Variations of Annual Turnover Cycles for Nutrients in the North Sea, German Bight Nutrients Turnover Cycles in The North Sea

The long term behavior of water chemical composition in the North Sea, German Bight is much influenced by the atmospheric, hydrographical as well as biological processes. We analyze the long term trends in annual turnover cycles of the silicate, phosphate, nitrate and nitrite for the period of 1966-2009. We provide a straightforward numerical procedure for determination of the nutrients depletion phases. It uses the rates of cumulative concentrations to distinguish among different phases of nutrients cycle. The silicate is one of the key nutrients used by diatoms. The amount of silicate also depends on the mixing of coastal and more marine water at Helgoland. We show that seasonal dynamics of silicate undergone a strong change since 1987 until 2009. It is characterized by an earlier onset of nutrients depletion and build-up phases. Also a shortening of nutrient depletion phases for both silicate and phosphate is observed during period 1966-1984/1985 For phosphate the start of depletion and build-up phases generally evolves towards later time of the year in the interval 1984-2009. Among other nutrients nitrate exhibits higher interannual and interseasonal variability over the whole time analyzed. The depletion cycle length for most of the nutrients (with exception of silicate) is negatively correlated with total annual concentrations. The annual sunshine hours are qualitatively compared with the onset and termination of depletion for phosphate and a good match between the variation of depletion phase and annual sunshine is observed.