Paleolimnological assessment of the impact of logging on small boreal lakes

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Limnologica 37 (2007) 193–207 www.elsevier.de/limno

Paleolimnological assessment of the impact of logging on small boreal lakes Johanna Ra¨sa¨nena,, Kaarle Kentta¨miesa, Olavi Sandmanb a

Finnish Environment Institute (SYKE), P.O. Box 140, FI-00251 Helsinki, Finland South-Savo Regional Environment Institute, Ja¨a¨ka¨rinkatu 14, FI-50100 Mikkeli, Finland

b

Received 12 July 2006; received in revised form 22 December 2006; accepted 27 December 2006

Abstract In Finland a great number of forest lakes are affected by silvicultural practices such as logging. Logging affects water chemistry and thus the ecological state of lakes by causing nutrient loads and increasing erosion and humic substances in water. Water quality assessment requires definition of natural background conditions and ecological status of water bodies. Therefore it is necessary to determine the impact of these practices on aquatic organisms. In the absence of long-term monitoring data, paleolimnological methods provide a powerful tool for determining human-induced changes in lakes. In this study diatom assemblages, diatom-inferred water total phosphorus and total organic carbon, and sediment chemistry were analyzed from the sediments of six lakes with a logged catchment area (11–53%). According to one-way analysis of similarities (ANOSIM) the diatom communities of three lakes were different before, immediately after and more than 10 years after logging and diatom assemblages in remaining three lakes did not show statistically significant differences between these times. However, all changes were minor, and at present the diatom assemblages and diatom-inferred water chemistry of all the lakes are close to the pre-logging conditions. The minor alterations are probably due to the wide protective zones around the lakes. r 2007 Elsevier GmbH. All rights reserved. Keywords: Paleolimnology; Diatoms; Logging; Water framework directive; Reference conditions

Introduction Forestry practices affect chemical characteristics and thus the ecological state of lakes by causing nutrient loads and increasing erosion and humic substances in water (Carignan & Steedman, 2000). In some areas of Finland, forestry activities such as forest harvesting, scarification and ditching are the most common types of land use. About 75% of the Finnish land area is covered Corresponding author. Tel.: +358 9 40300352; fax. +358 9 40300390. E-mail address: [email protected].fi (J. Ra¨sa¨nen).

0075-9511/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.limno.2006.12.003

by forest, of which 84% is sporadically under silvicultural management (Peltola, 2005). Consequently, many of the 50 000 forest lakes are impacted by forestry practices. This fact is relevant for the ongoing implementation of the EU Water Framework Directive (WFD; EU, 2000). The directive aims to achieve good chemical and good ecological status for all surface waters by the year 2015. To attain this goal the reference conditions for water bodies must be determined. Reference conditions are defined as background conditions that are minimally affected by anthropogenic disturbance. To qualify for good status only minor deviations from reference conditions are accepted.

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The directive emphasizes the use of biological elements in water quality monitoring. Hence, information about the impact of forestry practices on aquatic organisms needs to be gathered. Of all forestry practices in Finland, logging causes the greatest phosphorus and nitrogen loads to waters, 50% and 40%, respectively (Kentta¨mies, 2006). Forest harvesting has an unambiguous impact on the chemical parameters of lakes (Rask, Nyberg, Markkanen, & Ojala, 1998; Watmough, Aherne, & Dillon, 2003), but the influence on biological elements is more difficult to assess because different species can respond to the disturbances in different ways and on different time scales. In previous studies, the impact of forestry on aquatic life forms has varied from slight changes in species assemblages (Paterson, Cumming, Smol, Blairs, & France, 1998; Rask et al., 1998; Planas, Desrosiers, Groulx, Paquet, & Carignan, 2000; Patoine, PinelAlloul, Prepas, & Carignan, 2000; Laird, Cumming, & Nordin, 2001; Laird & Cumming, 2001, Bredesen, Bos, Laird, & Cumming, 2002) to considerable alterations in biological communities and water chemistry (Lott, Siver, Marsicano, Kodama, & Moeller, 1994; Simola, Kukkonen, Lahtinen, & Tossavainen, 1994; Ko¨ster et al., 2005). Although logging is expected to have the greatest impacts in the first and second year, previous studies have shown that the effects of forestry practices on water chemistry can be long, lasting from a decade (Ahtiainen & Huttunen, 1995; Kubin, 1998) to a century (Scully, Leavitt, & Carpenter, 2000). No long-term, i.e. decadal scale studies about forestry-induced chemical and especially biological changes in Finnish lakes have been carried out, except for a few paleolimnological investigations (Sandman, Turkia, & Huttunen, 1992, 1994; Turkia, Sandman, & Huttunen, 1998). Long-term monitoring data on small and remote lakes are generally not available. Accordingly, there is insufficient knowledge of the pre-disturbance conditions of such lakes. Data should be collected in order to help lake managers set realistic mitigation targets for future restoration and management decisions. Paleolimnology uses the physical, chemical and biological information archived in lake sediments to reconstruct and interpret past environmental conditions. The top-bottom approach is a frequently used method to analyze samples before and after a certain environmental incidence (e.g. logging) which is expected to have changed the water quality (Cumming et al., 1992; Michelutti, Laing, & Smol, 2001; Manca & Armiraglio, 2002; Bennion, Fluin, & Simpson, 2004; Miettinen, Kukkonen, & Simola, 2005). Diatoms (Bacillariophyceae) are part of the phytoplankton and phytobenthos mentioned as biological elements in WFD. Diatoms are widely used as bioindicators of environmental conditions in lakes (Stoermer & Smol, 2000). These micro-

