Effects of Biomanipulation on Fish and Plankton Communities in Ten

Hydrobiologia (2006) 553:67–88 Springer 2006 DOI 10.1007/s10750-005-0786-0

Primary Research Paper Eﬀects of biomanipulation on ﬁsh and plankton communities in ten eutrophic lakes of southern Finland

M. Olin1,2,*, M. Rask2, J. Ruuhija¨ rvi2, J. Keskitalo3, J. Horppila1, P. Tallberg4, T. Taponen5, A. Lehtovaara3 & I. Sammalkorpi4 1Department of Biological and Environmental Sciences, University of Helsinki, P.O. Box 65 FIN-00014, Finland 2Finnish Game and Fisheries Research Institute, Evo Fisheries Research Station, FIN-16970, Evo, Finland 3Lammi Biological Station, University of Helsinki, Pa¨a¨ja¨rventie 320, 16900, Lammi, Finland 4The Finnish Environment Institute, P.O. Box 140 FIN-00251, Helsinki, Finland 5Uusimaa Regional Environment Centre, P.O. Box 36 FIN-00521, Helsinki, Finland (*Author for correspondence: Tel.: +358-9-191-58446; Fax: +358-9-191-58257; E-mail: mikko.olin@helsinki.ﬁ)

Received 7 June 2004; in revised form 18 May 2005; accepted 24 May 2005

Key words: biomanipulation, ﬁsh community, zooplankton, phytoplankton, cyanobacteria, transparency

Abstract The effects of biomanipulation were studied in ten Finnish lakes to determine responses in fish and plankton communities and water quality after mass removal of cyprinids. From 1997 to 2001, the fish communities shifted from the dominance of large cyprinids to an explosion of small cyprinids and a higher proportion of piscivores in effectively biomanipulated lakes (>200 kg ha)1 3yr)1). The biomass of cyanobacteria decreased, and the duration of the blooms shortened and shifted towards the autumn. Decreased concentrations and slower cycling of nutrients and increased grazing by cladocerans probably affected the declined biomass of cyanobacteria. Less intensive sediment disturbance and increased phosphorus-retention in fast growing fish biomass may have turned the role of the fish assemblage from ‘nutrient recycler’ to ‘nutrient storage’. Increased potential grazing pressure, higher proportion of edible algae, and lower chlorophyll a:total phosphorus ratio indicated strengthened herbivore control. A high mass removal catch in relation to trophic state, low background turbidity, and bearable external loading favoured the successful biomanipulation, whereas intensive cyprinid reproduction, high nutrient loading and non-algal turbidity hindered the recovery. Three important issues should be noticed before biomanipulation in Finland: (1) careful selection of target lake, (2) well-planned, effective and long-lasting biomanipulation and (3) sustainable management of piscivores.

Introduction blooms are common, and in smaller, dystrophic lakes the biomass of Gonyostomum semen (Raph- Eutrophication is a severe problem in Finland idophyceae) is often high (Lepisto¨ et al., 1994). affecting over 2000 lakes (Tammi et al., 1999). In Biomanipulation by reducing cyprinid fish these lakes, the density of cyprinid fish is usually density can decrease the phytoplankton biomass high (Olin et al., 2002), the water is turbid, and the mainly by three routes: by reducing the availability biomass and thus grazing potential of herbivorous of suitable nutrients, by increasing the grazing zooplankton is low (Rask et al., 2002). High phy- pressure of large, herbivorous zooplankton toplankton biomass is likely the main factor (Daphnia sp.), and by reducing turbidity (e.g. reducing the recreational value of eutrophic Finnish Shapiro et al., 1975; Perrow et al., 1997; Hansson lakes. In the larger eutrophic lakes, cyanobacterial et al., 1998; Drenner & Hambright, 1999; Meijer 68 et al., 1999). If the concentrations and the cycling system (Mazumder, 1994; Meijer et al., 1999; of nutrients decrease, the phytoplankton commu- Sarvala et al., 2000). nity should shift towards species favouring less We hypothesise that if biomanipulation reduces eutrophic conditions, thus reducing the competi- the cyprinid biomass resulting in lower level of tive predominance of cyanobacteria. The grazing zooplanktivory, bioturbation and nutrient excre- effect is somewhat uncertain due to the poor edi- tion, the following responses should occur: (1) bility of cyanobacteria (de Bernardi & Giussani, expansion in fish, especially perch (Perca fluviatilis), 1990), and due to a possible expansion in zoo- fry production; (2) increase in the abundance of planktivory by fish fry or by invertebrate predators large cladocerans, inducing higher grazing pressure after the biomanipulation (Brabrand et al., 1986; and lower chlorophyll a:phosphorus ratio; (3) Romare & Bergman, 1999). Nevertheless, in some lower concentrations and/or slower cycling of studies small cladocerans can consume colonies of nutrients causing lower chlorophyll a:phosphorus cyanobacteria without a reduction in filtration rate, ratio; (4) decrease in algal, especially cyanobacterial which happens with large Daphnia sp. (de Bernardi biomass; and (5) increase in transparency. & Giussani, 1990). The reduction of water turbidity should favour macrophytes that hamper cyanobacteria by several mechanisms including reduc- Materials and methods tion of nutrient concentration and circulation, protection of herbivores, and production of allel- Study and control lakes opathic substances (Scheffer, 1998). In most studies, biomanipulation has been The ten study lakes were located in southern Fin- conducted in shallow lakes with high algal turbid- land (Fig. 1). One of the study lakes, A¨ ima¨ ja¨ rvi, ity and located in temperate regions of central consisted of two basins (A¨ 1andA¨ 2) and Hiidenvesi Europe and North America (Drenner & had four basins (H1–H4, Table 1) (Tallberg et al., Hambright, 1999). Biomanipulation in large or 1999; Rask et al., 2003). All the basins had different deep lakes with thermal stratification is less studied morphometric and water quality characteristics but the mechanisms are thought to be different and were treated as separate lakes in statistical from shallow lakes (Scheffer, 1998; Horppila et al., analyses. Hence, the study included 14 basins. All 1998). Compared to temperate regions, the effects the lakes were eutrophic, had a dense cyprinid fish of biomanipulation may differ in northern boreal stock (Olin et al., 2002), and suffered from high lakes having fewer species, shorter growing season algal biomass (Tallberg et al., 1999; Sammalkorpi, and stronger seasonality including long ice cover 2000; Rask et al., 2003). Cyanobacteria dominated and strong spring flow which brings nutrients and the phytoplankton community in eleven basins, but causes turbidity. In addition, it is not yet clear how in the three smallest lakes Gonyostomum semen the mass removal of cyprinids will affect the water (Raphidophyceae) was the most abundant species. quality in lakes with high background turbidity i.e. The area of the basins ranged from 0.15 to turbidity not due to algae. In southwestern Fin- 9.70 km2, and the mean depth from 0.9 to 11.2 m land, clay turbid lakes are very common: ca. 200 of (Table 1). Half of the basins had thermal stratifi- the lakes larger than 50 ha. Humic lakes are abun- cation in summer. Five basins were turbid mostly dant in the forested areas: ca. 1100 of lakes >50 ha due to algae. Seven basins had clay and algal tur- (Finnish Environment Institute, unpublished). bidity, and the two smallest lakes were mesohumic In this study, we explored how biomanipula- (summer average of water colour 40–71 mg Pt l)1) tion affected the biomass and community structure and had algal turbidity. Before the biomanipulation of fish, zooplankton and phytoplankton in ten of the lakes, the mean summer concentration of Finnish lakes, including both small and large lakes total phosphorus (TP) ranged from 32 to 107, total with high and low background turbidity. We also nitrogen (TN) from 578 to 1457, and chlorophyll a considered the changes in nutrient concentrations (chl a) from 16 to 83 lgl)1. Correspondingly, the and nutrient ratios. In addition, chlorophyll a.: Secchi depth varied between 0.5 and 2.5 m. nutrient ratios were examined because they readily In addition to biomanipulation, Pusulanja¨ rvi, reflect the changes in the functioning of the eco- Ena¨ ja¨ rvi and Tuusulanja¨ rvi were mixed artificially 69

