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Context: In Australia's Murray–Darling Basin, the invasive common carp (Cyprinus carpio) poses a significant ecological threat. Efforts to manage their population include the potential use of cyprinid herpesvirus (CyHV-3) as a biological control agent. The environmental benefits of carp elimination could be substantial in the long term; our knowledge of the immediate ecological consequences resulting from large-scale fish die-offs remains limited. Aim: This study sought to investigate the effect of a large-scale fish-mortality event on water quality. Methods: Experiments of an increasing scale (bucket, mesocosm and whole wetland) were conducted to assess how decomposing carp carcasses alter dissolved oxygen and nutrient concentrations in water. Key results: In mesocosms, dead carp decayed more rapidly at 18 than 12°C, yielding oxygen demands of 1.022 ± 0.029 and 0.496 ± 0.239 mg kg−1 min−1 respectively. This carp decay released phosphorus, yielding 2121.1 ± 140.4 mg kg−1. In the wetland experiment, carp addition of 2400 kg ha−1 resulted in anoxic conditions over the following 2 weeks. The release of dissolved organic carbon and lipids led to a peak biological oxygen demand of 95.3 mg L−1. Conclusions: Carp decomposition considerably contributes to biological oxygen demand and algal growth through nutrient enrichment and is strongly influenced by carp density. Implications: The finding highlights key factors to consider before using biocontrol agents causing mass carp mortality, including hypoxia, anoxia and increased risk of harmful algal blooms. Investigations into the decomposition of common carp across three experiments of an increasing scale (bucket, mesocosm, wetland) showed a considerable release of phosphorus, with faster decomposition rates in warm conditions. The addition of 2400 kg ha−1 of carp induced anoxia in the wetland, underscoring essential considerations deploying biocontrol agents (e.g. cyprinid herpesvirus, CyHV-3) causing mass carp mortality. [ABSTRACT FROM AUTHOR]

Despite the growing use of Aquatic Ecosystem Models for lake modeling, there is currently no widely applicable framework for their configuration, calibration, and evaluation. Calibration is generally based on direct data comparison of observed versus modeled state variables using standard statistical techniques, however, this approach may not give a complete picture of the model's ability to capture system‐scale behavior that is not easily perceivable in observations, but which may be important for resource management. The aim of this study is to compare the performance of "naïve" calibration and a "system‐inspired" calibration, an approach that augments the standard state‐based calibration with a range of system‐inspired metrics (e.g., thermocline depth, metalimnetic oxygen minima), to increase the coherence between the simulated and natural ecosystems. A coupled physical‐biogeochemical model was applied to a focal site to simulate two key state‐variables: water temperature and dissolved oxygen. The model was calibrated according to the new system‐inspired modeling convention, using formal calibration techniques. There was an improvement in the simulation using parameters optimized on the additional metrics, which helped to reduce uncertainty predicting aspects of the system relevant to reservoir management, such as the occurrence of the metalimnetic oxygen minima. Extending the use of system‐inspired metrics when calibrating models has the potential to improve model fidelity for capturing more complex ecosystem dynamics. Key Points: We assessed the use of system‐inspired metrics in a novel approach to calibrating Aquatic Ecosystem Models (AEMs)The use of system‐inspired metrics in calibration improved model performance relative to traditional calibration methodsImplementation of system‐inspired metrics has the potential to greatly improve model prediction of complex ecosystem dynamics [ABSTRACT FROM AUTHOR]

Shallow coastal lagoons with restricted connection to the ocean are often productive but can also be sensitive to nutrient enrichment and hydrologic changes. Resolving nutrient dynamics is important for their sustainable management, yet being able to accurately resolve nutrient budgets has remained a challenge due to their complex hydrological regimes and habitat heterogeneity. In this study, we undertake a systematic nutrient budget of a large shallow hypersaline lagoon (Coorong, South Australia), with assistance of a high‐resolution coupled hydrodynamic‐biogeochemical model, to demonstrate the conditions that lead to nutrient retention. Under current conditions, high rates of evapo‐concentration and limited water connectivity have led to a persistent accumulation of nutrients and poor water quality in substantial areas of the lagoon. The interplay between hydrological drivers and biogeochemical processes was quantified using an adjusted Damköhler number, comparing the timescales of nutrient flushing versus processing. This showed a general transition from hydrologic control to biogeochemical control with increasing distance from the main ocean connection, modified by episodes of increased flows and external loads. Whilst water age was a useful indicator of the factors controlling rates of nutrient retention, interannual variability in retention between areas of the lagoon was explained based on river flows and changes in mean sea level. As the system has been affected by reduced flows over past decades, the results provide evidence that increasing river flow to the lagoon would reduce the nutrient retention, and we discuss the potential for net nutrient export to the ocean under sustained high flows. Plain Language Summary: Nutrient loads to coastal waters have increased worldwide as a direct consequence of the growing human population and increased development, with the end result being degraded water quality. Coastal lagoons with topographically restricted connection to the ocean are particularly sensitive relative to other coastal ecosystems since poor flushing and evaporation promotes nutrient retention. Resolving processes and factors that "control" nutrient concentrations is therefore important for guiding water quality management and ecosystem restoration. We studied nutrient dynamics in the Coorong, a shallow hypersaline coastal lagoon located in South Australia, using a high‐resolution coupled hydrodynamic‐water quality model. We showed that reducing flows to the system, and low mean sea level anomalies, encourage increased nutrient recycling and retention within the lagoon, resulting in persistent accumulation of nutrients in the south lagoon where water quality has become degraded. Based on the results, we conclude that potential restoration measures must focus on increasing hydrologic control through managing flow volumes and/or ocean flushing in order to optimize nutrient export. Key Points: An adjusted Damköhler number was developed to quantitatively compare hydrologic versus biogeochemical controls on nutrient dynamicsA transition from hydrologic control to biogeochemical control with increasing water retention time led to higher nutrient retention ratesAnnual nutrient retention rates were co‐modulated by catchment inflows and mean sea levels [ABSTRACT FROM AUTHOR]