Ingestion, faecal pellet and egg production rates of Calanus helgolandicus feeding coccolithophorid versus non-coccolithophorid diets

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Abstract

Ingestion rates, faecal pellet and egg production were obtained in laboratory experiments with females of the copepod Calanus helgolandicus collected from the English Channel in November 1994. Five different algal monocultures were used as food: Prorocentrum micans (30 μm ESD), Thalassiosira weissflogii (13 μm ESD), Dunaliella tertiolecta (7 μm ESD), Emiliania huxleyi (5 μm ESD) and Coccolithus pelagicus (14 μm ESD). Results obtained suggest the low ingestion efficiency of the copepod when feeding on coccolithophorids during late autumn–early winter. From the five species offered, only the largest non coccolithophorid Prorocentrum micans and Thalassiosira weissflogii supported efficient feeding and calculated respiratory demand for C. helgolandicus. Both coccolithophorids, irrespective of cell size, were ingested at very low rates even when offered at high concentrations (233–468 μg C l−1). Besides low ingestion, no egg production was found in the copepods fed with Emiliania huxleyi, although unusual high gross efficiency (reaching 72%) was obtained in experiments performed with Coccolithus pelagicus. The late seasonal timing of the experiments (November) could explain the low ingestion and egg production rates.

Introduction

Coccolithophorids is a widely distributed phytoplankton group characterised by their calcium carbonate plates (coccoliths) surrounding the cell. The pelagic alga Emiliania huxleyi is the most widespread coccolithophore species and is considered to be the major producer of calcite in the biosphere. Extensive monospecific blooms of this alga have been described especially in mid-latitudes in both coastal and open oceanic waters. These blooms have an average annual area of 1.4×106 km2 of the world’s ocean (Brown and Yoder, 1994), and can persist for 3–6 weeks reaching densities higher than 106 cells l−1.

Emiliania huxleyi has been largely studied in the last years in relation to the global carbon cycle, due to the influence of calcite synthesis on the CO2 equilibrium in seawater and the production of the climate-related dimethylsulfide (DMS). Apart from that, it has also been suggested that coccoliths are important in carbon flux to the deep ocean, and the presence of coccoliths in sediment traps has been reported (Cadée, 1985). Sedimentation rates of individual coccoliths are very low (10 cm d−1, Honjo, 1976), as well as intact cells (1 m d−1, Smayda, 1971) so the formation of coccoliths or cell aggregates, such as zooplankton faecal pellets with higher sinking rates (>100 m d−1, Harris, 1994) may be one of the most important mechanisms contributing to this flux.

But the importance of this mechanism requires the existence of well-developed copepod populations, with animal abundance high enough to produce enough pellets to result in significant export. During monospecific Emiliania blooms, this alga may be the main source of food for zooplankton growth and reproduction. The present study analyses the capability of different coccolithophore diets (Emiliania huxleyi and Coccolithus pelagicus) to satisfy Calanus helgolandicus metabolic and reproductive requirements when offered as the only food, and also the capability to sustain high copepod abundance, when compared with non-coccolithophore diets such diatoms (Thalassiosira weissflogii), or flagellates (Prorocentrum micans and Dunaliella tertiolecta).

Section snippets

Material and methods

Copepods were collected from net tows made off Plymouth (English Channel) between 10th and 29th November 1994 using a WP2 net (60 cm diameter and 200 μm mesh). The cod end contents were transferred to the lab in surface seawater in less than 1 h. Ten Calanus helgolandicus females, actively swimming and apparently healthy (without broken appendices) were selected for each experiment and maintained for 24 h in filtered (0.2 μm) seawater to clean their gut contents. Each female was examined

Ingestion rates

Ingestion and clearance rates obtained with the five algal species are plotted against food concentration in Fig. 1, Fig. 2. Results suggest that Calanus helgolandicus only feed efficiently on two of the five algal species offered. Females fed the small coccolithophore E. huxleyi showed very low ingestion rates only apparent when offered more than 468 μg C l−1. Rates with the other coccolithophore, Coccolithus pelagicus, and the small non-coccolithophore Dunaliella tertiolecta were also very

Discussion

To estimate ingestion rates in copepods using the disappearance of cells in incubation bottles has some experimental problems. Roman and Rublee, 1980, Peters and Downing, 1984 and others discuss the most important problems associated with this method. These problems include: behavioural effects, enhanced growth of algae in experimental bottles caused by ammonium excretion by copepods, loss of faecal pellets due to breakage, ingestion of pellets (coprophagy) and eggs (cannibalism) and particle

Acknowledgements

Support for this study was provided by the European Commission, MAST II Programme, Contract MAST-CT92-0038. The authors are grateful to D. Pond and D. Lesly for their help in the design and performance of the experiments. We also thank R. González-Quirós for his useful comments in the final redaction of the article. [RW]

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