Impact of forest drainage on the macroinvertebrates of a small boreal headwater stream: Do buffer zones protect lotic biodiversity?

Biological Conservation 77 (1996) 87-95 Copyright © 1996 Elsevier Science Limited ELSEVIER

0006-3207(95)00123-9

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IMPACT OF FOREST D R A I N A G E ON THE MACROINVERTEBRATES OF A SMALL BOREAL HEADWATER STREAM: DO BUFFER ZONES PROTECT LOTIC BIODIVERSITY? Kari-Matti Vuori University of Joensuu, Department of Biology, PO Box 111, FIN-80101, Finland

& Ilona Joensuu University of Jyvaskyl?i, Department of Biology and Environmental Sciences, Yliopistonkatu 9, FIN-40100 Jyvaskyla, Finland

(Received 31 May 1995; accepted 6 September 1995)

INTRODUCTION

Abstract Forest drainage, utilizing protective buffer zones, caused clear changes in the habitat structure as well as in the species richness and composition of moss-dwelling invertebrates in a small headwater stream. The aquatic moss Fontinalis dalecarlica was the dominant habitat in the control riffle areas, whereas sand dominated the riffles impacted by forest ditches. The significance of the forest ditches as a source of inorganic material was studied by comparing the quality of both indigenous and transplanted moss habitats. The Fontinalis tufts in the affected sites were silted up and contained a significantly higher average amount of inorganic matter than the mosses in the control site. The species richness of invertebrates within the silted mosses was significantly lower than at the control site. Shredder-feeding stoneflies dominated the moss communities of the control site, whereas Simuliidae dominated the impacted riffles. We suggest that the drainage-induced impoverishment of the benthic communities is due to two combined disturbance factors: (1) deposition of the particles on benthic habitats and (2) particle movement along the surfaces. Further, it is concluded that the present buffer zones and sedimentation ponds are insufficient to protect the biodiversity of streams draining easily eroding lands. In order to protect these vulnerable lotic ecosystems effectively, the most erodible parts of the catchment area shouM be undrained, and more retentive buffer mechanisms should be utilized in the drainage areas. Copyright © 1996 Elsevier Science Lid

Habitat fragmentation and deterioration of water quality constitute increasing threats to lotic biodiversity. Most estuaries and lower reaches of large rivers have a long history of human impact, whereas many headwater streams have only more recently experienced significant anthropogenic changes. Headwater streams are particularly sensitive to changes in the land use practices of the catchment area. These changes have been so rapid and intense that both in Europe and North America it is nowadays difficult to find a stream in its original natural state (Vannote et aL, 1980; Benke, 1990; Zwick, 1992). In Finland, efficient forestry practices initiated mainly in the 1950s have often included the draining and clearing of even the smallest streams concomitantly with the logging and draining of the surrounding catchment area. Extensive draining and logging may drastically alter the structure and function of streams draining the impacted areas even though the streams themselves are left untouched. In Scandinavia, forest draining has changed the state of small headwater streams more than any other single factor (Anon., 1987; H/im~il/finen, 1987). Forest draining may cause substantial changes in the hydrology and water quality of streams (Heikurainen et al., 1978; Seuna, 1982; Ahtiainen, 1988). However, while both hydrological, limnological and ecological impacts of forest clear-cutting have been described in numerous studies (e.g. Gurtz & Wallace, 1984; Wallace & Gurtz, 1986; Golladay et al., 1989; Bilby & Bisson, 1992; Garman & Moring, 1993; Ormerod et al., 1993), only a few studies have dealt with the ecological impacts of forest draining on lotic ecosystems (Bergquist et al., 1984; Holopainen & Huttunen, 1992).

