Strong isolation-by-distance in the absence of genetic population structure in the eelpout (Zoarces viviparus,Linnaeus 1758)

The eelpout (Zoarces viviparus) is a benthic marine fish, which has long been assumed to be philopatric. This made it particularly suitable for environmental monitoring programs as it allows matching its content of pollutants to the area of catch. However, a recent small scale genetic study in the Baltic Proper suggested that gene-flow might be stronger than generally believed and may occur frequently up to a distance of at least 90 km. As challenging the assumption of philopatry might have strong implications for environmental monitoring studies, we tested the hypothesis of philopatry at a larger geographical scale using ten microsatellite loci. A total of 220 individuals sampled from eight locations covering almost the entire geographic range of the species was genotyped. Our results show that genetic diversity decreases from the North Sea to the northeastern Baltic Sea. No strong population structuring was found, but a highly significant isolation-by-distance pattern was detected, suggesting a stepwise migration pattern among populations. Thus, the hypothesis of long-distance migration can be falsified. It is more likely that only limited gene flow exists among adjacent populations without any barriers between them. Our results suggest that dispersal in the eelpout is weak enough to retain this species as an important bioindicator. However, we suggest that reference stations should be placed in an appropriate distance to avoid misleading results from migrating individuals. We conclude that a more precise knowledge on migration rates of the eelpout is required in order to get more reliable insights into the potential area over which the concentration of environmental pollutants is integrated.     

HETEROCAPSA ARCTICA SUBSP. FRIGIDA SUBSP. NOV. (PERIDINIALES,DINOPHYCEAE)—DESCRIPTION OF A NEW DINOFLAGELLATE AND ITS OCCURRENCE IN THE BALTIC SEA1

Characteristics important in identification of Heterocapsa species (i.e., thecal plate pattern, body scale structure, and shape and position of the nucleus and pyrenoid) are practically identical in the dinoflagellate investigated here and in Heterocapsa arctica T. Horig. described from the Canadian Arctic. Analysis of internal transcribed spacer (ITS) sequences confirms that the two dinoflagellates are very closely related; however, there is a clear difference in their size and shape. Our experiments show that the low‐salinity Baltic Sea brackish water does not reduce the size of the marine H. arctica to match that of the Baltic Sea morphotype. On the basis of these dissimilarities in general morphology and its geographic isolation in the Baltic Sea, we consider our material sufficiently differentiated from the typical H. arctica to warrant the status of a new subspecies, H. arctica subsp. frigida subsp. nov. Being of a distinct cell shape, the occurrence of subsp. frigida has been recorded in Algaline phytoplankton monitoring data collected since 1993. Although it has never been responsible for high biomass blooms, it commonly occurs in spring in the Northern Baltic Proper and in the western Gulf of Finland, when the water temperatures are <5°C.     

Effects of seasonal and latitudinal cold on oxidative stress parameters and activation of hypoxia inducible factor (HIF-1) in zoarcid fish

Acute, short term cooling of North Sea eelpout Zoarces viviparus is associated with a reduction of tissue redox state and activation of hypoxia inducible factor (HIF-1) in the liver. The present study explores the response of HIF-1 to seasonal cold in Zoarces viviparus, and to latitudinal cold by comparing the eurythermal North Sea fish to stenothermal Antarctic eelpout (Pachycara brachycephalum). Hypoxic signalling (HIF-1 DNA binding activity) was studied in liver of summer and winter North Sea eelpout as well as of Antarctic eelpout at habitat temperature of 0°C and after long-term warming to 5°C. Biochemical parameters like tissue iron content, glutathione redox ratio, and oxidative stress indicators were analyzed to see whether the cellular redox state or reactive oxygen species formation and HIF activation in the fish correlate. HIF-1 DNA binding activity was significantly higher at cold temperature, both in the interspecific comparison, polar vs. temperate species, and when comparing winter and summer North Sea eelpout. Compared at the low acclimation temperatures (0°C for the polar and 6°C for the temperate eelpout) the polar fish showed lower levels of lipid peroxidation although the liver microsomal fraction turned out to be more susceptible to lipid radical formation. The level of radical scavenger, glutathione, was twofold higher in polar than in North Sea eelpout and also oxidised to over 50%. Under both conditions of cold exposure, latitudinal cold in the Antarctic and seasonal cold in the North Sea eelpout, the glutathione redox ratio was more oxidised when compared to the warmer condition. However, oxidative damage parameters (protein carbonyls and thiobarbituric acid reactive substances (TBARS) were elevated only during seasonal cold exposure in Z. viviparus. Obviously, Antarctic eelpout are keeping oxidative defence mechanisms high enough to avoid accumulation of oxidative damage products at low habitat temperature. The paper discusses how HIF could be instrumental in cold adaptation in fish.     

