A survey of ciliates at the long-term sampling station “Helgoland Roads”, North Sea

This study presents a new checklist of ciliates at the long-term sampling station Helgoland Roads. The work is based on a microzooplankton monitoring programme from January 2007 to June 2009 and a ciliate monitoring programme from June 2010 to May 2012. The checklist includes 89 ciliate taxa from 46 different genera. The total abundance of the ciliate community at Helgoland Roads ranged between 0.14 and 67.7 × 10³ cells L^?1 with a distinct peak in June. The total carbon biomass ranged between 0.2 and 234.6 μg C L^?1. The ciliate community showed a clear temporal succession pattern during the monitoring periods. The present study is not only the first detailed and updated list of ciliates at Helgoland Roads, but also provides information on seasonality, i.e. the temporal variation of species composition, abundance and carbon biomass as well as information on the biogeographic distribution of dominant ciliates in comparison with other relevant studies.     

Dinoflagellates and ciliates at Helgoland Roads, North Sea

A monitoring programme for microzooplankton was started at the long-term sampling station “Kabeltonne” at Helgoland Roads (54°11.3′N; 7°54.0′E) in January 2007 in order to provide more detailed knowledge on microzooplankton occurrence, composition and seasonality patterns at this site and to complement the existing plankton data series. Ciliate and dinoflagellate cell concentration and carbon biomass were recorded on a weekly basis. Heterotrophic dinoflagellates were considerably more important in terms of biomass than ciliates, especially during the summer months. However, in early spring, ciliates were the major group of microzooplankton grazers as they responded more quickly to phytoplankton food availability. Mixotrophic dinoflagellates played a secondary role in terms of biomass when compared to heterotrophic species; nevertheless, they made up an intense late summer bloom in 2007. The photosynthetic ciliate Myrionecta rubra bloomed at the end of the sampling period. Due to its high biomass when compared to crustacean plankton especially during the spring bloom, microzooplankton should be regarded as the more important phytoplankton grazer group at Helgoland Roads. Based on these results, analyses of biotic and abiotic factors driving microzooplankton composition and abundance are necessary for a full understanding of this important component of the plankton.     

Year-to-year variation in larval fish assemblages of the Southern North Sea

In order to test the temporal stability within and the reproducibility of larval fish assemblages between years, the larval fish assemblage at Helgoland Roads, North Sea (NE Atlantic) was quantitatively sampled almost daily from January 2003 to December 2005. The survey resulted in a total of 462 samples containing 50,632 larval fish of at least 42 taxa. In winter the larval fish assemblage was mainly dominated by larvae emerging from demersal eggs. This changed gradually to larvae hatching from pelagic eggs. Larvae from pelagic eggs dominated the ichthyoplankton assemblage in summer. A remarkably stable seasonality in terms of dominance patterns with recurring, season-specific fish assemblages was observed over the 3 years, despite substantial variation in environmental conditions such as a temperature difference of almost 20°C between summer and winter. The lesser sandeel (Ammodytes marinus), was the only species which showed significant fluctuations in abundance between the years. After removal of this species from the analysis, the dominance patterns of the remaining fish species were almost identical between years.     

Species-Specific Bacterial Communities in the Phycosphere of Microalgae?

Specific associations of bacteria with phytoplankton have recently been reported in the literature. In our study, we analyzed bacterial communities of microalgal cultures related to algal growth phases. Seven freshly isolated key diatom and dinoflagellate species from Helgoland Roads, North Sea, were investigated. The community composition of associated bacteria as well as the cell numbers, the photosynthetic efficiency of the algae, and the depletion of inorganic nutrients in the medium were recorded over a period of 8 weeks in batch cultures. Diversity and succession of bacterial communities was analyzed by ribosomal intergenic spacer analysis. Phylogenetic analysis of bacterial populations was performed by denaturing gradient gel electrophoresis of 16S rRNA genes followed by DNA sequence analysis. Members of Alphaproteobacteria and Gammaproteobacteria and the Flavobacteria–Sphingobacteria group within the Bacteroidetes phylum predominated in the cultures. Differences in free-living and attached bacterial populations were observed between the phylogenetic groups. Shifts in the bacterial communities could not be correlated to changes of nutrient levels or algal growth phases. Regarding our results, it should not be generalized that the compositions of the bacterial communities are strictly species specific for microalgae. The importance of factors like the composition of exudates is apparent.     

