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RESEARCH ARTICLE
Contents Vol 74(18)

Ineffective artificial mouth-breaching practices and altered hydrology confound eutrophic symptoms in a temporarily closed estuary

Marelé Nel A B , Janine B. Adams https://orcid.org/0000-0001-7204-123X A B , Lucienne R. D. Human https://orcid.org/0000-0002-5673-4092 A C , Monique Nunes https://orcid.org/0000-0002-6538-1815 A B , Lara Van Niekerk https://orcid.org/0000-0001-5761-1337 A D and Daniel A. Lemley https://orcid.org/0000-0003-0325-8499 A B *
+ Author Affiliations
- Author Affiliations

A Department of Botany, Nelson Mandela University, PO Box 77000, Gqeberha 6031, South Africa.

B Department of Science and Innovation (DSI)–National Research Foundation (NRF) Research Chair in Shallow Water Ecosystem, Institute for Coastal and Marine Research, Nelson Mandela University, Gqeberha 6031, South Africa.

C South African Environmental Observation Network, Elwandle Coastal Node, Nelson Mandela University, Gqeberha 6031, South Africa.

D Council for Scientific and Industrial Research (CSIR), Coastal Systems, PO Box 320, Stellenbosch 7599, South Africa.

* Correspondence to: danielalan.lemley@mandela.ac.za

Handling Editor: Kerrylee Rogers

Marine and Freshwater Research 74(18) 1519-1535 https://doi.org/10.1071/MF23053
Submitted: 11 March 2023  Accepted: 27 November 2023  Published: 14 December 2023

© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing

Abstract

Context

Artificial breaching of intermittently closed estuaries has become more frequent in the face of global-change pressures.

Aims

This study aimed to determine whether the ecological health of the Great Brak Estuary has been affected by the prolonged loss of marine connectivity arising from below-average inflow and failed breaching attempts.

Methods

We characterised primary eutrophic symptoms (inorganic nutrients, dissolved oxygen, microalgae) typical of the various mouth states, i.e. open, closed and semi-closed.

Key results

Initially, low inflow and closed mouth conditions facilitated the widespread occurrence of macroalgal blooms (Cladophora glomerata). Phytoplankton bloom conditions (>20 μg Chl-a L−1) ensued only in response to favourable hydrodynamic conditions (e.g. increased water residency, halocline formation) and increased nutrient availability from fluvial sources and macroalgal dieback. These blooms occurred in brackish conditions and comprised numerous taxa, including Cyclotella atomus var. marina, Cryptomonas sp. and Prorocentrum cordatum. Widespread hypoxia (<2 mg L−1) occurred during the semi-closed mouth phase because of the reduced flushing potential associated with the preceding high flow conditions.

Conclusions

Global-change pressures and ineffective breaching practices will promote eutrophic conditions in intermittently closed estuaries in the future.

Implications

Allocating sufficient environmental flows is key to preventing ecosystem degradation.

Keywords: eutrophication, harmful algal blooms, hypoxia, macroalgae, microphytobenthos, mouth management, phytoplankton, water quality.

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