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Emissions of some greenhouse gases from aquatic and semi-aquatic ecosystems in the Netherlands and options to control them
The Science of the Total Environment, 126 (1992) 277-293 Elsevier Science Publishers B.V., Amsterdam
277
Emissions of some greenhouse gases from aquatic and semi-aquatic ecosystems in the Netherlands and options to control them Ron O.G. Franken*, Wim van Vierssen and Henk J. Lubberding Department of Environmental Engineering, International Institute for Hydraulic and Environmental Engineering (II-IE), P.O. Box 3015, 2601 DA Delft, The Netherlands (Received October 9th, 1991; accepted November 18th, 1991) ABSTRACT An inventory was made to quantify the emission of the waterborne greenhouse gases CH4, N20 and CO2 from fresh surface waters and wetlands in the Netherlands. The aim of the study was to give an overview of the existing fluxes and balances and to quantify the role of aquatic and semi-aquatic habitats in the total emissions of these gases in the Netherlands. The CO2 uptake of surface waters and wetlands turned out to be very low. Only 0.6% of the total annual CO2 emission from the Netherlands is absorbed by the ecosystems in these habitats. However, they are significant sources for N20 and CH£ respectively, 18% and 19% (including sewage treatment plants) of the total annual emission in the Netherlands. It is indicated that especially in the case of N20 and CH4 the reduction of the eutrophication will also reduce the output of these gases from the surface waters and the wetlands. However, attention should be paid to the operation of sewage treatment plants because reducing the discharge of nitrogen by extending the denitrification capacity could easily lead to more nitrous oxide release from these plants.
Key words: methone; nitrous oxide; carbon dioxide; emission; fresh water; wetlands
1. INTRODUCTION
The global climatic changes that will probably result from the steadily increasing amounts of the so-called greenhouse gases in the atmosphere are extensively and widely discussed. Apart from the efforts to model and quantitatively describe the different processes involved, government agencies are *Present address: National Institute for Public Health and Environmental Hygiene, P.O. Box 1, 3720 BA Bilthoven, The Netherlands. 0048-9697/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved
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seriously concerned as how to manage this problem in the (near) future. National governments are in the process of developing strategies to control the massive output of greenhouse gases. However, to be able to do so, reliable information is needed to base such strategies on. The present study was part of a broader framework of data collection and data evaluation on the production of greenhouse gases in the Netherlands, sponsored by the Dutch Ministry of Housing, Physical Planning and the Environment. A quick glance at the figures proves that the Netherlands population, which is roughly 0.003% of the world's population produces about 1% of the total world's output of greenhouse gases. This means that the Netherlands should clearly take up their responsibility to do something about the problem. The present study specifically focussed on the fluxes and balances of a number of the most important greenhouse gases which are produced by freshwater wetlands and fresh surface waters in the Netherlands. The ultimate goal of the study we report on here was to determine which role these aquatic and semi-aquatic ecosystems quantitatively play in the fluxes and balances of a number of greenhouse gases (CO2, CH4 and N20) on a national scale. Considering the fact that approximately 14% of the Dutch surface area is occupied by wetlands and surface waters, these habitats probably represent a large potential for greenhouse gas (CH4 and N20) production. Therefore, we also examined the possibility whether this information could lead towards specific environmental management guidelines by means of which the output of these gases could be reduced. Different management options had to be judged in the context of the whole set of adopted management policies for aquatic and semi-aquatic ecosystems in the Netherlands. Combating eutrophication and stopping the continuous lowering of groundwater tables in areas with sensitive ecosystems are major management goals at present as well as in the near future [1]. Major greenhouse gases are CO2, CH4, N20, CFK's and 03 with CO: being the far most important one. Each of these greenhouse gases has its own infrared absorption characteristic and therefore, a different potential for enhancing the greenhouse effect. The relative importance of 1 mol CH4 is 7.6 times that of 1 mol CO2 [2]. This figure is even more dramatic for N20 which has a potential of 290 times that of CO2. Based on the present worldwide concentration and output of these gases, an estimate has been made by the IPCC on the relative contribution of the different gases to the greenhouse effect in the next century. A contribution of 61% by CO2, 15% by CH4, 4% by N20 and 20% by miscellaneous components is anticipated
[21.
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In the present paper we will focus on Dutch freshwater wetlands and fresh surface waters as a source and sink for CO2, CH4 and N20. We adopted the distinction between wetlands and surface waters because of the important difference in aeration status and the fact that their management faces different types of problems. Surface waters are permanent aquatic habitats. Wetlands are semi-aquatic habitats. They may dry up or become flooded more or less frequently. This status is important because it determines to a large extent the major type of greenhouse gas emitted. We have collected the data on CH4 and N20 emissions from the international literature. The CO2 absorption capacity of wetlands we deduced from specific data on community consumption and production. 2. MATERIALS AND METHODS
To quantify the CO2, C H 4 and N 2 0 sources and sinks of the Dutch wetlands and surface waters we had to use different methods and types of data. First of all, the wetlands were categorized according to a predefined set of types (see below and Tables 1 and 2). Further, we used information on the total area that exists of each type and the emissions for the different categories to combine them into total outputs. To get an idea about the relative importance of the quantities being produced by these aquatic and semi-aquatic ecosystems, we used a review on the production of greenhouse gases from a number of major sources in the Netherlands [3]. They estimated the total output of the same gases for the Netherlands. They based themselves on other sources, and were not very specific about the output of these gases for the different aquatic ecosystem types. However, we used their total estimate for the Netherlands as a reference point and considered our figures as being part of their total estimates although they were not specified to that detail. For only a few Dutch fresh surface waters carbon balances appeared to be available. We largely based our estimates on the carbon budget that was made for the Dutch Lake Marken [4]. This lake is a rather shallow lake (average depth < 5 m) and represents one of the quantitatively important freshwater habitat types in the Netherlands. The net CO2 uptake was estimated by combining data on the inorganic binding of CO2 (important because of the high buffering capacity of many eutrophicated waters) and the organic binding capacity of CO2 (formation of semi-permanent detritus, transformation of CO2 via plant material and detritus into CH4). We calculated the physico-chemical absorption of CO2 by the Dutch surface water volume by combining the information on total water volume and its binding capacity per unit of volume. For the CO2 uptake by wetlands (mainly organic peatlands) we used other data [5]. For the quantification of
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the release of CO2 from drained peatlands (a major source of CO2) we used the data from [6]. We decided that these sources and sinks were the major ones to be discerned and neglected others. Only very little information was also available for CH4 production in Dutch surface waters and wetlands. Therefore we had to use data from other areas in the world. We used an up to date review on CH4 emissions from wetlands [7]. We have mainly based ourselves on some of their data and supplemented them with other data from the literature. With these data we arrived at an emission range for both the wetlands and the surface waters. Combining the surface area of the different habitats with the production figures, we could estimate the CH4 emission for Dutch aquatic and semi-aquatic ecosystems. For the collection of the N20 data we adopted a similar approach of data collection as for C H 4. However, one could suppose that the denitrification
28
88
56
A C 5 1 1 8 ~ ~ / ~
3
~
B I~
lakes ditches
~
large
rivers
and canals I I small rivers and canals P~I land
Fig. 1. Characteristics of Dutch surface waters. (A) Contribution of different aquatic habitats (%) to total area covered by surface waters. (B) Percentage of the Netherlands with different types of surface waters. (C) Percentage of total surface water volume for different types of surface waters.
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process in sewage treatment plants is very important within the present context. Unfortunately, as we found out, there is hardly any specific information available on the N20 production in these plants. However, we used the data we could lay our hands on and reviewed them. We eventually tried to predict how management may minimize the output of these greenhouse gases and tried to quantify how such a management may affect total greenhouse gas production. 3. RESULTS 3.1 Freshwater wetland and surface water area
In Fig. 1, the most important surface water types and their relative importance (data made available by RIZA; Institute for Inland Water Management and Waste Water Treatment) are given. As you can see from Fig. 1A, lakes are the most important habitat type considering the area they occupy. About 56% of the surface waters consist of lakes. Almost 11% of the Netherlands (approx. 33 000 km 2) comprises surface waters as can be seen in Fig. 1B. Considering the water volume (Fig. 1C) 74% of the total surface water volume is contained by lakes. In Fig. 2, different wetland categories and the relative size of the area they occupy in the Netherlands are summarized. The total area is less than that occupied by surface water but all the same adds another 3% to the total area of surface water. An important habitat type is the floodplain (40% of the wetlands); they consist of the riverine areas of the rivers Rhine, Waal, Meuse and IJssel, which are flooded during the annual peak discharges of the river.