Nucleotide sequence variation of the mitochondrial COI gene in several eelpout species of the genus Zoarces (Zoarcidae, Pisces)

The mitochondrial COI gene was for the first time sequenced in eelpout species of the genus Zoarces from the Taui Bay of the Sea of Okhotsk: notched-fin Z. elongatus Kner, 1868 and the new sympatric form Zoarces sp., differing in a large set of morphological traits. The two species were compared with European eelpout Z. viviparus Linnaeus, 1758 from the Finnish Bay of the Baltic Sea. Divergence and phylogenetic analyses demonstrated a greater genetic similarity between Zoarces sp. and Z. viviparous than between sympatric Zoarces sp. and Z. elongatus.     

Aerobic mitochondrial capacities in Antarctic and temperate eelpout (Zoarcidae) subjected to warm versus cold acclimation

Capacities and effects of cold or warm acclimation were investigated in two zoarcid species from the North Sea (Zoarces viviparus) and the Antarctic (Pachycara brachycephalum) by investigating temperature dependent mitochondrial respiration and activities of citrate synthase (CS) and NADP⁺ -dependent isocitrate dehydrogenase (IDH) in the liver. Antarctic eelpout were acclimated to 5°C and 0°C (controls) for at least 10 months, whereas boreal eelpout, Z. viviparus (North Sea) were acclimated to 5°C and to 10°C (controls). Liver sizes were found to be increased in both species in the cold, with a concomitant rise in liver mitochondrial protein content. As a result, total liver state III rates were elevated in both cold-versus and warm-exposed P. brachycephalum and Z. viviparus, with the highest rates in boreal eelpout acclimated to 5°C. CS and IDH activities in the total liver were similar in Z. viviparus acclimated to 5°C and 10°C, but decreased in those warm acclimated versus control P. brachycephalum. Enzyme capacities in the total liver were higher in eelpout from Antarctica than those from the North Sea. In conclusion, cold compensation of aerobic capacities in the liver seems to be linked to an increase in organ size with unchanged specific mitochondrial protein content. Despite its life in permanently cold climate, P. brachycephalum was able to reduce liver aerobic capacities in warm climate and thus, displayed a capacity for temperature acclimation.     

The fine structure of the adenohypophysis of Zoarces viviparus L.

Summary In the adenohypophysis of Zoarces viviparus L. three types of acidophils, three types of basophils and two types of pars intermedia cells have been described on the level of light- and electron microscopy. It is pointed out that the seasonal alteration in the activity of the two types of basophils is correlated with the activity of the gonads, and the gonadotropic function of them is postulated. Ultrastructural similarity between the adenohypophyseal cells of Z. viviparus and those of mammals is considered.

---

Zusammenfassung In der Adenohypophyse von Zoarces viviparus L. werden aufgrund licht- und elektronenmikroskopischer Untersuchungen drei Typen von acidophilen, drei Typen von basophilen und zwei Typen von Pars intermedia-Zellen beschrieben. Der jahreszeitliche Wechsel in der Aktivität von zwei basophilen Zelltypen ist mit der Aktivität der Gonaden korreliert; eine gonadotrope Funktion dieser beiden Zelltypen wird angenommen. Es wird diskutiert, inwieweit sich die Feinstruktur der Zellen der Adenohypophyse bei Zoarces viviparus und bei Säugern ähnelt.

---

---

The work reported here was carried out with financial support from NATO and the Deutsche Forschungsgemeinschaft.

---

This paper was written as a tribute in honour of the 60th birthday of Prof. Dr. med. W. Bargmann.     

