Micro-algal carbon and nitrogen uptake in post-coccolithophore bloom conditions in the northeast Atlantic, July 1991

Abstract

A study of carbon and nitrogen uptake by phytoplankton was conducted at two sites in the subpolar northeast Atlantic during July 1991. The main study site at 61°N, 20°W was located within a cyclonic warm core eddy, and the other site was at 62°30′N, 20°W. Based on evidence of coccolithophore sedimentation and the relatively slow movement of the eddy, this structure was probably within the bounds of a mesoscale coccolithophore bloom that was in decline 10–20 days before the present study. As little is known about the conditions leading to the onset or collapse of such a bloom, the data presented here are, to our knowledge, the first to quantify phytoplankton dynamics following such an event. In addition, data from the main site provide one of the first assessments of the carbon and nitrogen dynamics within a cyclonic eddy that is typically observed in the northeast Atlantic. The findings at both sites revealed relatively high rates of primary production and concomitant uptake of inorganic nitrogen, suggesting that production rates were not enhanced within the eddy relative to the surrounding waters. These rates were comparable with those noted in late spring in the northeast Atlantic at lower latitudes (47°N, 20°W; 0.7 g C m −2 day −1) and are slightly in excess of modelled estimates of production for the subpolar North Atlantic in midsummer (0.5–0.6 g C m −2 day −1), but less than estimates based on functional photosynthetically active radiation (PAR)-productivity relationships derived from the North Atlantic Bloom Experiment (NABE) (greater than 1.0 g C m −2 day −1). Such functional relationships, derived at lower latitudes, may not be applicable to the subpolar northeast Atlantic. The findings of this study concur with the previous suggestion that the subpolar Atlantic is indeed productive at this time and may deserve further attention from modellers. A comparison of physical, chemical and biological conditions during the coccolithophore bloom and post-bloom periods indicates little change in environment and suggests the role of some self-regulatory mechanism, such as cell self-shading, as the cause of the collapse of the bloom. Invoking such a non-environmental factor may also explain the observed high rates of post-bloom production.