10
7
11
2
~
floodplains
~
marshes bogs fens
40
~
other wetlands swamps
Fig. 2. Differentwetland categories for the Netherlands and their area as percentage of the total Dutch area.
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R.o.o. FRANKEN ET AL.
3.2 C02 sources and sinks
It is obvious that aquatic and semi-aquatic ecosystems are usually more important CO2 sinks than sources; CO2 is the sole carbon source for primary production and only a fraction is being consumed and digested at higher trophic levels. By far the most important semi-permanent sinks are the detritus and the process by which organic material is transformed into CH4. One could imagine that the increase of the CO2 concentration in the air leads to an increase in CO2 absorption by the water itself, however, these quantities are almost negligible. As mentioned earlier the biological absorption capacity for surface waters in the Netherlands was derived from the Lake Marken situation. We estimate the net annual detritus accrement at 56.2 ton C. km -2. Another fraction of the organic material is being mineralized anaerobically, resulting in a C H 4 production of 24.3 ton C • km -2 • a -l (see section 3.3). As indicated above, the physico-chemical absorption of CO2 by surface waters is considered to be low and is estimated at 81 g. km -2. a -~. On the basis of these figures, the total CO2 uptake by fresh surface waters in the Netherlands is estimated at about 295 kton • a-1. This is about 0.6% of the total annual CO2 emission in the Netherlands. Peatlands (mainly bogs) in which the water table is lowered are exposed to the air. As a result, the peat is aerobically mineralized and these peatlands become a net source of CO2. In the Netherlands this practice is estimated to result in a CO2 release of 232 ton C • km -2 • a -I [6]. We further have assumed that marshes and swamps are the major sinks at a level of 48 ton C • km -2. a -1 [5], which brings the total for this category at 13 kton • a-1. Other wetlands were not taken into account because their net contribution was considered to be very small. Adding up all the sources and sinks, we arrive at the conclusion that there is a small net CO: emission from wetlands in the Netherlands, which amounts to 0.06% of the total annual CO2 emission. 3.3 CH4 sources and sinks
The most important methane source in freshwater ecosystems is the methane production in the sediments. In shallow waters and wetlands, the CH4 production controlling parameters are still poorly understood. In spite of our basic knowledge it is very difficult to quantitatively predict the output of methane in relation to e.g. organic content of the sediment and the temperature. In relatively deep waters, about 99% of the methane entering the water phase is being oxidized. However, not all the methane spends sufficient time in the waterlayer to become oxidized. In very shallow waters and
EMISSIONS OF GREENHOUSEGASES FROM AQUATICECOSYSTEMS
TABLE
283
1
C H 4 e m i s s i o n s (mg' m - 2 " d - I ) f r o m d i f f e r e n t w e t l a n d s a n d s u r f a c e w a t e r s as d e r i v e d f r o m d i f f e r e n t l i t e r a t u r e s o u r c e s . (Te = t e m p e r a t e , T = t r o p i c a l , St = s u b t r o p i c a l , B = boreal). D a t a give t h e a v e r a g e ( u n d e r l i n e d ) as well a s / o r t h e r a n g e o f d a t a Gas
Habitat fie/T/St/B)
Location
CIG
Bogs
USA-Te
106(53-211)
Fens
UK-Te USA-Te Sweden-B USA-B Canada-Te USA-Te
23.2 (< 1-62) 3.2-11.2 4._.77(1.1-21) 4.__00(0.7-23) 0._3 (< 0-10) 12__~6(22-727)
USA-Te Sweden-B USA-B USA-B Canada-B Canada-B S-America-T USA-Te S-America-T USA-St USA-St
12__.~.(61-83) 2 95 (26-350) 59 (18-195) 39 (26-52) 18-46 3.6-27.5 192 (162-219) 83-155 10__8_8 (52-162) 59 (< 0-274) 3"/ [12-112]
USA-Te Canada-Te USA-Te S-America-T USA-Te USA-Te S-Amedca-T USA-B USA-St USA-Te USA-St USA-St USA-St
23.__22[7-73] 3.3-11.7 4__(1.9-9.0) 590(195-885) 57_~2(493-664) 304(168-505) 230 (158-302) 175 (106-289) 160 157(106-289) 52-90 (0-1017) 61 (0-624) 44 (20-97)
USA-Te USA-Te USA-St USA-Te S-America-T USA-Te USA-St USA-St USA-St S-America-T USA-Te Netberi,-T
350 307 282 130 120 87 74 (10) 4-9 34 27 22 5-40
Swanps
Marshes
Lakes
Emission
Reference Crill et al. (1988) Harriss et al. (1985) Clymo and Reddaway (1971) Yavitt et al. (1990) Svensson and Rosswail (1984) Sebacher et al. (1986) Moore and Knowles (1990) Crill et al. (1988) Harriss et al. (1985) Swain (1973) Svensson and Rosswail (1984) Sehacher et aL (1986) Whalen and Reeburgh (1988) Moore and Knowles (1987) Moore and Knowles (1990) Barlett et al. (1988) Wilson et al. (1989) Devol et al. (1988) Harriss et al. (1988) Barlett et al. (1985) Crill et al. (1988) Harriss and Sebacher (1981) Moore and Knowles (1990) Harriss et al. (1982) Devol et al. (1988) Crill et al. (1988) Baker-Blocker et al. (1977) Barlett et al. (1988) Sebacher et al. (1986) Delaune et a|. (1983) Swain (1973) Barber et aL (1988) Harriss et al. (1988) Crill et al. (1988) Barlett et al. (1988) Dacey and Klug (1979) Kelly-Robertson (1979) Sebacher et ai. (1983) Cicerone and Shetter (1981) Devol et al. (1988) Kelly-Robertson (1979) Harriss et al. (1988) Delaune et al. (1983) Barber et al. (1988) Barlett et al. (1988) Rudd and Hammilton (1978,1979) Cappenberg et al. (1984) Sweerts et al. (1990)
[12] [13] [14] [15) [16] [17] [18] [12] [13] [19] [16] [17] [20] [21] [18] [22] [23] [24] [25] [26] [12] [27[ [18] [28] [24] [12] [29] [22] [17] [30] [19] [31] [25] [12] [22] [10] [32] [33] [34] [24] [32] [25] [30] [31] [22] [35,36] [8] [37]
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R.O.G. FRANKEN ET AL.
TABLE 1 (Cont) Gas
Habitat
Location
Emission
Reference
fie/T/St/B) Rivers
USA-Te USA-Te USA-Te USA-St USA-Te USA-Te USA-St
132-145 16 (5.5-36) 7.6 4.3 2._4__(1.6-3.6) 0.9 0.4
Chanton and Martens 0988) Angelis and Lilley (1987) Barlett et al. 0985) Brooks (1976) Angelis and Lilley (1987) Lamontagne et al. (1973) Atkinson and Hall (1976)
[38] [39] [26] [40] [39] [41] [42]
wetlands, the time is sometimes extremely short and CH4 escapes into the atmosphere. We may distinguish three different processes by which methane enters the atmosphere; by diffusion, by bubble evolution from the sediments [8] and by transportation through air chambers of water plants [9]. Bubbling of
minimum
~
maximum
100
150
lakes
rivers
small w a t e r s
marshes
fens,bogs
O
50
200
CH4 emission (g.m2.a-1)
Fig. 3. Methane emission Netherlands.
rates
(g'm-2'a -1) for surface waters and wetlands in the
EMISSIONS OF GREENHOUSE GASES FROM AQUATIC ECOSYSTEMS
285
methane sometimes accounts for 50% of the methane emission [8]. In a shallow lake dominated by aquatic plants (Nuphar luteum), almost 50% of the total methane emission was observed to arise from these macrophytes
[10,1.11. In Table 1, the literature data on which we based our calculations on CH4 emissions for wetlands and surface waters are listed. Average values are underlined. As can be seen, the data are derived from a variety of habitat types and climate zones. Therefore, it is quite understandable that there are large differences between the data. To arrive at the total emission for the Netherlands, we combined these data with the surface areas of the different habitat types in the Netherlands. Moreover, we calculated the total emission on the basis of 365 emission days a year. By following this procedure we may seriously overestimate the emission of CH4, but the result should be considered as a worst case scenario. The annual emission rates can now be estimated by multiplying the daily rates with 365. The results are given in Fig. 3. The ranges are based on the average maximum and minimum values underlying the data in Table 1. In Fig. 4, the estimated total emissions from Dutch surface waters and wetlands are given in kilotons per annum. A maximum, minimum and average value is given. It is also indicated (Fig. 4B) how this amount compares the total emission in the Netherlands.
