Diatom assemblages in superficial sediments from the Gulf of Riga,eastern Baltic Sea

Twenty one superficial sediment samples from areas of high and moderate eutrophication in the Gulf of Riga were studied with respect to siliceous microfossils, mainly diatoms.The results seem to imply that the number of taxa and the abundance of the frustules are affected by runoff from rivers and the degree of eutrophication in different parts of the Gulf. Areas with high eutrophication, e.g. the river estuaries, have diatom assemblages of varying composition, while in areas with moderate eutrophication the composition is almost constant and the influx of freshwater diatoms and littoral periphyton small. The high abundance and low diversity of brackish-marine and brackish, planktonic diatoms seem to be a result of an influx of nutrients and pollutants in the water and bottom sediments. The sea ice diatoms occurring in the Baltic waters and also in the Gulf of Riga tend to be resistant to eutrophication or are even favoured by it.     

Predation by the isopod Saduria entomon on the amphipods Monoporeia affinis and Pontoporeia femorata: experiments on prey vulnerability

Summary Predation by Saduria entomon on the depositfeeding amphipods Monoporeia (=Pontoporeia) affinis and Pontoporeia femorata was studied in laboratory experiments. Prey vulnerabilities were compared in singleprey treatments and in mixed-prey treatments, where the proportions of the two species were varied at one total density. In a pilot experiment, P. femorata was the more vulnerable prey, both in single-prey and mixed-prey treatments. In later experiments, the amphipod species were equally vulnerable in single-prey treatments, while in mixed-prey treatments Saduria preyed preferentially on M. affinis, even when this prey was rare, i.e. it did not switch. We suggest that the different result in the later experiments may have been due to a change in the search mode of the isopods.     

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.     

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.     

Adaptability of the amphipod Pontoporeia affinis (Crustacea: Amphipoda) to salinity changes

Salinity tolerance of the amphipod Pontoporeia affinis from the Baltic Sea was examined after acclimation to increased and decreased salinity. There were adaptive shifts in tolerance related to ambient salinity. A sharp change in salinity tolerance of amphipods occurred after acclimation to extremely low salinity.     

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     

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.     

Seasonal variation in copper and zinc concentrations in three talitrid amphipods (Crustacea)

Cu and Zn concentrations were determined for three talitrid amphipods, Orchestia gammarellus (Pallas), O. mediterranea Costa and Talitrus saltator (Montagu), collected at two-monthly intervals in 1987 from sites on Great Cumbrae Island, Firth of Clyde, Scotland. To account for size effects, log transformed Cu and Zn concentrations were regressed against log dry weight for each bimonthly sample of each species, and compared by analysis of covariance. Copper concentrations in O. gammarellus were significantly raised in March 1987, and lowered in November 1987. Copper concentrations in O. mediterranea and T. saltator were significantly lowered in November 1987. Cu concentrations differed interspecifically in the order O. gammarellus > O. mediterranea > T. saltator. There was no significant intraspecific seasonal variation in zinc concentration in any of the three species. Zn concentrations differed interspecifically in the order T. saltator > O. gammarellus > O. mediterranea.     

Seasonal variation in trace metal concentrations in three talitrid amphipods from the Gulf of Gdansk, Poland

Talitrid amphipod crustaceans are increasingly being used as biomonitors of trace metal bioavailabilities in coastal waters, and it is important to understand how other factors, in addition to metal pollution, might affect their accumulated body concentrations. Seasonal variation (April-October 2000) in body concentrations of copper, zinc, cadmium, iron, manganese, lead and nickel has been investigated in three species, Orchestia cavimana, Talitrus saltator and Talorchestia deshayesii, from the region of the Gulf of Gdansk (including the Vistula Lagoon), Poland, an area with considerable temperature variation between summer and winter. Seasonal variation in the body metal concentrations of talitrid amphipod crustaceans was confirmed in each population, but only copper showed a consistent pattern. Copper concentrations fell from early to late summer, perhaps either as a result of the replacement of the older overwintering generation by the younger spring one, or as a result of the loss of haemocyanin in autumn amphipods entering a period of low metabolic activity. Metal concentration data for T. saltator from Gdynia-Osada Rybacka compared with equivalent data for 1996 and 1998 show a significant drop in metals associated with the influx of the Vistula river water, which occurred in 1998, following a major flood event in 1997.     

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.     

