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Project Report
Alien Invasive Species in the North-East Baltic Sea:
Monitoring and Assessment of Environmental Impacts
Henn Ojaveer, Jonne Kotta, Helen Orav-Kotta, Mart Simm,
Ilmar Kotta, Ain Lankov, Arno Põllumäe and Andres
Jaanus
Financed by the US State Department (grant award number SEN100-02-GR069)
Estonian Marine Institute, University of Tartu
Tallinn 2003
2
Abstract
Several alien species are important constituents of both planktonic and benthic
invertebrate communities in Estonian marine waters. These include, amongst others, the
predatory cladoceran Cercopagis pengoi, the polychaete Marenzelleria viridis, the zebra
mussel Dreissena polymorpha and the soft shelled clam Mya arenaria. The current study
was aiming at monitoring of selected alien species (incl. those named above) in order to
track spatio -temporal abundance and distribution patterns of these species. Field
experiments were carried out in order to reveal ecological impacts of some selected alien
benthic invertebrates. First laboratory experiments with Cercopagis for clarification of
taxonomic matters of the genus and getting insight into the feeding ecology of the species
were undertaken. Similarly, first biological sampling directly in one of the largest ports of
the Baltic Sea – Port of Tallinn (Muuga Harbour) – was performed.
3
Contents
Introduction 4
1. Monitoring of most important invasions 5
1.1. Material and Methods 5
1.2. Preliminary results 9
2. Monitoring of port areas (high-risk areas of primary invasions) 21
2.1. Material and Methods 21
2.2. Preliminary results 22
3. Experiments with selected alien species 27
3.1. Material and Methods 27
3.2. Preliminary results 30
4. Presentations of the project results at international conferences 37
5. Publications (in press, in review, in preparation) 38
6. References 39
4
Introduction
It is commonly accepted and agreed that invasion of alien species is one of the most
serious and gradually increasing threats to aquatic ecosystems. This human-aided process
has initiated significant, unpredictable and irreversible changes to both abiotic and biotic
environment in variety of waterbodies worldwide (e.g., Carlton, 1996, 1999; Karatayev et
al. 2002; Ojaveer et al. 2002 ; Vanderploeg et al. 2002) and may cause severe economic
damage to man.
There are about 100 alien species found in the Baltic Sea. However, fewer than 70
of them have been able to establish reproducing populations. About 60 alien species
recorded in the Baltic Sea are unintentionally introduced. Besides variety of ecological
changes caused by alien species, several of them have also caused economic damage,
incl. the hydrozoan Cordylophora caspia, the barnacle Balanus improvisus, the
cladoceran Cercopagis pengoi and the bivalve Dreissena polymorpha (e.g., Olenin and
Leppäkoski 1999; Leppäkoski et al. 2002).
Realizing global and local dimensions of invasions of alien species with
concomitant harm to marine environment and potential economical damage, International
Maritime Organization (IMO) organised recently Baltic Regional Workshop on Ballast
Water Management in Estonia in 2001. One of the outcomes of the project was
preparation and submission of 5-years co-operative scientific project proposal by Estonia,
Latvia and Russian Federation to local US embassies/consulates for consideration for
funding. Out of the three national proposals submitted, only Estonian application
received funding (in the requested amount of $ 34 000) for one year period (2002-2003).
In the current report we document both activities undertaken and preliminary
research findings for aquatic alien species studies in the Estonian marine waters
according to the following major project topics: (1) monitoring of selected most
important invasions, (2) biological sampling of port areas (high risk areas of primary
invasions) and (3) field/laboratory experiments of selected alien species. For
interpretation of the current project results, data collected within other studies of the
Estonian Marine Institute from earlier years have also been used.
5
1. Monitoring of most important invasions
1.1. Material and Methods
Zooplankton
Samples were collected from the stationary long-term sampling station in the north-
eastern Gulf of Riga by large Juday plank ton net (vertical hauls, mouth area 0.1 m2, mesh
size 90 µm) from bottom to surface on weekly basis during May-October. In addition to
that, the open part of the whole Gulf of Riga was sampled for zooplankton in 23 locations
(simultaneously to the fish pelagic trawl hauls) during July-August and shallow Pärnu
Bay in one station on weekly basis in 2003 by means of large Juday plankton net. The
collected material was stored in plastic bottles with formaldehyde solution. Zooplankters
are generally identified to the species level and abundance and biomass by species (as
ind. and g per m-3, respectively) are determined. The 2002 samples are processed and
data included into the current report whereas 2003 samples are at the processing stage as
yet.
In addition to studies in population abundance and biomass dynamics at various
time scales (multi-annual, seasonal, weekly), seasonal dynamics of Cercopagis
gamogenetic and parthenogenetic fecundity, by using both previously and newly
collected material from Pärnu Bay in the May-September period, was studied. In total, ca
1300 individuals were examined.
Spatial distribution and population abundance characteristics of the hydromedusae
Maeotias marginata in Väinameri Archipelago (Figure 1) was studied within the August
2003 survey. The medusa was sampled by Hensen trawl in 8 stations.
Figure 1. Locations of major sub-basins around Estonian coasts. The studied port areas
(Muuga Harbour and Port of Pärnu) are also shown.
Zoobenthos
The distribution area and abundance dynamics of benthic alien species was investigated
in relation to the key environmental parameters. Zoobenthos sampling was done in the
Gulf of Finland, the Gulf of Riga and in the Baltic Proper. The field sampling was done
according to the standards of HELCOM. Macrozoobenthos samples were collected by
van Veen grab or Tvärminne sampler. Material was sieved through a net of 0.5 mm mesh
size and then deep frozen at –20 °C. In the laboratory all animals were counted under a
binocular microscope. Dry weights were obtained (± 0.1 mg) after drying the material at
80 °C for 48 hours. Molluscs were weighed with shells.
The abundance of M. viridis in the water column was obtained by vertical Hensen
net (opening diameter 0.8 m, mesh size 0.4 mm) tows. The sampling was performed
every second hour covering the period from sunset to sunrise (reported time is local
summertime). In each sampling occasion three hauls were taken at 26 m station: from 5
60º
59º
57º
Tallinn
Gulf of Finland
Gulf ofRiga
Helsinki
Riga
BalticProper
VäinameriArchipelago
22° 24° 26° 28°
Port ofPärnu
Muuga Harbour
58º
7
m to surface, from thermocline to 5 m and from 25 m to thermocline. In the shallower
areas only one haul was taken from 1 m above the bottom to the surface. All samples
were stored in 4% buffered formaldehyde-seawater solution. Total length (± 0.1 mm),
abundance and dry weight (80 ºC, 48 h, ± 0.001 g) of the polychaetes were determined in
the laboratory.
