Abundance, feeding and reproduction of Calanus finmarchicus in the Irminger Sea and on the northern Mid-Atlantic Ridge in June

Measurements of hydrography, chlorophyll a, abundance, egg production rates and gut fluorescence of Calanus finmarchicus were made on the northern Mid-Atlantic Ridge, along a north–south transect extending from Iceland to the Azores (∼60–44°N, 25–35°W) in June 2004. In addition, similar data were sampled in the Irminger Sea and on the Reykjanes Ridge in June 2003. The results show marked differences in abundance, phytoplankton ingestion rates and egg production rates from north to south. C. finmarchicus was most abundant in the central Irminger Sea (∼80,000–160,000 individuals m−2, 0–100 m), with abundance declining drastically (<20,000 individuals m) south of ∼58°N. By far, the majority of the animals resided in the upper 100 m−2 of the water column. Egg production rates were generally the highest near the continental shelf slopes in the western and northeastern parts of the Irminger Sea and in slope waters south of Greenland (∼40–60 eggs female−1 d−1). A trend of increasing egg production near the frontal area of the Charlie–Gibbs Fracture Zone also was observed. Phytoplankton ingestion rates, as based on gut fluorescence analysis, were the greatest in the northern Irminger Sea and in slope waters south of Greenland (∼8–28 μg C female d−1). The relationships between egg production rates and ingestion rates on the one hand and egg production rates and chlorophyll a concentrations on the other were described by Ivlev curves. The possible causes for the observed spatial variability in abundance and productivity of C. finmarchicus are discussed.