Biodiversity of settled material in a sediment trap in the Gulf of Trieste (northern Adriatic Sea)

Phytoplankton succession and sinking rates were studied from January to December 2003 at a coastal station in the Gulf of Trieste (northern Adriatic Sea), 200 m offshore, in a relatively undisturbed area. A conical sediment trap, moored at 15 m depth (water depth 17 m), was used. The hypothesis if the presence of benthic and epiphytic diatoms can lead to an overestimation of the vertical fluxes was tested. To evaluate primary and secondary sedimentation contributions, planktonic, benthic and epiphytic diatoms were distinguished. Benthic species abundance varied throughout the year and it was related to resuspension that strongly influenced sinking rates. All over the year, diatoms were the prevailing class in the trap material accounting for 75.32% of the settled cells, while flagellates represented 24.11%. Dinophyceae and resting cells constituted minor components, accounting for 0.43% and 0.14%, respectively. The gross sedimentation rates ranged from 0.006 × 10⁸ cell m⁻² d⁻¹ in the second week of May to 6.30 × 10⁸ cell m⁻² d⁻¹ in the third week of January with a mean annual value of 1.09 ± 1.43 × 10⁸ cell m⁻² d⁻¹. To the primary sedimentation rate Pseudo-nitzschia seriata of the group “Nitzschia seriata complex” contributed for 49.77% followed by Chaetoceros spp. (23.88%). The major contributor to the secondary sedimentation rate was the diatom Paralia sulcata, accounting for 24.76%. Epiphytic diatoms contributed for 11.19% and 12.27% on annual average gross abundance and biomass, respectively, reaching even 72.04% of gross abundance and 56.06% of gross biomass in the second week of August. The correlation between temperature and the logarithm of the epiphytic biomass was statistically significant, with r = 0.66 and P < 0.001. Both in the cluster analysis and in the PCA four main groups were formed, where benthic and epiphytic species were separately gathered. Planktonic, benthic and epiphytic forms accounted for 50.78%, 36.95% and 12.27%, respectively, calculated on the annual average biomass. Therefore, vertical fluxes can be overestimated of 50% or more if benthic and epiphytic species are not rejected.     

Ecological consequences of biological invasions: three invertebrate case studies in the north-eastern Baltic Sea

Population dynamics and ecological impacts of the cirriped Balanus improvisus, the polychaete Marenzelleria neglecta and the cladoceran Cercopagis pengoi were investigated in the north-eastern Baltic Sea. After an increase during the first decade of invasion, the density of M. neglecta and C. pengoi declined afterwards. The studied abiotic environmental variables did not explain the interannual variability in the seasonal cycles of M. neglecta and C. pengoi indicating that the species are at their initial phase of invasion. The population dynamics of B. improvisus was best described by water temperature. B. improvisus promoted the growth of the green alga Enteromorpha intestinalis. M. neglecta enhanced the content of sediment chlorophyll a and reduced growth and survival of the polychaete Hediste diversicolor and growth of the amphipod Monoporeia affinis. Concurrent with the invasion of C. pengoi the abundance of small-sized cladocerans declined, especially above the thermocline. C. pengoi had become an important food for nine-spined stickleback, bleak, herring and smelt.     

Nitrogen Fixation During Blooms of Nodularia in Coastal Waters and Backwaters of the Arkona Sea (Baltic Sea) in 1974

Nitrogen fixation was investigated by means of the acetylene reduction method during the development of a water bloom of Nodularia in coastal waters of the Baltic Sea west of the island of Hiddensee and in backwaters showing different degrees of eutrophication. Depending on plankton density, the values found varied greatly. The maximum of nitrogen fixation values found in extremely dense water blooms under special conditions (Baltic Sea, 2250 μg N₂/l · h; Kleiner Jasmunder Bodden, 374 μg N₂/l · h) are up to 10³ times higher than from other parts of the Baltic Sea or from inland waters. The average nitrogenase activity determined for coastal water populations of the Baltic Sea is 2.15 pg N₂/heterocyst · h and that of the inmost backwaters 0.77 pg N₂/heterocyst · h. The relationship between N₂-fixation and nutrient content in water is discussed.     

Seasonal Variation in Community Structure of Macrobenthic Invertebrates in Sandy Beaches of Jordan Coastline,Gulf of Aqaba,Red Sea

Two Jordanian sandy beaches, one at the northern end and the other at the southern end near the Saudi-Jordanian border, were examined during November 1984 to October 1985. Sediments of the South station were made of coarser sand than the North station. In general, low organic carbon and calcium carbonate levels were found at both stations. A total of 15839 animals representing 45 taxa were collected from both stations. Major differences in community structure were found between the two stations. An average density of 7981/m², 39 taxa. an average diversity value of 1.78 and an evenness of 0.37 were characteristic of the South station. Lower number of taxa (39) and density (818/m²) were found on the North station. However, the average diversity (3.18) and evenness (0.74) were higher. Using the biological index value of MCCLOSKEY (1970), the archiannelid Saccocirrus sp. was the most dominant species at the South station and the bivalve Tellina perna at the North station.     

Preliminary study of summer phytoplankton in coastal waters of the Gulf of Taranto (Ionian Sea)

Abstract

Phytoplankton abundances and species composition of coastal waters of the Gulf of Taranto (Ionian Sea) were investigated in July 1995. Abundances were lower than those of other Mediterranean coastal areas. Higher densities values were found in the central part of the studied area. In surface waters, small phytoflagellates (<10μm) and dinoflagellates, such as Heterocapsa niei and Oxytoxum variabile, were the dominant species. Diatoms, mainly belonging to the genus Chaetoceros, were also present.     

