Model Results: Dynamics
This section shows some model results. In order to transmit a feeling for the dynamics in the whole system we present a number of movies that show aspects of system behaviour as they evolve in space and time. The model is still under development and the dynamics shown here will not necessarily coincide completely with observable reality.
The results presented here represent only a small sample of the full set of simulation results. We have limited ourselves to some examples of what we consider to be the central features of the model.
Using movies as the output medium, results can be shown in 2 space dimensions over time, which then represents the third dimension. Most variables are displayed in time over the surface field (x, y) of the North Sea. Some movies, however, show the dynamics of a variable versus water depth along an E-W transect between Denmark and the UK east-coast at a latitude of 57ºN.
Physics: Temperature, Salinity & Silt
Temperature Areal
| The water temperature near the surface is affected by the boundaries and river inputs, but strongly affected by solar energy input and the air temperature. |
E-W Transect: Temperature
| This clearly demonstrates that the heating by input of light energy leads to thermal vertical stratification during summer.This stratification minimizes the vertical transport between a bottom layer and the surface layer. The dynamics show that in shallow areas the water temperature is more affected by the air temperature than in deep areas. |
Salinity
| Salinity is a conservative constituent which is only controlled by the open boundaries with the Atlantic Ocean, the Baltic Sea and by input of fresh water from the rivers. |
Silt
| The primitive silt model which is now included, models the vertical silt distribution but not the horizontal transport of silt. This silt model is only meant to calculate the light climate in every vertical layer at every grid point in the model, in order to get more realistic dynamics of the primary production and hence of the phytoplankton dynamics (see further under Ecology). |
Ecology: Phytoplankton biomass & Nutrients
Chlorophyll
| Primary production is the formation of organic carbon from carbon dioxide, using light as a source of energy. All phytoplankton cells contain chlorophyll to harvest the light energy. Therefore, chlorophyll might be used to estimate the biomass of phytoplankton. However, the chlorophyll content per cell depends on the available light and the traits of a specific phytoplankton type. The chlorophyll content is therefore dynamically modeled for each phytoplankton type. | ||
| In the North Sea the growth of phytoplankton (= primary production) starts in March in the South where there is more light and the water temperature is higher. In April the growth starts in the northern part of the North Sea. In spring diatoms have the highest production and determine to a large extent phytoplankton biomass and hence the chlorophyll concentration. | ||
Phosphate
| Inorganic nutrients, such as nitrogen (N), phosphate (P) and silicate (Si) are essential nutrients for phytoplankton to build their cells. This movie also shows that when phytoplankton growth takes off in early spring in the South, it is accompanied by a rapid decline of the phosphate concentration. If phosphate or one of the other nutrient concentrations (N or Si) becomes limiting, this terminates the spring bloom. During summer phosphate is the most limiting nutrient in the coastal zone. | ||
| Inorganic nutrients, such as nitrogen (N), phosphate (P) and silicate (Si) are essential nutrients for phytoplankton to build their cells. This movie also shows that when phytoplankton growth takes off in early spring in the South, it is accompanied by a rapid decline of the phosphate concentration. If phosphate or one of the other nutrient concentrations (N or Si) becomes limiting, this terminates the spring bloom. During summer phosphate is the most limiting nutrient in the coastal zone. | ||
Phaeocystis
| The algae Phaeocystis appears after the diatom bloom when diatom growth becomes limited by silica only. If the other macro-nutrients (N and P) are still available there is a fair chance that Phaeocystis, which can reach very high concentrations by colony formation being too large for grazing by meso-zooplankton and which moreover protects the cells against virus infection, will bloom. This, in combination with the fact that Phaeocystis has a high light sensitivity, gives it a competitive advantage which is sufficient for this phytoplankton group to frequently reach high concentrations, especially in turbid coastal regions. |
E-W Transect Chla - Nitrate
| In this movie we show simultaneously the dynamics of diatoms and nitrate in an east-west section at 56o N in a vertical profile from the sea surface to the bottom. In winter when phytoplankton production is low a slow increase in nitrate is seen due to regeneration from the sediment and input from the ocean boundaries. After the start of the diatom bloom a fast decrease of the nitrate is seen. Subsequently at the end of the spring bloom the diatoms sink to the sediment and become accessible as food for the benthic filter-feeders. | ||
| At the same time due to input of light energy the temperature of the upper layers rapidly increases; this results in a thermal stratification of the water column, effectively separating the upper and lower layers (See movie 'E-W Transect: Temperature' under the section 'Physics'). The regenerated nutrients from the sediment and in the lower water column can not be mixed over the entire water column. This leads to a high nitrate concentration in the bottom layers and to a low diatom concentration in the upper layers. Only when the stratification (due to a wind event) is eroded a short algal growth peak can be seen in the shallow parts of the slice due to the event-driven mixing exchange between upper and lower layers. In autumn the surface water cools again and the stratification of the water column breaks down. This may lead to small algae blooms which utilise new nutrients in the upper layer. | ||
Model Results: Aggregated
We also present results which give a summary of the dynamics by presenting graphs of annual averages of a variety of state variables and processes. This way of presentation clearly illustrates the large spatial variability in the North Sea for most biotic and abiotic conditions.
