ECN site code: L16
ECN monitoring by: Agri-Food and Biosciences Institute

Lough Neagh covers 386 km2 and is by far the largest area of freshwater in the British Isles. Situated in north-east Ireland, it has a drainage basin of 4450 km2, which is shared between Northern Ireland (91%) and the Republic of Ireland (9%). The average water retention time is 15 months. Although large in area, the lake is relatively shallow with a mean depth of 8.9m (max. 25 m). This, combined with its great size and a mild and windy oceanic climate, ensures that the water column is generally well mixed. The lake supports commercial fisheries for eels, pollan (Coregonus autumnalis pollan Thompson), perch and trout of which the eel fishery is the most significant, with an annual catch in the region of 600 t. Lough Neagh is hypertrophic with a mean annual total phosphorus concentration of 160 mg P l-1. Attempts to lower P concentrations in the lough by curtailing point sources of P have been unsuccessful due to increasing inputs from diffuse sources. Levels of P in the lough support large phytoplankton populations with annual chlorophyll a concentrations typically in excess of 60 mg l-1. The dominant alga is the cyanophyte Planktothrix agardhii (Komarek) Anagnostides and the phytoplankton is now less diverse than in the late 1960s when regular monitoring began. Since then there has been regular monitoring of the plankton, lake and river nutrient concentrations, which have been used to produce nutrient budgets for the lake.