Dissolved Oxygen and Nitrification in the Upper Forth Estuary During Summer (1982–92): Patterns and Trends

Abstract

River discharge, wastewater inputs and water quality data from summer surveys in the upper Forth estuary have been reviewed for the period 1982–92. Particular attention has been given to dissolved oxygen and its possible links to the processes involved in the cycling of dissolved inorganic nitrogen species. Average dissolved oxygen (DO) concentrations for a particular year are linked with river flow; DO concentrations were lowest in years when river flow was low. Average DO concentrations in the upper Forth estuary have increased since 1959 when records began; however, the inter-annual variability in river flowprecludes the detection of any significant trend over the present study period. An important process contributing to the development of the oxygen minimum in the upper estuary is nitrification. A comparison has been made between total oxidized nitrogen (TON) concentrations (i.e. nitrate+nitrite) observed in river water, and those predicted by extrapolating down estuary TON/salinity relationships to zero. The latter are always larger than the former with the greatest discrepancies observed under low river flows. It is suggested that under low flow conditions, the longer water residence times in the upper estuary enable nitrification to proceed to a greater extent. Ample particulate organic nitrogen (PON) required for nitrification is demonstrated to exist in the turbidity maximum. This can originate from the river and/or the estuary and may be of natural or anthropogenic origin. Evidence is presented to show that the magnitude of the internally generated TON source has increased over the period under study. Maximum ammonia and nitrite concentrations in the estuary tend to be associated with years in which river flow is low. The mid-estuarine concentration maxima for nitrite are indicative of a benthic source, their locations are downstream of the nitrification-produced nitrate maximum. Their distributions are consistent with processes breaking down PON in reducing sediments (e.g. denitrification). Therefore, lower DO concentrations in the water column may be taken as indicative of more reducing conditions close to the surface sediments. An oxygen budget is constructed for the upper estuary. While gas exchange is identified as the major source of oxygen overall, DO from river water is likely to dominate at low salinity. The main sinks for oxygen in the estuary are nitrification, inputs of BOD and sediment oxygen demand. The link between high river flows and improved water quality is attributed not only to the oxygen contained in river water, but also to the associated shift of the turbidity maximum further downstream where the volume of the estuary is greater. A complex picture emerges of chemical and physical processes tending to act in concert towards low DO concentrations under low river flow conditions.