Model assessment of present-day Phaeocystis colony blooms in the Southern Bight of the North Sea (SBNS) by comparison with a reconstructed pristine situation

• We modeled present-day and pristine diatom-Phaeocystis blooms in the Southern Bight of the North Sea (SBNS).
• Diatoms are dominant in pristine time; Phaeocystis colonies are present but never reach undesirable cell densities (<4·106 L−1).
• Added anthropogenic nutrients do not affect the diatom-Phaeocystis succession per se.
• Anthropogenic nutrients favor Phaeocystis with little effect on diatoms maintained close to their pristine level.
• Some 72% of the present-day SNS is affected by undesirable Phaeocystis colonies.

We performed simulations with the existing MIRO&CO-3D ecological model to assess the present-day magnitude and geographical extent of undesirable Phaeocystis colony blooms in the Southern Bight of the North Sea (SBNS) receiving anthropogenic nutrient inputs from large European rivers that mix with the inflowing Atlantic waters. The criterion of 4·106Phaeocystis colonial cells L−1 of Lancelot et al. (2009) is used to scale the presence of undesirable bloom. These simulations are compared with a reconstructed pristine SBNS ecosystem making use of nutrient inputs calculated with the Seneque/Riverstrahler model of the river system when all human activities on the watershed have been erased. Interannual variability is considered by performing model runs for two contrasted meteorological years: wet (2001) and dry (2005). Results show a large excess of nitrogen (N) and phosphorus (P) delivery to the SBNS, i.e., 12 and 5 times the pristine situation respectively. In contrast, the total silicon (Si) input is decreased with respect to natural conditions due to freshwater diatom growth in some eutrophied rivers. Qualitatively, rivers deliver nutrients in large Si excess for pristine condition but N excess for both 2001 and 2005, when scaled to the N, P and Si requirement of coastal diatoms.Responding to the river nutrient inputs, either natural or of anthropogenic origin, phytoplankton blooms are simulated in the vicinity of the river mouths, especially in the eastern SBNS receiving 78–98% of the direct river inputs. In this area nutrients cumulate along a SW-NE gradient and the bloom is forming a wide ribbon parallel to the coast. A short time-delay is simulated between the western and eastern SBNS due to light limitation imposed by the amount of suspended particles carried by the Thames River. A spring diatom-Phaeocystis succession is simulated for both present-day and pristine condition. Diatoms dominate the bulk of the pristine phytoplankton community; Phaeocystis colonies develop in the whole domain, especially in the Thames and Scheldt river plume though their biomass remains low and never exceed the threshold of 4·106 cells L−1. In contrast, present-day Phaeocystis colonies start growing when diatoms attain their maximum in April, co-occur with and supplement them in May and June, reaching colonial cells densities >4·106 cells L−1 in many locations of the SBNS. Overall, Phaeocystis colonies are favored by wet meteorological conditions that enhance river N delivery. The geographical extent of undesirable Phaeocystis blooms thus varies between dry and wet years and affects 72% of the modeled SBNS domain distributed in three areas: the eastern band (90%), the Thames plume (6%) and a small offshore area (4%) possibly connected to the eastern band.