The biogeochemical properties of an extensive bloom (∼250,000 km2) of the coccolithophore, Emiliania huxleyi, in the north east Atlantic Ocean were investigated in June 1991. Satellite (NOAA‐AVHRR) imagery showed that the bloom was centered initially at 60°–63°N by 13°–28°W and lasted approximately 3 weeks. Spatial variations in satellite‐measured reflectance were well correlated with surface measurements of the beam attenuation coefficient, levels of particulate inorganic carbon, and coccolith density. Rates of both photosynthesis and calcification were typically relatively low within the coccolithophore‐rich waters, suggesting the population was in a late stage of development at the time of the field observations. Levels of dimethyl sulphide (DMS) in surface waters were high compared to average ocean values, with the greatest concentrations in localized areas characterized by relatively high rates of photosynthesis, calcification, and grazing by microzooplankton. The estimated spatially averaged flux of DMS to the atmosphere was 1122 nmol m−2 h−1, somewhat greater than that determined for the same region in June‐July 1987. Coccolith production (1 × 106 tonnes calcite‐C) had a significant impact on the state of the CO2 system, causing relative increases of up to 50 μatm in surface pCO2 in association with alkalinity and water temperature changes. Gradients in pCO2 were as great as 100 μatm over horizontal distances of 20–40 km. The environmental implications of these findings are discussed in relation to the spatial and temporal distributions of E. huxleyi.

[1] The relevance of aquifer heterogeneity for flow and transport is recognized broadly; however, its characterization is hampered by the inaccessibility of the subsurface. Time-lapse electrical resistivity tomography (ERT) offers the possibility of imaging noninvasively subsurface transport. We present results of two tracer tests that were carried out successively in a shallow aquifer at the Krauthausen test site (Germany). The breakthroughs of an electrically conductive and a resistive tracer were monitored with ERT and local multilevel groundwater samplers (MLS) along two cross sections perpendicular to the mean flow direction. Sinking of the conductive salt tracer due to density effects was observed with ERT. We applied a stream tube model to characterize the spatially variable transport. ERT-derived stream tube parameters showed similar patterns for the two tracer experiments, reflecting the effect of aquifer heterogeneity on transport. MLS data did not show similar spatial patterns, which indicates that these measurements may be prone to subtle changes of the flow field in the small sampling volume and mixing within screened wells. Between 50% and 10% of the tracer was recovered in the ERT-derived breakthrough curves. Compared with transport simulations in a homogeneous aquifer, ERT-derived time-integrated changes in electrical conductivity were locally larger but focused in a smaller area. MLS data indicated that in this area, ERT did not underestimate the tracer recovery. The relatively low tracer recovery was attributed to undetected tracer breakthrough in regions with low ERT sensitivity and in regions where the length of the tracer plume and the electrical conductivity contrast were small.

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