ESTIMATION OF THE VARIABILITY OF PRODUCTION BY SIMULATING ANNUAL CYCLES OF PHYTOPLANKTON IN THE CENTRAL NORTH-SEA

A physical and a biological one-dimensional upper layer model for the simulation of the annual cycles of both the physical and the phytoplan kton dynamics, are used to estimate the annual primary production in t he central North Sea. The simulations are driven with actual 3-hourly meteorological standard observations and estimated radiation data for the 25 years 1962 to 1986. The high variability of the forcing generat es a considerable variability in the physical and biological oceanic m ixed layer dynamics. As an example, the model results from two years w ith contrasting meteorological conditions, 1963 and 1967, are discusse d in detail. The mixing regimes generated are very different which res ult in different annual phytoplankton cycles. During 1963 when conditi ons were warm and windless, the early establishment of a calm upper la yer water mass enabled a strong spring plankton bloom; whereas in 1967 , which was stormy and cold, convective overturning continued until Ap ril, suppressing an early spring broom and prolonging the blooming int o summer. For the meteorological conditions observed in 1962 to 1986, the simulations yield an integrated annual water column gross producti on of 83.5-99.0 gC m(-2)a(-1) and an integrated annual water column ne t production ranging between 43.0 and 64.2 gC m(-2)a(-1) for the centr al North Sea Grazing by the prescribed copepod population ranges from 24.5 to 40.0 gC m(-2)a(-1). The production events are described irregu larly over the different years, total gross production varies only abo ut 17%, and total net production by about 21%. The nutrient taken up b y the algae is 2.6 to 3.2 times the winter concentration of that layer which in summer is situated above the seasonal thermocline. The addit ional nutrient is provided by local regeneration and by turbulent entr ainment from below the thermocline. Local regeneration in the upper la yer provides about 2.4 and 0.3 times the entrained amount of phosphate during spring and summer, respectively. In the 25 years 16 late summe r or early fall storm events entrained more than 1.2mmol P m(-2)d(-1) into the depleted upper layer, potentially initiating new production e vents. The simulated annual cycles can be validated with the available data only in the sense that the variability, but not single events, c an be compared to measurements. Such comparisons between simulated and field data show that the simulation reproduces the general features o f annual phytoplankton cycles. This establishes confidence in those ca lculated estimates, for which field data are not directly comparable. It is concluded that weather-induced variability can explain most of t he observed variability in phytoplankton in annual cycles. A typical a nnual cycle of phytoplankton biomass dynamics is presented. Ratios of daily process contributions show that the balances between the differe nt processes change during the annual cycle. Diagrams of the mean and seasonal phosphorus flow are derived from the simulations. Two thirds of the primary production are channelled through the copepods, and one third is lost by other processes. Organic matter corresponding to mor e than the initial amount of nutrients in the mixed layer is sedimenti ng out of the upper layer, and about the same amount is regenerated at the bottom and mixed into the water column at the end of the year.