Keywords." forest drainage, erosion, disturbance, benthic invertebrates, Fontinalis, stream biodiversity. Correspondence to: Kari-Matti Vuori. Tel: 358-73-151 3683; Fax: 358-73-151 35 90; e-mail (Internet): kvuori@joyl. joensuu.fi

87

88

K . - M . Vuori, I. Joensuu

During and after the digging of forest ditches, large amounts of suspended solids may enter the streams (e.g. Heikurainen et al., 1978; Seuna, 1982). Therefore, a number of protective schemes have recently been applied and recommended in Scandinavian forest drainage programmes. These schemes include the digging of sedimentation ponds and leaving untouched buffer zones between the ditches and the stream draining erodible lands (Joensuu,. 1990). No comprehensive studies on the ecological effects of these protective schemes have been conducted. Hence, it was our aim to study the capability of sedimentation ponds and buffer zones to protect the lotic communities in a small boreal headwater stream. As benthic invertebrates have a central role in the structure and function of stream ecosystems (Vannote et al., 1980), we focused our study on the potential effects of draining on macroinvertebrate communities and their habitats. STUDY AREA Our study area, the Pajuluoma stream, is one of the main tributaries of the River Isojoki in western Finland (Fig. 1). The drainage basin (1112 km 2) of the River Isojoki is part of the Project Aqua-basin of UNESCO. The river, which flows into the Gulf of Bothnia 10 km south of the town of Kristiinankaupunki, is one of only five in Finland that still has a naturally breeding Baltic trout S a l m o trutta m. trutta population. The mean discharge of the Pajuluoma stream is 499

SWEDEN~

litre s-' and the size of the drainage basin is 52.5 km 2. Fifty-six per cent of the basin is covered by peatlands, 41% by forest, 2% by arable land and 1% by water. The total length of forest ditches draining into the stream is 711 km, mainly dug in the 1960s. The pre-Quaternary rocks along the stream channel consist of granodiorite and quartz diorite. The stream flows mainly through even ground moraine with peat deposits. The area is easily eroded and the channel has changed its course at several sites due both to the natural and drainage-induced erosion of fine sand and silt material (Lipkin & Set~tl~i, 1989). The water of the stream is oligotrophic and characterized by acid surges and low alkalinity during flood periods (Table 1). Here, we studied the impact of two forest ditches on two riffle areas. Digging took place between 28 August and 12 September 1991. We considered the riffle areas situated above the ditches as controls (sites 1 and 2a in Fig. 1). The first upstream riffle (1) served as the source of experimental material, whereas the second riffle (2) was divided into control (2a) and impacted (2b) parts. This was done because the drainage waters from the ditch at this site enter the stream in the middle of the riffle as a surface flow through a 30 m wide buffer zone. The ditch drains easily-eroded soils and the buffer zone consists of a sparse vegetation of trees and bushes. The lower ditch, some 600 m downstream from the upper one, enters the stream through a sedimentation pond in a rather slowly flowing bend. There is also a 10 m wide bushy buffer zone between the pond and the

ROSSIA

Marjakeidas marshland

A uluoma stream

0 I

I I

2 I

3 km I

Fig. 1. Location of the study area and experimental riffles. Dense lines represent the old ditches. 1 and 2a, control riffles; 2b and 3, rifflesimpacted by the new ditches.

Forest drainage and lotic biodiversity Table 1. Mean, minimum and maximum values of the water quality parameters of the Pajuluoma stream during the period 1 April 1992 -31 December 1993

Variable

Mean

Min.

Max.

