Sediment-water exchange of nutrients in the intertidal zone of the Humber estuary, UK

Nutrient pore-water profiles, sediment-water exchange and sediment characteristics were measured for six intertidal mudflat sites throughout the Humber Estuary over the different seasons. The Humber is a highly dynamic, non steady state system and hence neither the biogeochemical zones nor the macrofaunal communities were sustained for long periods of time. Sediment mixing and resuspension on tidal, episodic or seasonal timescales was the predominant control on nutrients. Over an annual cycle, mean measured fluxes were -8.7 mmoles/m²/day nitrate, 3.7 mmoles/m²/day ammonia and 0.2 mmoles/m²/ day nitrite. Net phosphate and silicate fluxes were very small. The intertidal mudflats were a sink for nitrate (-1000 kmol/day), a major source of ammonia (430 kmol/day) and a minor source of nitrite (25 kmol/day). Nitrate influxes decreased in a seaward direction (-13.4 mmoles/m²/day in inner estuary, -11.0 mmoles/m²/day in mid-estuary, -5.2 mmoles/m²/day in outer estuary), but when the area of the mudflats in each area is taken into account, both the mid and outer estuary were sinks for approximately 40% of the total nitrate taken up. In contrast, the outer estuary was the source of c. 90% of the ammonia and 105% of the nitrite (the inner estuary being a minor sink of nitrite).