Occurrence of the Chinese mitten crab Eriocheir sinensis in commercial gillnet
catches was studied in Muuga Bay. The study covers both continuous recording of crab
individuals during the current project as well as summarising data from the hand-written
records of one fisherman from previous years. Mesh size, length and height of gillnets
was 40-55 mm, 1.5-1.8m and 60m, respectively. Mean number of fishing days per year
was 165 (variation from 119 to 198 days).
Fish
Feeding habits of the most abundant pelagic (the Baltic herring Clupea harengus
membras, three-spined stickleback Gasterosteus aculeatus and sprat Sprattus sprattus)
and bentho-pelagic fish (smelt Osmerus eperlanus) in the Gulf of Riga were examined.
This involved analysis of the already collected data from the bottom trawl surveys in
1994-1998 (for methodological details see Ojaveer et al. 1999) and laboratory analysis of
stored material – fish stomachs – from pelagic trawl surveys in 2001-2002. In laboratory
analysis, 36 samples of herring stomachs, 13 of three-spined sticklebacks, 6 of sprat and
2 of smelt were analysed.
In the 2003 cruise during July- August, 23 stations were trawled (Figure 2). In
these surveys, 39 samples of stomachs of herring (incl. 16 juveniles), 19 for three-spined
stickleback and two fo r sprat were collected.
8
Figure 2. Locations of experimental pelagic fish trawl survey and zooplankton sampling
stations in the Gulf of Riga in summer 2003.
One sample for fish feeding analysis consists usually of stomachs of 20
individuals. Stomachs are removed immediately after taking fish out of the water, packed
into liquid-permeable textile cloth, labelled and stored prior to analysis in formaldechyde
solution. Stomach content is determined to the species level (if possible) and measured in
weight units at a precision of 0.001g. The Ivlev’s electivity index was calculated as E =
(Nw-Ns)/(Nw+Ns), where Nw and Ns is the abundance of prey in the environment and in
fish stomachs, respectively.
For analysis of the percent contribution of Cercopagis in stomachs of different
size-classes of fish (grouped into 1 cm increments), all individuals from samples where at
least one fish had consumed Cercopagis (as an indication of availability of Cercopagis
for fish) were used. In all, 560 herring (length range 9-17 cm), 175 smelt (7-16 cm) and
105 sticklebacks (two species analysed together; length range 3-6 cm) were analysed.
Riga
Pärnu
9
Within the routine long-term fish gillnet surveys in Pärnu Bay (7 stations, gillnet
mesh sizes 16, 25, 30, 38, 45, 48, 50, 60 mm), presence of alien fish species was
recorded. Contacts with local commercial fishermen were used for new evidences of
occurrence of the round goby Neogobius melanostomus that was first found in Estonian
waters in Pärnu Bay early 2002.
1.2. Preliminary results
Zooplankton
The most important invasion concerning plankton communities of the Baltic Sea in the
recent decade has been the predatory cladoceran Cercopagis pengoi. This species of
Ponto-Caspian origin was first found in Baltic Sea in 1992 (Muuga and Pärnu bays) and
it has been since then continuously expanding in terms of spatial distribution, abundance
and biomass.
The long-term development of Cercopagis population in one of the first observed
locations, the NE part of the Gulf of Riga, points to continuation of already earlier
revealed tendencies also in 2002: population abundance increases at the multiannual
scale, the cladoceran continues to be present in plankton community for longer time per
season and achievement of an abundance level of 10 ind. m–3 tended to occur earlier over
the years studied (Figure 3). This event does not seem to be regulated directly by the
water temperature alone, as variation between the years is substantial. For instance, an
abundance of 50 ind. m–3 was achieved at 11 °C in 1993 but at 20 °C in 1994. Similarly,
seasonal declines in the population do not appear to be strictly related to temperature. For
example, the mean population abundance exceeded 150 ind. m–3 in September, 1999 at
11.6 °C whereas the mean Cercopagis abundance was 13 ind. m–3 at 15 °C in September
1994.
Long-term abundance dynamics of the most abundant small-sized cladoceran in
this area, Bosmina coregoni maritima, indicates significant differences between pre and
post invasion populations (log (x+1)-transformed data, t-test, p<0.001). Although the
abundance of Bosmina exhibited considerable variability before the Cercopagis invasion,
and was quite low in some years, post- invasion populations have been consistently low
(Figure 4).
10
Cercopagis fecundity data are very scarce in invaded ecosystems (Baltic Sea,
North American Great Lakes). This has resulted in the situation, where generalisations
are being made on the basis of one-month sampling. However, results of our studied
point that this may be severely misleading as both parthenogenetic (number of embryos
per female) and gamogenetic fecundity (number of resting eggs per female) exhibit
significant seasonal dynamics. Specifically, perthenogenetic fecundity of Cercopagis is
significantly higher in spring-early summer (when the population abundance is low)
whereas in summer-early autumn, fecundity is significantly lower (during the high
population abundance) (Figure 5).
Similarly, production of resting eggs is not similar throughout the season: it
reached the lowest level in late July – early August (Cercopagis population abundance is
in increasing stage, food should be not the limiting factor). Production of resting eggs
increases and remains at high level in August. The reason for that could be worsening of
feeding conditions (i.e. shortage of food) for Cercopagis that is very abundantly present
at this time and this situation may force the population to switch for production of resting
eggs.
As a seasonal average, individuals bearing two resting eggs slightly dominate
over those having 12 resting egg (53.4 and 45.4%, respectively) and Cercopagis with
three resting eggs is very rare in the NE Gulf of Riga.
11
Figure 3. Annual abundance dynamics (ind. m–3) of Cercopagis pengoi (upper
panel), duration of presence (weeks) of Cercopagis in zooplankton community at
densities >10 ind. m–3, shown as deviation from the long–term mean (intermediate panel)
and first month during which Cercopagis population density reached 10 ind. m–3 (lower
panel) in the Gulf of Riga during 1992–2002.
Figure 4. Development of abundance of Bosmina coregoni maritima as evidenced from
the long-term zooplankton sampling station in the NE Gulf of Riga in 1970-2002.