Temperature °C Suspended solids mg litre 1 Gran-alkalinity mmol litre I pH Turbidity FTU Colour mg Pt litre J COD Mn 02 mg litre ~ TOT P t~g litre TOT N/zg litre ~ TOT Fe/zg litre ~ TOT AI/zg litre t

7.4 4.2 0.039 5.3 3.4 159 28.9 26.9 604.3 898 494

0.1 0.9 -0.027 4.5 1.1 80 20.0 15.0 500.0 540 20

14.8 23.0 0.231 6.7 13.0 260 45.0 51.0 700.0 1300 1100

stream. The particle size of the ground is larger and the ditch erosion smaller than at the upper ditch. At this site the stream is deep and current speed relatively slow. Hence, the first suitable riffle area some 700 m downstream was chosen as the impacted riffle (site 3 in Fig. 1). All the riffle areas are characterized by swift currents (0.30-1.40 m s l), stony substrates and dense vegetation of mixed coniferous and deciduous trees and bushes shading the stream. The width of the stream at all study sites was 4-7 m. METHODS In order to distinguish the naturally varying factors from those caused by forest draining, we used two different approaches. First, the relative abundances of different habitat types upstream and downstream from the ditches were compared. We based habitat comparisons on mapping, which included the estimation of proportional (%) coverage of different habitat types in 10 randomly chosen 1 m × 1 m squares. Estimations were made by eye and hand in shallow, swiftly flowing riffle areas (20-100 cm s ~). Sites were mapped at the beginning of the forest drainage (2-3 September 1991) and one and two years after. In addition, tufts of Fontinalis dalecarlica growing at the control and impacted sites were collected for the estimation of inorganic material deposited within the tufts. Second, as we are aware that serious practical and theoretical problems in terms of spatial pseudoreplication (Hurlbert, 1984; Cooper & Barmuta, 1993) occur when field comparisons are made between upstream 'control' and downstream 'impact' sites, we used moss transplant units as a partial solution to this problem. While it was not possible to assign several sites for drainage treatments and controls, we used the experimental unit approach suggested by Cooper and Barmuta (1993). We transplanted unaffected, natural patches of mosses from control site 1 to site 2a and to the impacted sites 2b and 3. The risk of having other

89

gradients than those caused by forest drainage was minimized by the short distances between sites, particularly in the case where drainage waters entered the stream in the middle of a rime. Three experimental units each with three replicated moss habitats were assigned to each site treatment. These experimental units, and not the moss samples from within them, were used as replicates in statistical analyses. Within each unit the potential effect of location was minimized by placing three replicate moss tufts side by side in order to reach as similar an environmental condition as possible. The use of moss was selected as over 90% of the species found in the Pajuluoma stream occur in this habitat. Most also reach their greatest abundance in the moss microhabitat (Vuori & Joensuu, unpublished data). It is widely recognized that mosses support a high zoobenthic diversity and abundance compared to other lotic habitats (e.g. Glime & Clemons, 1972; Suren, 1991; Steinman & Boston, 1993). Unfortunately, we were unable to apply the BeforeAfter-Control-Impact design suggested by StewartOaten et aL (1992). Even so, our replicated, translocated microhabitat approach is able to test the null hypothesis: moss communities are similar in riffles above and below forest drains utilizing protective tools. The moss tufts used in the transplantation were collected by a dredge from the upper control (site 1, Fig. 1). One transplantation unit consisted of two tiles rolled into a plastic net and three replicate Fontinalis dalecarlica tufts attached to the netting with plastic bandages. The size and quality of the tufts was standardized by allocating a volume of 1 litre of fresh, green moss to each replicate. The three moss tufts were attached side by side, c. 10 cm apart from each other. On 3 September 1992, three replicates of such transplantation units were installed randomly within the swiftly flowing, stony bottom areas of each riffle. After the transplantation unit was placed in the river bed, current speeds 5 cm above the three moss tufts were measured with a Schiltknecht MiniAir2 (Germany) flowmeter. The weather during the transplantation was rainy and the temperatures were 10 and 6°C in the air and water, respectively. The tiles with netting and moss tufts were sampled on 21 May 1993. The transplantation unit was carefully lifted into a wooden sieve box and carried to the river bank. Each tuft was quickly put into a plastic bag. Animals which dropped into the box were taken out and preserved in 70% ethanol. The amount of these 'outsider' individuals appeared to be small apart from a large number of Simuliidae in some transplantation units. However, the relative proportion of individuals collected from the box was insignificant compared to the total number collected from the transplantation units. Hence, the material collected from the box was ignored in further analysis and only moss-dwelling animals were treated.

