14 Apr 2022
14 Apr 2022
Status: this preprint is open for discussion.

A proxy of subsurface Chlorophyll-a in shelf waters: use of density profiles and the below mixed layer depth (BMLD)

Arianna Zampollo1, Thomas Cornulier1, Rory O'Hara Murray2, Jacqueline F. Tweddle1, James Dunning1, and Beth E. Scott1 Arianna Zampollo et al.
  • 1School of Biological Sciences, University of Aberdeen, Aberdeen, AB24 2TZ, UK
  • 2Marine Scotland Science, Aberdeen, AB11 9DB, UK

Abstract. Primary production dynamics are strongly associated with vertical density profiles, which dictate the depth of stratification and mixed layers. Climate change and artificial structures (e.g. windfarms) are likely to modify the strength of stratification and vertical distribution of nutrient fluxes, especially in shelf seas where fine scale processes are important drivers, affecting the vertical distribution of phytoplankton. To understand the effect of physical changes on primary production, identifying the linkage between density and phytoplankton profiles is essential. Here, the ecological relevance of eight density layers (DLs) obtained by multiple methods that define three different portions of the pycnocline (above, centre, below) was evaluated to identify a valuable proxy for subsurface Chlorophyll-a (Chl-a mg m-3) concentrations. The associations of subsurface Chl-a with surface and deep mixing were investigated by hypothesizing the occurrence at the same depth of any DL and the maximum Chl-a layer (DMC) using Spearman correlation, linear regression, and a Major Axis analysis. Out of 1237 observations of the water column exhibiting a pycnocline, 78 % reported DMCs above the bottom mixed layer depth (BMLD). This suggests that the BMLD is a boundary trapping Chl-a in shallow waters (≤ 120 m). BMLD constantly described Chl-a vertical distribution despite surface mixing indicators, suggesting a significant contribution of deep mixing processes in supporting subsurface production under specific conditions (e.g. prolonged stratification, tidal cycle, and bathymetry). Using BMLD for defining subsurface Chl-a could be a valuable tool for understanding the spatiotemporal variability of Chl-a in shelf seas, representing a potential variable for ecological assessments.

Arianna Zampollo et al.

Status: open (until 09 Jun 2022)

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Arianna Zampollo et al.

Arianna Zampollo et al.


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Short summary
The proposed paper gives new insight into the relevance of deep mixing in sustaining subsurface primary production in shelf waters. The identification of a strict correlation between subsurface Chlorophyll-a (a proxy of primary production in seawater) and the base of the pycnocline (a layer of the water column with a rapid change of density) is likely to impact the scientific community working on site-specific or large scales, offering a new perspective on primary production in shelf seas.