Diversity of Aphanizomenonflos-aquae (Cyanobacterium) Populations along a Baltic Sea Salinity Gradient

Colony-forming cyanobacteria of the genus Aphanizomenon form massive blooms in the brackish water of the Baltic Sea during the warmest summer months. There have been recent suggestions claiming that the Baltic Sea Aphanizomenon species may be different from Aphanizomenon flos-aquae found in lakes. In this study, we examined variability in the morphology and 16S-23S rRNA internal transcribed spacer (ITS) sequences of A. flos-aquae populations along a salinity gradient from a string of lakes to a fjord-like extension of the Baltic Sea to the open Baltic Sea. Morphological differences among the populations were negligible. We found that the Baltic Sea was dominated (25 out of 27 sequences) by one ITS1-S (shorter band of ITS 1 [ITS1]) genotype, which also was found in the lakes. The lake populations of A. flos-aquae tended to be genetically more diverse than the Baltic Sea populations. Since the lake ITS1-S genotypes of A. flos-aquae are continuously introduced to the Baltic Sea via inflowing waters, it seems that only one ITS1 genotype is able to persist in the Baltic Sea populations. The results suggest that one of the ITS1-S genotypes found in the lakes is better adapted to the conditions of the Baltic Sea and that natural selection removes most of the lake genotypes from the Baltic Sea A. flos-aquae populations.     

Nucleotide sequence variation of the mitochondrial COI gene in several eelpout species of the genus Zoarces (Zoarcidae, Pisces)

The mitochondrial COI gene was for the first time sequenced in eelpout species of the genus Zoarces from the Taui Bay of the Sea of Okhotsk: notched-fin Z. elongatus Kner, 1868 and the new sympatric form Zoarces sp., differing in a large set of morphological traits. The two species were compared with European eelpout Z. viviparus Linnaeus, 1758 from the Finnish Bay of the Baltic Sea. Divergence and phylogenetic analyses demonstrated a greater genetic similarity between Zoarces sp. and Z. viviparus than between sympatric Zoarces sp. and Z. elongatus.     

Testing the hypothesis of tolerance strategies in Hiatella arctica L. (Mollusca: Bivalvia)

The physiological and biocenotic optima of Hiatella arctica L. inhabiting shallow water fouling communities of the White Sea were compared. The biomass and proportion of H. arctica in communities were used for the estimation of biocenotic optima or community success. The physiological state of populations was assessed by means of the fluctuating asymmetry. The fluctuating asymmetry of H. arctica was calculated using the valve weights. It was determined that the shell of H. arctica possesses a slight directional asymmetry, the right valve usually being larger (and heavier) than the left one. The relationship between fluctuating and directional asymmetries is discussed. High biomass and proportion of H. arctica in the community generally correspond with high levels of fluctuating asymmetry. Thus, a discrepancy between physiological and ecological optima is observed, which is recognised as being characteristic of a tolerance strategy. However, in the absence of pressure from major filter-feeding competitors such as the mussel Mytilus edulis L. and the solitary ascidian Styela rustica L., populations of H. arctica possess a high biomass and low levels of fluctuating asymmetry. It appears that H. arctica prefers to inhabit shelters or depressions, or to be covered by other organisms. Populations of H. arctica existing in the absence of shelter had extremely high levels of fluctuating asymmetry despite the absence of filter-feeding competitors. Thus, the strategy adopted by H arctica L. in the investigated upper 5 m layer of water in the White Sea can be described as a stress- and competitor-tolerant strategy.     

Fortpflanzung und Oocyten-Entwicklung bei der Aalmutter(Zoarces viviparus) (Pisces,Osteichthyes)

Zusammenfassung 1. Nach dem histologischen Aspekt liegt die Paarungszeit vonZoarces viviparus L. etwa im August. Ab September sind Embryonen im Ovarlumen nachweisbar.2. Die Anzahl der Embryonen ist vor allem von der Körperlänge des Muttertieres abhängig und überschreiter 100 nicht wesentlich. Die Jungtiere werden zum größten Teil im Februar/März geboren.3. Während der Reifung werden die Oocyten so weit in das Ovarlumen hineinverlagert, daß sie mit dem Ovarialgewebe nur noch durch den Funiculus verbunden sind. Durch diesen und eine Haftzone an der Ovarwand befestigt, schwimmt die Oocyte ansonsten frei in einem sich vergrößernden Lymphraum.4. Das am Aufbau des Follikels beteiligte Bindegewebe ist stark vascularisiert. Kapillaren ziehen bis an die Basalmembran zwischen Theca folliculi und Follikelepithel. Die Dicke der Hüllschichten ist gering, die Grundkonstruktion des Cortex stimmt jedoch mit der bei oviparen Fischen überein.5. Nach der Ovulation bleiben Reste des Follikels als Zotte stehen, die bis zur nächsten Fortpflanzungszeit reduziert wird.6. Die Dauer der Entwicklung im Vergleich zu anderen Arten und die Bedeutung der Schichten des Follikels werden diskutiert.

