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Managing the Seemingly Unmanageable: Water Quality and Phytoplankton Dynamics in a Heavily Urbanised Low-Inflow Estuary

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Abstract

Anthropogenic nutrient loading from urbanised coastal watersheds can negatively impact downstream estuarine ecosystems. The urbanised and low-inflow Swartkops Estuary in South Africa is one such example of an ecosystem suffering severe anthropogenic water quality alterations. Substantial inorganic nutrient loads (209 kg DIN d−1; 127 kg DIP d−1) are discharged into the estuary from three upstream wastewater treatment works and an adjacent stormwater canal. As a result, previous studies have indicated an increasing frequency and magnitude of phytoplankton bloom conditions (> 20 µg Chl-a l−1), yet the determinants of this are not fully understood. As such, the drivers of phytoplankton community dynamics–including harmful algal blooms (HABs)–in the Swartkops Estuary were evaluated by investigating seasonal and twice-weekly data collected over a two-and-a-half-year period. Eutrophic levels of dissolved inorganic nitrogen (DIN: > 100 µM) and phosphorus (DIP: > 85 µM) entered the estuary throughout the study period. Despite low DIN:DIP ratios (< 1:1), high-biomass HABs of Heterosigma akashiwo (max. 2120 µg Chl-a l−1) and Cyclotella sp. (max. 687 µg Chl-a l−1) were frequently recorded in the meso- to polyhaline (salinity: 5–23) middle and upper reaches. These events facilitate extreme dissolved oxygen conditions (supersaturated to hypoxic), increased turbidity, and occasional mass fish mortality events. This shift to an undesirable alternate stable state is attributed to the synergistic interaction between drought conditions, altered hydrodynamics, and anthropogenic nutrient loading. Innovative management interventions (e.g. wastewater reclamation, constructed wetlands, dual nutrient reduction strategies) are required to reduce the severity of these eutrophic symptoms.

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Acknowledgements

Opinions expressed and conclusions arrived at are those of the authors and do not necessarily represent the official views of the funding agencies. Dr Monique Nunes is thanked for providing the study site map. The Zwartkops Conservancy is also thanked for providing logistical support. We are grateful to the South African Environmental Observation Network (SAEON) Elwandle node for logistic assistance in the field and laboratory through the Shallow Marine and Coastal Research Infrastructure (SMCRI) programme.

Funding

This work is based on research financially supported by the Water Research Commission (Project: C2020/2021–00076), a Communities of Practice grant from the National Research Foundation (NRF) (GUN: 110612), and the DSI/NRF Research Chair in Shallow Water Ecosystems (UID: 84375). The Nelson Mandela University is thanked for providing postdoctoral fellowship funding to the lead author, Dr Daniel A. Lemley.

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Authors and Affiliations

  1. Botany Department, Nelson Mandela University, Gqeberha, 6031, South Africa

    Daniel A. Lemley, Lucienne R. D. Human, Emily Whitfield & Janine B. Adams

  2. DSI/NRF Research Chair in Shallow Water Ecosystems and the Institute for Coastal and Marine Research, Gqeberha, South Africa

    Daniel A. Lemley, Gavin M. Rishworth, Emily Whitfield & Janine B. Adams

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

    Lucienne R. D. Human

  4. Zoology Department, Nelson Mandela University, Gqeberha, 6031, South Africa

    Gavin M. Rishworth

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  1. Daniel A. Lemley
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Correspondence to Daniel A. Lemley.

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Communicated by Hongbin Liu

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Lemley, D.A., Human, L.R.D., Rishworth, G.M. et al. Managing the Seemingly Unmanageable: Water Quality and Phytoplankton Dynamics in a Heavily Urbanised Low-Inflow Estuary. Estuaries and Coasts 46, 2007–2022 (2023). https://doi.org/10.1007/s12237-022-01128-z

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