K.-M. VuorL I. Joensuu

90

In the laboratory, the benthic invertebrates were sorted from the moss and other material trapped within the tufts. The moss tufts and associated materials were dried for 24 h at 60°C, weighed and then ashed at 550°C for 5 h and weighed again. Samples were cooled in a desiccator before reweighing. The amount of inorganic material in the moss tufts was determined as the proportion of ash dry weight to the total dry weight. Differences in the habitat characteristics above and below the drainage sites were tested with one-way ANOVA followed by a pair-wise comparison procedure of Tukey's HSD test. Within-treatment variances in the species number and abundance in the moss tufts were compared with between-treatment effects by two-way ANOVA. Logarithmic transformations of species numbers and abundances were conducted to achieve a normal distribution. The homogeneity of variances was verified with the Scheff6-Box test (Sokal & Rohlf, 1981). RESULTS The whole stream became turbid several km downstream from the drainage area during draining, and to a lesser extent during the heavy autumnal rainfalls of 1992 (Joensuu, field observation). Elevated turbidity and concentrations of suspended solids were also observed during the spring floods. Furthermore, increases of aluminium, iron and nutrient concentrations and a decrease in pH occurred during the floods. The maximum values for the chemical oxygen demand and colour of the water also reflected the substantial increases in the discharge of dissolved and particulate organic matter during the floods (Table 1). There were significant differences in the average cover of mosses and sand between the sites (one-way ANOVA, p <0.01). After the beginning of the drainage works in the autumn of 1991, the stones covered by tufts of Fontinalis dalecarlica were the dominant habitat at the upstream control site 2a, whereas sand dominated the downstream sites (Fig. 2). The average cover of sand was significantly higher and that of mosses significantly lower below the ditches than above them (Tukey's HSD test, p <0.01). This was most obvious in riffle 2, which received runoff from the ditch through the buffer zone of sparse bush and tree vegetation.

Site 2a

Immediately below the ditch, indigenous mosses growing in lower current velocities (25-10 cm s 1) were often almost totally buried under sand. However, even in faster currents (30-70 cm s-l), the amount of sand and silt was in some cases so great that the number of green moss shoots had declined. The fact that the buried moss tufts were still partially alive, as indicated by green leaves, suggested that the burying had occurred recently. Hence, draining was hypothesized to increase the load of sand and silt carried into the stream. The proportion of sand increased and that of Font# nalis decreased slightly below the ditches in autumn 1992, but differences between years were not significant (t-test, p >0.09). There were no significant between-year differences in the average proportions of different habitat types at the control site either (t-test, p >0.1). Current velocities within the habitat mapping squares varied between 15 and 110 cm s-I. Considering both years, there were no significant differences in average current velocities in the habitat mapping squares between replicates or sites (one-way ANOVA, p >0.05). The proportion of ash dry weight to total dry weight of Fontinalis collected from the rime stones in September 1992 and in transplanted moss tufts in May 1993 was significantly higher below the ditches than above them (Tukey's HSD test, p <0.001). The average proportion of inorganic material in the indigenous Fontinalis-tufts was 35, 57 and 47% at sites 2a, 2b and 3, respectively. These values were in accordance with those measured in the transplanted moss tufts. The averages for the proportion of ash dry weight to total dry weight were 38, 63 and 64% in the tufts transplanted to control site 2a and the impacted riffles 2b and 3, respectively (Fig. 3(a)). There were significant differences in the average number of benthic invertebrate species between sites (Fig. 3(b), ANOVA F -- 14.3, p <0.001). Both the average abundance and species number of the mossdwelling macroinvertebrates were significantly lower below the ditches compared to the upstream sites (Tukey's test, p <0.01). The species composition of the tufts transplanted below the ditches also differed from those transplanted above the ditches. At the downstream sites Simuliidae dominated the invertebrate community, whereas Amphinemura borealis, Protone-

Site 2b

Site 3

FONTINALIS

,RAVEL STONES>20 CM

SAND

Fig. 2. The average cover percentage (n= 10) of different habitat types in the control and impacted riffles.