---

Reproduction and oocyte-development inZoarces viviparus (Pisces, Osteichthyes)

The oocytes ofZoarces viviparus L., obtained from south-eastern parts of the North Sea (Helgoland, Sylt), undergo four cytological stages. They are ready for fertilization in July–August. In the ovaries of gravid females, embryos appear in September. The number of embryos depends mainly on the female body length. During oogenesis the ripening oocytes and the adjacent cell layers are transferred into the lumen of the ovary. Each of the ripening eggs is fixed to the germ epithelium by a funiculus and an adhering region. A cleft filled with liquor originates in the theca folliculi. The oocyte and its surrounding layers are embedded in this liquor. The layers are the same as in oviparous fishes: cortex radiatus internus and externus, follicle epithelium, basal lamina and theca folliculi. The main difference is the smaller thickness of the cortical layers inZoarces oocytes. Another difference is the higher grade of vascularization in the connective tissue of the follicle. Both features are adaptations characteristic to ovoviviparity. The duration of development and the possible function of the follicle layers are discussed.

---

---

Mit dankenswerter Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Different stability of posttranslationally modified brain microtubules isolated from cold-temperate fish

Microtubule proteins were isolated by a temperature-dependent assembly-disassembly method from brain tissue of for cold-temperate fish; one fresh water fish (Oncorhynchus mykiss), and three marine fish (Labrus berggylta, Zoarces viviparus andGadus morhua). The -tubulins from all four fish species were acetylated. The -tubulins from the marine fish were composed of a mixture of tyrosinated and detyrosinated tubulin, while the fresh water fish tubulin only reacted with an antibody against detyrosinated tubulin. The isolated microtubules had a similar MAP composition. A 400 kD protein and a MAP2-like protein were found, but MAP1 was missing. All microtubules disassembled upon cooling to 0°C. In spite of these common characteristics, the assembly of microtubules fromLabrus berggylta was inhibited by colchicine and calcium, in contrast to the assembly of microtubules fromOncorhynchus mykiss andZoarces viviparus. For the latter, colchicine was not completely inhibitory even at a concentration as high as 1 mM, and calcium induced the formation of both loosely and densely coiled ribbons. The effects of calcium and colchicine on microtubules fromOncorhynchus mykiss andZoarces viviparus were modulated by either fish or cow MAPs, indicating that the effects are due to intrinsic properties of the fish tubulins and not the MAPs. In view of these findings, our results suggest that there is not correlation between colchicine sensitivity, inability of calcium to inhibit microtubule assembly, and acetylation and detyrosination.     

Neurosecretory processes projecting from the preoptic nucleus into the third ventricle of Zoarces viviparus L.

Summary The cellular processes loaded with neurosecretory elementary granules penetrate the ependyma and project into the third ventricle at the level of preoptic nucleus of Zoarces viviparus L. These cellular processes seem to be arising from the neurosecretory cells. At this zone, not all but some of the ependymal cells have a cilium. In this paper the possible function of these neurosecretory processes and the ciliated ependymal cells are discussed.This work was aided by a Grant from NATO and the Deutsche Forschungsgemeinschaft.     

Comparative analysis of the karyotype of the sculpin Myoxocephalus ochotensis (Pisces: Cottidae) from the Odyan Bay of the Sea of Okhotsk

The karyotype of the endemic Okhotsk Sea sculpin Myoxocephalus ochotensis Schmidt (Cottidae) from Odyan Bay was studied. The number and morphology of chromosomes were determined, 2n = 42 (2 metacentric, 20 subtelocentric, and 20 acrocentric chromosomes), NF = 44. Variability of chromosome number was not revealed; no difference between male and female karyotypes was found. The karyotype of the Okhotsk sculpin M. ochotensis was compared with karyotypes of the Far East Steller’s M. stelleri (Tilesius), snow M. brandti (Steindachner), and plain M. jaok (Cuvier) sculpins, and to the European shorthorn sculpin M. scorpius (Linnaeus) from White Sea. Their similarities and distinctions were shown.     

Papposphaera iugifera nov. sp. from West Greenland,Svalbard, and the Baltic Sea

We are currently revisiting coccolithophore genera and species described from high latitudes in both hemispheres, and also in the process describing new taxa when appropriate, with the aim of providing the best possible framework for polar species segregation based on external morphological features only. The present paper thus introduces Papposphaera iugifera nov. sp. from West Greenland (Disko – type locality), Svalbard (Isfjorden) and the Baltic Sea (Bothnian Sea). P. iugifera is clearly related to P. sagittifera, P. sarion and P. arctica and forms with these a continuum of species that are, with the exception of P. sarion, on the one hand much similar with respect to calicate spine details, while on the other hand clearly differentiated with respect to the complexity of central area calcification. While this is extensive in P. sagittifera it is reduced to just a single transverse bar or even completely absent in P. iugifera.     