-6-4-202468
Dev
iatio
n in
dex
5
6
7
8
1992
1994
1996
1998
2000
2002
Mon
th
Bosmina coregoni maritima
0
10
20
30
40
50
1970
1974
1978
1982
1986
1990
1994
1998
2002
Year
Abu
ndan
ce in
d. m
-3
0
200
400
600
800
1000
1200
1400
ind.
m-3
12
Figure 5. Seasonal dynamics of parthenogenetic (number of em bryos) and gamogenetic
(number of resting eggs) fecundity of Cercopagis pengoi. The upper panel represents the
mean number of embryos (with s.e.) for the May-September period; the intermediate
panel shows the mean number of resting eggs from the 4th week of June until 3rd week of
September and the lower panel evidences the switch between production of 1 and 2
resting eggs for the period of 4th week of June - 3rd week of September. Grey: 1 resting
egg; white: 2 eggs and black: 3 eggs.
Another species that rather recently started to dominate in the zooplankton community in
the NE Gulf of Riga is Marenzelleria viridis, specifically its pelagic larvae. The 13-years
dataset indicates gradual increase of the abundance of Marenzelleria larvae in the 1990s,
followed by rapid decline afterwards. The relatively low level was also kept in 2002
0 %
20%
40%
60%
80%
100%
Jun 4 Jul 1 Jul 2 Jul 3 Jul 4 Aug 1 Aug 2 Aug 3 Aug 4 Sep 1 Sep 2 Sep 3
Week
1,00
1,20
1,40
1,60
1,80
2,00
2,20
Jun 4 Jul 1 Jul 2 Jul 3 Jul 4 Aug 1 Aug 2 Aug 3 Aug 4 Sep 1 Sep 2 Sep 3
Week
Mea
n ab
solu
te fe
cund
ity
0
4
8
12
16
V V I VII VIII IX
Month
Num
ber o
f em
bryo
s
13
(Figure 6). Although being abundantly present in the plankton, the larvae haven’t been
found in stomachs in planktivorous fish.
Figure 6. Mean abundance of larvae of Marenzelleria viridis for May-October in the NE
Gulf of Riga in 1990-2002.
Only two specimen of the hydromedusae Maeotias marginata in Väinameri
Archipelago were found. The finding sites situated rather closely to those of the year
2002 and of the first findings (Väinöla and Oulasvirta 2001). Thus, based on our data it
could be stated that the hydromedusae Maeotias marginata probably forms self-
reproducing population in the Baltic Sea (Väinameri Archipelago), but due to low
population abundances, its ecological impact to pelagic invertebrates is likely
insignificant.
Zoobenthos
M. viridis was observed for the first time in the northern Baltic Sea near the mouth of the
Daugava River, the Gulf of Riga in 1988 (Lagzdins and Pallo 1994). The following four
years the polychaete densities rose more than 100 times reaching the values of 1400 ind
m-2. In the northern part of the Gulf of Riga and the Väinameri Archipelago Sea M.
viridis was found only in 1995 although the larvae of the polychaete were found since
1991. The salinity values were relatively stable at the beginning of the 1990s whereas
average summer temperature was much higher in 1994 than in previous years. Higher
0
1000
2000
3000
4000
5000
1989 1991 1993 1995 1997 1999 2001 2003
ind.
m-3
14
temperature likely resulted in a higher reproductive output of M. viridis, and hence
enhanced its dispersal to northwards. This is also indicated by a strong positive
correlation between the temperature and the number of polychaete larvae in the study
area (r2 = 0.96, p < 0.001) (Figures 7 and 8).
Figure 7. The invasion success of Marenzelleria viridis in the northeastern Baltic Sea.
In the Väinameri Archipelago Sea the polychaete established only in deeper parts
of the archipelago (7–11 m). The area is homogeneous both in terms of sediment and
macrovegetation: the sandy clay substrate is covered with a loose layer of the red algae
Furcellaria lumbricalis. The infauna below the algal mat is poorer when compared to the
sediments in unvegetated areas. The biomass of M. viridis, however, increased with the
coverage of F. lumbricalis (Figure 9).
15
Figure 8. Mean annual densities of M. viridis at salinities above and below 5 psu in the
northern Gulf of Riga in the 1990s and 2000s. The abundance of larvae is indicated by
dotted line and the dry weight of adults by solid line. Interannual changes in the summer
temperature are shown for both areas.
16
Figure 9. Relationship between the coverage of the red alga F. lumbricalis, the biomass
of infauna and M. viridis in the Väinameri Archipelago Sea.
The first observation of M. viridis at the northern coast o f the Gulf of Finland was
made in 1990 (Norkko et al. 1993). During 1990–1993 M. viridis expanded its
distribution into the eastern parts of the gulf. However, anti-clockwise circulation of the
currents would not permit M. viridis to spread from the northern side of the Gulf of
Finland towards its southern side. Only one specimen was recorded in the south-eastern
coast of the Gulf of Finland, in 1994. Until 1996 the polychaete was not observed along
the southern coast of the Gulf of Finland. Some occasiona l findings of M. viridis in the
westernmost bays of the Gulf of Finland suggest the Väinameri Archipelago Sea as a
donor region. Within the following four years M. viridis has practically established the
whole coasts of the Gulf of Finland.
When all data was pooled together the establishment of M. viridis has been more
successful either in more eutrophicated regions (e.g. river estuaries) or in more uniform
17
biotopes (e.g. deeper water or under the mat of F. lumbricalis). In the shallower areas (<
20 m) the success of the establishment increased with the number of macrozoobenthic
species in the community whereas in the deeper sites (> 20 m) the relationship was
insignificant. In the shallower areas M. viridis preferred sand or gravel bottoms. Deeper
down M. viridis was confined to silty clay bottoms. The abundance of the polychaete
larvae and adults increased with increasing total phosphorus in the water.
Following the establishment in the NE Gulf of Riga the densities of the
polychaete larvae significantly increased. The fluctuation in the polychaete larvae was
fully synchronized with the density of adults in the open sea but not at salinities below 5
psu. Instead, the biomass of adult M. viridis in less saline areas was mainly a function of
the adults in the open sea. Hence, the recruitment of M. viridis in less saline areas was
likely due to the immigration of benthic stages.
M. viridis performed diel vertical migrations (Figure 10). Regardless of the depth
the polychaetes reached to sea surface. The length of the migrating population varied
between 2–33 mm. The response to light was an important factor in the migrations. The
migration began at sunset, reached a peak at 4:00 a.m. The share of smaller polychaetes
in the vertical migrations was higher than bigger ones. Smaller individuals migrated
closer to the surface than bigger polychaetes.
Figure 10. Diel vertical migration of M. viridis at three depths in the Gulf of Riga during
summer. Average abundance, length and dry weight of the migrating population are
shown.
