ALBERT

Beschreibung: During the past few decades, the green crab Carcinus maenas, a native to Europe, has invaded the North American Pacific coast. In this new habitat, C. maenas encounters North American periwinkles of the genus Littorina that differ from European Littorina spp. in size, shape and shell strength. We hypothesize that the ability to handle prey never encountered previously is a prerequisite for successful invasion of novel habitats. In a first approach to testing our hypothesis, we compared European (native) to Canadian (invaded) C. maenas in feeding trials with Littorina spp. from Europe as well as Canada. Canadian crabs had significantly larger crusher claws than European crabs of the same size. Prey handling by Baltic crabs, but not by North Sea crabs, significantly depended on shell morphometry and strength of European periwinkles. By contrast, neither European nor Canadian crabs were affected by shell characteristics of the relatively soft-shelled Canadian periwinkles. Baltic and Canadian crabs did not differ in terms of handling time for, and handling success of, different periwinkle species, but North Sea crabs needed more time for, and were less successful in, crushing periwinkles. We conclude that C. maenas exhibits plasticity in both claw morphometry and feeding behaviour that enables this predator to handle novel prey organisms, and contributes to its success as an invader.

Beschreibung: Climate change has the potential to profoundly influence the community structure and function of marine ecosystems. Prior to testing the consequences of altered environmental conditions on ecosystem functioning, it is first necessary to better understand how the functioning of an ecosystem is affected by its structure. Using phytoplankton communities with 4 naturally co‑occurring coccolithophores including species of Emiliania, Gephyrocapsa, and Calcidiscus collected off the Azores, we experimentally tested whether varying initial dominance leads to different competitive outcomes and consequently affects community functioning, such as biomass and carbon accumulation. We manipulated initial community structure by creating 5 different dominance scenarios: (1) all species contributing evenly to total initial biomass, and (2–5) one of each species contributing 4× that of the remaining 3 species to total initial biomass. All 4 species were simultaneously grown in monocultures starting with the same total initial biomass as the communities. Monocultures differed significantly in total final biomass, particulate inorganic carbon, and particulate organic carbon content. Priority effects in the communities caused the initially dominant species to remain dominant during the stationary phase in 3 out of 4 cases. However, despite varying dominant species and different outcomes in the monocultures, community functioning was unaffected. We suggest that selective and facilitative effects are responsible for the equalization of community functioning. We conclude that monoculture experiments are not sufficient to predict whole-community responses, since species interactions can significantly alter the expected functional outcome.

Beschreibung: Droop’s cell-quota model is the most successful description of phytoplankton growth in laboratory cultures and is increasingly being introduced into the ecosystem components of biogeochemical models. Although the Droop model’s parameters can be easily interpreted in biological terms, it was nevertheless derived empirically and lacks a sound mechanistic foundation. Here we derive Droop’s model from a simple optimality condition which maximises net growth rate. Our approach links the maximum cell quota to the cost of nutrient acquisition and suggests that respiration is influenced more strongly by C fixation than by N assimilation.

Beschreibung: The growth and development of the aragonitic CaCO3 otoliths of teleost fish could be vulnerable to processes resulting from ocean acidification. The potential effects of an increase in atmospheric CO2 on the calcification of the otoliths were investigated by rearing Atlantic cod Gadus morhua L. larvae in 3 pCO2 concentrations—control (370 µatm), medium (1800 µatm) and high (4200 µatm)—from March to May 2010. Increased otolith growth was observed in 7 to 46 d post hatch (dph) cod larvae at elevated pCO2 concentrations. The sagittae and lapilli were usually largest in the high pCO2 treatment followed by the medium and control treatments. The greatest difference in mean otolith surface area (normalized to fish length) was for sagittae at 11 dph, with medium and high treatments being 46 and 43% larger than the control group, respectively. There was no significant pCO2 effect on the shape of the otoliths nor were there any trends in the fluctuating asymmetry, defined as the difference between the right and left sides, in relation to the increase in otolith growth from elevated pCO2.