algae have siliceous cell walls that are taxonomically diagnostic and they are well preserved in most lake sediments. In addition, diatom species generally have well-defined tolerances to many environmental variables, such as total phosphorus, pH and water color. These characteristics allow the development of transfer functions, mathematical models that describe the relationships between species assemblages and environmental variables. By applying these models, former values of an environmental variable can be inferred from the composition of fossil assemblages. Six small and remote lakes in eastern Finland were selected for this study. Logging was carried out in the catchment areas during the early 1990s and no other human impact (excluding atmospheric fall-out) had affected the lakes. The aim of the study was to assess the impact of logging by comparing diatom assemblages before, a few years after and more than 10 yr after the logging. Both changes in species composition and diatom-inferred water chemistry variables such as total phosphorus and total organic carbon were considered. Chemical properties of the sediments were also examined. Finally, the findings were used to evaluate the extent of the management actions required in order to fulfill the WFD requirements.

Materials and methods The study lakes The six study lakes (Erilampi, Palolampi, Kalliolampi, Ma¨ntyja¨rvi, Valkeisja¨rvi and Saukkolampi) are situated in Kuhmo, eastern Finland close to the Russian border (631530 –641300 N, 281480 –301060 E; Fig. 1). Their surface area ranges from 2 to 55 ha (Table 1). The lakes are located in a wilderness in coniferous forested catchment’s areas. All the catchments lay on granite bedrock overlain by moraine and peat deposits. Logging took place in the catchment’s areas during 1991–1994, and additionally around Lake Ma¨ntyja¨rvi in 1988. All logging expect that around Lake Ma¨ntyja¨rvi was clearcut logging. Protective zones (ca. 50 m) around the lakes were created and after the logging new trees were planted in the area. The proportion of the logged area of the total catchment’s area ranged from 11% to 53% (Table 1).

Sediment sampling and chemical analysis Lakes were visited twice for sediment coring. During the first coring operation performed in winter 1997, i.e. 3–6 yr after the logging, a 20 cm long sediment core was taken from each lake. During the second coring visit in 2005 only surface sediment (0–2 cm) was sampled to study the present conditions of the lake biota. All lakes

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Fig. 1. Location of the study lakes (1 ¼ Erilampi, 2 ¼ Saukkolampi, 3 ¼ Ma¨ntyja¨rvi, 4 ¼ Kalliolampi, 5 ¼ Palolampi, 6 ¼ Valkeisja¨rvi). Table 1.

Summary of some physical and chemical variables of the study lakes and the percentages of the catchment areas logged

Erilampi Palolampi Kalliolampi Ma¨ntyja¨rvi Valkeisja¨rvi Saukkolampi

Area (ha) Catchment/ Lake ratio

Depth (m)

TotP (mg L-1)

pH

Colour Secchi Logging (mg Pt L 1) depth (m) year

Catchment logged (%)

9.4 8.6 1.7 52 55 2.6

8 9 10 4.5 10 8

22 12 16 6.4 4.9 17

5.9 5.4 5.6 5.8 5.5 6.0

107 106 100 55 30 125

24 15 53 21 11 33

8 5 22 3 4 23

were cored from the deepest part of the lake with a Limnos gravity corer (Kansanen, Jaakkola, Kulmala, & Suutarinen, 1991). Following the Finnish standard SFS 3008 (1981), the water content and loss-on-ignition (LOI) of the sediment were measured from the 20 cm long sediment cores taken in 1997. Sediment total nitrogen (TotN) and total phosphorus (TotP) contents were determined by colddigestion as described by Zink-Nielsen (1975). For these analyses the sediment cores were sliced at 1 cm intervals (20 samples from each lake). Due to the fixed times of the sampling events, no sediment dating was needed. The surface sediment always represents recent conditions. To make a rough estimation of the sedimentation rate and to make sure that no sediment reworking had occurred, a soot particle (spherical carbonaceous particles, SCP) analysis was carried out for the 20 cm profile sample of Lake Saukkolampi, sliced in 1 cm sub-samples (20 samples). Analysis followed the procedure described in Renberg and Wik (1984). SCPs are derived from oil and coal burning that increased intensively in the 1960s. Based on