Forest drainage and lotic biodiversity

91

60

mura intricata, Nemoura avicularis and Baetis spp. dominated the upstream site (Table 2). While c.20% of the total abundance of benthic invertebrates at control site 2a comprised shredder-feeding stonefiies, more than 70% of the invertebrate abundance at the impacted sites 2b and 3 consisted of filter-feeding blackflies. Average current velocities above the experimental units did not differ significantly either between or within sites (two-way ANOVA, p >0.05).

Z

50

DISCUSSION

0 Z

40

0

30

.w all

o

Z m [-,

<

70 -

Physical disturbances play a central role in determining the community structure of benthic invertebrates in many lotic ecosystems. As in our study, mechanisms involved in this disturbance-mediated community often

Z

2 [-

20

O O

10

.<

Table 2. The average abundance (nos/g dry weight Fontinalis) of the macroinvertebrates in the transincated moss tufts at the control (site 2a) and impacted sites (2b and 3) Standard deviation in the parentheses. Species

SlTE2a

SlTE2b

SITE3

Oligochaeta Isopoda Asellus aquaticus

Site 2a

Site 2b

Site 3

0.02 (0.04)

0.01 (0.02)

1.4 (0.5)

0.2 (0.1)

0.3 (0.1)

2.3 (1.0) 0.8 (0.1) 0.5 (1.0) 0.4 (0.1) 0.2 (0.5) 3.6 (4.1) 0.5 (1.0) 1.0 (0.1) 0.01 (0.04)

0.1 (0.3)

0.1 (0.2)

0.1 (0.1) 0.1 (0.04)

0.2 (0.1) 0.3 (0.1)

Plecoptera

SITE

Amphinemura borealis Nemoura avicularis Nemoura cinerea Nemoura sp. Nemurella picteti Protonernura intricata? Leuctra nigra Leuctra sp. Diura nanseni

<

0.04 (0.08) 0.2 (0.3) 0.02 (0.1)

Ephemeroptera Leptophlebia marginata Leptophlebia vespertina Metretopus borealis Baetis niger Baetis rhodani Baetis sp. Heptagenia dalecarlica

•" 20 O tL

Z r.o

m

15

Z m

Rhyacophila nubila Hydropsyche angustipennis Hydropsyche siltalai Ceratopsyche silfvenii Polycentropusflavomaculatus Plectrocnemia conspersa Ithytrichia lamellaris Anabolia laevis Halesus sp. Potamophylax latipennis Lepidostoma hirtum Micrasema gelidum Goerapilosa Silo pallipes

10

5

< <

0.4 (0.3) 0.1 (0.2) 0.3 (0.2) 0.02 (0.2) 0.9 (0.9) 0.8 (0.9) 0.03 (0.08) 0.01 (0.07) 0.01 (0.1) 0.1 (0.6) 0.001 (0.01) 0.3 (0.6) 0.004 (0.05) 0.01 (0.01)

0.2 (0.3) 0.001 (0.02)

0.04 (0.1)

1.2 (1.8)

0.5 (0.9)

4.1 (4.6) 2.1 (3.3) 5.1 (3.3)

4.1 (5.2)

2.4 (21.8)

28.5 (5.7) 2.5 (4.1)

20.3 (8.2) 3.4 (2.0)

0.03 (0.04) 0.01 (0.02) 0.01 (0.06) 0.002 (0.03) 0.001 (0.01)

Neuroptera Sialis lutaria

0 SITE2a

f-

0.1 (0.2)

