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The Meltdown of Biogeographical Peculiarities of the Baltic Sea: TheInteraction of Natural and Man-made ProcessesAuthor(s): Erkki Leppäkoski and Sergej OleninSource: AMBIO: A Journal of the Human Environment, 30(4):202-209. 2001.Published By: Royal Swedish Academy of SciencesDOI: http://dx.doi.org/10.1579/0044-7447-30.4.202URL: http://www.bioone.org/doi/full/10.1579/0044-7447-30.4.202

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202 © Royal Swedish Academy of Sciences 2001 Ambio Vol. 30 No. 4–5, August 2001http://www.ambio.kva.se

INTRODUCTIONBiogeography traditionally operates at species and higher lev-els of biological organization. A major issue is time. Living na-ture is never in a state of equilibrium; biotic assemblages areconstantly changing. All ecosystems are dynamic and charac-terized by a continual succession of species both in time andspace.

The present Baltic Sea exists as an ecological continuum, be-ing a result of significant natural environmental alterations dur-ing the past 10␣ 000 years of its post-glacial history. The present,naturally formed continuum, was described and discussed as aspecific biogeographical feature of the Baltic in the 1950s and1960s, when Ekman (1), Segerstråle (2) and Zenkevich (3) pub-lished their classic reviews. They distinguished the followingmain components of the Baltic fauna: i) North Atlantic borealmarine fauna; ii) Arctic relicts; iii) brackish water species ofNorth Sea origin; iv) brackish water species of Sarmatian ori-gin; and v) freshwater species. Shurin (4) discriminated 7 zoo-geographical groupings (complexes), among them the Yoldianand boreal Atlantic complexes and the complex of newly intro-duced species (see 5). Since the 1960s, the biogeography of theBaltic Sea has not been a key research item. An Internet searchon the Baltic Marine Environment Bibliography (6) for 1970–1998 yielded 14 hits on “biogeogr*”, 13 on “phytogeogr*” andonly 2 on “zoogeogr*”.

The Baltic Sea is exposed to both gradual and stepwise (manytimes fully stochastic) changes related to human influences. Inthis review, we discuss the biogeographical aspects of the re-cent environmental changes induced by both anthropogenic andnatural processes, or by synergistic effects of both these drivingforces. As a starting point we use the concept of biological in-tegrity, defined as “the capability of supporting and maintain-ing a balanced, integrated, adaptive biological system having afull range of elements (genes, species, assemblages) and proc-esses (mutations, biotic interactions, nutrient and energy dynam-ics, and metapopulation processes) expected in a natural habitatof a region” (7). We will consider changes at species level andtheir effects on the biogeographical integrity of the Baltic Sea

at different scales, from global through inter- and intracontinentalto regional ones.

BIOLOGICAL INVASIONS: A GLOBAL CHANGEThe global scale of biological invasions associated with humanactivities, resulting in large-scale mixing of previously isolatedbiotas, became more and more evident at the end of the 20thcentury. This process is recognized as an important element ofglobal change (8–13). Many natural barriers to dispersal for bothterrestrial and aquatic species have been weakened and, conse-quently, both the number of potential invaders and the numberof remote donor areas have increased through human-mediateddispersal (12, 13). Thereby, geographic isolation of seas and con-tinents as a creator and conservator of global biodiversity wasbreached several centuries ago and has continued to decrease atan increasing rate in recent decades.

Results of this global exchange of species are evident in allEuropean brackish-water seas, including the Baltic (Fig. 1).Much of their present biological diversity is foreign origin, i.e.composed of species intentionally or unintentionally transportedby humans over intrinsic environmental barriers (14–17). Wehave defined this human-mediated addition of nonindigenousspecies and even higher taxa as xenodiversity (Gr. xenos =strange) to indicate structural and functional diversity caused bynonindigenous (nonnative, alien, exotic, introduced) species (18).

Dozens of nonindigenous species have been introduced, in-tentionally or unintentionally, into the Baltic Sea during historictime. Invasion rates appear to have increased in the past 50 years,due to changes in factors that once prevented the introductionof aquatic species. Without doubt this apparent increase also re-flects increasing awareness and research efforts. By the year2000, 99 species of animals and plants that are known or be-lieved to be nonnative have been recorded in the Baltic Sea (19).These include 41 zoobenthos, 29 fish, 9 phytoplankton, 9 phyto-benthos, 4 zooplankton, 3 nektobenthos, 3 parasitic invertebrates,2 mammals and 1 bird species (1). Of these species, less than70 have been able to establish reproducing populations (18, 19).However, we have to look at the invaders over at least severaldecades to determine which species failed or were successful,i.e. whether or not they became naturalized. The failures are notusually documented and even our knowledge of winners in this“ecological roulette” (20) is still in its cradle.

Generally, estuaries and embayments have been affected byalien invaders more frequently than the outer coast (11). In Eu-rope, this appears to be true for whole brackish-water seas, suchas the Baltic, Black, and Caspian Seas (14, 16, 21). Character-istically, the recent stage (since ca 500 BP) of the Baltic has beendescribed as the “Mya Sea” (22) decades before the status ofthe nominator species (Mya arenaria, the soft shell clam) wasrecognized as a globally successful marine invader from NorthAmerica, probably transported by the Vikings (23, 24).

