Seasonal and short-term variation in denitrification and anammox at a coastal station on the Gulf of Finland,Baltic Sea

Benthic processes were measured at a coastal deposition area in the northern Baltic Sea, covering all seasons. The N₂ production rates, 90–400 μmol N m⁻² d⁻¹, were highest in autumn-early winter and lowest in spring. Heterotrophic bacterial production peaked unexpectedly late in the year, indicating that in addition to the temperature, the availability of carbon compounds suitable for the heterotrophic bacteria also plays a major role in regulating the denitrification rate. Anaerobic ammonium oxidation (anammox) was measured in spring and autumn and contributed 10% and 15%, respectively, to the total N₂ production. The low percentage did, however, result in a significant error in the total N₂ production rate estimate, calculated using the isotope pairing technique. Anammox must be taken into account in the Gulf of Finland in future sediment nitrogen cycling research. Handling editor: J. Cole     

Abiotic factors influencing cyanobacterial bloom development in the Gulf of Finland (Baltic Sea)

Blooms of cyanobacteria are a recurrent phenomenon in the Baltic Sea, including the Gulf of Finland. The spatial extension, duration, intensity and species composition of these blooms varies widely between years. Alg@line data collected regularly from ferries as well as weather service and marine monitoring data from 1997 to 2005 are analysed to determine the main abiotic factors influencing the intensity and species composition of cyanobacterial blooms in the Gulf of Finland. It is demonstrated that the development of the Nodularia spumigena Mertens bloom is highly dependent on weather conditions such as photosynthetically active radiation and water temperature. Nutrient conditions, especially the surplus of phosphorus (according to Redfield ratio) related to the pre-bloom upwelling events in the Gulf, affect the intensity of Aphanizomenon sp. (L.) Ralfs blooms. Differences in bloom timing and duration indicate that, if the preconditions (like nutrient ratio/concentration and weather conditions) for bloom formation are favourable, then the Aphanizomenon bloom starts earlier, the overall bloom period is longer and the Nodularia peak might appear in a wider time window. Handling editor: K. Martens     

Binding of phosphate in sediment accumulation areas of the eastern Gulf of Finland,Baltic Sea

The relationships between P and components binding P were studied by analysing the concentrations of N, P, Fe, Mn, Ca and Al in sediments and pore water along the estuarine transect of the River Neva in August 1995. The high sediment organic matter concentration resulted in low surface redox potential and high pore-water o-P concentration, whereas the abundance of amphipods resulted in high surface redox potentials and low pore-water o-P concentration. However, despite the variation in sediment organic matter and the abundance of amphipods, very reduced conditions and slightly variable concentrations of Tot-P (0.7–1.1 mg g^–1 DW) were observed in the 10–15 cm sediment depth along the estuarine gradient, indicating that the pools of mobile P were largely depleted within the depth of 0–15 cm. Multiple regression analysis demonstrated that organic matter and Tot-Fe concentration of the sediment were closely related to the variation in Tot-P concentration of the sediments (r ² = 0.817, n=32). In addition, the high total Fe:P ratio suggested that there is enough Fe to bind P in sediments along the estuarine gradient. However, low Fe_diss concentrations in the pore water of reduced sediment (redox-potential <–50 mV) indicated efficient precipitation of FeS (FeS and FeS₂), incapable to efficiently bind P. Consequently, the low Fe_diss:o-P ratio (< 1) recorded in pore water in late summer implied that Fe³⁺ oxides formed by diffusing Fe_diss in the oxic zone of the sediments were insufficient to bind the diffusing o-P completely. The measured high o-P concentrations in the near-bottom water are consistent with this conclusion. However, there was enough Fe_diss in pore water to form Fe³⁺ oxides to bind upwards diffusing P in the oxic sediment layer of the innermost Neva estuary and the areas bioturbated by abundant amphipods.     

Effects of nonindigenous species on diversity and community functioning in the eastern Gulf of Finland (Baltic Sea)

An increase of xenodiversity in plankton and benthos in the eastern Gulf of Finland was observed from 1998 to 2004. Nonindigenous species account for 4.8% of all species found and up to 96% of total biomass. Invasive benthic omnivores, the alien amphipods Gmelinoides fasciatus and Pontogammarus robustoides and the predaceous fish Perccottus glenii with their versatile diets strongly affect the community structure. Invasive sessile seston-feeders that directly (through grazing and water clearance) and indirectly (through recycling of nutrients) interact with other ecosystem components, are mainly represented by the zebra mussel Dreissena polymorpha, which affect the structure of benthic and planktonic communities as well as benthic-pelagic coupling. The invasive predatory cladocerans Cercopagis pengoi and Evadne anonyx and larvae of D. polymorpha are only temporary components in the zooplankton, which is limiting their overall effect. Alien benthic bioturbators, the polychaetes Marenzelleria neglecta and the oligochaete Tubificoides pseudogaster account for a high proportion of total abundance and biomass but their effects on native species need further research.     

