Bladderwrack Fucus Vesiculosus Research Articles

Effects of isolation on invertebrate biodiversity among the bladderwrack Fucus vesiculosus

Geographic isolation is a factor that is known to affect species composition and diversity in marine systems. In this study, we tested the effects of geographic isolation on invertebrate fauna living among the belt-forming foundation species Fucus vesiculosus in the northern Baltic Sea. Sampling was carried out on reefs with varying levels of isolation (0.5, 1 and 2 km) in July 2021 in the Åland islands (between 59.96°N 20.24°E and 59.75°N 20.84°E). The total number of taxa, abundance, Shannon-Wiener index, Pielou’s evenness and biomass were calculated and the effects of isolation were tested using generalised linear models. Multivariate methods were used to analyse differences in the invertebrate community composition. Although the number of taxa and Shannon´s diversity index did not vary along the isolation gradient, the total abundance of fauna decreased with increasing isolation. In addition, some taxa-specific patterns were observed, e.g. the abundance of insect larvae decreased with isolation while isopods were more numerous in the most isolated locations. As diversity and the number of taxa were similar regardless of isolation, shallow Fucus vesiculosus belts likely act as important biodiversity refuges and potential stepping-stones for invertebrates in remote outer archipelago areas. In a changing climate, these kinds of refuges aiding species dispersal are likely to be even more important, and should be considered in conservation planning.

Apparent return of free-living Fucus vesiculosus to the Polish Baltic waters

The underwater meadows of the Puck Bay, once thriving with eelgrass Zostera marina, bladderwrack Fucus vesiculosus and black carrageen Furcellaria lumbricalis, experienced a decline in water quality during the 1960s and 1970s due to untreated sewage pollution. This, together with commercial exploitation, led to the disappearance of bladderwrack in 1977, with unsuccessful attempts at reintroduction in the early 2000s. In December 2023, a SCUBA survey near Rzucewo revealed a numerous bladderwrack in a benthic free-living form after 46 years of absence. The algae were found between 1.7 and 2.7 m depth, loosely positioned on the seabed, often within Z. marina beds, and with blue mussels Mytilus edulis attached. This reappearance suggests a positive trend in seawater quality and overall state of the Puck Bay, especially when combined with recent recovery of other algae species. The apparent return of bladderwrack could enhance ecosystem functionality, benefiting fish recruitment, grazer and algal biomass. Further investigations on bladderwrack's reappearance are needed in order to verify whether this is the only ecotype currently present in the Puck Bay.

A Manipulative Field Experiment to Evaluate the Impact of Discarded Plastic Bags on Shallow Vegetated Benthic Communities

Coastal regions are biologically active areas with significant ecological and socioeconomic values. These regions are increasingly being affected by marine litter. The impact of macro-sized marine litter on biomass and net primary production of hard and soft bottom communities was investigated by using a manipulative field experiment. Plastic bags were used to mimic the disturbance caused by litter to benthic vegetation and fauna. The experiment was carried out on a soft substrate community dominated by sago pondweed Stuckenia pectinata (L.) Boerner and bladder wrack Fucus vesiculosus L. as a foundation species of a hard substrate. A rapid negative impact of the plastic bag cover on vegetation biomass of soft-bottom community was detected, while the impact on the biomass of hard bottom vegetation was non-significant. Plastic bag cover substantially reduced the net production rates of hard substrate species Fucus and the biomass of associated zoobenthos. The difference in net production rates of Stuckenia with and without plastic cover was negligible.

