15 |
Methods to Evaluate Whole Aquatic and Terrestrial Systems |
| H. E. Evans | |
| RODA Environmental Research Limited, Canada | |
| P. J. Dillon | |
| Ontario Ministry of the Environment, Canada |
The traditional approach to assess the effects of chemicals on ecosystems involves the examination of either one or a few components of the system (such as an individual species), or it involves the study of only a physical portion (i.e., a micro- or mesocosm) of the system either in situ (e.g., enclosures) or in the laboratory. Unfortunately, the data derived from laboratory experiments apply only to the experimental conditions used; thus, they cannot be extrapolated necessarily to the natural environment. Similarly micro- and mesocosm experiments are inappropriate in situations where the effects of the chemical on organisms that are large relative to the size of the physical system are under study.
Micro- and mesocosms are unsuitable also in long-term experiments, because the natural floral and faunal assemblages may become unbalanced as a result of the experimental conditions. For example, Schindler (1987) reported that for experiments lasting more than a few weeks, even the very large aquatic enclosures (limnocorrals) used in the Experimental Lakes Area (ELA), Canada, developed abnormally high populations of periphyton.
Thus, the experimental manipulation of whole aquatic and terrestrial ecosystems is the most effective mechanism for assessing the fate of chemicals. Provided the time, the money, the manpower, and, most importantly, the appropriate "system" is available, whole system experiments can provide the opportunity to test the biological effects and responses of chemicals and to apply the scientific method in situ.
A long history exists for the experimental manipulation of whole ecosystems. Among limnologists, Juday et al. (1938) were among the earliest to manipulate a whole lake experimentally. Over a period of five years, they enriched Weber Lake, Wisconsin, with an assortment of six organic and inorganic fertilizers including phosphate, lime, potash and soybean meal. Suggesting that soybean meal was the most effective fertilizer in terms of increasing phytoplankton standing crop and stimulating fish growth, their results led them to conclude that "organic content is the limiting factor in the plankton production."
Subsequent to the experiments of Juday et al. (1938), Hasler et al. (1951) added hydrated lime to two lakes in Wisconsin, in order to test whether the transparency of the water would be improved. Based on pretreatment data and also on data from relatively similar reference lakes nearby, Hasler and his co-workers found that light penetration and alkalinity were greatly increased in Cather Lake; however, in Turk Lake, which was limed in a slightly different manner, only a small effect of the treatment was observed. These results clearly demonstrated the need for improved controls during whole-lake manipulations.
A new classic experiment involving a whole-lake manipulation was conducted in the early 1950s. Peter-Paul Lake on the Michigan-Wisconsin border was a small, brown-water lake with two basins connected by a narrow, shallow channel. In 1951, Johnson and Hasler constructed an earthen dam across the narrow part of the lake and separated it into two lakes, Peter Lake and Paul Lake. Peter Lake was treated with hydrated lime (Ca(OH)2) beginning in 1951 and continuing until 1954, while Paul Lake remained as a control or reference lake. The significant feature of this experiment is that it was perhaps the first instance in which a whole ecosystem "control" was part of the experimental design.
As a result of these early experiments employing whole-lake manipulations, many other scientists have been inspired by the "ecosystem" approach for the study of the fate and behaviour of chemicals in the natural environment. The treatment of double-basin lakes as two lakes has been attempted in several other situations, e.g., in Lakes 226 and 302 of the Experimental Lakes Area of Canada (Schindler, 1974, 1975; Schindler and Fee, 1974; Schindler et al., 1980a) and in Little Rock Lake, Wisconsin (Brezonik et al., 1986). In addition, the experimental manipulation of systems other than lakes has become prevalent in the literature. One example is the Reducing Acidification In Norway (RAIN) project in which two pristine catchments were acidified by the addition of H2SO4 and H2SO4 plus HNO3, respectively; while at an acidified catchment, ambient acidic precipitation was excluded by means of a "roof" and clean precipitation was added beneath.
In this chapter, some of the techniques currently in use for manipulating whole terrestrial and aquatic systems for the purposes of assessing the fate of chemicals are reviewed. These techniques will be categorized in terms of the system being manipulated; i.e., lakes, streams, and forest catchments. Since the method for manipulating a system is in many ways dependent on the chemical being tested, the discussion for each system will focus on certain categories of chemicals that have been tested historically on whole ecosystems. These include nutrients, acidifying chemicals, metals and organic contaminants. Neutralizing agents will be discussed only briefly, because generally these are added to lakes, streams, and catchments to mitigate the effects of other chemicals (i.e., strong mineral acids), and not to test the effects of the chemical itself.