Diversity of Oil-Degrading Microorganisms in the Gulf of Finland (Baltic Sea) in Spring and in Summer

Diversity of the oil-degrading microbial strains isolated from the water and sediments of the Gulf of Finland (Baltic Sea) in winter and in summer was studied. Substrate specificity of the isolates for aliphatic and aromatic hydrocarbons was studied. The isolates belonged to 32 genera of the types Proteobacteria (alpha-, beta-, and gammaproteobacteria), Actinobacteria,Firmicutes, and Bacteroidetes. Seasonal variations of the oil-degrading microbial communities was revealed. The presence of the known genes responsible for the degradation of oil aliphatic and aromatic hydrocarbons was determined. The alkB sequence of the alkane hydroxylase gene was found in ~16% of the studied strains. The sequence of the phnAc phenanthrene 3,4- dioxygenase was found in Sphingobacterium sp. and Arthrobacter sp. isolates retrieved in winter and summer. In five Pseudomonas sp. strains from winter samples, the classical operons of naphthalene degradation (nah) were localized in catabolic plasmids, of which three belonged to IncР-9, one, to IncР-7, and two to an unidentified incompatibility group. Burkholderia and Delftia strains contained the operons for naphthalene degradation via salicylate and gentisate (nag). The presence of nag genes has not been previously reported for Delftia spp. strains. The sequences of the nagG salicylate 5-hydroxylase gene were also found in Achromobacter, Sphingobacterium, and Stenotrophomonas strains.     

European smelt Osmerus eperlanus in the eastern Gulf of Finland,Baltic Sea: Stock status and fishery

In the eastern Gulf of Finland, European smelt Osmerus eperlanus occurs as an anadromous ecological form that spawns in coastal, low-salinity zones and in several rivers entering the gulf, most importantly in the River Neva. Osmerus eperlanus is a key commercial fish species for the population of St. Petersburg, and the city's fish symbol. However, the state of the smelt stock has considerably deteriorated over the past few decades. Monitoring shows that annual catches during 1965–1992 averaged 2274 t, compared to 292 t in 2002–2017, although some improvement is inidicated by a gradual increase in catches in recent years (e.g., to 595 t in 2017). This paper addresses long-term and recent changes in the fisheries for smelt in the eastern Gulf of Finland, including both commercial and recreational fisheries, and the range of fishing gears used. The commercial smelt fishery is mainly conducted during the spawning period (April–May) on migrating fish, using trap nets of various designs and beach seines. There has been an increase in recreational fisheries for smelt; in years when ice cover on the gulf during winter (December–April) is steady, recreational fishers from St. Petersburg, using hook and line, may catch quantities that are comparable to those of commercial catches. Several factors may account for the stock dynamics and associated changes in catches of smelt: these include a decrease in prey availability in the gulf; loss and degradation of spawning and nursery habitats, partly associated with large-scale hydroengineering operations in Neva Bay; and illegal, unreported and unregulated (IUU) fishing which became widespread in post-Soviet decades. In spite of a gradual increase in smelt abundance in the most recent 15 years, a failure to accurately monitor and manage the extent of IUU and recreational fishing for smelt, to assess the status of the stock, or to forecast catches, increases the risk of overfishing of the spawning stock.     

Microbiological Parameters for Differentiating between Coastal and Open Waters in the Northern Baltic Proper and the Gulf of Finland

The abundance of selected types of micro-organisms was compared between seven sites in inshore and offshore areas in the northern Baltic proper and at the entrance to the Gulf of Finland. The aim was to find out which of the microbiological parameters indicate terrestrial and littoral influences on the sites investigated and which of them indicate offshore conditions. During four cruises in 1977–78, samples were collected from 1-0 m depth, within one or a few days, at five offshore and two inshore sites.
Microbial numbers decreased consistently with increasing distance from the shore. The influence of terrestrial runoff, outflows from littoral areas and sediments was clear in the two Tvärminne inshore areas but not in the Ajax area situated about 10 km seaward of the outermost skerries of the Tvärminne archipelago. The influence of these factors was most distinctly indicated by viable bacteria at 18°C and polymyxin-resistant bacteria, rods and bacterial spores. Offshore characteristics were indicated by vibrios and bacteria obviously in a starved state. Proteolytic bacteria and yeasts did not indicate either of these conditions.     