Figure 1. The location of the study lakes. The size of the circles refers but to the size order of the lakes (not in relation to area). The lakes are numbered in the same size order than in Table 1. to improve their hypolimnetic oxygen conditions biomanipulation of L. Vesija¨ rvi (Horppila et al., during summer stagnation in 1999–2001, 1998– 1998), we set the target catch of mass removal to 1999 and 1998–2001, respectively. 200 kg ha)1 in three years. Because in some lakes The control lakes (Ctrl) were selected from the fishing could not be started efficiently in the first database of the Finnish Environment Institute. All year, a fourth fishing period was added in 2000– available lakes (16) with continuous monitoring in 2001. The mass removal of cyprinids was con- 1997–2001, and with location, size and nutrient ducted during 1997–2001 by motorised seining in concentration within the range of the study lakes the autumn or winter and fyke netting in the spring. were chosen (Table 1). The control lakes were not In seining, the shoals were first located by echo- biomanipulated but had a normal recreational sounder and then seined (Turunen et al., 1997; fishing. Only physical and chemical data were Sammalkorpi, 2000). If no fish aggregations ex- available in the control lakes. isted, fishing was cancelled. In Ena¨ ja¨ rvi, the mass removal started already in 1993 and the highest Weather conditions catches were caught before this study: 87, 72 and 88 kg ha)1 between the growing seasons in 1994– The weather conditions varied notably between 1995, 1995–1996 and 1996–1997, respectively. the years. The summers 1997, 1999 and 2001 were The weight of the mass removal catch (MRC) warm and quite dry, whereas the summer 2000 was was estimated from the number of specific barrels less warm and the summer of 1998 was cold and the weight of which was measured as full of fish. rainy (Fig. 2). However, the weather conditions in To explore the species distribution, subsamples of the beginning (1997) and in the end (2001) of the 10–30 kg were collected on each or every second research were quite similar enabling comparisons. fishing day from each basin. From the subsamples, Mass removal fish species were assorted, counted and weighed. Whether the target catch was achieved or not, the In the biomanipulated lakes of central Europe, the basins were divided into two groups: loC with )1 general aim is to reduce the fish biomass severely mass removal catch smaller than 200 kg ha and )1 (>75%) during the first year, and subsequent hiC having catch higher than 200 kg ha in three fishing is conducted only if necessary (Jeppesen years. Both lake groups included seven basins. et al., 1990; Meijer et al., 1999). In large and/or deep lakes it can take longer to attain substantial Experimental gillnetting reduction in fish biomass due to higher catch need and demands on the fishing technique (Horppila & Responses in the structure and abundance of fish Peltonen, 1994). On the basis of the successful assemblages were followed up by test fishing with Table 1. Lake characteristics 70

Lake Surface Mean Mixed/stratiﬁed Catchment Retention Sediment P load TP TN chl-a Secchi ) ) ) ) ) area (ha) depth (m) area (km2) time (d) type (kg ha 1 yr 1) (g l 1) (g l 1) (g l 1) depth (m)

Takaja¨ rvi 15 2.1 strat. 2.46 380 mud 9 43 767 31 1.2 Etuja¨ rvi 16 3.2 strat. 3.52 540 mud 2 37 728 27 1.2 Otalampi 31 3.3 strat. 1.44 998 sand <3 33 650 83 1.5 Rusutja¨ rvi 133 2 mix. 9.6 350 sand 2 52 978 34 0.8 Pusulanja¨ rvi 207 4.5 strat. 226 51 clay 14 45 640 – 1.2 Ena¨ ja¨ rvi 492 3.4 mix. 34 580 clay 3 104 1003 49 0.5 Tuusulanja¨ rvi 592 3.2 mix. 92 250 clay 8 107 1172 47 0.5 Lehija¨ rvi 704 6 strat. 83 1152 sand 3 32 578 16 2.5 A¨ ima¨ ja¨ rvi 852 2.6 mix. 93 330 mud 2 75 1061 53 1.0 A¨ima¨ja¨rvi, A¨1 370 2 mix. 53 120 mud 3 98 1457 82 0.7 A¨ima¨ja¨rvi, A¨2 480 3 mix. 40 450 mud 1 51 665 24 1.3 Hiidenvesi 2910 6.6 mix./strat. 934 270 clay 10 69 1035 25 0.7 Hiidenvesi, H1 160 0.9 mix. ––clay – 94 1084 35 0.5 Hiidenvesi, H2 260 2 mix. ––clay – 99 1272 – 0.6 Hiidenvesi, H3 360 2.6 mix. ––clay – 50 885 23 0.8 Hiidenvesi, H4 970 11.2 strat. ––clay – 33 900 16 1.0 Stora Lonoks 48 – – 48.2 – – – 82 864 – 0.5 Valkja¨ rvi Vitsjo¨ n 72 3.4 – 2.94 – – – 23 458 7 1.8 Vuorenselka¨ 92 – – – – – – 79 1054 107 0.8 Ka¨ lltra¨ sket 105 – – – – – – 36 436 25 1.9 Tjustra¨ sk 114 4.4 – 410.7 – – – 74 1000 33 0.7 Averia 138 3.2 – 232.1 – – – 80 1016 – 0.8 K. Pyha¨ ja¨ rvi 138 0.3 – – – – – 22 570 – 1.4 Viktra¨ sk 187 4.4 – 477.5 – – – 59 985 31 0.8 Tiila¨ a¨ nja¨ rvi 213 4.4 – 38.1 – – – 53 950 29 0.5 Sakara 231 – – 132.3 – – – 22 340 6 2.8 Kyta¨ ja¨ rvi 267 4.5 – 138.7 – – – 48 846 22 0.9 Humalja¨ rvi 429 4.0 – 11.7 – – – 32 404 13 0.9 Kernaalanja¨ rvi 446 – – – – – – 45 978 25 1.1 Nuijamaanja¨ rvi 528 – – – – – – 26 666 9 1.5 Punelia 819 – – 101.8 – – – 21 240 4 4.3 Vanajavesi* 1030 – – – – – – 58 1484 36 0.9

Nutrient and chlorophyll a concentrations and the Secchi depth are summer mean values in 0–2 m depth in 1992–1996, except in control lakes in 1997–2001. *The whole area of the lake is not included in the study. 71

(a)

(b)

Figure 2. a: Temperature in the study lakes in 1997–2001 including all observations and the moving average. b: The ﬂow (grey area) and the phosphorus (P, black circles) and nitrogen (N, white triangles) load of River Vantaa in 1997-2001 (Uusimaa Regional Environment Centre, unpublished). The observations of R. Vantaa reﬂect the changes in external loading of the study lakes.

