for natural prey, such as latex beads or ink particles.…”