A
B ~a~b~c
~a~b~c I ~. 0
cO
300
._~
30
200
cO
20
c 0
D
a
c 0
8
100 4~
0
o T 0
0
SW W L
T
SW W L
T
habitat
type
habitat
type
Fig. 4. Total methane emissions from Dutch surface waters and wetlands. (SW = surface water; W L = wetland; T = total; a = minimum estimate, b = average estimate; c = maximum estimate). (A) Absolute emissions. (B) Contribution to total emissions in the Netherlands.
286
R.O.G. FRANKENET AL.
TABLE 2 Overview of the literature data pertaining the emission of N20 from wetlands and fresh surface waters (mg N-N20 • m -2" h-l). Gas
Habitat
Location
Emission average
Reference
N20
Wetland
USA-Te USA-Te Denmark-Te USA-Te Denmark-Te USA-Te USA-Te USA-Te
0.149 0.()65 0.049 0.031 0.007 0.001 0.2__~5(0.08-2.5) 0.12-0.24
[44] [44] [45] [44] [45] [44] [46]
USA-Te USA-T¢
0.14 0.084
USA-Te
0.01 (0.0002-0.032) 0.006
Goodroad and Keeney (1984) Goodroad and Kecney (1984) Struwe and Kjoller (1989) Goodroad and Keeney (1984) Struwe and Kjoller (1989) Goodroad and Keeney (1984) Hemond and Duran (1989) Deck (1981) in: Hemond and Duran (1989) Lemon and Lemon (1981) Elkins (1978) Hemond and Duran (1989) De Angelis and Gordon (1985) Smith and Delaune (1983)
[49]
Surface Water
USA-Te
F~
minimum
[~
[46] [47] [48] [46] [39l
maximum
surface w a t e r s
wetlands
0.00
0.50
1.00
1.50
2.00
N 2 0 emission (g.m-2.a- 1) Fig. 5. N20 emission rates (g' m -2" a - l ) for surface waters and wetlands in the Netherlands.
287
EMISSIONS OF GREENHOUSE GASES FROM AQUATIC ECOSYSTEMS
At least 9% o f the total C H 4 emission is accounted for by these ecosystems. A worst case scenario estimates 27% o f the total emission to come from these aquatic and semi-aquatic habitats. The average is 18%, which means that (see Fig. 1) since the area covered by these habitats makes out 14% of the Netherlands, the contribution is relatively high.
3.4 N20 sources and sinks The most relevant processes as related to the N20 production are the nitrification and denitrification process. At low oxygen saturation values ( < 30 /~M), the production o f N20 is at its maximum. High nitrogen loadings of surface waters will increase the N20 emission [43]. Stressed conditions seem also to increase the N20 emission. The latter was concluded from the fact that at unusual conditions, such as when stormwaters occurred, maximum N20 production and emission were observed in sewage treatment plants. It is not known which factors are responsible for that phenomenon, although it is clearly correlated with the occurrence o f so-called stress factors (e.g. stormwaters).
A
B I
]3
g
z
-
I
0
O4 Z E o
_~ ~C" o CO "-E
¢
O o4 Z
10
~
40
8
Z
3O
6
_c:
4
~ 4-
z: 4~
20 -
2
10
4~ O 4-~
.......
0
SWWL sp habitat
T
type
o
0 S W W L SP habitat
T
type
Fig. 6. Total N20 emissions from Dutch surface waters and wetlands. (SW = fresh surface water; WL = freshwater wetland; SP = sewage treatment plant; T = total; a = minimum estimate; b = average estimate; c = maximum estimate). (A) Absolute emissions. (B) Contribution to total emissions in the Netherlands.
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R.O.G. FRANKEN ET A L
In Table 2, literature data are summarized on which we based our calculations for the Dutch habitats. We added one extra habitat; the sewage treatment plant for reasons indicated above. In Fig. 5, the annual emission rates are given for the wetlands, and surface waters. The emissions from sewage treatment plants are not given, but they are estimated to be 5% of the nitrogen input. In Fig. 6, total N20 emissions from Dutch surface waters, wetlands and sewage treatment plants are given (Fig. 6A). Further, it is indicated how much these N20 sources contribute to the total Dutch emissions (Fig. 6B). In the worst case scenario up to 34% of the total N20 emission arises from freshwater ecosystems (including sewage plants). Because of the seemingly erratic behaviour of sewage treatment plants as far as the N20 production is concerned, the range of estimated levels is rather wide. DISCUSSION AND CONCLUSIONS The bulk of the major greenhouse gas, the CO2, can hardly be influenced by actively managing aquatic and semi-aquatic ecosystems. They have a total CO2 absorbing capacity of 0.6% of all the CO2 that is produced annually in the Netherlands. Taking into consideration that it is expected that 61% of the greenhouse effect in the next century is caused by CO2, it is quite obvious that optimizing the management of aquatic and semi-aquatic habitats towards the goal of enlarging the CO2 absorbing capacity of the aquatic world is quite useless. Moreover, the ways to do that would be quite incompatible with the longterm policies of water managers in the Netherlands. The dosing of lime to increase the physico-chemical absorption capacity as well as artificially increasing the trophic status of the ecosystem to speed up primary productivity would be totally incompatible with the long-term water quality management policies in the Netherlands [1]; a further reduction of nutrients. Since wetlands are also very productive, another option would be to enlarge the total area of wetland ecosystem. An expansion of wetland area is foreseen in the Netherlands within the framework of new nature conservation policies for the next decade. The envisaged expansion comprises 650 km 2. This is only 2% of the Netherlands but means an expansion of the existing wetland area of 70% and brings the total area with wetlands and surface water areas to more than 15% of the Netherlands' territory. However, as we have seen, the CH4 emission will increase accordingly. To bring the net annual CO2 uptake to the level of 1% of the total amount being produced annually in the Netherlands, one would need an expansion
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of wetland of at least 1040 km 2 (48 ton C. km -2. a-l; based on data [5]. However, the methane being produced by the same area would be in the range of 15-58 (average 29) ton C. a -l (fens) or 55-165 (average l l0) ton C. a- 1 for marshes. This production of CH4 is equivalent to a CO2 emission of about 221-840 ton C. a -1. This means that the overall effect would be clearly negative as far as the control of greenhouse-enhancing gases is concerned. Another way of influencing the atmospheric CO2 concentration is to diminish the output of CO2. Although very modest in its effect, one could decide to stop the lowering of the water tables in peatlands in order not to expose them to the air. This would certainly stop the large-scale oxidation of peatlands and would diminish the production of CO: from this source. Others [50] found a linear correlation between CO2 emission and the extent of lowering of the groundwater table, which deafly supports this hypothesis. A little more promising seems the situation as related to the emission of CH4 and N20. Both greenhouse gases are produced by aquatic and semiaquatic ecosystems in the Netherlands in considerable quantities. Their production equals, respectively, 18% and 19% of the total annual emissions. It has to be stated that since there are no major industrial activities associated with freshwater ecosystems and wetlands, the fluxes and balances are largely the result of purely biological processes. Of course they are influenced by man, but unlike on land, the influence on the ecosystems is indirect (by means of management-directed nutrient flows). First of all it would be feasible to try to reduce the CH 4 emission. It is strongly correlated with the composition (C/N ratio) of the biomass from which the detritus originates. It is estimated [7] that 2-7% of the biomass in an ecosystem will finally be converted into CH4. When eutrophication diminishes, more macrophytes may be expected to occur at the cost of phytoplankton. Macrophytes have a relatively high C/N ratio compared to phytoplankton and as a result, mineralization will be slower and less CH4 will be produced [51]. Suppose that as a result of a lower trophic level less biomass (more macrophytes but considerably less phytoplankton) would be bound to the system. It is very likely that this negative effect of lower CO2 absorption would be overcompensated by the lower CH 4 emission. Further, the reduction of nitrogen in surface waters would be very effective in lowering the emission of N20. The Dutch water quality policies for the future as related to nitrogen loadings of surface waters aim at a reduction of 50% in 1995 with the year 1985 as a reference. The most important nitrogen sources are the wash-out and wash-off from agricultural land. It is estimated that they can be reduced with 30% if the application of N-based fertilizers is reduced with 50%. The removal capacity of nitrogen from sewage water has to be expanded
290
R.O.G. FRANKEN ET AL.
