Bio‐optical variability associated with phytoplankton dynamics in the North Atlantic Ocean during spring and summer of 1991

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Abstract
Bio‐optical data recorded from April 30 to July 19, 1991, using a mooring located in the open ocean (59°35.6°N, 20°57.9°W) are described and interpreted. Five multi‐variable moored systems (MVMS) were deployed in the upper 90 m to obtain concurrent, co‐located measurements of horizontal currents, water temperature, photosynthetically available radiation (PAR), transmission of light at 660 nm (c660), and stimulated chlorophyll fluorescence. In addition, meteorological and subsurface temperature data (12 depths from 80 to 310 m) were collected. When the mooring was deployed, surface waters were weakly stratified and there was little evidence of a phytoplankton bloom. Soon after the deployment, a marked increase in phytoplankton concentration occurred simultaneously with an increase of near‐surface water temperature. The most striking observation was a period (year days 128–140) of strong mixed layer depth variability (daily amplitude of about 40 m) during which phytoplankton standing stock reached its maximum. During this period, phytoplankton biomass was mixed down to deeper waters at nighttime. As a result, the variability of the bio‐optical parameters was extremely high, and deepwater phytoplankton concentration was much greater than would have been expected from the productivity estimates. Later, phytoplankton concentrations declined sharply in response to extremely stormy weather around year day 140. Once the storm passed (after day 143), surface waters stratified and the phytoplankton stock increased again, but the depth integrated biomass concentration did not reach as high values as before the storm. During this strong thermal stability period, fluorescence and c660 signals in near‐surface waters were much higher than at depth, and displayed a diel cycle which was well correlated with PAR.