1.3 1.3 0.7 1.7 1.5 1.4

1992 1992 1992 1994 1991–92 1992

SCPs and the literature information of the dated cores from oligotrophic lakes in boreal region, a sediment depth of 25–30 cm represents pre-industrial conditions (Johansson, 1989; Huttunen et al., 1990; Pettersson, Renberg, Gelandi, Lindberg, & Lindgren, 1993). Therefore, a sediment depth 49–10 cm must safely represent the time before forest cuttings, i.e. more than 3–6 yr in time. It is not relevant for the aim of the study to know the exact dates of the lowermost background samples. Most important it is to know that these samples are not affected by the forestry practices. Water chemistry variables were extracted from the National Water Quality Database (maintained by the Finnish Environment Institute). Sampling and analyzing followed the standard procedures (Niemi et al., 2000).

Diatom analysis An extended top-bottom analysis was conducted. In this procedure 12 sub-samples from each lake were

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investigated for diatoms. The 20 cm long sediment core taken in 1997 was sliced into sub-samples representing the estimated time immediately after logging (0–0.5, 0.5–1, 1–1.5, 1.5–2, 2–2.5 and 2.5–3 cm from the sediment top) and the time before logging (9–10, 11–12, 14–15 and 19–20 cm). These samples were treated with an acidic mixture of HNO3 and H2SO4, and they were mounted with Hyrax. The surface sediment subsamples taken in 2005 (0–0.5 and 1–1.5 cm) representing present conditions were prepared for microscopy by oxidizing the organic matter with 35% H2O2 and the acidic mixture described above (Battarbee, 1986; Battarbee, Jones, Flower, Cameron, & Bennion, 2001). Diatom slides were mounted with Naphrax and 400–500 valves were identified from each sub-sample. Diatoms were counted along randomly selected transects using a Leitz Diaplan microscope with 1250  final magnification and phase contrast. The main taxonomic sources were Krammer and Lange-Bertalot (1986, 1988, 1991a, b), Lange-Bertalot and Moser (1994) and some individual references (e.g. Gibson, Anderson, & Haworth, 2003). The diatom counts were expressed as relative abundances.

Statistical methods The diatom assemblages were divided into three groups in time: (i) before the logging (9–10, 11–12, 14–15 and 19–20 cm samples), (ii) immediately after the logging (0–0.5, 0.5–1, 1–1.5, 1.5–2, 2–2.5, and 2.5–3 cm samples) and (iii) 11–14 yr after the logging, i.e. at present (0–0.5 and 1–1.5 cm samples taken in 2005). The composition of each diatom assemblage group (i–iii) was calculated as an average for each lake. In the following these abundances are designated as ‘‘an average diatom assemblage’’. The Bray-Curtis distance coefficient was used as a similarity measure giving the degree of floristic change between average diatom assemblages of groups (i–iii). The values of the similarity coefficient range from 0 to 1, where 1 indicates that two samples are identical and 0 indicates that they are totally different (Michelutti et al., 2001). Tests for significant differences in diatom-species composition in each lake between groups (i), (ii) and (iii) were performed using the one-way analysis of similarities (ANOSIM). ANOSIM is a non-parametric test that assesses significant differences between groups using a distance measure (Clarke, 1993). The distances are converted to ranks. Various dissimilarity or similarity measures can be employed. In this study, Bray–Curtis distance was used. ANOSIM gives a statistical value (Global R), which can range from 1 to 1. Groups are completely different when R is +1. The significance (pvalue) is computed by permutation of group membership with 5000 replicates.

