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As part of the North Atlantic Bloom Experiment (NABE), CTD/transmissometer profiles were made on two cruises at 47N, in spring 1989 to quantify thee increase in particles during a phytoplankton bloom in relationship to biophysical forcing factors. Water samples were filtered to obtain particle mass concentrations for calibration of beam attenuation (beam c) from the transmissometer. Shipboard experiments demonstrated that beam c resulted primarily from the concentration of small particles (<20 m), with little signal coming from the aggregates or dregs that settle to the bottom of water bottles. Profiles of beam attenuation showed a three-fold increase in the surface mixed layer over a 2-week period. Superimposed on this increase were daily variations with evening highs and morning lows. Increases in beam attenuation resulted primarily from increases in particle mass generated through primary production. Some portion of the diel variation may be a physiological response to light. Nighttime decreases could be caused by remineralization, biological consumption and vertical migration, dilution by diurnal increases in the mixed-layer thickness, and large-particle production with subsequent settling (fecal pellets, aggregates). Linear regression of beam attenuation values versus time at multiple depths over the first 2-week period suggested a three-layer system with particle production in the top 50 m, particle loss down to 400 m (most rapid loss between 50 and 200 m) and a slow increase in particles from 400 to 2000 m. Caution is urged, however, in blindly treating the data as time-series measurements of conditions in a homogenous parcel of water, since the ship had to relocate as it followed buoys drogued at different depths drifting in divergent directions in a mesoscle eddy field. Wind mixing, stratification and solar radiation were obvious influences on the rate of particle increase in surface waters. Depth-integration of beam attenuation over 20 and 50 m revealed that the increase in derived particle organic carbon (POC) can account for only about 18-28% of the drawdown of total CO2. Fluxes measured by drifting sediment traps at 150 m can be satisfied by the settling of only 2-4% day-1 of the standing crop in the upper 100 m, but the particles would have to be converted into larger, more rapidly settling particles to be collected effectively in sediment traps. Based on the drawdown of CO2 and the standing crop of particles, significant primary production must have occurred before we arrived on day 115. Our data demonstrate that the transmissometer is an effective optical tool in monitoring the dynamics of particles in surface waters and its signal can be quantitatively related to biological processes in the ocean. 1992.
Deep Sea Research Part II: Topical Studies in Oceanography
author list (cited authors)
Gardner, W. D., Walsh, I. D., & Richardson, M. J.