according to the Dutch policy makers from 60% in 1985 to at least 70% in 1995. This will be done by expanding the denitrification capacity. Although the exact nature of the relationship between denitrification and N20 emission volume is still unknown, this will certainly lead to an increase of the N20 emission from sewage treatment plants. The net effect of more nitrogen removal from sewage water and less nitrogen in surface waters is not necessarily positive as far as the production of greenhouse gases and their effects are concerned. It is very difficult to make reliable predictions about the effects of certain measures. However, a number of recommendations may be made. The reduction of fertilizers (either N or P) should have priority. This will lower the primary productivity and detritus formation but will definitely diminish CH4 and N20 emissions. Since aquatic and semi-aquatic ecosystems quite considerably contribute to the production of greenhouse gases in the Netherlands, it is worthwhile to pay more attention to understanding the production processes which are in fact still poorly understood. The operation of sewage treatment facilities should get more attention in this respect. However, when the relative contribution in the Netherlands o f the different greenhouse gases to the greenhouse effect in the future would be similar to that predicted by the IPCC ([2]; 61% contribution by CO2, 15% by CH4 and 4% by N20) we have to ask ourselves whether the priorities to take action in order to lower the emissions of greenhouse gases should lay in the domain of water quality management. First of all, the CO2 absorption capacity is negligible. Further, suppose that the average contribution of C H 4 and N20 from aquatic and semiaquatic habitats to the total emission of these gases in the Netherlands would be 25% (a sensible percentage considering our results). This would still mean that, supposing that the IPCC [2] figures given above are also valid for the Netherlands (they are probably not), only about a maximum of 5% (19% from 25%) of the Dutch contribution to the greenhouse effect would be theoretically manageable via our aquatic and semi-aquatic ecosystems. This seems to be not very much but the possibility deserves closer thought and attention. ACKNOWLEDGEMENTS This study was sponsored by the Ministry of Housing, Physical Planning and the Environment. We would like to thank Mrs. Ir. M.J.M. Hootsmans and Dr. L.H.J.M. Janssen of this Ministry for their support. REFERENCES 1 Anonymus, Water voor nu en later. Derde nota waterhuishouding. Ministerie van Verkeer en Waterstaat, SDU uitgeverij, Den Haag, 1989.
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2 IPCC (Intergovernmental Panel on Climate Change) 1990, Section 1, "Greenhouse gases and aerosols" (draft) 1990. 3 G.J. Van den Born, A.F. Bouwman, J.G.J. Olivier, R.J. Swart, The emission of greenhouse gases in the Netherlands (draft report). RIVM report 222901003, 1990. 4 F.J. Los and A.H. Bakema, Her general ecomodel. Analyse en toepassing als balansmodel. Waterloopkunding Laboratorium, rapport T 248, 1988. 5 T.V. Armentano and E.S. Menges, Patterns of change in the carbon balance of organic soil-wetlands of the temperate zone. J. Ecol., 74 (1986) 755-774. 6 C.J. Schothorst, Het zakkingsproces bij ontwatering van de westelijke veenweidegronden. Landbouwkundig tijdschrift, 90 (1978) 167-175. 7 I. Asselman and P.J. Crutzen, Global distribution of natural freshwater wetlands and rice paddies: their net primary productivity, seasonality and possible methane emissions. J. Atmospheric Chem., 8 (1989) 307-358. 8 Th.E. Cappenberg, C.A. Hordijk and C.P.M.M. Hagenaars, A comparison of bacterial sulfate reduction and methanogenesis in the aerobic sediments of a stratified lake ecosystem. Arch. Hydrobiol. Beiheft, 19 (1984) 191-199. 9 D.I. Sebacher, R.C. Harriss and K.B. Barlett, Methane emissions to the atmosphere through aquatic plants. J. Environ. Qual., 14 (1985) 40-46. 10 J.W.H. Dacey and M.J. Klug, Methane efflux from lake sediments through water lilies. Science, 203 (1979) 1253-1255. 11 J.W.H. Dacey, Internal winds in water lilies: an adaptation for life in aerobic sediments. Science, 210 (1980) 1017-1018. 12 P.M. Crill, K.P. Bartlett, R.C. Harriss, E. Gorham, I.S. Ferry, D.S. Ferry, D.I. Sebacher, E.L. Madsar and W. Sanner, Methane flux from Minnesota peatlands. Global Biogeochem. Cyc., 2 (1988) 371-384. 13 R.C. Harriss, E. Gotham, D.I. Sebacher, K.B. Bartlett and P.A. Flebbe, Methane flux from northern peatlands. Nature, 315 (1985) 652-653. 14 R.S. Clymo and E.J.F. Reddaway, Productivity of Sphagnum (bog-moss) and peat accumulation. Hydrobiologia, 12 (1971) 181-192. 15 J.B. Yavitt, D.M. Downey and E. Lancaster, Methane consumption in decomposing Sphagnum-derived peat. Soil Biol. Biochem., 22 (1990) 441-447. 16 B.H. Svensson and T. Rosswall, In situ methane production from acid peat in plant communities with different moisture regimes in asubaretic mire. Oikos, 43 (1984) 341-350. 17 D.I. Sebacher, R.C. Harriss, K.B. Bartlett, S.M. Sebacher and S.S. Grice, Atmospheric methane sources: Alaskan tundra bogs, an alpine fen and a subarctic boreal marsh. Tdlus, 38B (1986) 1-10. 18 T.R. Moore and R. Knowles, Methane emissions from fen, bog and swamp peatlands in Quebec. Biogeochemistry, 11 (1990) 45-61. 19 F.M. Swain, Marsh gas from the Atlantic coastal plain, U.S., in B. Tissot and F. Bremer (Eds.), Advances in Organic Geochemistry, 1973, pp. 671-687. 20 S.C. Whalen and W.S. Reeburgh, A methane flux time series for tundra environments. Global Biogeochem. Cyc., 2 (1988) 399-409. 21 T.R. Moore and R. Knowles, Methane and carbon dioxide evolution from subarctic fens. Can. J. Soil Sci., 67 (1987) 77-81. 22 K.A. Barlett, P.M. Crill, D.I. Sebacher, R.C. Harriss, J.O. Wilson and J.M. Melack, Methane flux from the central Amazonian floodplain. J. Geophys. Res., 93 (1988) 1571-1582. 23 J.O. Wilson, P.M. Crill, K.B. Bartlett, D.I. Sebacher, R.C. Harriss and R.L. Sass, Seasonal variations of methane emissions from a temperate swamp. Biogeochemistry, 8 (1989) 55-71.
