Annual cycles of horizontal distributions of temperature and salinity,and of concentrations of nutrients,suspended particulate matter and chlorophyll on the northwest european shelf

During the years 1993 to 1996 the MAST project “Northwest European Shelf Programme (NOWESP)p ]During the years 1993 to 1996 the MAST project “Northwest European Shelf Programme (NOWESP)” has compiled most available observations for nine key state variables, namely temperature, salinity, phosphate, nitrate, nitrite, ammonia, silicate, suspended particulate matter (SPM) and chlorophyll, for the Northwest European Shelf. One important aim of NOWESP was to derive mean spatial distributions for each month by merging the available data to construct climatological monthly mean distributions to picture the climatological annual cycle, disregarding possible trends, e.g. in the coastal areas. In these areas the horizontal distributions have a bias towards the last fifteen years of observation because of the trends and of the increasing number of observations, and the monthly mean data are representative of the decade 1984 to 1993. Maps of horizontal distributions of temperature and salinity and of the concentrations of the nutrients phosphate, nitrate, nitrite, ammonia and silicate, of SPM and of chlorophyll are presented here for the months February, May, August and November. The properties of a few averaging (interpolation) methods are discussed and the method chosen, i.e. exponentially distance weighted averaging, is compared to the other methods. More details and maps for every month are given in the corresponding technical report by Radach and Gekeler [1997].     

Review of three-dimensional ecological modelling related to the North Sea shelf system: Part 1: models and their results

The state-of-the-art in modelling the marine ecosystem of the greater North Sea is reviewed, providing an overview especially about three-dimensional models that describe and predict how the marine ecosystem of the greater North Sea area functions and how concentrations and fluxes of biologically important elements vary in space and time, throughout the shelf and over years, in response to physical forcing. Articles with a strong concentration on modelling were selected from the available literature, and all articles around the existing “ecological modelling groups” dealing with the area of the North Sea were sorted in chronological order of their appearance in the literature. We found eleven of such groups and described their different modelling efforts. Selecting the seven three-dimensional models (NORWECOM, GHER, ECOHAM, ERSEM, ELISE, COHERENS and POL3dERSEM), we characterized the complexity of the models, by comparing the resolution in time and space, and the resolution of the trophic structure by discussing the number and kind of state variables and of the processes relating these state variables to each other.The review of biogeochemical/ecological modelling for the greater North Sea shows that important findings by model simulations have either confirmed existing knowledge derived from field work or have given new insight into the mechanisms of the functioning of the North Sea system: the temporal and spatial development and magnitude of primary production, its spreading from the coasts to the north-west over the open North Sea, its mechanisms of limitation, the functioning of the pelagic small food web and of the benthic web, the mechanisms of nutrient regeneration, the effects of riverine and atmospheric nutrient inputs causing eutrophication of coastal waters, the extent of eutrophication in the North Sea, and the budgets for nitrogen, phosphorus, and silicon. The three-dimensional ecological models of the greater North Sea have provided consistent distributions and dynamics of the lower trophic levels on their regional, annual and decadal scales which cannot be derived to this degree of coverage by observations.The state-of-the-art in validation for these models is presented in part 2     

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 stimualtion of the annual cycles of both the physical and the phytoplankton dynamics, are used to estimate the annual primary production in the 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 generates a considerable variability in the physical and biological oceanic mixed layer dynamics.As an example, the model results from two years with contrasting meteorological conditions, 1963 and 1967, are discussed in detail. The mixing regimes generated are very different which result in different annual phytoplankton cycles. During 1963 when conditions were warm and windless, the early establishment of a calm upper layer water mass enabled a strong spring plankton bloom; whereas in 1967, which was stormy and cold, convective overturning continued until April, suppressing an early spring bloom and prolonging the blooming into summer.For the meteorological conditions observed in 1962 to 1986, the simulations yield an integrated annual water column gross production of 83.5–99.0 gC m⁻²a⁻¹ and an integrated annual water column net production ranging between 43.0 and 64.2 gC m⁻²a⁻¹ for the central North Sea. Grazing by the prescribed copepod population ranges from 24.5 to 40.0 gC m⁻²a⁻¹. The production events are described irregularly over the different years, total gross production varies only about 17%, and total net production by about 21%. The nutrient taken up by 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 additional nutrient is provided by local regeneration and by turbulent entrainment from below the thermocline. Local regeneration in the upper layer provides about 2.4 and 0.3 times the entrained amount of phosphate during spring and summer, respectively. In the 25 years 16 late summer or early fall storm events entrained more than 1.2mmol P m⁻²d⁻¹ into the depleted upper layer, potentially initiating new production events.The simulated annual cycles can be validated with the available data only in the sense that the variability, but not single events, can be compared to measurements. Such comparisons between simulated and field data show that the simulation reproduces the general features of annual phytoplankton cycles. This establishes confidence in those calculated estimates, for which field data are not directly comparable. It is concluded that weather-induced variability can explain most of the observed variability in phytoplankton in annual cycles.A typical annual cycle of phytoplankton biomass dynamics is presented. Ratios of daily process contributions show that the balances between the different 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 more than the initial amount of nutrients in the mixed layer is sedimenting 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.The critical points in the model: grazing, recycling of nutrients and mixing in the bottom boundary layer, are discussed. The model still needs to be refined with respect to these processes in order to achieve the delicate balances required to generate fall blooms. A series problem is the appropriateness of primary production measurements for a comparison with simulated quantities. Attempts should be made to establish a one-to-one correspondence between model-derived production quantities and measurements.Single events are important, so both sampling strategies and the estimation of fluxes from data should take account of the possible occurrence of such events, which may have been missed in the observations, by presenting ranges covering the realistic variance rather than mean values.     

