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Invertebrate Occurence in Relation to Water Permanence and Fish in Shallow Wetlands at Öland, Sweden Jennie Niesel Biology 160 p Examination Project Work in Biology 20 points for Master of Science Förord Det här examensarbetet har skrivits i samarbete med LIFE-projektet Strandängar och våtmarker i det öländska odlingslandskapet. Projektets mål är att arealen välhävdade, fuktiga och våta miljöer i det öländska odlingslandskapet ska öka. Författaren är ensam ansvarig för innehållet i rapporten, ståndpunkterna behöver alltså inte representera länsstyrelsens eller högskolans synsätt. Annigun Wedin Projektledare för LIFE-projektet Strandängar och våtmarker i det öländska odlingslandskapet. Invertebrate Occurrence in Relation to Water Permanence and Fish in Shallow Wetlands at Öland, Sweden Jennie Niesel Biology 160 p Examination Project Work in Biology: 20 points for Master of Science Supervisor: Börje Ekstam, Ph D Senior Lecturer in Biology Department of Biology and Environmental Science, University of Kalmar Examiner: Lars-Eric Persson, Assoc. Prof. Senior Lecturer in Biology Department of Biology and Environmental Science, University of Kalmar Abstract: Activity traps were used to study the effects of water permanence and fish predation on the ratio of invertebrate predator and prey species and overall invertebrate abundance in June 2002 in eight shallow calcareous lakes and two small ephemeral water bodies at Öland, southeastern Sweden. The invertebrate predator prey ratio was expected to decrease with decreasing water permanence since invertebrate predators are highly sensitive to drying. Fish was not expected to affect the ratio, since fish selectively feed on large prey species with no care taken to whether the prey is an invertebrate predator or not. The total abundance of invertebrates was expected to decrease both with decreasing water permanence and presence of fish because of drying mortality and selective predation by fish on large invertebrates. Parametric analyses of covariance (ANCOVA) on the lake data revealed no significant effect of fish or maximum depth (used as a measure of permanence) on the ratio of invertebrate predators and prey in the studied wetlands. Fish significantly negatively affected the invertebrate abundance and the mean abundance was twice as high in fishless wetlands as in wetlands with fish. The significant fish effect is however dependent on only one value and is therefore no longer significant when the interaction variable is excluded in the analysis. Maximum depth did not significantly affect the abundance even if the relationship was close to significant. Since both maximum depth and the interaction variable were close to significant, increased replication of fishless wetlands might produce significant effects of these variables on the invertebrate abundance. Although tench (Tinca tinca) is an effective benthic forager, the overall invertebrate abundance was not more affected in the one local with tench than it was in the other wetlands with northern pike (Esox lucius). This indicates that one cause of the decrease in waterfowl density and diversity observed during the 20th century might be found in the benthic invertebrate community (e.g. chironomids and others). Sammanfattning: Effekten av vattenbeständighet (maxdjup) och fiskpredation på kvoten evertebrata (ryggradslösa) predatorer och bytesdjur samt totala abundansen evertebrater studerades med hjälp av aktivitetsfällor i åtta grunda kalkrika sjöar och två små temporära vattenområden i juni 2002 på Öland. Kvoten förväntades att minska med minskande maxdjup (vattenbeständighet) eftersom evertebrata rovdjur är mycket känsliga för uttorkning. Kvoten förväntades vara i princip opåverkad av fisk eftersom fisk selektivt äter stora bytesdjur utan hänsyn till ifall bytet är en evertebrat predator eller inte. Slutligen förväntades abundansen evertebrater att minska både med minskande maxdjup och närvaro av fisk på grund av dödlighet beroende på uttorkning och predation av fisk på både evertebrata predatorer och bytesdjur. Covariansanalys (ANCOVA) visade ingen effekt av vare sig maxdjup eller fiskpredation på kvoten. Däremot fanns en signifikant negativ effekt av fisk på abundansen och medelabundansen i fisktomma våtmarker var mer än dubbelt så hög som i våtmarker med fisk. Den signifikanta fiskeffekten var dock beroende av ett enda värde och försvann då interaktionsvariabeln togs bort ur analysen. Maxdjup hade ingen signifikant effekt på abundansen, trots att det var nära. Med ökad replikering av fisktomma våtmarker kan förmodligen effekterna av både fisk, maxdjup och interaktionsvariabeln bli signifikanta. Även om sutare (Tinca tinca) anses vara en effektiv bottenfuragerare så var den totala abundansen evertebrater inte nämnvärt mindre i lokalen med sutare än i lokalerna med gädda (Esox lucius), trots att snäckor saknades helt i fällfångsterna. Detta betyder att en orsak till nedgången i täthet och diversitet hos bl.a. sjöfågel som observerats under 1900-talet kanske kan hittas bland sedimentlevande bottenfauna som t.ex. chironomider (fjädermygglarver). 