Trichoptera

r~

O

0.5 (0.7) 0.01 0.1 (0.2) 0.002 (0.01) 0.2 (0.3) 1.0 (0.5) 0.2 (0.3) 0.4 (0.2) 0.1 (0.I) 0.01 (0.2)

S1TE2b

SITE3

SITE

Fig. 3. The average (+SE, n=9) proportion (%) of the inorganic matter (ash dry wt) to the total dry weight of Fontinalis dalecarlica (a) (top) and the average (+SE, n=9) number of moss-dwelling invertebrate species (b) (bottom) in control (site 2a) and impacted (sites 2b and 3) rimes.

Coleoptera Elmis aenea Limnius volckrnari Oulimnius tuberculatus

Diptera Sirnuliidae Chironomidae Ceratopogoniidae Tipulidae

7.0 (6.6) 0.9 (0.8) 0.03 (0.02) 0.05 (0.08)

0.001 (0.002)

92

K.-M. Vuori, L Joensuu

include changes in the type and amount of suitable habitat patches (Gurtz & Wallace, 1984; Townsend, 1989; Scrimgeour et aL, 1994). Our results imply that despite the buffer zones, the load of sand and silt from the forest ditches changed both the relative proportion of different habitat types as well as the quality of the most favourable habitat, the Fontinalis-tufts, in the impacted riffles. This resulted in the impoverishment of the moss-dwelling macroinvertebrate community and drastic changes in the relative abundances of the taxa. Above the forest drainage area the species composition represented a typical fauna of acid headwater streams with a high abundance of shredder-feeding stoneflies in particular (Townsend et aL, 1983; Nyman et aL, 1986; H/~m~l~iinen & Huttunen, 1990). It also contained a relatively high abundance of many plecopteran species that are common in Finnish Lapland but rare or threatened in southern Finland including Protonemura intricata, Nemurella picteti and Diura nanseni (Meinander, 1965; Rassi et aL, 1992). The silting of the moss tufts downstream of the forest ditches resulted in the disappearance of many sensitive species and increased the abundance of tolerant species, especially Simuliidae. The elevated COD and colour values during floods imply a significant increase in organic matter discharge which favours the suspension-feeding simuliids (Wotton, 1987; SchrOder, 1988). Simuliids are also quite tolerant of physical disturbances (Hemphill, 1988) and hence are likely to be favoured by the drainage-induced increase in the discharge of both organic and inorganic matter. Other taxa prevailing downstream of the drainage included such species as Asellus aquaticus, Nemoura cinerea, Hydropsyche angustipennis, Plectrocnemia conspersa, Oulimnius tuberculatus and Sialis lutaria, which are all known to be tolerant of acidification, eutrophication and metal contamination (Townsend et al. 1983; Nyman et al. 1986; H~im~il~iinen & Huttunen, 1990; Vuori, 1995). In general, the impact of draining on lotic zoobenthos depends greatly on the type and amount of suspended material entering the stream during and after draining. Sedimentation and retention of mineral particles in streams is much greater than that of organic material and has been suggested to have more adverse effects on stream biota (Gray & Ward, 1982; Bergquist et al., 1984). In particular, a load of fine mineral particles has been observed to cause a decline in benthic invertebrate abundance and species number (Chutter, 1969; Nuttall, 1972; Gray & Ward, 1982; Culp et aL, 1986) as well as in fish population densities and reproduction (Wickett, 1958; Shapley & Bishop, 1965; Burns, 1972). In addition to the physical disturbance of the habitats, sand sedimentation also restricts the capability of many riffle insects to move on the substrate (Luedtke & Brusven, 1976). Culp et al. (1986) emphasized that the sliding and bouncing of fine sediment particles along the surface