The enrichment of the Baltic Sea fauna and flora bynonindigenous species can, from a biogeographer’s point ofview, be interpreted as a successive recovery from the last gla-

Erkki Leppäkoski and Sergej Olenin

The Meltdown of Biogeographical Peculiaritiesof the Baltic Sea: The Interaction of Natural andMan-made Processes

The biogeographical peculiarities of the Baltic Sea havedeveloped since the last glacial period. The characteristicmixture of marine, brackish water, and freshwater species,and relicts from previous periods in the Baltic, is threat-ened by ongoing environmental changes. This reviewfocuses on the recent impacts of nonindigenous species,eutrophication, and a temporary oxygen deficit in the deepbasins, on the biogeographical integrity of the Baltic ondifferent spatial and time scales. Today the biota ofbrackish waterbodies are exposed to each other becauseof the breakdown in geographical barriers due to shippingtraffic, leading to an exchange of species and furtherhomogenization of aquatic animal and plant life worldwide.

203Ambio Vol. 30 No. 4–5, August 2001 © Royal Swedish Academy of Sciences 2001http://www.ambio.kva.se

ciation, anthropogenic dispersal being a spreading mechanismcomplementary to natural ways of dispersal (25, 26). However,human activities have opened invasion corridors, e.g. via shiptraffic and intentional introductions, which were not possible atall by spontaneous range expansion across the oceans (27) orcould be expected to take hundreds of thousands or even mil-lions of years. In addition, global communications have increasedthe speed of human-mediated species transfers from one coastor sea to another. Today, rapid transportation increases the sur-vival probability of the sea stowaways and, thus, there are eco-logical and economic risks associated with these invasions. Atrip with a modern cargo ship across the Atlantic Ocean takesless than 10 days, therefore, the survival rate of ballast or hullfouling travellers is much higher than in the sailing vessel orsteamship era (28).

At present, ship traffic is the most important vector for thespread of aquatic organisms into the coastal waters of northwest-ern Europe, including the Baltic Sea (29, 30). More ships nowarrive with larger volumes of ballast water from more regionsin less time than was the case 50 years ago (11). The largestdeep-sea vessels have a ballast capability of 250␣ 000 tonnes (t)(31). The first European study of species transported by ship-ping, undertaken in 1992–1996, collected more than 400 spe-cies in the ballast tanks or on the hulls of 211 ships visiting Ger-man ports. More than 60% of these species (ranging frommicroalgae to crustaceans, mollusks and fish 15-cm long) werenonindigenous to German waters (32). The number of species

transported by ships has, on a global scale, been estimated at3000 to 7000 (or even 10␣ 000) at any time (27, 28, 32). A glo-bal homogenization of aquatic biota is underway due to the es-tablishment of nonnative species, and the environmental effectsof these species continue to accrue (11, 28, 33, 34).

INTERCONTINENTAL EXCHANGE OF SPECIESSpecies introduced into the Baltic Sea originate from all conti-nents except Antarctica. North America, predominantly its eastcoast, is the most important donor area, contributing approxi-mately 30% of all known introductions (Table 1). One third ofnonnative free-living invertebrates, one fourth of fish species,the only bird species and both species of mammals are of Ameri-can origin. These species originated from unintentional (58%)rather than intentional (42%) introductions (29). This might berelated more to the successive opening of routes of commercein the post-Columbian era than to the adaptability of the poten-tial invaders from America. Ongoing Americanization, reviewedhere from a biogeographical point of view is thus one of the mostimportant processes contributing to the xenodiversity of allsemienclosed European seas, including the Baltic (29, 30).

From Europe to America and Vice VersaThe Eastern and Western hemispheres are not remote. Bidirec-tional exchange of species has, in many cases, formed the basisfor their agriculture-based economies. The success of emigrat-

Table 1. Origin of xenodiversity in the Baltic Sea. Not all species are known to reproduce in their newarea. Compiled from different sources. Species list is available at http://www.ku.lt/nemo/mainnemo.htm.

Area of origin North America Ponto-Caspian Other regions Cryptogenic Total

Phytoplankton 1 0 2 5 8Macrophytes 1 0 8 0 9Invertebrates (parasites) 0 0 3 0 3Invertebrates (plankton) 2 2 0 0 4Invertebrates (benthos, 13 14 16 0 43nektobenthos, wood borers)Fish 5 6 18 0 29Birds 1 0 0 0 1Mammals 2 0 0 0 2

25 22 47 5 99

Shipping and particularly ballastwater are increasingly importantas vectors for intercontinentalexchange of nonnative species.Photo: C. Boström.

204 © Royal Swedish Academy of Sciences 2001 Ambio Vol. 30 No. 4–5, August 2001http://www.ambio.kva.se

ing Europeans largely depended on their ability to Europeanizethe fauna and flora of the New World (35, 36).

The intercontinental exchange of animal species between theOld and New Worlds has been anything but even. Terrestrial spe-cies introduced from Europe into North America are 10 timesas numerous as those transported in the opposite direction (37).In olden times, sailing vessels went on their way west almostexclusively with solid ballast, transporting soil fauna andpropagules of terrestrial plants. Ships have used water as bal-last since the 1880s (31, 38); with this global conveyer, moreaquatic species have been and continue to be transported.

