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The ecosystem function of local fisheries holds great societal importance in the coastal zone of Cartagena, Colombia, where coastal communities depend on artisanal fishing for their livelihood and health. These fishing resources have declined sharply in recent decades partly due to issues of coastal water pollution. Mitigation strategies to reduce pollution can be better evaluated with the support of numerical hydrodynamic models. To model the hydrodynamics and water quality in Cartagena Bay, significant consideration must be dedicated to the process of light attenuation, given its importance to the bay’s characteristics of strong vertical stratification, turbid surface water plumes, algal blooms and hypoxia. This study uses measurements of total suspended solids (TSS), turbidity, chlorophyll-a (Chla) and Secchi depth monitored in the bay monthly over a 2-year period to calculate and compare the short-wave light extinction coefficient (Kd) according to nine different equations. The MOHID-Water model was used to simulate the bay’s hydrodynamics and to compare the effect of three different Kd values on the model’s ability to reproduce temperature profiles observed in the field. Simulations using Kd values calculated by equations that included TSS as a variable produced better results than those of an equation that included Chla as a variable. Further research will focus on evaluating other Kd calculation methods and comparing these results with simulations of different seasons. This study contributes valuable knowledge for eutrophication modelling which would be beneficial to coastal zone management in Cartagena Bay. © 2019, Springer Nature Switzerland AG.

A bay's capacity to buffer fluvial fluxes between the land and sea is sensitive to hydrological changes that can affect its water renewal rates. In Cartagena Bay, Colombia, pollution issues have been associated with freshwater fluxes which are projected to increase in future years. This has led to plans to reduce freshwater flows by constructing upstream hydraulic doors. Given the influence of freshwater discharge on coastal water renewal, it is important to assess how these upstream changes will affect the bay's hydrodynamic processes. This study calibrated the 3D MOHID Water model, configured with a high-resolution mixed vertical discretization to capture the bay's characteristic processes of vertical stratification and mixing. A Lagrangian transport model was used to analyze the flow of passive particle tracers and calculate water renewal time scales. Mean residence times of 3–6 days and flushing times of 10–20 days for canal water were found, while mean residence times of 23–33 days and flushing times of 70–99 days were calculated for the bay's complete water volume. An assessment of future scenarios showed that increases in freshwater runoff would result in faster water renewal in the bay, while plans to decrease freshwater discharge would result in slower water renewal in the bay. It is therefore imperative that any plans for reducing fluvial fluxes into the bay be accompanied by the control of local pollution sources, which are abundant and could worsen the bay's water quality issues should water renewal times become longer. © 2019 Elsevier Ltd

The identification and prioritization of pollution sources is essential to coastal zone management. This task is complicated when a variety of pollution sources are found and by limited data availability, which can result in an inconclusive assessment and differing public perceptions, ultimately hindering the progress of management actions. This is the case in Cartagena Bay (Colombia), a Caribbean hot-spot of pollution, which receives large freshwater discharges from the Magdalena River drained via the Dique Canal along with coastal industrial effluents and untreated domestic wastewater from parts of the coastal population. This study presents a methodology for the integrated assessment of anthropogenic pollution sources discharged into the coastal zone by estimating their loads and comparing their relative contributions to receiving coastal waters. Given the lack of available data on discharges and water quality, an integrated approach is applied by combining various methods of load estimation while emphasizing the importance of calculating confidence intervals for each load value. Pollution loads from nearby sources of domestic wastewater, coastal industrial effluents and continental runoff were assessed with respect to their contributions of coliforms, total suspended solids, nitrogen, phosphorus, and biological oxygen demand (BOD). Loads from the canal's surface runoff were calculated with monthly discharge and water quality data. Domestic loads were computed using GIS analyses of population and sewerage coverage in combination with export coefficients of daily load per capita. Industrial loads were estimated based on previous studies. Results show that each type of land-based source is responsible for different pollution impacts observed in Cartagena Bay. Occasionally, inadequate recreational water quality can be attributed to nearby sources of domestic wastewater, which contribute the highest coliform load (6.7 ± 3.9 × 1015 MPN/day). Continental runoff via the

Cartagena Bay, one of the Caribbean's hot spots of pollution, is an estuarine system connected to the Caribbean Sea by two straits. Large freshwater discharges from the Dique Canal into the south of the bay produce estuarine conditions strongly related to the seasonal variability of runoff from the Magdalena River watershed. The bay's seasonal conditions may be characterized by three seasons: strong winds/low runoff (Jan.–April), weak winds/intermediate runoff (May–Aug.), and weak winds/high runoff (Sept.–Dec.). This coastal zone is known to be impacted by land-based sources of pollution, including continental runoff, industrial effluents and domestic wastewater. However, previous studies have not sufficiently ascertained the spatio-temporal extent of this pollution. This study addresses the following research question: What is the current extent of water and sediment pollution in Cartagena Bay and which factors control its seasonal variability? Monthly seawater samples (Sept.2014–Aug.2015) were taken from surface and bottom depths at 16 stations in and around Cartagena Bay and analyzed for physical, chemical, and biological parameters. Surface sediments were sampled from the bay's bottom every three months and analyzed for various trace metals. Seasonal variability was observed in nearly all of the water quality parameters, with higher concentrations usually coinciding with the high runoff season. Potential pollution impacts are shown by wet-season averages of total suspended solids (45.0 ± 89.5 mg/l), turbidity (26.1 ± 59.7 NTU), biological oxygen demand (1.20 ± 0.91 mg/l), chlorophyll-a (2.47 ± 2.17 µg/l), nitrate (171.1 ± 112.6 µg/l), phosphate (43.1 ± 63.5 µg/l), total phosphorus (85.3 ± 77.2 µg/l), phenol (2.9 ± 17.4 mg/l), faecal coliforms (798 ± 714 MPN/100 ml) and enterococci (32 ± 30 CFU/100 ml) in excess of recommended threshold values for marine conservation and recreational adequacy. The bay's hypoxic conditions are evident with low dissolved oxygen con

The Caribbean Sea provides significant ecosystem services to the livelihood and well-being of countries in the region. Protection of the marine ecosystem requires policy on coastal water quality that considers ecologically-relevant thresholds and has a scientific foundation linking land-based discharges with seawater quality. This study demonstrates a practical method for setting local-scale coastal water quality targets by applying this approach to Cartagena Bay, Colombia, and setting targets for end-of-river suspended sediment loads to mitigate offshore coral reef turbidity. This approach considers reef thresholds for suspended sediments and applies a field-calibrated 3D hydrodynamic-water quality model (MOHID) to link the marine thresholds to fluvial loads. Monitoring data showed that suspended sediments were consistently above the coral reef ecosystem threshold of 10 mg/l, and the model adequately reproduced field observations. It was shown that ecosystem thresholds could be maintained within the extent of the bay by reducing suspended sediment loads in the Dique Canal from current load estimates of 6.4 × 103 t/d (rainy season) and 4.3 × 103 t/d (transitional season) to target loads of 500–700 t/d, representing reductions of ~80–90%. These substantial reductions reflect ongoing issues in the Magdalena watershed which has experienced severe erosional conditions and intense deforestation over the past four decades. The presented method is practical for countries without access to long-term datasets, and could be applied to other parameters or discharge types. The method is particularly beneficial for developing site-specific targets, which are needed considering the natural and anthropogenic variability between different coastal zones and water bodies. © 2019 Elsevier Ltd