1. Lough Neagh is a large eutrophic lake covering 387 km(2) with a mea
n depth of 8.9 m. It is an important natural resource, being the large
st single source of potable water for Belfast, Northern Ireland. 2. Th
is report examines the causes of the year-to-year variation in the Apr
il-June (spring) algal biomass, measured as chlorophyll a, for the per
iod 1974-92. 3. The spring chlorophyll a declined following the introd
uction of a phosphorus (P) reduction programme at major sewage treatme
nt works in 1981. However, since 1990 the chlorophyll a concentrations
in the spring have increased. 4. Time series methodology was employed
to develop a model which explained 76% of the annual variation in spr
ing chlorophyll a concentrations. 5. The independent variables used in
the multiple regression model were the previous year's spring chlorop
hyll a concentration, soluble reactive P inputs for April-June and the
particulate P concentration in the Lough during the previous summer.