Water and sediment quality in Cartagena Bay, Colombia: Seasonal variability and potential impacts of pollution

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 mu g/l), nitrate (171.1 +/- 112.6 mu g/l), phosphate (43.1 +/- 63.5 mu g/l), total phosphorus (85.3 +/- 77.2 mu g/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 concentrations (<4 mg/l) found at bottom depths during the wet season, moderate concentrations in the windy season, and low concentrations approaching surface waters during the transitional season, showing a seasonality related to the variability of water circulation and vertical stratification. Lower chlorophyll-a levels found in the water column during the wet season suggest that primary productivity in this eutrophic system is not limited by nutrients, which are abundant due to land-based effluents, but rather by water transparency which is significantly reduced during the wet season due to large sediment loads discharged from the Dique Canal. Sediments from the bay's bottom were found to have concentrations of mercury, cadmium, chromium, copper and nickel in excess of the Threshold Effects Levels (TEL) used as an indicator of potential impacts on marine life. (C) 2017 Elsevier Ltd. All rights reserved.