Analogically, a successive Europeanization of the aquatic lifeof North America has occurred both intentionally and uninten-tionally. Of all aquatic species introduced to the United States,29% are native to Europe or Eurasia (39). For some US estuar-ies, the number of known nonnative species ranges from 60 to212, with Asia and Europe being the most common areas of ori-gin (11). Approximately 55% of 139 nonindigenous aquatic orsemiaquatic species established in the Great Lakes since the1800s are native to Eurasia (40). Some 90 of these species canbe regarded as true aquatic organisms, i.e. wetland species ex-cluded; more than 40% of them are native to Europe (29, 41).These include the Eurasian watermilfoil (Myriophyllum spi-catum), faucet snail (Bithynia tentaculata), common carp(Cyprinus carpio) and rudd (Scardinius erythrophthalmus; 41).The most recent Neo-Americans of European origin in the lakes,the predatory water fleas Bythotrephes cederstroemi andCercopagis pengoi, zebra mussel (Dreissena polymorpha), ruffe(Gymnocephalus cernuus) and round goby (Neogobius mela-nostomus), have appeared to be highly aggressive invaders thathave had a heavy impact on the ecology of the lakes (42, 43).All of these species occur in the coastal waters of the brackishBaltic Sea also.

Undeclared “Biological War” and Other UnwantedConsequencesThe 20th century was characterized by several accidental intro-ductions that had tremendous ecological and economic impactson both continents. The first episode in an undeclared (and un-intentional) “biological war” between the Old and New Worldswas the invasion of the Colorado beetle (Leptinotarsa decemli-neata) into Europe. From the early 1920s, this species infestedall countries of Western and Eastern Europe and caused greatdamage to their potato-based agricultural production. Currentlyits distribution covers about 6 million km2 in Europe and Asia(44).

The European “response” took place 6 decades later, but itwas equally important. The zebra mussel invaded the NorthAmerican Great Lakes in the mid-1980s with the epizootic di-mension of ecological impact costing the US and Canadianeconomies USD 3.1 to USD 5 billion by the year 2000 (12).

An assessment of the importance of introductions from an eco-system or economic point of view cannot be based merely onthe number of nonnative species present nor on their abundancein the invaded ecosystems. Some of them can, on a rather sub-jective basis, be regarded as nuisance species. Of the Neo-Eu-ropeans of American origin living in or along the brackish wa-ters of Europe, the comb yelly Mnemiopsis leidyi, the polychaeteMarenzelleria viridis, the barnacle Balanus improvisus, themuskrat Ondatra zibethicus and the American mink Mustelavison have had a significant ecological or economic impact onthe invaded areas (29). The recent deterioration of the Black Seaecosystem and the collapse of its commercial fisheries offers adramatic example of an ecological catastrophe caused bynonindigenous species. Within 10 years (since 1982), a decreasein the anchovy catch took place simultaneously with the expan-sion of Mnemiopsis (21). In its donor area (the Atlantic coast of

the US) the comb jelly does not pose a major problem whereasin the recipient area, the Black Sea, traditional fisheries and fishindustries were severely affected (21).

Introduced species are known to take roles that the native spe-cies do not have. Several new life forms, and thus an “Ameri-can style” of aquatic animal life, have been introduced into Eu-ropean brackish water seas. In many cases an invader representsa new functional group in the invaded community and differssubstantially from natives in life form and efficiency of resourceutilization (17). The only barnacle species living in the BalticSea, B. improvisus, is native to the Americas, as is the dwarfcrab Rhithropanopeus harrisii, one of only two crabs found inPolish coastal lagoons. The muskrat is the only herbivorousmammal seen in coastal bays and inlets.

Successful animals in stressed ecosystems have been arguedto be small and exotic (45). However, some of the Neo-Europe-ans of American origin in European enclosed seas have appearedto be large and aggressive habitat conquerors in comparison withpreviously dominant native organisms in the same habitats. Therewere no polychaetes in the inner parts of the Baltic coastal la-goons until the appearance of M. viridis in the mid-1980s. ThisNorth American polychaete has significantly altered the com-munity structure of the bottom fauna in the Baltic coastal la-goons. The biomass of benthos has increased up to 10 times dueto this species, which became dominant within less than 5 yearsafter its invasion (17, 46, 47). Marenzelleria, which digs downto 40 cm into the sediment, is a giant compared to native bur-rowing organisms (chironomid larvae and oligochaetes) dwell-ing in muddy bottoms of the Baltic coastal lagoons (17). Onrocky shores, Balanus exceeds by hundreds of times the size ofother sessile animals, such as the bryozoan Electra crustulenta.The soft-shell clam Mya arenaria grows bigger and digs moredeeply into the bottom sediment than other (native) bivalves(Macoma balthica and Cerastoderma spp.).

Thus, the nonnative species, native to transoceanic biogeo-graphical realms, contribute not only to the species diversity bothwithin and between habitats but also to the functional diversity,i.e. the range of functions performed by organisms in a system(8, 17). The most successful invaders in enclosed European seashave, without doubt, been capable of altering fundamental eco-system level processes and thus control the functioning of wholeecosystems (14, 28). The newcomers also affect the ecosystemservices available for man, such as primary production, fish pro-duction, degradation capacity, recreational services, and ameni-ties (48).