Beschreibung: Differences with respect to anti-herbivore defense were investigated in invasive and native populations of the seaweed Gracilaria vermiculophylla. Specimens from 6 native populations in East Asia and from 8 populations invasive in Europe and the Mexican Pacific coast were maintained under identical conditions and offered to herbivorous snails from both the native range (Littorina brevicula) and Europe (L. littorea) in no-choice feeding assays. L. brevicula consumed in total significantly larger amounts of G. vermiculophylla tissue than did L. littorea. Further, both snail species least consumed the seaweed specimens originating from either non-native populations or from populations native to the Korean East Sea/Sea of Japan. The Korean East Sea/Sea of Japan had previously been identified as putative donor region of all the invasive populations of G. vermiculophylla. Thus, populations in the donor region as well as non-native populations in different invaded realms feature an increased capacity to resist feeding pressure. Differences in nutrient content did not account for the observed patterns of consumption, as palatability and carbon to nitrogen (C:N) ratio were not significantly correlated. Thus, mechanical or chemical defenses or the content of feeding cues influenced the behavior of the snails. We suggest that low palatability contributed to the invasion success of the species.

Beschreibung: Repeated invasions of European waters by the ctenophore Mnemiopsis leidyi offer a unique opportunity to study population dynamics and dispersal in gelatinous zooplankton. Here we followed population establishment in two recently invaded areas, the North and Baltic Sea, and analysed changes in population structure during a 3-yr interval using 7 highly polymorphic microsatellites (representing 191 alleles). A second goal was to reconstruct routes of recent invasive range expansion into the Mediterranean Sea During the study period (2008-2010) populations in North Sea and Western Baltic Sea maintained their allelic composition with virtually unchanged levels of genetic diversity and between-population differentiation. This demonstrates that gene flow between the two regions was limited and indicates successful reproduction in both areas. In contrast, at the eastern distribution limit in the central Baltic (Bornholm Basin) the same measures fluctuated between years and genetic diversity decreased from 2008-2010. In concordance with prior ecological observations, this supports the view that here M. leidyi formed a sink population. In the area of recent range expansion (Mediterranean Sea) we observed high population differentiation for a holoplanktonic species. Among Mediterranean samples collected at sites in Spain, France and Israel pairwise differentiation was between Fst = 0.04-0.16. Despite such differentiation, Bayesian clustering and phylogeographic analysis support the hypothesis that all Mediterranean M. leidyi result from a secondary introduction originating in the Black Sea. Our study contributes to growing evidence that multiple invasions of the same species can vary in their degree of genetic diversity and demonstrates how genetic markers can help to resolve whether gelatinous plankton species form self-sustaining populations.

Beschreibung: Changing seawater chemistry towards reduced pH as a result of increasing atmospheric carbon dioxide (CO2) is affecting oceanic organisms, particularly calcifying species. Responses of non-calcifying consumers are highly variable and mainly mediated through indirect ocean acidification effects induced by changing the biochemical content of their prey, as shown within single species and simple 2-trophic level systems. However, it can be expected that indirect CO2 impacts observed at the single species level are compensated at the ecosystem level by species richness and complex trophic interactions. A dampening of CO2-effects can be further expected for coastal communities adapted to strong natural fluctuations in pCO2, typical for productive coastal habitats. Here we show that a plankton community of the Kiel Fjord was tolerant to CO2 partial pressure (pCO2) levels projected for the end of this century (〈1400 µatm), and only subtle differences were observed at the extremely high value of 4000 µatm. We found similar phyto- and microzooplankton biomass and copepod abundance and egg production across all CO2 treatment levels. Stoichiometric phytoplankton food quality was minimally different at the highest pCO2 treatment, but was far from being potentially limiting for copepods. These results are in contrast to studies that include only a single species, which observe strong indirect CO2 effects for herbivores and suggest limitations of biological responses at the level of organism to community. Although this coastal plankton community was highly tolerant to high fluctuations in pCO2, increase in hypoxia and CO2 uptake by the ocean can aggravate acidification and may lead to pH changes outside the range presently experienced by coastal organisms.

Beschreibung: The notion that excess phosphorus (P) and high irradiance favour pelagic diazotrophy is difficult to reconcile with diazotroph behaviour in laboratory experiments and also with the observed distribution of N2-fixing Trichodesmium, e.g. in the relatively nitrogen (N)-rich North Atlantic Ocean. Nevertheless, this view currently provides the state-of-the-art framework to understand both past dynamics and future evolution of the oceanic fixed N inventory. In an attempt to provide a consistent theoretical underpinning for marine autotrophic N2 fixation we derive controls of diazotrophy from an optimality-based model that accounts for phytoplankton growth and N2 fixation. Our approach differs from existing work in that conditions favourable for diazotrophy are not prescribed but emerge, indirectly, from trade-offs among energy and cellular resource requirements for the acquisition of P, N, and carbon. Our model reproduces laboratory data for a range of ordinary phytoplankton species and Trichodesmium. The model predicts that (1) the optimal strategy for facultative diazotrophy is switching between N2 fixation and using dissolved inorganic nitrogen (DIN) at a threshold DIN concentration; (2) oligotrophy, especially in P and under high light, favours diazotrophy; (3) diazotrophy is compatible with DIN:DIP supply ratios well above Redfield proportions; and (4) communities of diazotrophs competing with ordinary phytoplankton decouple emerging ambient and supply DIN:DIP ratios. Our model predictions appear in line with major observed patterns of diazotrophy in the ocean. The predicted importance of oligotrophy in P extends the present view of N2 fixation beyond a simple control by excess P in the surface ocean.

