Estuarine Nutrient Fluxes to the Humber Coastal Zone, UK, during June 1995

Measurements are presented of salinity, temperature and dissolved inorganic nutrient concentrations from an intensive survey of the near-mouth region of the Humber Estuary and coastal zone during a tidal cycle on 6 June 1995. The tidal movement of water in this area is illustrated with model results and used to interpret the observations. Variable–salinity plots showed that while estuarine-derived nitrate and silicate mixed conservatively with seawater, nitrite and phosphate showed evidence of sources within the mouth region of the estuary. A temperature–salinity scatter plot showed elevated temperatures within the shallow, estuarine waters due to solar heating. Despite the relatively small range of salinities encountered, from 34.2 to 29.6 progressing from the coastal zone into the estuary mouth, the suspended particulate matter concentration increased from 2 to 80 mg l−1 and the temperature from 10°C to 14°C. Nitrate concentration increased from 1.4 to 106 μM; nitrite, 0.1–0.4 μM; phosphate, 0.3–1.7 μM and silicate, 0.3–11.3 μM. The residence time of the Humber was approximately 60 days during summer 1995. Using freshwater inflows averaged over the residence time gave estimates of the seaward transport rates for nitrate–nitrogen, phosphate–phosphorus and silicate–silicon of 9.9 × 103, 73 and 1.1 × 103 k mol d−1. However, significantly greater nutrient fluxes from the Humber system are likely to occur during winter and spring, high freshwater inflow periods.