Ongoing AmericanizationIt has been generally thought that the isolation between the East-ern and Western Hemispheres was broken and the 2 worlds reu-nited on 12 October, 1492. With increasing maritime connec-tions between the Old and New Worlds, a vector was establishedfor bilateral exchange of aquatic fauna and flora. Along this vec-tor, 2 previously isolated biotas have been exposed to each otherfor 500 years. Ongoing Americanization of European brackish-water seas gives continued evidence of post-Columbian ex-change between North America and northwestern Europe. How-ever, the history of marine introductions from North Americato Europe is believed to go back to the pre-Columbian era, atleast to the 13th century (23).

Many of the prehistoric, and since long fully acclimatized, in-troductions will remain unknown forever, due to the difficultyin distinguishing which species were or were not transferred intothe coastal seas by early man. These species will remain as cryp-togenic, neither clearly native or exotic (49). Their contributionto the early succession and long-term dynamics of the young(less than 10 000 years) ecosystem of the Baltic Sea remains un-documented as well.

205Ambio Vol. 30 No. 4–5, August 2001 © Royal Swedish Academy of Sciences 2001http://www.ambio.kva.se

Ecologically and biogeographically, the uncontrolled intercon-tinental transportation of nonindigenous species is resulting inlarge-scale mixing of and greater uniformity among both terres-trial and aquatic faunas and floras – a “McDonaldization” of thebiosphere (10). The post-Columbian exchange continues; todaythere is a built-in piece of America in the European enclosedseas, and a piece of Europe on the other side of the Atlantic, inthe Great Lakes.

PONTO-CASPIAN SPECIES: INTRACONTINENTALINVADERS

One-way Invasion Corridors?Species endemic to Ponto-Caspian basins (the Black and Cas-pian Seas, the Sea of Azov and their catchment areas) havespread and become established in inland Europe, the Baltic Seaand the North American Great Lakes. More than 40 Ponto-Cas-pian species have expanded their ranges into central and west-ern Europe (17, 50, 51). This active or passive intracontinentaldispersal was facilitated by the construction of numerous canals(opened between 1775 to 1952) and reservoirs on Ponto-Cas-pian rivers that allowed species to disperse to Central and WestEuropean river systems through previously disconnected water-ways by active migration, attachment to barge hulls or transportin ballast tanks (50). In addition, several Ponto-Caspian crusta-ceans were transplanted between the 1950s and 1980s to stimu-late fish production in western lakes and reservoirs of the USSR;more than 30 species of amphipods and opossum shrimps fromthe “Caspian complex” were used for these acclimatization ex-periments (52).

Migration of Ponto-Caspian species upstream is an ongoingprocess in all great European rivers. Astonishingly, there are noknown introductions in the opposite direction (14, 53). Thepolychaete Hypania invalida spread from the lower reaches tothe middle Volga in the 1980s and 1990s (54); in 1990s it was

also found in the River Rhine, where it came through the canalconnection with Danube (55).

For a long time Caspian elements in fauna of the Volga Riverwere considered to be relicts of former sea transgressions of thePonto-Caspian Basin (56); probably that is why the Ponto-Cas-pian invaders, with their genetically “build-in euryhalinity”, soeasily conquered northwestern European rivers and brackish-water estuaries. The process is considered to be a natural rangeexpansion if it takes place within the limits of the maximal trans-gressions (57). However, the major European inland waterwaysystems constructed during the last 2 centuries have bridgedintracontinental barriers between formerly isolated water basinsand their biotas.

Centers of XenodiversityIn the Baltic Sea, species of Ponto-Caspian origin constitute thesecond largest part of xenodiversity: 22 have been recorded; ofthem a dozen have been able to establish self-reproducingpopulations (19). Their proportion is greatest in the sheltered,low-salinity coastal lagoons along the southern and southeast-ern coast of the Baltic and in the eastern Gulf of Finland, butdiminishes westward (Fig. 1). Estuaries and coastal lagoons havefunctioned as stepping stones that may have aided in the estab-lishment of numerous nonindigenous species in nonestuarinecoastal areas. Sheltered coastal waterbodies are subject to a broadvariety of anthropogenic stressors that interact with invasionprocesses (11). However, the presence of nonindigenous speciescannot be taken as a cause, but only as a consequence of envi-ronmental deterioration.

There are fewer introduced species on the open coast of theBaltic than in the sheltered bays and estuaries. Baltic centers ofxenodiversity, known to host a high number of established non-native species, are the Curonian, Vistula, and Szczecin Lagoons,German Boddens and the Neva Estuary (17, 58, 59). The vec-tors that have served to introduce most of the nonnative species

Figure 1. Origin and composition of Balticxenodiversity. Donor areas of the Baltic Seaalien species: 1) Ponto-Caspian Region; 2)North America; 3) Southeast Asian seas; 4)other regions of the world and unknownorigin.CL, VL and OL - Curonian, Vistula and Odra/Oder lagoons, respectively. Isolines showmean salinity of the surface water layer.