2 Introduction The occurrence of many free-living freshwater animals in lentic (i.e. standing) freshwater habitats generally depends on habitat permanence or predator distribution, in its turn affected by habitat permanence (Wellborn et al. 1996). This permanence-dependent distribution of freshwater species may create distinct communities along a gradient from small temporary bodies of water to large and permanent lakes. Among the most important factors influencing organisms living in temporary habitats is the length of the aquatic phase (Williams 1997), how the water disappears and if this pattern of disappearance is predictable or not (Collinson et al. 1995). Predators like fish and predatory invertebrates are very sensitive to drying because of a slower developmental rate to adult stage compared to their invertebrate prey (Wellborn et al. 1996). Temporary habitats are therefore often characterised by few predators, which instead are limited to deeper, more permanent habitats (Collinson et al. 1995; Wellborn et al. 1996; Williams 1997). Predation in temporary waters is mainly by invading invertebrates from other places (Macan 1977). The foraging behaviour of animals is often affected by the need to avoid predators (Begon et al. 1996). Many animals rely on movement when foraging. Increased foraging activity increases the risk of detection, causing potential prey species that coexist with fish having lower activity levels than those living in habitats without fish (Wellborn et al. 1996). Fish, especially cyprinids, is well known to greatly affect the densities of invertebrates (Andersson 1981; Gilinsky 1984; Giles 1991; Brönmark 1994; Wagner 1997; Wagner & Hansson 1998; Svensson et al. 1999). Mainly large invertebrates are subject to fish predation. This is caused by (1) the majority of fishes are larger relative their invertebrate prey, making few invertebrates large or fast enough to avoid predation by fish and (2) size selectivity of fish on the potential prey available to consume, with larger species probably favouring maximum energy intake (Wellborn et. al 1996). However, in order to isolate the effect of fish presence from other factors on invertebrate organisms, the effect of fish on invertebrate prey must be compared at various places along the permanence gradient (Wellborn et al. 1996), but those studies found that hitherto deal with this (Pearman 1995; Smith 1983; Roth & Jackson 1987; Skelly 1995; Smith & Van Buskirk 1995; Corti et al. 1997) are conducted in other systems than those in this study. Since the year of 2000, the County Administrative Board of the Kalmar County (abbreviated from now on as CABK) runs a nature conservation project concerning restoration of coastal meadows and wetlands in Natura 2000 areas on Öland, an Island situated in southeastern Sweden. This project is to a large extent financed by the LIFE-Nature fund of the European Commission. One important aim is to improve the present unclear preservation status of these habitats and the plant- and animal species (mainly birds) dependent upon them (CABK 2001). The knowledge of the invertebrate fauna in lentic wetlands on Öland is poor (see however Franzén 1996, 1998). Available invertebrates are important food for aquatic birds (e.g. Danell & Sjöberg 1982; Blindow et al. 1991; Giles 1991; Elmberg et al. 1993, 1994; Wagner 1997; Wagner & Hansson 1998). With this background, research on invertebrate-eating aquatic birds in relation to occurrence of available invertebrate food and the common phenomenon of summer drought is needed in order to maintain these habitats at a favourable condition or restore them to favourable sites for species listed in the bird (EEC 1979) and habitat directive (EEC 1992) respectively. In a second report, I therefore intend to interpret my results in relation to occurrence of invertebrate-eating ducks that were found in these habitats in the bird inventories during spring and early summer 2002. This study aims to examine differences in (1) the ratio of invertebrate predators and prey and (2) the abundance of all invertebrates, both in relation to occurrence of fish and water permanence by using activity traps in shallow permanent calcareous lakes and small temporary waterbodies of various sizes on Öland. This Island provides the entire permanence gradient from temporary to more permanent habitats, ideal for these types of studies. The invertebrate predator/prey ratio is expected to decrease with decreasing water permanence since invertebrate predators, as mentioned above, are highly sensitive to drying. Only adult invertebrate predators, like flying waterbeetles, migrating into the temporary habitats will survive during drought. The ratio will be largely unaffected by fish since fish selectively feed on large prey species with no care taken to whether the invertebrate prey is a predator or not. The total abundance of invertebrates is however expected to decrease both with decreasing water permanence and presence or absence of fish because of drying mortality and predation by fish on both invertebrate predators and prey. 3 Methods Study area This survey is to a large extent based on field studies in 10 relatively shallow lentic wetlands (standing water habitats) on Öland, situated in the Baltic Sea at the southeast coast of Sweden (Fig. 1). Two types of lentic wetlands (e.g. lakes, rock pools, fens and others) were of interest for this study. First the temporary Alvar swamps (Königsson 1968), i.e. small waters that normally dry out annually, mostly in late summer, and are refilled by the autumn rains. Second, the permanent Alvar lakes, which are larger waterbodies that normally keep water through the entire year. Those habitat types are further discussed by Königsson (1968). A short description of the investigated wetlands follows in Appendix 1. The information about the wetlands is to a large extent from the wetland inventory project on Öland 1993 (Hylander 1994), but I have also cited other sources. The field measurements consist of my own invertebrate inventories and depth measurements, present knowledge about fish populations in the wetlands and my own field observations of fish presence. a. b. Figure 1. The study was conducted at Öland (framed and black coloured), situated in southeastern Sweden (a) in 10 wetlands (b) from north to south. The wetlands are (1) Knisa mosse, (2) Petgärde träsk, (3) Lenstad mosse, (4) Dröstorps mosse, (5) Möckelmossen, (6) Frösslundamossen, (7) Triberga mosse, (8) Heljemossen, (9) Kvarnkärret and (10) Stormaren. Invertebrate sampling and analysis To be able to sample all 10 wetlands within the shortest possible time (i.e. avoid that differences between the wetlands would depend on the sampling dates), I randomly choose half of the wetlands to be sampled between day one and three. The