Changing perspectives on the importance of urban phosphorus inputs as the cause of nutrient enrichment in Lough Neagh

The scale and chronology of the phosphorus (P) enrichment of Lough Neagh, a large hypertrophic lake, was assessed using, as enrichment proxies, published palaeocological studies on diatoms and chironomid head capsules preserved in the lake sediments and, from 1974, monitoring data from the lake and its inflowing rivers. Enrichment commenced in 1880, and the scale and chronology of the diatom and chironomid records were similar up to 1960, but in the 1960s, chironomid accumulation rates increased dramatically, whereas diatom rates remained unchanged. From subsequent lake monitoring, the absence of a diatom response after 1960 was attributed to silica limitation so that P could be considered as the driver of increased diatom production only up to 1960. Using a coefficient for the demand for P by diatoms of 4 μg P mg SiO2−1, it was calculated that the increased diatom production between 1881 and 1961 required an increase in lake P of 26 μg P l−1. This value is close to the predicted increase of 22.5 μg P l−1 in the lake caused by inputs of P from sewage treatment works and septic tanks. There was no evidence that diffuse source P contributed to enrichment over this period. Enrichment up to 1960 appears modest in comparison to subsequent years. From 1961 to 1974, lake P was estimated to have increased by 62 μg P l−1, but only 25 μg P l−1 of this increase was attributable to greater loadings of P from urban and septic tank sources. The enrichment response of chironomids at this time was also much greater than expected from a regression-derived relationship between urban P inputs and chironomid accumulation rates suggesting additional sources of P. The larger than expected increase in lake P was attributed to the onset of a significant internal loading of P and the commencement of an increase in diffuse source loadings of P. River monitoring has shown that diffuse P has increased steadily since 1974, more than counterbalancing a 40% reduction in urban P loadings that has taken place since 1980. By the end of the 20th century urban sources contributed only 19.7% of inflow P to Lough Neagh but lake P was 145 μg P l−1 compared to an estimated concentration of 20 μg P l−1 in 1881. Failure to control diffuse P sources has therefore been instrumental in the continued increase of lake P in Lough Neagh.