Since the first experiments of Juday et al. (1938) on Weber Lake in Wisconsin, dozens of experiments have been conducted using whole-lake manipulations. These experiments can be divided into two categories: (1) those involving only a "single" lake to which the test chemical is added and (2) those involving a "double-basin" lake which has been mechanically separated into two lakes that are then manipulated independently.
The most common approach to manipulate lakes is to add the test chemical to a single lake. The major problem with this type of lake manipulation (as indeed is the case with all whole system manipulations) is in establishing a suitable "control" or "reference." As Likens (1985) has pointed out, in whole system experiments, "reference" is perhaps the more appropriate word to use because the complexity of natural ecosystems precludes the use of an absolute experimental "control."
For single-lake manipulations, two ways exist to establish a control. The first is to use the lake itself as a reference. This approach requires that the lake be studied for one to several years prior to the experiment. Establishing the length of time needed before the treatment begins can be a problem, because determining a priori the natural variation in the system is impossible. A long-term (i.e., ongoing) monitoring programme is ideal, provided it is inexpensive and simple. Also, it should include measurements for abiotic and biotic parameters that are sensitive not only to changes in the chemical currently under study, but also to changes in chemicals that might be tested in the future.
Alternately, or in addition to using the lake itself as a reference, other relatively similar lakes can be used as "controls," while the study lake is being manipulated. While this approach has the advantage that potentially fewer years of background data may be required prior to the commencement of the experiment, finding two or more lakes having identical morphometries, water renewal rates, food web structures, etc., is virtually impossible. Ideally, both long-term data should be collected for the study lake prior to its manipulation, and other lakes should be monitored as references during the course of the experiment. Finally, data should continue to be collected even after the experiment has ceased.
An alternate approach to the manipulation of a single lake is to mechanically divide a lake into two or more basins, and then to manipulate each basin individually. Peter-Paul Lake was separated into two lakes by an earthen dam, while at ELA and in Little Rock Lake, Wisconsin, a vinyl curtain reinforced with nylon (ELA) or Dacron (Little Rock Lake) was installed to separate the basins. In ELA Lake 226, the north basin received annual additions of C, N, and P, while the south basin received only C and N (Schindler, 1974, 1975, 1991; Schindler and Fee, 1974; Findlay and Kasian, 1987); and in Lake 302, P, N, and C were added to the hypolimnion of the north basin of the lake, and the south basin was left as a reference (Schindler et al., 1980a). In Little Rock Lake, the north basin was acidified with H2SO4 while the south basin remained as a reference.
The separation of a single lake into two provides a natural reference or "control." At ELA Lake 226, a problem developed with leakage of water between the two basins and movement of water over the surface collar (Findlay and Kasian, 1987). Alternately, multibasin lakes that behave as individual lakes could be used. An example is Kennedy Lake, British Columbia, Canada, which is composed of two discrete arms, separated by a very narrow, shallow sill with a depth of <10 m. Stockner and Shortreed (1985) fertilized Clayoquot Arm with P from 1978 to 1984 inclusive, while the Main Arm was fertilized only in 1979 and 1980. Similarly, at ELA, the natural sill between the two basins of Lake 302 was so effective at separating the basins, that installation of the curtain in 1974 had little effect on the chemistry or the phytoplankton of the two basins (Schindler et al., 1980a).
Whatever the approach selected to manipulate the whole-lake system, the method to add the chemical to the lake is dependent on the chemical being tested. A great deal of information is available on the addition of nutrients to lakes, both in North America and Europe. For example, at ELA in Canada, Lake 227 has been fertilized with N and P since 1969; Lake 304 was treated with N, P, and C in 1971 and 1972, with N and C in 1973 and 1974, and with p and N in 1975 and 1976; and Lake 303 was treated during the summer of 1975 and 1976 with N and P, while Lake 230 was treated during the winter of 1974 and 1975 with N and P (Schindler, 1991). In other parts of Canada, P has been added to three subarctic lakes in Schefferville, Quebec (Smith et al., 1984), while in British Columbia, many lakes have been fertilized with both N and P (e.g., Stockner, 1981; Stockner and Shortreed, 1985; Shortreed and Stockner, 1990). In Scandinavia, six lakes in the Telemark region of Norway were treated with P and/or N (Johannessen et al., 1984) to improve fish stocks, while in Sweden N and/or P were added to Lakes Magnusjaure (1974-75), and Gunillajaure (1978-1979) (Jansson, 1978; Holmgren, 1983 as cited by Findlay and Kasian, 1987).
