Effect of dissolved organic carbon from a point- and non-point-source of nutrients on oxygen demand of a subtropical estuary

Nutrient and organic matter inputs can drive hypoxia in estuarine waters. However, the relative effect of point- and non-point sources has not been directly compared, which can be crucial for developing targeted management strategies to address the specific impacts of each source. This study compared the effect of three nutrient sources, i.e., simulated soil erosion runoff (soil slurries), sewage treatment plant (STP) and aquaculture effluent, on dissolved oxygen (DO) demand in a subtropical estuary. We also chemically characterized the organic compounds in these sources. Using three-day incubations in the dark as a measure of the potential demand, we found that nutrient source-specific factors drove DO demand. For soil slurry treatments, dissolved organic carbon (DOC) concentration was most highly correlated with DO demand (R² = 0.52). For STP treatments, a combination of concentrations of total nitrogen (TN), phosphate (PO4-P), and ratios of TN: total phosphorus (TN:TP) and DOC: ammonium (DOC:NH4) were most correlated with DO demand (R² = 0.5). The combination of chlorophyll-a concentrations and DOC: TDN (total dissolved nitrogen) explained most of the DO demand for aquaculture treatments (R² = 0.80). Chemical characterization revealed that soil slurries contained organic compounds that were more bioavailable than STP and aquaculture effluents, with glucose and fatty acids being the dominant forms. Conversely, STP effluent comprised small organic acids that were likely derived from the breakdown of organic compounds in sewage, whilst aquaculture effluent had more lipophilic compounds. DOC bioreactivity, measured as mg DO mg^-1 DOC d^-1, of the three sources correlated with C:N ratios and N:P ratios. These results suggest that microbes in the estuarine water were organic carbon and nutrient limited, depending on the source and concentrations. Our findings highlight the importance of considering organic carbon concentration, form, and source in monitoring and managing nutrient and organic matter inputs to waterways.

Presentation Preference: Oral

Primary Presenter: Jing Lu, Griffith University (jing.lu1124@gmail.com)

Authors:

Jing Lu, Griffith University  (jing.lu@griffith.edu.au)

Michael Newham, Queensland Department of Environment, Science and Innovation  (michael.newham@des.qld.gov.au)

Ann Chuang, Griffith University  (a.chuang@griffith.edu.au)

Joanne Burton, Council of Mayors, South East Queensland  (joanne.burton@seqmayors.qld.gov.au)

Stephen Faggotter, Griffith University  (s.faggotter@griffith.edu.au)

Joshua Hayton, Griffith University  (j.hayton@griffith.edu.au)

Anthony Carroll, Griffith University  (a.carroll@griffith.edu.au)

Alexandra Garzon-Garcia, Queensland Department of Environment, Science and Innovation  (alexandra.garzon-garcia@des.qld.gov.au)

Michele Burford, Griffith Univerisity  (m.burford@griffith.edu.au)