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24 A.H. Devol, J.E. Richey, W.A. Clark, S.L. King and L.A. Martinelli, Methane emissions to the troposphere from the Amazon floodplain. J. Geophys. Res., 93 (1988) 1583-1592. 25 R.C. Harriss, D.I. Sebacher, K.B. Bartlett, D.S. Bartlett and P.M. Crill, Sources of atmospheric methane in the south Florida environment. Global Biochem. Cyc., 2 (1988) 231-243. 26 K.A. Barlett, R.C. Harriss, and D.I. Sebacher, Methane flux from coastal salt marshes. J. Geophys. Res., 90 (1985) 5710-5720. 27 R.C. Harriss and D.J. Sebacher, Methane flux in forested freshwater swamps of the southern United States. Geophys. Res. Lett., 8 (1981) 1002-1004. 28 R.C. Harriss, D.J. Sebacher and F.P. Day, Methane flux in the Great Dismal Swamp. Nature, 297 (1982) 673-674. 29 A. Baker-Blocker, T.M. Donohue and K.H. Mancy, Methane fluxes from wetland areas. Tellus, 29 (1977) 245-250. 30 R.D. Delaune, Ch.J. Smith and W.H. Patrick, Methane release from Gulf Coast Wetlands. Tellus, 35B (1983) 8-15. 31 T.R. Barber, R.A. Burke and W.M. Sackett, Diffusive flux of methane from warm wetlands. Global Biochem. Cyc., 2 (1988) 411-425. 32 C. Kelly-Robertson, Quantitative comparison of the significance of methane in the carbon cycles of two small lakes. Arch. Hydrobiol. Beiheft, 12 (1979) 123-135. 33 D.I. Sebacher, R.C. Harriss and K.B. Bartlett, Methane flux across the air-water interface: air velocity effects. TeUus, 35B (1983) 103-109. 34 R.J. Cicerone and J.D. Shetter, Sources of atmospheric methane: measurements in rice paddies and a discussion. J. Geophys. Res., 86 (1981) 7203-7209. 35 J.W.M. Rudd and R.D. Hammilton, Methane cycling in an autotrophic shield lake and its effects on whole lake metabolism. Limnol. Ocean., 23 (1978) 337-348. 36 J.W.M. Rudd and R.D. Hammilton, Methane cycling in Lake 227 in perspective with some components of carbon and oxygen cycles. Arch. Hydrobiol. Beiheft, 12 (1979) 115-122. 37 J.P.R.A. Sweerts, T.M.J. Dekkers and Th.E. Cappenberg, Methane oxidation at the sediment-water interface of a shallow eutrophic lake (Lake Loosdrecht) and a deep m e s o eutrophic lake (Lake Vechten). Arch. Hydrobiol., in press. 38 J.P. Chanton and C.S. Martens, Seasonal variation in ebollutive flu~ and carbon isotopic composition of methane in tidal freshwater estuary. Global Biogeochem. Cyc., 2 (1988) 289-298. 39 M.A. De Angelis and L.I. Gordon, Upwelling and river runoff as sources of dissolved nitrous oxide to the Alsea Estuary, Oregon. Est. Coast. Mar. Shelf Sci., 20 (1985) 375-386. 40 J.M. Brooks, The flux of light hydrocarbons into the Gulf of Mexico via runoff, in H.L. Windom and R.A. Duce (Eds.), Marine Pollution Transfer, 1976, pp. 185-200. 41 R.A. Lamontagne, J.W. Swynnerton, V.J. Lynnenbom and W.D. Smiths, Methane concentrations in various marine environments., J. Geophys. Res., 78 (1973) 5317-5324. 42 L.P. Atkinson and J.R. Hall, Methane distribution and production in the Georgia Salt Marsh. Est. Coast. Mar. Sci., 4 (1976) 677-686. 43 W.A. Kaplan and S.C. Wofsy, The biogeochemistry of nitrous oxide. A review. Adv. Aquat. Microbiol., 3 (1985) 181-206. 44 L.L. Goodroad and D.R. Keeney, Nitrous oxide emissions from forest, marsh and prairie ecosystems. J. Environ. Qual., 13 (1984)448-452. 45 S. Struwe and A. Kjoller, Field determination of denitrification in water logged forest soils. FEMS Microbiol. Ecol., 62 (1984) 71-78.
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46 H.F. Hemond and A.P. Duran, Fluxes of N20 at the sediment-water and wateratmosphere boundaries of a nitrogen-rich river. Water Resour. Res., 25 (1989) 839-846. 47 E. Lemon and D. Lemon, Nitrous oxide in freshwaters of the Great Lake Basin. Limnol. Ocean., 26 (1981) 867-879. 48 J.W. Elkins, S.C. Wofsy, M.B. McElroy, C.E. Kolb and W.A. Kaplan, Aquatic sources and sinks for nitrous oxide. Nature, 275 (1978) 602-606. 49 C.J. Smith and R.D. De Laurie, Nitrogen loss from freshwater and saline estuarine sediments. J. Environ. Qual., 12 (1983) 514-517. 50 T.R. Moore and R. Knowles, The influence of water table levels on methane and carbon dioxide emissions from peatland soils. Can. J. Soil Sci., 69 (1989) 33-38. 51 J.I. Sprent, The Ecology of the Nitrogen Cycle, Cambridge University press, Cambridge, U.K., 1987.
277
Emissions of some greenhouse gases from aquatic and semi-aquatic ecosystems in the Netherlands and options to control them Ron O.G. Franken*, Wim van Vierssen and Henk J. Lubberding Department of Environmental Engineering, International Institute for Hydraulic and Environmental Engineering (II-IE), P.O. Box 3015, 2601 DA Delft, The Netherlands (Received October 9th, 1991; accepted November 18th, 1991) ABSTRACT An inventory was made to quantify the emission of the waterborne greenhouse gases CH4, N20 and CO2 from fresh surface waters and wetlands in the Netherlands. The aim of the study was to give an overview of the existing fluxes and balances and to quantify the role of aquatic and semi-aquatic habitats in the total emissions of these gases in the Netherlands. The CO2 uptake of surface waters and wetlands turned out to be very low. Only 0.6% of the total annual CO2 emission from the Netherlands is absorbed by the ecosystems in these habitats. However, they are significant sources for N20 and CH£ respectively, 18% and 19% (including sewage treatment plants) of the total annual emission in the Netherlands. It is indicated that especially in the case of N20 and CH4 the reduction of the eutrophication will also reduce the output of these gases from the surface waters and the wetlands. However, attention should be paid to the operation of sewage treatment plants because reducing the discharge of nitrogen by extending the denitrification capacity could easily lead to more nitrous oxide release from these plants.
Key words: methone; nitrous oxide; carbon dioxide; emission; fresh water; wetlands
1. INTRODUCTION
The global climatic changes that will probably result from the steadily increasing amounts of the so-called greenhouse gases in the atmosphere are extensively and widely discussed. Apart from the efforts to model and quantitatively describe the different processes involved, government agencies are *Present address: National Institute for Public Health and Environmental Hygiene, P.O. Box 1, 3720 BA Bilthoven, The Netherlands. 0048-9697/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved
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seriously concerned as how to manage this problem in the (near) future. National governments are in the process of developing strategies to control the massive output of greenhouse gases. However, to be able to do so, reliable information is needed to base such strategies on. The present study was part of a broader framework of data collection and data evaluation on the production of greenhouse gases in the Netherlands, sponsored by the Dutch Ministry of Housing, Physical Planning and the Environment. A quick glance at the figures proves that the Netherlands population, which is roughly 0.003% of the world's population produces about 1% of the total world's output of greenhouse gases. This means that the Netherlands should clearly take up their responsibility to do something about the problem. The present study specifically focussed on the fluxes and balances of a number of the most important greenhouse gases which are produced by freshwater wetlands and fresh surface waters in the Netherlands. The ultimate goal of the study we report on here was to determine which role these aquatic and semi-aquatic ecosystems quantitatively play in the fluxes and balances of a number of greenhouse gases (CO2, CH4 and N20) on a national scale. Considering the fact that approximately 14% of the Dutch surface area is occupied by wetlands and surface waters, these habitats probably represent a large potential for greenhouse gas (CH4 and N20) production. Therefore, we also examined the possibility whether this information could lead towards specific environmental management guidelines by means of which the output of these gases could be reduced. Different management options had to be judged in the context of the whole set of adopted management policies for aquatic and semi-aquatic ecosystems in the Netherlands. Combating eutrophication and stopping the continuous lowering of groundwater tables in areas with sensitive ecosystems are major management goals at present as well as in the near future [1]. Major greenhouse gases are CO2, CH4, N20, CFK's and 03 with CO: being the far most important one. Each of these greenhouse gases has its own infrared absorption characteristic and therefore, a different potential for enhancing the greenhouse effect. The relative importance of 1 mol CH4 is 7.6 times that of 1 mol CO2 [2]. This figure is even more dramatic for N20 which has a potential of 290 times that of CO2. Based on the present worldwide concentration and output of these gases, an estimate has been made by the IPCC on the relative contribution of the different gases to the greenhouse effect in the next century. A contribution of 61% by CO2, 15% by CH4, 4% by N20 and 20% by miscellaneous components is anticipated
[21.