Climatological annual cycles of nutrients and chlorophyll in the North Sea

A large amount of nutrient and chlorophyll data from the North Sea were compiled and organised in a research data base to produce annual cycles on a relatively fine spatial resolution of 1° in each horizontal direction. The data originate from many different sources and were partly provided by the ECOMOD data base of the Institut fur Meereskunde in Hamburg and partly by ICES in Copenhagen to cover the time range from 1950 to 1994. While the annual cycles of nutrients and chlorophyll derived for the continental coastal zone are representative for the decade 1984–1993 only, those for the remaining parts of the North Sea may be considered climatological annual cycles based on data from more than four decades. The composite data set of climatological annual cycles of medians and their climatological ranges is well suited to serve for validational and forcing purposes for ecosystem models of the North Sea, which have a resolution larger than or equal to 1° in both longitude and latitude. The annual cycles of the macronutrients and chlorophyll presented here for 1° × 1° squares in the North Sea show especially that sufficient observational data exist to provide initial, forcing and validational data for the simulations with the 130-box setup (ND130) of the ecosystem model ERSEM. The annual cycles presented give a clear picture for the whole of the North Sea. The highest concentrations occur at the continental coasts as a result of continued river input, which is added to the ongoing atmospheric input over the North Sea. Also, from the Atlantic Ocean water with relatively high nutrient concentrations enters the North Sea via the northern boundary. In the productive areas on and around the Dogger Bank nutrient concentrations are lower than in the other parts of the North Sea, even in winter. The areas with seasonal stratification have very different annual cycles in the upper (0–30 m) and lower layers (30 m-bottom). The shallow boxes are fully mixed and exhibit a relatively fast increase of nutrient concentrations caused by summer regeneration of nutrients.     

Long-term simulation of the eutrophication of the North Sea: temporal development of nutrients, chlorophyll and primary production in comparison to observations

The ecosystem model ERSEM II has been used to hindcast the development of the ecosystem of the North Sea during the years 1955 to 1993. The simulation was driven by the box-aggregated output from a general circulation model of the North Sea of corresponding duration; radiation, river inputs, atmospheric input and boundary conditions at the borders to the Atlantic Ocean and to the Baltic Sea were applied as realistically as possible. The general features of the eutrophication process are reproduced in the hindcast: the coastal areas show strong changes in nutrient concentrations in the hindcast as well as in the observations. Eutrophication not only shows up in the nutrient concentrations, but also in primary production. The simulated spatial distributions of phosphate, nitrate and primary production compare well with the observed ones. In addition, the hindcast simulates considerable trend-like changes of the nutrients in the southern part of the North Sea, where the nutrients are transported from the continental coastal strip to the southern central North Sea. The line from the river Humber to southern Norway separates the region of noticeable anthropogenic influence of riverine and atmospheric input from the northern area, which is mainly influenced by the Atlantic nutrient inflow. The observed annual cycles in the central and northern North Sea are quite well reproduced by the hindcast. The comparison of the hindcast with the long-term observations at two sites in the continental coastal zone of the North Sea shows that the long-term behaviour of phosphate, nitrate and silicate is simulated well. Primary production is increased in summers during the main period of eutrophication, 1975 to 1989, in the hindcast and in the observations. The flagellates at Helgoland, however, experience much more pronounced annual cycles with much less interannual variability in the hindcast than in the observations.     

Variability of continental riverine freshwater and nutrient inputs into the North Sea for the years 1977–2000 and its consequences for the assessment of eutrophication

We determined the monthly and annual riverine freshwater, nitrogen (N) and phosphorus (P) loading into the North Sea from Belgium, The Netherlands, and Germany for the years 1977–2000. An average of 133 km³ yr⁻¹ of the 309 km³ yr⁻¹ precipitation into the watershed is carried by the rivers into the sea. Total freshwater discharge fluctuates with a strong 6–7 yr periodicity, is strongly correlated with precipitation, and exhibits a slight long-term decrease. The temporal changes of regional patterns of precipitation lead to changing ratios of annual discharge of the western rivers compared to the eastern rivers, varying between 2.2 and 3.5. The long-term oscillations in discharge were more pronounced as discharge increased. The annual means of total and dissolved inorganic N and P loads were estimated to be 722 and 582 kt N yr⁻¹ and 48 and 26 kt P yr⁻¹, respectively. The monthly N loads were much more strongly correlated with discharge, compared to the monthly P loads. Total N and P as well as dissolved inorganic N also demonstrated a 6–7 yr periodicity. The annual N loads decreased by about 17 kt N yr⁻¹ from 1977 to 2000. The total phosphorus and phosphate loads decreased from about 80 and 50 kt P yr⁻¹ in the 1980s to 25 and 12 kt P yr⁻¹, respectively, in the 1990s. The western rivers contributed the major part of the nutrient loads. The long-term oscillations in their nutrient loads were much more pronounced, compared to the eastern rivers. The area-specific loading rates estimated for all rivers are comparable to earlier estimates using shorter data records, smaller sample sizes, and a less complete watershed monitoring program. The monthly and annual average N:P ratios and their variability increased considerably for individual rivers during the study interval. These results confirm that the water quality of European continental rivers is strongly influenced by intense land use. They demonstrate the necessity for using long time series monitoring results to assess change and evaluate the effects of climate change on the North Sea coastal ecosystems, using ecosystem models on decadal time scales.