Factors affecting the spatial pattern of the deep chlorophyll maximum in the region of the Azores front

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Abstract Results are given of two surveys of the deep chlorophyll maximum (DCM) in the vicinity of an oceanic front south-west of the Azores. The surveys, made in late April and May 1981, used mainly a towed undulating fish (Seasoar) fitted with CTD and in situ fluorometer. In the latter survey observations of underwater irradiance, nitrate concentration and primary production were also made. The front was baroclinic and is considered to be an arm of the Gulf Stream return flow in the North-east Atlantic. The current associated with the front made a large meander through the area and during the observation period shed an eddy which later drifted westwards. The front separated two distinct water types which were termed East Atlantic Water (EAW) and West Atlantic water (WAW). The DCM in WAW was on average 20 m deeper and between 50% and 60% of the magnitude of that in EAW. However the 1% light level coincided with the depth of the DCM in both water types and this fact, together with observations on the different nitrate concentrations in the two water types, led to the theory that the differences in the DCM could be explained by assuming an overall lower level of primary productivity in WAW, although differences in vertical mixing may have been a contributory factor. The measurements of primary production imply that the specific growth rate of phytoplankton in the DCM was less than 0.1 day−1. In the Front itself there was some evidence for small scale (∼ 20 km) eddy-like features which produced patchiness of the same scale in the DCM. There was also evidence for advective intrusions along isopycnals of EAW across the front into WAW. In two cases this process produced deep subsidiary maxima below the DCM with chlorphyll a concentrations of 0.1 mg m−3 as deep as 170 m. In the late April survey two of the Seasoar sections showed higher phytoplankton chlorophyll concentrations on the EAW side of the front but in general on both surveys there was little evidence for increased phytoplankton biomass associated with the front.