Shannon, Simpson and Hill’s N2 diversity indices were used to estimate diversity of the diatom assemblages (Harper, 1999). The Shannon index characterizes the entropy in assemblages taking into account the number of individuals as well as the number of taxa. The Simpson index measures the evenness of the community and varies from 0 to 1. Hill’s N2 diversity index was calculated with the program C2 (Juggins, 2003). This diversity index determines the effective number of occurrences of a taxon. The more even the relative abundances and the number of occurrences of a species are, the higher are the Hill’s N2 values. Multivariate techniques were used to summarize the compositional changes in diatom assemblages in each lake over time. For each lake, an exploratory detrended correspondence analysis (DCA; Hill & Gauch, 1980) was used to measure the species turnover (gradient length expressed as standard deviatation units, SD) in order to determine whether to use linear or unimodal ordination technique. For most lakes DCA resulted in rather short (o2 SD) gradient lengths, suggesting that linear response-based principal component analysis (PCA; Gauch, 1982) would be an appropriate multivariate technique. Consequently, PCA was employed and only those taxa that attained at least 1% relative abundance in at least one sample were taken into the analysis. Bray–Curtis similarity coefficient calculations, ANOSIM analyses, Shannon and Simpson diversity index calculations, DCA and PCA were performed with the program Past 1.4 (Hammer, Harper, & Ryan, 2001). Water total phosphorus concentrations were inferred from sedimentary diatom assemblages (DI-TP) using the model of Kauppila, Moisio, and Salonen (2002). The calibration set for the model consists of 61 southern Finnish lakes with autumnal epilimnic TP range of 3–89 mg L1. The model is generated with simple weighted averaging regression (WA; ter Braak & van Dam, 1989) and classical deshrinking. The root mean squared error of prediction (RMSEP) for the model is 0.16 log mg TP L1. Diatom-based water total organic carbon concentration (DI-TOC) was calculated with the TOC optima for diatoms developed by Huttunen and Turkia (1990) and simple weighted averages. TOC represents the quantity of humic matter, which makes the water dark. The optima are based on 89 lakes, with a TOC range of 0.6–17.1 mg L1.

Results Sediment chemistry In the sediments of the studied lakes the percentage organic matter, measured by LOI, fluctuated between 32% (Ma¨ntyja¨rvi) and 65% (Kalliolampi) [Fig. 2(a)].

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Fig. 2. Sediment chemistry of the 20 cm profile samples taken in 1997. (a) Loss-on-ignition, (b) carbon–nitrogen ratio, (c) total nitrogen, (d) total phosphorus. The grey bars represent the estimated time of logging.

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Lake Ma¨ntyja¨rvi differed in its increasing organic matter towards the sediment surface whereas in other lakes LOI values decreased up core. In deeper parts of Lake Saukkolampi sediment LOI increased but there was a turn at about 14–15 cm, from where the value decreased towards the surface. Excluding Lake (Valkeisja¨rvi) the C/N-ratio decreased at the topmost 5 cm of the sediment [Fig. 2 (b)]. The most notable decrease in C/N-ratio occurred in Lake Saukkolampi, where allocthonous material began to increase earlier than in other lakes. Sediment TotN concentrations varied from 12 (Ma¨ntyja¨rvi) to 24 mg g1 (Erilampi) [Fig. 2(c)]. The trend of TotN was similar to that of the LOI, because there is a correlation between organic matter and nitrogen in sediment. The minimum of sediment TotP was found in Erilampi (approximately 1 mg g1) [Fig. 2(d)]. High, widely differing values were found in background samples of Lake Saukkolampi, where the maximum concentration was 4 mg g1 approximately. The background samples in lake Saukkolampi did not contain SCPs (Fig. 3). This indicates that the samples represented the time before the 1900s. SCP concentra-

tions in the topmost samples ranged from 270 to 640 grains g1 (dry weight).

Diatoms A total of 238 species were found from the small forest lakes. Frustulia rhomboides (Ehrenb.). De Toni, Brachysira brebissonii Ross, Navicula radiosa Ku¨tzing, Cymbella gracilis (Rabenh.) Cleve, Navicula subtilissima Cleve and Navicula mediocris Krasske were present in every lake in all samples. Planktic species were much scarcer than the benthic ones. The average composition of each diatom assemblage group (i) before, (ii) immediately after and (iii) 10 yr after logging, i.e. at present showed that the most abundant species, Frustulia rhomboides, increased in all the lakes immediately after cuttings. Another common species which increased towards the present was Tabellaria flocculosa (Roth) Ku¨tzing, which has a broad ecological range. However, the amounts of most other abundant species (42% abundance before logging) decreased immediately after logging (Fig. 4). According to ANOSIM, three lakes (Erilampi, Kalliolampi and Palolampi) showed statistically significant difference in species assemblage between the three diatom groups (Table 2). R-values for the lakes ranged from 0.08 to 0.51. The R-value was highest in Lake Erilampi (0.51), indicating that the sample groups had the greatest differences in this lake. According to Bray–Curtis similarities calculated between ‘‘average diatom assemblages’’, the lakes appeared to have changed to a rather similar extent (Table 3). Before and immediately after the cuttings the most similar diatom assemblages were found in Lake Valkeisja¨rvi. On the other hand, the greatest dissimilarity was found between average assemblages immediately after logging and at present in Lake Ma¨ntyja¨rvi. In general, the differences were greatest between the average assemblages immediately after logging and these at present.

Erilampi

Fig. 3. Soot particle (spherical carbonaceous particles, SCP) results from the 20 cm profile sample of Lake Saukkolampi.

