Stable isotopes trace estuarine transformations of carbon and nitrogen from primary- and secondary-treated paper and pulp mill effluent

Stable isotope analysis of a novel combination of carbon and nitrogen pools traced inputs and processing of primary-treated (PE) and secondary-treated effluent (SE) from a paper and pulp mill (PPM) in a temperate Australian estuary. Distinct carbon stable isotope ratios of dissolved organic carbon (DOC) near the PPM outfall indicated large PE and reduced SE inputs of DOC. DOC was remineralized to dissolved inorganic carbon regardless of season, but rates were lower in winter. PE discharge in winter elevated DOC concentrations along much of the estuary. Distinct stable isotope ratios confirmed particulate organic matter (POM) input from PE and SE to the water column and into the sediment. This was relatively localized, indicating rapid POM settlement regardless of season. SE discharge increased nutrient inputs and enhanced algal productivity, particularly in summer when chlorophyll-a concentrations were elevated throughout the estuary. SE discharge reduced pCO₂ from levels associated with PE discharge. However, the estuary remained heterotrophic as subsequent respiration or decomposition of algal material offset reductions in PPM organic matter input. The influence of the PPM was apparent throughout the estuary, demonstrating the ability of anthropogenic inputs, and changes to these, to affect ecosystem functioning.