Seasonal variations in sources and cycling of nitrogen and carbon in a tropical mangrove-lined creek impacted by treated sewage effluent

  • Kanchana Niwanthi Warnakulasooriya

    Student thesis: Doctor of Philosophy (PhD) - CDU


    Sewage effluent can be a major point source of nutrients in receiving waterways if conventional sewage treatment methods cannot effectively reduce nutrient loads. I investigated the dispersion and ecosystem uptake of nutrients from treated sewage discharged into Buffalo Creek, a macro-tidal mangrove ecosystem near Darwin, Northern Australia. Prior to this study, the dispersion and assimilation of sewagederived nutrients in Buffalo Creek ecosystem were poorly constrained. This study combines for the first time a multi-isotope approach to detect and quantify the influence of treated sewage effluent, which is the main nutrient source in Buffalo Creek. Nutrient dynamics in the creek were mainly governed by physical processes on spring tides and wet season neap tides, while both physical and biogeochemical processes were important during dry season neap tides. On spring tides, the creek was well flushed and showed an improved water quality compared to neap tides. High loads of stormwater inputs during wet season neaps and minimum in-situ processing of ammonium during dry season neaps, allowed transport of effluentderived nutrients downstream, and export to Shoal Bay. Water column denitrification and benthic nutrient fluxes were important biogeochemical processes during this period. Sedimentation of freshwater phytoplankton and sediment resuspension were important physical process during dry season neaps. Thus, sediment in the creek acts as a sink for sewage-derived N and a source of dissolved inorganic N to the water column. Mangrove leaves and gastropods contained a 15N/14N enriched sewage signal along the creek compared to samples in the control creek, indicating that uptake by mangroves and benthic feeders are an important pathway for sewagederived nitrogen. Sewage-derived nitrogen was retained in sediment and transferred to mangroves along the entire length of the creek but in the lower reaches of the creek tidal mixing supplied additional nitrogen sources in marine organic matter. Nitrogen isotope ratios measured in gastropods showed a down-creek gradient similar to mangrove leaves and sediment. However for gastropods, intra- and interspecific isotopic variations were influenced by spatial differences in available food sources. This variability in gastropod snails did not compromise their potential as early bio-indicators of sewage pollution. Mangrove leaves and gastropod snails are suitable bio-indicators to track dispersal and assimilation of sewage-derived nutrients. This study provided a more comprehensive and robust assessment of impacts of nutrient loading on mangrove ecosystems that will help guide future management practices on macro-tidal ecosystems.

    Note: Please note that Appendix D has been removed from the online version due to copyright restrictions.
    Date of Award2017
    Original languageEnglish
    SupervisorNiels Munksgaard (Supervisor)

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