EMISSIONS OF GREENHOUSE GASES FROM AQUATIC ECOSYSTEMS
279
In the present paper we will focus on Dutch freshwater wetlands and fresh surface waters as a source and sink for CO2, CH4 and N20. We adopted the distinction between wetlands and surface waters because of the important difference in aeration status and the fact that their management faces different types of problems. Surface waters are permanent aquatic habitats. Wetlands are semi-aquatic habitats. They may dry up or become flooded more or less frequently. This status is important because it determines to a large extent the major type of greenhouse gas emitted. We have collected the data on CH4 and N20 emissions from the international literature. The CO2 absorption capacity of wetlands we deduced from specific data on community consumption and production. 2. MATERIALS AND METHODS
To quantify the CO2, C H 4 and N 2 0 sources and sinks of the Dutch wetlands and surface waters we had to use different methods and types of data. First of all, the wetlands were categorized according to a predefined set of types (see below and Tables 1 and 2). Further, we used information on the total area that exists of each type and the emissions for the different categories to combine them into total outputs. To get an idea about the relative importance of the quantities being produced by these aquatic and semi-aquatic ecosystems, we used a review on the production of greenhouse gases from a number of major sources in the Netherlands [3]. They estimated the total output of the same gases for the Netherlands. They based themselves on other sources, and were not very specific about the output of these gases for the different aquatic ecosystem types. However, we used their total estimate for the Netherlands as a reference point and considered our figures as being part of their total estimates although they were not specified to that detail. For only a few Dutch fresh surface waters carbon balances appeared to be available. We largely based our estimates on the carbon budget that was made for the Dutch Lake Marken [4]. This lake is a rather shallow lake (average depth < 5 m) and represents one of the quantitatively important freshwater habitat types in the Netherlands. The net CO2 uptake was estimated by combining data on the inorganic binding of CO2 (important because of the high buffering capacity of many eutrophicated waters) and the organic binding capacity of CO2 (formation of semi-permanent detritus, transformation of CO2 via plant material and detritus into CH4). We calculated the physico-chemical absorption of CO2 by the Dutch surface water volume by combining the information on total water volume and its binding capacity per unit of volume. For the CO2 uptake by wetlands (mainly organic peatlands) we used other data [5]. For the quantification of
280
R.O.G. FRANKEN ET AL.
the release of CO2 from drained peatlands (a major source of CO2) we used the data from [6]. We decided that these sources and sinks were the major ones to be discerned and neglected others. Only very little information was also available for CH4 production in Dutch surface waters and wetlands. Therefore we had to use data from other areas in the world. We used an up to date review on CH4 emissions from wetlands [7]. We have mainly based ourselves on some of their data and supplemented them with other data from the literature. With these data we arrived at an emission range for both the wetlands and the surface waters. Combining the surface area of the different habitats with the production figures, we could estimate the CH4 emission for Dutch aquatic and semi-aquatic ecosystems. For the collection of the N20 data we adopted a similar approach of data collection as for C H 4. However, one could suppose that the denitrification
28
88
56
A C 5 1 1 8 ~ ~ / ~
3
~
B I~
lakes ditches
~
large
rivers
and canals I I small rivers and canals P~I land
Fig. 1. Characteristics of Dutch surface waters. (A) Contribution of different aquatic habitats (%) to total area covered by surface waters. (B) Percentage of the Netherlands with different types of surface waters. (C) Percentage of total surface water volume for different types of surface waters.
EMISSIONS OF GREENHOUSE GASES FROM AQUATIC ECOSYSTEMS
281
process in sewage treatment plants is very important within the present context. Unfortunately, as we found out, there is hardly any specific information available on the N20 production in these plants. However, we used the data we could lay our hands on and reviewed them. We eventually tried to predict how management may minimize the output of these greenhouse gases and tried to quantify how such a management may affect total greenhouse gas production. 3. RESULTS 3.1 Freshwater wetland and surface water area
In Fig. 1, the most important surface water types and their relative importance (data made available by RIZA; Institute for Inland Water Management and Waste Water Treatment) are given. As you can see from Fig. 1A, lakes are the most important habitat type considering the area they occupy. About 56% of the surface waters consist of lakes. Almost 11% of the Netherlands (approx. 33 000 km 2) comprises surface waters as can be seen in Fig. 1B. Considering the water volume (Fig. 1C) 74% of the total surface water volume is contained by lakes. In Fig. 2, different wetland categories and the relative size of the area they occupy in the Netherlands are summarized. The total area is less than that occupied by surface water but all the same adds another 3% to the total area of surface water. An important habitat type is the floodplain (40% of the wetlands); they consist of the riverine areas of the rivers Rhine, Waal, Meuse and IJssel, which are flooded during the annual peak discharges of the river.
10
7
11
2
~
floodplains
~
marshes bogs fens
40
~
other wetlands swamps
Fig. 2. Differentwetland categories for the Netherlands and their area as percentage of the total Dutch area.
282
R.o.o. FRANKEN ET AL.
