Northern Baltic Sea Research Articles

Impacts of flocculation on the distribution and diagenesis of iron in boreal estuarine sediments

Abstract. Iron (Fe) plays a key role in sedimentary diagenetic processes in coastal systems, participating in various redox reactions and influencing the burial of organic carbon. Large amounts of Fe enter the marine environment from boreal river catchments associated with dissolved organic matter (DOM) and as colloidal Fe oxyhydroxides, principally ferrihydrite. However, the fate of this Fe pool in estuarine sediments has not been extensively studied. Here we show that flocculation processes along a salinity gradient in an estuary of the northern Baltic Sea efficiently transfer Fe and OM from the dissolved phase into particulate material that accumulates in the sediments. Flocculation of Fe and OM is partially decoupled. This is likely due to the presence of discrete colloidal ferrihydrite in the freshwater Fe pool, which responds differently from DOM to estuarine mixing. Further decoupling of Fe from OM occurs during sedimentation. While we observe a clear decline with distance offshore in the proportion of terrestrial material in the sedimentary particulate organic matter (POM) pool, the distribution of flocculated Fe in sediments is modulated by focusing effects. Labile Fe phases are most abundant at a deep site in the inner basin of the estuary, consistent with input from flocculation and subsequent focusing. The majority of the labile Fe pool is present as Fe (II), including both acid-volatile sulfur (AVS)-bound Fe and unsulfidized phases. The ubiquitous presence of unsulfidized Fe (II) throughout the sediment column suggests Fe (II)-OM complexes derived from reduction of flocculated Fe (III)-OM, while other Fe (II) phases are likely derived from the reduction of flocculated ferrihydrite. Depth-integrated rates of Fe (II) accumulation (AVS-Fe + unsulfidized Fe (II) + pyrite) for the period 1970–2015 are greater in the inner basin of the estuary with respect to a site further offshore, confirming higher rates of Fe reduction in near-shore areas. Mössbauer 57Fe spectroscopy shows that refractory Fe is composed largely of superparamagnetic Fe (III), high-spin Fe (II) in silicates, and, at one station, also oxide minerals derived from past industrial activities. Our results highlight that the cycling of Fe in boreal estuarine environments is complex, and that the partial decoupling of Fe from OM during flocculation and sedimentation is key to understanding the role of Fe in sedimentary diagenetic processes in coastal areas.

Allochthonous Organic Matter Supports Benthic but Not Pelagic Food Webs in Shallow Coastal Ecosystems

Rivers transport large amounts of allochthonous organic matter (OM) to the ocean every year, but there are still fundamental gaps in how allochthonous OM is processed in the marine environment. Here, we estimated the relative contribution of allochthonous OM (allochthony) to the biomass of benthic and pelagic consumers in a shallow coastal ecosystem in the northern Baltic Sea. We used deuterium as a tracer of allochthony and assessed both temporal variation (monthly from May to August) and spatial variation (within and outside river plume). We found variability in allochthony in space and time and across species, with overall higher values for zoobenthos (26.2 ± 20.9%) than for zooplankton (0.8 ± 0.3%). Zooplankton allochthony was highest in May and very low during the other months, likely as a result of high inputs of allochthonous OM during the spring flood that fueled the pelagic food chain for a short period. In contrast, zoobenthos allochthony was only lower in June and remained high during the other months. Allochthony of zoobenthos was generally higher close to the river mouth than outside of the river plume, whereas it did not vary spatially for zooplankton. Last, zoobenthos allochthony was higher in deeper than in shallower areas, indicating that allochthonous OM might be more important when autochthonous resources are limited. Our results suggest that climate change predictions of increasing inputs of allochthonous OM to coastal ecosystems may affect basal energy sources supporting coastal food webs.