remaining wetlands were sampled between day three and five. All wetlands were in that way sampled during five days. All wetlands were sampled in June, July and August to get an overall picture of the invertebrate fauna (Table 1). Table 1. Date for invertebrate sampling and depth measurements in the investigated wetlands, and number of traps used in each wetland. The wetlands Kvarnkärret and Heljemossen are omitted because no trap sampling was performed here. Local Knisa mosse Petgärde träsk Lenstad mosse Dröstorps mosse Möckelmossen Frösslundamossen Triberga mosse Stormaren Depth 26/6 26/6 25/6 25/6 26/6 26/6 25/6 26/6 Invertebrates 12-14/6, 9-11/7, 26-28/8 10-12/6, 9-11/7, 26-28/8 12-14/6, 7-9/7, 26-28/8 10-12/6, 9-11/7, 26-28/8 12-14/6, 7-9/7, 26-28/8 10-12/6, 9-11/7, 28-30/8 12-14/6, 7-9/7, 28-30/8 10-12/6, 7-9/7, 28-30/8 No. of traps 12 7 12 7 12 12 12 7 4 Nektonic and climbing macroinvertebrates were caught both with activity traps and standardised hand net sampling. Hand netting was only performed as a complement and will be included only in the second report. Since further analyses will focus only on trap data, according to relevant literature on invertebrates and ducks (see the statistical analyses section), only the trap sampling will be described here. The traps consisted of 1.23 litre plastic jars with conical funnels (entrance diameter 17 mm) in the front and plain bottoms, both consisting of a net material with mesh size 1 mm. Depending on the area of the open water in each wetland during the sampling period, different numbers of traps were used to get an equal sampling effort per area. Seven traps were used in wetlands smaller than 6 ha and 12 in those larger than or equal to 6 ha. In two wetlands (Kvarnkärret and Heljemossen), however, traps were not used at all. Heljemossen was too shallow and the traps were exposed to trampling cattle. Due to time limitations, I was not able to investigate Kvarnkärret with both traps and hand netting. However it would still be possible to compare the invertebrate fauna between wetlands using the hand net samples together with the trap catches in the second report. Traps were placed in the shallow (less than 1 metre) open water areas of each wetland so that the main types of submerged vegetation, bottoms and shore vegetations were represented during all three sampling periods. A shallow depth (less than 1 m) was chosen in order to connect the results to a discussion about food availability for dabbling ducks in the wetlands in a second report. According to Elmberg et al. (1992, 1993) dabbling ducks are more or less restricted to shallow littoral areas when foraging. Mallard (Anas platyrhynchos), for example, forage in the shallow areas until they are complete depleted (Guillemain et al. 2000). Each trap was horizontally placed (depending on occurrence) in submerged vegetation, in the outer border of the shore vegetation and around vegetation stands in the open water area connected to a 32 cm metal stick. A couple of stones (from land to avoid possible by-catches) were placed in each trap to prevent the trap to rise to the surface. Depth varied between 0.20 to 0.50 metres depending on sampling period and placement. Traps were emptied after 48 hours according to current practice (Elmberg et al. 1992, 1993; Hyvönen & Nummi 2000) by carefully pass the content in each trap through a 1 mm sieve and conserve the catches in 70% ethanol as a single sample. All trap samples were analysed as single samples and the invertebrates were determined according to Gärdenfors et al. (1988), Wallace et al. (1990), Edington & Hildrew (1995), and Nilsson (1996, 1997), mostly to families, but also to genus (e.g. diving beetles, Coleoptera) or, in some cases, species and thereafter counted. For each taxonomic group, the number of individuals within each wetland and sampling period were combined to a mean, calculated as number of individuals per trap for each wetland and sampling period. Since one of the purposes was to investigate the response of invertebrate predators and prey to fish presence and water permanence, it raised the need to identify invertebrate predators and prey that are subject to fish predation according to the literature. It was however somewhat difficult to find literature data on invertebrate predator species and their food preferences. According to found literature data, I have divided my invertebrate material into invertebrate predators and invertebrate prey (Table 2), from now on also referred to as ’Pred’ and ’Prey’ respectively. The waterspider Argyroneta aquatica, larval odonates, all waterbugs of the genera Notonecta, Nepa, Ranatra and Ilycoris, larval and adult large waterbeetles found were put together in the invertebrate predator group. I was not able to find any literature on Gammarus pulex, why I considered this species as prey. Furthermore, their small abundance would not have a large influence on the statistical outcome. Therefore, crustaceans (Asellus aquaticus and G. pulex), mayflies, snails and mussels were put in the prey category. The small body size of whirling beetles (Gyrinidae) and some dytiscids (e.g. representatives of the subfamilies Laccophilinae and Hydroporinae) may limit their choice of prey. This has in fact been reported for larval dytiscids (Nilsson 1996), and they are hence omitted in the predator category. Also waterboatmen and larval caddisflies were omitted because of the reported large variation in feeding habits (Nilsson 1996). Waterboatmen are seldom eaten by fish (Macan 1962, 1965) and their distribution is not recorded to vary with predator distribution either (Wellborn et al. 1996), which further support my exclusion. 