disturbs benthic communities even at lower water velocities when particles are not suspended in the water column. We did not distinguish between the impact of sedimented and moving inorganic matter. Presumably the filling of the interstices of Fontinalis by sedimented materials reduced the current velocity as well as the availability of food and oxygen (Rosenberg & Wiens, 1978). These factors may be the main reasons for the absence of many sensitive shredder species, such as Protonemura intricata, Nemurella picteti and Nemoura avicularis from the silted tufts. The shredder species comprised over 20% of the total abundance of zoobenthos at the upstream control site, whereas their contribution to the abundance of the moss-dwelling invertebrates at the impacted sites was only approximately 1%. Shredder-feeding detritivores depend greatly on the efficiency of the riffle to retain organic matter (Dobson & Hildrew, 1992). Hence, changes in the quality of Fontinalis tufts caused by the load of inorganic material may be the major reason for the significant decrease in the abundance and species number of shredders. Since the current velocity in the experimental riffles was quite high (up to 140 cm s 1), it is likely that the movement of fine sand particles over and within the moss tufts may also have induced physical stress to macroinvertebrates. For example, the bed load of inorganic material with a grain size of 0.2 mm, which is common in our study area, is initiated at about 15 cm s ~ (Seuna, 1982). Indeed, disrupted Fontinalis leaves were observed in the shoots growing at the upper parts of the tufts transplanted to the impacted sites, thus indicating shearing stress. Further, the almost total absence of hydropsychid larvae from the mosses of the impacted riffles may be a consequence of the disruptive effects of bed load on the nets of these filter feeders. Hydropsychids typically spin their webs on the exposed parts of the Fontinalis tufts (Vuori, personal observation). Hence, in such a small stream as the Pajuluoma, the bed load in combination with the burying effect of the deposited particles may disturb a whole range of microhabitats (Culp et aL, 1986). This may have a more severe impact on the benthos than a minor flood or ice scouring, for instance, which leaves patches of untouched refuges (Lake & Schreiber, 1991; Scrimgeour et al., 1994). In many small streams, aquatic mosses are the predominant producers and support a high zoobenthic diversity and abundance compared to other lotic habitats (Glime & Clemons, 1972; Friberg et aL, 1977; Triska et aL, 1982; Naiman, 1983; Englund, 1991; Suren, 1991; Steinman & Boston, 1993). In general, substrate heterogeneity increases and substrate mobility decreases the abundance of Fontinalis (McAuliffe, 1983; Englund, 1991; Steinman & Boston, 1993; Muotka & Virtanen, 1995). Furthermore, phosphorus concentrations, detritus accumulation and epiphyte overgrowth may also limit the distribution of Fontinalis (Finlay &

Forest drainage and lotic biodiversity

Bowden, 1994). Our results indicate that the smothering and disrupting effect of sand and silt may also be an important factor regulating the distribution of both Fontinalis dalecarlica and some of the moss-dwelling invertebrates. By increasing the retrieval of organic matter and physical complexity of the riffle habitat, mosses may act as ecosystem engineers (sensu Jones et al., 1994) which both directly and indirectly affect the structure and function of the whole community. Hence, the duration and extent of the smothering and disruptive effect of mineral particles on the moss habitats is likely to be a key factor determining the overall impact of forest draining on the biodiversity of the streams which drain easily-eroding lands. Although the present results do not allow clear evaluation of the magnitude and duration of the draininduced changes on the moss communities, they indicate that the adverse effects of inorganic material prevailed in subsequent years following draining. This is manifested by the trend of decrease in the moss cover percentages. Furthermore, field observations and correlative data indicate that past drainage schemes (1960-1990) still significantly increase the load of suspended solids and may contribute to the decreased abundance of moss vegetation in several streams within the study area (Vuori et al., 1995). In general, stream communities tend to have low resistance to unpredictable disturbances, and resilience, i.e. recovery to pre-disturbance conditions, varies from months to decades depending on the type of disturbance (Cairns et al., 1971; Yount & Niemi, 1990; Tikkanen et al., 1994). Lack of recovery after 5-25 years has been observed in connection with a long-lasting load of sand following extensive logging or mining in the catchment area (Yount & Niemi, 1990). As the negative effects of the increasing load of mineral material on the fish and other biota have long been known, utilization of sedimentation ponds and buffer zones has been recommended, especially when easily eroding forest lands are drained (Joensuu, 1990). However, our results imply that despite these efforts in protection such a large load of fine mineral material may occur in streams that both the quantity and quality of favourable habitats decrease. In general, sedimentation ponds effectively retrieve the coarse fraction of suspended solids, whereas retrieval of finer matter is ineffective (Joensuu, 1990). The measured load of suspended solids on rivers is in most cases an underestimate of the true impact of solid matter entering streams from forest ditches. Hence, it seems probable that, despite protective efforts, the load of both suspended solids and those moving along the river bed may constitute a long-lasting threat to the macroinvertebrates of small streams. More research is needed on the relative importance and spatio-temporal variation of the ecological effects of the suspended and bed loads of solids in streams.