Patterns of vertical cyst distribution and survival in 100-year-old sediment archives of three spring dinoflagellate species from the Northern Baltic Sea

The history of expansion of bloom-forming cold water dinoflagellates in the Northern Baltic Sea was studied using 100-year-old sediment archives of their resting cysts. Vertical cyst distributions of Biecheleria baltica and Apocalathium malmogiense, two dinoflagellates indistinguishable by light microscopy and not recognized as distinct species in monitoring, and chain-forming Peridiniella catenata were analysed in Pb210 and Cs137 dated layers of a sediment core from deep, hypoxic accumulation bottoms of the Gulf of Finland. Cyst profiles showed that B. baltica and A. malmogiense were already present in the Baltic spring phytoplankton community at the beginning of the 20th century. This confirms that B. baltica, which was only recognized in the late 1980s, is a native species in the area. A drastic increase in B. baltica cyst concentrations in the 1930s to 1960s coincided with the acceleration of anthropogenic eutrophication. Large cyst deposits accumulated over several decades in the sediment which, by the 1980s, amounted to the seed stock necessary to inoculate dominant blooms. In the cyst records A. malmogiense always contributed a minor fraction of the two species. P. catenata had a relatively short cyst record in Gulf of Finland sediments despite demonstrated long-term presence in the plankton, which emphasizes that cyst-based historic surveys are not suitable for all cyst-forming dinoflagellates. This was corroborated by correspondence analyses of long-term plankton and cyst records which validated the trends from the sediment archive for B. baltica and A. malmogiense, but failed to do so for P. catenata. Germination experiments with 100-year-old cysts revealed a remarkable long-term survival capacity of A. malmogiense, making this species a suitable model for resurrection studies testing adaptation in heavily impacted systems such as the Baltic Sea.

Non‐Redfieldian Dynamics Explain Seasonal pCO2Drawdown in the Gulf of Bothnia

AbstractHigh inputs of nutrients and organic matter make coastal seas places of intense air‐sea CO2exchange. Due to their complexity, the role of coastal seas in the global air‐sea CO2exchange is, however, still uncertain. Here, we investigate the role of phytoplankton stoichiometric flexibility and extracellular DOC production for the seasonal nutrient and CO2partial pressure (pCO2) dynamics in the Gulf of Bothnia, Northern Baltic Sea. A 3‐D ocean biogeochemical‐physical model with variable phytoplankton stoichiometry is for the first time implemented in the area and validated against observations. By simulating non‐Redfieldian internal phytoplankton stoichiometry, and a relatively large production of extracellular dissolved organic carbon (DOC), the model adequately reproduces observed seasonal cycles in macronutrients and pCO2. The uptake of atmospheric CO2is underestimated by 50% if instead using the Redfield ratio to determine the carbon assimilation, as in other Baltic Sea models currently in use. The model further suggests, based on the observed drawdown of pCO2, that observational estimates of organic carbon production in the Gulf of Bothnia, derived with themethod, may be heavily underestimated. We conclude that stoichiometric variability and uncoupling of carbon and nutrient assimilation have to be considered in order to better understand the carbon cycle in coastal seas.

Reliability of Local Scale Human Pressure Modeling at the Seafloor of the Baltic Sea

ABSTRACTEfforts to understand anthropogenic stress in marine environments have introduced methods of cumulative human pressure modeling in the broad-scale mapping of large sea areas. This paper examines the usability of such modeling in the shallow seafloor and complex shoreline conditions of the Archipelago Sea in the northern Baltic Sea. We employed public spatial data sources to describe the spatial patterns of 24 different human pressures with their normalized values in a 20 m × 20 m pixel size. Public data were also used to classify the seafloor into six different environmental types. The model output was assessed against environmental data from regular seafloor monitoring. Visual examination shows general agreement between the cumulative human pressure model and the health of the benthic fauna, seabed oxygen and, to a degree, also the tributyltin. However, although none of these field parameters show high statistical correlation with the cumulative pressure status, multiple parameters assessed simultaneously could provide a sufficient reference. Local-scale disagreements between the cumulative model and the field parameters particularly occur near the largest harbor in the region where regular dredging is practiced. Model sensitivity to different input variables was further tested by comparing 36 dissimilar variants. The comparisons reveal a general stability across the inclusion, exclusion, or modification of such spatially restricted human pressure data, which only induced local-scale details. High instabilities occurred when the tested input data had a large spatial coverage. As the cumulative human impact model reduces several human pressures into one index, it should be considered as a broad scale descriptor of the likely overall health status of the sea. When implemented with accurate local-scale data it can help coastal and marine spatial planning but it should not be considered as a predictive depiction of any particular environmental characteristic. The cumulative pressure map is particularly powerful in providing unforeseen insights into the distributions of overall anthropogenic influences within the study region, and through these it can also contribute to environmental policies.

