Characterising the response of inter-tidal zone ecohydrology, to coastal hydrodynamics and anthropogenic nutrient loads

Gayan Lakendra Gunaratne Delkandura Arachchige

    Research output: ThesisDoctoral Thesis

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    Abstract

    Coastal embayments are important ecosystems with high biodiversity and social values often under threat from human activities. Biota living in the intertidal zone of coastal embayments are adapted to a dynamic environment dominated by the tidal flooding and ebbing of water with periods of tidal exposure. Seasonal changes in key biota, their influence on intertidal zone primary production, and the consequences for intertidal ecology, appear to be driven by a combined effect of anthropogenic nutrient delivery, climate factors and intertidal zone hydrodynamics.

    The objective of this project was to apply a research approach in field surveys with the involvement of the community and a range of modelling techniques to increase understanding of how sub-tropical catchment hydrology, nutrient delivery together with physical forcings that control intertidal zone hydrodynamics and environment variables affects the growth behaviour of marine cyanobacteria, Lyngbya Majuscula (hereafter Lyngbya) in an hypertidal coastal embayment. Lyngbya is a cyanobacterium that is found worldwide in tropical and subtropical coastal habitats, typically in low levels. During some parts of its life cycle it is toxic and has the potential to affect nearly the entire food web. The main research question was why Lyngbya occurs in an hypertidal coastal environment which generally discourage algal bloom with strong vertical mixing and strong flushing capacities. Finding answers for this question is challenging because the causes of toxic Lyngbya blooms are complex and multifaceted. This is more challenging when logistical issues confronted in remote and ungauged environments on top of the scientific problems. This study provides insight into the catchment, climate, ecological and physical processes controlling light and nutrients affecting Lyngbya blooms.

    A remote, ungauged catchment and intertidal zone, of Roebuck Bay, near Broome in sub-tropical Western Australia was the study site of this research. This Ramsar listed hypertidal coastal embayment is under pressure from Broome’s urbanisation with the discharge of nutrients from waste water effluent and fertilisers occurring through surface and groundwater pathways. This increasing wetland nutrient loads, is threatening the sensitive ecosystem of Roebuck Bay. In recent years, blooms of Lyngbya have risen in frequency and severity in the intertidal zone. The overall aim of this research was to investigate how changes in catchment hydrology, nutrient delivery, hydrodynamics and environmental conditions combine to affect nutrient dynamics, and influence the risk of Lyngbya growth. In order to achieve this, a two-year stormwater quality and quantity sampling campaign was undertaken to address knowledge and information gaps in the historical catchment and embayment data. These data were then analysed using statistical analyses and numerical modelling tools.

    The research was arranged into four parts. In the first part, Broome catchment was instrumented to quantify nutrient export to Roebuck Bay with a community participated sampling campaign. This led to a quantitative estimate of surface hydrology and nutrient dynamics from this region that displays distinct episodic wet season conditions. Nutrient delivery to the Roebuck Bay embayment was identified to be greatest from the original Broome town site sub-catchments compared to sub-catchments that have been progressively urbanised since 2000. Most sub-catchments showed a distinct seasonal first flush with an initial 30% of runoff volume containing 40-70% of the nutrient load, resulting in periodic shock loading of nutrients to the bay.

    The second research component analysed spatiotemporal changes in impervious surface areas over time to understand how a progressive increase in urbanisation has changed nutrient wash-off. With the increase in impervious surfaces, annual stormwater runoff volume and nutrient concentrations were predicted as having increased by 57% between 1981 and 2012. Nutrient loads increased from 378 g/ha/year and 64 g/ha/year in 1981 to 588 g/ha/year and 100 g/ha/year in 2012 for total nitrogen and total phosphorous respectively.

    The third research component focuses on understanding the significance of physical and environmental variables on the seasonal occurrence of Lyngbya blooms in the intertidal zone of Roebuck Bay. A three-dimensional coupled hydrodynamic and water quality model, TUFLOWFV-AED, was used to investigate the hydrodynamics and nutrient dynamics in intertidal zone. Findings suggested that nutrient availability is a necessary precondition for blooms but does not guarantee blooms will occur. The timing of nutrient loading in the context of other temporally varying parameters is critical. These parameters include physical and environmental properties such as temperature, turbulence intensity, tidal asymmetry, tidal exposure, photosynthetic active radiation, and water retention time.

    The final study offers a mechanistic description of the next generation of ecohydrological model required to predict Lyngbya biomass in an intertidal zone of a coastal ecosystem. The model structure is based on three key state variables of Lyngbya biomass; 1) benthic submerged, 2) floating and 3) benthic exposed. It links these three states with key environmental factors for growth: bioavailability of nutrients (including iron, phosphorus, nitrogen and dissolved organics); light, salinity and temperature regimes; biological and physical processes such as growth, mortality, respiration, sloughing, floating and beach wracking. Data requirements for improved setup and validation of the model for supporting ongoing management decisions are outlined in this study.

    This thesis highlights the importance and challenges of the integrated approach applied in this study to a remote subtropical coastal environment to achieve an improved understanding of how interacting driving factors affect nutrient pathways and lead to Lyngbya blooms. It is envisioned that this approach can be use in other similar systems under threat from increased nutrient loads and subsequent algal blooms. A key finding on Lyngbya is that the tidally exposed niche that favours it in competition for light is consistent with the nutrient rich thin submarine groundwater layer in tidally dominated aquatic environments.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - Feb 2015

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