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.

Highlights

  • The Chlorophyll-a (Chl-a) concentrations in estuaries often show regions where Chl-a accumulates, resulting in zones with locally elevated Chl-a concentrations

  • Our results suggest that a change 345 in mortality rate is the main factor to explain the appearance and disappearance of Chl-a accumulation in the brackish region and that other parameters (e.g., suspended particulate matter (SPM)) alone cannot explain this observed evolution in Chl-a

  • Assuming a system-constant mortality rate, we found mortality rate values ranging from 0.83 − 2.64 × 10−6 s−1, which complies with the value of approximately 1.1 × 10−6 s−1 presented in Desmit et al (2005) who studied a real-case in the Scheldt estuary near km 115

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Summary

Introduction

The Chlorophyll-a (Chl-a) concentrations in estuaries often show regions where Chl-a accumulates, resulting in zones with locally elevated Chl-a concentrations Such accumulation results from a complex interplay between physical, transport-related processes and chemical-biotic processes that determine net local phytoplankton growth. The dynamics of suspended particulate matter (SPM) that determine the light climate are crucial to understanding the phyto plankton dynamics. Examples of such light-limited systems are the Gironde estuary (Irigoien and Castel, 1997), Seine estuary (Garnier et al, 2001), Ems estuary (Liu et al, 2018), and Scheldt estuary (Desmit et al, 2005)

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