such as "Introduction", "Conclusion"..etc
Nadezhda A. Berezina
Russian Academy of Sciences,
Universitetskaya emb. 1, 199034 St. Petersburg, Russia.
OCEANOLOGIA, 49 (1), 2007.
At present, the number of nonindigenous invertebrates in different parts
of the world has increased, resulting in structural and functional changes
of aquatic ecosystems (Lozon & MacIsaac 1997, Cohen & Carlton 1998).
A wide variety of human-mediated vectors such as deliberate and accidental
introductions, natural migration via constructed inland waterways and high
rates of spread, survival and reproduction in these species have facilitated
the rapid dispersal and successful establishment of new species in Europe
(Lepp¨akoski, Olenin & Gollasch (eds.) 2002). As a rule, the dispersion of
euryoecious species of amphipods in diverse directions is a rapid process,
owing to the ability of a species to migrate great distances and successfully
establish themselves under new conditions (Berezina 2004).
The amphipod Gammarus tigrinus Sexton, 1939, a species native to
estuaries of the Atlantic seaboard of North America, has an extensive
invasion history. It was introduced into Northern Ireland with ballast water
transport (Hynes 1955). G. tigrinus was first reported in 1931 in England in
fresh waters contaminated by natural brine seepage (Sexton & Cooper 1939)
and in purely freshwater sites in Northern Ireland (Hynes 1955). The wide
spread of G. tigrinus in inland European waters began after its intentional
introductions to supplement fish feeding: from a brackish lake in England to
the salt-polluted River Werra, Germany in 1957 (Schmitz 1960), and from
the freshwater Lough Neagh in Northern Ireland to the freshwater Ijsselmeer
in the Netherlands in 1960 (Nijssen & Stock 1966, Pinkster et al. 1977).
In the Baltic Sea it was first found in the Schlei Fjord in 1975 (Bulnheim
1976). In 1994 G. tigrinus was recorded in the Mecklenburg area (Rudolph
1994) and the Odra Estuary (Gruszka 1999, Jazz¿ewski & Konopacka 2000).
Soon, it spread along the entire Baltic Sea shore of northeastern Germany
(Zettler 2001). During the next decade the species reached Puck Bay
(Szaniawska et al. 2003) and the Vistula Lagoon (Jazdzewski et al. 2002,
Ezhova et al. 2005). In 2002 the Vistula Lagoon was the easternmost
point of the range of G. tigrinus in the Baltic (Ja¿d¿ewski et al. 2002).
Nevertheless, already in 2003 it was recorded in the Gulf of Riga (Kotta
2005), off the Finnish coast in the Gulf of Finland (Pienim¨aki et al. 2004)
and later in the Curonian Lagoon (Daunis & Zettler 2006), where it was
probably introduced accidentally with the ballast water of ships. The high
risk of G. tigrinus expanding to the easternmost part of the Baltic Sea, the
Neva Estuary, had been predicted earlier (Berezina 2004). From 2004 to
2006 intensive midsummer surveys were carried out in the littoral zone of
the eastern Gulf of Finland (Neva Estuary) in order to monitor the invasion
of nonindigenous species.
One of the largest estuaries (3600 km2) in the Baltic Sea, the Neva
Estuary consists of three main parts: Neva Bay, and the inner and outer
estuary. Neva Bay is the freshwater part with a low salt content in the
water (42.75 mg dm-3); it has been separated from the other parts of the
estuary by a storm-surge barrier (dam) since the early 1980s (Fig. 1). The
water salinity ranges from 210 to 2500 mg dm.3 in the inner estuary and is
influenced by mesohaline waters from open part of the Gulf of Finland. The
summer daytime temperature varied from 14 to 25oC in 2004 and from 16
to 25oC in 2005 and 2006. The substrate at the locations studied consists
of coarse sand, gravel and stones. In different parts of the Neva Estuary,
the macroalgal communities growing on hard substrates consist of the green
algae Cladophora glomerata and Enteromorpha intestinalis, the brown algae
Pilayella litoralis and Ectocarpus spp., and the red alga Ceramium rubrum.
