The University of Southampton
University of Southampton Institutional Repository

Modelling the impact of macronutrients on the eutrophication status of small estuaries

Modelling the impact of macronutrients on the eutrophication status of small estuaries
Modelling the impact of macronutrients on the eutrophication status of small estuaries
Small, semi-enclosed coastal basins have always been attractive locations for human settlements. However, as human populations have rapidly increased in recent decades, this has subjected these water bodies to extensive anthropogenic use, potentially negatively impacting their water quality and making them more susceptible to nutrient enrichment leading to eutrophication. An understanding of the hydrodynamics driving circulation and flushing times, and corresponding biogeochemical interactions, of small, eutrophic, temperate estuaries is less advanced than that of larger estuaries due both to a lack of available environmental data sets and difficulties in accurately modelling small scale systems. The overall aim of the research presented in this thesis is to
establish which physical drivers influence the circulation and water flushing times in small estuaries and to investigate the influence this has on the biogeochemistry and thus water quality of the case study microtidal Christchurch Harbour estuary in Dorset UK. A coupled depth-averaged hydrodynamic-biogeochemical model has been configured of the estuary using MIKE 21 software to define the physical controls on circulation and flushing times of the estuary and thus improve understanding of how these processes drive declines in water quality and dictate eutrophication development in small shallow basins. Results indicated circulation control changes from tidally to fluvially driven as riverine inputs to the estuary increase. Flushing times, calculated using a
particle tracking method, revealed the system can take as long as 132 hours to flush when river flow is low in summer months, or as short as 12 hours when riverine input is exceptionally high.

When total river flow into the estuary is less than 30 m3s−1, tidal flux is the dominant hydrodynamic control, which results in long flushing times during neap tides. Conversely, when riverine input is greater than 30 m3s−1 the dominant hydrodynamic control is fluvial flux and flushing times during spring tides are longer than at neaps. Instances of summer oxygen undersaturation and increased levels of chlorophyll were found to coincide with regions in the estuary yielding long residence times, even under low nutrient river water concentrations. Eutrophic and hypoxic conditions, defined in this instance as dissolved oxygen concentrations falling below 4 mg/L and 2 mg/L, respectively, for a duration of at least 4 hours, were observed in summer month simulations. Inverse relationships between time of oxygen undersaturated and both river flow and river nutrient concentration were observed but with no significant correlation between time undersaturated and summer solar irradiance which is attributed to the estuary’s shallow nature. The results showed that although river flow controls estuarine renewal, river nutrient concentration plays the greatest role in driving eutrophication development in small, shallow semi-enclosed basins like Christchurch Harbour. The methodology presented in this thesis shows modelling at small spatial scales is possible with the findings suggesting easy application to other similar estuaries across the world to infer conditions which may present deleterious effects on ecosystem health.
University of Southampton
Huggett, Rebecca
4c318b0e-e287-4c67-a0c6-a0cb23581df9
Huggett, Rebecca
4c318b0e-e287-4c67-a0c6-a0cb23581df9
Purdie, Duncan
18820b32-185a-467a-8019-01f245191cd8

Huggett, Rebecca (2021) Modelling the impact of macronutrients on the eutrophication status of small estuaries. University of Southampton, Doctoral Thesis, 335pp.

Record type: Thesis (Doctoral)

Abstract

Small, semi-enclosed coastal basins have always been attractive locations for human settlements. However, as human populations have rapidly increased in recent decades, this has subjected these water bodies to extensive anthropogenic use, potentially negatively impacting their water quality and making them more susceptible to nutrient enrichment leading to eutrophication. An understanding of the hydrodynamics driving circulation and flushing times, and corresponding biogeochemical interactions, of small, eutrophic, temperate estuaries is less advanced than that of larger estuaries due both to a lack of available environmental data sets and difficulties in accurately modelling small scale systems. The overall aim of the research presented in this thesis is to
establish which physical drivers influence the circulation and water flushing times in small estuaries and to investigate the influence this has on the biogeochemistry and thus water quality of the case study microtidal Christchurch Harbour estuary in Dorset UK. A coupled depth-averaged hydrodynamic-biogeochemical model has been configured of the estuary using MIKE 21 software to define the physical controls on circulation and flushing times of the estuary and thus improve understanding of how these processes drive declines in water quality and dictate eutrophication development in small shallow basins. Results indicated circulation control changes from tidally to fluvially driven as riverine inputs to the estuary increase. Flushing times, calculated using a
particle tracking method, revealed the system can take as long as 132 hours to flush when river flow is low in summer months, or as short as 12 hours when riverine input is exceptionally high.

When total river flow into the estuary is less than 30 m3s−1, tidal flux is the dominant hydrodynamic control, which results in long flushing times during neap tides. Conversely, when riverine input is greater than 30 m3s−1 the dominant hydrodynamic control is fluvial flux and flushing times during spring tides are longer than at neaps. Instances of summer oxygen undersaturation and increased levels of chlorophyll were found to coincide with regions in the estuary yielding long residence times, even under low nutrient river water concentrations. Eutrophic and hypoxic conditions, defined in this instance as dissolved oxygen concentrations falling below 4 mg/L and 2 mg/L, respectively, for a duration of at least 4 hours, were observed in summer month simulations. Inverse relationships between time of oxygen undersaturated and both river flow and river nutrient concentration were observed but with no significant correlation between time undersaturated and summer solar irradiance which is attributed to the estuary’s shallow nature. The results showed that although river flow controls estuarine renewal, river nutrient concentration plays the greatest role in driving eutrophication development in small, shallow semi-enclosed basins like Christchurch Harbour. The methodology presented in this thesis shows modelling at small spatial scales is possible with the findings suggesting easy application to other similar estuaries across the world to infer conditions which may present deleterious effects on ecosystem health.

Text
Huggett, Rachel_PhD Thesis_Dec_2021 - Version of Record
Available under License University of Southampton Thesis Licence.
Download (269MB)
Text
Huggett, Rebecca_Permission to deposit
Restricted to Repository staff only
Available under License University of Southampton Thesis Licence.

More information

Published date: 6 December 2021

Identifiers

Local EPrints ID: 452916
URI: http://eprints.soton.ac.uk/id/eprint/452916
PURE UUID: 5a6c93bf-89ac-438e-bcd1-8d962f5c9686
ORCID for Duncan Purdie: ORCID iD orcid.org/0000-0001-6672-1722

Catalogue record

Date deposited: 06 Jan 2022 17:49
Last modified: 17 Mar 2024 02:32

Export record

Contributors

Author: Rebecca Huggett
Thesis advisor: Duncan Purdie ORCID iD

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×