Mesozooplankton community composition, feeding, and export production during SOIREE

Abstract

The community composition and feeding rates of mesozooplankton (>200 μm length) were determined using plankton hauls, bottle incubations and gut pigment determinations during Southern Ocean Iron RElease Experiment (SOIREE) in the Southern Ocean in February 1999. Upper-ocean (0–65 m) mesozooplankton biomass (4.2 and 3.2 g m−2, inside and outside the iron-fertilised patch, respectively) was dominated by large copepodites (>1.5 mm). Salps and euphausiids were absent and very rare, respectively. Incubations using large copepods showed no significant difference in clearance rates of nano- (2–20 μm) and net- (>20 μm) plankton. Mean clearance rates inside and outside the iron-fertilised patch also did not differ and were very low (ca. 50 ml mg DW−1 d−1). Mean ingestion rate, however, was significantly greater in the patch due to higher algal and heterotrophic nanoflagellate (HNAN) biomass there. Gut pigment analysis showed that most ingestion by large, medium and small copepods occurred at night, and that specific ingestion was greatest in small copepods. Daily integrated ingestion rates determined by the incubation and gut pigment methods were similar for comparable large copepods. Phytoplankton and HNAN ingestion met only 14% of the estimated daily respiratory carbon requirements of the large copepods inside the patch, and 4% outside. Little ciliate or detrital carbon was available in the system, which could have further supplemented the food supply. A number of other studies have found a similar disparity between ingestion and nutritional requirements in copepods. Reasons for this include the possibility that fine-scale aggregations of copepods and their food have not been adequately sampled, or that measured metabolic rates have been systematically overestimated. Ingestion of phytoplankton by the total copepod community was low, with <1% of standing stock removed per day (inside and outside the patch) and 4% and 8% of primary production removed (inside and outside the patch, respectively). Estimated egestion accounted for only 4% of the total particulate carbon flux found in sediment traps, within the patch. Little of the POC observed in traps was copepod faecal material. This supported the finding that feeding rates were low, and combined with the finding of an apparently weak diel vertical migration, suggested that the copepod assemblage contributed little passive and active export flux during SOIREE.