18
The available data on the Chinese mitten crab Eriocheir sinensis since 1991
(Table 1) point to the increased abundance of the species in the NE Baltic in 2002-2003.
This is also confirmed from the personal communications of commercial fishermen in
other coastal regions of Estonia.
Table 1. Records of findings of the Chinese mitten crab Eriocheir sinensis in commercial
gillnet fishing by one fisherman in Muuga Bay since 1991.
Date Duration of catch
Total length of nets
Number of crabs
28.06.91 Start of recording 16.12.94 114 180 1 28.10.95 48 240 1 30.10.95 42 240 1 19.10.97 48 240 1 18.11.97 66 180 1 10.08.00 54 300 1 04.07.02 24 150 1 15.10.02 54 180 1 24.10.02 48 210 1 25.10.02 24 210 1 28.10.02 24 210 1 12.11.02 72 180 1 20.05.03 24 150 1 23.05.03 24 150 1 22.06.03 54 240 1 19.08.03 24 270 1 02.11.03 48 210 1 06.11.03 24 240 1
Fish
Relative biomass contribution of Cercopagis to fish diet was higher for adult herring,
sticklebacks and adult smelt (ca 6%) whereas juvenile herring almost rejected this
cladoceran. The dominant biomass contributors of herring diet were copepods (over
70%), whereas sticklebacks and juvenile smelt consumed mostly Eurytemora (ca. 50%)
and Bosmina (12-14%) (Table 2).
19
Table 2. Percent contribution (%, wet weight, mean±s.e.) of the main prey taxa in the diet
of planktivorous fish in the main feeding areas in the NE Gulf of Riga during the main
feeding period (June-September) in 1994-1998 (from: Ojaveer et al. in press).
Eurytemora Acartia Cercopagis Bosmina Pleopsis Mysis Fish Herring ad. (51)
63.3±5.1 11.6±2.9 6.3±3.0 5.5±2.3 1.6±1.0 6.8±2.9 -
Herring juv. (20)
51.2±8.1 33.6±8.0 0.1±0.1 4.9±2.3 2.1±1.4 5.1±5.0 -
Smelt ad. (25)
3.6±1.6 4.9±4.1 6.3±4.4 3.0±2.0 4.0±3.9 43.0±8.7 26.2±5.2
Smelt Juv (30)
56.0±6.9 4.39±3.2 3.7±2.7 11.9±4.8 0.6±0.3 18.0±5.5 2.0±0.9
Stickl. (27) 48.0±6.6 4.0±1.7 6.1±2.6 13.7±4.5 6.1±1.4 - 2.9±1.9 Herring, ad. June (n=16)
76.1±6.7 19.8±6.5 - 0.5±0.3 3.7±3.1 0.1±0.1 -
July (n=14)
72.1±8.9 6.3±2.3 0.1±0.1 0.3±0.1 1.4±0.6 3.9±3.2 13.9±8.2
August (n=14)
52.0±10.6 6.7±4.0 16.5±8.2 16.6±7.7 0.3±0.3 6.3±4.2 1.0±1.0
September (n=8)
57.9±15.7 12.2±10.2 11.4±11.4 4.3±2.8 0.1±0.1 14.5±12.2 -
ad.– adult, juv. – juvenile. Stickl – two species of sticklebacks (three-spined stickleback, Gasterosteus aculeatus and nine-spined stickleback, Pungitius pungitius). Numbers in parentheses in the fist column denote the number of samples that consist of stomachs from 20 fish.
In the warm summer, when environmental conditions were favourable for
development of Cercopagis, the cladoceran formed 59% (on the wet wt basis, wwt) of the
contents of herring stomachs in the open Gulf of Riga and 66% of herrings had fed on it.
In July 2001 and 2002, Cercopagis formed 21 and 28% wwt of the herring stomachs,
respectively. In July 2002 this cladoceran formed 13% wwt of the sticklebacks’
stomachs.
Minimal size of fish with Cercopagis in stomachs varied between the species
more than two-fold: sticklebacks 3.2, herring 4.1 and smelt 7.1 cm. The threshold size of
fish is a species-specific and substantially lower for smaller adult size (sticklebacks) than
for species of bigger adult size (herring and smelt). In difference from the common
pattern for herring and smelt, smaller sticklebacks (3-4 cm) tended consume more
Cercopagis than bigger individuals (5-6 cm) showing that fish size is not a limiting factor
for predation of Cercopagis (Figures 11 and 12).
20
Based on the data from the bottom trawl surveys, herring, smelt and stickleback
do not select for Cercopagis. This is evidenced by a negative Ivlev’s electivity indices:
the higher mean electivity index was observed for adult herring (0.13) while 0-group
herring totally avoid Cercopagis. The picture can be somewhat different for the warmest
period for the open Gulf of Riga where adult herring and stickleback may select for
Cercopagis (evidenced by the electivity indices values for adult herring +0.54 and
stickleback +0.10). This process may be intensified because of high densities of the
cladoceran in the upper water layers. Electivity indices for smelt and juvenile herring in
upper layers in same time were –0.36 and –0.67, respectively.
Figure 11. The percentage contribution (wet wt. basis, mean+s.e.) of Cercopagis in
stomachs of herring Clupea harengus membras, smelt Osmerus eperlanus and
sticklebacks Gasterosteus aculeatus and Pungitius pungitius by different length classes in
the Gulf of Riga. Individuals from only those samples (usually 20 fish per sample) where
at least one fish was found to consume Cercopagis, were used (source data aggregated
from bottom trawl surveys in 1994-1998 and pelagic trawl surveys in 1999-2000).
Herring
020406080
100
7 8 9 10 11 12 13 14 15 16 17Fish length (cm)
mas
s %
Smelt
0204060
80100
7 8 9 10 11 12 13 14 15 16 17
Fish length (cm)
mas
s %
Sticklebacks
0
20
40
60
80
100
3 4 5 6Fish length (cm)
mas
s %
21
Figure 12. Relative importance (%) of Cercopagis pengoi in herring diet (by wet weight,
mean, S.E.) in the open Gulf of Riga in July 2001-2002.
Of alien fish species, only the gibel carp Carassius gibelio was encountered in
experimental gillnet fishing in Pärnu Bay. Based on the data from 2002 and from three
earlier years it could be concluded that abundance of the gibel carp was rather low. The
fish was found in four stations out of seven sampled and annual catch (each station was
fished twice, fishing was performed in autumn) varied between 1 and 15 individuals.
Another alien cyprinid, which is to some extent present in commercial fishing catches –
the common carp Cyprinus carpio - was not found in experimental gillnet catches.
