Organic matter and nitrogen cycling in a heavily modified coastal lagoon
Date of Award
Doctor of Philosophy
School of Science
Coastal waterbodies and their catchments have been highly modified, leading to altered flushing and eutrophication. Strategies to manage water flow to either maintain water levels or reduce salt-water intrusion and mitigate impacts to coastal waterbodies include engineering approaches such as the construction of surge barriers and river diversions and manipulation of sandbars. Climate change is increasingly impacting coastal waterbodies with predictions of increased drying and significant changes to rainfall patterns. Consequently, engineering management strategies are likely to increase, but it is unclear how biogeochemistry and benthic cycling in coastal waterbodies will be affected, and how to manage the likely eutrophication issues that ensue. Therefore, the aim of this project was to determine how organic matter and nutrients are transported and cycled within a heavily modified intermittently closed/open lakes and lagoons (ICOLL). The Vasse Wonnerup Wetland System (VWWS) is a modified eutrophic ICOLL in southwestern Australia. It has been managed for over 100 years and has multiple surge barriers, river diversions, an oxygenation plant, and an artificially managed sandbar. In addition, significant portions of the VWWS seasonally dry out, making it an ideal system to study the effects of climate change to coastal systems which are likely to experience similar modifications as the VWWS. Stable isotope analyses and mixing models showed that the particulate organic matter (POM) in the system is derived mainly from autochthonous sources (fringing vegetation and aquatic macrophytes). Similarly, compound-specific stable isotopes showed that the sources of dissolved organic matter (DOM) are mainly autochthonous and dominated by dissolved organic nitrogen (DON). The extremely low ( < detection limit) concentrations of dissolved inorganic nitrogen (DIN; nitrate and ammonium) in the basin water column suggests that DON is crucial to sustaining a DIN supply in the VWWS through decomposition and tight cycling between DON and DIN. Currently, national and international management guidelines focus on inorganic nutrient concentrations as indicators of unacceptable concentrations (trigger values) and management strategies are generally focused upon reducing allochthonous (external) dissolved inorganic nutrients (i.e., nitrate, ammonium, and phosphate). This study shows that the focus of management on inorganic nutrients may not be well placed in this type of system. Benthic flux experiments demonstrated that water column DO and seasonal drying of the sediment did not affect dissolved organic C, N or P fluxes significantly but did influence benthic metabolism with higher rates occurring in high water column DO conditions. Despite this, benthic metabolism remained anaerobic. Surprisingly, decreasing water column DO did not influence net greenhouse gas (GHG) emissions indicating increasing water column DO will not decrease GHG emissions. Oxygenation of the water column did increase N removal, with higher net N2 effluxes with increasing water column DO. Bioavailable nitrogen pools the water column were supplemented in low DO conditions by N2O, with consumption of N2O occurring during dark hours. The lack of significant effects from DO manipulation treatments on many of the measured nutrient species indicate that maintenance of water column oxic conditions, regardless of the concentrations are unlikely to be effective in promoting removal or storage of nutrients in eutrophic systems. Increasing drying out of coastal waterbodies will have impacts on benthic metabolism, however this issue may become system specific depending on sandbar and surge barrier management strategies influencing water levels. Overall, this study confirmed the importance of autochthonous OM contributions and cycling in an ICOLL, whilst highlighting the impacts of engineered modifications in this type of coastal waterbody and its catchment.
Access to this thesis is embargoed until 28 June 2024.
McCallum, R. (2022). Organic matter and nitrogen cycling in a heavily modified coastal lagoon. https://ro.ecu.edu.au/theses/2552