The bottom mixed layer depth as an indicator of subsurface Chlorophyll a distribution
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Published:2023-08-29
Issue:16
Volume:20
Page:3593-3611
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Zampollo AriannaORCID, Cornulier Thomas, O'Hara Murray Rory, Tweddle Jacqueline Fiona, Dunning James, Scott Beth E.
Abstract
Abstract. Primary production dynamics are strongly associated with vertical density
profiles in shelf waters. Variations in the vertical structure of the
pycnocline in stratified shelf waters are likely to affect nutrient fluxes and
hence the vertical distribution and production rate of phytoplankton. To
understand the effects of physical changes on primary production,
identifying the linkage between water column density and Chlorophyll a
(Chl a) profiles is essential. Here, the vertical distributions of density
features describing three different portions of the pycnocline (the top,
centre, and bottom) were compared to the vertical distribution of
Chl a to provide auxiliary variables to estimate Chl a in shelf waters. The
proximity of density features with deep Chl a maximum (DCM) was tested using
the Spearman correlation, linear regression, and a major axis regression over 15
years in a shelf sea region (the northern North Sea) that exhibits
stratified water columns. Out of 1237 observations, 78 % reported DCM
above the bottom mixed layer depth (BMLD: depth between the bottom of the
pycnocline and the mixed layer underneath) with an average distance of 2.74 ± 5.21 m from each other. BMLD acts as a vertical boundary above which
subsurface Chl a maxima are mostly found in shelf seas (depth ≤ 115 m).
Overall, DCMs were correlated with the halfway pycnocline depth (HPD) (ρS = 0.56) which, combined with BMLD, were better predictors of the locations
of DCMs than surface mixed layer indicators and the maximum squared buoyancy
frequency. These results suggest a significant contribution of deep mixing
processes in defining the vertical distribution of subsurface production in
stratified waters and indicate BMLD as a potential indicator of the Chl a
spatiotemporal variability in shelf seas. An analytical approach integrating
the threshold and the maximum angle method is proposed to extrapolate BMLD,
the surface mixed layer, and DCM from in situ vertical samples.
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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