Seasonal nutrient co-limitation in a temperate shelf sea: A modelling approach

Nutrient limitation on phytoplankton growth plays a critical role in ocean productivity, the functioning of marine ecosystems, and the ocean carbon cycle. In the Celtic Sea, a temperate shelf sea, many studies have shown the importance of nitrate on phytoplankton growth focusing on the seasonal cycle of nitrate and feedbacks with the physical environment; but only recently has it been demonstrated, through discrete measurements, that dissolved iron also plays an important role in the ecosystem of the region. A well -established one-dimensional model has been developed to analyse the nutrient co-limitation between dissolved iron and dissolved inorganic nitrogen in the Celtic Sea. This model allows us to study the full seasonal cycle and inter-annual variability of these two nutrients. Simulations show that dissolved iron is an important nutrient for the development of the spring bloom, while nitrate plays a more important role during the summer season. Sensitivity analyses show that these results are robust when varying the nutrient-related parameters; the largest variability observed for primary production was observed when varying the nutrient sediment flux rates for dissolved iron and nitrate while less impact on phytoplankton production occurs when changing the half saturation constants. Here, we demonstrate that dissolved iron is an important nutrient for the development of the spring bloom and it should not be neglected as a state variable when modelling the Celtic Sea or other temperate shelf seas.