The estuary is impacted by a number of human activities, such as discharges
of large amounts of waste waters from St. Petersburg and its hinterland,
intensive ship traffic, the development of new ports and oil terminals,
commercial and sporting fishery, and recreation.
During the survey in 2004 G. tigrinus was not found at all the monitoring
sites. The first individuals of this North American amphipod were recorded
at only one site in the eastern Gulf of Finland, near a new oil terminal (site 1
. 60o20'27''N, 28o41'54''E; Fig. 1) on July 12, 2005. The density of the
newcomer averaged 27 indiv. m-2 (+/-15 SE). A year later, on July 11, 2006,
it was found at two sites (site 1, and site 2 . 60o09'44''N, 29o09'34''E) with
respective average densities of 99 (+/-50 SE) and 126 indiv. m-2 (+/-21 SE).
In 2006 a total of 39 G. tigrinus individuals were found, 36% of them males,
38% female, and the remainder juveniles. The body lengths of males and
females were 10.13 and 7.11 mm respectively. 60% of females were fecund
with 16.30 eggs of stage 2-3 in their marsupia. Native species of amphipods
(Gammarus zaddachi, G. salinus and G. oceanicus) and isopods (Jaera
albifrons praechirsuta and Saduria entomon), the nemertine Cyanophthalma
obscura, the mysid Neomysis integer, the gastropods Theodoxus fluviatilis,
Bithynia tentaculata and Lymnaea ovata, together with some oligochaete
and aquatic insect species (Chironomidae, Ephemeroptera and Trichoptera)
were present in the benthic communities at the sites. In 2006 G. tigrinus
contributed 1.5% (site 1) and 2% (site 2) to the total density of the
Past predictions (Bulnheim 1976, Pinkster et al. 1977) on the eventual
range of expansion of G. tigrinus have come true, owing to the high
ecological potency and reproductive capacity of this species. This invader
has a high salinity tolerance. Native to the tidal estuaries in the northwest
Atlantic Ocean, it is widely distributed from the St. Lawrence River
in Quebec to the east coast of Florida, and occurs in salinities of up
to 25 PSU (Bousfeld 1973, Kelly et al. 2006). The species is known
from German freshwaters with a high ion content. It has been shown
experimentally that G. tigrinus is able to regulate extracellular K+ in river
waters with high potassium concentrations, and to tolerate stress levels
(>380 mmol dm-3) of Na+ and Cl-, which makes it more successful in saltpolluted
rivers than other amphipods, including its competitor Gammarus
pulex (Koop & Grieshaber 2000). G. tigrinus has become a member of the
benthic community in many freshwater systems. Since 2002 G. tigrinus has
been recorded in samples from the Laurentian Great Lakes, where it has
reproduced successfully (Grigorovich et al. 2005, Kelly et al. 2006).
A study of the invasion pathways between the source and introduced
populations of the amphipod G. tigrinus using a molecular phylogeographical
approach has shown that the most divergent clades occurred
in the British Isles and mainland Europe and were sourced from the St.
Lawrence and Chesapeake/Delaware Bay estuaries. G. tigrinus did not
occur in freshwater at putative source sites, but colonized both fresh and
brackish waters in the British Isles, Western Europe and Eastern Europe.
Populations consisting of admixtures of the two invading clades were found
principally in recently invaded fresh and brackish water sites in Eastern
Europe, and were characterized by a higher genetic diversity than the
putative source populations (Kelly et al. 2006).
The Gulf of Finland is a transition zone on waterways from Europe
and other continents to the inland waters of Russia. The further dispersal
of G. tigrinus to the lakes of Eastern Europe via the canal-river systems
from the easternmost Baltic area is currently possible, which may result
in unforeseeable consequences for ecosystem stability, something that
happened in the largest European lake, Lake Ladoga, after the successful
establishment there of another successful invader, the Baikalian amphipod
Gmelinoides fasciatus (Berezina in press). Comprehensive risk assessments
of new G. tigrinus invasions should be based on possible vector pathways,
the invasibility of systems, and the ecological requirements of the new
invader, including information on its tolerance to environmental factors,
life cycle traits, reproduction rate, food habits, energy requirements, and
strength of interactions with other species.
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