Despite of our efforts with commercial fishermen, there are no further evidences on
the presence of the round goby Neogobius melanostomus in Pärnu Bay, the area of the
first finding of the species in Estonian waters early 2002.
2. Monitoring of port areas (high-risk areas of primary invasions)
2.1. Material and Methods
Zooplankton
The composition of zooplankton was investigated in most important port areas - as the
high-risk areas in terms of biological invasions. Muuga Harbour (Port of Tallinn, the Gulf
of Finland) and Port of Pärnu (Gulf of Riga) was sampled regularly during the ice-free
0
10
20
30
40
50
60
10 11 12 13 14 15 16 <17Length (L, cm)
ww
t, %
20012002
22
season – generally twice per month. In each occasion three (Muuga Harbour, see Figure
13) and one (Port of Pärnu) predefined sites were visited. Vertical sampling was
performed directly from port terminal (Muuga Harbour) or on boat (Port of Pärnu) by
large Juday net (mesh size 90 µm) The samples were preserved until laboratory analysis
in formaldechyde solution.
Zoobenthos
Macrozoobenthos was sampled in Muuga Harbour and Port of Pärnu regularly (once per
month) during the ice- free season. The sampling procedure was similar as in the
monitoring of the most important invasions. The sampling was done with bottom grabs
(Lenz or Petersen types) and benthic sledge. In Muuga harbour, the sites were selected to
cover the most prevailing depth zones – shallow sites at 1 m (site 1), intermediate depths
(site 3) at 10 m and deep sites at 14 m (site 2). The depth of the station in Pärnu Bay was
6 m.
2.2. Preliminary results
Zooplankton
The full species list is given in the Table 3 where alien species are indicated in bold.
There are four zooplankton species present in the port area, which were not been
encountered in the national monitoring station in Muuga Bay during the last decade:
Chydorus sphaericus, Diaphanosoma brachyurum, Asplanchna sp. and Argulus
foliaceus. The first three are typical freshwater species and appear sometimes in the
littoral zone of the Baltic Sea. The fish louse Argulus foliaceus is rarely encountered in
zooplankton samples, however this species is very common in the littoral zone.
The most frequent and abundant zooplankton species in the port area and outside
(Muuga Bay) are the same: Acartia bifilosa, Eurytemora affinis and Synchaeta baltica.
23
Figure 13. Sampling sites in Muuga Harbour (indicated by black arrows, from left to
right): Site 1 – Ro-Ro and Container terminal; Site 2 – Grain terminal; Site 3 – Oil
terminal.
24
Table 3. List of zooplankton species found in three sampled sites in Muuga harbour (for
location of sites see Figure 13). Alien species are given in bold.
Site 1 Site 2 Site 3
Copepods Limnocalanus macrurus Limnocalanu s macrurus Limnocalanus macrurus Acartia bifilosa Acartia bifilosa Acartia bifilosa Eurytemora affinis Eurytemora affinis Eurytemora affinis Centropages hamatus Centropages hamatus Temora longicornis Pseudocalanus minutus
elongates Pseudocalanus minutus elongates
Pseudocalanus minutus elongates
Mesocyclops leuckarti Mesocyclops leuckarti Mesocyclops leuckarti Harpacticoida (Ectinosoma
curticorne) Harpacticoida (Ectinosoma curticorne)
Harpacticoida (Ectinosoma curticorne)
Cladocerans Bosmina coregoni maritima Bosmina coregoni maritima Bosmina coregoni maritima Cercopagis pengoi Cercopagis pengoi Cercopagis pengoi Chydorus sphaericus Chydorus sphaericus Daphnia sp. Diaphanosoma brachyurum Evade normanni Evade normanni Evade normanni Pleopsis polyphemoides Pleopsis polyphemoides Pleopsis polyphemoides Podon intermedius Podon intermedius Rotifers Asplanchna sp. Asplanchna sp. Keratella cochlearis
recurvispina Keratella cochlearis recurvispina
Keratella cochlearis recurvispina
Keratella cruciformis eichwaldi
Keratella cruciformis eichwaldi
Keratella quadrata quadrataKeratella quadrata quadrataKeratella quadrata quadrata Synchaeta baltica Synchaeta baltica Synchaeta baltica Synchaeta monopus Synchaeta monopus Synchaeta monopus Meroplankton Polychaeta larvae Polychaeta larvae Polychaeta larvae Balanus improvisus nauplii Balanus improvisus nauplii Balanus improvisus nauplii Gastropoda larvae Gastropoda larvae Gastropoda larvae Lamellibranchiata larvae Lamellibranchiata larvae Lamellibranchiata larvae Others Fritillaria borealis acuta Fritillaria borealis acuta Fritillaria borealis acuta Argulus foliaceus Zoobenthos
Muuga Harbour. Field surveys indicated that the benthic communities in the port area
significantly distinguished from the sites in the adjacent sea. Adjacent to the port area
very strong water currents were measured. Strong currents were generated by very high
25
wave energy input to the system. Hence, there was a clear relationship between depth and
species composition, abundance and biomass of macrozoobenthos. Shallowest sites had
low species number, abundance and biomass values. Deeper down the diversity,
abundance and biomass of macrozoobenthos gradually increased (Figure 14).
0200400600800
10001200
10 12 14 16 18 20
Depth, m
Ab
un
dan
ce, i
nd
./m2
0
50
100
150
200
10 12 14 16 18 20
Depth, m
Bio
mas
s, g
/m2
Figure 14. Depth distribution of abundance and biomass of macrozoobenthos in the sea
area adjacent to the port of Muuga.
On the other hand, sedimentation processes are very active in the port area. Clay
particles dominate in the sediment. Moderate mixing took place at the surface layer of
sediment due to the shipping. The concentration of food particles were significantly
higher in such accumulation areas, hence, abundance and biomass values of benthic
invertebrates were much higher in port areas as compared to any coastal site of the Gulf
of Finland. These diverse and dense macrozoobenthic communities (150-550 g wet
weight m2) supported the presence of three nonindigeneous species – the cirriped Balanus
improvisus, the bivalve Mya arenaria and Potamopyrgus antipodarum. However, their
26
share to the total abundance and biomass values was relatively low (< 10%). It is likely,
however, that owing to moderate disturbance and favourable feeding conditions the risk
of establishment is very high for the most estuarine species in the port of Muuga.
Table 4. Species composition of macrozoobenthos in the three sites of the Muuga
Harbour during the ice-free season. Alien species are shown in bold.