5 Table 2. The division of invertebrates into predator and prey categories according to found literature. Predatory invertebrates Spiders (Arachnida) Argyroneta aquatica Damsel- and Dragonflies (Odonata) Lestidae indet. Aeschnidae indet. Libellulidae indet. True waterbugs (Hemiptera) Notonecta sp. Nepa cinerea Ranatra linearis Ilycoris cimicoides Waterbeetles (Coleoptera) Colymbetes sp. Agabus sp. Hydaticus sp. Graphoderus sp. Dytiscid larvae indet. Reference Invertebrate prey Crustaceans (Crustacea) Asellus aquaticus Gammarus pulex Mayflies (Ephemeroptera) Snails (Gastropoda) Planorbidae indet. 7-9, 11, 13-16, 21 Lymnaeidae indet. Physidae indet. Hydrobidae indet. Mussels (Bivalvia) Sphaeridae indet. 3, 10, 11 1 2-6, 12 Reference 12, 15, 18-20 9, 17, 18, 20 20, 22 20, 22 References: 1) Gärdenfors 1988, 2) Blaustein 1995, 3) Norland & Mulla 1975, 4) Hampton & Gilbert 2001, 5) Nilsson 1997, 6) Burks et al. 2001, 7) Briers & Warren 1999, 8) Barry 1997, 9) Koperski 1997, 10) Nilsson 1994, 11) Nilsson 1996, 12) Henrikson 1993, 13) Blois & Cloarec 1983, 14) Van-Buskirk 2001, 15) Casas & Hulliger 1994, 16) Cockrell 1984, 17) Macan 1977, 18) Dahl 1998, 19) Petridis 1990, 20) Muus & Dahlström 1981, 21) Akhmetbekova 1986, 22) Brönmark 1994. Leeches were excluded because they were not found in the literature as important fish food. Water mites, oligochaetes and chironomids were also excluded (although oligochaetes and chironomids are subject to predation by many invertebrate and fish species) because mites and oligochaetes were able to escape from the traps (small body size) and thus not caught properly. Neither chironomids were caught properly. Depth measurements I conducted depth measurements to obtain variables that were thought to describe water permanence in the wetlands, subject to my investigation. Depth was measured at 10 metre intervals along three transects in all wetlands in June (Table 1). In those cases that the wetland was too deep or too difficult to force by feet, the depth was measured from a rubber dinghy. A permanent station for continuous water level measurements was established in each wetland to obtain depth values during July and August. Depending on depth and bottom structure, this measurement station consisted either by a 2 metre long graduated wooden stick (in shallower wetlands with soft, deep sediments), or a 2 metre long graduated cord connected to a weight at the bottom end and a float at the surface end (in deeper wetlands with hard sediments). The maximum June depth in each wetland was noted and the transect depth data from each wetland in June were combined to a mean depth. The depth was also noted in July and August, but these depth measures were not further used in the analyses. Fish presence Survey fishing was unfortunately not done at all depending on various reasons out of my control. The only available fish density information in the investigated wetlands is the inventory fishing with multimesh gillnets conducted by students at the University of Kalmar in Knisa mosse in May 2002, who found only tench (Tinca tinca) and nine-spined stickleback (Pungitius pungitius) (Grage et al., unpublished). I was therefore forced to use this knowledge and my field observations of the presence of the larger fish species tench (T. tinca) and northern pike (Esox lucius, hereafter named pike), in my analyses with the idea that the presence of larger fishes, independent of species, affects the invertebrate faunal composition. 6 Nine-spined stickleback P. pungitius was not regarded as a “true” fish and was therefore excluded in my analyses. This is because the small adult size limits the range of prey possible to ingest. In fact, Corti et al. (1995) were not able to detect strong predation effects on benthic invertebrate community structure by small fish and thus concluded that predator effects appeared to be attributable primarily to the activities of large fish. Statistical analyses The analysis was based on comparisons of depth data, invertebrate data and fish presence between the investigated wetlands and within sampling period (June). I used a Spearman correlation test of the measures mean depth, third maximum depth (the third highest depth), and wetland area to search for the factor that best described the persistence of water in each wetland. Third maximum depth was chosen instead of real maximum depth to exclude the influences of extremely small deep areas. The result was assessed using two-tailed probabilities and gave a strong significant correlation between mean depth and third maximum depth (Spearman Correlation Coefficient = 0.997, P = 0.000). Wetland area was neither significantly correlated with mean depth nor with third maximum depth (Spearman Correlation Coefficient = 0.456 (P= 0.186) and 0.466 (P= 0.174) respectively). Wetland area was therefore multiplied with mean depth to obtain a volume, which in turn was correlated with mean depth and third maximum depth respectively. The best correlation was between volume and third maximum depth (Spearman Correlation Coefficient = 0.628, P= 0.052). However, despite a non- significant correlation, the linear regression showed a remarkably high R2 value (0.85), meaning that 85% of the variation in volume was explained by third maximum depth. Species richness has in fact been observed to decrease with increasing water depth (Nilsson et al. 1994; also discussed by Corti et al. 1995) and Collinson et al. (1995) found a significant correlation between water depth and permanence, so the use of maximum depth in the analyses is further supported by literature data. The invertebrate trap, fish and depth data were analysed in parametric analyses of covariance (ANCOVA) using the statistical software package SPSS. The combined effects of maximum depth and presence (1)/ absence (0) of fish on (1) the ratio (Pred/Prey) of predatory invertebrates (Pred) and prey (Prey) (or also named only ratio) and (2) on the abundance of invertebrates (all invertebrate groups in table 2) were tested with the ratio and abundance separately in two analyses as dependent variables, fish as fixed factor (binary variable) and maximum depth as independent covariable (see Table 3). The probability level in the analyses was set to 0.05. I only used the trap data to be able