Phosphorus can be added to the lake in several forms. Hakanson et al. (1990) used the emissions from fish-cage farms to add P (and other nutrients) to two lakes in Sweden. Sodium phosphate (Na2HPO4) was added to Lake 227 in Canada (Schindler et al., 1987) and to Lake Hymenjaure in Sweden (Jansson, 1978) in the first year of study (1969 and 1972, respectively); but in both studies, phosphoric acid (H3PO4) was substituted in subsequent years. Phosphoric acid would appear to be the preferred source of P for lake water additions because of its greater solubility (Schindler et al., 1973). If nitrogen is being added to the lake in conjunction with the P, the two nutrients can be added as ammonium phosphate ((NH4)3PO4) or as a commercial fertilizer (e.g., Häkanson and Andersson, 1992). Nitrogen alone is added as either ammonium chloride (NH4Cl) (Schindler, 1975, Schindler et al., 1980a) or, more commonly, as ammonium nitrate (NH4NO3) (Schindler, 1975; Jansson, 1978; Stockner, 1981; Johannessen et al., 1984; Stockner and Shortreed, 1985; Shortreed and Stockner, 1990). Carbon was added to ELA Lakes 226,227, 302N and 304 as sucrose (Schindler, 1975) or to ELA Lake 224 as 14C-labelled NaHCO3 (Hesslein et al., 1980a).
During the ice-free season, the cheapest and simplest method for adding these nutrients (with the exception of the radioisotope) is by pouring the liquid or predissolved fertilizer (in the case of dry chemicals) onto the surface of the lake through the prop-wash of the boat while cris-crossing the lake. To simulate point- source additions, the chemicals can be added using a semi-continuous trickle feed from a large (e.g., 200 L) barrel or drum (Findlay and Kasian, 1987; Levine and Schindler, 1989) situated on a raft in the middle of the lake or on the shore. If the funds are available, the fertilizer can be spread over the lake from an airplane (Stockner and Shortreed, 1985). During the winter, nutrients can be dispensed through a hole in the ice or through the lake inflows (Smith et al. 1984). However, often nutrients are not added during the winter months because productivity is low at that time of year.
While nutrients are loaded generally onto the surface or epilimnetic waters of the lake, Schindler et al. ( 1980a) injected P, N, and C into the hypolimnion of the north basin of Lake 302 to test the hypothesis that the phosphorus would be permanently transferred to the sediments before it could reach the surface waters and cause algal blooms.
In the hypolimnetic experiment of Schindler et al. (1980a), as in many other whole-lake fertilization experiments, the nutrient additions commonly are made weekly during the ice-free season. Loadings can be uniform throughout the season (Schindler et al. 1973), or they can be adjusted according to the P, N, or C concentration in the lake or the outflow discharge. If an immediate or dramatic increase in concentration is required, a large pulse of fertilizer is added at the beginning of the ice-free season. For example, after ice-out in June 1979, Smith et al. (1984) added 8 kg of P as H3PO4 to the surface of Lejeune Lake in order to double the pre-treatment concentration of total P in the lake water. Thereafter, the lake was loaded at a rate of 17.6 kg P/yr at weekly intervals.
A final point concerns the importance of pre-treatment, post-treatment, and "during" treatment sampling. The sampling regime, including the frequency of collection and the location of the sampling station(s), and the methodologies involved for both the chemical and the biological analyses must be well established prior to the commencement of the experiment, otherwise many years and much money will be wasted.
Good sampling techniques are important, not only for whole-lake manipulations involving nutrients, but also for those experiments in which other chemicals are added to the lake system. For chemicals such as sulphuric and nitric acid, some information is available on whole-lake manipulations as a result of the heightened concern over the effects of acidic precipitation. Pioneering work in this area was conducted at the ELA in Canada, where experimental acidification of Lake 223 (in the form of H2SO4) was conducted beginning in 1976, acidification of Lake 114 (in the form of H2SO4) was conducted from 1979 to 1986 (Al2SO4 was added in 1984), and acidification of Lake 302 (in the form of H2SO4 in one basin, HNO3 in the other) had been carried out from 1982 (Schindler 1991). At Little Rock Lake, Wisconsin, the north basin was acidified with H2SO4 from 1985 to 1990, while the south basin remained as a reference (Sampson et al. 1993).
