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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. Finally I would like to thank
Jan Herrmann for helpful comments on an earlier version of this manuscript. The County Administrative Board
of the Kalmar County placed a rental car at my disposal for the field sampling.
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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.