to compare with relevant studies on invertebrates and ducks (i.e. Elmberg et al. 1993, 1994; Nilsson et al. 1994; Nummi et al. 1994, 1995; Wagner 1997; Wagner & Hansson 1998). This procedure was thought to reflect the availability of sampled invertebrate groups for ducks. Since fish affect the activity of invertebrates (Wellborn et al. 1996), the use of activity traps may further highlight the effect of fish in these wetlands. Further, the wetlands Heljemossen and Kvarnkärret were excluded in the analyses because they were not sampled with activity traps. Results Invertebrate ratio vs. fish and maximum depth The ratio did not change with maximum depth, and was not affected by the presence of fish (Fig. 2). Neither the individual effects of fish and maximum depth, nor the interaction effect were however significant (Table 4). The results did not differ when the interaction was omitted in the analysis, neither for fish nor for maximum depth. Noteworthy is the very high ratio in Triberga mosse compared with the other wetlands (see also table 3), but the statistical non- significant outcome (Table 4) did not change when it was omitted in the analysis. 7 Invertebrate predator / prey ratio 3,5 Tri 3 2,5 2 1,5 1 Frö 0,5 Kni Mö Drö Pet 0 0 0,2 Len Mar 0,4 0,6 0,8 1 1,2 1,4 Maximum depth (m) Figure 2. The ratio of invertebrate predators and prey in the presence (filled squares) and absence of fish (open squares) in relation to increasing maximum depth. The wetlands are (1) Knisa mosse, (2) Petgärde träsk, (3) Lenstad mosse, (4) Dröstorps mosse, (5) Möckelmossen, (6) Frösslunda mosse, (7) Triberga mosse and (8) Stormaren. Table 3. Loaded variables from each wetland in the ANCOVA analyses. Local Knisa mosse Lenstad mosse Möckelmossen Frösslundamossen Triberga mosse Petgärde träsk Dröstorps mosse Stormaren Fish presence Max.depth (m) Ratio Pred/Prey Abundance (ind./trap) yes yes yes yes yes no no no 0.63 1.25 0.33 0.33 0.43 0.23 0.40 0.43 0.58 0.42 0.88 0.33 3.10 0.03 0.20 0.21 3.40 4.50 1.30 8.70 3.40 17.00 3.40 7.30 Table 4. Analysis of covariance (ANCOVA) for the two summary variables (a) the ratio of predatory invertebrates and prey (Pred/Prey) and (b) the invertebrate abundance. The probability level used in the analyses was 0.05. a) Source of variance SS df F P 0.485 1 0.396 0.563 maxdepth 4.592E-05 1 0.000 0.995 fish x maxdepth 7.44E-02 1 0.061 0.817 4.895 4 Ratio Pred / Prey fish Error 8 Table 4. Continued b) Source of variance SS df F P fish 102.438 1 8.827 0.041 maxdepth 79.143 1 6.820 0.059 fish x maxdepth 78.049 1 6.726 0.060 Error 46.420 4 Invertebrate abundance Invertebrate abundance in relation to fish and maximum depth The invertebrate abundance was significantly lower in wetlands with fish (Table 4, Figure 3). The significant fish effect on the abundance was however dependent on only one value (Petgärde) and disappeared when the interaction variable was excluded. Maximum depth did not significantly affect the abundance even if the relationship was close to significant (Table 4). 18 Invertebrate abundance (ind./trap) 2 16 14 12 10 6 8 8 6 3 4 4 2 7 1 5 0 0 0,2 0,4 0,6 0,8 1 1,2 1,4 Maximum depth (m) Figure 3. The invertebrate abundance in the absence (open squares) and presence (filled squares) of fish in relation to increasing maximum depth. The wetlands are (1) Knisa mosse, (2) Petgärde träsk, (3) Lenstad mosse, (4) Dröstorps mosse, (5) Möckelmossen, (6) Frösslunda mosse, (7) Triberga mosse and (8) Stormaren. The mean value for invertebrate abundance was more than twice as high in wetlands without fish (9.23) as in wetlands with fish (4.26), which was due to the two groups crustaceans and snails. The mean abundances were not changed when Knisa mosse were excluded. The invertebrate abundance in Knisa mosse, although it houses tench, does not differ considerably from the other wetlands with pike. Fish louse (Argulus sp., Branchiura) was only found in Knisa mosse. The complete absence of snails, the very low abundance of mussels and the relatively high abundance of mayflies are also characteristic for the trap catches in this wetland. The traps caught more than invertebrates. Juveniles of tench (T. tinca) were caught in Knisa mosse in August, juveniles of pike (E. lucius) in Triberga mosse in June and adults and juveniles of ninespined sticklebacks (P. pungitius) were caught at all sampling occasions in almost all wetlands. No other fish species were caught (neither with traps nor by hand net) or observed. The abundance of corixids (not included in the statistical analysis), snails and crustaceans was higher in some wetlands compared to others. Petgärde träsk had a very high abundance of corixids (about 14 times as high here as in the other wetlands together), but also high abundances of crustaceans and 9 snails compared to the other wetlands (Fig. 4). In fact, the higher mean abundance in fishless wetlands in general was due to the two groups crustaceans (mainly A. aquaticus) and snails (Table 3, Fig. 4). Note that all abundances are Log10- transformed in Fig. 4 in order to show the corixid abundance together with the other groups. 