Similar to the methods employed for nutrient additions, in each lake the acid was added by slowly pouring the concentrate from a moving boat, into the prop-wash of an outboard motor. Physical mixing studies suggest that this method is sufficient to mix the acid into the epilimnion within a few hours after addition. Furthermore, despite the high density of the acid, no evidence of it sinking through the thermocline to the bottom of the lake was found (Schindler et al., 1980b).
As with nutrient additions, the acidification regime for the ELA lakes was designed to reduce the pH to a predetermined value early in the ice-free season, and then to hold it at that value until the following spring. Thus, large quantities of acid were added to the lakes early in the season, followed by weekly loadings. This regime is analogous to a large pulse of acid entering the lake during spring snowmelt, followed by episodic additions from summer rains.
When acid is added to lakes, it must be relatively free of impurities such as heavy metals, otherwise, the effects of the metal contaminants might be confused with the effects of the acid. When metals have been tested on whole-lake systems, radioisotopes have been used with some success. For example, the gamma-emitting isotopes, 75Se, 203Hg, 134Cs, 59Fe, 65Zn, and 60Co were added to ELA Lakes 224 and 226 (northeast and southwest basin) (Hesslein et al., 1980b; Hesslein, 1987) and the alpha-emitting isotope 226Ra was added to Lakes 224,226,227, and 261 (Emerson and Hesslein, 1973; Emerson et al., 1973; Hesslein and Slavicek, 1984). The isotopes were dispensed as either chloride salts or nitrate salts 203Hg only) with the exception of 75Se which was supplied as Na2SeO3. In all studies, the radionuclides were combined in a metal drum containing 10 L of water (per lake or lake basin). The drum was then mounted on a raft and towed around the lake for a period of about one hour while the contents drained out. The barrel was then refilled with water, and the operation was repeated. This procedure is the same used to dispense 14°C in the nutrient addition experiment conducted on Lake 224 (Hesslein et al., 1980a).
Unlike the whole-lake manipulations involving nutrients and acids, no weekly additions were made of the metal radiotracers in the experiments discussed above. Malley et al. (1989) conducted an experiment in which both stable Cd (as CdCl2) and the radiotracer 109Cd were added to the epilimnion of Lake 382 in 33 weekly additions during the period 23 June to 29 October 1987. The large cost of radioisotopes and the hazard involved in using both radioisotopes and metals precludes their widespread use in many other lakes. Nonetheless these types of whole-lake experiments provide the opportunity to monitor the pathway of metals from the water to the biotic and the abiotic compartments.
Häkanson et al. (1990) and Häkanson (1991) added Se in conjunction with lime to five lakes in Sweden. The Se was added either using mixed lime, or it was encapsulated in rubber tubes placed into nets from which the selenium was successively released. However, the purpose of their work was to reduce Hg concentrations in fish by means of precipitating the Hg from the lake water as HgSe, and not to examine the behaviour of the Se. Similarly, radionuclides other than metals have been added to some ELA lakes (e.g., 3H was added to Lakes 227 and 224; Hesslein, 1980; Quay et al., 1980, respectively) to determine diffusion coefficients and not to evaluate the effect of the chemical itself.
Experiments conducted on streams are somewhat different from those conducted on lakes, because manipulation of an entire stream is virtually impossible unless the chemical is added at the source of each tributary. Consequently, the chemical being tested is dispensed usually at a certain point in the stream, and its effects are noted downstream. Thus, the control or reference site for the experiment must be upstream of the addition. Furthermore, minimal variability (in terms of mixing characteristics, substrate type, riffle-pool sequences, etc.) must exist between the reference site and the experimental section of the study site. As with lakes, the additions of the chemical can be made over a short (i.e., less than 24 hours) or over a long period of time.
Probably the greatest number of studies on stream manipulations have involved the experimental acidification of streams. Study sites have been located in New Hampshire (Hall et al., 1980, 1987; Hall and Likens, 1981, 1984), Maine (Norton et al., 1992) and Colorado (McKnight and Bencala, 1989) in the United States, in Wales (Ormerod et al., 1987), and in Norway (Henriksen et al., 1984, 1988; Norton et al., 1987; Wright et al., 1988b). In many cases, the acid is added as H2SO4 (Hall and Likens, 1981, 1984; Norton et al., 1987; McKnight and Bencala, 1989), although Hall et al. (1980, 1987) used HCl because the concentration of Cl+ was low in both the biologic and geologic material at their study site and because Cl+ is a good chemical tracer of groundwater movement (Hall et al. , 1987). For similar reasons, McKnight and Bencala (1989) injected LiCl (in conjunction with H2SO4) as a conservative tracer in their experimental stream.