Cormorants have negligible seascape-scale impacts on benthic vegetation communities

Great cormorant Phalacrocorax carbo sinensis populations in the Baltic Sea have rapidly expanded since the 1990s, raising concerns about their ecosystem impacts. Nutrient runoff from colonies, as well as cormorant predation on fish, can affect surrounding producer communities. Past studies have found cormorant impacts on producers in the immediate vicinity of colonies, but the importance of cormorants over a larger spatial scale is unknown, especially compared to other environmental variables. We used an extensive underwater vegetation inventory dataset (~18000 data points along the Finnish coast) to determine the effects of cormorant colonies on macroalgae and plants. We compared community structure and species abundance/occurrence in near-colony (<5 km from a colony) and control (>10 km from a colony) points, and determined the importance of cormorant influence (using an index incorporating colony size and distance from the colony) in near-colony sites. We found no significant differences in community structure between near-colony and control points in most habitats, and adding the cormorant index only infinitesimally improved statistical models after incorporating other environmental factors. However, the abundance of several species did differ; in particular, the foundation species bladderwrack Fucus vesiculosus and eelgrass Zostera marina were either less likely to occur in near-colony points or were negatively correlated with cormorant index, possibly due to the effects of nutrient enrichment from colonies. Our findings confirm that cormorants can have effects on some producer species, but highlight that these effects are negligible when taking into account the scale and magnitude of other bottom-up and top-down processes occurring in the Baltic Sea.

It takes two to stay afloat: interplay of morphology and physiological acclimation ensures long-term floating dispersal of the bladderwrack Fucus vesiculosus (Phaeophyceae, Fucales)

ABSTRACTFloating algae can be found in high abundances at mid and high latitudes, their prolonged positive buoyancy allowing long-distance dispersal. However, despite their importance to dispersal and ecological and evolutionary meta-population dynamics, little is known about the buoyancy responses of high latitude algae to the conditions at the sea surface. Indeed, even at 60°N environmental conditions during spring/summer can be challenging, and may cause the demise and sinking of floating algae. The bladderwrack Fucus vesiculosus from the Northern Baltic Sea floats on the sea surface when detached from the benthic substratum. We conducted a field experiment with tethered individuals during their reproductive period to measure variation in floating time and how that is related to morphological traits such as occurrence of vesicles and/or receptacles, and to measure growth and photosynthesis while afloat. Algal individuals with receptacles tended to sink quickly, whereas the longest floating time was evident for individuals carrying vesicles but lacking receptacles. While afloat, all individuals grew in size, showed photosynthetic acclimation to sea surface conditions and had a few invertebrates associated with them. Our results showed that rafts of F. vesiculosus were physiologically viable until their day of sinking and that morphological traits such as the occurrence of vesicles and receptacles modified their floating time. Accordingly, floating algae with a similar morphological set-up, and thus also reproductive phenology, to F. vesiculosus can have a high floating persistence, but, depending on their reproductive structures, they may mostly serve as long-range dispersal vehicles for associated organisms.

Geographic variation in fitness-related traits of the bladderwrack Fucus vesiculosus along the Baltic Sea-North Sea salinity gradient.

In the course of the ongoing global intensification and diversification of human pressures, the study of variation patterns of biological traits along environmental gradients can provide relevant information on the performance of species under shifting conditions. The pronounced salinity gradient, co‐occurrence of multiple stressors, and accelerated rates of change make the Baltic Sea and its transition to North Sea a suitable region for this type of study. Focusing on the bladderwrack Fucus vesiculosus, one of the main foundation species on hard‐bottoms of the Baltic Sea, we analyzed the phenotypic variation among populations occurring along 2,000 km of coasts subjected to salinities from 4 to >30 and a variety of other stressors. Morphological and biochemical traits, including palatability for grazers, were recorded at 20 stations along the Baltic Sea and four stations in the North Sea. We evaluated in a common modeling framework the relative contribution of multiple environmental drivers to the observed trait patterns. Salinity was the main and, in some cases, the only environmental driver of the geographic trait variation in F. vesiculosus. The decrease in salinity from North Sea to Baltic Sea stations was accompanied by a decline in thallus size, photosynthetic pigments, and energy storage compounds, and affected the interaction of the alga with herbivores and epibiota. For some traits, drivers that vary locally such as wave exposure, light availability or nutrient enrichment were also important. The strong genetic population structure in this macroalgae might play a role in the generation and maintenance of phenotypic patterns across geographic scales. In light of our results, the desalination process projected for the Baltic Sea could have detrimental impacts on F. vesiculosus in areas close to its tolerance limit, affecting ecosystem functions such as habitat formation, primary production, and food supply.