NORDIC multimesh gillnets (Appelberg et al., Aspius aspius and burbot, Lota lota), and other fishes 1995; Olin et al., 2004) in years 1997–2001, except (mainly ruffe, Gymnocephalus cernuus;smelt, in Tuusulanja¨ rvi where the fishing years were 1996, Osmerus eperlanus; vendace, Coregonus albula and 1998–2001. The sampling procedure was random introduced peled white fish, Coregonus peled). The and stratified including bottom, surface and percentage of piscivores (piscivore%) consisted of mid-water gillnets (see Olin et al., 2002 for details). the combined proportion of >15 cm perch and Every basin was sampled from July to September otherpiscivoresfromthetotalcatch. 2–5 times per year. The sampling effort was adjusted to the area and depth and ranged from 6 to Water chemistry, zooplankton and phytoplankton 60 net nights per year per basin. The nets were set in evening for 12 h except in Lehija¨ rvi and The sampling location for water chemistry, zoo- A¨ ima¨ ja¨ rvi where fishing time was 12–16 h. plankton and phytoplankton monitoring was the The catch was assorted to species and then coun- deepest part of each basin. Samples for physical ted and weighted. Fish lengths were measured (TL, and chemical analyses were taken during growing 1 cm size-classes) and the weights of the length classes season (May–September) in 1997–2001 from the were calculated from length–weight curves from surface (0–2 m). The analyses of TP, TN and chl a separate samples (at least 70 specimens). The data was were conducted according to the Finnish standard combined into eight different species and size groups methods (SFS standard). The number of the including: cyprinids <10, 10–15 and >15 cm, perch observations per growing season ranged from 5 to <10, 10–15 and >15 cm; other piscivores (all sizes of 12, except for Hiidenvesi in 1997 where 1–3 TP pikeperch, Sander lucioperca;pike(Esox lucius); asp, and TN samples were collected. Secchi depth was 72 measured 5–17 times during growing season. The tion as a link function (McCullach & Nelder, 1989). control lakes had approximately the same range of Each basin was treated separately. The fixed vari- sampling effort and similar analysing methods ables in the model were year (1997–2001), depth compared to the study lakes. The results are given zone (2–10 levels) and fish group (8 levels). Water as month weighted averages of whole growing temperature (1 m depth) and transparency affect season (V–IX) or late summer (VII–VIII). fish activity and catchability (Hamley, 1975) and Zooplankton (3–6 samples per basin each year) were included as covariates in the model. In addi- was collected mainly in July–August in 1997 and tion, fishing time was included as a covariate in 2001. The samples (28–105 l) were taken with tube Lehija¨ rvi and A¨ ima¨ ja¨ rvi because of higher varia- samplers from the whole water column in shallow tion in fishing time. The best models were selected lakes or from surface water (0–4 m) in deep basins on the basis of deviance in relation to chi-square (see Tallberg et al., 1999; Rask et al., 2002, 2003 and the visual fit of residuals. Due to overdisper- for details). After filtration (50-lm net) and pres- sion, the parameter estimates were adjusted by ervation (formaldehyde 4%), the crustacean zoo- dispersion parameter given by Pearson’s chi-square plankton were identified, calculated and measured statistic divided by the degrees of freedom by microscopy (·20–80 magnification). The results (McCullach & Nelder, 1989). Wald chi-square test were transformed into carbon contents according was used for pairwise comparisons. Due to re- to Luokkanen (1995). peated measures, Bonferroni correction was used. Phytoplankton samples (5–10 per basin per The responses in the characters of fish groups, year) were taken during the growing season (May- zooplankton, phytoplankton, nutrients and trans- September) in 1997–2001 from 0 to 2 m depth with parency were analysed with Anova. The values a tube sampler and preserved with Lugol-solution before the measures (1997) were compared to (see Tallberg et al., 1999; Rask et al., 2002, 2003 values after the measures (2001). The lakes were for details). Phytoplankton taxa were identified divided into the loC and hiC groups (see above). In and cells counted by using an inverted microscope nutrients and transparency, the control lakes were technique (modified from Utermo¨ hl, 1958; see e.g. included as the third lake group. The model had Tikkanen & Wille´ n, 1992). The results were con- the variables year, lake group and their interac- verted to wet weight (Edler, 1979; Tikkanen & tion. The data was ln-transformed except for per- Wille´ n, 1992). Month weighed averages of V–IX cent values that were angular transformed and fish and VII–VIII were calculated. variables for which the means were produced by To evaluate the effect of zooplankton on algal the generalised linear model (see above). For Sec- biomass, the potential grazing pressure (PGP) was chi depth, nutrients and phytoplankton, V–IX and estimated (Jeppesen et al., 1994). PGP is based on VII–VIII comparisons were possible. For zoo- the assumption that herbivorous cladocerans can plankton and fish, only late summer values were consume their own body weight per day: PGP = available. Tukey’s test was used for pairwise ) cladoceran biomass (lgCl 1)/phytoplankton comparisons. Tuusulanja¨ rvi was excluded from ) biomass (lgCl 1) ·100. Wet weight of algal bio- the fish analyses because the first test fishing of ) mass (mg l 1) was converted to carbon contents Tuusulanja¨ rvi was done already in 1996 with dif- (lgCl)1) by equation: carbon contents = wet ferent methods inducing highly deviating results weight · 1000 · 0.2 · 0.4 (Behrendt, 1990). compared to later years. The zooplankton data included only six basins in loC group since no data Statistical analyses was available from Ena¨ ja¨ rvi in 2001. The dependences between the change of cypri- The gillnet data was highly negatively skewed and nid BPUE (from 1997 to 2001) and the change of the variances were similar to the means. Thus, to cladoceran biomass, between the cladoceran and explore the changes in the BPUEs of the fish phytoplankton biomasses, and between MRC and groups, we used a generalised linear model (PROC G. semen biomass were studied with linear regres- GENMOD, SAS Institute Inc., 1999) with sion. Correlation analyses was used for explaining assumption of Poisson distribution, and log-func- Secchi depth with chl a. 73

Results between-year variation in the gillnet catches was high in every basin (Fig. 3, Wald v2, p < 0.05, Mass removal catches except in A¨ 1 and H1, p<0.1 and p = ns, respectively). However, only in A¨ 2 and H2, the gillnet The target catch (200 kg ha)1 3yr)1) was attained BPUE decreased clearly (Wald v2, p < 0.05) after in seven basins of five lakes (Table 2). In 1997– high MRC and stayed at a lower level than in 1997. 2001, the highest MRC was obtained in Tu- The intended 75% reductions were not observed. ) usulanja¨ rvi, 472 kg ha 1 and the lowest in H1, In many basins, the gillnet BPUEs were higher in 44 kg ha)1. The proportion of cyprinids in the the warm summers (1997, 1999 and 2001) com- mass removal catch of the basins varied between pared to the colder ones (1998 and 2000). 63 and 97%. Roach (Rutilus rutilus) and bream When comparing the gillnet catches in the ba- (Abramis brama) were usually the most important sins with high or low MRC before (1997) and after removed species. In addition, small perch and (2001) the biomanipulation, the total BPUE in- bleak (Alburnus alburnus) were among the major creased in loC basins but decreased in the hiC ba- species in less eutrophic lakes. In Hiidenvesi, blue sins (Fig. 4, Anova, MRC*YEAR interaction: bream (Abramis ballerus) and smelt were among p<0.1). Similarly, the BPUE of large (>15 cm) the most important species after roach. and mid-sized (10–15 cm) cyprinids responded negatively in the hiC basins and positively in the Responses in fish communities loC basins (Anova, MRC*YEAR interaction: p<0.1 and ns, respectively). Lake Tuusulanja¨ rvi The arithmetic average of total BPUE in the study was clear exception among the hiC basins and was lakes ranged between 1.19 and 5.15 kg in 1997 and excluded from the analyses. The BPUE of small from 1.28 to 4.92 kg in 2001. The corresponding cyprinids increased in both lake groups though not values for the percentage of cyprinids were 31.5– significantly. In year 2001, the hiC basins contained 79.8 in 1997 and 51.7–79.8 in 2001 and the domi- more small cyprinids compared to the loC basins nance of cyprinids remained in the lakes (Fig. 3). (Tukey, p<0.1 and ns, respectively) as in 1997 the As analysed with the generalised linear model, the lake groups did not differ. The perch BPUE did not