93

In summary, the effective protection of the biodiversity of streams draining easily eroding forestry lands requires the development of more effective protection schemes. The most erodible parts of the catchment area should be left undrained. Further, a comprehensive prediction and understanding of the ecological effects of forestry on lotic ecosystems calls out for long-term experiments spanning larger spatial and temporal scales of the stream ecosystem. In particular, it is important to note that at the landscape level mosaics of forests and streams maintain complex food-webs and aquatic-terrestrial interactions. While benthic communities have a central role in processing the forest-derived allochtonous material, they also contribute significantly to the local decomposition processes, metabolic activity of streams and transfer of energy to many aquatic and terrestrial consumers. Hence, protecting the integrity of the lotic communities is also a key issue in the biodiversity management of the whole boreal forest ecosystems. ACKNOWLEDGEMENTS This is a subproject contribution of the Joint Research Project on the Adverse Effects of Forest Management on the Aquatic Environment (METVE). Funding by the Ministry of Agriculture and Forestry and the Ministry of the Environment, Finland, is gratefully acknowledged. We thank Mr Kimmo Aronen for his help in the tedious task of carrying and installing the experimental tiles, and the laboratory personnel of Vaasa District Office of Waters and Environment for the water analyses. Comments by Erkki Ahti, Heikki H~im/~l~iinen, Timo Muotka and two anonymous referees improved the paper and were much appreciated. REFERENCES Ahtiainen, M. (1988). Effects of forest clear-cutting and drainage on water quality in the Nurmes-study. Publ. Acad. Finland, 411988, 206-19. Anon. (1987). Metsfi- ja turvetalouden vesiensuojelutoimikunnan mietint0 (Report of the Water Protection Committee of Forestry and Peat Industry. KomiteanmietintO, 1987:62, Maa-ja mets~talousministeri0, Helsinki (in Finnish). Benke, A. C. (1990). A perspective on America's vanishing streams. J. N. Amer. Benthol. Soc., 9, 77-88. Bergquist, B., Lundin, L. & Andersson, A. (1984). Hydrologiska och limnologiska konsekvenser av skogs- och myrdikning (Hydrological and limnological effects of forest and peatland draining. Uppsala Univ., Limnol. Inst., Res. Rep. No. 9, LIU 1984 B:4 (in Swedish). Bilby, R. E. & Bisson, P. A. (1992). Allochthonous versus autochtonous organic matter contributions to the trophic support of fish populations in clear-cut and old-growth forested streams, Can. J. Fish. Aquat. Sci., 49, 540-51. Burns, J. W. (1972). Some effects of logging and associated road construction on northern California streams. Trans. Amer. Fish. Soc., 101, 1-17. Cairns, J. Jr, Crossman, J. S., Dickson, K. L. & Herricks, E. E. (1971). The recovery of damaged streams. Ass. South. Biol. Bull., 18, 79-106.

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