Whole Baltic

Donor areas

206 © Royal Swedish Academy of Sciences 2001 Ambio Vol. 30 No. 4–5, August 2001http://www.ambio.kva.se

to the Baltic bring species from and deliver them to estuaries.Most of the major harbors of the world are located at rivermouths. Somewhere along their estuarine gradient these harborsexhibit salinity conditions matching those of the Baltic; environ-mental matching plays a major role in invasion success (38).Fewer or no vectors are available to transport species betweenbeaches or rocky shores of the world (11). In the Baltic Sea,eutrophicated inlets, coastal lagoons and artificial hard substratehabitats (e.g. harbor constructions), especially in the southernpart of the area, serve as examples of subsystems that have beenhighly modified by introduced species, whereas the open coastand pelagic systems were relatively free from them until the mid-1980s (17). For example, the Ponto-Caspian complex is repre-sented by only 5 species in the Baltic outside lagoons and rivermouths (Cercopagis pengoi, the hydrozoan Cordylophoracaspia, Dreissena polymorpha, the opossum shrimp Hemimysisanomala and the round goby Neogobius melanostomus).

Ecosystems have assimilated nonindigenous Ponto-Caspianspecies to a certain extent. Even if most of them appear to berather benign in their new Baltic areas of occurrence, there isevidence of large-scale effects on structural and functional di-versity with marked food-web impacts in the most heavily in-vaded waterbodies (17, 25, 60). Ponto-Caspian species have hadthe highest impact on benthic communities; the impact of re-cently invaded C. pengoi on the pelagic subsystem remains tobe assessed in more detail (60–62). The zebra mussel invadedfreshwater parts of the southeastern Baltic coastal lagoons nearly2 centuries ago. In the 1990s, it spread further in the oligohalineparts of the Gulf of Finland and the Gulf of Riga, where it co-occurs with the native bivalve Mytilus edulis (63, 64).

The Ponto-Caspian predacious water flea Cercopagis pengoiwas first found in 1992 in the Gulfs of Riga and Finland (62;A. Laine, pers. comm. Finnish Institute of Marine Research). By1999 it had invaded nearly the whole Baltic proper (61, 65, 66),the Gulf of Bothnia up to the Vaasa area (K.-E. Storberg, pers.comm. West Finland Regional Environment Centre), theCuronian and Vistula Lagoons (67, 68) and the Gulf of Gdansk(65).

The Ponto-Caspian round goby Neogobius melanostomus wasfirst recorded in the Gulf of Gdansk, Poland, in 1990 (69). By1999, the round goby had spread to the mouth of the VistulaRiver, adjacent canals, the Vistula Lagoon (70) and the Rügenarea (71). The most recent representatives for Ponto-Caspian in-vasion into the Baltic are the sevruga sturgeon Acipenserstellatus and the hydromedusa Maeotias marginata, both ofwhich were recorded in the northern Baltic in 1999 (72, 73).

CHANGES AT THE REGIONAL LEVEL

Ongoing Eutrophication and Shifts towards“Limnification” in Coastal AreasOverall consequences of eutrophication reflected in the changesin the biogeographical composition of brackish-water biota arebecoming evident at the regional and local levels. Generally,there seems to be an increasing predominance of freshwater spe-cies with increasing eutrophication (74) – a successive “limni-fication” of the pristine brackish ecosystems.

In bottom fauna, the North American barnacle Balanusimprovisus is abundant in eutrophicated harbor areas; its abun-dance is generally one or two orders of magnitude greater thanthe numbers in more natural environments (18, 75). Green al-gae of freshwater origin (e.g. Cladophora glomerata andEnteromorpha spp.) benefit from increasing nutrient levels andreplace marine algae in eutrophicated coastal waters (76). In thestraits off the city of Turku, southwestern Finland, the structureof both benthic and fish fauna was largely determined byeutrophication in the early 1970s when nutrient loading from

municipal sources was a major environmental issue in this area(sewage treatment plants were started between 1967 and 1972).In the most eutrophicated inner parts of the inlets, the dominanceof freshwater species among bottom fauna (expressed in termsof density, the marine/freshwater species ratio being 0.04 : 1)was very apparent (77). In fish fauna, freshwater cyprinids suchas roach Rutilus rutilus and white bream Blicca björkna, com-prised 40 to 65% of the biomass (78).

From the 1950s on, the populations of Arctic tern Sternaparadisaea increased and spread towards the inshore archipelagoof southwestern Finland, obviously benefiting from the increasein emerging midges (Chironomus plumosus) as food objects, thebottom-living larvae of which became very numerous with in-creasing eutrophication (79). Other species of water fowl thatbenefit from eutrophication in the archipelago waters are typi-cal inhabitants of eutrophic lakes, among them the pochardAythya ferina, coot Fulica atra, great crested grebe Podicepscristatus (80) and the mute swan Cygnus olor (81).

Hypoxia Excludes Arctic RelictsAnother process with consequences at the regional level hastaken place in the deep basins of the Baltic, frequently exposedto hypoxia and anoxia due to water stagnation below the pri-mary halocline (82). This process has resulted in the extinctionof several Arctic relicts in the subhalocline bottom fauna. In the1950s and 1960s, the bivalve Astarte borealis was a dominantspecies comprising up to 100% of community biomass in theBornholm Basin (southern Baltic), and reaching the southernslope of the Gotland Basin in the east (83, 84). The boundarybetween this Arctic relict community and the zoogeographicallymore heterogeneous Macoma balthica community, typical ofmost of the Baltic Sea, was situated at approximately 85 m depth.Since the early 1970s, cosmopolitan and Atlantic-boreal species,probably being more resistant to hypoxia and/or having a higherrecolonization capacity, took over in the subhalocline areas ofthe southern Baltic. The previously bivalve-dominated benthiccommunities progressively become polychaete-dominated; in theearly 1970s, cosmopolitan and Atlantic-boreal species comprisedmore than 90% of the total density (84).