Effective and easy to use extraction method shows low numbers of microplastics in offshore planktivorous fish from the northern Baltic Sea

Although the presence of microplastics in marine biota has been widely recorded, extraction methods, method validation and approaches to monitoring are not standardized. In this study a method for microplastic extraction from fish guts based on a chemical alkaline digestion is presented. The average particle retrieval rate from spiked fish guts, used for method validation, was 84%. The weight and shape of the test particles (PET, PC, HD-PE) were also analysed with no noticeable changes in any particle shapes and only minor weight change in PET (2.63%). Microplastics were found in 1.8% of herrings (n=164) and in 0.9% of sprat (n=154). None of the three-spined sticklebacks (n=355) contained microplastic particles.

Chemical and microbiological evaluation of novel chemical treatment methods for acid sulfate soils

Naturally occurring sulfide rich deposits are common along the northern Baltic Sea coast that when exposed to air, release large amounts of acid and metals into receiving water bodies. This causes severe environmental implications for agriculture, forestry, and building of infrastructure. In this study, we investigated the efficiency of ultrafine-grained calcium carbonate and peat (both separately and in combination) to mitigate acid and metal release. The experiments were carried out aerobically that mimicked summer conditions when the groundwater level is low and acid sulfate soils are exposed to oxygen, and anaerobically that is similar to autumn to spring conditions. The ultrafine-grained calcium carbonate dissipated well in the soil and its effect alone and when mixed with peat raised the pH and reduced pyrite dissolution while peat alone was similar to the controls and did not halt metal and acid release. High throughput 16S rRNA gene sequencing identified populations most similar to characterized acidophiles in the control and peat treated incubations while the acidophilic like populations were altered in the calcium carbonate alone and calcium carbonate plus peat treated acid sulfate soils. Coupled with the geochemistry data, it was suggested that the acidophiles were inactivated by the high pH in the presence of calcium carbonate but catalyzed pyrite dissolution in the controls and peat incubations. In conclusion, the anaerobic conditions during winter would likely be sufficient to mitigate acid production and metal release from acid sulfate soils and in the summer, treatment with calcium carbonate was the best mitigation method.

The environmental niche separation between charophytes and angiosperms in the northern Baltic Sea

Charophytes and angiosperms are the most important primary producers and habitat modifiers in shallow soft-bottom fresh and brackish water bodies. The sheltered bays of the low-salinity northern Baltic Sea provide unique habitats, where euryhaline macrophytes of both freshwater and marine origin can thrive. The regional cohabitation of several species, with generally similar biological traits, raises questions about the mechanisms underlying the coexistence. Generally, sympatric populations rely on niche separation to reduce competition pressure that, in turn, facilitates long-term coexistence. A large benthos biomass data-set, covering the whole extent of the Estonian sea area, the northern Baltic Sea, together with a set of 13 environmental variables (bathymetrical, hydrodynamical, chemical, biological) was used to test the potential niche separation between regionally coexisting charophytes and angiosperms. A constrained correspondence analysis indicated both the grouping of some species and a distinct niche separation of others. The niche centers of angiosperms Zostera marina, Zannichellia palustris, and Potamogeton perfoliatus diverged most strongly from all the other species, while those of Chara connivens and Myriophyllum spicatum nearly coincided. Depth, salinity, and duration of ice were the most influential environmental gradients that discerned the environmental niches of the species. Comparison of the breadths of the environmental niche spaces, occupied by the studied species, showed highly species-specific results. Within the studied species, Z. palustris and Z. marina had the lowest niche specialization, and C. connivens had the highest specialization. Altogether, among the angiosperms the variability of the environmental niche breadth was higher compared to the charophytes.