Site 1 Site 2 Site 3 Crustaceans Worms Bivalves Insects
Neomysis integer Gammarus salinus Corophium volutator Oligochaeta Mya arenaria Cerastoderma glaucum Theodoxus fluviatilis Hydrobia ulvae Chironomidae
Monoporeia affinis Balanus improvisus Gammarus oceanicus Corophium volutator Jaera albifrons Hediste diversicolor Halicryptus spinulosus Oligochaeta Prostoma obscurum Mytilus edulis Macoma balthica Cerastoderma glaucum Potamopyrgus antipodarum Hydrobia ulvae Hydrobia ventrosa Chironomidae
Balanus improvisus Corophium volutator Oligochaeta Prostoma obscurum Hediste diversicolor Mytilus edulis Macoma balthica Cerastoderma glaucum Mya arenaria Hydrobia ventrosa Hydrobia ulvae
Port of Pärnu. The macrobenthic communities in the port of Pärnu were characterised as
moderately disturbed due to municipal pollution of Pärnu Town and wave exposure. The
abundance and biomass of macrozoobenthos in the port area are slightly lower than in the
adjacent sea. However, due to high nutrient input to Pärnu Bay in general, the values
exceeded those in the most Estonian coastal sea areas. The abundance and biomass of
macrozoobenthos ranged between 2900-4200 ind m-2 and 135-185 g wet weight m-2. The
following species were found in the study site (aliens are marked as bold): Oligochaeta,
Hediste diversicolor, Marenzelleria viridis, Corophium volutator, Balanus improvisus,
Macoma balthica, Mya arenaria, Dreissena polymorpha and Hydrobia ulvae.
Corophium volutator, Macoma balthica, Oligochaeta prevailed in abundance and
Macoma balthica and Mya arenaria in biomass, respectively. The relative share of alien
species in the invertebrate communities is shown in the following table (Table 5).
27
Table 5. The share of alien species within invertebrate (%) communities in the port of
Pärnu.
Species Abundance Biomass Marenzelleria viridis 1.7 0.1 Mya arenaria 2.4 69.3 Dreissena polymorpha 0.7 1.5 Balanus improvisus 0.3 1.2
3. Experiments with selected alien species
3.1. Material and Methods
Zooplankton
For testing the hypothesis postulated by us earlier that the two morphological forms
(atypical - spring form and typical - summer form) of Cercopagis identified represent
different ontogenetic stages of the species Cercopagis (Cercopagis) pengoi and,
therefore, only one species from the genus Cercopagis occurs in the Baltic Sea (Simm
and Ojaveer 1999), two experiments were carried out. Firstly, we have collected resting
eggs from Cercopagis and hatched them in laboratory conditions. The resting eggs were
collected from typical (summer form) Cercopagis individuals caught form Pärnu Bay in
summer, 2002 and kept them at ca 6°C in darkness until hatching in February, 2003.
Another experiment was set up in spring 2003 when we collected atypical (spring form)
individuals from the Gulf of Riga and kept them in laboratory conditions (20°C, natural
light conditions) until embryos were released from brood pouch in early June.
For studies on Cercopagis feeding habits, we have performed several runs of
feeding experiments (in total over 125 successful experiments) where both adults and
newly born youngs of Cercopagis were provided with various prey items (larvae of the
barnacle Balanus improvisus, copepods Acartia spp. and Eurytemora hirundoides,
copepod nauplii and Bosmina coregoni maritima) and their mixtures (copepod nauplii
and adult copepods; copepod nauplii and Balanus). The experimental medium was
natural filtered seawater. Experiments were run in 1- liter glass jars for 8 hours with pre-
adapted predator and prey. The predator density varied between 1 and 2 ind/m-3, that of
prey from 10 to 60 ind/m-3. All zooplankters were counted individually prior and after the
experiment. Only alive and healthy individuals were used. For testing reliability of the
obtained data, experimental control runs for all the prey taxa were performed.
28
Zoobenthos
We estimated the role of human-mediated introductions in the structure and development
of biotic assemblages and marine food webs. The field experiments on the biology of
selected invasive species were carried out in order to assess their impact on natural
communities.
To test whether the introduced polychaete M. viridis had a potential to
outcompete the native fauna we examined the effects of the polychaete on the
biodiversity of benthic communities. Besides, we evaluated the grazing impact of
D. polymorpha on the phytoplankton community.
An in situ experiment, combining natural densities of the native species and the
introduced polychaete, was carried out at a shallow semi-enclosed bay in the north-
eastern Baltic Sea (Figure 15). Altogether 40 mesocosms of 3 l were used to permit 8
treatments replicated 5 times. The test organisms were added in accordance with their
values in the field. The mesocosms were closed by a mesh-net to minimise the risk of
migration but at the same time assure sufficient water exchange. The experiment lasted
for 18 days. Prior to the experiment the length (± 0.1 mm) and dry weight (80 ºC, 48 h, ±
0.001 g) of 20 individuals of each studied species (representing the same cohort as used
in the experiment) were determined. At the end of the experiment the sediment in the
buckets were sampled (Ø=9.6 mm, sampling depth 50 mm) for chlorophyll a and
phaeopigments. Living animals were counted and the length and dry weight of all
experimental animals were determined.
The grazing impact of the alien bivalve Dreissena polymorpha and the native
bivalve Mytilus edulis was studied on three transects in the littoral zone of the Gulf of
Riga (GOR) and two transects in the Gulf of Finland (GOF) (Figure 15). Northern GOR
was characterised by a wide coastal zone with a diverse bottom topography and extensive
reaches of boulders. Depending on the salinity values a scattered population of M. edulis
or D. polymorpha occurred on the boulders. The southern transect had a narrow coastal
zone. Coarse sandy substrate prevailed down to a depth of 4 m being replaced by
boulders at greater depths. The boulders housed a dense population of D. polymorpha.
Hard substrate prevailed at the northern GOF site. The coverage of M. edulis was almost
100 % along this transect. The southern GOF was characterised by a mixture of sand,
29
pebbles and boulders above 3 m depth. Deeper down only sandy substrate is found and,
hence, the area was practically devoid of filter- feeding bivalves.
Figure 15. Study area. The transects of M. edulis are indicated by crosses and that of D.
polymorpha by open circles.
Samples were collected from the seashore down to 12 m depth at a step of 1 m. Metal
frames of 20×20 cm surface area were placed randomly on the bottom by diver. All filter-
feeders within the frame were collected. Three replicates were taken at each location. The
length of the filter- feeders was measured to the nearest 0.1 mm using vernier callipers.