Hall et al. (1987) added AlCl3 instead of HCl in some of their experiments to compare neutralization mechanisms by the stream during acidification by a weak (AlCl3) and a strong (HCl) acid, and also to produce Al levels representative of those that occur after experimental deforestation of catchments in the Hubbard Brook Watershed.
Most of the stream acidification experiments mentioned above were relatively short term (i.e., the addition of the acid lasted less than 24 hours). First-, second-, and third-order streams were manipulated, with the size of the experimental sections (i.e., the distance to the downstream sites) ranging between about 50 and 200 m (McKnight and Bencala, 1989; Norton et al., 1992; Hall et al., 1980, 1987; Hall and Likens, 1981, 1984). The injection rate, the concentration of acid used and the duration of the input of acid to the streams were determined for each experiment according to the flow rate of the stream and the decrease in pH required. For example, McKnight and Bencala (1989) injected a 7.25 mol/L H2SO4 solution into a headwater stream in Colorado at a rate of 950 mL/min for a period of 3 hours to achieve a decrease in pH from about pH 4 to about pH 3.
Samples must be collected as soon as possible and as frequently as possible at both the upstream (i.e., reference) site and at the downstream sites. To know a priori when steady state might be expected to occur and also to avoid unnecessary
sampling, travel or mixing times and distances in these short-term experiments .
should be determined using a dye such as rhodamine WT, rhodamine B, or fluorescein, despite the possibility that the dyes might cause an effect independently.
For acidification experiments requiring that the pH be depressed for long period of time, frequent measurements of pH downstream of the injection site must be made so that the flow rate of the acid into the stream can be modified to account for changes in stream discharge. Hall et al. (1980) found that when the discharge was variable, monitoring of the pH was needed at short time intervals (i.e., every five min) to maintain a constant pH in the experimental section of Norris Brook (in the Hubbard Brook Watershed, New Hampshire, US). However, when the discharge was relatively constant, monitor pH was needed only at 6- to 8-hour intervals. They maintained approximately pH 4 in Norris Brook for a period of five months in 1977 by manually adding dilute (0.05-1 N) H2SO4 from a carboy, and modifying the drip rate of the acid into the stream with a Teflon stem needle valve in borosilicate.
The effects of pesticides in streams also have been studied by direct additions. For example, the effects of methoxychlor (1,1,1-trichloro-2,2-bis-(p-methoxyphenyl) ethane), a replacement for DDT, were investigated in the Coweeta Basin, North Carolina (Wallace et al., 1987, 1989), where two first-order streams were treated with methoxychlor on nine (Wallace et al., 1989) and four (Wallace et al., 1987) separate occasions, and also in the province of Quebec, Canada (Wallace and Hynes, 1975). Wallace et al. (1989) used two hand sprayers to apply a 25 percent emulsifiable concentrate of methoxychlor for a 2- to 4-hour period at a rate of 10 mg/L (based on discharge). Wallace and Hynes (1975) compared two methods for stream manipulations. They used a Piper Pawnee 235 fixed-wing aircraft equipped with a standard boom and nozzle spray rig to spread a 15 percent methoxychlor solution from an altitude of 45 m to stream M-26, whereas stream M-11 was treated from the ground at a calculated rate of 0.075 mg/L for 15 min.
Regarding nutrient manipulations, some work has been conducted on streams located in the Walker Branch Watershed, Tennessee (Elwood et al., 1981), and also in the Hubbard Brook Experimental Forest, New Hampshire (Meyer, 1979). Elwood et al. (1981) conducted a long-term (95 day) P-addition experiment in which they divided a 340 m reach of Walker Branch (a second-order woodland stream) into three sections: a 70 m upstream section was kept as a control (<10mg/L PO4-P), a 150 m section was continuously enriched to 100 mg/L PO4-P, and a 200 m section downstream was enriched to 1000mg/L PO4-P. Phosphorous solutions were prepared weekly in 2 x 200 L drums containing H3PO4 mixed with stream water that was siphoned through surgical tubing at a rate of ~20 mL/min to the head of each stream section and introduced at points where rapid mixing occurred. As with the acidification experiments discussed above, the phosphorus concentration in the drums (~60 and 450 mg PO4-P/L) was based on the discharge of the stream. Another similarity with the acidification experiments was the addition of a conservative tracer by Meyer (1979) who added NaCl in conjunction with the P (as KH2PO4) to Bear Brook, New Hampshire, over a period of 27 hours. NaCl was added as an inert tracer to monitor changes in phosphorus concentration due to dilution over the reach.