A Fuzzy Logic Model to Describe the Cyanobacteria Nodularia spumigena Blooms in the Gulf of Finland, Baltic Sea

A fuzzy logic model to describe the seasonal evolution of Nodularia spumigena blooms in the Gulf of Finland was built and calibrated on the basis of monitoring data. The model includes three phosphate sources: excess phosphate after the annual spring bloom and parameterised phosphate transport to the upper mixed layer by turbulent mixing and upwelling events. Surface layer temperature and wind mixing form the physical conditions controlling the growth of N. spumigena. Model simulations revealed that phosphate input caused by turbulent mixing and upwelling have to be taken into account to achieve the best fit with observed data. Testing the fuzzy model for early prediction of maximum N. spumigena biomass about a month before the usual occurrence of blooms, gave good results. The potential use of the model for prediction of bloom risk at a certain location along the Estonian or Finnish coast was tested. The bloom transport velocities used in the fuzzy model were pre-calculated by a 3D numerical circulation model for different wind regimes.     

The influence of weather conditions (temperature and wind) on cyanobacterial bloom development in the Gulf of Finland (Baltic Sea)

Cyanobacterial blooms are common in the Baltic Sea. They are dominated by Aphanizomenon flos-aquae and Nodularia spumigena and take place in July–August. Investigations of bloom development using different approaches have been carried out in the Gulf of Finland during recent years. The ship-of-opportunity technique allows to observe the upper layer dynamics from meso- to basin-wide scale with high temporal and spatial frequency at low cost. Unattended measurements on board a commercial ferry along a transect between Tallinn and Helsinki have been conducted for 3 years (1997–1999). The influence of weather conditions—temperature and wind—on the cyanobacterial bloom development was investigated. The formation of cyanobacterial blooms was favoured by warm and calm weather, while in cold and windy conditions other species formed mass occurrences. Water temperature has been found to be the main factor controlling the initiation of the bloom, in general, while vertical stratification appeared to be the critical factor determining the intensity of the bloom at species level. The spatial distribution of the cyanobacterial bloom was determined rather by the wind-forced advection than by the possible vertical transport of nutrients in the areas of the observed upwelling events.     

Community development and modes of phosphorus utilization in a late summer ecosystem in the central Gulf of Finland, the Baltic Sea

The development of a filamentous, nitrogen-fixing cyanobacterial bloom was followed during July–August 1990 in a stratified basin in the central Gulf of Finland, Baltic Sea. Hydrography, dissolved inorganic, particulate and total nutrients, chlorophyll a, alkaline phosphatase activity, ³²PO₄-uptake and phytoplankton species were measured. The study period was characterized by wind-induced mixing events, followed by marked nutrient pulses and plankton community responses. Phosphate uptake was highest throughout the study period in the size fraction dominated by bacteria and picocyanobacteria (< 2 µm) and the proportion of uptake in the size fraction 2–10 µm remained low (2–6%). Higher phosphate turnover times were observed in a community showing signs of enhanced heterotrophic activity. The bloom of filamentous, nitrogen-fixing cyanobacteria Aphanizomenon flos-aquae was promoted by a nutrient pulse with an inorganic nutrient ratio (DIN:DIP) of 15. The results show that the quality, frequency and magnitude of the physically forced nutrient pulses have an important role in determining the relative share of the different modes of phosphorus utilization and hence in determining the cyanobacterial bloom intensity and species composition in the Baltic Sea.     

Genetic diversity in strains of the genus Anabaena isolated from planktonic and benthic habitats of the Gulf of Finland (Baltic Sea)

Late summer cyanobacterial blooms in the Baltic Sea contain Anabaena sp. together with Nodularia spumigena and Aphanizomenon flos-aquae. Although Anabaena is common especially in the Gulf of Finland, very little is known about its genetic diversity. Here we undertook a molecular phylogenetic study of 68 Anabaena strains isolated from the brackish Gulf of Finland. We sequenced the 16S rRNA genes from 54 planktonic and 14 benthic Anabaena strains, and rbcL and rpoC1 genes from a subset of these strains. Phylogenetic trees showed that Anabaena strains, from both planktonic and benthic habitats, were genetically diverse. Although the Anabaena strains were morphologically diverse, in our study only one genetically valid species was found to exist in the plankton. Evolutionary distances between benthic Anabaena strains were greater than between planktonic strains, suggesting that benthic habitats allow for the maintenance of greater genetic diversity than planktonic habitats. A number of novel lineages containing only sequences obtained in this study were compiled in the phylogenetical analyses. Thus, it seemed that novel lineages of the genus Anabaena may be present in the Baltic Sea. Our results demonstrate that the Baltic Sea Anabaena strains show surprisingly high genetic diversity.     