The in situ grazing rates of M. edulis and D. polymorpha were estimated by
quantifying the egestion of Chl a by the mussels. Bivalves of 9-31 mm shell length were
collected by diver in the vicinity of deployment. Three individuals were placed on the net
of the funnel allowing biodeposits to sediment to the collecting vial below. During
deployment the temperature and salinity was monitored. After deployment the shell
lengths were recorded, the sedimented material in the vials was sorted under a dissecting
microscope, faeces was collected with a pipette and filtered on Whatman CF/F filters
within 4 h of retrieval. Filters were extracted in dark in 96% ethanol overnight. Chl a was
quantified fluorometrically correcting for phaeopigments (Pha). The values of Chl a
equivalent or total Chl a (Chl a eq) were calculated as Chl a eq = Chl a + 1.52 × Pha.
30
Algal grazing by the mussel population was estimated from the functional relations after
correction for loss of Chl a during gut passage. When considering their effect at the
population level the data on ambient temperature, salinity, Chl a concentration, mussel
abundance and size distribution were taken into account. Grazing by individuals of
different size (Gl) was scaled by shell length, i.e. Gl = G20 × l2/202, where G20 is the
grazing rate of 20 mm individua ls and l the shell length. For more detailed description of
the methods see Kotta & Møhlenberg (2002).
3.2. Preliminary results
Zooplankton
Hatching experiments with resting eggs showed that from the resting eggs, collected from
the ‘summer form’ specimen of Cercopagis pengoi characterised by a relatively long
caudal process with S-loop and backwardly bent or straight tips of barbs, ‘spring form’-
like individuals were hatched. These had a straight and relatively short caudal process
with forwardly bent tips of barbs (Figure 16). Hatching experiments with the ‘spring
form’- like adult Cercopagis individuals, caught from Pärnu bay in spring, confirmed our
hypothesis that the released youngs are, in fact, typical (or ‘summer form’-like)
Cercopagis individuals. These experimental results suggest that at least in the Gulf of
Riga, there is only one species of the genus Cercopagis – Cercopagis pengoi. Similar
conclusion has been made earlier on the basis of genetic studies with Cercopagis in the
North American Great Lakes (Makarewicz et al. 2001). However, these results are in
contradiction to those of Gorokhova et al (2000) who have suggested presence of several
species/forms of Cercopagis in the Baltic Sea.
31
Figure 16. ‘Spring form’ of Cercopagis pengoi: straight and relatively short caudal
process with forwardly bent tips of barbs (four pairs).
Although the feeding data haven’t been analysed in detail as yet, several basic
conclusions on feeding ecology of Cercopagis could be made:
1. Cercopagis, both adult and newly born young stages, are able to consume other prey
than small sized cladocerans ( Bosmina);
2. Newly born youngs are unable to prey on adult copepods, presumably due to size
problems;
3. Feeding intensity of Cercopagis is higher for the prey with limited escape response
(e.g., Balanus larvae an copepod nauplii) than for more evasive prey (adult
copepods);
4. Copepod nauplii are more preferred prey than adult copepods and Balanus larvae;
5. Cercopagis may cause reductions of food resource for larval fish that may, in turn,
result in declined year-class abundances of many commercial fish species
Zoobenthos
Effect of Marenzelleria viridis on the native fauna
In the shallower sites of the Gulf of Riga we found a significant negative correlations (p <
0.05) between the biomass of M. viridis and the bivalve M. balthica. Adjacent to the
rivermouth the positive correlation between M. viridis the amphipod C. volutator was
found. Concurrent with the invasion of M. viridis the deep water amphipod M. affinis has
32
notably decreased in the Gulf of Riga (Figure 17). The occurrence of the polychaete H.
diversicolor might facilitate the establishment of M. viridis. In the Väinameri
Archipelago Sea M. viridis established itself successfully in the areas where H.
diversicolor prevailed among the polychaetes. Already one year later, the density of H.
diversicolor had dropped from about 500 ind m-2 to almost nil.
Figure 17. Relationships between the biomass of M. viridis, C. volutator, M. balthica in
the shallower water and between M. viridis and M. affinis in the deeper sites of the Gulf
of Riga in the 1990s and 2000s.
An in situ experiment, combining natural densities of the shallow water species
and the introduced polychaete, showed that sediment chlorophyll a content in the
treatment with M. viridis was significantly higher than in all other treatments (ANOVA p
< 0.001). M. viridis reduced the survival of H. diversicolor (p < 0.05). As suggested by
the field observations the survival and growth of M. viridis was higher when co-occurring
with H. diversicolor compared to the treatment with only M. viridis. The presence of M.
viridis increased the growth of the bivalve M. balthica (p < 0.001). The survival of M.
viridis was significantly reduced by the presence of M. balthica (p < 0.05) (see also
Figure 18)
33
Figure 18. Survival (% of initial numbers) of M. viridis and H. diversicolor and the
growth of M. balthica at different treatments (mean value ± S.E.)
A laboratory experiment showed that food, density of M. affinis and the presence
of M. viridis had a significant effect on the growth of the amphipods (3-way ANOVA,
Food: p<0.001, Density: p<0.05, Marenzelleria : p<0.05). Growth was significantly
higher in microcosms where food was added. The amphipod growth was density
dependent in the absence of the polychaetes and not so in the presence of the polychaetes.
At average field densities (2000 ind m-2) and with food addition the amphipods grew
faster in the absence of the polychaetes than in their presence. The survival of M. affinis
was significantly affected by food addition (Figure 19). The survival was not affected by
amphipod density or the presence of the polychaetes (3-way ANOVA, Food: p<0.05,
Density: p>0.05, Marenzelleria : p>0.05).
34
To conclude, owing to its unprecedented invasion success, dominance in many
biotopes, and the effects on the native macrofauna the polychaete may be ranked among
the most influential exotics in the northern Baltic Sea. Competitive interactions between
M. viridis and M. balthica appear a key factor limiting the further expansion of M. viridis
in the study area.
Figure 19. Mean growth in length (± SE) of M. affinis in relation to food availability,
amphipod density and presence of M. viridis.
Grazing impact of Dreissena polymorpha and Mytilus edulis.
Biodeposition rates of the bivalves (µg Chl a eq ind-1 h-1) were mainly a function of
ambient temperature and Chl a eq. The biodeposition values increased curvlinearly with
temperature and ambient Chl a eq. The effect of temperature interacted with Chl a eq.
There were statistically significant differences in the regression coefficients between
different basins, sites within a basin and seasons. The two studied bivalve did not differ
35
in their biodeposition rates. In general, the biodeposition values were higher at GOF sites
than at GOR sites. The winter values were significantly lower from other seasons.
