Primary production in the deep chlorophyll maximum of the central North Sea

Deep chlorophyll maxima (CM) are commonly observed in the summer stratified North Sea. This feature was studied north of Dogger Bank in August and showed high chlorophyll a (Chl a) concentration (∼6 mg m−3) relative to surface waters. Photosynthesis versus irradiance (PvE) relationships were determined and showed the deep chlorophyll maximum accounted for 58% of water column primary productivity with average water column-integrated primary productivity of 424 mg C m−2 day−1. Quantum yield (ϕ) also showed more favourable phytoplankton growth conditions at the thermocline (mean ϕ = 0.058). Phytoplankton nitrate and ammonium uptake rates were also higher in the deep CM (mean = 0.18 mmol and 0.44 mmol m−3 h−1, respectively) relative to the surface mixed layer (SML) (mean = 0.03 and 0.22 mmol m−3 h−1, respectively). Primary production associated with the maximum was supported in part by nutrient flux from the nutrient rich bottom mixed pool resulting in continuous new production throughout the summer. Using these measurements annual new production associated with the deep chlorophyll maximum was estimated at 37% (5.7 × 106 t C) of annual new production for the summer stratified North Sea. These subsurface maxima are not detectable using remote sensing and therefore these highly significant regions of production are potentially neglected.