Table 2. Mass removal catches between growing seasons during 1997–2001

Lake Mass removal catch (kg ha)1) Total Main species

1997–1998 1998–1999 1999–2000 2000–2001

Takaja¨ rvi* 37 60 96 54 295 Ro, Br Etuja¨ rvi* 50 59 72 75 348 Ro, Br, Pe Otalampi – 70 44 6 119 Ro Rusutja¨ rvi* 6 119 76 – 201 Ro, Br, Bl Pusulanja¨ rvi 36 44 53 41 182 Ro, Bl, Br Ena¨ ja¨ rvi 68 18 58 47 190 Ro, Br, Bl Tuusulanja¨ rvi* 190 83 131 63 472 Ro, Br Lehija¨ rvi 1 1 26 61 90 Ro, Bl, Pe A¨ ima¨ ja¨ rvi, A¨ 1* 23 47 80 106 257 Ro A¨ ima¨ ja¨ rvi, A¨ 2* 51 110 38 27 226 Ro Hiidenvesi, H1 11 24 6 3 44 Ro, Bb Hiidenvesi, H2* 61 65 147 138 411 Ro, Bb Hiidenvesi, H3 15 24 56 34 153 Sm, Ro Hiidenvesi, H4 42 23 29 34 121 Sm, Bl

The basins where the target catch (200 kg ha)1 3yr)1) was achieved are indicated with *. Total = catches during 1997–2001 (4 years) and also includes catches during growing seasons. Main species comprise at least 20 % of the total catch. Ro = roach, Br = bream, Pe = perch, Bl = bleak, Bb = blue bream, and Sm = smelt. 74

Figure 3. The late summer gillnet BPUEs of the different fish groups and mass removal catch (MRC) between the growing seasons in 1997–2001. a = loC basins and b = hiC basins; within the groups, basins are in order of increasing MRC. The BPUEs are Poisson means from the generalised linear models (best models, Appendix A). Error bars denote 95% confidence limits for the total catch. Some upper confidence limits are shown as numbers. Basin abbreviations: H1–H4 = basins of Hiidenvesi, Lehi = Lehija¨ rvi, Ota = Otalampi, Pus = Pusulanja¨ rvi, Ena¨ = Ena¨ ja¨ rvi, Rus = Rusutja¨ rvi, A¨ 1–A¨ 2 basins of A¨ ima¨ ja¨ rvi, Taka = Takaja¨ rvi, Etu = Etuja¨ rvi, and Tuu = Tuusulanja¨ rvi.

) increase in either lake group (Fig. 4). In Takaja¨ rvi lgCl 1 in 1997 and between 84 and 335 (mean ) and Etuja¨ rvi the perch BPUE decreased from 1997 200) lgCl 1 in 2001 (Fig. 5). The corresponding to 2001. The percentage of piscivores decreased values for cladocerans were 23 and 151 (mean 82) more in the loC than in the hiC lakes (Anova, lgCl)1 in 1997, and 13 and 212 (mean 100) MRC*YEAR interaction: p < 0.1). lgCl)1 in 2001. The relatively high proportions of Chydorus sp. in the cladocerans and cyclopoids Responses in zooplankton in the copepods were distinctive in the lake set. From 1997 to 2001 the total crustacean bio- The mean biomass of planktonic crustaceans var- mass increased in most of the basins but the dif- ied between 88 and 322 (mean of all lakes 190) ferences between the years or between the hiC and 75

0.8 Cyprinids Cyprinids Cyprinids <10 cm 2.0 10-15 cm 2.0 >15 cm 0.6 1.5 1.5

0.4 1.0 1.0

0.2 0.5 0.5

0.0 0.0 0.0 0.0 0.2 0.4 0.6 0.8 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

ter 0.4 Perch 0.6 Perch 0.5 <10 cm 10-15 cm cm 0.5 0.4 0.3

)or%af 0.4 0.3

-1 0.2 0.3 0.2 0.2 0.1 0.1 0.1 0.0 0.0 0.0 BPUE (kg net 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5

6.0 Total 35 Piscivore% 5.0 30 25 4.0 20 3.0 15 2.0 10 1.0 5 0.0 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0 5 10 15 20 25 30 35 -1 BPUE (kg net ) or % before Figure 4. The total BPUE, the BPUEs of the different fish groups and percentage of piscivores before (1997) and after (2001) the biomanipulation. Below the black line the response was negative and above the line it was positive. Open and closed circles are loC and hiC basins, respectively. Note that the scale differs between the fish groups.

Figure 5. Zooplankton biomass and potential grazing pressure (PGP) in 1997 and 2001. Closed circles denote for MRC between 1997 and 2001. a: loC basins and b: hiC basins. Abbreviations as in Figure 3. 76

Total 250 Cladocera 300 200

200 150 100 100 ter 50 0 0 0 100 200 300 0 50 100 150 200 250

60 Daphnia sp. 100 Bosmina sp. 60 Chydorus sp. 50 80 50 40 40 60 30 30 40 20 20 ), size (mm) or grazing (%) af

-1 10 20 10

gCl 0 0 0 µ 0204060 0 50 100 0204060

Cladoceran 150 PGP Biomass ( 0.6 median size 100 0.4

0.2 50

0.0 0 0.0 0.2 0.4 0.6 0 50 100 150

-1 Biomass (µ g C l ), size (mm) or grazing (%) before Figure 6. The characteristics of zooplankton before (1997) and after (2001) the biomanipulation. For further information see Figure 4. loC groups were not significant (Fig. 6). The total Changes in the BPUE of small and mid-sized biomass of Cladocerans increased from 1997 to cyprinids explained the responses in the zooplank- 2001 in both lake groups but not significantly. The ton biomass indicating high implication of plank- biomass of Daphnia sp. responded positively in five tivory. With increasing BPUE of <10 cm of seven hiC basins, but the response was not cyprinids, the biomass of cladocerans decreased significantly different from loC basins. The bio- from 1997 to 2001: change in cladoceran biomass of Bosmina sp. increased or remained the mass = 34.3 )199.8 · change in cyprinid BPUE same in all basins and no divergence between lake (r2 = 0.307; F = 4.427; p = 0.062). Similarly, groups occurred. The biomass of Chydorus sp. was when the BPUE of 10–15 cm cyprinids increased higher in the hiC basins than in the loC basins in the biomass of cladocerans decreased: change in both 1997 and 2001 (Tukey, p < 0.05 and <0.1, cladoceran biomass = 21.3–31.7 ( change in cyp- respectively). From 1997 to 2001, the average rinid BPUE (r2 = 0.262; F = 3.909; p = 0.074). biomass of Chydorus sp. decreased in hiC basins Rusutja¨ rvi was a clear outlier and not included in and increased in loC basins, yet not significantly. the first analysis. The changes in the BPUEs of large The median size of cladocerans was higher in the cyprinids or any other fish group did not signifi- loC basins compared to the hiC basins in both cantly affect the change of cladoceran biomass. years (Tukey, p < 0.01). The response of the cladoceran median size was on the average, but not Responses in nutrients significantly, positive in the loC basins while no responses took place in the hiC basins. The The mean summer TP in the basins varied between potential grazing pressure (PGP) mostly increased 23 and 111 lgl)1 (mean of all basins = 54 lgl)1) in both lake groups but not significantly. The in 1997 and between 22 and 102 (mean 58 lgl)1) highest increases were observed in Otalampi, in 2001 (Fig. 7). Generally, the warm years 1997, Lehija¨ rvi and Takaja¨ rvi. 1999 and 2001 had higher concentrations of TP 77