Habitat-specific gut microbiota of the marine herbivore Idotea balthica (Isopoda)

We hypothesised that the generalist herbivore Idotea balthica may harbour intestinal symbiotic bacteria that aid in food utilisation by the host, and that the number and/or identities and thus the contribution of such symbionts may vary between the habitats. We investigated the gut microbiota and its contribution to host use ability of the I. balthica originating from two host assemblages, dominated by either the bladderwrack Fucus vesiculosus or the seagrass Zostera marina. In I. balthica, the host use abilities of populations were habitat-specific suggesting divergent evolution for digestion mechanism between different selective environments. We found bacteria both in the hindgut of I. balthica and in the digestive midgut glands, demonstrating for the first time the presence of bacterial midgut symbionts in a marine isopod. To study the effects of gut microbes on isopod performance, we reared isopods on two distinct diets, Fucus and Zostera, and manipulated their gut microbes with antibiotic treatment. The effect of antibiotic treatment on the bacterial counts depended on the isopod origin: antibiotics reduced bacterial counts in isopods originating from the Fucus assemblage, but had no effect on isopods originating from the Zostera assemblage, supporting the hypothesis that the bacterial communities in the gut differ between the habitats. On the Fucus diet, isopods with natural gut bacteria grew less well than those treated with antibiotics, whereas no such difference in growth was found on the much lower quality Zostera diet. Symbiotic bacteria, depending on the diet, may thus be harmful or indifferent to their isopod host.

Testing the potential for predictive modeling and mapping and extending its use as a tool for evaluating management scenarios and economic valuation in the Baltic Sea (PREHAB).

We evaluated performance of species distribution models for predictive mapping, and how models can be used to integrate human pressures into ecological and economic assessments. A selection of 77 biological variables (species, groups of species, and measures of biodiversity) across the Baltic Sea were modeled. Differences among methods, areas, predictor, and response variables were evaluated. Several methods successfully predicted abundance and occurrence of vegetation, invertebrates, fish, and functional aspects of biodiversity. Depth and substrate were among the most important predictors. Models incorporating water clarity were used to predict increasing cover of the brown alga bladderwrack Fucus vesiculosus and increasing reproduction area of perch Perca fluviatilis, but decreasing reproduction areas for pikeperch Sander lucioperca following successful implementation of the Baltic Sea Action Plan. Despite variability in estimated non-market benefits among countries, such changes were highly valued by citizens in the three Baltic countries investigated. We conclude that predictive models are powerful and useful tools for science-based management of the Baltic Sea.

Seaweed traits linked to wave exposure determine predator avoidance

Greater attention to habitat-specific species interactions should improve our ability to predict the context-dependency of species interactions. Predator avoidance depends upon resource traits that can vary predictably between habitats, but few studies have linked predator avoidance to habitat-specific resource traits. For example, predator avoidance in herbivores may depend upon wave exposure, because wave-exposed and wave-protected seaweeds display strong intraspecific differences in morphology and physiology that likely influence their value to herbivores. In 2 sets of laboratory mesocosm experiments we compared predator avoidance in the herbivorous snail Littorina obtusata offered the bladder wrack Fucus vesiculosus from either wave-exposed or protected shores. The effect of predation risk by the green crab Carcinus mae- nas on avoidance by L. obtusata depended on wave exposure, with a stronger effect of predation risk for snails on wave-exposed F. vesiculosus. To explore the traits responsible for this pattern, we conducted multi-choice feeding experiments with F. vesiculosus from 3 wave-exposed and 3 pro- tected sites. F. vesiculosus from protected sites (where avoidance behaviors were weak) was more palatable than F. vesiculosus from wave-exposed sites. Although several morphological (e.g. blade width and vesicle number) and physiological traits (e.g. C:N ratio) of F. vesiculosus depended upon wave exposure, these were unrelated to snail feeding and predator avoidance. Therefore, L. obtusata displayed stronger predator avoidance on less palatable, wave-exposed seaweed. We suggest that the different effects of predators on rocky shores may partly reflect the influence of exposure-specific seaweed traits that determine predator avoidance.

