Johannes Pätsch

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

Abstract 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.

Climatological annual cycles of nutrients and chlorophyll in the North Sea

Abstract 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.

An investigation into the variability of circulation and transport on the north-west european shelf using three hydrodynamic models

Recent modelling studies of the circulation of the north-west European shelf have been undertaken at three institutes; Proudman Oceanographic Laboratory (POL), Institut fur Meereskunde (IfM), and Institute of Marine Research (IMR) in which long-term meteorological forcing data have been applied to drive each institute’s own hydrodynamic model. These models have been run for periods of up to 39 years (1955–1993) — made possible by the use of a long set of good quality meteorological data from the Norwegian Meteorological Institute (DNMI). These studies have provided a valuable insight into the long-term shelf circulation and the nature of its variability. The quantification of volume transports through key shelf sections has been completed and, where possible, comparisons have been made with other modelling and observational studies. Calculated transports for North Sea sections confirm the generally accepted circulation pattern although difficulties arise in the Irish Sea due to model limitations (low resolution and unsuitable advection schemes). The three models show similar patterns of variability in the water volume transports calculated for the sections. Mean values compare well in the southern North Sea where the water is well mixed, however large differences in mean transport values occur for northern North Sea sections due to the differences in model physics. IfM and IMR models are baroclinic and as such provide a more realistic representation of the transport through these deep water sections where baroclinic processes are important. Spectral analysis of the 39 year model runs shows a dominant annual cycle and less significant longer period signals of 7–10 years.

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.

Variability in fluxes of nutrients (N, P, Si) into the North Sea from the Atlantic Ocean and skagerrak caused by variability in water flow

Fluxes of nutrients (dissolved phosphate, nitrate and silicate) from the Atlantic Ocean (Northern Atlantic and Channel) and Skagerrak to the North Sea were calculated for the upper 30 meters and for the total water depths. Monthly average water flows and their standard deviations were taken from the model calculations of the If M (HAMSOM model) and IMR (NORWECOM model) for the period 1955–1993 and 1976–1995, respectively. Climatological monthly mean nutrient data were collected for the same transects from the NOWESP Research Data Base, and extracted from additional literature. With the water flow from the models and the available nutrient data rather precise fluxes can be calculated. The aim is to compare these fluxes with those published in the literature which are often based on yearly averaged water flow and some nutrient concentrations. The northern Atlantic inflow (total depth) of the mean fluxes of nitrate, phosphate and silicate is 3,972 ±1,604, 713 ±310 and 3,847 ±1,527 ktonnes/year. About 20% of these fluxes transported in the top 30 m water layer (IMR model). For the upper 30 m the IfM model calculated higher fluxes for these nutrients: 1,654 ±621, 301 ±111 and 1,549 ±565 ktonnes/year, respectively. For the Skagerrak the IMR model estimated a net mean outflow to the Baltic, while the IfM model estimated a net mean inflow of nutrients to the North Sea. Both models agree rather well in the mean inflow of nutrients through the Channel, but with rather high standard deviations as compared to the other sections. For example, for the IMR total depth estimation: 162 ±236, 27 ±39 and 161 ±237 ktonnes/year for N, P, Si respectively. In the northern sections, literature data on nutrient fluxes are at the lower end of the range estimated in this study, while in the Channel the literature data are in the upper range. It will be discussed that the largest improvements in nutrient flux estimations are to be expected when the precision and accuracy of water flow data become better.

Radium isotopes as a tracer of sediment-water column exchange in the North Sea

Sediment-water column exchange plays an important role in coastal biogeochemistry. We utilize short-lived radium isotopes (224Ra and 223Ra) to understand and quantify the dominant processes governing sediment-water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats and North Sea sediments as the dominant source. Pore waters show 100-fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment-water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation-based Ra flux estimates and the majority of previously reported Ra flux estimates for other regions. Ra-based estimates of benthic alkalinity fluxes compare well to observed values, and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.

A Novel Modeling Approach to Quantify the Influence of Nitrogen Inputs on the Oxygen Dynamics of the North Sea

Oxygen (O2) deficiency , i.e., dissolved O2 concentrations below 6mgO2L-1, is a common feature in the southern North Sea. Its evolution is governed mainly by the presence of seasonal stratification and production of organic matter, which is subsequently degraded under O2 consumption. The latter is strongly influenced by riverine nutrient loads, i.e., nitrogen (N) and phosphorus (P). As riverine P loads have been reduced significantly over the past decades, this study aims for the quantification of the influence of riverine and non-riverine N inputs on the O2 dynamics in the southern North Sea. For this purpose, we present an approach to expand a nutrient-tagging technique for physical-biogeochemical models – often referred to as ‘trans-boundary nutrient transports’ (TBNT) – by introducing a direct link to the O2 dynamics. We apply the expanded TBNT to the physical-biogeochemical model system HAMSOM-ECOHAM and focus our analysis on N-related O2 consumption in the southern North Sea during 2000–2014. The analysis reveals that near-bottom O2 consumption in the southern North Sea is strongly influenced by the N supply from the North Atlantic across the northern shelf edge. However, riverine N sources – especially the Dutch, German and British rivers – as well as the atmosphere also play an important role. In the region with lowest simulated O2 concentrations (around 56°N, 6.5°E), riverine N on average contributes 39% to overall near-bottom O2 consumption during seasonal stratification. Here, the German and the large Dutch rivers constitute the highest riverine contributions (11% and 10%, respectively). At a site in the Oyster Grounds (around 54.5°N, 4°E), the average riverine contribution adds up to 41%, even exceeding that of the North Atlantic. Here, highest riverine contributions can be attributed to the Dutch and British rivers adding up to almost 28% on average. The atmospheric contribution results in 13%. Our results emphasize the importance of anthropogenic N inputs ans seasonal stratification for the O2 conditions in the southern North Sea. They further suggest that reductions in the riverine and atmospheric N inputs may have a relevant positive effect on the O2 levels in this region.

The influence of winter convection on primary production: A parameterisation using a hydrostatic three-dimensional biogeochemical model

In the recent past observational and modelling studies have shown that the vertical displacement of water parcels, and therefore, phytoplankton particles in regions of deep-reaching convection plays a key role in late winter/early spring primary production. The underlying mechanism describes how convection cells capture living phytoplankton cells and recurrently expose them to sunlight. This study presents a parameterisation called ‘phytoconvection’ which focusses on the influence of convection on primary production. This parameterisation was implemented into a three-dimensional physical–biogeochemical model and applied to the Northwestern European Continental Shelf and areas of the adjacent Northeast Atlantic. The simulation was compared to a ‘conventional’ parameterisation with respect to its influence on phytoplankton concentrations during the annual cycle and its effect on the carbon cycle. The simulation using the new parameterisation showed good agreement with observation data recorded during winter, whereas the reference simulation did not capture the observed phytoplankton concentrations. The new parameterisation had a strong influence on the carbon export through the sinking of particulate organic carbon. The carbon export during late winter/early spring significantly exceeded the export of the reference run. Furthermore, a non-hydrostatic convection model was used to evaluate the major assumption of the presented parameterisation which implies the matching of the mixed layer depth with the convective mixing depth. The applied mixed layer depth criterion principally overestimates the actual convective mixing depth. However, the results showed that this assumption is reasonable during late winter, while indicating a mismatch during spring.

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.