3.2 C02 sources and sinks
It is obvious that aquatic and semi-aquatic ecosystems are usually more important CO2 sinks than sources; CO2 is the sole carbon source for primary production and only a fraction is being consumed and digested at higher trophic levels. By far the most important semi-permanent sinks are the detritus and the process by which organic material is transformed into CH4. One could imagine that the increase of the CO2 concentration in the air leads to an increase in CO2 absorption by the water itself, however, these quantities are almost negligible. As mentioned earlier the biological absorption capacity for surface waters in the Netherlands was derived from the Lake Marken situation. We estimate the net annual detritus accrement at 56.2 ton C. km -2. Another fraction of the organic material is being mineralized anaerobically, resulting in a C H 4 production of 24.3 ton C • km -2 • a -l (see section 3.3). As indicated above, the physico-chemical absorption of CO2 by surface waters is considered to be low and is estimated at 81 g. km -2. a -~. On the basis of these figures, the total CO2 uptake by fresh surface waters in the Netherlands is estimated at about 295 kton • a-1. This is about 0.6% of the total annual CO2 emission in the Netherlands. Peatlands (mainly bogs) in which the water table is lowered are exposed to the air. As a result, the peat is aerobically mineralized and these peatlands become a net source of CO2. In the Netherlands this practice is estimated to result in a CO2 release of 232 ton C • km -2 • a -I [6]. We further have assumed that marshes and swamps are the major sinks at a level of 48 ton C • km -2. a -1 [5], which brings the total for this category at 13 kton • a-1. Other wetlands were not taken into account because their net contribution was considered to be very small. Adding up all the sources and sinks, we arrive at the conclusion that there is a small net CO: emission from wetlands in the Netherlands, which amounts to 0.06% of the total annual CO2 emission. 3.3 CH4 sources and sinks
The most important methane source in freshwater ecosystems is the methane production in the sediments. In shallow waters and wetlands, the CH4 production controlling parameters are still poorly understood. In spite of our basic knowledge it is very difficult to quantitatively predict the output of methane in relation to e.g. organic content of the sediment and the temperature. In relatively deep waters, about 99% of the methane entering the water phase is being oxidized. However, not all the methane spends sufficient time in the waterlayer to become oxidized. In very shallow waters and
EMISSIONS OF GREENHOUSEGASES FROM AQUATICECOSYSTEMS
TABLE
283
1
C H 4 e m i s s i o n s (mg' m - 2 " d - I ) f r o m d i f f e r e n t w e t l a n d s a n d s u r f a c e w a t e r s as d e r i v e d f r o m d i f f e r e n t l i t e r a t u r e s o u r c e s . (Te = t e m p e r a t e , T = t r o p i c a l , St = s u b t r o p i c a l , B = boreal). D a t a give t h e a v e r a g e ( u n d e r l i n e d ) as well a s / o r t h e r a n g e o f d a t a Gas
Habitat fie/T/St/B)
Location
CIG
Bogs
USA-Te
106(53-211)
Fens
UK-Te USA-Te Sweden-B USA-B Canada-Te USA-Te
23.2 (< 1-62) 3.2-11.2 4._.77(1.1-21) 4.__00(0.7-23) 0._3 (< 0-10) 12__~6(22-727)
USA-Te Sweden-B USA-B USA-B Canada-B Canada-B S-America-T USA-Te S-America-T USA-St USA-St
12__.~.(61-83) 2 95 (26-350) 59 (18-195) 39 (26-52) 18-46 3.6-27.5 192 (162-219) 83-155 10__8_8 (52-162) 59 (< 0-274) 3"/ [12-112]
USA-Te Canada-Te USA-Te S-America-T USA-Te USA-Te S-Amedca-T USA-B USA-St USA-Te USA-St USA-St USA-St
23.__22[7-73] 3.3-11.7 4__(1.9-9.0) 590(195-885) 57_~2(493-664) 304(168-505) 230 (158-302) 175 (106-289) 160 157(106-289) 52-90 (0-1017) 61 (0-624) 44 (20-97)
USA-Te USA-Te USA-St USA-Te S-America-T USA-Te USA-St USA-St USA-St S-America-T USA-Te Netberi,-T
350 307 282 130 120 87 74 (10) 4-9 34 27 22 5-40
Swanps
Marshes
Lakes
Emission
Reference Crill et al. (1988) Harriss et al. (1985) Clymo and Reddaway (1971) Yavitt et al. (1990) Svensson and Rosswail (1984) Sebacher et al. (1986) Moore and Knowles (1990) Crill et al. (1988) Harriss et al. (1985) Swain (1973) Svensson and Rosswail (1984) Sehacher et aL (1986) Whalen and Reeburgh (1988) Moore and Knowles (1987) Moore and Knowles (1990) Barlett et al. (1988) Wilson et al. (1989) Devol et al. (1988) Harriss et al. (1988) Barlett et al. (1985) Crill et al. (1988) Harriss and Sebacher (1981) Moore and Knowles (1990) Harriss et al. (1982) Devol et al. (1988) Crill et al. (1988) Baker-Blocker et al. (1977) Barlett et al. (1988) Sebacher et al. (1986) Delaune et a|. (1983) Swain (1973) Barber et aL (1988) Harriss et al. (1988) Crill et al. (1988) Barlett et al. (1988) Dacey and Klug (1979) Kelly-Robertson (1979) Sebacher et ai. (1983) Cicerone and Shetter (1981) Devol et al. (1988) Kelly-Robertson (1979) Harriss et al. (1988) Delaune et al. (1983) Barber et al. (1988) Barlett et al. (1988) Rudd and Hammilton (1978,1979) Cappenberg et al. (1984) Sweerts et al. (1990)
[12] [13] [14] [15) [16] [17] [18] [12] [13] [19] [16] [17] [20] [21] [18] [22] [23] [24] [25] [26] [12] [27[ [18] [28] [24] [12] [29] [22] [17] [30] [19] [31] [25] [12] [22] [10] [32] [33] [34] [24] [32] [25] [30] [31] [22] [35,36] [8] [37]
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R.O.G. FRANKEN ET AL.
TABLE 1 (Cont) Gas
Habitat
Location
Emission
Reference
fie/T/St/B) Rivers
USA-Te USA-Te USA-Te USA-St USA-Te USA-Te USA-St
132-145 16 (5.5-36) 7.6 4.3 2._4__(1.6-3.6) 0.9 0.4
Chanton and Martens 0988) Angelis and Lilley (1987) Barlett et al. 0985) Brooks (1976) Angelis and Lilley (1987) Lamontagne et al. (1973) Atkinson and Hall (1976)
[38] [39] [26] [40] [39] [41] [42]
wetlands, the time is sometimes extremely short and CH4 escapes into the atmosphere. We may distinguish three different processes by which methane enters the atmosphere; by diffusion, by bubble evolution from the sediments [8] and by transportation through air chambers of water plants [9]. Bubbling of
minimum
~
maximum
100
150
lakes
rivers
small w a t e r s
marshes
fens,bogs
O
50
200
CH4 emission (g.m2.a-1)
Fig. 3. Methane emission Netherlands.
rates
(g'm-2'a -1) for surface waters and wetlands in the
EMISSIONS OF GREENHOUSE GASES FROM AQUATIC ECOSYSTEMS
285
methane sometimes accounts for 50% of the methane emission [8]. In a shallow lake dominated by aquatic plants (Nuphar luteum), almost 50% of the total methane emission was observed to arise from these macrophytes
[10,1.11. In Table 1, the literature data on which we based our calculations on CH4 emissions for wetlands and surface waters are listed. Average values are underlined. As can be seen, the data are derived from a variety of habitat types and climate zones. Therefore, it is quite understandable that there are large differences between the data. To arrive at the total emission for the Netherlands, we combined these data with the surface areas of the different habitat types in the Netherlands. Moreover, we calculated the total emission on the basis of 365 emission days a year. By following this procedure we may seriously overestimate the emission of CH4, but the result should be considered as a worst case scenario. The annual emission rates can now be estimated by multiplying the daily rates with 365. The results are given in Fig. 3. The ranges are based on the average maximum and minimum values underlying the data in Table 1. In Fig. 4, the estimated total emissions from Dutch surface waters and wetlands are given in kilotons per annum. A maximum, minimum and average value is given. It is also indicated (Fig. 4B) how this amount compares the total emission in the Netherlands.
A
B ~a~b~c
~a~b~c I ~. 0
cO
300
._~
30
200
cO
20
c 0
D
a
c 0
8
100 4~
0
o T 0
0
SW W L
T
SW W L
T
habitat
type
habitat
type
Fig. 4. Total methane emissions from Dutch surface waters and wetlands. (SW = surface water; W L = wetland; T = total; a = minimum estimate, b = average estimate; c = maximum estimate). (A) Absolute emissions. (B) Contribution to total emissions in the Netherlands.
286
R.O.G. FRANKENET AL.
TABLE 2 Overview of the literature data pertaining the emission of N20 from wetlands and fresh surface waters (mg N-N20 • m -2" h-l). Gas
Habitat
Location
Emission average
Reference
N20
Wetland
USA-Te USA-Te Denmark-Te USA-Te Denmark-Te USA-Te USA-Te USA-Te
0.149 0.()65 0.049 0.031 0.007 0.001 0.2__~5(0.08-2.5) 0.12-0.24
[44] [44] [45] [44] [45] [44] [46]
USA-Te USA-T¢
0.14 0.084
USA-Te
0.01 (0.0002-0.032) 0.006
Goodroad and Keeney (1984) Goodroad and Kecney (1984) Struwe and Kjoller (1989) Goodroad and Keeney (1984) Struwe and Kjoller (1989) Goodroad and Keeney (1984) Hemond and Duran (1989) Deck (1981) in: Hemond and Duran (1989) Lemon and Lemon (1981) Elkins (1978) Hemond and Duran (1989) De Angelis and Gordon (1985) Smith and Delaune (1983)
[49]
Surface Water
USA-Te
F~
minimum
[~
[46] [47] [48] [46] [39l
maximum
surface w a t e r s
wetlands
0.00
0.50
1.00
1.50
2.00
N 2 0 emission (g.m-2.a- 1) Fig. 5. N20 emission rates (g' m -2" a - l ) for surface waters and wetlands in the Netherlands.