A comparative study of the pumping activity of some fouling animals from the White Sea

The pumping activity of the bivalve Mytulus edulus, Hiatella arctica, and solitary ascidian Styela rustica from the White Sea has been studied. The dependence of the pumping activity on the mass and size of animals has been analyzed. It was shown that the mussel is the most active filter-feeder, that the least value of the pumping activity has been registered for H. arctica, and that S. rustica occupies an intermediate position according this index. The results obtained are examined in connection with the competitive relationship of the animals in the fouling community.     

Alien or endangered? Historical development of Chara connivens in the Baltic coastal waters and its implication for the species management

The occurrence of Chara connivens (Charophyta, Characeae) and its status in the Baltic Sea may raise controversies regarding its origin and historical dispersal pathways in the area. This study critically revises the protection status of C. connivens in the countries around the Baltic Sea, as well as its status on the red lists of endangered plant species (including the HELCOM Red List). The first reports on the presence of C. connivens in the Baltic Sea area were published in the aftermath of Carl Baenitz’s talks given in the early 1870s. Already then, the scientific community was well aware of the fact that C. connivens had been introduced as a ballast plant to the known Baltic areas of occurrence – the first known record of the species is from 1829. Since Poland is the only country where C. connivens is protected, the historical and contemporary distribution of this charophyte in the Polish coastal waters is presented against the background of the available historical and recent records of the species in the Baltic Sea. Recent reports from the second half of the 20th century and the beginning of the 21st century have confirmed a fairly common occurrence of C. connivens in Estonia, Sweden and Poland. This species still occurs on the German coast and has also been reported from Finland (the Åland archipelago). In recent decades, however, the species was considered rare in the Baltic Sea area. In Poland, C. connivens was even classified as extinct, despite earlier data on its occurrence in the Vistula Lagoon in the 1970s, where it was rediscovered in 2011. It was also found in the Szczecin Lagoon a year later. Both localities well suit Luther’s pattern of C. connivens occurrence in areas with intensive shipping and ballast discharge operations in historical times. Based on this in-depth revision of historical and current distribution, it is postulated that C. connivens, as non-indigenous, should not be red-listed in the Baltic Sea area, following the example of Finland. Moreover, its legal status in Poland of a strictly protected species should be reconsidered.     

Uptake of dissolved organic nitrogen by size-fractionated plankton along a salinity gradient from the North Sea to the Baltic Sea

The Baltic Sea is known for its ecological problems due to eutrophication caused by high nutrient input via nitrogen fixation and rivers, which deliver up to 70% of nitrogen in the form of dissolved organic nitrogen (DON) compounds. We therefore measured organic nitrogen uptake rates using self produced ¹⁵N labeled allochthonous (derived from Brassica napus and Phragmites sp.) and autochthonous (derived from Skeletonema costatum) DON at twelve stations along a salinity gradient (34 to 2) from the North Sea to the Baltic Sea in August/September 2009. Both labeled DON sources were exploited by the size fractions 0.2–1.6 μm (bacteria size fraction) and >1.6 μm (phytoplankton size fraction). Higher DON uptake rates were measured in the Baltic Sea compared to the North Sea, with rates of up to 1213 nmol N l^?1 h^?1. The autochthonous DON was the dominant nitrogen form used by the phytoplankton size fraction, whereas the heterotrophic bacteria size fraction preferred the allochthonous DON. We detected a moderate shift from >1.6 μm plankton dominated DON uptake in the North Sea and central Baltic Sea towards a 0.2–1.6 μm dominated DON uptake in the Bothnian Bay and a weak positive relationship between DON concentrations and uptake. These findings indicate that DON is an important component of plankton nutrition and can fuel primary production. It may therefore also contribute substantially to eutrophication in the Baltic Sea especially when inorganic nitrogen sources are depleted.     

