External and internal control of winter concentrations of nutrients (N, P and Si) in north-west European shelf seas

Nutrient data from five winter cruises since 1994 and the 40-year time series from the CYPRIS station in the Irish Sea are used to assess the controls of winter concentrations in the Celtic and Irish Seas and the Malin Shelf region of the north-west European shelf. At the Malin Shelf break, boundaries between ocean water, the shelf edge current and shelf waters were well defined by changes in salinity, temperature and concentrations of nutrients (nitrate+nitrite (N), phosphate (P) and silicate (Si)). Concentrations fell from oceanic values of 11.0 mM N, 0.68 mM P and 4.75 mM Si to 7.4 mM N, 0.53 mM P and 3.3 mM Si in open shelf sea waters. Across the Celtic Sea shelf break boundaries are less well defined. Areas of water with concentrations of nutrients below those off-shelf are detected in certain areas. Denitrification is a ubiquitous process on the shelf. The accumulated loss of nitrate relative to phosphate increases moving from the shelf break into the Irish Sea. Comparisons of estimated rates of denitrification with measured N: P ratios suggest waters on the Malin Shelf have an age of 400 days relative to ocean water crossing the shelf break. Across the Celtic Sea the transit time for water from the Atlantic Ocean into the central Irish Sea is six years, consistent with the complexity of water types and frontal structures. Concentrations at CYPRIS are consistent with the degree of nitrate loss from the water on route to the central Irish Sea. Relatively high concentrations of silicate measured at CYPRIS station indicate this location is more influenced by waters with an origin south of 53.5(N than from the eastern Irish Sea.