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Volume 52, Issue 1 p. 354-369
Article
Free Access

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

Lynda Bunting

Corresponding Author

Lynda Bunting

Limnology Laboratory, Department of Biology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada

School of Geography, Archaeology and Palaeoecology, Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland

Corresponding author ([email protected]).Search for more papers by this author
Peter R. Leavitt

Peter R. Leavitt

Limnology Laboratory, Department of Biology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada

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Christopher E. Gibson

Christopher E. Gibson

Agricultural and Environmental Sciences Division, Department of Agriculture and Rural Development for Northern Ireland, Belfast BT9 5PX, Northern Ireland

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Edward J. McGee

Edward J. McGee

Experimental Radiation Research Laboratory, Department of Experimental Physics, University College Dublin, Dublin 4, Ireland

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Valerie A. Hall

Valerie A. Hall

School of Geography, Archaeology and Palaeoecology, Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland

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First published: 16 January 2007
Citations: 93

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.