Short sea migration and precocious maturation in reared Atlantic salmon post-smolts in the northern Baltic Sea

Abstract Atlantic salmon (Salmo salar L.) display significant variation in life history traits, including migration patterns and age at maturity. Hatchery rearing has been shown to affect the life history, and rearing-induced changes may include unfavourable consequences, e.g. shortened sea migration period and smaller size at maturity. We report on a new phenomenon of life history of reared Atlantic salmon in the Baltic Sea area: small-sized salmon returning to freshwater only a few months after release as smolts. These “one-sea-summer (1SS)” salmon were ca. 35 cm in length and weighed ca. 400 g, being clearly larger than smolts, but substantially smaller than one-sea-winter (1SW) salmon from the same cohorts. Almost all 1SS salmon were mature males and, at release, had been longer than the overall mean. Stable isotope analysis suggested that the 1SS salmon had been feeding in different sea areas than 1SW and multi-sea-winter salmon, likely in nearby Bothnian Bay, which is typically not a salmon feeding area. If an increasing proportion of the released salmon are not undertaking a normal marine migration (≥1SW) and are returning to estuaries and rivers as 1SS fish, the success and profitability of the reared salmon releases will decline even more than the reduced post-smolt survival is suggesting. We suggest that alternative rearing practices (e.g. enriched rearing environments and advanced diets) should be considered in hatchery production for shaping the reared smolts towards a closer resemblance to wild smolts.

A model for oil spill scenarios from tanker collision accidents in the Northern Baltic Sea

A model for oil spill scenarios from tanker collision accidents in the Northern Baltic Sea

The impact of an invasive mud crab on brood success of nest-building fish in the Northern Baltic Sea

Native fauna in species poor communities, such as those of the Baltic Sea, may be particularly vulnerable to the effects of species invasions. However, the interspecific interactions that result in the negative impacts on native species tend to be poorly understood. One contributing factor to this knowledge gap may be that the vulnerability of native species can vary between different life-history stages. For example, the parental care phase is often risky both for the developing offspring and care-giving parents. Accordingly, we investigated the interactions between invasive mud crabs, Rhithropanopeus harrissii, and native nest-building littoral fish, with a special focus on the sand goby, Pomatoschistus minutus. Firstly, our field survey demonstrated that while the recently established mud crab and native nest-building fish have different habitat and depth optima, their distributions also have a considerable overlap, with a high potential for interactions between them. Secondly, our laboratory experiments indicate that the crabs are likely to impact the fish negatively, especially by taking over occupied nests, as well as pre-occupying nesting resources that are of very limited supply in the studied population. We did not find evidence for successful predation on eggs guarded by male sand gobies naive to the mud crabs. Collectively, the results suggest that the invasive crabs have high potential to negatively impact native fish. Furthermore, the results support the scenario that the parental phase can be a particularly vulnerable life-history stage in face of novel selection pressures, such as species invasions.

Seasonal population dynamics of the invasive polychaete genus Marenzelleria spp. in contrasting soft-sediment habitats

Three species of the invasive polychaete genus Marenzelleria are among the dominant benthic taxa in many, especially deeper, areas in the Baltic Sea. The population dynamics of the polychaetes in the Baltic are, however, still largely unknown. We conducted monthly samplings of the benthic communities and environmental parameters at five sites with differing depths and sediment characteristics in the northern Baltic Sea (59°50.896′, 23°15.092′) to study the population dynamics, productivity and growth of Marenzelleria spp. from April 2013 to June 2014. The species of Marenzelleria occurring at the study sites were identified by genetic analyses. At the deepest site (33m) only M. arctia was present, while all three species were found at the shallower, muddy sites (up to 20m depth). At the shallow (6m) sandy site only M. viridis and M. neglecta occurred. The sites differed in the seasonal dynamics of the Marenzelleria spp. population, reflecting the different species identities. The muddy sites up to 20m depth showed clear seasonal dynamics, with the population practically disappearing by winter, whereas more stable populations occurred at the deepest site and at the sandy site. The highest density, biomass and production were observed at the 20m deep, organic-rich muddy site where all three species recruited. The seasonally very high densities are likely to have important consequences for organic matter processing, and species interactions at these sites. The observed high productivity of the populations has possibly facilitated their establishment, and considerably increased secondary production in especially the deeper areas.