Occurrence of <Emphasis Type="Italic">Vibrio parahaemolyticus</Emphasis> and <Emphasis Type="Italic">Vibrio alginolyticus</Emphasis> in the German Bight over a seasonal cycle

Bacteria of the genus Vibrio are an important component of marine ecosystems worldwide. The genus harbors several human pathogens, for instance the species Vibrio parahaemolyticus, a main cause for foodborne gastroenteritis in Asia and the USA. Pathogenic V. parahaemolyticus strains emerged also in Europe, but little is known about the abundance, pathogenicity and ecology of V. parahaemolyticus especially in Northern European waters. This study focuses on V. parahaemolyticus and its close relative Vibrio alginolyticus in the North Sea (Helgoland Roads, Germany). Free-living, plankton-attached and shellfish-associated Vibrio spp. were quantified between May 2008 and January 2010. CFUs up to 4.3 × 10³ N l⁻¹ and MPNs up to 240 N g⁻¹ were determined. Phylogenetic classification based on rpoB gene sequencing revealed V. alginolyticus as the dominant Vibrio species at Helgoland Roads, followed by V. parahaemolyticus. We investigated the intraspecific diversity of V. parahaemolyticus and V. alginolyticus using ERIC-PCR. The fingerprinting disclosed three distinct groups at Helgoland Roads, representing V. parahaemolyticus, V. alginolyticus and one group in between. The species V. parahaemolyticus occurred mainly in summer months. None of the strains carried the virulence-associated genes tdh or trh. We further analyzed the influence of nutrients, secchi depth, temperature, salinity, chlorophyll a and phytoplankton on the abundance of Vibrio spp. and the population structure of V. parahaemolyticus. Spearman Rank analysis revealed that particularly temperature correlated significantly with Vibrio spp. numbers. Based on multivariate statistical analyses we report that the V. parahaemolyticus population was structured by a complex combination of environmental parameters. To further investigate these influences is the key to understanding the dynamics of Vibrio spp. in temperate European waters, where this microbial group and especially the pathogenic species, are likely to gain in importance.     

LATITUDINAL TRENDS IN THE GROWTH AND REPRODUCTIVE SEASONALITY OF DELESSERIA SANGUINEA,MEMBRANOPTERA ALATA,AND PHYCODRYS RUBENS (RHODOPHYTA)1

The seasonality of Delesseria sanguinea, Membranoptera alata, and Phycodrys rubens (Rhodophyta) was studied at Helgoland (North Sea, Germany) and Roscoff (Brittany, France). Plants were collected at bimonthly intervals, and growth and reproduction were monitored. Growth of blades was observed mainly in spring, although small blades were found on plants of M. alata and P. rubens all year round. In summer, plants started to degenerate and in autumn they became fertile. The reproductive season of D. sanguinea lasted from October to February/April at both locations, whereas reproductive plants of M. alata and P. rubens were found until April at Helgoland and until June and August, respectively, in Roscoff_: Lower winter temperatures at Helgoland than at Rascoff may have caused these differences in the duration of the reproductive season. Using published data, the seasonal patterns at Helgoland and Roscoff were compared to those found at other locations (e.g. Barents Sea; Maine, USA; Isle of Man, UK) and local temperature/daylength conditions. Blade growth was synchronized across all populations and occurred in spring, when temperatures were usually still suboptimal for growth. Maximum reproduction was generally found in the colder half of the year but started earlier in autumn in the Barents Sea. Adaptive strategies in the seasonal control of growth and reproduction are discussed. Adequate timing of the history events (e.g. appearance of juveniles in spring) appears more important than maximal growth and reproduction of adults during the season with the most favorable temperatures.     

The diatom Mediopyxis helysia Kühn, Hargreaves & Halliger 2006 at Helgoland Roads: a success story?