1000.0 Log abundance (ind./trap) 100.0 corixids crustaceans mayflies snails mussels 10.0 1.0 0.1 1 2 3 4 5 6 7 8 Local Figure 4. The Log10- transformed abundances of certain invertebrate groups in the investigated wetlands (1) Knisa mosse, (2) Petgärde träsk, (3) Lenstad mosse, (4) Dröstorps mosse, (5) Triberga mosse, (6) Möckelmossen, (7) Frösslundamossen and (8) Stormaren. Note that corixid abundance was not statistically analysed. Discussion Invertebrate predator / prey ratio vs. fish and maximum depth Neither the individual effects of fish and maximum depth, nor the interaction effect significantly affected the ratio. The fish effect is in clear accordance with my hypothesis based on Wellborn et al. (1996) that the ratio will be largely unaffected by fish. If fish do not affect the ratio, the ratio will not differ significantly between the two types of wetlands. The high variation in the ratio in wetlands with fish causing the non-significant fish effect might be due to my choice of predators and prey within the invertebrate fauna. I was not able to find a significant effect of maximum depth on the ratio. Although the ratio seemed to increase linearly in fishless wetlands with maximum depth, this pattern was not significant which may be due to poor replication of fishless wetlands. The linear relationship in fishless wetlands is weakened or even destroyed by the high variation in ratio in those wetlands with fish. The very high invertebrate predator prey ratio in Triberga mosse is astonishing and difficult to explain. In order to do so one might need to look at individual species. Invertebrate abundance vs. fish and maximum depth Fish significantly affected the invertebrate abundance when the interaction was included in the analysis and the mean abundance in fishless wetlands was more than twice that in wetlands with fish. However, since the significant fish effect is dependent on only one value (Petgärde träsk), it is therefore no longer significant when the interaction variable is excluded from the analysis. Since both maximum depth and the interaction variable were close to significant, increased replication of fishless wetlands might produce significant effects of these variables on the abundance. 10 I found it interesting to compare Knisa mosse with the other wetlands since Knisa mosse houses tench and the other wetlands houses pike according to my field observations. The similar invertebrate abundance in Knisa compared to the other wetlands, suggests that tench does not affect the overall invertebrate abundance more than pike does. This is probably due to the fact that adult tench mainly forage on benthic invertebrates (Muus & Dahlström 1981), with a strong preference for snails and bivalves (Brönmark 1994), while pike, although generally considered as a piscivore, may exhibit a generalistic foraging on damselfly larvae and amphipods (Little et al. 1998). In ponds without larger prey species (except small pike), pike feed on different insect larvae and the freshwater isopod Asellus aquaticus (Muus & Dahlström 1981). The results of Brönmark (1994) (i.e. the strong preference for snails and bivalves) further explain the absence of snails in the trap catches from Knisa mosse. The high abundance of mayflies in Knisa mosse compared to the other wetlands may depend on a lower productivity in this lake. For example, Macan (1977) mention that mayflies make a substantial contribution to the biomass in unproductive lakes. The absence of mayflies from productive habitats is according to the author due to predation by fish. In Knisa mosse the most plausible explanation is probably that the trapped mayflies are pelagic and thus not subject to tench predation. Because of the similar invertebrate abundance in Knisa mosse compared to the other wetlands, it is not surprising that the mean abundance was unaffected when this wetland was excluded in the ANCOVA. The higher mean abundance in fishless habitats was due to crustaceans (i. e. Asellus aquaticus) and snails. This might indicate that these two groups are more affected by fish or maximum depth than other invertebrates in these wetlands. The high abundance of corixids in Petgärde träsk is striking. According to Macan (1962) a high diversity of corixids generally occurs in places rich in biotopes or in unstable places, for example places that dry up or suffer deoxygenation. Potential predators are unable to colonise these habitats as quickly as flying corixids, which thus are offered a secure habitat and are able to fly away when the habitat is dried. Furthermore, regular drying out leads to remineralisation of nutrients, which promotes a productive habitat (Collinson et al. 1995). Temporary habitats are also characterised by snails and a high diversity of Hemiptera (including corixids) and Coleoptera (Williams 1997). Asellus aquaticus is also reported from habitats with productive conditions (Macan 1977). The higher abundance of snails and crustaceans (A. aquaticus) in Petgärde träsk can thus be related to temporary habitat characteristics. My division of wetlands into the two categories with and without fish was based upon field observations of fish. This division is of course doubtful and the presence or absence of fish must be verified by standardised survey fishing. However, no juvenile tench or other carp fish species were observed or caught with traps in other wetlands than Knisa mosse. Fish louse mainly parasites on carp fishes (Olsen & Svedberg 1999) and the fact that fish louse was only found in Knisa mosse, further indicates that either carp fishes, or fish louse, or both were absent in the other wetlands. No juvenile pike were, as with tench, caught in other wetlands than those with observed pike. These observations might support my division into locals with and without fish. The strong decrease in abundance with increasing maximum depth was remarkable and highly unexpected, but is consistent with Corti et al. (1995) who found that overall invertebrate richness and abundance decreased with increasing pond permanence. According to them, the interplay of (1) periodic drying and flooding providing organic matter to detritivores and other consumers and (2) increased abundance and diversity of invertebrate predators may produce this pattern. However, they also argued that invertebrate predators are able to exert a high predation pressure on invertebrate prey species in the absence of fish. This has been shown in experiments where predatory invertebrates common in fishless habitats have been introduced into habitats where fish has been excluded. Wellborn et al. (1996) has discussed this matter, but they concluded that the effect probably is weaker in the fishless habitats native to the introduced predatory invertebrates. There was however no clear increase in the abundance of predatory invertebrates with increasing maximum depth in the investigated wetlands. 