200 150 TP 180

125 160 ) 140 -1 -1 100 ha

120 g µg l 75 100 80

50 60 MRC (k 40 25 20 0 0

1600 TN 1400 1200 -1 1000 µg l 800 600 400 200 0

40 TN:TP 35 30

ratio 25 20

TN:TP 15 10 5 0 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 97 99 01 H1 Lehi Ota H4 H3 Pus Enä Rus Ä2 Ä1 Taka Etu H2 Tuu LoC HiC Figure 7. The summer averages of total phosphorus (TP), total nitrogen (TN) and TN:TP ratio in loC basins (white columns) and hiC basins (grey columns) in 1997–2001. Closed circles denote MRC between the growing seasons. In TN:TP, TN limits the phytoplankton production below the solid horizontal line, and TP is limiting above the dashed horizontal line. Between the lines both nutrients can be limiting. Abbreviations as in Figure 3. than 1998 and 2000. The range of average TN either nutrient can limit the production (Smith, concentration in 1997 was 603–1523 lgl)1 (mean 1982), but in the hiC basins, the ratio was closer to of all basins = 860), and in 2001, 498–1373 (mean the TN-limitation. 947 lgl)1). In 1997 and 1999, the TN concentra- In hiC group, both the summer average of TP tions were generally low. In both nutrients, the and TN concentration remained or decreased from average concentration increased but both mini- 1997 to 2001 (Fig. 8). H2, Tuusulanja¨ rvi and mum and maximum concentrations decreased Takaja¨ rvi in the hiC group had the highest from 1997 to 2001. In almost all basins, the TN:TP reductions in the TP concentration: (10 lgl)1.In was below the limit 29:1, which may indicate high loC and especially in control basins the concen- probability of the domination of cyanobacteria tration increased. The between-year responses (Smith, 1983). When concerning the nutrient lim- within the lake groups were, however, not signiﬁ- itation, the ratio ranged between the lines where cant. In 1997, the hiC group had higher TP and 78

150 TP, V-IX 150 TP, VII-VIII

100 100

50 50

)after

-1 0 0

µ 0 50 100 150 0 50 100 150

n( TN, V-IX TN, VII-VIII 1500 1500

Concentratio 1000 1000

500 500

0 0 0 500 1000 1500 0 500 1000 1500

Concentration (µ g l-1) before Figure 8. TP and TN before (1997) and after (2001) the biomanipulation. Both the growing season (V–IX) and late summer (VII–VIII) averages are shown. Cross symbols denote the control lakes. For further information see Figure 4.

TN concentration compared to control lakes The biomass of cyanobacteria reduced in the (Tukey, p < 0.05 for both) but in 2001 the dif- following summer after a high MRC (>100 kg ) ferences were no longer significant. ha 1) in both basins of A¨ ima¨ ja¨ rvi and in Tuusulanja¨ rvi (Fig. 10). In B2 of Hiidenvesi, the Responses in phytoplankton cyanobacterial biomass was low in the first year after large MRC but increased again in 2001. The average phytoplankton biomass in the study In Rusutja¨ rvi, no reduction of cyanobacteria basins ranged between 1.7 and 21.8 mg l)1 in 1997 occurred despite of the high MRC. Besides and between 1.6 and 14.8 mg l)1 in 2001 (Fig. 9). decreasing magnitude of the cyanobacterial bio- The corresponding values for cyanobacteria were mass, the duration of blooms shortened and shifted ) ) 0.1 and 13.8 mg l 1 in 1997 and 0.03 and 9.6 mg l 1 towards autumn in both basins of A¨ ima¨ ja¨ rvi and in in 2001. The share of cyanobacteria during the Tuusulanja¨ rvi. In addition, Ena¨ ja¨ rvi had the peak growing season varied between 4–62% in 1997 and of cyanobacteria only in late September 1997 after between 1–65% in 2001. In 1997, the major species the MRC of 88 kg ha)1, but the peak returned producing blooms were Aphanizomenon flos-aquae back to mid-summer in the following years. (n = 21 bloom observations from all lakes), In three smallest lakes, the biomass of G. semen Anabaena spiroides (n = 15) and A. circinalis was high in the summers following an effective fish (n = 11). Other species engaged in the blooms less removal (r2 = 0.371, F = 7.659, p = 0.016). than five times. In 2001, several dominating species From 1997 to 2001, the average of total algal were observed. Anabaena curva and Microcystis biomass decreased in hiC group but remained the aeruginosa had 14 and 10 observations of blooms, same or increased in loC group (Fig. 10). This respectively. Altogether eight species including effect was clear especially in late summer (VII– M. wesenbergii, M. flos-aquae, M. viridis, Anabaena VIII, Anova, MRC*YEAR interaction: p < 0.1). spiroides, A. flos-aquae, A. perturbata, A. mucosa The only diverging basin in the hiC group was H2, and Aphanizomenon yezoense were found 8–9 times where the algal biomass increased despite of the in bloom samples. high MRC. The cyanobacterial biomass decreased 79

100 100 H1 H3 Rus Taka 01 97 100 97 100 01 97 01 97 10

10 10 10 1 10 100 10 100 10 100 10 100

100 100 Lehi 97 Pus Ä2 Etu 01 100 01 97 10 97 01 97 10 10 ) 01 -1 µ (gl

a 1 1 10 1 10 100 10 100 10 100 10 100

100 100 Ota Enä Ä1 H2 Chlorophyll 97 100 01 97 100 97 01 01 10 01

1 10 10 10 10 100 10 100 10 100 10 100

H4 Ctrl Tuu 97

97 100 10 100 01 10 01

1 1 10 10 100 10 100 10 100 Total phosphorus (µ g l-1) Figure 9. Biomass of cyanobacteria (black area), Gonyostomum semen (white area) and other phytoplankton (grey area) in the study lakes during the growing seasons in 1997–2001. Note that the scale is either 0–50 or 0–25. In Tuusulanja¨ rvi, the highest observed biomass was 60 mg l)1 in 1997. Closed circles denote MRC between the growing seasons. to less than half in most of the hiC basins, though the line of large herbivores. Especially in the three the decrease within the group was not significant. smallest lakes, Takaja¨ rvi, Etuja¨ rvi and Otalampi, In 1997, the cyanobacterial biomass was higher in the ratio was high. From 1997 to 2001, the ratio the hiC than in the loC group (Tukey, p < 0.05), moved closer to the line of large herbivores in five whereas no significant difference occurred in 2001. hiC basins (Fig. 11). In Etuja¨ rvi and H2, the ratio The percentage of cyanobacteria decreased in both did not change. In Rusutja¨ rvi, the ratio moved lake groups, yet not significantly. towards the line of large herbivores in 1997–1999 but was partly restored in the later years. In loC Responses in the chl a:TP ratio basins the ratio decreased in two basins, H3 and Otalampi. In Ena¨ ja¨ rvi, the ratio was already close When comparing the chl a:TP ratio to the regres- to the line of large herbivores in 1997, increased in sion lines of Mazumder (1994), the ratio was, in all 1998 and gradually returned after that. In control basins, closer to the line of small herbivores than to lakes, no clear trends could be seen except for the 80

50 H1 50 Rusutjärvi 200

40 40 150 30 30 100 20 20 10 10 50 0 0 0

25 Lehijärvi 25 Ä2 200 20 20 150 15 15 100 10 10 5 5 50 0 0 0

25 Otalampi 50 Ä1 200 20 40 150 15 30 100 10 20 5 10 50 0 0 0 )

25 25 Takajärvi 200 )

