Degradation of water quality in Lough Neagh, Northern Ireland, by diffuse nitrogen flux from a phosphorus-rich catchment
Limnol. Oceanogr., 52(1), 2007, 354-369 | DOI: 10.4319/lo.2007.52.1.0354
ABSTRACT: Annually resolved fossil records of nitrogen (N) inputs (as sedimentary δ15N, N content), aquatic production (δ13C, 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 δ15N signatures but elevated δ13C values and chlorophyte microfossil concentrations. In contrast, Zone II (ca. 1955-1995) exhibited greatly increased contents of 15N, N, C, and algal pigments, combined with strongly reduced δ13C 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 δ15N; r2 = 0.916, p < 0.0001), colonial cyanobacterial abundance(as myxoxanthophyll; r2 = 0.837, p < 0.0001), and total algal standing crops (as b-carotene; r2 = 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 (r2 δ15N = 0.503, p < 0.0001; r2cyanobacteria = 0.296, p < 0.0001; r2total algae = 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.