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

Abstract

Annually resolved fossil records of nitrogen (N) inputs (as sedimentary d 15 N, N content), aquatic production (d 13 C, 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 d 15 N signatures but elevated d 13 C values and chlorophyte microfossil concentrations. In contrast, Zone II (ca. 1955II (ca. -1995 exhibited greatly increased contents of 15 N, N, C, and algal pigments, combined with strongly reduced d 13 C 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 d 15 N; r 2 5 0.916, p , 0.0001), colonial cyanobacterial abundance (as myxoxanthophyll; r 2 5 0.837, p , 0.0001), and total algal standing crops (as b-carotene; r 2 5 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 2 d15N 5 0.503, p , 0.0001; r 2 cyanobacteria 5 0.296, p , 0.0001; r 2 total algae 5 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.

Temporal dynamics of uncultured viruses: a new dimension in viral diversity

“…Apparently, in this eutrophic lacustrine ecosystem the main limiting factor is different, which 435 is in accordance with previous findings that in Lough Neagh nitrogen loading can have a 436 stronger long-term impact than phosphorus on lake eutrophication (Buntig et al, 2007). VCs (large-size clustering level).…”

The past and future of phytoplankton in the UK's largest lake, Lough Neagh

“…As was evident from Figure 6, this effect was enhanced even more when the reduction in nutrient loads was greater, and therefore nitrate became more scarce. Nitrate limitation appears to be more frequent in the Lough in recent years (McElarney et al, 2015a) and this nutrient has had more of an effect on the phytoplankton than phosphorus (Bunting et al, 2007). Our results suggest that even with decreased nitrogen and phosphorus loading from the catchment, nutrient dynamics will still play a dominant role in deciding phytoplankton species composition in the Lough.…”

“…The PROTECH model also reflects this low-light specialization in its modelling of Planktothrix, hence its continued dominance in most of the scenarios. Thus, whilst it has been speculated that its rise to dominance may have been greatly enhanced by nitrogen pollution to the lake (Bunting et al, 2007), a reduction in this nutrient does not guarantee a marked improvement in the lake. Such an effect is not uncommon in the recovery of shallow, nutrient-rich lakes (i.e.…”

“…Due to the resources required to further reduce catchment export of nutrients, it is useful to understand how the future trajectory of phytoplankton change may develop in the lake. Diffuse pollution from agriculture in the Neagh catchment has been difficult to ameliorate mainly due to agricultural intensification and a high proportion of land in the catchment used for farming (Bunting et al, 2007;Foy et al, 2003). Sixty-six percent of the catchment is currently under agricultural use, the majority for grassland grazing.…”

Comparative effects of urea, ammonium, and nitrate on phytoplankton abundance, community composition, and toxicity in hypereutrophic freshwaters

“…Historically, regulatory efforts have focused on P control because surveys of, and experiments in, boreal lakes indicate that phytoplankton production is regulated mainly by the influx of P (Schindler 2006), because historically it has been less costly to eliminate P than N from urban wastewater, and because diversion of N from P-limited lakes may have little effect on primary production . In contrast, analysis of some highly eutrophic lakes suggests that pollution with N can degrade water quality beyond levels attributable to P alone Bunting et al 2007). In these ecosystems, predictive relationships between P influx and lake production are weak (Allan 1980), soluble reactive P (SRP) is abundant despite elevated algal biomass (Foy et al 1995;Patoine et al 2006), fixation of atmospheric N 2 does not meet algal requirements (Patoine and Leavitt 2008;Paerl and Scott 2010), lakes retain N but not P (Allan 1980;Patoine et al 2006), and diversion of P does not necessarily reduce algal abundance .…”