From this example it becomes clear that some of thepostglacial Arctic relict species are threatened by worsened oxy-gen conditions in the deepest parts of the Baltic Sea. Their oc-currence is restricted to a transition zone between the oxygen-poor deep waters and the low-saline waters above the halocline(16, 26, 84–86). With this, the deeper parts of the southern Bal-tic have lost their most fascinating biogeographical peculiarity,namely that of offering hospitable conditions for the marine gla-cial relicts, and thereby also their close zoogeographical connec-tion with Arctic shallow-water biomes. At present, the BornholmBasin (if not lifeless due to oxygen deficiency) is linked withboreal areas by the most euryhaline cosmopolitan and Atlantic-boreal soft-bottom fauna (82, 85).

DISCUSSION

Impact of Global and Regional FactorsBiogeography aims at understanding the history of formation offlora and fauna, dealing with both qualitative and quantitativeaspects of this process. The scale of the factors that presentlyaffect the biogeographical integrity of the Baltic Sea ranges fromglobal to regional ones. More than 20-years ago Leppäkoski (87)listed the most important anthropogenic impacts on the BalticSea ecosystem, among them the introduction of nonindigenousspecies. Since then more than 20 new species have been broughtinto the Baltic Sea (19), among them Neogobius, Marenzelleria,and Cercopagis, which have had a much more noticeable im-

207Ambio Vol. 30 No. 4–5, August 2001 © Royal Swedish Academy of Sciences 2001http://www.ambio.kva.se

Table 2. Vectors for introduction of nonnative species into the Baltic Sea.

Area of origin North America Ponto-Caspian Other regions Cryptogenic Total

IntentionalOrnamental purposes 2 0 1 0 3Stocking and aquaculture 9 12 20 0 41

UnintentionalAssociated with aquaculture 1 0 7 2 10Ship’s ballast water and hulls 12 10 17 2 41Mechanism unknown 1 0 3 0 4

Total 25 22 48 4 99

pact on the Baltic ecosystem than any earlier newcomers. To-day, nonindigenous species act on the all-Baltic (whole region)basis, affecting the biogeographical integrity of the sea from theBothnian Bay in the north to the Kattegat in the southwest, andfrom shallow coastal lagoons to the sub-halocline depths. Theirsecondary dispersal within the waterbody is largely a naturalprocess (18), which, however, may be accelerated by intensiveexchange of ballast water between the Baltic sub-regions (88).From the biogeographical point of view, eutrophication seemsto cause changes mainly at the local level, giving benefits tofreshwater species.

Loss of Biogeographical IntegrityNonindigenous species are increasingly affecting the biologicaland even biogeographical integrity of coastal waters all over theworld. In Europe, all the brackish seas (the Baltic, Black, Cas-pian, and Aral Seas) are highly pervaded by nonindigenous spe-cies (14, 29, 30). Today, the biota of brackish-waterbodies areexposed to each other because of the breakdown of geographi-cal barriers by shipping traffic, leading to an exchange of spe-cies.

The losses of biogeographical integrity described above seemto be more pronounced in the southeastern Baltic lagoons com-pared with the Baltic proper and the northern Baltic archipelagowaters (17). Their susceptibility to invasions may be due to i)their topography; ii) their repeatedly early successional statussubsequent to stochastic changes of abiotic environmental fac-tors (fluctuations and especially sudden salinity fluctuations inthe outer parts of the lagoons; unstable ecosystems have beenpostulated to be more open for nonnative species than stableones); iii) their relative poverty in species numbers and, conse-quently, the high number of imaginary “vacant” niches (89); iv)environmental changes such as increasing eutrophication or otherdisturbance; and v) stochastic inoculation events (e.g. intentionalintroductions of forage species to nearby freshwater reservoirsin the Baltic republics of the former USSR) (17, 49, 90, 91).

Different subsystems of the stratified Baltic Sea are not equallyexposed to species invasions either. Most of the newcomers be-long to the littoral or shallow sublittoral subsystems. The hy-drozoan Cordylophora caspia, the barnacle Balanus improvisusand the zebra mussel Dreissena polymorpha are, in places, com-mon members of the biofouling community in shallow waters.Deeper water layers below the primary halocline at 30–70 mdepth have maintained much of their biogeographical integrity.The soft shell clam Mya arenaria can occasionally be founddown to 45 m depth; on sheltered sedimentation bottoms,Polydora redeki has been recorded at 29 m depth. Marenzelleria,the North American newcomer, has been recorded at 50–78 mdepth (18).

Because of its ecological and evolutionary history, the BalticSea seems to be predominantly a receiver area of introduced spe-cies, donor areas of which are to be found both in the adjacentinland waters and oceanic coasts but also in remote seas (38).Recently, the appearance of 2 zooplanktonic species in the North

American Great Lakes has been attributed to the existing inva-sion corridor between the eastern Baltic and the lakes: Bytho-trephes longimanus in the early 1980s and Cercopagis pengoiin 1998 (92, 93).

