Abstract

1. Lough Neagh is one of the most important non-estuarine sites in the British Isles for overwintering wildfowl. A change in the waterbird assemblage following the winter of 2000/2001 was driven mainly by a rapid decline in the population of overwintering diving ducks. Sudden and discrete changes in resident as well as migratory waterbirds may suggest an intrinsic cause. 2. We compared the density and biomass of benthic macroinvertebrates, the food of overwintering diving ducks, in 2010 (following the diving duck population decline) with values from a baseline survey conducted in 1997/1998 (before the decline in diving ducks). 3. The mean total density of macroinvertebrates declined significantly by c. 65% from 15300m(-2) in 1997/1998 to 5136m(-2) in 2010. There was a concomitant c. 70% decline in mean macroinvertebrate biomass from 15667mg m (-2) to 5112mgm(-2). In terms of taxonomic composition, the relative contribution of Tanypodinae, Glyptotendipes spp. and Tanytarsini declined, while the relative contribution of Chironomus spp. increased. 4. We describe a shift in chlorophyll-a concentration, a proxy of phytoplankton biomass, in the largest freshwater lake in the British Isles coincident with a significant reduction in macroinvertebrate density and biomass, with potential implications for ecosystem processes and ecologically and economically important consumer populations, including waterbirds and fishes.