Mortality of Baltic sea trout (Salmo trutta) after release from gillnets

We explore the mortality rate of disentangled sea trout in whitefish fishery using gillnets with a 35–43 mm bar length. The study was conducted during the main fishing seasons in the Gulf of Bothnia in the northern Baltic Sea. Overall 59.5% of the sea trout were alive at the end of a 2–7 day observation period following release from the gillnets. Altogether, 12.1% of the captured fish were found dead in gillnets and 28.4% died due to injuries during an extended observation period. The average length of the captured sea trout was 435 mm, indicating that the majority were spending their first or second year at sea. The proportion of the survived and not injured (no observed damage) fish was highest in larger fish, >450 mm. The injured and not injured fish died equally frequently. Post capture survival was not connected to the removal time from gillnets or type of observed injuries but to the type of entanglement. Most of the fish were entangled by a mesh around the body, which caused extensive scale loss and open sores on the skin. The smallest fish may have had internal wounds that were not registered in this study. These results can be used in fisheries management to estimate the mortality of multi‐species gillnet fishing to sea trout populations in relation to management actions.

Atmospheric Transport and Deposition of Bromoanisoles Along a Temperate to Arctic Gradient

Bromoanisoles (BAs) arise from O-methylation of bromophenols, produced by marine algae and invertebrates. BAs undergo sea-air exchange and are transported over the oceans. Here we report 2,4-DiBA and 2,4,6-TriBA in air and deposition on the Swedish west coast (Råö) and the interior of arctic Finland (Pallas). Results are discussed in perspective with previous measurements in the northern Baltic region in 2011-2013. BAs in air decreased from south to north in the order Råö > northern Baltic > Pallas. Geometric mean concentrations at Pallas increased significantly (p < 0.05) between 2002 and 2015 for 2,4-DiBA but not for 2,4,6-TriBA. The logarithm of BA partial pressures correlated significantly to reciprocal air temperature at the coastal station Råö and over the Baltic, but only weakly (2,4-DiBA) or not significantly (2,4,6-TriBA) at inland Pallas. Deposition fluxes of BAs were similar at both sites despite lower air concentrations at Pallas, due to greater precipitation scavenging at lower temperatures. Proportions of the two BAs in air and deposition were related to Henry's law partitioning and source regions. Precipitation concentrations were 10-40% of those in surface water of Bothnian Bay, northern Baltic Sea. BAs deposited in the bay catchment likely enter rivers and provide an unexpected source to northern estuaries. BAs may be precursors to higher molecular weight compounds identified by others in Swedish inland lakes.

Distribution patterns of epilithic diatoms along climatic, spatial and physicochemical variables in the Baltic Sea

The species richness and community composition of the diatom communities were studied in the Baltic Sea, Northern Europe, to enhance knowledge about the diversity of these organisms in a brackish water ecosystem. Many organisms in the Baltic Sea have been studied extensively, but studies investigating littoral diatoms are scarce. The goal of this study was to examine the importance of climatic, spatial and water physicochemical variables as drivers of epilithic diatoms in the Gulf of Finland and the Gulf of Bothnia. The variation in species richness was best explained by pH, total phosphorus and total nitrogen. Redundancy Analysis indicated that the most important factors correlating with species composition were air temperature, silicon, total phosphorus, water temperature, salinity and pH. Variation Partitioning showed that the species composition was mostly affected by climatic and spatial variables, whereas physicochemical variables had little impact. However, the strongest factor was the combined influence of climatic, spatial and physicochemical variables. The results suggest that diatom species richness in the northern Baltic Sea is primarily regulated by local factors, while climatic and spatial variables have little impact on richness. Species composition is mostly affected by climatic and spatial variables. We conclude that understanding the distribution patterns of Baltic Sea diatoms requires the inclusion of climatic, spatial and water chemistry variables.

