Ecosystems - Modelling of the surface system

surface‑bedrock interactions

4.3 Modelling of the surface system

4.3.2 Ecosystems

In order to build up the knowledge about transport of elements within and between ecosystems, elemental transport models have been set up for the terrestrial, limnic and marine ecosystems in the Forsmark area. For this purpose, ecosystem models have been used to describe major pathways for flow of matter within the trophic web in each ecosystem and to reveal the major abiotic and biotic pools within the ecosystem.

In order to describe the flows of elements to and from ecosystems, as well as the permanent burial of elements in sediments and soils, the mass balance concept is used. The concept focuses on fluxes of matter, assuming that the difference between inflow and outflow represents the result of the biotic and abiotic processes occurring within the ecosystem. This is a more robust approach than ecosystem budgets as less data are needed, but it reveals much less detail. The mass balance identifies the major fluxes in and out of an ecosystem. Similarly to ecosystem budgets, the mass balance indicates, for example, whether a lake functions as a source or a sink for different elements. As a complement to the mass balance analysis, the amount of matter in “pools” within the ecosystem can be estimated.

Figure 4‑7. Discharge points from a model case where particles were released at a depth of 150 m inside an area corresponding to the planned repository; the simulations are reported in /Bosson et al. 2008/.

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Bolundsfjärden

Eckarfjärden

Fiskarfjärden Nuclear Power Plant

1628000

1630000

1632000

1634000

1636000

6696000 6696000

6698000 6698000

6700000 6700000

6702000 6702000

±

0 0.5 2 km

© Lantmäteriverket Gävle 2007 Consent I 2007/1092 2008-01-14, 10:45

^Exit points after 300 years ^{^}^_Exit points after 5000 years

The chemical behaviour of many bioavailable contaminants and radionuclides is similar to that of other bioavailable compounds or elements, such as macronutrients or trace elements /Whicker and Schultz 1982, Sterner and Elser 2002/ and these so called analogues can thus be utilised for modelling purposes. Similarly, there are elements that are subjected to passive uptake by plants and the transport of these elements may therefore be better described by water flow through the plant, e.g. transpiration /Greger 2004/. However, in general, a multitude of bioavailable radionuclides with various behaviours, assimilated into living tissue, will ultimately follow the fluxes of organic matter.

Consequently, the production of organic material, the Net Primary Production (NPP), may define an upper limit to the incorporation of different elements in primary producers /Kumblad et al. 2006/.

Fluxes to other trophic levels than the dominant vegetation, such as large herbivores, predators and humans, can be used to evaluate their potential exposure from food intake.

Models for element transport in the terrestrial ecosystem

Pools and fluxes of organic matter and water (Figure 4-8) were investigated and compiled for three localities, representing vegetation types that can be regarded as being important both in respect to area coverage and as potential sinks for organic matter, see Table 6-1 in /Löfgren (ed) 2008/. Two conifer forests, representing the dominant vegetation type, and one forested wetland were studied.

All three investigated localities were net carbon sinks, where most of the carbon accumulated in the vegetation. The net primary production (NPP) was between 429 and 537 g C/(m²·y). The forested wetland accumulated 74 g C/(m²·y) in the soil organic carbon pool, whereas the two forests were close to zero with regard to soil carbon balance. This model described three ecosystems using site-specific field measurements of pools and fluxes, and served as a baseline for comparison with the results of dynamic modelling, see section 4.1.3 and chapter 7 in /Löfgren (ed) 2008/ and with more general literature data that may be used to describe pools and fluxes in long-term perspectives in the safety assessment.

The dynamic vegetation model LPJ-GUESS /Sitch et al. 2003/ was used to make a regional descrip-tion of carbon balances for a number of different vegetaThe dynamic vegetation model LPJ-GUESS /Sitch et al. 2003/ was used to make a regional descrip-tion types dominating the investigaThe dynamic vegetation model LPJ-GUESS /Sitch et al. 2003/ was used to make a regional descrip-tion area.

Some areas (sea shore, wetlands and forested wetlands) were not covered due to the extensive work required to adapt the model for these vegetation types. For these cases, ground-based measurements as well as literature data were presented, see chapter 7 in /Löfgren (ed) 2008/. The model results cover the vegetation types; young (25 y) and old (80 y) stands of Norway spruce, Scots pine, deciduous trees (Pedunculate oak (Sw. ek) and Silver birch (Sw. våtbjörk)), mixed forests, dry pine on acid rocks, meadows and arable land. The model results were validated against ground-based estimates, which confirmed that estimated carbon balances are realistic in relation to measurements.

Estimates of carbon balances for 2005 for the different vegetation types were made using the model (Table 7-4 in /Löfgren (ed) 2008/). The modelled vegetation types were all net sinks except for the clear-cut that was a carbon source, mainly due to decomposition of the large litter pool originating from the residues after the clear-cut. NPP was between 461 and 664 g C/(m²·y) for the forested vegetation types. A similar calculation of the net ecosystem production (NEP) as above showed that all vegetation types were accumulating carbon in the SOC pool or were close to zero, except for the dry pine forest and the previously mentioned clear-cut. The spatial variation of NPP was studied in the regional model area by combining remote sensing and dynamic vegetation modelling (Table 7-6 in /Löfgren (ed) 2008/). Finally, the temporal variation of a number of ecosystem properties was investigated by modelling 400 years of forest succession using a set of 100 y of site-specific climate data repeated four times.