Until the 1980s, the nonindigenous element occupied onlymarginal areas and, in most cases, only marginal niches (25, 89).Biogeographically, however, this element signifies a great ad-dition to the Baltic fauna and, in Kattegat, even flora, with re-spect to the naturally low number of species present. Nonindi-genous species have contributed to species diversity and com-munity structure, introduced novel functions and created newinterspecific relationships (17, 53). For a conservationist, how-ever, this addition indicates contamination of the biota bynonindigenous elements, reflecting the history of man’s eco-nomic interests and activities. Unlike chemical pollutants, estab-lished nonindigenous species can become a permanent andexponentially growing problem: they reproduce and spread withunpredictable and irreversible consequences, prey on native spe-cies, compete for food and space, degrade habitats, food websand water quality, and often transport and spread diseases andparasites (93, 94).

Nonindigenous species are a major threat to indigenousbiodiversity leading to restructuring of communities. Until now,not one species has become extinct in the Baltic Sea due to theintroduction of nonindigenous species, but there is no guaran-tee that this will be the case in the future. This international con-cern has been included in the Convention on Biological Diver-sity (Article 8, h ): “Contracting Parties shall, as far as possibleand appropriate, prevent the introduction of, control or eradicate

The soft-shell clam ( Mya arenaria ) is believed to be the first human-mediated introduction into the Baltic Sea, carried by Vikings across theAtlantic. Photo: E. Leppäkoski.

208 © Royal Swedish Academy of Sciences 2001 Ambio Vol. 30 No. 4–5, August 2001http://www.ambio.kva.se

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those alien species which threaten ecosystems, habitats or spe-cies.”

All trophic levels are represented among the nonidigenous or-ganisms in the Baltic Sea but not equally subjected to a biogeo-graphical change by them. In certain coastal waters, food chainsand whole macrobenthic communities can be based upon intro-duced species. In the early 1960s, the zebra mussel made up 88%of the total benthic biomass in the Szczecin Firth, Poland. Herethe North American freshwater crayfish Orconectes limosus fedmainly upon the Ponto-Caspian coelenterate Cordylophoracaspia, which used planktonic larvae of Dreissena as food (25).In Dead Vistula (a cut-off arm of the Vistula River in Poland),the adult dwarf crab Rhithropanopeus harrisii, native to NorthAmerica, fed mainly upon Dreissena, whereas young individu-als consumed Balanus larvae and Cordylophora (25, 95).

Most of the recent invaders originate from warmer climates

and hydroclimates. If global warming continues, not only spon-taneously spreading European invaders but also more nonindi-genous species from warmer regions of the world can be ex-pected to establish in the Baltic. Two target species are poten-tially able to spread with climatic warming. The zebra musselhas not been recorded in the Gulf of Bothnia yet. In NorthAmerica, its northward distribution is expected to be assisted bywarming, provided that other requirements (calcium, pH) arefavorable (96). Due to several striking similarities between theBaltic and Black Seas such as brackish water and the annual tem-perature range (16), the Baltic is to be regarded as an area ofspecial concern related to a Mnemiopsis outbreak (21, 38, 97).

LONG-TERM SOLUTIONSNot all windows of introduction can be closed. More than 90%of the transported ballast water originates from marine areas (32).

209Ambio Vol. 30 No. 4–5, August 2001 © Royal Swedish Academy of Sciences 2001http://www.ambio.kva.se

63. Orlova M.I., Khlebovich V.V.and Komendantov A. Yu. 1998. Potential euryhalinityof Dreissena polymorpha (Pallas) and Dreissena bugensis (Andr.). Russ. J. Aquat. Ecol.7, 17–28.

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75. Vuorinen, I., Laihonen, P. and Lietzén, E. 1986. Distribution and abundance of inver-tebrates causing fouling in power plants on the Finnish coast. Mem. Soc. Fauna FloraFenn. 62, 123–125.

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78. Anttila, R. and Niinimäki, J. 1975. Fisheries in the sea area off Turku in 1973-1974.Lounais-Suomen Vesiensuojeluyhdistyksen julk. 23, 1–75. (In Finnish).

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plankton in ballast water on a ship’s voyage from the Baltic Sea to the open Atlanticcoast of Europe. Int. Rev. Hydrobiol. 85, 577–596.

89. Abiotic factors and interspecific relationships will determine the realised niche of anintroduced species in its novel environment. Niche dimensions will vary in differentecosystems. Any introduced species will have an impact on the recipient community(11) and adapt to resources that are not fully exploited or “trophic opportunities“. Thisdoes not necessarily mean that the niche was “vacant”. Leach, J.H. 1995. Non-indig-enous species in the Great Lakes: Were colonization and damage to ecosystem healthpredictable? J. Aquat. Ecosyst. Health 4, 117–128.

90. Di Castri, F. 1990. On invading species and invaded ecosystems: the interplay of his-torical chance and biological necessity. In: Biological Invasions in Europe and the Medi-terranean Basin. Di Castri, F., Hansen, A.J. and Debussche, M. (eds). Monogr. Biol.65, Kluwer Academic Publishers, pp. 3–15.

