Published 08 June 2018
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The value of urban estuaries
As with the nexus between rivers and seas, estuaries (bodies of water usually found where rivers meet the sea) are important for thriving ocean systems and human populations. Healthy estuaries are biodiversity hotspots, carbon stores, and filters for contaminants (Potter et al., 2010, Tan et al., 2016, Johnston et al., 2015, Teuchies et al., 2013, Kayranli et al., 2010). Estuaries are also culturally and economically significant, contributing to local economies through tourism, transport, recreation, and extractive industry (Banks et al., 2016, Aguirre et al., 2016, Fernández et al., 2016). A large proportion of today’s major cities – including New York City, London, and Sydney – are located on highly modified estuaries, further highlighting the importance of these systems.
Threats to estuary health
Historically, the evolution of harbour cities around the world resulted in the degradation of estuary systems due to processes such as land clearing, reclamation, and the unregulated disposal of industrial effluent (Aguirre et al., 2016, Banks et al., 2016, Fernández et al., 2016, Birch, 2006). Despite regulations on such activities, estuary health remains threatened by anthropogenic pressures (Mayer-Pinto et al., 2015). One of these pressures is the direct input of metal contaminants (such as lead and arsenic) from urban stormwater runoff and groundwater leachate (Mayer-Pinto et al., 2015, Birch et al., 2013, Aguirre et al., 2016).
Metal contamination is problematic in estuary systems because sediments act as a contaminant sink and source, accumulating toxic levels of trace metals, and re-releasing metals into the water column when sediments are disturbed and system conditions are altered (Spooner et al., 2003, Birch, 2006). Once metal particulates are freed they become bioavailable, meaning metal contaminants can be taken up by organisms, often with lethal effects and, ultimately, detrimental ecosystem-wide impacts (Spooner et al., 2003). It is, therefore, necessary to address contaminated sediments in order to ensure overall estuary health.
Oyster reef restoration as a bioremediation approach
Oysters are sessile bivalves that form dense aggregations – or reefs – made up of living organisms and accumulations of dead shells that evolve with changes in sea-level rise and salinity over geological time (Mann and Powell, 2007, Gillies et al., 2018, Zhang et al., 2012). As filter feeders, oysters depend on the tidal flushing that occurs in tropical and temperate near shore coastal systems, such as bays and estuaries (Gillies et al., 2018, O’neil, 2016). Oysters diversify the benthic landscape, provide habitat for other organisms, stabilise the seashore, store carbon, and improve water quality through filtration, making them a potential tool for remediating degraded estuaries (Grabowski et al., 2012, Forrest et al., 2009). Problematically, however, overall estimates suggest that a total of 85% of oyster reefs throughout the world have been lost from urban estuaries (Gillies et al., 2018, Beck et al., 2011).
Oyster reef restoration projects (such as New York City’s Billion Oyster Project) aim to directly address the decimation of these biological structures in an attempt to restore the ecosystem services they provide (O’neil et al., 2016, Grabowski et al., 2012). The value that these projects contribute to their respective ecosystems has been estimated between $10,325 and $99,421 per hectare per year (Grabowski et al., 2012). However, the effect of oyster reef restoration projects on metal contaminated surface sediments, natural geochemical cycles, and local hydrodynamics is relatively unknown.
Research into the effects of oysters on their surrounding environment is mostly limited to water quality monitoring or aquaculture perspectives (Forrest and Creese, 2006, Forrest et al., 2009, Edge et al., 2014, Lee et al., 2016). From such research, it is known that, in addition to performing various ecosystem services, oysters have the ability to accumulate toxic metal contaminants without suffering lethal effects (Spooner et al., 2003). However, it is unknown if oysters can reduce the number of metal contaminants in surface sediments. In addition, oyster farms have been found to alter surrounding sediments through the deposition of fine-grained, organic-rich wastes and by altering water flow, thus potentially producing long-term environmental consequences to estuarine systems (Forrest and Creese, 2006, Forrest et al., 2009). Therefore, the aim of my honours thesis is to assess oyster reefs as a bioremediation strategy for metal contaminated sediments in urban estuaries.
In order to achieve this aim, I will be examining the surface sediments and wave regime surrounding a group of established oyster reefs in Sydney’s Botany Bay, an extensively modified estuary home to an international airport and commercial port. Examining surface sediments for metal contaminants, nutrients, and organics will provide an indication of the local effects of oyster reefs on the benthic environment. Investigating wave regime at an established oyster reef will elucidate the relationship between local hydrodynamics, sediment particle size, and the flushing of contaminants. I hope my research will contribute to ensuring future estuary health in Sydney and beyond, for the good of the oceans and the people that depend on them.
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Carolina Mora is an Honours Student with the School of Geoscience’s Geocoastal Research Group. Under the supervision of Associate Professor Ana Vila-Concejo and Dr Bree Morgan, Carolina is investigating the effects of established oyster reefs on surrounding sediments and local hydrodynamics in urban estuaries. Carolina’s thesis will contribute to the Sydney Institute of Marine Science’s work on improving harbour health, with implications for global harbours. Carolina is passionate about ocean ecosystems and has volunteered for marine research centres locally and overseas.
This blog is a part of SEI’s Student Blog Series, which features original content by Honours, Masters and PhD students at the University of Sydney who are undertaking research on environmental issues and topics. If you are a current postgraduate student at the University of Sydney who would like to participate in the series, click here for details.