THE BOFS 1990 SPRING BLOOM EXPERIMENT - TEMPORAL EVOLUTION AND SPATIAL VARIABILITY OF THE HYDROGRAPHIC FIELD

The overall background to the U.K. BOFS (Biogeochemical Ocean Flux Study) Project, designed to investigate oceanic carbon flux processes throughout the water column, is briefly described together with the strategy for the 1990 BOFS Spring Bloom Experiment. The Experiment involved two ships and was carried out in the northeast Atlantic between 46-50-degrees-N, 14-22-degrees-W in the period 18 April - 25 June 1990 with the objective of monitoring and quantifying the major carbon flux changes associated with the succession of the spring bloom. Sampling was carried out over a 7 week period adjacent to a Lagrangian buoy drogued at 30m. The spatial fields of the major variables were characterised from box grid surveys around the position of the marker drogue at the beginning and end of the time series observations with the time series hydrographical changes being related to features observed in the spatial surveys. The hydrographical and core biological observations made in the Experiment are described and interpreted. The reference drogue was deployed within an anticyclonic eddy in which initially there was little evidence of seasonal thermocline or phytoplankton development. The majority of an array of 30m drogues placed around the reference drogue drifted between 75-150km north and east of their origin, probably exiting from the original eddy system after the first 6 days of deployment. The reference drogue moved anticyclonically around the eddy centre for the first 13 days before exiting from the eddy system and becoming entrained in a discontinuity zone located between discrete warmer and cooler water bodies defined between 50-200m. During this latter period, which continued through to the end of the Experiment, the drogue tracked SE overall and alternately grazed the margins of the two water bodies with greater drift speeds being associated with the influence of the cooler water. Phytoplankton development proceeded slowly over the period that the drogue remained in the original eddy and paralleled the gradual development of the seasonal thermocline. A marked increase in phytoplankton occurred concurrently with the exit of the buoy from the eddy system into the boundary region of the cooler water where increased stratification prevailed. The phytoplankton increase persisted for only 6 days and declined sharply, primarily owing to advective influences, as the buoy moved away from the cool water influence with chlorophyll values remaining low for the remainder of the Experiment. Mesoscale influences were observed to have a major influence on the development sequence of the spring bloom in this area of the northeast Atlantic.