Coastal Fisheries in the Eastern Baltic Sea (Gulf of Finland) and Its Basin from the 15 to the Early 20th Centuries

The paper describes and analyzes original data, extracted from historical documents and scientific surveys, related to Russian fisheries in the southeastern part of the Gulf of Finland and its inflowing rivers during the 15- early 20^th centuries. The data allow tracing key trends in fisheries development and in the abundance of major commercial species. In particular, results showed that, over time, the main fishing areas moved from the middle part of rivers downstream towards and onto the coastal sea. Changes in fishing patterns were closely interrelated with changes in the abundance of exploited fish. Anadromous species, such as Atlantic sturgeon, Atlantic salmon, brown trout, whitefish, vimba bream, smelt, lamprey, and catadromous eel were the most important commercial fish in the area because they were abundant, had high commercial value and were easily available for fishing in rivers. Due to intensive exploitation and other human-induced factors, populations of most of these species had declined notably by the early 20^th century and have now lost commercial significance. The last sturgeon was caught in 1996, and today only smelt and lamprey support small commercial fisheries. According to historical sources, catches of freshwater species such as roach, ide, pike, perch, ruffe and burbot regularly occurred, in some areas exceeding half of the total catch, but they were not as important as migrating fish and no clear trends in abundance are apparent. Of documented marine catch, Baltic herring appeared in the 16^th century, but did not become commercially significant until the 19^th century. From then until now herring have been the dominant catch.     

Scales of horizontal patchiness in chlorophyll a, chemical and physical properties of landfast sea ice in the Gulf of Finland (Baltic Sea)

Horizontal variation of first-year landfast sea ice properties was studied in the Gulf of Finland, the Baltic Sea. Several scales of variation were considered; a number of arrays with core spacings of 0.2, 2 and 20 m were sampled at different stages of the ice season for small-scale patchiness. Spacing between these arrays was from hundreds of meters to kilometers to study mesoscale variability, and once an onshore–offshore 40-km transect was sampled to study regional scale variability. Measured variables included salinity, stable oxygen isotopes (¹⁸O), chlorophyll a (chl-a), nutrients and dissolved organic carbon. On a large scale, a combination of variations in the under-ice water salinity (ice porosity), nutrient supply and the stage of ice development control the build-up of ice algal biomass. At scales of hundreds of meters to kilometers, there was significant variability in several parameters (salinity, chl-a, snow depth and ice thickness). Analyses of the data from the arrays did not show evidence of significant patchiness at scales <20 m for algal biomass. The results imply that the sampling effort in Baltic Sea ice studies should be concentrated on scales of hundreds of meters to kilometers. Using the variations observed in the study area, the estimate for depth-integrated algal biomass in landfast sea ice in the Gulf of Finland (March 2003) is 5.5±4.4 mg chl-a m^–2.     

Growth of enterobacteria on algal mats in the eastern part of the Gulf of Finland

Model experiments on a possibility that pathogenic enterobacteria Salmonella enteritidis (Gartneri) can grow on decaying algal mats with prevalence of the filamentous algae Cladophora glomerata (L.) Kütz were carried out. Samples of algal mats have been collected in the eastern part of the Gulf of Finland in the Baltic Sea. A bacterial culture of Salmonella enteritidis was placed into tubes containing samples of mats. The intensive growth of salmonella was noted in alga samples collected in the freshwater zone (salinity 0.2–1.5‰); growth was practically absent in the samples of algae collected in a zone with salinity 2–3‰, while salmonella remained viable in the control tubes with water without algae. The growth of coliform enterobacteria initially inhabited in the algal mats was discovered in all experiments. Studies carried out in 2009 show that the thickness of the algal mats in the costal zones of the Gulf of Finland reached 20 cm and their biomass reached a few tons per 1 km². These experiments showed that dead algal mats stimulate the growth of enterobacteria in the littoral zone of the Baltic Sea, especially in the freshwater part, and can promote the development of these pathogenic microorganisms.     

