Assessing eutrophic conditions in local inshore areas

Abstract

In this study, the presence of eutrophic conditions in Grand Harbour, Marsamxett Harbour and Marsaxlokk Bay was investigated using the TRIX index. Surface and subsurface water samples were collected from 10 to 11 fixed stations at each location. Seasonal field surveys were carried out from December 2009 to December 2010. Seawater quality parameters including temperature, salinity, dissolved oxygen, water transparency and chlorophyll a were measured on site during the field surveys. Additionally, seawater samples were collected and analysed in the laboratory for nitrate, phosphate and total phosphorus concentrations, as well as for faecal coliform levels. The Kriging interpolation method of the ArcGIS Spatial Analyst toolbar was also used to create the Kriging models from TRIX index values for all three areas of study. Additionally, the boat drift observed during the field survey was used so as to deduce surface water current direction. At Grand Harbour mediocre and poor water quality were recorded mostly in inshore stations 6 and 10 located in the Senglea - Vittoriosa region, and in the Marsa - Kordin region respectively, especially in January 2010. At the water surface in station 6 a TRIX value of 5.215 was recorded, and at the water surface in station 10 a TRIX value of 6.203 was obtained in January 2010. In this same sampling session a TRIX value of 5 .088 was recorded for waters at Sm depth at station 6. Both locations are under intense anthropogenic pressures, and accommodate a number of land- and marine-based sources of nutrient and sewage input. Also, Marsa valley which is interconnected to several other valleys empties its nutrient-rich rainwater runoff directly into the inshore areas of the Grand Harbour. At Marsamxett Harbour mediocre and poor water quality were recorded mostly in inshore stations located in Msida Creek, Pieta' Creek and Sliema Creek. In January 2010, TRIX values of 5.967 and 5.501 were recorded in Msida Creek, a TRIX value of 5.243 was monitored in Pieta' Creek, while a TRIX value of 5.055 was obtained at Sliema Creek, at the surface water level. In April 2010, a TRIX value of 6.014 was recorded in Pieta' Creek, and in July 2010 a TRIX value of 5.027 was obtained in the surface waters of the same sampling station. Eutrophic conditions were most evident at Pieta' Creek. This area accommodates the berths for the Malta-Gozo channel ferry, while the Haywharf, Sa Maison coastline located close to the Creek is used by the Maritime Unit of the Armed Forces, which operate a number of patrol boats. Such vessels result in sewage discharge, thus threatening the marine quality of the enclosed area. Recurrent spring 'red tides' have previously occurred in this creek. Additionally, a sewage pumping station is located in Pieta' Creek. TRIX index values, and thus eutrophic conditions, were observed to diminish gradually from the field survey carried out in January 2010 till the monitoring session carried out in July 2010. At Marsaxlokk Bay the overall water quality recorded in both inshore and offshore stations was mainly classified as excellent or good for all four monitoring sessions carried out during this investigation. This was indicated by the low TRIX index values, ranging from 2 to 5, obtained for this area, and was further confirmed by the Kriging models produced for Marsaxlokk Bay. However, certain inshore areas such as Pretty Bay, St. George's Bay and Marsaxlokk Bay, as well as the waters along the Malta Freeport Terminal are at risk of eutrophication, unless immediate action is taken. It is clear that the main causes which are leading to eutrophic conditions in these areas are the release of untreated sewage and surface runoff containing nutrients. Such pollutants are leaving their negative impact mostly on inshore areas where there is lack of water circulation, especially during the winter months when runoff increases due to increased precipitation. Eutrophic conditions in offshore open waters may be attributed to illegal sewage and organic waste discharge from vessels at berth, as well as while leaving the harbour. Eutrophication is characterised by low dissolved oxygen levels, as well as high chlorophyll a levels, increased water turbidity and elevated nutrient concentrations. It is evident that eutrophic conditions are more pronounced in Grand Harbour, followed by Marsamxett Harbour and Marsaxlokk Bay, where eutrophic conditions are the least present.