Spatiotemporal Modelling of Nitrate and Orthophosphate Concentrations in Aquatic Ecosystems

Abstract

A study of the temporal and spatial variations of Total Oxidized Nitrogen (TON) and orthophosphate in freshwater bodies within the boundaries of the Loch Lomond and Trossachs National Park (LLTNP) is described. A hierarchy of temporal and spatiotemporal additive models (AM) and additive mixed models (AMM), using spline smoothing, were fitted to the data to explore trends, seasonality and variation. Individual site analyses of both determinands show different temporal behaviour across sites. Temporal and seasonal patterns also differ by site group - rivers, lochs and sewage treatment works (STW). Overall, TON concentrations in the rivers and lochs are declining but increasing at the STW sites. Orthophosphate time trends are similar in the three groups except that concentrations in rivers and lochs show signs of an increase from 2006. Seasonal patterns are apparent, with the concentration of orthophosphate at a maximum in summer and lowest in winter in rivers and STW sites. In contrast, the seasonal peak in the lochs is in late winter/spring. TON is higher in winter with lowest levels in the late summer/autumn in the rivers and lochs. At the STW sites, the highest levels were found in the summer and the lowest levels in the winter. The effects of nitrogen deposition and total weekly rainfall on TON depend on their interactions with different categories of land cover. This study is part of an assessment of the effects of regulatory efforts of Environmental Agencies to improve water quality in the region.