Molecular identification key based on PCR/RFLP for three polychaete sibling species of the genus <Emphasis Type="Italic">Marenzelleria</Emphasis>, and the species’ current distribution in the Baltic Sea

Studies of Marenzelleria species were often hampered by identification uncertainties when using morphological characters only. A newly developed PCR/RFLP protocol allows a more efficient discrimination of the three species Marenzelleria viridis, Marenzelleria neglecta and Marenzelleria arctia currently known for the Baltic Sea. The protocol is based on PCR amplification of two mitochondrial DNA gene segments (16S, COI) followed by digestion with restriction enzymes. As it is faster and cheaper than PCR/sequencing protocols used so far, the protocol is recommended for large-scale analyses. The markers allow an undoubted determination of species irrespective of life stage or condition of the worms in the samples. The protocol was validated on about 950 specimens sampled at more than 30 sites of the Baltic and the North Sea, and on specimens from populations of the North American east coast. Besides this test we used mitochondrial DNA sequences (16S, COI, Cytb) and starch gel electrophoresis to further investigate the distribution of the three Marenzelleria species in the Baltic Sea. The results show that M. viridis (formerly genetic type I or M. cf. wireni) occurred in the Öresund area, in the south western as well as in the eastern Baltic Sea, where it is found sympatric with M. neglecta. Allozyme electrophoresis indicated an introduction by range expansion from the North Sea. The second species, M. arctia, was only found in the northern Baltic Sea, where it sometimes occurred sympatric with M. neglecta or M. viridis. For Baltic M. arctia, the most probable way of introduction is by ship ballast water from the European Arctic. There is an urgent need for a new genetic analysis of all Marenzelleria populations of the Baltic Sea to unravel the current distribution of the three species.     

Diversity and abundance of “Pelagibacterales” (SAR11) in the Baltic Sea salinity gradient

The candidate order “Pelagibacterales” (SAR11) is one of the most abundant bacterial orders in ocean surface waters and, periodically, in freshwater lakes. The presence of several stable phylogenetic lineages comprising “Pelagibacterales” correlates with the physico-chemical parameters in aquatic environments. A previous amplicon sequencing study covering the bacterial community in the salinity gradient of the Baltic Sea suggested that pelagibacteral subclade SAR11-I was replaced by SAR11-IIIa in the mesohaline region of the Baltic Sea. In this current study, we investigated the cellular abundances of “Pelagibacterales” subclades along the Baltic Sea salinity gradient using catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). The results obtained with a newly designed probe, which exclusively detected SAR11-IIIa, were compared to CARD-FISH abundances of the marine SAR11-I/II subclade and the freshwater lineage SAR11-IIIb (LD12). The results showed that SAR11-IIIa was abundant in oligohaline–mesohaline conditions (salinities 2.7–13.3), with maximal abundances at a salinity of 7 (up to 35% of total Bacteria, quantified with a universal bacterial probe EUB). As expected, SAR11-I/II was abundant (27% of EUB) in the marine parts of the Baltic Sea, whereas counts of the freshwater lineage SAR11-IIIb were below the detection limit at all stations. The shift from SAR11-IIIa to SAR11-I/II was confirmed in the vertical salinity gradient in the deeper basins of the Baltic Sea. These findings were consistent with an overlapping but defined distribution of SAR11-I/II and SAR11-IIIa in the salinity gradient of the Baltic Sea and suggested the adaptation of SAR11-IIIa for growth and survival in mesohaline conditions.     

The Native Bacterioplankton Community in the Central Baltic Sea Is Influenced by Freshwater Bacterial Species

The Baltic Sea is one of the largest brackish environments on Earth. Despite extensive knowledge about food web interactions and pelagic ecosystem functioning, information about the bacterial community composition in the Baltic Sea is scarce. We hypothesized that due to the eutrophic low-salinity environment and the long water residence time (>5 years), the bacterioplankton community from the Baltic proper shows a native “brackish” composition influenced by both freshwater and marine phylotypes. The bacterial community composition in surface water (3-m depth) was examined at a single station throughout a full year. Denaturing gradient gel electrophoresis (DGGE) showed that the community composition changed over the year. Further, it indicated that at the four extensive samplings (16S rRNA gene clone libraries and bacterial isolates from low- and high-nutrient agar plates and seawater cultures), different bacterial assemblages associated with different environmental conditions were present. Overall, the sequencing of 26 DGGE bands, 160 clones, 209 plate isolates, and 9 dilution culture isolates showed that the bacterial assemblage in surface waters of the central Baltic Sea was dominated by Bacteroidetes but exhibited a pronounced influence of typical freshwater phylogenetic groups within Actinobacteria, Verrucomicrobia, and Betaproteobacteria and a lack of typical marine taxa. This first comprehensive analysis of bacterial community composition in the central Baltic Sea points to the existence of an autochthonous estuarine community uniquely adapted to the environmental conditions prevailing in this brackish environment.