Summary
Carlingford Lough is a coastal embayment and supports a wide range of aquaculture and fishing interests, much of which is protected by a range of conservation schemes. With regards to this, the aims of this project were to investigate whether eutrophic conditions currently exist in Carlingford by the collection of water quality and hydrographic data, and the calculation of nutrient loads to the system.
Direct inputs of nutrients are predominantly to the inner Lough. On an annual basis the Newry River contributes 84, 24, and 57 % of the total loads of NO3, NH4 and SRP respectively, and is clearly the most important source of nutrients. STWs are the largest source of NH4 although ammonium contributes < 2 % of the total load of DIN. The load of NO3 is seasonal with the load in winter months typically being > 100 T month-1, but during the summer loads are generally reduced to < 50 T month-1.
Freshwater discharge from the Newry River did affect the salinity of the Lough, although generally it was well mixed both vertically and horizontally. Nutrient concentrations in the Lough decreased on moving seawards, due to the major sources discharging at the head of the Lough, and subsequent dilution with nutrient poor water from the Irish Sea. Nutrients showed the characteristic seasonal patterns of summer minima and winter maxima found in temperate waters. Following the initial spring bloom of phytoplankton, NO3 concentrations were generally reduced to < 1 μM in the mid and outer Lough until September and the N:P ratio fell below 15. At some stations in the inner Lough, concentrations of NO3 were higher than the mid and outer during the summer probably due to a combination of the nutrient sources discharging to this zone and remineralization of nutrients in the shallow water and sediments.
The phytoplankton population was dominated by diatoms, but dinoflaggelates contributed significantly in June. Throughout the Lough chlorophyll concentrations increased from winter minimum concentrations to between 5 and 10 μg l-1 during March and April. Following this initial ‘bloom’, the stations could be broadly grouped into two according to the trends observed. Seaward of station 10 chlorophyll concentrations decreased to < 3 μg l-1, whereas concentrations were sustained at > 3 μg l-1 at stations landward of station 10. The evidence presented suggests that N is the primary limiting factor for phytoplankton growth, and therefore the sustained levels of chlorophyll in the inner Lough are probably caused by continued inputs of nutrients to this zone. There is no evidence to suggest that the Lough is currently eutrophic.
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