91. Lodge, D.M. 1993. Biological invasions: lessons for ecology. TREE 8, 133–137.

Erkki Leppäkoski has a PhD in zooecology. He is professorof ecology and environmental protection at Åbo AkademiUniversity, Finland. He has been involved in research intoman’s impacts on brackish water ecosystems since the1960s. His research interests are focused on nonindigenousspecies. His address: Department of Biology,Environmental and Marine Biology, Åbo AkademiUniversity, FIN-20500 Turku, Finland.E-mail: eleppakoski@abo.fi

Sergej Olenin received his MSc in biology from the KazanState University, and PhD in biology from the A.N.Severtsov Institute of Animal Evolutionary Morphology andEcology in Moscow, Russia. He is a senior scientist at theCoastal Research and Planning Institute, KlaipedaUniversity, Lithuania. He is interested in ecology of marinebottom fauna, benthic biotopes and invasive alien species.His address: CORPI, Klaipeda University, H. Manto 84,LT-5808, Klaipeda, Lithuania.E-mail: s.olenin@corpi.ku.lt

92. MacIsaac, H.J., Grigorovich, I.A., Hoyle, J.A., Yan, N.D. and Panov, V. 1999. Inva-sion of Lake Ontario by the Ponto-Caspian predatory cladoceran Cercopagis pengoi.Can. J. Fish. Aquat. Sci. 56, 1–5.

93. Panov, V., Ojaveer, H. and Leppäkoski, E. 1999. Introduction of alien species into theGulf of Finland – an increasing environmental problem. In: Annual Assessment on theState of the Gulf of Finland. http://www.zin.ru/projects/invasions/ (Issued in April,1999).

94. Efford I.E., Garcia, C.M. and Williams, J.D. 1997. Facing the challenges of invasivealien species in North America. Global Biodiversity 7, 25–30.

95. Turoboyski, K. 1973. Biology and ecology of the crab Rhithropanopeus harrisii ssp.tridentatus. Mar. Biol. 23, 303–313.

96. Leach, J.H. 2000. Climate change and the future distribution of aquatic organisms inNorth America. In: Nonindigenous Freshwater Organisms. Vectors, Biology, and Im-pacts. Claudi, R. and Leach, J.H. (eds). Lewis Publishers, pp. 399–400.

97. A project called “Risk Assessment for Marine Alien Species in the Nordic Area” wasfunded in 1997–1999 by the Nordic Council of Ministers. In the first report from thisproject (41), individual physical, chemical and biological profiles for five harbours areprovided (Bergen area, Norway; Stenungsund area, Sweden; Klaipeda, Lithuania; Turku/Åbo, Finland; St. Petersburg, Russia). A list of target species with high invasion po-tential to be considered as probable future immigrants to the Baltic Sea is presented.

98. The International Maritime Organization’s (IMO) Marine Environment Protection Com-mittee has concluded that voluntary guidelines were the appropriate first step in ad-dressing the ballast water problem. In 1997 the IMO Assembly adopted ResolutionA.868 (20) “Guidelines for the Control and Management of Ship’s Ballast Water toMinimise the Transfer of Harmful Aquatic Organisms and Pathogens”.

99. ICES 1995. ICES Code of Practice on the Introductions and Transfers of Marine Or-ganisms 1994. ICES Co-oper. Res. Rep. 204.

100.Ricciardi, A. and Rasmussen, J.B. 1998. Predicting the identity and impact of futurebiological invaders: a priority for aquatic resource management. Can. J. Fish. Aquat.Sci. 55, 1759–1765.

101.We thank Karsten Reise and an anonymous referee for valuable comments on the manu-script. Members of the Baltic Marine Biologists’ Working Group 30 are acknowledgedfor their input to the Baltic Sea Alien Species Database. This study was funded in partby grants from the Nordic Council of Ministers and the Academy of Finland.

Therefore, on the global and intercontinental scales, managementpractices for ballast water are the first step to be taken to mini-mize the risks associated with species introductions (38).

Mid-ocean exchange of ballast water is currently the most re-liable (but not fully effective) method to reduce the number oftank travellers (98). Several shipboard treatment options consid-ered and tested to date include both physical and chemical meas-ures (31): filtration, ozonization, treatment by ultraviolet radia-tion, ultrasound, electric pulses, heat, etc. The preventive meth-ods to be developed must be effective and applicable but alsoenvironmentally safe and sound. There is a need to develop ef-fective and environmentally safe methods to prevent hull foul-ing as well. In research and aquaculture, quarantine proceduresand import restrictions (99) should be followed. Advanced meth-ods for risk assessment, monitoring programs and warning sys-tems are needed (97), as well as guidelines for identifying po-

tential high-risk invaders, donor regions and dispersal pathwaysof future invaders (38, 100).

Xenodiversity tends to reach and even exceed nativebiodiversity in terms of number of species and life forms, as wellas number and rate of ecosystem functions. Much of what wascreated over millions of years of evolutionary separation and spe-cialization of animal and plant life on both sides of the AtlanticOcean has been lost forever over the last 500 years because ofthe activities of agricultural, industrialized and maritime man.This trend towards homogeneity is one of the most importantaspects of the geography of life since the retreat of the conti-nental glaciers (35). This issue must be placed on all relevantagendas worldwide, not only in the fields of environmental andmarine biology and water-dependent technology but also in sec-tors of human activities such as trade, transportation and tour-ism, food, and human health security.