Diurnal fluctuations of metals have been observed in the South Tyne river catchment, UK, in both upland tributaries and major river reaches. Zinc exhibits the most pronounced cyclicity, with concentrations increasing during the night to a maximum near 05:00 before decreasing during the day. This trend is the inverse of pH and temperature observations, which are thought to be the predominant drivers behind the cyclicity. Photosynthetic reactions of biomass and algae alter the pH within the river systems, therefore promoting hydrous metal oxide precipitation during the daylight which consequently allows divalent cations including zinc to sorb onto them. This mechanism may be supported by direct uptake of zinc by algae and other biogeochemical reactions which combine to create large differences in zinc concentrations; during base flow zinc concentrations increased by 326% from the minima over 48 hours. Maximum concentrations are not being captured during routine water quality analysis resulting in inaccurate and misleading EQS results and total flux estimations, for example the annual total zinc flux in a small tributary increases from 17 to 76 tonnes/year when routine grab sample data are supplemented with 24-hour sampling results.
Keywords: diurnal, flux, monitoring, water quality, zinc