Environmental niche separation between native and non-native benthic invertebrate species: Case study of the northern Baltic Sea

Knowledge and understanding of geographic distributions of species is crucial for many aspects in ecology, conservation, policy making and management. In order to reach such an understanding, it is important to know abiotic variables that impact and drive distributions of native and non-native species. We used an existing long-term macrobenthos database for species presence-absence information and biomass estimates at different environmental gradients in the northern Baltic Sea. Region specific abiotic variables (e.g. salinity, depth) were derived from previously constructed bathymetric and hydrodynamic models. Multidimensional ordination techniques were then applied to investigate potential niche space separation between all native and non-native invertebrates in the northern Baltic Sea. Such an approach allowed to obtain data rich and robust estimates of the current native and non-native species distributions and outline important abiotic parameters influencing the observed pattern. The results showed clear niche space separation between native and non-native species. Non-native species were situated in an environmental space characterized by reduced salinity, high temperatures, high proportion of soft seabed and decreased depth and wave exposure whereas native species displayed an opposite pattern. Different placement of native and non-native species along the studied environmental niche space is likely to be explained by the differences in their evolutionary history, human mediated activities and geological youth of the Baltic Sea. The results of this study can provide early warnings and effectively outline coastal areas in the northern Baltic Sea that are prone to further range expansion of non-native species as climate change is expected to significantly reduce salinity and increase temperature in wide coastal areas, both supporting the disappearance of native and appearance of non-native species.

Holocene environmental history of the Ångermanälven Estuary, northern Baltic Sea

The Baltic Sea has experienced a complex geological history, with notable swings in salinity driven by changes to its connection with the Atlantic and glacio‐isostatic rebound. Sediments obtained during International Ocean Drilling Program Expedition 347 allow the study of the effects of these changes on the ecology of the Baltic in high resolution through the Holocene in areas where continuous records had not always been available. Sites M0061 and M0062, drilled in the Ångermanälven Estuary (northern Baltic Sea), contain records of Holocene‐aged sediments and microfossils. Here we present detailed records of palaeoecological and palaeoenvironmental changes to the Ångermanälven Estuary inferred from diatom, palynomorph and organic‐geochemical data. Based on diatom assemblages, the record is divided into four zones that comprise the Ancylus Lake, Littorina Sea, Post‐Littorina Sea and Recent Baltic Sea stages. The Ancylus Lake phase is initially characterized as oligotrophic, with the majority of primary productivity in the upper water column. This transition to a eutrophic state continues into the Initial Littorina Sea stage. The Initial Littorina Sea stage contains the most marine phase recorded here, as well as low surface water temperatures. These conditions end before the Littorina Sea stage, which is marked by a return to oligotrophic conditions and warmer waters of the Holocene Thermal Maximum. Glacio‐isostatic rebound leads to a shallowing of the water column, allowing for increased benthic primary productivity and stratification of the water column. The Medieval Climate Anomaly is also identified within Post‐Littorina Sea sediments. Modern Baltic sediments and evidence of human‐induced eutrophication are seen. Human influence upon the Baltic Sea begins c. 1700 cal. a BP and becomes more intense c. 215 cal. a BP.

