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

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.

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ORCID for Duncan Purdie: orcid.org/0000-0001-6672-1722

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