Summary
Larne Lough is a coastal embayment on the north-east coast of Ireland, and is an important economic and environmental resource supporting a wide range of interests. The aims of this project were to investigate the trophic status of Larne Lough with regards to European legislation. This was achieved by the calculation of nutrient loads to the system, the collection of water quality, algal biomass, and hydrographic data. The data was then analysed using mathematical modelling.
The load of Dissolved Inorganic Nitrogen (DIN) to Larne Lough is dominated by contributions by riverine inputs. STWs contributed an average of 18% to the total load of DIN to the Lough during the spring and summer months. Salinity within the Lough is generally greater than 33 psu demonstrating the predominantly marine nature of the Lough. Soluble Reactive Phosphorous (SRP), Silicate (SiO2), Nitrate (NO3) and Ammonium (NH4) concentrations are highest in the winter and lowest in the summer, due to greater loads during the winter and biological uptake during the summer. Winter concentrations of DIN and DIP show that they are not elevated above the background levels for this area. Winter N:P and N:Si ratios are not elevated. Chl a (Chlorophyll a) concentrations do not exceed > 10 μg l-1. Higher concentrations are generally observed towards the southern part of the Lough.
Mathematical modelling shows that the flushing time of the Lough is less than 1 day, and therefore nutrient concentrations within the Lough are predominantly a reflection of concentrations in the North Channel. Predicted nutrient concentrations within the Lough were close to observed values, suggesting that the effect of internal processes on nutrient concentrations is small relative to the flushing time of the Lough. Significant negative relationships between Chl a and DIN were only observed on 2 sampling dates. This in combination with sufficient nutrient concentrations to maintain algal growth suggest that physical exchange is the predominant limitation to phytoplankton production in Larne Lough. It is suggest that the residence time is greater in the southern part of the Lough which gives rise to higher Chl a concentrations observed in this region. Classification of the Lough under criteria to assess trophic status (DEFRA, 2002) show that the Lough is presently not eutrophic or likely to become eutrophic in the future.
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