In German inland and coastal waters of the North and Baltic Sea, about 141 aquatic alien species from different taxonomic groups have established permanent populations so far. First data analyses show that German waters are colonized primarily by alien invertebrate species, partly on a considerable scale. About every second species has already spread successfully across a larger area, about every fifth species can be defined as invasive.
Between the three German aquatic ecosystems (inland waters, North and Baltic Sea coasts) major natural hydrographical and topograhical differences exit, which is also reflected in a distinct occurrence of the alien species. Several alien species have become established in inland and coastal waters, because they can adapt to a wide range of salinities including freshwater (e.g. the Caspian zebra mussel Dreissena polymorpha and the Chinese mitten crab Eriocheir sinensis).
In Europe the invasive Chinese mitten crab Eriocheir sinensis
was first observed in the German river Aller in 1912.
At least 95 species are recognized as established aliens in the freshwater environment of Germany.
In the ecologically important group of phytoplankton no alien species could be observed yet. For several years, however, Cylindrospermopsis raciborskii, a toxic freshwater cyanobacterium of tropical origin, is increasingly found in Germany and in other temperate regions (Mischke 2001). This species shows a distinct distribution pattern and occurs mainly in non-interconnected lakes, ponds and reservoirs. Therefore, it is very likely that C. raciborskii, which can develop spore-like dormant forms (akinetes), has reached new areas with the assistance of migratory birds as natural vector (attached to feet or feathers or by internal transport in the digestive system) as it is reported for several other planktonic species (Nehring 1998). Taking into account that its human mediated introduction is unlikely, at present C. raciborskii can not be regarded as alien.
Information about the occurrence of introduced higher freshwater plants are rare. Currently, sixteen alien species are considered as established. Two of them, the invasive North-American waterweeds Elodea canadensis and E. nuttallii, are wideley distributed and are found primarily and frequently with nuisance densities in standing waters and in slow flowing creeks and rivers (Kowarik 2003).
Among the zooplankton, various “exotic” Daphnia species have been observed during the last decades, however, no information on their present distribution and current status of establishment in the area is available. Doubts have been raised about their status as alien species, also.
The majority of aliens in German freshwater environments are macroinvertebrates. According to Tittizer et al. (2000), 35 macrozoobenthic species are of allochthonous origin in federal waterways. Additionally, some other alien invertebrate species are found in inland waters (a.o. Geiter et al. 2002), of which seventeen species have established permanent populations. The most important molluscan invader is the the Caspian zebra mussel Dreissena polymorpha. In Germany this invasive species lives in lakes, dam reservoirs and large rivers, as well as in brackish water bodies connected to the North and Baltic Sea. Increasing water pollution in the mid-20th century strongly reduced Dreissena populations. The continuous improvement of water quality since the 1980s has allowed the populations to recover; they nowadays have again attained densities of up to 40,000 individuals per m² (Böhmer et al. 2001). Native mussels belonging to the genera Unio and Anodonta, of which some are listed in German Red Lists, are co-opted by zebra mussels as hard subrate for successful settlement. Those mussels that serve as “hosts” for Dreissena are effectively starved, because undisturbed filter feeding is no longer possible (Böhmer et al. 2001).
The sucessfull dispersal of the invasive Asiatic calm Corbicula fluminea in
Germany is correlated with the heat pollution of the rivers by power
plant stations (Schöll 2000, modified).
In scientific literature about 26 alien fish species were recorded, for which natural reproduction in German waters could not be excluded. However, most of these freshwater species have a questionable invasion status (Geiter et al. 2002). Only fifteen fish species are considered as aliens yet, which occur in self-sustaining populations (Geiter et al. 2002; Kowarik 2003; Nehring et al. in prep.). Most of these species have a limited distribution and do not seem to expand their ranges. Only the American pumpkinseed Lepomis gibbosus and the Pontocaspian gobbies are known to colonise new habitats by active migration (Schadt 2000; Freyhof 2003).
Various reports of exotic amphibians in freshwaters, being intentionally released by their owners are known. Due to climatic conditions they have no chance to establish permanent populations in Germany yet. However, a first exception to the rule is the bullfrog Rana catesbeiana, imported from America as a faunistic addition in private garden ponds and aquaria. For several years this invasive species reproduced successfully in natural ponds and lakes nearby Karlsruhe (LfU 2001). There is a serious risk that native species (a.o. crabs, snails, fish, amphibians) will be displaced by bullfrogs throughout Germany, if this invasive species continue to spread. Therefore every year control measures are conducted to combat the spread of the population nearby Karlsruhe (Reinhardt et al. 2003. The second exception to the rule is the Italian Crested Newt Triturus carnifex, which was released intentionally about 1990 in the German alps.