The most abundant diatom species in Lake Erilampi were Frustulia rhomboides and Brachysira brebissonii, accounting on average for 18% and 10% of the diatom communities, respectively [Fig. 5(a)]. No marked changes occurred in their abundance, but the amount of Brachysira vitrea (Grun.) Rossii decreased after logging and remained rather low in the uppermost samples. Navicula difficillima Husted appeared in the samples immediately after logging, whereas it was absent in the background samples as well as in samples representing the present conditions. However, at its maximum it represented only 2% of all the diatom

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(b)

(c)

(f)

(e)

(d)

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Fig. 4. Species that comprise on average 42% of the ‘‘average diatom assemblage’’ before logging (see text) excluding very dominant species. A linear trend line was drawn from background samples to aid interpretation. (a) Erilampi, (b) Palolampi, (c) Kalliolampi, (d) Ma¨ntyja¨rvi, (e) Valkeisja¨rvi and (f) Saukkolampi. Abbreviations: Achn_imp, Achnanthes impexa, Achn_min ¼ Achnanthes minutissima, Achn_spp ¼ Achnanthes spp., Achn_sto ¼ Achnanthes stolida, Achn_sub ¼ Achnanthes subatomoides, Aula_amb ¼ Aulacoseira ambigua, Aula_Digr ¼ Aulacoseira distans group, Aula_ten ¼ Aulacoseira tenella, Brac_bre ¼ Brachysira brebissonii, Brac_vit ¼ Brachysira vitrea, Cycl_oce ¼ Cyclotella ocellata, Cymb_gra ¼ Cymbella gracilis, Cymb_per ¼ Cymbella perpusilla, Euno_GR ¼ Eunotia argus/exigua/meisterii/rhynchephala, Euno_spp ¼ Eunotia spp., Frag_con ¼ Fragilaria constricta, Frag_exi ¼ Fragilaria exigua, Frag_spp ¼ Fragilaria spp., Navi_GR ¼ Navicula aboensis/absoluta/ modica, Navi_ind ¼ Navicula indifferens, Navi_med ¼ Navicula mediocris, Navi_rad ¼ Navicula radiosa, Navi_spp ¼ Navicula spp., Navi_sub ¼ Navicula subtilissima, Nitz_fon ¼ Nitzschia fonticola, Stau_anc ¼ Stauroneis anceps, Tabe_flo ¼ Tabellaria flocculosa.

Table 2.

Results of analysis of similarity (ANOSIM)

Erilampi Palolampi Kalliolampi Ma¨ntyja¨rvi Valkeisja¨rvi Saukkolampi

R

p

0.51 0.42 0.36 0.26 0.12 0.08

0.001** 0.009** 0.02* 0.06 0.18 0.25

*po0.05; **po0.01.

species. Another species of which the amount changed completely was Aulacoseira islandica (Mu¨ller) Simonsen, which made its first appearance in the present samples. Aulacoseira perglabra (Oestrup) Haworth was present in only one background sample but after logging its amount remained regular, although small. Aulacoseira

Table 3. Bray–Curtis similarity coefficients calculated from ‘average diatom assemblages’ (see text for details)

Ma¨ntyja¨rvi Erilampi Kalliolampi Saukkolampi Palolampi Valkeisja¨rvi

After–before After–present

Before–present

0.72 0.77 0.82 0.82 0.82 0.86

0.74 0.63 0.78 0.87 0.75 0.70

0.66 0.73 0.76 0.80 0.79 0.78

distans v. tenella (Nygaard) Simonsen was more abundant in the present day samples than in either of the earlier groups of samples. First axis PCA coefficients and diversity indices showed only minor variation [Fig. 5(a)]. A small increase in diversity was observed after logging.

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(b)

(c)

Fig. 5. Stratigraphic diagrams showing the main diatom taxa (45%) and some selected taxa (percentage relative abundance). Shannon, Simpson and Hills N2 diversity indices and the first PCA axis scores are also shown. The grey bars represent the estimated time of logging. (a) Erilampi, (b) Palolampi, (c) Kalliolampi, (d) Ma¨ntyja¨rvi, (e) Valkeisja¨rvi, (f) Saukkolampi. Please note the different scales in the x-axis for different species.

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(e)

(f)

Fig. 5. (Continued)

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Palolampi Only a few species dominated the diatom assemblage of Lake Palolampi. As in Erilampi, Frustulia rhomboides and Brachysira brebissonii were the most abundant species [Fig. 5(b)]. Immediately after logging the amount of Frustulia rhomboides slightly increased and that of Brachysira brebissonii decreased. Other abundant species were Navicula subtilissima, Tabellaria flocculosa and Aulacoseira distans (Ehr.) Simonsen, the abundance of the two latter species generally increasing towards the present. Aulacoseira alpigena (Grun.) Krammer became more common immediately after logging, but it was absent in the present samples. Aulacoseira ambigua (Grun.) Simonsen appeared with low abundance immediately after logging. All diversity indices and coefficients for the first PCA axis indicated a very stable diatom community in the background conditions [Fig. 5(b)]. Logging caused a minor shift in the assemblages but the diatom community has become stable again in recent years.

