Annual cycles of nutrients in a shallow inshore area, Kiel Bight. — variability and trends

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Abstract
The annual cycle of nutrients in relation to biological processes and seasonality of the physical environment of a coastal ecosystem, Kiel Bight, is presented. Distinct patterns of nutrient cycling succeed each other in the course of the annual cycle in the ecosystem; five such phases are identified as recurring features of the system, although there is interannual variability in temporal scales of these phases. During spring, autumn and winter pelagic regeneration is low. Whereas pelagic uptake exceeds sedimentary nutrient release in spring, the reverse pattern prevails in autumn. In the transition from spring to summer nutrients released by the benthos are efficiently retained by the pelagic food web. In summer, pelagic uptake is balanced by pelagic and benthic regeneration in sediments above the thermocline. In the subthermocline environment nutrient release from the sediments is increasingly influenced by the declining oxygen content and this situation prevails throughout the autumn phase. Winter nutrient equilibrium is achieved 4–6 weeks after the end of the growth season. Thereafter concentrations are maintained at constant levels largely by non-biological mechanisms till the onset of spring growth. Winter concentrations of dissolved inorganic nitrogen have been remarkably constant since 1969, whereas for silicate a decline is indicated. In contrast to these nutrients, phosphate winter concentrations exhibit an increase, which has led to almost a doubling of winter levels during the last two decades.