287
EMISSIONS OF GREENHOUSE GASES FROM AQUATIC ECOSYSTEMS
At least 9% o f the total C H 4 emission is accounted for by these ecosystems. A worst case scenario estimates 27% o f the total emission to come from these aquatic and semi-aquatic habitats. The average is 18%, which means that (see Fig. 1) since the area covered by these habitats makes out 14% of the Netherlands, the contribution is relatively high.
3.4 N20 sources and sinks The most relevant processes as related to the N20 production are the nitrification and denitrification process. At low oxygen saturation values ( < 30 /~M), the production o f N20 is at its maximum. High nitrogen loadings of surface waters will increase the N20 emission [43]. Stressed conditions seem also to increase the N20 emission. The latter was concluded from the fact that at unusual conditions, such as when stormwaters occurred, maximum N20 production and emission were observed in sewage treatment plants. It is not known which factors are responsible for that phenomenon, although it is clearly correlated with the occurrence o f so-called stress factors (e.g. stormwaters).
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In Table 2, literature data are summarized on which we based our calculations for the Dutch habitats. We added one extra habitat; the sewage treatment plant for reasons indicated above. In Fig. 5, the annual emission rates are given for the wetlands, and surface waters. The emissions from sewage treatment plants are not given, but they are estimated to be 5% of the nitrogen input. In Fig. 6, total N20 emissions from Dutch surface waters, wetlands and sewage treatment plants are given (Fig. 6A). Further, it is indicated how much these N20 sources contribute to the total Dutch emissions (Fig. 6B). In the worst case scenario up to 34% of the total N20 emission arises from freshwater ecosystems (including sewage plants). Because of the seemingly erratic behaviour of sewage treatment plants as far as the N20 production is concerned, the range of estimated levels is rather wide. DISCUSSION AND CONCLUSIONS The bulk of the major greenhouse gas, the CO2, can hardly be influenced by actively managing aquatic and semi-aquatic ecosystems. They have a total CO2 absorbing capacity of 0.6% of all the CO2 that is produced annually in the Netherlands. Taking into consideration that it is expected that 61% of the greenhouse effect in the next century is caused by CO2, it is quite obvious that optimizing the management of aquatic and semi-aquatic habitats towards the goal of enlarging the CO2 absorbing capacity of the aquatic world is quite useless. Moreover, the ways to do that would be quite incompatible with the longterm policies of water managers in the Netherlands. The dosing of lime to increase the physico-chemical absorption capacity as well as artificially increasing the trophic status of the ecosystem to speed up primary productivity would be totally incompatible with the long-term water quality management policies in the Netherlands [1]; a further reduction of nutrients. Since wetlands are also very productive, another option would be to enlarge the total area of wetland ecosystem. An expansion of wetland area is foreseen in the Netherlands within the framework of new nature conservation policies for the next decade. The envisaged expansion comprises 650 km 2. This is only 2% of the Netherlands but means an expansion of the existing wetland area of 70% and brings the total area with wetlands and surface water areas to more than 15% of the Netherlands' territory. However, as we have seen, the CH4 emission will increase accordingly. To bring the net annual CO2 uptake to the level of 1% of the total amount being produced annually in the Netherlands, one would need an expansion
EMISSIONS OF GREENHOUSE GASES FROM AQUATIC ECOSYSTEMS
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of wetland of at least 1040 km 2 (48 ton C. km -2. a-l; based on data [5]. However, the methane being produced by the same area would be in the range of 15-58 (average 29) ton C. a -l (fens) or 55-165 (average l l0) ton C. a- 1 for marshes. This production of CH4 is equivalent to a CO2 emission of about 221-840 ton C. a -1. This means that the overall effect would be clearly negative as far as the control of greenhouse-enhancing gases is concerned. Another way of influencing the atmospheric CO2 concentration is to diminish the output of CO2. Although very modest in its effect, one could decide to stop the lowering of the water tables in peatlands in order not to expose them to the air. This would certainly stop the large-scale oxidation of peatlands and would diminish the production of CO: from this source. Others [50] found a linear correlation between CO2 emission and the extent of lowering of the groundwater table, which deafly supports this hypothesis. A little more promising seems the situation as related to the emission of CH4 and N20. Both greenhouse gases are produced by aquatic and semiaquatic ecosystems in the Netherlands in considerable quantities. Their production equals, respectively, 18% and 19% of the total annual emissions. It has to be stated that since there are no major industrial activities associated with freshwater ecosystems and wetlands, the fluxes and balances are largely the result of purely biological processes. Of course they are influenced by man, but unlike on land, the influence on the ecosystems is indirect (by means of management-directed nutrient flows). First of all it would be feasible to try to reduce the CH 4 emission. It is strongly correlated with the composition (C/N ratio) of the biomass from which the detritus originates. It is estimated [7] that 2-7% of the biomass in an ecosystem will finally be converted into CH4. When eutrophication diminishes, more macrophytes may be expected to occur at the cost of phytoplankton. Macrophytes have a relatively high C/N ratio compared to phytoplankton and as a result, mineralization will be slower and less CH4 will be produced [51]. Suppose that as a result of a lower trophic level less biomass (more macrophytes but considerably less phytoplankton) would be bound to the system. It is very likely that this negative effect of lower CO2 absorption would be overcompensated by the lower CH 4 emission. Further, the reduction of nitrogen in surface waters would be very effective in lowering the emission of N20. The Dutch water quality policies for the future as related to nitrogen loadings of surface waters aim at a reduction of 50% in 1995 with the year 1985 as a reference. The most important nitrogen sources are the wash-out and wash-off from agricultural land. It is estimated that they can be reduced with 30% if the application of N-based fertilizers is reduced with 50%. The removal capacity of nitrogen from sewage water has to be expanded
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according to the Dutch policy makers from 60% in 1985 to at least 70% in 1995. This will be done by expanding the denitrification capacity. Although the exact nature of the relationship between denitrification and N20 emission volume is still unknown, this will certainly lead to an increase of the N20 emission from sewage treatment plants. The net effect of more nitrogen removal from sewage water and less nitrogen in surface waters is not necessarily positive as far as the production of greenhouse gases and their effects are concerned. It is very difficult to make reliable predictions about the effects of certain measures. However, a number of recommendations may be made. The reduction of fertilizers (either N or P) should have priority. This will lower the primary productivity and detritus formation but will definitely diminish CH4 and N20 emissions. Since aquatic and semi-aquatic ecosystems quite considerably contribute to the production of greenhouse gases in the Netherlands, it is worthwhile to pay more attention to understanding the production processes which are in fact still poorly understood. The operation of sewage treatment facilities should get more attention in this respect. However, when the relative contribution in the Netherlands o f the different greenhouse gases to the greenhouse effect in the future would be similar to that predicted by the IPCC ([2]; 61% contribution by CO2, 15% by CH4 and 4% by N20) we have to ask ourselves whether the priorities to take action in order to lower the emissions of greenhouse gases should lay in the domain of water quality management. First of all, the CO2 absorption capacity is negligible. Further, suppose that the average contribution of C H 4 and N20 from aquatic and semiaquatic habitats to the total emission of these gases in the Netherlands would be 25% (a sensible percentage considering our results). This would still mean that, supposing that the IPCC [2] figures given above are also valid for the Netherlands (they are probably not), only about a maximum of 5% (19% from 25%) of the Dutch contribution to the greenhouse effect would be theoretically manageable via our aquatic and semi-aquatic ecosystems. This seems to be not very much but the possibility deserves closer thought and attention. ACKNOWLEDGEMENTS This study was sponsored by the Ministry of Housing, Physical Planning and the Environment. We would like to thank Mrs. Ir. M.J.M. Hootsmans and Dr. L.H.J.M. Janssen of this Ministry for their support. REFERENCES 1 Anonymus, Water voor nu en later. Derde nota waterhuishouding. Ministerie van Verkeer en Waterstaat, SDU uitgeverij, Den Haag, 1989.
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