As in lake manipulations, the importance of a rigorous sampling regime cannot be overemphasized. Adequate abiotic and biotic samples must be collected before, during, and after the treatment at both the reference and the downstream sites so that the effect of the chemical can be properly evaluated.
Two common approaches exist to manipulate whole catchments and forests. In "addition" experiments, the test chemical is added to the system (similar to whole-lake and stream manipulations), whereas in "exclusion" experiments the chemical, in conjunction with other nutrients and contaminants, is excluded from entering the system.
Many whole-catchment and forest manipulation studies have dealt with the addition and/or exclusion of acids. Most notable among these is the RAIN project conducted in Norway (Wright, 1985; Wright and Gjessing, 1986; Wright et al., 1986, 1988a). The project comprised two parallel large-scale experimental manipulations, representing both addition experiments (Sogndal) and exclusion (Risdalsheia) experiments. At Sogndal, four pristine headwater catchments were selected; two of the sites acted as controls (SOG1 and SOG3), a third site was acidified with H2SO4 (SOG2), and the fourth site was acidified with a 1:1 mixture of H2SO4 + HNO3 (SOG4). Acid addition, which began in April 1984, consisted of application to the snowpack of 0.02 mm of water at pH 1.9 and four or five events of 11 mm at pH 3.2 during the snow-free months. Acid was mixed with lakewater from SOG1 and applied at a rate of 2 mm/hour using commercial irrigation equipment. Before and after each acid addition, 2 mm of unacidified lake water were added to "wet" and "wash" down the vegetation, respectively (Wright et al., 1988a). In conjunction with this RAIN project experiment, a similar methodology was employed by Wright et al. (1988b) to acidify a Sogndal catchment with sea salt, except that seawater instead of H2SO4 was mixed with the lakewater from SOG1.
At Risdalsheia, an acidified area in southern Norway, three natural headwater catchments were selected; a site (ROLF) acted as a control and two sites (KIM and EGIL) were covered with transparent roofs. At KIM and EGIL, precipitation was collected by means of a gutter and cistern system. At KIM, the water was pumped through a filter and ion exchange system, seawater was added to increase the concentration of salts to ambient levels, and then the water was pumped back out to a sprinkler system mounted beneath the roof. The system at the EGIL catchment was similar to that at KIM except that the water was not treated; rather, it was recycled back beneath the roof. Both catchments were watered at a rate of 2 mm/hour. During winter the systems were shut down. In 1985, artificial "acid" snow was made using commercial snow-making equipment; from 1986 onwards, ambient snow was used and added beneath the roofs with a snowblower .
Other whole-catchment experiments involving acid additions have been conducted in two subcatchments of Gärdsjön Lake, as part of the Gärdsjön Project in Sweden (Hultberg and Grennfelt, 1986), in Bear Brook Watershed, Maine (Norton et al., in press) as part of the Watershed Manipulation Project (WMP) in the United States, and also in Hoglwald, Germany (Rasmussen et al. 1992), as part of the Experimental Manipulation of Forest Ecosystems Project (EXMAN) (Rasmussen et al., 1992) in Europe. At Gärdsjön, catchment Ll received 90 kg S/ha in October 1985 and 108 kg S/ha in October 1986 (as Na2SO4) while catchment F5 received 112 kg S/ha (as elemental S) each year. At Bear Brook, dry (NH4)2SO4 (1880 eq/ha yr) was applied by helicopter to the West Bear catchment while the East Bear catchment remained as a reference. At Hoglwald, the catchment is being irrigated with acid in the form of H2SO4.
Irrigation was also the method used by Bayley et al. (1987) to experimentally acidify a fen located in ELA. The fen (MIRE 239) was irrigated with a 1:1 mixture of H2SO4 and HNO3 using a pipe distribution network to deliver water to 160 sprinklers, an irrigation pump to supply the water, a hydrant to regulate delivery of the water, an in-line meter to monitor pumping rates, and pressure gauges on the pump and on the distribution lines. Acid was added to the water in the experimental part of the bog, resulting in a spray with a pH of about 3.0. Irrigation lasted four to five hours, followed by 20-30 min of rinse at higher pH (5-7).