ja¨rvi was Frustulia rhomboides. Its abundance increased considerably immediately after logging but decreased again towards the present. Navicula radiosa showed a similar trend. Brachysira brebissonii and Brachysira vitrea formed about equal amounts (3–4%) of the diatom assemblages and both became slightly more abundant immediately after logging. Navicula indifferens Husted was present in small amounts in two lowermost background samples, but immediately after logging its abundance collapsed and in the present samples the species was totally absent. Generally, a quite large variation was observed in the samples before and immediately after logging. PCA coefficients for the first axis showed changes after logging, but the present samples were similar to background samples [Fig. 5(d)]. The diversity indices confirmed the changes after logging [Fig. 5(d)].

Valkeisja¨rvi

The diatom community of Lake Kalliolampi was also dominated by Frustulia rhomboides and Brachysira brebissonii [Fig. 5(c)]. There were no marked changes in their abundances over time. The amount of Navicula mediocris declined immediately after logging but that of Navicula subtilissima remained rather stable. No clear changes occurred in those species forming only a minor part of the diatom assemblage. The only species that completely disappeared from the present samples was Aulacoseira distans v. tenella, which was present before and immediately after logging. Achnanthes impexa LangeBertalot became scarcer towards the present whereas Cymbella gracilis became slightly more abundant. Coefficients for the first PCA axis showed subtle changes immediately after logging [Fig. 5(c)]. Both indices exhibited a similar pattern, i.e. diversity generally decreased immediately after the logging but then became stable again towards the present. According to the Simpson index, changes in the diversity were small, but the Shannon and Hills N2 indices showed greater variation [Fig. 5(c)].

In Lake Valkeisja¨rvi, the most abundant diatoms were Frustulia rhomboides and Brachysira brebissonii [Fig. 5(e)]. On average, the amount of Frustulia rhomboides increased and that of Brachysira brebissonii decreased towards the present [Figs. 4(e) and 5(e)]. Other common species of which the abundances generally decreased towards the present were Navicula subtilissima, Navicula mediocris, Brachysira vitrea and Cymbella perpusilla Cleve. The abundance of Navicula radiosa slightly increased towards the present, but the variation between different samples was high as in the case of N. mediocris. Eunotia exigua (Breb.) Rabenhorst was rather common in all samples and its abundance remained relatively stable over time. In less common species a few clear changes were observed. Aulacoseira crenulata Thwaites became more abundant towards the present, as did Cymbella hebridica (Grun.) Cleve. Some species such as Achnanthes stolida Krasske, Pinnularia divergens Smith and Actinella punctata Lewis were not present in background conditions but appeared in small quantities immediately after logging. The coefficients for the first PCA axis and diversity indices showed variation in background conditions and immediately after logging [Fig. 5(e)]. The two topmost samples representing the present conditions were homogenous.

Ma¨ntyja¨rvi

Saukkolampi

Lake Ma¨ntyja¨rvi differed from the other lakes mainly due to its large quantity of Cyclotella ocellata Pantocsek. This species was most abundant in background samples, decreased immediately after logging but increased again in the present samples [Fig. 5(d)]. However, the most abundant species in Lake Ma¨nty-

The main difference in Lake Saukkolampi compared to the other lakes was that Aulacoseira subarctica Mu¨ller, a planktic, mesotrophic diatom was the most abundant diatom species over time. At its maximum it comprised more than 40% of the community [Fig. 5(f)]. Its abundance decreased immediately after logging but

Kalliolampi

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at present this species is on average more abundant than before logging. Frustulia rhomboides and Brachysira brebissonii were abundant but not as dominant as in other lakes. Fragilaria-species were also rather common in Lake Saukkolampi and they generally increased after logging [Fig. 4(f) and [Fig. 5(f)]. The most visible feature in less abundant species was the slight increase of Achnanthes pusilla Grunow immediately after cuttings. It was present in two background samples but in samples immediately after logging, it’s amount increased representing a maximum of about 2% of the diatom community. Achnanthes minutissima Ku¨tzing also became more abundant immediately after logging. Variation in the background diatom communities can be seen in the PCA coefficients and diversity indices [Fig. 5(f)]. The greatest variation was in the Hills N2 and Shannon indices. However, according to these metrics, the two oldest samples and the most recent sample were similar.

