Degradation of water quality in Lough Neagh, Northern Ireland, by diffuse nitrogen flux from a phosphorus-rich catchment

Annually resolved fossil records of nitrogen (N) inputs (as sedimentary delta N-15, N content), aquatic production (delta C-13, C content), and algal abundance and gross community composition (pigments, nonsiliceous microfossils) from Lough Neagh, Northern Ireland (NI), were compared with annual records of climatic variability, atmospheric and urban nutrient loading, whole-catchment nutrient budgets, and limnological monitoring data to identify the unique effects of N on the eutrophication of a phosphorus (P)-rich lake during ca. 1933-1995. Cluster analysis revealed two major biostratigraphic zones. Zone I (ca. 1933-1955) was characterized by moderate lake production, as inferred from low concentrations of most fossil pigments and reduced delta N-15 signatures but elevated delta C-13 values and chlorophyte microfossil concentrations. In contrast, Zone II (ca. 1955-1995) exhibited greatly increased contents of N-15, N, C, and algal pigments, combined with strongly reduced delta C-13 ratios and chlorophyte fossil abundance, a pattern consistent with recent severe eutrophication. Overall, microfossils of diazotrophic cyanobacteria were most abundant during the transition period between zones (ca. 1955-1964). Regression analysis revealed that past N influx to the lake (as delta N-15; r(2) = 0.916, p < 0.0001), colonial cyanobacterial abundance (as myxoxanthophyll; r(2) = 0.837, p < 0.0001), and total algal standing crops (as beta-carotene; r(2) = 0.388, p < 0.0001) were all strongly correlated to agricultural inputs of N to NI farmland, weakly correlated to P inputs to NI farmland (r(delta 15N)(2) = 0.503, p < 0.0001; r(cyanobacteria)(2) = 0.296, p < 0.0001; r(total algae)(2) = 0.046, p > 0.05), and uncorrelated to most measures of climatic variability and atmospheric or urban nutrient inputs. Thus, degradation of water quality during the 20th century resulted from excessive loading of diffuse N to the lake from P-rich agricultural lands.