The regional ecosystem description was combined with estimates of site-specific element concentra-tions for different pools and elemental fluxes in order to determine mass balances for a number of different elements using the discharge areas as the geographical extents over which the mass balances were determined, see chapter 8 in /Löfgren (ed) 2008/.

Models for element transport in the limnic ecosystem

Ecosystem carbon models have been developed for both larger and smaller lakes in the area, see chapter 5 in /Nordén et al. 2008/. An example of an ecosystem carbon budget for one of the larger lakes, Lake Eckarfjärden, is presented in Figure 4-9. In both the larger and the smaller lakes, biomass is mainly associated with the benthic habitat, where more than 90% of total biomass in the lake is found. Also, the primary production is concentrated in the benthic habitat, which can be expected in shallow hard-water lakes. This makes the Forsmark lakes different from Swedish lakes in general, where phytoplankton is an important group of primary producers. The reason for the difference is, of course, the shallow water depth of the Forsmark lakes which, together with the clear water, makes all bottom areas available for primary producers.

The processing of organic matter in freshwater ecosystems has been found to significantly affect the overall carbon balance of freshwater-rich landscapes such as the Swedish boreal forest. Both the carbon budget and the carbon mass balance show that, contrary to typical Swedish lakes, production exceeds respiration in the larger Forsmark lakes, making them carbon sinks. In some of the smaller lakes, the respiration and primary production are of similar sizes. As many elements follow the carbon flow in ecosystems, the same flows can often be attributed to other elements that are incorporated into biota.

Figure 4‑8. The major pools and fluxes of carbon and water in an Alder-Norway spruce swamp forest. Figures within boxes describe pools, whereas figures outside are fluxes. A change in a pool is expressed with a figure preceded with a ± (decrease/increase). Pools are in g C/m² and fluxes are g C/(m²·y). The sizes of the water fluxes are illustrated by the length of the arrow (data from Table 6-10 in /Löfgren (ed) 2008/).

CO₂

Root uptake 4543

+74±222

Transpiration

NPP

Heterotrophic respiration

166±108 5411±2120

+130±43 326±305

68±67

404±120

25±22

225±190

108±32

25±22 Run off

Figure 4‑9. Carbon flow (kg C/y) in the ecosystem model for Eckarfjärden /Nordén et al. 2008/. Sizes of boxes and arrows are relative to the sizes of pools and fluxes in the model. The consumption of phyto-, zoo- and bacterioplankton by benthic fauna is included in the consumption of sediment carbon in the figure. Note that consumption within the same functional group is not shown, e.g. the consumption of fish by carnivorous fish. DOC = Dissolved Organic Carbon, DIC = Dissolved Inorganic Carbon, POC = Particulate Organic Carbon.

DIC DOC

Benthic primary producers

Benthic bacteria

Fish

13 100

Zooplankton Bacterioplankton

Phytoplankton

sediment carbon

15 900

300 2 400

50 500

40 9

940 1 400

Primary

Producer Consumer

Respiration and Excess Abiotic

POOLS

Consumption or Net Primary Production FLUXES

Eckarfjärden

11 800 12

9 110

9 230

15 3 670 476

26 900 4 910

203 1 490

410 80 Benthic fauna

17 200 693

5 770

1 000

Element mass balances have been calculated for a number of other elements, together with an assessment of the major pools of each element, see chapters 5 and 7 in /Nordén et al. 2008/. As an example, the mass balance for phosphorus in Lake Bolundsfjärden is presented in Figure 4-10.

The mass balance shows, for example, that the sediment is by far the largest phosphorus pool in the lake. Biota contains considerably more phosphorus than the water pools. Moreover, all fluxes (inflow, deposition, outflow, and sediment accumulation) are of the same order of magnitude. The phosphorus pools in the lake are larger than the annual inputs (inflow and deposition) and outflows (outflow and sediment accumulation), and thus the lake is probably more influenced by internal circulation of phosphorus than by phosphorus entering the lake from the catchment.

Models for element transport in the marine ecosystem

The marine ecosystem and its characteristics are conceptualised in an ecosystem model. The model was calculated on a spatial domain consisting of 1,500×1,500 grid-cells, each with a 20×20 m size.

To enable the model calculations, a database including important ecosystem components with a 20 m resolution was created, using the site-specific data and various interpolation methods described in

Five basins, localised in the immediate vicinity of the target area for the repository, were studied in more detail. The sediment pool in the studied basins, and indeed most of the Öregrundsgrepen, is the dominant pool for carbon (and for many other elements). Among biotic pools, macrophytes and benthic fauna, especially detrivores, are the largest. The dominant flux within the food-web model is uptake of carbon by these functional groups, including bacteria. The flux of elements from one trophic level to another is small. Instead, the major part follows other paths, such as respiration or excretion. In most basins, the flux to and from the basin through advective transport is many times larger than the fluxes within the system.

The results show that although most parts of the area are heterotrophic, the total mean of the whole area is autotrophic, i.e. more carbon is fixed in biomass by primary producers than is released from respiration by all organisms. The mean Net Ecosystem Production (NEP) in the model area is 14.8 g C/(m²·y), but ranges between –55 to 228 g C/(m²·y) in separate basins. The inner, shallow coastal areas tend to be autotrophic, whereas the outer are heterotrophic. A difference between inner and outer coastal areas can also be seen in biomass. This is mainly an effect of the benthic component, especially primary producers, being more abundant in shallow areas.

In document Site description of Forsmark at completion of the site (Page 79-83)