Diatom inferred water chemistry DI-TP reconstructions were considered most reliable in Lakes Ma¨ntyja¨rvi and Valkeisja¨rvi due to the water

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colour [Fig. 6(a)]. In these lakes the water colour was o60 mg Pt L1, whereas in other lakes the water colour exceeded 100 mg Pt L1. The training set of Kauppila et al. (2002) includes lakes with water colour o80 mg Pt L1. Water colour has a strong effect on diatoms, and high colour possibly hampers the DI-TP reconstructions. In lakes Ma¨ntyja¨rvi and Valkeisja¨rvi, the DI-TP (3–8 mg L1) was in good agreement with the actual measured water TP concentrations (4–8 mg L1) (data not shown). Changes immediately after logging were seen in all lakes, but they were small (1 mg L1 or less) [Fig. 6(a)]. The greatest change was found in background samples of Lake Saukkolampi, where the DI-TP concentration was over 13 mg L1 in the two lowest samples, but then suddenly decreased to 9 mg L1. The DI-TP concentration for present-day samples was again about 13 mg L1. Generally, the DI-TOC concentrations were similar to the TOC concentrations measured from water samples during the 1990s (data not shown). DI-TOC in Lake Saukkolampi showed some deviation from measured TOC: the maximum measured TOC concentration was 17 mg L1 (August 1997), whereas the maximum DI-TOC was about 8 mg L1. The DI-TOC may

(b)

Fig. 6. (a) Diatom inferred total phosphorus (DI-TP) and (b) diatom-inferred total organic carbon (DI-TOC) reconstructions for each lake. The grey bars represent the estimated time of logging.

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underestimate the TOC-concentrations, because Lake Saukkolampi has very brown water (125 mg Pt L1). In Lake Ma¨ntyja¨rvi, the DI-TOC concentration increased from the background 6.8–7.7 mg L1, which is the largest change observed among the study lakes [Fig. 6(b)]. Otherwise the changes were negligible, less than 1 mg L1 in the other lakes.

Discussion The sedimentary diatom assemblages of the study lakes were generally similar, as expected due the close geographical location of the lakes. Acidophilic species derived from the littoral zones of the lakes, such as Frustulia rhomboides and Brachysira brebissonii, were the most important group of diatoms found in the sediment cores. Planktic diatoms were scarcer than benthic diatoms, probably due to the low pH and shallowness of the lakes (Willen, 1991). The diatom species assemblages observed in this study were typical for unproductive forest lakes in Finland (Merila¨inen, Huttunen, & Pirttiala, 1982; Huttunen & Merila¨inen, 1983). The most common species, Frustulia rhomboides has been found to decrease together with increasing pH, conductivity and total phosphorus (Merila¨inen, Huttunen, & Pirttiala, 1982). In the study lakes the amount of F. rhomboides increased in every lake immediately after logging. This response describes that the trophic status of the study lakes probably did not become markedly more eutrophic due to logging. The amount of some scarcer species was found to change in some lakes immediately after logging. For example, Achnanthes stolida, Pinnularia divergens and Actinella punctata increased in Lake Valkeisja¨rvi. The increases were small and could be insignificant or due to natural variation, but also due to forestry practices. An increase in the amount of Fragilaria spp. as in lakes Erilampi and Saukkolampi may well also be due to forestry practices because occurrences of Fragilaria spp. have been associated with environment instability (Denys, 1990). Forest clearance has physical and chemical effects on lake ecosystems. For example, small lakes in particular become more exposed to the wind and the depth of the epilimnion increases, resulting in improved conditions for phytoplankton growth during summer stratification. Denudation of the landscape can increase watershed erosion and alter mineral and nutrient balances, which may of course affect the biological communities in the lake (e.g. Carignan & Steedman, 2000). In this study, the results of sediment chemistry were difficult to interpret since there were already great fluctuations before the forestry practices. However, there were some general features that could be connected to logging. The carbonnitrogen ratio (C/N) indicates the ratio of allochtonic material derived from the catchment area and auto-