Beschreibung: Species identification based on morphological characteristics has caused misidentifications and led to twisted views of abundances and roles of ctenophores. Based on extensive field studies from 2007 to 2010, the occurrence of the arctic ctenophore Mertensia ovum was genetically verified in the southern, central and northern Baltic Sea, and its egg production, distribution and abundance were studied in relation to physical factors. Genetic analyses indicate that M. ovum is by far the most abundant small ctenophore in the Baltic Sea. Specimens from a 20 yr old ctenophore collection were also genetically identified as M. ovum, contrary to their previous morphological identification as another ctenophore species, Pleurobrachia pileus. Thus, earlier reports on P. pileus in the Baltic Sea may actually refer to M. ovum. The abundance of M. ovum was regulated by both salinity and temperature, with highest abundances found in sea areas and water layers at temperatures 〈7°C, salinities 〉5.5 and oxygen levels 〉4 ml l-1. During summer, the highest abundances of ctenophores and their eggs were found near the halocline, while the distribution was more uniform throughout the water column during winter. Only ctenophores 〉3.5 mm (oral-aboral length) produced eggs in the experiments, with an average rate of 2.2 eggs ind.-1 d-1. Finally, comparison with published data from the 1980s (assuming that those data refer to M. ovum) indicates that the present-day ctenophore abundance is ~80% lower in the north and ~55% higher in the southern parts of the Baltic Sea, due to reasons yet to be established.

Beschreibung: Coral reefs are under threat, exerted by a number of interacting effects inherent to the present climate change, including ocean acidification and global warming. Bioerosion drives reef degradation by recycling carbonate skeletal material and is an important but understudied factor in this context. Twelve different combinations of pCO2 and temperature were applied to elucidate the consequences of ocean acidification and global warming on the physiological response and bioerosion rates of the zooxanthellate sponge Cliona orientalis—one of the most abundant and effective bioeroders on the Great Barrier Reef, Australia. Our results confirm a significant amplification of the sponges’ bioerosion capacity with increasing pCO2, which is expressed by more carbonate being chemically dissolved by etching. The health of the sponges and their photosymbionts was not affected by changes in pCO2, in contrast to temperature, which had significant negative impacts at higher levels. However, we could not conclusively explain the relationship between temperature and bioerosion rates, which were slightly reduced at both colder as well as warmer temperatures than ambient. The present findings on the effects of ocean acidification on chemical bioerosion, however, will have significant implications for predicting future reef carbonate budgets, as sponges often contribute the lion’s share of internal bioerosion on coral reefs.

Beschreibung: Pelagic zooplankton were monitored from 2000 to 2012 at a permanent location near the Svalbard archipelago, at the boundary between the central Arctic Ocean and the Greenland Sea in the eastern Fram Strait. The temporal results reveal the first evidence of successful reproduction in Arctic waters by an Atlantic pelagic crustacean from temperate waters. The Atlantic hyperid amphipod Themisto compressa is shown to have expanded its range from more southerly and warmer waters from 2004 onwards. Successful reproductive activity by T. compressa in Arctic waters was confirmed in 2011, indicated by the presence of a complete temporal series of developmental stages including ovigerous females and recently hatched juveniles. The Arctic amphipod community is currently in transition and a continuing northward spread of southern invaders could cause a biodiversity shift from large Arctic to smaller Atlantic species.

Beschreibung: To accurately assess community composition of invertebrates, both active and dormant life stages should be considered. Dormant stages are typically produced as a strategy to overcome inhospitable environmental conditions and can also facilitate species dispersal. While they often sink and accumulate in sediment of natural habitats forming ‘egg banks,’ dormant stages are also found in the sediments accumulated in ships’ ballast tanks. Recent studies have used 2 different methods to separate dormant stages from ballast sediment to assess invasion risk associated with ballast tanks: the colloidal silica sol Ludox HS 40 and sugar flotation (i.e. the Onbé-Marcus method). It has been assumed that the Ludox HS 40 method is most effective for separation but reduces dormant stage viability whereas sugar flotation has lower separation efficacy but higher resulting viability. We conducted a comparative assessment of the 2 methods by separating dormant stages from 160 ballast sediments and examining resulting abundance counts, hatching results, DNA extractions and PCR amplifications. We found no difference in the results between the methods. The financial cost of sugar flotation is lower than that of Ludox HS 40, and costs can be further reduced by using only 1 method instead of both due to lower labour costs, particularly for a large number of samples.