The filtration rates (l ind-1 h-1) of the studied species increased curvlinearly with
ambient temperature. There was a significant interaction between temperature and Chl a
eq. The effect of Chl a eq varied between sites and seasons. The filtration rate of D.
polymorpha decreased with increasing salinity. M. edulis had significantly higher
filtration rate than D. polymorpha. Similarly to the biodeposition values the filtration
values were higher at GOF sites than at GOR sites and the winter values were lower from
other seasons.
The major variability in population grazing potential (% of overlaying water
filtered m-2 h-1) was due to the spatial differences in the density of bivalves. The filter
feeders removed daily on average from 3 to 2426% of phytoplankton stock in the coastal
area. Population grazing decreased with increasing Chl a eq i.e. eutrophication level. The
effect of the bivalves was highest in July owing to low phytoplankton biomass and high
filtration activity.
Figure 20. Potential of the populations of M. edulis and D. polymorpha to filter the
overlying water column (% of overlaying water filtered m-2 h-1).
36
Figure 21. Relationships between water Chl a eq and the average population grazing of
bivalves in the littoral zone (0-12 m).
37
4. Presentations of the project results at international conferences
Kotta, J., Orav-Kotta, H., Kotta, I. and Simm, M. 2003. Effects of the introduced
polychaete (Marenzelleria viridis) on the simple ecosystem of the northern Baltic
Sea. Third International Conference on Marine Bioinvasions. La Jolla, California,
March 16-19, 2003 & ICES Annual Science Conference, Tallinn, Estonia,
September 24-27, 2003.
Lankov, A., Simm, M. and Ojaveer, H. 2003. Role of the three-spined stickleback
Gasterosteus aculeatus in the food-web of the Gulf of Riga. Baltic Sea Science
Congress. Helsinki, Finland, August 24-28, 2003.
Ojaveer, H., Simm, M., Lankov, A. and Kotta, J. 2003. Population dynamics and
ecological impacts of the Ponto-Caspian predatory cladoceran (Cercopagis pengoi)
in the Baltic Sea. Third International Conference on Marine Bioinvasions. La Jolla,
California, March 16 – 19, 2003.
Panov, V., Kotta, J., Laine, A., Berezina, N. and Maximov, A. 2003. Research on alien
species in the Gulf of Finland: current state and perspectives. Baltic Sea Science
Congress, Helsinki, Finland, August 24-28, 2003.
Põllumäe, A. and Väljataga, K. 2002. “Cercopagis pengoi (Cladocera) in Southern Gulf
of Finland: horizontal distribution, seasonal pattern and interaction with other
zooplankton. International Symposium: The Changing State of the Gulf of Finland
Ecosystem, Tallinn, October 28-30, Tallinn
Simm, M., Kotta, J. and Kotta, I. 2003. Larvae of zoobenthos, meroplankton, in the NE
Gulf of Riga. Baltic Sea Science Congress, Helsinki, Finland, August 24-28, 2003.
Simm, M. and Ojaveer, H. 2003. Taxonomic status and development of Cercopagis –
bioinvasion induced improvement of basic knowledge. Baltic Sea Science
Congress, Helsinki, Finland, August 24–28, 2003.
38
5. Publications (in press, in review, in preparation)
Kotta, J., Torn, K., Martin, G., Orav-Kotta, H. and Paalme, T. Seasonal variation of
invertebrate grazing on Chara connivens and C. tomentosa in Kõiguste Bay, NE
Baltic Sea. Helgoland Mar. Res. (submitted).
Kotta, J., Orav-Kotta, H., Kotta, I. and Simm, M. Effect of the introduced polychaete
Marenzelleria viridis on the simple ecosystem of the northern Baltic Sea.
Biological Invasions (submitted).
Kotta, J., Orav-Kotta, H. and Vuorinen, I. Field measurements on the variability in
biodeposition and grazing pressure of suspension feeding bivalves in the northern
Baltic Sea. In: R. Dame & S. Olenin (eds) The Comparative Roles of Suspension
Feeders in Ecosystems. Kluwer Academic Publishers, The Netherlands, Dordrecht
(submitted).
Simm, M., Kukk, H. and Viitasalo, M 2003. Pelagic larvae of the invader Marenzelleria
viridis (Polychaeta; Spionidae) in the plankton community of the NE part of the
Gulf of Riga, Baltic Sea. Proceedings of the Estonian Academy of Sciences.
Biology, Ecology (in press).
Ojaveer, H., Simm, M. and Kotta, J. 2003. Importance of alien species in the globalizing
world: aquatic ecosystems. In: Kaasaegse ökoloogia probleemid. Eesti IX
Ökoloogiakonverentsi lühiartiklid, 185 – 192 (in Estonian).
Ojaveer, H., Simm, M. and Lankov, A. Population dynamics and ecological impact of the
non- indigenous Cercopagis pengoi in the Gulf of Riga (Baltic Sea). Hydrobiologia
(in press).
Põllumäe, A and Väljataga, K. Cercopagis pengoi (Cladocera) in Southern Gulf of
Finland: horizontal distribution, seasonal pattern and interaction with other
zooplankton (in preparation).
39
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Ojaveer, H., Leppäkoski, E., Olenin, S., and Ricciardi, A. 2002. Ecological impacts of Ponto-Caspian invaders in the Baltic Sea, European inland waters and the Great Lakes: an inter-ecosystem comparison. In (eds. E. Leppäkoski, S. Gollasch and S. Olenin) Invasive Aquatic Species of Europe: Distribution, Impacts and Management. Kluwer Scientific Publishers, Dorthrecht, The Netherlands, pp. 412-425.
Ojaveer, H., Simm, M. and Lankov, A. Population dynamics and ecological impacts of the non- indigenous Cercopagis pengoi in the Gulf of Riga (Baltic Sea). Hydrobiologia (in press).
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Vanderploeg, H.A., Nalepa, T.F, Jude, D.J, Mills, E.L., Holeck, K.T., Liebig, J.R., Grigorovich, I.A. and Ojaveer, H. 2002. Dispersal and emerging ecological impacts of Ponto-Caspian species in the Laurentian Great Lakes. Canadian Journal of Fisheries and Aquatic Sciences, 59: 1209-1228.
Väinölä, R. and Oulasvirta, P. 2001. The first record of Maeotias marginata (Cnidaria, Hydrozoa) from the Baltic Sea: a Pontocaspian invader. Sarsia 86: 401-404.