Evidently extensive, intensive, and expensive sampling networks are necessary to implement and maintain a whole-catchment manipulation study. At least one year (e.g., the RAIN project) and preferably several years (Norton et al., in press) of baseline data are required before the manipulation can begin. This situation has resulted in the development of several co-operative programmes in Europe, directed towards the assessment and prediction of the impact of environmental change on whole-catchment systems. ENCORE (European Network of Catchments Organised for Research on Ecosystems) (Hornung, 1992) comprises a network of 18 sites and ~40 catchments in seven countries. As part of the baseline programme, background environmental data (e.g., vegetation, soil type) and input and output flux information are being collected at each site using standard protocols. Furthermore, a more intensive process or mechanistic study or a whole catchment manipulation experiment must be performed at each site. Past and future ENCORE whole catchment manipulations include acidification and de-acidification experiments (i.e., the RAIN project), the addition of neutral salts and fertilizers, and also liming experiments.
In addition to ENCORE, other co-operative European programmes using whole catchment manipulations include:
EXMAN in which two manipulations (i.e., fertilization, liming, irrigation, acidification or roof construction) must be performed at each site;
NITREX (NITRogen saturation EXperiment) which involves nine separate large-scale nitrogen addition or exclusion experiments (Wright et al., 1992); and
CLIMEX (CLIMatic change EXperiment) which is a proposed project to experimentally enrich with CO2 and raise the temperature at two entire forested headwater ecosystems (Jenkins et al., 1992).
These projects are beyond the scope of this chapter; therefore, the reader is referred to Teller et al. (1992) and the references therein for further information.
While the addition of lime or limestone (Ca(OH)2 or CaCO3) has long been practised as a means to increase lake productivity (Hasler et al. , 1951; Stross and Hassler, 1960; Stross et al., 1961), the addition of lime and other neutralizing agents as a mitigation technique for restoring or protecting biota in acidified waters has become widespread only in recent years. Lake neutralization has been practised on a large scale in Sweden (Wilander and Ahl, 1972; Svendrup and Bjerle, 1983; Lindmark, 1982, 1984; Hultberg and Andersson, 1982; Hultberg and Grennfelt, 1986; Alenås et al., 1991) and Norway (Wright, 1985; Hindar and Rosseland, 1988), and to a lesser extent in other European countries such as Italy (Calderoni et al., 1991) and the United States (Young et al., 1989; Porcella, 1989, 1991; Bukaveckas and Driscoll, 1991; McAvoy and Driscoll, 1989), and Canada (Yan and Dillon, 1984; Molot et al., 1986).
While several agents have been used to neutralize lakes (Grahn and Hultberg, 1975), lime has been widely adopted, because it is readily available in different size grades, is safe to handle, and is relatively inexpensive. Although pure calcite (CaCO3) may be preferred because it does not cause the pH in the lake to increase beyond the equilibrium value upon dissolution as does Ca(OH)2 and CaO (Molot, et al. , 1986), limestone, dolomite, lime slags, and olivine all have been used with reasonable success (Bengsston et al., 1980).
The disadvantage of using certain calcite-based materials is that they are not readily soluble. Some researchers (Svendrup and Bjerle, 1983) have suggested that if the CaCO3 reaches the sediments after application to the lake's surface, it may form metal and humic complexes (especially in humic lakes) that decrease the dissolution rate of the calcite and render it ineffective. Consequently, they suggest that, when applying the lime, measures should be taken to maximize the dissolution of the sinking calcite and thus minimize losses to the sediments. These include using a small particle size, dispersing the lime over as large an area as possible in proportion to lake volume, and applying the lime as a slurry to separate the particles (Svendrup and Bjerle, 1983; Molot et al., 1986). If the latter method is used, a small amount of surfactant (~0.15-0.20 percent by weight) such as sodium polyacrylate (Molot et al., 1986; Driscoll et al., 1989) is added to facilitate the suspension of the calcite and ultimately promote its wetting and dispersion in the water column.
An alternative approach is to apply a larger particle size of lime to the lake surface (McAvoy and Driscoll, 1989) in the hope that it will reach the sediments and release a slow diffusive Ca2+ flux across the sediment-water interface. Gubala and Driscoll (1991) compared the effectiveness of a water column application of CaCO3 (i.e., mean particle size = 2mm) to a water column-sediment application of CaCO3 (i.e., a 1-2 mixture of 6-44mm and 40-400 mm CaCO3) in Woods Lake, New York. They found that while the water column-sediment application involved a 50 percent greater dose of calcite than the water treatment alone, both treatments appeared to have a similar effect in terms of the net amount of runoff and acidic inputs that were neutralized.