-1 H4 20 20 -1

150 ha

15 15 g 100 ( 10 10 50 5 5 MRC k Biomass (mg l 0 0 0

50 H3 25 Etujärvi 200 40 20 150 30 15 100 20 10 10 5 50 0 0 0

25 Pusulanjärvi 50 H2 200 20 40 150 15 30 100 10 20 5 10 50 0 0 0

50 Enäjärvi 50 60 Tuusulanjärvi 200 40 40 150 30 30 100 20 20 10 10 50 0 0 0 1997 1998 1999 2000 2001 1997 1998 1999 2000 2001 Figure 10. Total phytoplankton biomass (Total), biomass of cyanobacteria (Cyano) and the share of cyanobacteria (Cyano %) before (1997) and after (2001) the biomanipulation. Both the growing season (V–IX) and late summer (VII–VIII) averages are shown. Note that the scale diﬀers. For further information see Figure 4. 81

25 Total, V-IX 40 Total, VII-VIII

20 30 15 20 10 5 10 0 0

ter 0 5 10 15 20 25 0 10203040

age af 15 Cyano, V-IX Cyano, VII-VIII 30 10 20 ) or percent -1 5 10

0 0 0 5 10 15 0102030 Biomass (mg l Cyano %, V-IX Cyano %, VII-VIII 60 80

60 40 40 20 20

0 0 0204060 0 20406080 Biomass (mg l-1) or percentage before Figure 11. The chl a:TP ratio in the study and control lakes (ctrl) in 1997–2001. Basins are in order of increasing MRC from up to down. The dotted and dashed lines represent the regression lines of small or large herbivores for mixed or stratified lakes (Mazumder, 1994). most eutrophic control lake, where the ratio de- Secchi depth was dependent on the chl a con- creased. centration in six basins: Otalampi, Rusutja¨ rvi, En- a¨ ja¨ rvi, Lehija¨ rvi, A¨ 1andA¨ 2 (correlation coefficients: )0.603, )0.728, )0.526, )0.821, )0.622 and )0.652, Responses in the Secchi depth respectively; p < 0.001 in all cases). In other basins, Secchi depth was not significantly related to chl a. The average summer Secchi depth ranged from 0.3 to 2.3 (mean of all lakes 1.1) m in 1997 and from 0.4 to 2.5 (mean 1.1) m in 2001. Increased Secchi Discussion depth after high MRC occurred only in H2 and Tuusulanja¨ rvi and these changes were only tem- Fish community porary (Fig. 12). No MRC-induced clear water phases were observed in early summer. The Secchi The gillnet catches of this study indicated that the depth did not differ between loC and hiC groups structure of the fish community changed from the or between 1997 and 2001. dominance of relatively large cyprinids to an 82

H1 1.0 2.0 Rusutjärvi 200 0.8 1.5 150 0.6 1.0 100 0.4 0.2 0.5 50 0.0 0.0 0

6.0 Lehijärvi 3.0 Ä2 200 5.0 150 4.0 2.0 3.0 100 2.0 1.0 50 1.0 0.0 0.0 0

5.0 Otalampi Ä1 200 1.5 4.0 150 3.0 1.0 100 2.0 0.5 1.0 50 0.0 0.0 0

1.5 H4 2.0 Takajärvi 200 ) -1

1.5 150 ha 1.0 g k 1.0 100 ( 0.5 0.5 50 MRC

Secchi depth (m) 0.0 0.0 0

1.2 H3 Etujärvi 200 2.0 1.0 150 0.8 1.5 0.6 1.0 100 0.4 50 0.2 0.5 0.0 0.0 0

2.0 Pusulanjärvi 1.0 H2 200 1.5 150 1.0 0.5 100 0.5 50 0.0 0.0 0

Enäjärvi Tuusulanjärvi 200 2.0 1.0 150 1.5 100 1.0 0.5 0.5 50 0.0 0.0 0 Figure 12. The Secchi depth in the study lakes during the growing seasons in 1997–2001. Black cross-lines are the summer averages (V–IX). Closed circles denote MRC between the growing seasons. explosion of small-sized cyprinids and a higher decreasing mean size of cyprinids increase the po- proportion of piscivores. Both the smaller biomass tential for predator control. The role of the fish and the changed fish assemblage may have wide- community in the nutrient balance of the lakes can ranging effects on the ecosystem functioning (Tol- shift from ‘nutrient recycler’ to ‘nutrient storage’ onen et al., 2000). Since the piscivores are often due to several mechanisms. First, given that the gape-limited (e.g. Kitchell et al., 1994), the large cyprinids are benthivorous (Vinni et al., 83