Several parasite species are reported as introduced by human activities to German freshwaters, of which nine are assumed to be established. The introduced invasive parasitic fungus Aphanomyces astaci causes the crayfish plague, which has contributed to the decimation of indigenous crayfish species. The seven parasites are associated with fish, e.g. the invasive swim-bladder nematode Anguillicola crassa, imported with infected eels from Asia in the 1980s and which occur in the European eel with an infection rate up to 90 % today (Geiter et al. 2002).
North Sea coast
At the German North Sea coast, along with the Wadden Sea and several estuaries, a total of 52 established alien species are known.
Eight alien species belong to the phytoplankton, whereby some of them form mass occurrences every year (e.g. the invasive Indopacific diatom Coscinodiscus wailesii), some others are regarded as potentially toxic (e.g. the invasive Pacific flagellat Fibrocapsa japonica) (Nehring 2001).
The occurrence of alien macrophytes is mainly restricted to the rocky shores of the island of Heligoland. Here five introduced species are recorded which also settle partially in low abundances in adjacent sea areas (Nehring 2001). The invasive cord-grass Spartina anglica, a fertile hybrid of S. maritima and S. alterniflora, was introduced into the Wadden Sea in the 1920s to promote sediment accretion. Recently it is a widespread species and grows as a pioneer plant in the tidal zone, where it displaces several native plants (e.g. the glasswort Salicornia stricta) and their associated benthic invertebrate species (Reise et al. 2005).
The invasive cord-grass Spartina anglica was introduced into the German
Wadden Sea in the 1920s to promote sediment accretion.
Only two species of zooplankton (the pacific copepod Acartia tonsa and the western Atlantic ctenophora Mnemiopsis leidyi) are among the assumed aliens of the German North Sea coast.
Recent summaries of introduced species revealed about 33 alien macrozoobenthic species occurring in the area today (e.g. Buschbaum unpubl.; Nehring & Leuchs 1999, 2000; Reise et al. 2002). The share of alien macrozoobenthos species increases from the offshore part towards the coast, and there it increases further on from the open coast towards the estauries. In the latter, the percentage of alien species compared to the total number of macrozoobenthic species amounts to 20 (Nehring 2003a). Several alien species attain locally high abundances (e.g. the American razor clam Ensis americanus), however, significant interactions with native species have only been recorded from small areas yet (Reise et al. 2002). The alien barnacle Elminius modestus is capable of marked habitat alteration through the construction of dense crusts on hard surfaces. This overgrowth and pre-emption of space dimish other epifaunal species such as the native barnacle Balanus balanoides (Nehring & Leuchs unpubl.). Before the arrival of the invasive Chinese mitten crab Eriocheir sinensis in European waters in 1912, no native brachyuran crab migrated between brackish and freshwater habitats. For most of its life E. sinensis lives in freshwater and up to 700 km from coastal waters away. During August adults crabs migrate seawards and gather in large swarms to bread in estuaries. In spring 1998, 850 kg of juveniles (ca. 75,000 crabs) were caught in the Elbe estuary by hand in two hours only (Gollasch et al. 1999). Juveniles and adults mitten crabs create burrows on the river banks that provide a refuge and protection from dessication. In areas where E. sinensis is particularly abundant, burrows can cause damages to banks and dykes and might affect flood defences (Gollasch et al. 1999).
The life cycle of the invasive Chinese mitten crab Eriocheir sinensis - adult individuals
were found up to 700 km upstream the river Elbe
Up to now, no alien vertebrate species occur in self-sustaining populations on the German North Sea coast. Here, only single individuals of alien freshwater fish species were found occasionally.
During the last decades several parasitic protozoans and copepodes have been found in mussels in North European coastal waters. They might have an exotic origin and were possibly introduced by human activities, however, their current status on the German North Sea coast is unkown (Nehring 2001). However, the parasitic swim-bladder nematode (see above) is observed in migrating European eels regulary. And, on 2 November 2011, a very dense, well established population of the North American toxic fungal pathogen G3 Claviceps purpurea was found on the common cord-grass Spartina anglica C.E. Hubbard at two localities on the German North Sea coast in the Wadden Sea (Nehring et al. 2012).
Baltic Sea coast
In the Baltic Sea, the world’s largest brackish-water sea area, only very few primary introductions of alien species are known. This is likely due to the fact that in the Baltic only minor aquaculture activities and intercontinental shipping are existent. The Baltic has been, and still is, subject to secondary introductions from both the North Sea area and adjacent inland waters. It is assumed that only 28 alien species have been able to establish permanent populations on the German Baltic coast yet.
In ecologically important groups, such as phyto- and zooplankton, macrophytes as well as parasites, only one to four alien species were recorded (a.o. Nehring 2003b; Olenin et al. 2004).