Evaluating eutrophication management scenarios in the Baltic Sea using species distribution modelling

Summary Eutrophication is severely affecting species distributions and ecosystem functioning in coastal areas. Targets for eutrophication reduction have been set in the Baltic Sea Action Plan (BSAP) using Secchi depth, a measure of water transparency, as the main status indicator. Despite the high economic costs involved, the potential effects of this political decision on key species and habitats have not been assessed. In a case study including species central to coastal ecosystem functioning, we modelled the effects of changing Secchi depth on the distribution of bladderwrack Fucus vesiculosus and eelgrass Zostera marina vegetation as well as recruitment areas of the main predatory fish species, perch Perca fluviatilis and pikeperch Sander lucioperca. Specifically, we explored the effects of changing Secchi depth on species distributions under a set of scenarios based on the BSAP, using three fundamentally different modelling techniques: maximum entropy, generalized additive and random forest modelling. Improved Secchi depth (reduced eutrophication) was predicted to cause a substantial increase in the distribution of bladderwrack, while the distribution of eelgrass remained largely unaffected. For the fish, a large increase in perch recruitment areas was predicted and a concurrent decrease in recruitment areas of pikeperch. These changes are likely to have effects on biodiversity and ecosystem services. The three modelling methods exposed differences in the quantitative predictions for species with a weaker coupling to Secchi depth. Qualitatively, however, the results were consistent for all species. Synthesis and applications. We show how ecological effects of environmental policies can be evaluated in an explicit spatial context using species distribution modelling. The model‐specific responses to changes in eutrophication status emphasize the importance of using ensemble modelling for exploring how species distributions may respond to alternative management regimes. A pronounced difference in response between species suggests that eutrophication mitigation will have consequences for ecosystem functioning, and thus ecosystem goods and services, by inducing changes in the simple food webs of the Baltic Sea. These model predictions form a basis for spatially explicit cost‐benefit estimates under different scenarios, providing valuable information for both decision‐makers and the wider society.

Prey selectivity by juvenile Atlantic cod Gadus morhua in three coastal habitat types

Coastal habitats are heavily subjected to eutrophication and commercial fisheries, and such alterations can affect organism interaction strengths and potentially influence trophic dynamics. A key species inhabiting coastal environments in temperate waters is the Atlantic cod Gadus morhua, which utilises coastal areas for food and shelter. We used an experimental mechanistic approach to assay prey selectivity by juvenile cod when foraging on grass shrimp Palaemon elegans and brown shrimp Crangon crangon, under light and dark conditions, in 3 of the most abundant habitat types in temperate coastal environments-sand, eelgrass (artificial vegetation mimicking Zostera marina), and bladderwrack Fucus vesiculosus. Using functional response relationships from single-prey experiments, we calculated the energetically best foraging strategy for cod in the 3 habitats, i.e. feeding selectively on either of the shrimp species or on a combination of both. These predictions were tested in experiments where the cod predator was offered both prey species. Cod selected both prey species in accordance with our predictions in eelgrass and in bladderwrack under light conditions, but a lower than predicted consumption of grass shrimp was found in sand and in bladderwrack under dark conditions. Cod decreasingly selected grass shrimp with increasing habitat complexity, i.e. the highest selectivity was in sand and the lowest selectivity was in bladderwrack. As the 2 shrimp species have different trophic roles, cod selective predation may have effects on lower trophic levels. We provide a quantitative prediction of cod selective predation in habitat types that undergo degradation, and suggest that such predation can influence the trophic consequences from environmental change. (Less)

Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 456:245-253 (2012) - DOI: https://doi.org/10.3354/meps09705 Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats Anders Persson*, Peter Ljungberg, Magnus Andersson, Elin Götzman, P. Anders Nilsson Department of Biology, Aquatic Ecology, Lund University, Ecology Building, 223 62 Lund, Sweden *Email: anders.persson@biol.lu.se ABSTRACT: Overfishing and eutrophication affects coastal communities worldwide, leading to dwindling fish stocks and deteriorated habitats. Hence, attempts to rebuild overfished stocks to past fish productivities need to account for functional relations between habitat types and fish performance. Here we quantified resource availability, foraging performance, and anti-predator behavior of juvenile Atlantic cod Gadus morhua (hereafter ‘cod’) to assess the costs and benefits associated with different coastal habitats. In the laboratory, cod foraged more efficiently in sand habitats compared to the structurally more complex habitats of eelgrass Zostera marina and the canopy forming bladderwrack Fucus vesiculosus. Presence of chemical cues from a cannibal reduced cod consumption rates in all habitats, but most strongly in the sand habitat. Field observations in the 3 habitats showed highest resource density in the bladderwrack habitat and lowest in the sand habitat, irrespective of season. Habitat profitability, calculated by combining data from field estimates of prey density and experimental quantifications of foraging performance, revealed the bladderwrack habitat to be the most profitable, independent of season. The difference in profitability between the complex habitats was relatively small, suggesting that cod in the field contributed to drive habitat profitability towards equalization. The results strengthen the view that the ongoing loss of seagrass and macroalgae habitats may have significant ramifications for juvenile cod performance, which ultimately may lower the productivity of entire stocks. Consequently, future and ongoing rebuilding of commercial fish stocks should revise the expectations of stock productivity (and hence harvesting intensity) accordingly. KEY WORDS: Cod · Habitat choice · Macroinvertebrates · Functional response · Seagrass · Bladderwrack · Fucus vesicolosus · Zostera marina Full text in pdf format PreviousNextCite this article as: Persson A, Ljungberg P, Andersson M, Götzman E, Nilsson PA (2012) Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats. Mar Ecol Prog Ser 456:245-253. https://doi.org/10.3354/meps09705 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Effects of limitation stress and of disruptive stress on induced antigrazing defense in the bladder wrack Fucus vesiculosus

We assessed the effects of light limitation and temperature shift on palatability and induced antiherbivore defense in the brown alga Fucus vesiculosus L. Incubation for 2 wk at light intensities above the compensation point of photosynthesis and in the absence of grazers increased the palatability of F. vesiculosus and its subsequent consumption by the omnivorous isopod Idotea baltica Pallas. This effect correlated with an increased C:N ratio and mannitol content in the algal tissue, presumably due to increased photosynthetic carbon fixation. Mannitol, the primary product of photosynthesis in F. vesiculosus, proved to be a feeding cue for I. baltica, and depletion of the mannitol pool may therefore account for the reduced palatability during light limitation. At light intensities above the compensation point of photosynthesis, F. vesiculosus responded with decreasing palatability when it was exposed to I. baltica grazing. Irrespective of the preceding light regime, such defense induction was prevented during incubation under light limitation. Thus, under low light, defense induction is not only inhibited, but also less necessary due to the relative absence of feeding cues. Upward or downward shifts in water temperature by approximately 10°C also inhibited inducible defense in F. vesiculosus. However, such shifts did not affect algal growth and were therefore the consequence of an impairment of specific defense-related components rather than of resource limitation, unless compensatory growth was given priority over defense.

Ecology of antifouling resistance in the bladder wrack Fucus vesiculosus: patterns of microfouling and antimicrobial protection