2000), their smaller biomass denotes lower Only one hiC basin, A¨ 2 had no expansion in small bioturbation and release of nutrients from the bot- cyprinids. It is not clear whether this was due to tom sediment (e.g. Ta´ trai & Istvanovits, 1986; decreased spawning stock, low external loading or Horppila & Kairesalo, 1990). Second, during rapid some other reason. The expansion of cyprinid fry growth, juvenile fish excrete mainly nitrogen and may hinder or delay the effects of biomanipulation can be an important storage of phosphorus (Kraft, (Meijer et al., 1995; Romare & Bergman, 1999). 1991). As the proportion of juvenile fish and the individual growth rate of cyprinids increased in the Zooplankton community effectively biomanipulated study lakes (Rask et al., in press), the fish induced P cycling has probably As the responses in zooplankton biomass and in decreased. Thirdly, the nutrients bound to fish tissue the potential grazing pressure were positive in were withdrawn from the cycling through the MRC many basins, increased grazing may also have catch. Assuming that ca. 0.8% of cyprinid wet contributed to the decreased phytoplankton bio- weight is phosphorus (Schreckenbach et al., 2001), mass. Also, the observed reduction in chl a:TP ra- the study basins with the highest loss of phosphorus tio in late summer can indicate the strengthening of per volume were H2, Tuusulanja¨ rvi and Takaja¨ rvi: the herbivore control (Mazumder, 1994; Sarvala 164, 118, and 112 lgl)1, respectively, during 1997– et al., 2000; Jeppesen & Sammalkorpi, 2002). Even 2001. The same basins had the highest reductions in though zooplankton would not directly regulate TP concentrations: )38, )23 and )10 lgl)1, cyanobacterial biomass, the role of grazing be- respectively. This indicates that, in some cases, it comes more important if the phytoplankton com- may be possible to reduce the TP concentration by munity shifts towards other algal groups due to biomanipulation if the loss of fish-bound phos- changes in nutrient circulation or transparency. phorus is high enough (P100 lgl)1) compared to Although the response in the cladoceran biomass the volume of the lake. However, as P-concentra- was modest, the productivity of cladocerans tion in the water of a shallow lake can highly be probably has increased due to higher proportion of dependent on sediment–water interactions edible algae and lower abundance of harmful cy- (Søndergaard et al., 2003), the release from the anobacteria. This is supported by the fact that the sediment should have been restricted, as well. cladoceran biomass had a positive response in Meijer et al. (1999) found no success of bi- some lakes even though the abundance of small omanipulation in lakes where fish reduction was cyprinids and thus zooplanktivory increased. Fur- lower than 75% of the original biomass. We did not thermore, the egg ratio of Daphnia sp. increased in find such reductions according to our gillnet cat- five hiC basins and in three loC basins, indicating ches. Our moderate decrements in BPUE can, higher productivity per se or better survival of however, indicate much higher changes in actual gravid females (Horppila & Rask, unpublished). fish biomass since the relatively low changes in The grazing pressure may thus have increased cyprinid BPUE induced high responses in the cla- without a notable response in cladoceran biomass. doceran biomass. Olin et al. (2004) demonstrated that, in lakes with high fish abundance, the catch- Phytoplankton community ability of gillnets strongly decreases due to fish accumulation. When the fish population is thinned In this study, the most evident positive response to out, a decrease in the gillnet catch is not compa- biomanipulation was the weakening of cyanobac- rable with the real fish reduction if the catchability terial blooms. The biomass of cyanobacteria of gillnets increases simultaneously. Thus, it is dif- decreased, and the duration of the blooms short- ficult to say how much the original fish biomass has ened and shifted towards the autumn. All these decreased in the study lakes. Instead, the increased responses were crucial for the recreational use of gillnet BPUE even in some heavily harvested lakes the lakes. Decreased amount of available nutrients indicates high fish production potential of the is a possible explanation for the depressing of eutrophic Finnish lakes. The removal of cyanobacterial dominance. The total nutrient 100 kg fish ha)1 yr)1 can be compensated within 1 concentrations did not clearly decrease except for or 2 years after the biomanipulation has ceased. the formerly mentioned three basins with high loss 84 of fish-bound phosphorus. However, the TP con- sonable to bind the target catch into the original centration does not necessarily give a good esti- cyprinid biomass. The mass removal target catch mate of the nutrients available for phytoplankton can be roughly estimated from the BPUE of the (e.g. Scheffer, 1998). Instead, the late summer Nordic gillnet. Based on the MRC in the lakes where reduction of chl a:TP ratio in most of the hiC cyprinids decreased and the gillnet BPUE of cypri- basins can indicate decreased or changed nutrient nids in 1997, each kg of cyprinid BPUE should mean cycling due to increased sedimentation, reduced a removal of at least 100 kg/ha. If the original fish resuspension or harder competition for nutrients biomass cannot be determined, the target catch can (Meijer et al., 1999; Sarvala et al., 2000). The de- also be estimated from TP concentration. Jeppesen lay of the cyanobacterial biomass peak may reflect & Sammalkorpi (2002) calculated a catch need for nutrient shortage that is not replenished until the European lakes: catch need (kg ha)1 yr)1)= fall turnover. The colonisation of macrophytes can 16.9 · TP0.52. When comparing to this catch need, be one reason for the changes in nutrient cycling the target catch was achieved (at least during 1 year) (Scheffer, 1998) though we have a direct observa- in five of our basins: Takaja¨ rvi, Rusutja¨ rvi, Tu- tion only from L. Tuusulanja¨ rvi (Venetvaara usulanja¨ rvi, A¨ 2 and H2. As in four of these basins et al., 2003). However, as macrophytes bind the gillnet BPUE of large cyprinids decreased, the nitrogen effectively (Van Donk et al., 1993), their equation of Jeppesen & Sammalkorpi (2002) invasion often cause a strong reduction in TN seemed to predict the target catch reasonably well. (Scheffer, 1998; Meijer et al., 1999) such as we A low or moderate external loading is generally observed in some basins. The late summer reduc- considered as a prerequisite for success in bioma- tion in cyanobacteria took place in spite of the nipulation (Scheffer, 1998; Hansson et al., 1998). decrease of TN:TP ratio, which should increase the This is supported by our results too although the competitive advantage of cyanobacteria (Smith, rates of external loading, as well as the nutrient 1983). According to the studies of Jeppesen et al. concentrations, were low compared to the bioma- (1998a, b), decreasing TN or low TN:TP ratio does nipulated lakes in the central Europe (e.g. Jeppesen not necessarily promote cyanobacteria. et al., 1999; Gulati & van Donk, 2002). Of the seven The high chl a:TP ratio in the three smallest lakes cases with reduced phytoplankton biomass, five was likely due to the high biomass of Gonyostomum had an external loading lower than or close to the semen, which is capable of retrieving nutrients from critical level, as defined by Vollenweider (1976). In hypolimnion (Eloranta & Ra¨ ike, 1995). The re- the other too basins Takaja¨ rvi and Tuusulanja¨ rvi, sponse of G. semen to biomanipulation seemed to be the exceptionally high mass removal catch possibly positive. The alga might get a competitive advan- enabled the reduction in the phytoplankton bio- tage if the biomass of cyanobacteria is reduced but mass in spite of the high external loading. the grazing pressure remains low. This may explain According to Scheffer (1998) high background the increase of G. semen in humic lake Etuja¨ rvi. In turbidity can reduce the potential for successful Otalampi, the reduction in the biomass of G. semen biomanipulation. In our study, all the five basins can be explained by the increase in cladoceran with no clay turbidity or with low concentration of biomass. In addition, the increased transparency humic substances had reduction in the phyto- might have some negative effects on the light- plankton biomass. In spite of high background avoiding G. semen (Eloranta & Ra¨ ike, 1995). turbidity, Tuusulanja¨ rvi and Takaja¨ rvi had a negative response in phytoplankton biomass Factors enabling or restricting the responses to probably because of the effective biomanipulation. biomanipulation Artificial mixing during summer stagnation may have affected the responses in three lakes. Both the High mass removal catch was likely the reason for biomanipulation and mixing can reduce the internal the positive responses in the basins where the target loading and depress the cyanobacteria and sepa- catch was achieved. In addition in Otalampi, the rating the effects of these measures is difficult. The target catch was not attained but the original mixing was most efficient in Tuusulanja¨ rvi pre- amount of cyprinids seemed to be low enough to venting the thermal stratification during the grow- enable the reduction in cyprinid biomass. It is rea- ing season. This might have averted the blooms of 85

Microcystis sp. (Gulati & van Donk, 2002) and fa- but, against our hypothesis, mainly for roach and voured other cyanobacteria as the blooms dimin- to lesser extent for perch. No clear responses in the ished and consisted of several genera. The mixing size of cladocerans were detected, but the response also kept the bottom sediment oxygenated appeared as increased biomasses. Lower chloro- restraining the dissolving of phosphorus. However, phyll a : phosphorus ratios were recorded in most the mixing can also increase the shear stress, tem- effectively manipulated lakes and in a few cases perature and pH near the bottom sediment which all also some decrease in P concentrations but not in can enhance the release of phosphorus from the water turbidity. Algal biomasses and especially sediment (see Scheffer, 1998). Thus, artificial mixing those of cyanobacteria decreased in some lakes as might also have negative effects on the water qual- supposed. The delay in the timing of cyanobacte- ity, which may have affected the situation in Pus- rial blooms was one of the most striking responses. ulanja¨ rvi and Ena¨ ja¨ rvi. When these three lakes were Altogether, the responses were fairly slight, excluded from the analyses, the main results still which apparently is a sum of several factors, like remained: the average of total phytoplankton bio- differences in background turbidity due to clay or mass decreased in the hiC basins and increased in humic substances, short growing season, long the loC basins so that the biomass was higher in hiC lasting ice cover, and strong spring flow due to basins in 1997 (Tukey, p < 0.01) but not in 2001. snow melting and extensive network of drainage The sustainability of the effects of biomanipu- ditches affecting the nutrients and turbidity lation in our study lakes is questionable. This is (Kortelainen & Saukkonen, 1998). Furthermore, mainly due to lack of substantial increases in liberal fishing policy, enabling expansive recrea- Secchi depth, which are considered crucial for the tional gillnet fishing and angling creates high fish- longevity of responses. With increased transpar- ing pressure on piscivores, thus supporting the ency, macrophytes can invade the lake enabling dominance of cyprinids. Despite of these restric- the alternative stable state characterised by clear tions, biomanipulation can be successful in the water and low algal biomass (Scheffer, 1998). boreal lake types we examined in this study. The Missing positive response of perch could be target lakes of biomanipulation should be carefully explained besides by cyprinid fry expansion, also selected, the mass removal of fish should be well- by the remaining turbid conditions with sparse planned, effective and long-lasting. The restoration macrophyte stands, which make perch fry ineffec- programme should also include restrictions in tive zooplanktivores as compared to cyprinids fishing of piscivores and even additional stocking (Winfield, 1986; Diehl, 1988). The recovery of to support their populations. Water protection perch and pike, which prey on small cyprinids, is measures in the catchment area to reduce the load important for long-lasting results. Thus it seems of nutrients and solid matter must not be forgotten. that by 2001, none of the study lakes reached the alternative, macrophyte-dominated stable state, if Acknowledgements such exists in Finnish lakes (Sarvala et al., 2000). Instead, signs of fading recovery were observed in We thank all the collectors and analysers of water some basins in 2001 concerning e.g. fish abun- and phytoplankton samples. Special thanks to dance and nutrient concentration. Furthermore, excellent gillnetting crew: Kari Puranen, Anssi the weather conditions in the study period might Toivonen and Sami Vesala. This work was sup- have boosted the eutrophication. In the colder and ported by the involved organizations and by the rainy years, the inflows brought a lot of nutrients Biological Interactions Graduate School. in the lakes partly enabling the high production during the next, warm summer.

