Preprints
https://doi.org/10.5194/bg-2021-59
https://doi.org/10.5194/bg-2021-59
31 Mar 2021
 | 31 Mar 2021
Status: this discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

Evolution of the long-term and estuary-scale phytoplankton patterns in the Scheldt estuary: the disappearance of net growth in the brackish region

Dante M. L. Horemans, Yoeri M. Dijkstra, Michèle Tackx, Patrick Meire, and Tom J. S. Cox

Abstract. Estuaries often show regions in which Chlorophyll-a (Chl-a) accumulates. The location and magnitude corresponding to such accumulation result from a complex interplay between processes such as river flushing, salinity, nutrients, phytoplankton grazing, and the light climate in the water column. Of particular interest is the long-term evolution of the estuary-scale Chl-a distribution in the Scheldt estuary (Belgium/Netherlands) in spring. From 2004–2007, we observed a limited spring-bloom in the brackish region. This bloom intensified in 2008–2014 and disappeared after 2015. This long-term evolution in Chl-a has been linked to simultaneous long-term trends in the suspended particulate matter (SPM) distribution and the improvement of the water quality, which affects grazing of Chl-a by zooplankton. However, this hypothesis has not been systematically investigated. In this paper, we apply two approaches to test this hypothesis. In the first approach, we analyze long-term in situ observations covering the full estuary. These observations include the SPM concentration, zooplankton abundance, and other variables affecting the Chl-a concentration, and show a long-term estuary-scale evolution in not only the SPM distribution but also in zooplankton abundance, freshwater discharge, and maximum photosynthetic rate. In the second approach, we apply a model approach supported by these observations to determine which of the changed conditions may explain the observed change in Chl-a. Our results suggest that a change in SPM alone cannot explain the Chl-a observations. Instead, mortality rate and grazing by zooplankton mainly explains the long-term estuary-scale evolution of Chl-a in spring. Our results highlight that insight into the zooplankton dynamics is essential to understand the phytoplankton (cf. Chl-a) dynamics in the Scheldt estuary.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Dante M. L. Horemans, Yoeri M. Dijkstra, Michèle Tackx, Patrick Meire, and Tom J. S. Cox

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-59', Huib E. de Swart, 17 Apr 2021
    • AC2: 'Reply on RC1', Dante Horemans, 10 Jun 2021
  • RC2: 'Comment on bg-2021-59', Anonymous Referee #2, 21 Apr 2021
    • AC1: 'Reply on RC2', Dante Horemans, 11 May 2021
  • CC1: 'Comment on bg-2021-59', Josette Garnier, 18 May 2021
    • AC3: 'Reply on CC1', Dante Horemans, 10 Jun 2021
  • RC3: 'Comment on bg-2021-59', Anonymous Referee #3, 19 May 2021
    • AC4: 'Reply on RC3', Dante Horemans, 15 Jun 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-59', Huib E. de Swart, 17 Apr 2021
    • AC2: 'Reply on RC1', Dante Horemans, 10 Jun 2021
  • RC2: 'Comment on bg-2021-59', Anonymous Referee #2, 21 Apr 2021
    • AC1: 'Reply on RC2', Dante Horemans, 11 May 2021
  • CC1: 'Comment on bg-2021-59', Josette Garnier, 18 May 2021
    • AC3: 'Reply on CC1', Dante Horemans, 10 Jun 2021
  • RC3: 'Comment on bg-2021-59', Anonymous Referee #3, 19 May 2021
    • AC4: 'Reply on RC3', Dante Horemans, 15 Jun 2021
Dante M. L. Horemans, Yoeri M. Dijkstra, Michèle Tackx, Patrick Meire, and Tom J. S. Cox

Data sets

Scheldt data observed in the Netherlands Rijkswaterstaat https://waterinfo.rws.nl

Scheldt data observed in Belgium De Vlaamse Waterweg http://www.omes-monitoring.be/en/data

Model code and software

iFlow version 2.9 Yoeri M. Dijkstra, Ronald L. Brouwer, and Dante M. L. Horemans https://doi.org/10.5281/zenodo.4560637

Dante M. L. Horemans, Yoeri M. Dijkstra, Michèle Tackx, Patrick Meire, and Tom J. S. Cox

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Short summary
We study the appearance in 2008–2014 and disappearance after 2015 of a phytoplankton  spring-bloom in the brackish region in the Scheldt estuary. To this end, we analyze long-term in situ observations covering the full estuary and apply a model approach to determine which of the changed conditions may explain the observed change in phytoplankton. Our results suggest that insight into the zooplankton dynamics is essential to understand the phytoplankton dynamics in the Scheldt estuary.
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