11 Conclusions and further studies This study demonstrates that fish does not affect the ratio of invertebrate predators and prey. The study also suggests, in contrast to my hypothesis that fish do not affect the invertebrate abundance. Since the effect of fish on the abundance was not significant, I was not able to reject the possibility that fish do not affect the abundance, even though Knisa mosse houses tench, a carp fish. The invertebrate abundance in Knisa was not different from the other investigated wetlands. I must therefore conclude that tench, although snails were absent in Knisa mosse, does not affect the overall abundance of investigated invertebrates in Knisa mosse more than pike does in the other studied wetlands, except snails. This is interesting, since decreasing bird (including waterfowl) densities and diversities, in Knisa mosse has been observed during the 20th century (Breife 1981; Ekstam 1981; Forslund 2001) (see Appendix 1). The decrease seems to be a continuing phenomenon according to the bird inventory here previous year (Holm et al., unpublished). If the decrease in waterfowl density and diversity rely on food shortage, it probably results from low abundances of snails or other benthic invertebrates like chironomids. Adult tench is like other carp fishes a benthic forager, with strong preference for molluscs (Brönmark 1994), but switches to other food sources when molluscs are rare (Scheffer 1998). The fact that snails were absent in Knisa mosse may also be due to sedimentation, caused by the foraging activities of tench. According to Martin & Neely (2001), snails, together with Coleoptera, Diptera and Odonata larvae, are adversely affected by sedimentation. However, in order to survive in Knisa mosse, tench must feed on other benthic invertebrates. This is supported by the findings of Petridis (1990) who found that tench utilized a diet dominated by red chironomids in sites, which were depleted of vegetation and lacked gastropods and Asellus aquaticus. The fact that submerged vegetation is missing in Knisa mosse supports this idea. Further studies are therefore needed concerning for example availability of chironomids to breeding waterfowls in Knisa mosse. The almost significant effect of maximum depth and the interaction between fish and maximum depth on the invertebrate abundance suggests that increased replication primarily of fishless wetlands will lead to significant effects. If so, water permanence may be important in structuring the invertebrate fauna in these habitats. Neither the fish community structure nor the depth in these wetlands was known before my investigation and unfortunately no survey fishing was done because of various reasons out of my control. In this study care was therefore unfortunately not taken neither to which fish species were present nor community structure, only the presence of 'large' fish species. The lack of knowledge about the fish community structure in these wetlands clearly needs further attention. The division into predators and prey rely on literature data. In order to give a more proper picture on the trophical relationships between the studied invertebrates, one method is to use stable isotope analyses on nitrogen-isotope ratios (Sarakinos et al. 2002) in the scope of a larger study. It is important to mention that this year was the driest since 100 years, and wetlands on Öland that normally keep water dried out this summer. It would therefore be interesting to investigate the influence of this extreme summer drought next summer on the invertebrate fauna composition in the investigated wetlands. When sampling and analysis design are accounted for, it will be relevant to investigate whether predatory invertebrates in absence of fish are able to structure invertebrate eating ducks in those wetlands. Even though my present data material does not allow this, it is still possible to relate the invertebrate abundance in the studied wetlands to occurrence of breeding pairs of aquatic birds. I intend to do so in the second report mentioned in the introduction, with focus on invertebrate-eating dabbling ducks, in order to valuate the wetlands as breeding habitats for dabbling ducks. Next step would be to include other wetlands, especially shallow temporary wetlands. Then it is possible to value the wetlands for other aquatic invertebrate-eating birds as well. 12 Acknowledgements I wish to thank my supervisor Börje Ekstam for help with analyse design and helpful comments during the process, but also for the bunch of relevant articles and other publications he provided. I am also grateful to Patrik Dinnetz for helpful comments on statistics. Irene Bohman helped me with sampling design and lended her activity traps. Also Ulf Bjelke and Görgen Göransson provided helpful literature. 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Temporary ponds and their invertebrate communities. Aquatic Conservation: Marine and Freshwater Ecosystems 7: 105-117. 15 Appendix 1. Studied wetlands Knisa mosse The wetland is the northernmost wetland included in my study (Fig. 1). It is one of few wetlands on Öland that has not been ditched since the end of the 17th century (Hylander 1994). The fen and some of the surroundings has been a bird protection area since 1931 and nature reserve since 1968 to create a refuge for some of the wetland birds on Öland (Ekstam 1981). The fen itself is 61 ha and the largest open water area in the southern part where the study has been conducted is 15,3 ha in size. The bird life was richest from the last century to the beginning of the 1930s, but deteriorated during the later part of the 20th century (Forslund 2001). The changes have been related to overgrowing, because of ceased usage of reed- and sedges and ceased grazing by cattle in the surroundings when the area was placed under protection (Breife 1981), to a drastic decrease in the occupation by charophytes (Chara spp.) in the open