Concluding remarks References

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Appendix A

Table A.1. Best models of the generalised linear modelling of gillnet data

Lake Source df v2 p Lake Source df v2 p

H1 Year 4 8.58 0.1450 Rusutja¨ rvi Year 4 6.97 0.2748 H1 Zone 1 2.18 0.2792 Rusutja¨ rvi Zone 1 14.16 0.0004 H1 Year · zone 4 11.63 0.0406 Rusutja¨ rvi Year · zone 4 8.43 0.1542 H1 Fishes 7 246.18 0.0002 Rusutja¨ rvi Fishes 7 433.52 0.0002 H1 Year · fishes 28 35.05 0.3368 Rusutja¨ rvi Year · fishes 28 281.94 0.0002 H1 Zone · fishes 7 23.93 0.0024 Rusutja¨ rvi Zone · fishes 7 25.25 0.0014 Rusutja¨ rvi Temp. 1 10.33 0.0026 Continued on p. 88 88

Table A.1. (Continued)

Lake Source df v2 p Lake Source df v2 p

Lehija¨ rvi Year 4 10.83 0.1425 A¨ 2 Year 4 24.68 0.0003 Lehija¨ rvi Zone 7 160.94 0.0005 A¨ 2 Zone2 4 39.98 0.0003 Lehija¨ rvi Year · zone 28 110.76 0.0005 A¨ 2 Fishes 7 493.84 0.0003 Lehija¨ rvi Fishes 7 33.61 0.0005 A¨ 2 Year · fishes 28 70.66 0.0003 Lehija¨ rvi Year · fishes 28 89.02 0.0005 A¨ 2 Zone2 · fishes 28 162.03 0.0003 Lehija¨ rvi Zone · fishes 49 318.84 0.0005 A¨ 2 Temp. 1 3.85 0.1494 Lehija¨ rvi Temp. 1 2.66 0.5145 A¨ 2 Secchi 1 3.09 0.2364 Lehija¨ rvi Temp. · fishes 7 28.30 0.0010 A¨ 2 Time 1 6.96 0.0249

Otalampi Year 4 5.16 0.5416 A¨ 1 Year 4 9.41 0.1032 Otalampi Zone 2 23.70 0.0002 A¨ 1 Fishes 7 38.81 0.0002 Otalampi Year · zone 8 23.33 0.0060 A¨ 1 Year · fishes 28 43.29 0.0652 Otalampi Fishes 6 105.83 0.0002 A¨ 1 Temp. 1 0.79 0.7470 Otalampi Year · fishes 24 43.76 0.0162 A¨ 1 Temp. · fishes 7 10.38 0.3360 Otalampi Zone · fishes 12 19.44 0.1568 A¨ 1 Time 1 3.64 0.1126

H4 Year 4 31.81 0.0004 Takaja¨ rvi Year 4 27.16 0.0002 H4 Zone 6 250.22 0.0004 Takaja¨ rvi Zone 2 3.85 0.2920 H4 Year · zone 24 50.10 0.0056 Takaja¨ rvi Year · zone 8 21.47 0.0120 H4 Fishes 7 130.82 0.0004 Takaja¨ rvi Fishes 7 52.76 0.0002 H4 Year · fishes 28 75.53 0.0004 Takaja¨ rvi Year · fishes 28 107.29 0.0002 H4 Zone · fishes 42 235.71 0.0004 Takaja¨ rvi Zone · fishes 14 43.55 0.0002

H3 Year 4 21.21 0.0006 Etuja¨ rvi Year 4 286.50 0.0002 H3 Zone 2 33.60 0.0002 Etuja¨ rvi Zone 2 19.24 0.0002 H3 Year · zone 8 24.39 0.0040 Etuja¨ rvi Year · zone 8 53.63 0.0002 H3 Fishes 7 171.63 0.0002 Etuja¨ rvi Fishes 6 131.71 0.0002 H3 Year · fishes 28 97.07 0.0002 Etuja¨ rvi Year · fishes 23 5084.02 0.0002 H3 Zone · fishes 14 53.36 0.0002 Etuja¨ rvi Zone · fishes 12 38.66 0.0002

Pusulanja¨ rvi Year 4 8.69 0.2768 H2 Year 4 7.52 0.2218 Pusulanja¨ rvi Zone 5 277.15 0.0004 H2 Zone 1 7.10 0.0154 Pusulanja¨ rvi Year · zone 20 81.00 0.0004 H2 Year · zone 4 16.91 0.0040 Pusulanja¨ rvi Fishes 7 584.73 0.0004 H2 Fishes 7 446.94 0.0002 Pusulanja¨ rvi Year · fishes 28 231.27 0.0004 H2 Year · fishes 28 58.88 0.0012 Pusulanja¨ rvi Zone · fishes 35 172.69 0.0004 H2 Zone · fishes 7 54.97 0.0002 Pusulanja¨ rvi Temp. 1 3.58 0.2344 H2 Temp. 1 7.93 0.0098

Ena¨ ja¨ rvi Year 4 46.38 0.0005 Tuusulanja¨ rvi Year 4 82.02 0.0005 Ena¨ ja¨ rvi Zone 2 162.32 0.0005 Tuusulanja¨ rvi Zone 3 107.68 0.0005 Ena¨ ja¨ rvi Year · zone 8 56.92 0.0005 Tuusulanja¨ rvi Year · zone 12 176.33 0.0005 Ena¨ ja¨ rvi Fishes 7 753.88 0.0005 Tuusulanja¨ rvi Fishes 7 25.79 0.0025 Ena¨ ja¨ rvi Year · fishes 28 137.64 0.0005 Tuusulanja¨ rvi Year · fishes 28 149.91 0.0005 Ena¨ ja¨ rvi Zone · fishes 14 240.63 0.0005 Tuusulanja¨ rvi Zone · fishes 21 214.70 0.0005 Ena¨ ja¨ rvi Temp. 1 7.96 0.0240 Tuusulanja¨ rvi Temp. 1 5.03 0.1245 Tuusulanja¨ rvi Temp. · fishes 7 19.28 0.0370

Basins are in the order of increasing MRC. Year = year of gillnetting, zone = depth zone, fishes = fish group, temp. = water temperature, Secchi = Secchi depth, time = fishing time.