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 411:33-48 (2010) - DOI: https://doi.org/10.3354/meps08644 Ecology of antifouling resistance in the bladder wrack Fucus vesiculosus: patterns of microfouling and antimicrobial protection M. Wahl1,*, L. Shahnaz1,2, S. Dobretsov1,3, M. Saha1, F. Symanowski1, K. David1, T. Lachnit1, M. Vasel1, F. Weinberger1 1Leibniz Institute of Marine Sciences, IFM-GEOMAR, Duesternbrookerweg 20, 24105 Kiel, Germany 2Present address: Department of Botany, University of Karachi, Karachi, Pakistan 3Present address: Department of Marine Science and Fisheries, Sultan Qaboos University, Muscat, Oman *Email: mwahl@ifm-geomar.de ABSTRACT: The impact of moderate environmental stress may be modulated by stress-induced shifts of biotic interactions such as host–epibiont relationships. We studied the stress regime in shallow Western Baltic habitats, the variability of fouling at different temporal and spatial scales, and whether common stressors—low light, high temperature and grazing—affect the abundance and composition of the biofilm on a regionally important macroalga, the bladder wrack Fucus vesiculosus. We further explored the alga’s capacity to chemically modulate the recruitment of microfoulers and analyzed whether this ability is impacted by stress. In laboratory, mesocosm and field experiments, fouling pressure and epibiotic cover on the alga varied strongly with changing environmental conditions such as temperature, irradiance, depth and grazing. The expectation that abiotic stress affects the fouling-modulating ability of the alga and thus indirectly produces the observed variability of epibiosis was not generally confirmed. Indeed, while the strength of chemical antifouling resistance varied seasonally, with a maximum in winter–spring and a minimum in late summer, this could not be related to temporal patterns of environmental stress, fouling pressure, or growth of F. vesiculosus. Only the seasonal variation in reproduction seemed to be in phase with antifouling activity. Controlled experiments confirmed that resistance strength was not affected by temperature or grazing, and only moderately by light. We conclude that the fouling modulation ability of F. vesiculosus may suffer from light reduction (e.g. by eutrophication effects), but is not sensitive to predicted warming or enhanced grazing. KEY WORDS: Marine algae · Stress · Epibiosis · Fouling modulation · Interacting stressors · Fucus vesiculosus Full text in pdf format Supplementary material PreviousNextCite this article as: Wahl M, Shahnaz L, Dobretsov S, Saha M and others (2010) Ecology of antifouling resistance in the bladder wrack Fucus vesiculosus: patterns of microfouling and antimicrobial protection. Mar Ecol Prog Ser 411:33-48. https://doi.org/10.3354/meps08644 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 411. Online publication date: July 29, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

GENETIC STRUCTURE IN POPULATIONS OF FUCUS VESICULOSUS (PHAEOPHYCEAE) OVER SPATIAL SCALES FROM 10 M TO 800 KM1

Recent studies showing consequences of species’ genetic diversity on ecosystem performance raise the concern of how key ecosystem species are genetically structured. The bladder wrack Fucus vesiculosus L. is a dominant species of macroalga in the northern Atlantic, and it is particularly important as a habitat‐forming species in the Baltic Sea. We examined the genetic structure of populations of F. vesiculosus with a hierarchical approach from a within‐shore scale (10 m) to a between‐seas scale (Baltic Sea–Skagerrak, 800 km). Analysis of five microsatellite loci showed that population differentiation was generally strong (average FST = 12%), being significant at all spatial scales investigated (101, 103, 104–5, 106 m). Genetic differentiation between seas (Baltic Sea and Skagerrak) was substantial. Nevertheless, the effects of isolation by distance were stronger within seas than between seas. Notably, Baltic summer‐reproducing populations showed a strong within‐sea, between‐area (70 km) genetic structure, while Baltic autumn‐reproducing populations and Skagerrak summer‐reproducing populations revealed most genetic diversity between samples within areas (<1 km). Despite such differences in overall structure, Baltic populations of summer‐ and autumn‐reproducing morphs did not separate in a cluster analysis, indicating minor, if any, barriers to gene flow between them. Our results have important implications for management and conservation of F. vesiculosus, and we raise a number of concerns about how genetic variability should be preserved within this species.

