Wilfried Kühn

Interannual variability of carbon fluxes at the North Atlantic Station ESTOC

Abstract The impact of sea surface temperature and wind stress on primary production, export production, and CO2 air–sea exchange at the ESTOC station (29°N, 15.5°W) north of the Canary Islands is the focus of our investigations. A one-dimensional carbon and nitrogen cycling model was applied for the 10-year period 1987–1996. The simulation results compare well with upper layer observations for 1994–1996. Our simulated deep-water particle fluxes mostly overestimate the originally observed values for 1992–1996. On the other hand, the simulated fluxes underestimate the 230Th corrected particle fluxes (Scholten et al., Deep Sea Res. 48 (2001) 1413). Identifying the original observations as lower and the corrected values as upper estimate for the particle flux the simulation results falls in the range between these estimates. The large simulated interannual variability of carbon fluxes is in apparent contrast to the low interannual variability of the meteorological forcing typical for this subtropical regime. The key to this phenomenon lies in the sensitivity of this ecosystem to nutrient supply: depending on the meteorological situation, in different years the mixed-layer depth can or cannot reach the nitracline.

Modelling Water Column Processes in the North Sea [and Discussion]

In the North Sea advective transports are not negligible. Nevertheless, physical properties like sea surface temperature (SST) can be hindcasted with sufficient precision by vertical process water column models. Annual cycles of SST in the southern, central, and northern North Sea can be simulated using physical upper layer models with relatively small RMS errors. For the Fladenground Experiment (FLEX’76) in the northern North Sea the RMS error is less 0.3 °C for the 2 months of the experiment. This justifies the initial use, at least, of vertical process water column models in simulations for investigating transfer processes in the planktonic ecosystem. Experiments have shown that the simulated entrainment velocities at the bottom of the mixed layer during summer are critically dependent on the resolution of the forcing variables. The effects of this resolution on the annual phytoplankton dynamics will be discussed. Phytoplankton dynamics are strongly influenced by those of the zooplankton, and vice versa. Several field investigations have shown that, seemingly, phytoplankton cannot sustain the observed stock of zooplankton in the northern North Sea: there exists a gap between the abundance of phytoplankton and the need for it to maintain the zooplankton. Revisiting FLEX’76, the simulations with water column models of increasing complexity concerning detritus suggest that pelagic detritus can fill the gap in food availability for the zooplankton. If it is assumed that the zooplankton feeds also on detritus, the zooplankton experiences no food shortage.

Modelling water column processes in the North Sea

In the North Sea advective transports are not negligible. Nevertheless, physical properties like sea surface temperature (sst) can be hindcasted with sufficient precision by vertical process water column models. Annual cycles of sst in the southern, central, and northern North Sea can be simulated using physical upper layer models with relatively small rms errors. For the Fladenground Experiment (FLEX’76) in the northern North Sea the rms error is less 0.3 °C for the 2 months of the experiment. This justifies the initial use, at least, of vertical process water column models in simulations for investigating transfer processes in the planktonic ecosystem. Experiments have shown that the simulated entrainment velocities at the bottom of the mixed layer during summer are critically dependent on the resolution of the forcing variables. The effects of this resolution on the annual phytoplankton dynamics will be discussed.

Cadmium in the North Sea—a mass balance

Abstract Based on recently published data on cadmium concentrations in the different compartments of the North Sea ecosystem, i.e. water, suspended particulate matter, biota and sediments, and based on 3D circulation simulations, a (preliminary) mass balance for cadmium in the North Sea was worked out by estimating the contents in the compartments as well as the advective and other fluxes into/out of and within the system. It turns out that the total cadmium content in the water of 810 ± 320 t is in the same order of magnitude as the amount of cadmium in the sediment 1000 ± 500 t. The net deposition flux of cadmium to the sediments (8 t/yr) amounts only to about 3% of the annual anthropogenic input. The amount of cadmium in the biota is about 63 ± 12 t, but the annual turnover of cadmium due to primary production, herbivorous grazing and remineralisation is at least of the same order. The anthropogenic cadmium input from rivers, atmosphere and direct discharges of 310 ± 185 t/yr would add again the total cadmium content in the water in about 2.6 yr. The flushing of the North Sea circulation is very effective: in about 0.7 yr the cadmium contained in the water of the North Sea would be washed out, if no new cadmium would be introduced. The net outflow of cadmium across the northern boundary (307 t/yr) is equal to the anthropogenic input. Thus, in the mid eighties, the mass balance of cadmium in the North Sea has reached a steady-state within the error bars given.

Recent Change—North Sea

This chapter discusses past and ongoing change in the following physical variables within the North Sea: temperature, salinity and stratification; currents and circulation; mean sea level; and extreme sea levels. Also considered are carbon dioxide; pH and nutrients; oxygen; suspended particulate matter and turbidity; coastal erosion, sedimentation and morphology; and sea ice. The distinctive character of the Wadden Sea is addressed, with a particular focus on nutrients and sediments. This chapter covers the past 200 years and focuses on the historical development of evidence (measurements, process understanding and models), the form, duration and accuracy of the evidence available, and what the evidence shows in terms of the state and trends in the respective variables. Much work has focused on detecting long-term change in the North Sea region, either from measurements or with models. Attempts to attribute such changes to, for example, anthropogenic forcing are still missing for the North Sea. Studies are urgently needed to assess consistency between observed changes and current expectations, in order to increase the level of confidence in projections of expected future conditions.