Primary production on the Faroe shelf: Temporal variability and environmental influences

Abstract

Primary production and environmental influences on its intensity were studied on the Faroe shelf during 2004 and 2005. The study area is a small neritic ecosystem, which is relatively well (although variably) separated from the surrounding oceanic environment. Diatoms dominated during the spring bloom in 2004, but due to a combination of low concentrations of essential nutrients, and an apparent shift in the primary nitrogen source from nitrate to regenerated nitrogen, the phytoplankton species composition changed to a dominance of the small flagellate Phaeocystis pouchetii in July. In 2005, the nutrient concentrations did not reach the same low values as in summer 2004, and diatoms dominated the phytoplankton species composition during the productive season. Primary production, based on the 14C-technique, showed a linear relationship to primary production, based on the Fast Repetition Rate Fluorometer-technique (FRRF), with a regression slope of 1.5 and an r 2 of 0.85. The timing and magnitude of the spring bloom on the shelf is highly variable interannually. Favourable conditions for phytoplankton growth, in terms of the critical depth, were met in March. However, the spring bloom did not occur before mid-May. The specific primary production was high in April and throughout the productive season but was not reflected in the phytoplankton biomass. On the other hand the pattern in the phytoplankton biomass was reflected in the total primary production, indicating the importance of phytoplankton accumulation for bloom development. Advection, most likely, caused most of the apparent loss of primary production during pre-bloom, while grazing by the copepod community accounted for ~ 10% of the loss. Thus, spring bloom formation in the area depends highly on the spatial dispersal of phytoplankton biomass. The annual primary production on the shelf, based on the 14C-technique, was 201 g C m − 2 y − 1 . Approximately 77 g C m − 2 y − 1 was based on nitrate as nitrogen source, corresponding to a f-ratio of approximately 0.4.