Taxonomic Status and Reproduction Dynamics of the Non-Indigenous Cercopagis in the Gulf of Riga (Baltic Sea)

Overwintering population of Cercopagis is represented by resting eggs which hatch generally in May. Although representatives of the first parthenogenetic generation (the spring form individuals) differ morphologically from individuals of the subsequent generations (the summer form individuals) and could be keyed to a separate species – Cercopagis (Apagis) ossiani – our analysis confirm that there is only one Cercopagis species in the Gulf of Riga: Cercopagis pengoi. Notable seasonal dynamics was observed for the gamogenetic mode of reproduction, being strongly associated with the total population density. Gamogenetic females develop since the end of June and reach the maximum absolute abundance in July whereas their proportion is generally the highest in fall. The gamogenetic absolute fecundity drops to the lowest level late July but increased afterwards until the late August. One resting egg was, an average, found in 45.4%, two in 53.4% and three in 1.2% of females. Parthenogenetic fecundity was significantly higher in May–June compared to other months studied. Brood pouch of parthenogenetic females was found to contain 11.6 ± 1.0 and 10.2 ± 0.3 embryos in the spring and summer form individuals, respectively. Variation in the two modes of reproduction and fecundity is probably solely not controlled by temperature, but also by food availability and population density.     

Linking denitrifier community structure and prevalent biogeochemical parameters in the pelagial of the central Baltic Proper (Baltic Sea)

The oxic-anoxic interface of the water column of the Gotland Basin (central Baltic Sea) is characterised by defined biogeochemical gradients and is hypothesised to be a zone of pronounced denitrification. Our aim was to analyse the composition and distribution of pelagic denitrifying microorganisms in relation to the physico-chemical gradients in the water column. PCR-amplified nirS genes--coding for dissimilatory nitrite reductase--were analysed as functional markers by terminal restriction fragment length polymorphism and cloning. The overall nirS diversity was low, with the lowest levels found at the oxic-anoxic interface. Only a few terminal restriction fragments dominated the denitrifier communities throughout the water column, and these could be assigned to several new Baltic Sea clusters that were revealed by phylogenetic analysis. The novel clusters were separated in two groups corresponding to the oxygen concentrations within specific layers of the water column. Gradients of prevalent biogeochemical parameters (H(2)S, NH(4) (+), NO(3) (-) and O(2)) largely determined the composition of the nirS-type denitrifier communities within the water column of the Gotland Basin.     

Population dynamics and ecological impact of the non-indigenous Cercopagis pengoi in the Gulf of Riga (Baltic Sea)

Abundance of the exotic predatory cladoceran Cercopagis pengoi (Ostroumov, 1891) in the Gulf of Riga has increased during the first ten years of invasion (1992–2001) and peaked with an annual mean of 420 ± 109 (s.e.) ind. m^?3 in 2001. As a long-term mean, Cercopagis population reached its highest abundance in the first week of August (mean 269 ind. m^?3) and the seasonal population development followed the course of water temperature. Presence of Cercopagis in the plankton community tended to last longer over the years by shifting its population development to earlier in the year. The latter correlated significantly (r ²=0.88, p<0.01) with a similar pattern of Eurytemora. After the invasion of Cercopagis, the annual mean abundance of Bosmina was significantly lower than during the pre-invasion time (t-test, p<0.01). Although populations of the other mesozooplankton taxa did not follow this pattern at the annual scale, seasonal population development of copepod nauplii and Acartia has changed, which can be caused by the invasion. The mean share of Cercopagisin the diet of most abundant planktivorous fish remained low (<7%) but the cladoceran constituted a substantial portion in the fish diet during warm months. Consumption of Cercopagis by herring, Clupea harengus membras, and smelt, Osmerus eperlanus, increased with increasing fish size. This was not evident in the case of sticklebacks, Gasterosteus aculeatus and Pungitius pungitius. The study points out that the invasion of Cercopagis has caused changes in zooplankton dynamics. Therefore, dietary overlap with fish larvae and planktivorous fish may occur in the Gulf of Riga.     

Stratigraphy of Cladocera (Crustacea) and Chironomidae (Insecta: Diptera) in Three Sediment Cores from the Central Baltic Sea as Related to Paleo-salinity

In three sediment cores from the Central Baltic Sea (Bornholm Basin, Gdansk Bay) corresponding horizons with maximum abundance of remains of a limnetic fauna (Cladocera, Chironomidae) were found. These associations indicate the existence of an oligotrophic freshwater lake. However, this horizon does not coincide with the Ancylus stage but is located in sediment layers from the very end of that period, or from the Mastogloia stage, which was a transitional brackish water phase.