chtonic material from production within the lake in sediment. Excluding Lake Valkeisja¨rvi the proportion of allocthonic matter increased in the topmost 5 cm of sediment (decreasing C/N-ratio), which may well be a sign of increased erosion caused by the forest harvesting. However, we suggest that diatom assemblages reflected changes after logging more logically than sediment chemistry, although all the changes were minor. This is in good agreement with previous studies of the effects of logging (Laird & Cumming, 2001; Laird et al., 2001). The main changes have occurred in the abundances of species rather than in the appearances of species, which is also in accordance with the previous studies. For example, Laird et al. (2001) concluded that the differences in diatom assemblages before and after forest harvesting generally resulted from small shifts in the percentages of the species. Three lakes showed statistically significant differences in ANOSIM analysis (Erilampi, Palolampi and Kalliolampi). All these lakes had very brown water color. It is possible that the diatom assemblages were so strongly affected by water color that the slightly increased leaching from the catchment’s area after logging did not change the assemblages. A general feature for all the lakes that did not show statistically significant differences according to ANOSIM (Ma¨ntyja¨rvi, Valkeisja¨rvi and Saukkolampi) was the variation in their background conditions. Natural variation is normal for all biological populations and can be for example a result of climate, seasonality, predation or their combinations. The area of forest harvesting did not appear to affect the magnitude of changes in the diatom assemblages. Namely, the diatom assemblages in lake Kalliolampi, where 53% of the catchment’s area was logged, showed similar changes to those in lake Valkeisja¨rvi where only 11% of the catchment’s area was logged. The ratio of catchment’s area to lake area also did not appear to explain the differences between lakes. Diatom-inferred TP increased very slightly after forest harvesting in Erilampi, Palolampi, Ma¨ntyja¨rvi and Valkeisja¨rvi. In Lake Kallioja¨rvi TP decreased towards the present and in Lake Saukkolampi variation between all samples was high. Changes in diatom-inferred TOC were minimal, which indicates that humic substances did not affect the water color after logging. It is questionable, whether these small changes in diatom-inferred water chemistry variables are ecologically meaningful. Reconstructed parameters are most suitable for revealing trends and directions of changes, not for displaying the actual values. In the present samples, some individual species showed recovery, i.e. returned back to their pre-logging levels in some lakes. Examples of such species are Navicula radiosa and Brachysira vitrea. However, according to Bray–Curtis similarity coefficients calculated between average diatom assemblages, the

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dissimilarities were greatest between diatom assemblages before cuttings and at present, i.e. 11–14 yr after cuttings. This indicates that the effects of logging may also be long term. Several different forestry practices usually follow one another in the same areas and therefore the effects of individual practices are difficult to distinguish. A concrete problem for lake managers is that information about forestry practices is often difficult to obtain. When the forest belongs to the state, details about silvicultural management are available. For the current study, lakes with only logging and subsequent soil preparations in the watershed were chosen. If multiple practices had been employed, the response of lake ecosystems might have been more apparent. However, it is notable that the catchment’s areas of the study lakes were intensively logged. Such large areas logged around lakes are not typical in Finland. Usually, about 1–4% of the catchment’s area is cut at one time. Three of the study lakes (Erilampi, Palolampi and Saukkolampi) were earlier examined by Rask et al. (1998). Water chemistry, zooplankton and -benthos, periphyton, phytoplankton and fish were monitored during 1991–1994. Phytoplankton species composition suggested slight eutrophication and a cyanobacterial bloom was observed in one lake after logging. Lake Saukkolampi, which had the largest part of the catchment’s area cut of these three lakes (33%), showed the clearest reaction to forestry treatments. Generally, however, the limnological responses to forestry were modest. Rask et al. (1998) concluded that the protective zones, i.e. about 20–50 m wide forested areas left on shores of the lakes mitigated the effects of forestry practices. Without those zones, the load of substances to the lakes might have been greater and have caused more severe alterations. Paterson et al. (1998) also explained the subtle postlogging changes in scaled chrysophyte populations by the buffer strips and rapid regrowth of vegetation in the catchment’s area. By contrast, Steedman and Kushneriuk (2000) reported that a buffer strip did not prevent decline in water clarity after clearcut logging. In the lakes studied, the buffer zones were wider than in the Finnish recommendations (10 m). Clearly this had a considerable effect on the magnitude of impacts. For example, Ahtiainen and Huttunen (1999) studied the effects of logging on small brooks in eastern Finland. The difference in phosphorus and nitrogen leaching was ten times higher in a brook without protective zones than in a brook where wide (30–50 m) protective zones were left on the shores. The findings are a positive signal for the implementation of WFD. The study indicates that logging did not markedly change the biological elements of the lakes, and that despite cuttings, all the studied lakes are currently close to their background conditions (probably with good or high status). Here, only diatoms were

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studied and the results cannot be generalized to other biological elements included in the directive such as fish, macroinvertebrates and macrophytes. However, diatoms have been demonstrated to be sensitive indicators of many kinds of disturbances in the lake or in the catchment’s area, as they are rapidly growing, smallsized algae that respond quickly to changes in water quality (Stoermer & Smol, 2000). Therefore, it is likely that changes would be seen in diatom assemblages, but not necessarily in other biological groups. Studying lakes by paleolimnological methods gives valuable information of ecological changes. Results may not always be uniform, but by combining ecological information with chemical results it is possible to estimate the magnitude of the changes. Without paleolimnological studies, not much would be known about the long-term changes on the studied boreal lakes induced by logging. Therefore, paleolimnological methods should be included in the research plans having questions about the ecosystem changes of these kinds.

Acknowledgements J. Turkia for coring effort and contribution in 1997; T. Kauppila for doing the TP-reconstructions; E. Ra¨sa¨nen for coring effort in 2005; M. Nyman, H. Vuoristo, V-P. Salonen and two anonymous referees for comments on the manuscript.

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