water area (Ekstam 1981), and finally, introduction of tench (Tinca tinca) (Grage et al., unpublished). From the 1950s the bird fauna stopped to decrease and Knisa wetland can still be regarded as a valuable bird habitat (Breife 1981). The bottom sediment is very homogenous; consisting of calcareous mud that closer to the shore is firm and mixed with vegetation remains in the shoreline. In the open water area the sediment is very loose and easily movable. The sediment is sometimes very deep (40-50 cm). In a strict sense, submerged vegetation is missing nowadays, at least in the southern water area, which probably depends on the sediment condition caused by benthic foraging tench. Petgärde träsk This marsh belongs to a 74 ha area that includes two other wetlands. Large parts of this area are nature reserve since 1978. Between 1917 and 1921 extensive ditching undertakings were conducted in the area and from the middle of the 1970s the water level of the area has been raised somewhat and is since then regulated (Rodebrand 1979). An open water area of 2,1 ha is situated in the middle, which has been created to favour the bird life (Rodebrand 1979). Submerged vegetation is scarce, at least in the western water area. Reed grows both in the open water and along the shore, sometimes in dense stands. The sediment is very loose and muddy and hard to force by feet. Only nine-spined stickleback (Pungitius pungitius) was observed during the survey. Lenstad mosse The fen is almost 21 ha and before diggings were conducted 1990 it was a uniform tufted sedge fen (Pehrsson 1992). Now there are large water-filled areas all the year round (Hylander 1994). These areas are about 9 ha in size. The sediment consists of partly thick layers of calcareous mud and is bound by submerged vegetation like pondweed (Potamogeton sp.), charophytes (Chara sp.) and Canadian waterweed (Elodea canadensis), where the waterweed constitutes a considerable part. Observed fish species is northern pike (Esox lucius) and nine-spined stickleback. Dröstorps mosse The Alvar lake Dröstorp mosse has got its name from the ruins of the devastated village Dröstorp, situated close northeast by the wetland. The 24 ha wetland is not influenced by any ditching or digging and is situated in the Great Alvar, south of Öland. The water-filled area consists both of at times flooded land (9 ha) and a central water area (4 ha) rich in charophytes (Hylander 1994). The survey was concentrated to the central water area. In the middle there is an area with reed, surrounded by osiers (Salix spp.) The wetland houses only nine-spined stickleback. Möckelmossen This wetland is the largest wetland (62 ha) that has been included in the study and is situated out on the Great Alvar of Öland. It does not normally dry out in the summer. The study has been conducted in the northernmost water area (about 7,5 ha). The submerged vegetation is quite widespread. Northern pike and nine-spined stickleback constitutes the fishing stock and probably migrate into the wetland from the Baltic Sea through the Frösslunda wetland close nearby. Frösslunda mosse Frösslunda mosse is situated in the eastern part of the Great Alvar and connected with Möckelmossen by a small stream that dries out periodically and flows into the Baltic Sea. The open water area is about 6 ha. The sediment is sometimes deep and the bottom is rich in charophytes. A reed bed is situated in the central part surrounded by tufted sedge. The tufted sedge also surrounds the open water area. The wetland is largely unaffected, except for a smaller dam in the northeastern part. Triberga mosse The fen of 9 ha is dammed by the eastern Ancylus shore bank and drains a large part of the Great Alvar (Hylander 1994). The open water area is 6 ha. The wetland was restored from 1999 to 2002. Before, it was a uniform tufted sedge-fen with a reed bed in the wettest parts and charophytes between the sedge tufts. The sediment on the restored bottoms was during the survey covered by charophytes and some pondweed stands. Northern pike and nine-spined stickleback were observed during my investigation. 16 Heljemossen The wetland is small, only 8 ha, which consists of a fen (4 ha) and a wet meadow (4 ha). It does not contain an open water area. A small stream drains the fen through the wet meadow and flows (probably) into the Baltic Sea. The survey was concentrated to the fen. Tufted sedge and other sedges with small stands of charophytes dominate the vegetation. There is no fish in the wetland, at least not in the summer, when the small stream is dried and the water levels are low in the fen. Kvarnkärret As with Heljemossen, this wetland does not contain an open water area. It consists of a fen (5 ha) and periodically flooded land (6 ha). The survey was conducted in the fen, which lacks fish. An existing water level was noted in June and July, but not in August. The dominating vegetation is tufted sedge, but also other sedges and charophytes can be found. The fen is, with the exception of a smaller pond construction for cattle, unaffected. It has though been subject of overgrowing. Stormaren (Maren) Stormaren is also a small wetland (9 ha), but it normally holds water all the year round (a so called Alvar lake). Open water areas (1 ha) are situated in the north and south, where the bottoms are covered by charophytes and pondweed. In the central part there is a large reed area, surrounded by smaller area with tufted sedge. Restoration efforts have been made round the lake during year 2000. This wetland has high ornithological values and is therefore a bird protection area. Gradually, the intention is to include the area in a nature reserve. During my survey I have noticed a drastic production by green filamentous algae in the open water mass together with a rich occurrence of Eurasian water milfoil (Myriophyllum spicatum), which grows in nutrient-rich habitats (Mossberg et al. 1997). The only fish species inhabiting Stormaren is nine-spined stickleback.