Seasonal changes in situ grazing of the mesoherbivores Idotea baltica and Gammarus oceanicus on the brown algae Fucus vesiculosus and Pylaiella littoralis in the central Gulf of Finland, Baltic Sea

The in situ grazing experiments were performed in the shallow water rocky habitat of the northern Baltic Sea during ice-free season 2002. In the experiments the effects of algal species and choice on the grazing of the mesoherbivores Idotea baltica (Pallas) and Gammarus oceanicus Segerstråle were tested. Salinity, temperature, concentration of nutrients in water and macroalgae and net production of macroalgae were considered as random effects in the analysis. The invertebrate feeding rate was mainly a function of the net photosynthetic activity of Pylaiella littoralis (L.) Kjellman and Fucus vesiculosus L. Feeding rate increased significantly with decreasing algal photosynthetic activity. When the two algal species were incubated together invertebrates fed primarily on P. littoralis. Low selectivity towards P. littoralis coincided with its high photosynthetic activity. The presence of F. vesiculosus did not modify the invertebrate feeding on P. littoralis. The results indicated that (1) the grazing on F. vesiculosus depended on the availability of P. littoralis, (2) the photosynthetic activity of algae explained the best the variation in grazing rate and (3) the grazers are not likely to control the early outbreak of filamentous algae in the northern Baltic Sea by avoiding young and photosynthetically active algae. The likely mechanism behind the relationship is that the increased photosynthetic activity of macroalgae coincides with their higher resistance to herbivory.     

Blue mussels (Mytilus edulis) in the diet of roach (Rutilus rutilus) in outer archipelago areas of the western Gulf of Finland,Baltic Sea

The blue mussel (Mytilus edulis) is one of the key species in the Baltic Sea ecosystem and it is living at the edge of its range in the western Gulf of Finland. Roach (Rutilus rutilus) is a freshwater fish species that has benefited from recent coastal eutrophication and is at present highly abundant in the outer archipelago of the Gulf of Finland. In 2000 and 2001, a total of 516 roach were sampled for diet analysis in three study areas. Shelled molluscs formed over 95% of the diet of roach, blue mussels being the dominant single species. The proportion of this species in the food of roach in the three study areas ranged between 38–61% for smaller roach (<225 mm) and 39–85% for larger (>225 mm) roach, indicating that blue mussel is a highly important food source for roach in outer archipelago areas of the western Gulf of Finland, in contrast to reports from other parts of the northern Baltic Sea. The scarcity of large blue mussels in mussel beds in the easternmost study area was reflected in the lower proportion of blue mussels in the diet of larger roach. However, the growth of roach was not affected by the availability of blue mussels. The twofold differences observed in the annual growth of roach between warm and cold years demonstrated that temperature is an important factor controlling the growth of roach in the western Gulf of Finland.     

Distribution,population structure and salinity tolerance of the invasive amphipod Gmelinoides fasciatus (Stebbing) in the Neva Estuary (Gulf of Finland,Baltic Sea)

In the early 1970s, the Baikalian amphipod Gmelinoides fasciatus (Stebbing) was intentionally introduced into several lakes in the Gulf of Finland basin in order to enhance fish production. By 1996, G. fasciatus successfully colonized the littoral zone of Lake Ladoga and, via the Neva River, invaded the Neva Bay, the freshwater part of the Neva Estuary. In 1999, G. fasciatus was first registered in the inner Neva Estuary, the very first record of the Baikalian amphipod in brackish waters of the Baltic Sea. Distribution, abundance, reproduction and population structure of G. fasciatus in the Neva Estuary were studied during 1998–2000. In some locations of the Neva Estuary, maximum densities of G. fasciatus reached 3500 ind. m⁻². In general, density and biomass of G. fasciatus in the freshwater part of the Neva Estuary were higher (around 1.5 fold) than in the brackish-water part. Fecundity of this amphipod averaged 10–20 eggs per female, depending on body size of females and season. In order to assess the possibility of further spread of G. fasciatus in the Baltic Sea, the salinity tolerance of this species was determined in a series of laboratory experiments. Our results showed that the invasive amphipod G. fasciatus is potentially able to colonize shallow coastal habitats of, for example, the Gulf of Bothnia, Gulf of Riga and other parts of the Baltic Sea with water salinities ranging from 1 to 5 psu.