Beschreibung: The Kasatochi volcanic eruption that occurred in the central Aleutian Islands in Alaska, USA, in August 2008 is thought to have induced a massive diatom bloom in the iron-limited waters of the Gulf of Alaska, which potentially affected the oceanic food web by increasing the abundance of zooplankton and sockeye salmon Oncorhynchus nerka in the northeast Pacific Ocean. We report the first seawater experiments involving volcanic ash ejected from the Kasatochi eruption, showing that the ash released 61 to 83 nmol Fe, 374 to 410 nmol NO3-, 5 to 6 nmol PO43- and 170 to 585 nmol SiO2 when it contacted seawater. Our study suggests that the amount of iron released from Kasatochi ash (an increase of 2.0 to 2.8 nM Fe) was indeed sufficient to cause the observed phytoplankton bloom in the northeastern Pacific Gyre, while the impact of macronutrient release was minimal. We further evaluated the multiple, interdependent processes in the oceanic food web related to the diatom bloom, involving the ocean survival of juvenile salmon that entered the northeast Pacific Ocean in the summer of 2008.

Beschreibung: Concentrations of heme b, the ironcontaining prosthetic group of many hemoproteins, were measured in 6 species of marine phytoplankton (Dunaliella tertiolecta, Emiliania huxleyi, Thalassio - sira weissflogii, T. oceanica, Phaeodactylum tricor - nutum and Synechococcus sp. WH7803) that were subjected to variations in iron concentration. Changes in heme b in response to reduced light and nitrate were also ex amined for E. huxleyi and T. oceanica. Results from laboratory cultures were compared with heme b determined in particulate material in the North Atlantic. In cultures, heme b made up 18 ± 14% (SE) of the total iron pool. Reduced iron and nitrate concentrations resulted in a decreased intracellular heme b concentration, expressed as per mole carbon. Chlorophyll a (chl a) to heme b ratios in E. huxleyi and D. tertiolecta in creased in response to limited light and nutrient availability, but slightly decreased or did not change in the diatoms and the cyanophyte Synechococcus sp. WH7803. The heme b:particulate organic carbon (POC) and chl a:heme b ratios in the North Atlantic were within the range observed in phytoplankton cultures. In the surface mixed layer, decreases in heme b:POC ratios were linked to decreases in nutrient concentrations. Chl a:heme b ratios increased with depth and were thus primarily affected by light availability. Relative relationships between heme b, chl a and POC in the North Atlantic likely represented a change in the ability of cells to undertake cellular processes driven by chl a (light harvesting) and heme b (e.g. electron transport) according to ambient light and nutrient conditions.

Beschreibung: Recent studies have discussed the consequences of ocean acidification for bacterial processes and diversity. However, the decomposition of complex substrates in marine environments, a key part of the flow of energy in ecosystems, is largely mediated by marine fungi. Although marine fungi have frequently been reported to prefer low pH levels, this group has been neglected in ocean acidification research. We present the first investigation of direct pH effects on marine fungal abundance and community structure. In microcosm experiments repeated in 2 consecutive years, we incubated natural North Sea water for 4 wk at in situ seawater pH (8.10 and 8.26), pH 7.82 and pH 7.67. Fungal abundance was determined by colony forming unit (cfu) counts, and fungal community structure was investigated by the culture-independent fingerprint method Fungal Automated Ribosomal Intergenic Spacer Analysis (F-ARISA). Furthermore, pH at the study site was determined over a yearly cycle. Fungal cfu were on average 9 times higher at pH 7.82 and 34 times higher at pH 7.67 compared to in situ seawater pH, and we observed fungal community shifts predominantly at pH 7.67. Currently, surface seawater pH at Helgoland Roads remains 〉8.0 throughout the year; thus we cannot exclude that fungal responses may differ in regions regularly experiencing lower pH values. However, our results suggest that under realistic levels of ocean acidification, marine fungi will reach greater importance in marine biogeochemical cycles. The rise of this group of organisms will affect a variety of biotic interactions in the sea.