| Net Primary Production | Higher trophic levels |
| Net Bacterial Production | Nutrient tracking |
| Secondary Production | Nutrient Tracking in sediments |
| Net Primary Production |
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Net primary production is the production of new phytoplankton biomass. It is the gross production minus excretion and respiration. |
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| Net Bacterial Production |
| The net bacterial production is half of the primary production. This means that all carbon originating from primary production directly (excretion) or indirectly (mortality, feces) is first taken up by bacteria before it is available as food for the consumers. The bacterial production in the southern part of the North Sea, follows the spatial distributions of the primary production. |
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| Secondary Production |
| Zooplankton and the benthic suspension-feeders are the primary consumers in the system. Their main food is phytoplankton. Zooplankton utilise the whole water column for food, by migrating vertically; towards the surface at night, towards the sea-bottom during daytime. The benthic suspension-feeders, are limited to food which rains down on the sediment. In our model we assume that at the end of the diatom bloom (due to nutrient depletion) the diatoms become sticky and form macro aggregates which rapidly sink to the sediment. Thanks to this manna raining down from the water sky filter feeder production is of the same order of magnitude as zooplankton production. The main difference between zooplankton and filter feeder production is the spatial distribution. The fact that in the deeper areas less nutrients is available for production leads to a relatively smaller fraction sinking to the sediment in the deeper areas. |
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| Higher trophic levels |
| The model does not explicitly simulate pelagic or benthic fish. However it is possible to estimate how much food is available for the higher trophic levels. This is done by making use of the “predation closure” in the model. In our model carnivorous zooplankton represents the highest trophic level in the water column and infaunal predators in the sediment. In order to constrain these predators to a realistic range of observations, we have defined density-dependent loss processes which regulate their abundance. These processes form the closure of the model and are self-regulating: density-dependent mortality and grazing by the top-predators on themselves. We then calculate the total loss of these processes in the pelagic as well as in the benthic, and assume that these losses are a measure for the food available to the higher trophic levels.
The results show that much more food is available for pelagic fish than for benthic fish. This is logical because in the sediment there is (almost) no primary production, as there is a permanent absence of sunlight. As a consequence the benthic system is completely dependent on the pelagic system and moreover in this model the number of intervening trophic levels between the primary producers and benthic fish is one or two more than between primary producers pelagic and fish. |
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| Nutrient tracking |
| For an OSPAR workshop on Trans-Boundary Nutrient Transports (TBNT) we tracked Nitrogen from a number of rivers. In order to do this we added a new state variable for each state variable which represents a concentration of N (dissolved, particulate) in the model. In this set of new state variables we only track the nutrients which originate from a river or a set of rivers. In this way the spreading of N from a river as affected by the ecological and transport processes can be tracked over the whole model domain. |
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| With the figures the distribution of nutrients originating from the rivers in the UK can be compared with the Dutch rivers Rhine and Meuse. It shows that the nutrients from theUK rivers do not reach the continental coast up to the tip of Denmark. The nutrients from the Dutch rivers stay close to the Dutch coast. At the tip of Denmark still 10% of the total N originates from the Dutch rivers. |
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| Nutrient tracking in sediments |
| The distribution of particulate N in the sediment as it is now in the model, is mainly controlled by the suspension feeders. The reason is that in the absence of a particulate matter transport model, we can model only net sedimentation. The suspension feeders filter detritus particles and phytoplankton cells out of the water column and put a part (20%) of the excretion products into the sediment. According to the model large sedimentation takes place in this way near the Northern point of Jutland (Denmark), but also in the Oyster grounds and in the German Bight. A number of these areas (e.g. Oyster grounds) are known temporary sedimentation areas. In reality these areas are ‘cleaned’ sometimes during a heavy storm when all small detritus particles are resuspended. These fine detrital particles are transported to the Skagerrak and settle there. |
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| The tracked N, originating from the river Meuse and Rhine are sedimentated along the continental coast, and much less in offshore areas such as the Oyster grounds (80 km North of the Netherlands) . |
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