Distribution of dissolved and suspended particulate molybdenum, vanadium, and tungsten in the Baltic Sea

Abstract In natural waters, dissolved oxyanions often dominate over the particle-bound element fraction. Still, the scavenging of oxyanions by suspended particles might contribute significantly to their dynamic cycling and distribution. To investigate how oxyanions are affected by manganese (Mn) redox cycling, detailed depth profiles across the pelagic redox zone at the Landsort Deep, Baltic Sea, were collected for molybdenum (Mo), vanadium (V), and tungsten (W), for both dissolved ( 0.22 μm) fractions. All three oxyanions show a non-conservative behavior in the stratified Landsort Deep. Strong linear correlations with Mn in the particulate fraction in the redox zone of the Landsort Deep suggest that Mn redox cycling influences their distribution. In the dissolved fraction, Mo, V, and W exhibited rather different behavior. Molybdenum was depleted below the redox zone, while V was depleted only within the redox zone. Tungsten concentrations increased within the redox zone, being three times higher in the sulfidic zone than in the surface water. Unlike Mo, W shows no tendency for adsorption or co-precipitation under the prevailing weak sulfidic conditions in the deep water of the Landsort Deep and is, therefore, not exported to the underlying sediment. The Landsort Deep data were compared with data from the northern Baltic Sea (Bothnian Bay, Kalix River and Rane River estuaries), where particulate iron (Fe) occurs in high abundance. The particulate fractions of Mo, V, and W decreased during mixing in these estuaries. Vanadium showed the most drastic reduction, with a decrease in dissolved and particulate fractions, indicating that different processes influence the distribution of these oxyanions.

Epifaunal community composition in five macroalgal species – What are the consequences if some algal species are lost?

Anthropogenic disturbances such as eutrophication and climate change are affecting the distribution and coverage of macroalgae in coastal areas worldwide. How these changes will affect the littoral food webs is challenging to predict as we still lack basic knowledge of epifaunal communities in different macroalgal species. The aim of this study was therefore to compare the epifauna in five common macroalgal species in the northern Baltic Sea. Samples of macroalgae and the associated epifauna were collected in mesh bags at 2 m depth in July–August 2014. The epifaunal abundance data were analyzed with univariate and multivariate methods. The results revealed significant differences in the epifaunal composition among the studied macroalgal species. Ceramium tenuicorne hosted the significantly highest and Fucus vesiculosus the lowest abundance of epifauna per algal dry weight. When comparing the relative epifaunal abundances in percentage, we found that different epifaunal taxa representing different functional groups dominated Pylaiella littoralis (Chironomidae, deposit feeder), Cladophora rupestris (Gammarus spp., herbivorous/omnivorous) and Furcellaria lumbricalis (Mytilus trossulus, suspension feeder). However, most of the epifaunal taxa were found in all algal species studied. We conclude that the loss or decline of specific macroalgal species will affect the ecosystem functions and energy flows to the higher trophic levels, but that none of the studied algal species seems to be crucial for the existence of single taxa or functional group of epifauna.

Rapid fluctuations in the northern Baltic Sea H 2 S layer

Abstract Hydrogen sulfide (H2S) is linked to water quality deterioration in the Baltic Sea, with widespread seafloor hypoxia. We examined the vertical and temporal variability of in situ [H2S], oxygen concentration ([O2]), temperature (T) and pH at weekly, hourly and minute intervals at 13 locations in the western Gulf of Finland in 2013–2014. The main target was the 60–100 m water depth range, containing 3.2–290 μM O2 and 6.3–22.6 μM H2S. Where gas was detected by acoustic surveys, the structure of the H2S layer was more complex compared to stations devoid of gas. Local minima and maxima in pH frequently occurred near the H2S upper boundary (redox transition zone). Except for the homogeneous, tranquil zone above the seafloor at some stations, substantial rapid changes in hydrographic conditions were common. Typically, a layer of marked temporal T variability was present atop or within the topmost H2S layers. The largest temporal changes over a weekly period were − 0.44 °C/− 10.8 μM H2S/− 0.12 pH units (at seafloor level), + 0.18 °C/+7.9 μM H2S between casts (1 h) and + 0.03 °C/− 2.5 μM H2S per minute (high resolution logging). Abrupt [H2S] changes were recorded at two stations with sediments containing free gas. The T and [H2S] changes were synchronous at several layers, reflecting water movement. We conclude that rapid changes occur in hydrographic conditions in the near-bottom H2S layer in the northern Baltic Sea, especially at locations where free gas is present in the underlying sediments.