The majority of established aliens are macroinvertebrates. Recently, 17 macrozoobenthic species have been identified as aliens in the coastal waters (e.g. Nehring 2000, Olenin et al. 2004, Sikorski & Bick 2004). The invasive wood boring bivalve Teredo navalis was probably brought to Europe from East Asia several centuries ago and is now widespread in the southwestern Baltic region. This species has had major direct negative economic impacts in the Baltic. It caused approximately US$ 25 million damage to wooden installations between 1995 and 2001 along the German Baltic coast (Leppäkoski et al. 2002). The soft-bottom community structure was totally changed by the invasive polychaete Marenzelleria neglecta (= cf. viridis) in nearshore waters when it became a dominant species on sandy and muddy habitats in the end of the 1980s. It reached high biomasses (400 g wet weight m-2) and comprised up to 96% of the total community biomass (Zettler 1997). It seems that only the planktonic copepod Ameira divagans, the macroalgae Fucus evanescens and the polychaete Marenzelleria cf. viridis were directly introduced by ocean shipping to the German Baltic Sea coast. The occurrence of all other alien species was faciliated by the construction of numerours canals on Ponto-Caspian rivers, which allow organisms to disperse to Central and West European river systems, which flow into the Baltic. Or they were at first introduced into the North Sea and arrived the German Baltic Sea coast via Danish Belts or Kiel Canal by active migration or by passive transport in water currents or by ships.
The spreading of the invasive polychaete Marenzelleria neglecta (= cf. viridis)
in the Baltic Sea.
In mid-1980s the round goby (Neogobius melanostomus) was unintentionally introduced from the Ponto-Caspian probably in ballast water to the Baltic Sea (Gulf of Gdansk) and Moscow river (Corkum et al. 2004). In the Baltic Sea the spread westwards continued and in 1999 this alien fish species was reported first in German Baltic waters at Rugia Island (H. Winkler, University of Rostock, unpubl. data cited in Corkum et al. 2004). In May 2003, the round goby had been reported at several locations along the entire Baltic Sea coast of Germany, west to Rostock (K. Skora, University of Gdansk, unpubl. data cited in Corkum et al. 2004).
Established populations of alien amphibians could not be observed on the German Baltic Sea coast to the present day.
Estuaries as a habitat for alien species
In the German river Elbe, from its source to its outer estuary including the full marine milieu of the open German Bight, in total 31 alien macrozoobenthic species (inclusive the archaeozoan Mya arenaria) have been identified as occurring in self-sustaining populations today (Nehring 2006). From the headwater region up to the German Bight, major natural hydrographical and topograhical differences exit, which is also reflected in a distinct occurrence of alien species. In addition to pollution of water and sediments as well as engineering impacts (damming, stream straightening, deepening, dredging and dumping), another fundamental factor for the occurrence of organisms is the changing hydrological regime. Highly variable water runoff in the stream and, especially on the North Sea coast, the tide with all its consequences - above all development of eulittoral areas and of a transition zone between limnetic and marine conditions with a wide salinity gradient - play an important role for the establishment of permanent populations of macroinvertebrate species.
Due to these specific characteristics and great range of opportunities, generalists should have the greatest chance for settlement. Among the aliens established in German waters and especially in the Elbe euryoecious and potent competitors predominate consequently. However, in the headwater region of the Elbe no alien species have been observed so far (Schöll & Fuksa 2000). This might be due to the fact that this region is relatively undisturbed by anthropogenic activities, such as shipping, commercial fisheries or direct connections to canals. Nevertheless, it is highly probable that first aliens will arrive there in the near future. Within the last decades a constant increase of alien species have been observed in the lower sections of the Elbe (Petermeier et al. 1996; Tittizer et al. 2000; Nehring 2003a). Therefore it is only a question of time when the first alien specimens will appear in the topmost region of the Elbe by active migration, by biovectors (e.g. birds, fishes) or by intentional anthropogenic introduction, which can never be excluded completely. How far species can establish permanent populations there can not be predicted exactly. On account of characteristic hydromorphological conditions in this mountainous region, usually only specifically specialized species are able to live and reproduce there (Schöll & Fuksa 2000). However, such alien species do not occur in the lower Elbe sections yet. Nevertheless, it can not be excluded that aquatic generalists living in plain areas with a high adaptation potential can colonize the headwater region permanently.
Among aquatic environments, estuaries have been common sites of biological invasions worldwide. It seems to be that they have been invaded more frequently than rocky or sandy shores of the outer coast (e.g. Ruiz et al. 1997; Reise et al. 2002) as well as inland waters (e.g. Cohen & Carlton 1998). In the Elbe, most of the known introduced macroinvertebrate species have established permanent populations in the saltwater influenced area of the estuary as well (21 species in total) (Nehring 2006).
This can be explained by the following reasons:
Species number and salinity preference of indigenous and alien macrozoobenthic
species in the Elbe river, from its source in the Czech Republic
to the German North Sea (modified after Nehring 2006).
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