Biogeochemical Nutrient Cycling in the Upper Great Ouse Estuary, Norfolk, U.K.

Abstract

The cycling of dissolved inorganic nutrients in the Great Ouse estuary, Norfolk, U.K. was studied over a 50-day period in the early summer of 1994. The cycling of nutrients within the estuary and the export of nutrients to the surrounding coastal waters is quantified using mass balancing and conservative mixing considerations. Overall, the estuary was a minor sink for nitrate+nitrite (8%), a modest sink for silicate and phosphate (12 and 22%, respectively), a large source of ammonium (95%), and, overall, a small sink for dissolved inorganic nitrogen (2%). Water-column processes account well for the observed silicon and phosphorus removals; however, sedimentary processes are required to account for most of the ammonium source and some of the nitrate sink. The most significant water-column processes were primary production and nutrient regeneration from particulate material. The former process appears to be driven mainly by nitrogen imported directly at the head of the estuary, not recycled nitrogen. Regenerative processes returned approximately half of the silicate assimilated in primary production to the water column, and reduced the inferred respiratory releases of nitrogen and phosphorus by 40 and 60%, respectively. The two approaches to quantifying export, mass balancing and conservative mixing considerations, complement each other well, with the mixing analysis suggesting magnitudes and locations for nutrient-cycling processes not quantified by the mass-balance model.