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Estuaries are productive ecosystems that provide important ecosystem functions such as the storage and cycling of dissolved organic matter (DOM) and nutrients. Intermittently closed/open lakes and lagoons (ICOLLs) can significantly impact biogeochemical processing and release of terrestrial nitrogen and carbon into the coastal environment due to longer residence times that can extend nutrient processing within the ICOLL. Pulses of nutrient release then occur when there is connectivity between the catchment and coastal waters. It remains unclear how modifications to estuaries and their catchments impact internal processes. To better understand the balance between autochthonous and allochthonous nutrients in a heavily modified ICOLL, multiple stable isotopes (δ13C, δ15N) of dissolved nutrients were used to evaluate seasonal and spatial changes to nitrogen sources and sinks in a southwest Australian ICOLL. The eutrophic status of water bodies has traditionally been based on concentrations of inorganic nitrogen (DIN) (particularly NH4+ and NOx) due to its presumed higher bioavailability and association with anthropogenic pollution. However, both NH4+ and NOx concentrations were low (0 – 12.5 µM) throughout the study area in both wet and dry seasons. Despite low surface water DIN concentrations, the system suffers from eutrophication issues such as algal blooms, low dissolved oxygen, and fish kills. The differences between the surface and porewater dissolved organic nitrogen (DON) and carbon (DOC) pools decreased in the wet season (high connectivity), suggesting that internal DOM turnover sustains eutrophication. This work demonstrates that including DON and its isotopic signature can be an effective way to study N in waterbodies with low DIN concentrations. It also highlights the need for DON, as a major constituent of total dissolved nitrogen, to be included in N studies and ultimately may have significant impact on the current understanding of the global N budget. [ABSTRACT FROM AUTHOR]