Geographical divergence in host use ability of a marine herbivore in alga–grazer interaction

When the selective environment differs geographically, local herbivore populations may diverge in their host use ability and adapt locally to exploit the sympatric host population. We tested whether populations of the marine generalist herbivore Idotea baltica have diverged in host us ability and whether they locally adapted to exploit the sympatric population of their main host species, the bladderwrack Fucus vesiculosus. We fed isopods from three local populations reciprocally with the sympatric and two allopatric populations of the host. The bladderwrack populations varied in their quality as food for isopods suggesting variation in the selective environment. The ability to exploit the main host showed considerable divergence among the isopod populations. There was no significant interaction between host and isopod origin, indicating that the patterns observed in the reciprocal feeding experiment could be explained by differences in overall suitability of the hosts and differences in overall performance of the isopod populations. Isopod population that was sympatric to a bladderwrack population with low phlorotannin content showed high performance on the algae from the sympatric but low performance on the algae from the two allopatric populations. Performance of isopods, especially in this population, decreased quickly with the increasing phlorotannin content of food algae. We therefore hypothesize that the isopods adapted to a low phlorotannin content were unable to utilize high-phlorotannin algae efficiently. Isopod populations sympatric to the high-phlorotannin bladderwrack populations may be generally better adapted to deal with phlorotannins, being thereby able to utilize a range of bladderwrack populations.

Decreased depth penetration of Fucus vesicolosus (L.) since the 1940's indicates eutrophication of the Baltic Sea

Although nutrient inputs to the Baltic Sea have increased drastically since the end of the last century, there is still little hard biological evidence of a general eutrophication of the Baltic Sea outside locally polluted areas. A revisit after 40 yr to some well-documented diving stations in the outer archipelago of the h a n d Sea gave us an opportunity to register any changes in benthic vegetation that could be linked to eutrophication, By mapping the vertical distribut~on in coverage of bladder wrack Fucus vesiculosus (L.) at 11 stations we observed that the lower limit of this alga had moved upwards at 10 stations from maximally 11.5 m in 1943/44 to 8.5 m in 1984. Also the depth of maximum development had withdrawn from 5 to 6 m in 1943/44 to 3 to 4 m in 1984, while coverage a t these depths was about the same: 58 % and 51 % respectively. The deepest specimens today at 8.5 m had the same dwarfed appearance as those found at 11.5 m in the 1940's; at that time growth at 8 .5 m was luxuriant During both studies the decrease in F. vesiculosus coverage with depth towards the lower limit could be approximately fitted to an exponentially decreasing light attenuation curve. Since the structure of Baltic hardbottom communities is almost totally governed by abiotic factors, the changes in depth penetration are probably caused by decreased transparency of the water column due to eutrophication. Results indicate that the decreased water transparency arises from a 40 to 50 % increase in summer values of chlorophyll a and nutrients in the offshore surface water of the Baltic Sea since the 1940's.

Plutonium and americium in seaweed from the channel islands

Samples of the seaweed bladder wrack Fucus vesiculosus collected from the Channel Islands and North Cornwall have been examined for alpha radioactivity. Using a combination of chemical separation techniques and high resolution low background alpha spectrometry, the levels of activity of the most significant naturally-occurring nuclides were determined in all the samples. The Channel Islands seaweed also contained measurable quantities of the artificial nuclides 238Pu, 239/240Pu and 241Am, ranging from 0·1 to 20 pCikg −1 (dry weight) of each nuclide. The activity of these transuranic nuclides was about 3% of the total alpha activity due to natural sources. This appears to confirm earlier conclusions that alpha-emitting nuclides discharged from the nuclear fuel reprocessing plant at Cap de la Hague, France, are now appearing at low but significant amounts in the marine ecosystem of the Channel Islands.

An Outdoor Model Simulating a Baltic Sea Littoral Ecosystem

Plastic pools (surface6.6m2, volume4.2 M3) were equippedwith aflow-through system providing 2.5 1 min-1. Except for fish predators the main components of the flora and fauna of the Baltic littoral zone were introduced into the pools to form a model of the ecosystem. During 8 weeks the macroscopic epifauna and infauna of the bladder wrack Fucus vesiculosus L. were found to be qualitatively and quantitatively fairly stable, and the number of aerobic heterotrophic bacteria showed little variation. Oxygen concentration, temperature and pH were recorded and compared with values measured in the littoral zone. The results indicate good agreement between the characters of the model system and of the natural littoral ecosystem. This together with the observed stability and the possibilities forcontrollingand measuringthe conditions inthe system makes us believe that the model is a valuable tool for assessing toxic effects on the littoral ecosystem.