Temperature and dissolved oxygen trends in English estuaries over the past 30 years

Water temperature and dissolved oxygen are two important parameters which affect the ecosystem functions which estuaries provide. As such, we need to understand the long-term change of these parameters in English estuaries to better understand those areas which may be vulnerable in the present and future. Detecting these trends in monitoring data is difficult however, since temporally and spatially heterogeneous sampling has the potential to obscure any long-term component of change. In this study, we use a generalized additive model (GAM) approach to isolate the inter-annual change of water temperature and dissolved oxygen in spot sampled monitoring data across English estuaries. This technique can account for the potentially non-linear variability caused by temporal and spatial variations in sampling throughout the day and year. We find that water temperature in all modelled estuaries has warmed, with an average warming of 0.037 degree celsius/yr between 1990 and 2022, in line with temperature change in rivers and the sea surface around England. For the same period, dissolved oxygen has increased for roughly half of modelled estuaries and decreased for the other half. The variation across English estuaries in these two conditions is best explained by the geographical location of an estuary. Many of the estuaries warming at the greatest rate are those where sea surface temperature increase has also been greatest. We suggest the observed distribution of changes in dissolved oxygen can be explained by three processes: (1) temperature driven change, (2) dissolved oxygen improvement (e.g., reduced nutrient inputs through wastewater management regulation) and (3) changes in the number of primary productivity-induced super saturation events. This study provides a comprehensive look at temperature and dissolved oxygen change across English estuaries over the past similar to 30 years, which may provide information on the processes behind how these water quality parameters have changed and will continue to do so.