The method to apply the lime directly onto the lake's surface can be either from a pontoon boat equipped with high-pressure tanks (Hasselrot and Hultberg, 1984; Calderoni et al., 1991) or manually from a small boat (Yan and Dillon, 1984; Alenås et al., 1991), although, recently, a boat has been developed in Sweden specifically for the purpose of spreading lime (Water/Engineering and Management, March 1992, p. 14). In remote and relatively inaccessible areas or if a large number of lakes are to be treated, helicopters (Porcella, 1989; Bukaveckas and Driscoll, 1991; Gubala and Driscoll, 1991; Häkanson and Andersson, 1992) or other aircraft (Molot et al., 1986) can be used. In addition, lime has been put onto the ice of lakes (Wilander and Ahl, 1972; Wright, 1985; Häkanson and Andersson, 1992) and onto the snow along the edge of the lake (Wright, 1985).
The successful neutralization of lake acidity by the addition of lime directly to the lake's surface requires a knowledge of the goals for the treatment (e.g., the final acid neutralizing capacity, ANC, of the water or the depth in the sediments to which neutralization should occur), the type of lime used, the method of application, the initial water column and sediment acidity, and the physical features of the lake including surface area, water depth, temperature regime, and hydrodynamics. While the amount of lime or the frequency of application required to achieve a desired ANC in Sweden cannot be stated with absolute precision, between 10 and 30 g of lime (as CaCO3)/m3 are needed to neutralize typical acid lake water (Bengsston et al., 1980; Hasselrot and Hultberg, 1984).
In addition to applying lime directly onto the lake's surface, lime has been added to streams, for example in Canada (Keller and Gunn, 1982), in Sweden (Hasselrot and Hultberg, 1984), and in Norway (Abrahamsen and Matzow, 1984). An alternative approach to liming the lake directly is to apply the lime to the entire catchment of the lake or to the wetland areas within the catchment. For example, in June 1984, 1500 kg of finely ground dolomite (0-0.2 mm) was spread onto catchment F2 as part of the Gärdsjön Project in Sweden (Hultberg and Grennfelt 1986). In the "liming-mercury-cesium" project conducted in Sweden between 1986 and 1989, several types of lime or potash or Se or fertilizers were added to many lakes, wetlands, and catchments in Sweden in an attempt to reduce the Hg and 137Cs concentrations in the fish (Häkanson et al., 1990; Häkanson, 1991; Häkanson and Andersson, 1992). Fertilizers were added in conjunction with the lime to help mitigate the effects of the lake acidity, a procedure that has been employed by others (Yan and Lafrance, 1984).
While the magnitude of this project precludes a complete discussion here, Häkanson and his co-workers have provided a comprehensive cost-benefit analysis of these remedial measures, and they have compared the advantages and disadvantages of whole-lake versus wetland versus catchment lime additions. Häkanson and Andersson (1992) point out that wetland and catchment liming is superior to lake liming for several reasons: (1) the effect is prolonged, (2) the potential "liming shock" in the lake is avoided, (3) the biota in the streams and rivers in the catchment also benefit, and (4) the transport of metals such as Fe and Al is reduced into the lake from the catchment.
Yet, in certain types of lakes, such as humic lakes and those have a short retention time, liming has a limited effect. Therefore, Lindmark (1982, 1984) proposed an alternate approach for neutralizing lakes. The CONTRACID method, which is based on the cation exchange properties of the lake sediment, involves injecting a sodium carbonate (soda ash) solution into the sediment (by a harrow) so that the sediment becomes sodium stocked. During acidification, a reverse ion- exchange process occurs (i.e., the Na in the sediments is replaced by H+). This process provides a long-lasting neutralizing capacity in addition to biological stimulation (from P release) that may be preferable to frequent liming. Unfortunately, no consensus is apparent in the literature as to the efficacy of this technique.
The methods to manipulate whole aquatic and terrestrial ecosystems are as diffuse and abundant as the number of studies involving them. Common among all whole- lake, stream, and catchment manipulations is the necessity for good quality data before treatment, during treatment, and also post-treatment so that the effects of the chemical being tested can be accurately assessed.
While whole system manipulations provide an excellent opportunity to apply the scientific method in situ, co-operative programmes both among different agencies within a country and also among different countries are essential if future whole system manipulations are to be carried out to any great extent.
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