The Helgoland Roads phytoplankton long-term data set is one of the longest and the most detailed data sets in Europe, having provided continuous work-daily observations of phytoplankton abundance since 1962. These high frequency counts have undergone and are continuously subject to a high level of quality control. The Helgoland data set thus is useful in the evaluation of new records in phyto- and zooplankton. Here, we report the first appearance of the relatively recently described diatom Mediopyxis helysia in the Helgoland Roads counts. This species was first detected in Helgoland samples in March 2009. Importantly, it has rapidly become a prominent member of the Helgoland phytoplankton community. While in 2009 it only produced a moderate spring peak of 4,000 cells?l^?1, it was one of the dominant diatoms in the 2010 spring bloom with Mediopyxis reaching cell densities above 300,000 cells?l^?1 and total chlorophyll concentrations exceeding 40?μg?l^?1. In 2010, this species was repeatedly present throughout the year. There was no clear relationship between temperature and cell abundance with all Mediopyxis cells occurring at temperatures between 3 and 12°C. However, the extensive peak in 2010 was associated with a sudden drop in salinity, which could indicate that this bloom might have been the result of inflow of low salinity water into the area. This was supported by a laboratory growth experiment in which a clonal culture of M. helysia grew fastest at a salinity of 27 and slowest at a salinity of 31.5. Further long-term observations will be required to establish whether this species will become a regular feature at Helgoland and how this might affect the local food web.     

Gymnodinium blooms in the Helgoland bight (North Sea) during August, 1968

Summary 1. As in other parts of the North Sea, dinoflagellate red tides occurred in Helgoland waters in August, 1968. Measurements of plankton, and physical and chemical water properties at the permanent station Helgoland Roads were analyzed to describe the blooms. In addition, planktological and hydrographical investigations at three areas south, southwest and northwest of Helgoland on 27 and 28 August, as well as at two drifting stations off the mouths of the Elbe and Eider rivers on 6 and 8 August, were used for this work.2.Gymnodinium sp. was abundant at all these localities, forming blooms near Helgoland from 14 to 30 August, with a maximum of 3 to 3.25 × 10⁶ cells/l (and 18 to 19µg chlorophylla/l) on 28 and 30 August at Helgoland Roads. The primary production was as high as 0.98 mg C/l in 6 hours in a suspension of 3.7 × 10⁶ Gymnodinium/l, where diatoms had been removed. This means that one millionGymnodinium produced 0.265 mg C in 6 hours. At Helgoland Roads nitrate and nitrite were depleted at times, but not phosphate.3. About 3 × 10⁶ Gymnodinium sp./l were found in the upper 16.5 m of water investigated around Helgoland on 27 and 28 August (maximal 7.8 × 10⁶ cells/l at 3 m). There was a marked vertical stratification ofGymnodinium with a concentration towards the surface during the day. This was particularly the case in the turbid water off the Elbe estuary, where numbers up to 0.3 × 10⁶/l were counted.Gymnodinium sp. formed 96 to 99 % of the phytoplankton biomass during maximal development. The chlorophylla content of one millionGymnodinium was only 3.5µg. An extinktion of E=0.083/1 m was measured in a suspension of 10⁶ Gymnodinium/l in samples with minimal other plankton and detritus.4. The water masses containingGymnodinium blooms, which reached Helgoland after a change of wind direction, were characterized by lower salinity and higher temperature. This indicates that blooms developed in the coastally influenced water masses east of Helgoland. The red tides occurred during a period of minimal discharge of Elbe river water and of relatively high salinity of the coastal water in the Inner German Bight. They developed after a long period of calm winds. There were no records of reported fish or shellfish poisoning.

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Gymnodinium-Wucherungen in der Helgoländer Bucht (Nordsee) im August 1968

Kurzfassung Wie in anderen Teilen der Nordsee traten auch in der Helgoländer Bucht im August 1968 starke Wucherungen von Dinoflagellaten auf. Untersuchungen des Planktons und der Wassereigenschaften auf der Helgoland-Reede sowie auf weiteren Stationen in der Helgoländer Bucht wurden für die Erklärung dieser Erscheinung ausgewertet. In allen untersuchten Gebieten (Fig. 1) fand sichGymnodinium sp. als dominante Planktonform. Gymnodinien-Wucherungen traten auf der Helgoland-Reede in der Zeit vom 14.–30. August auf und erreichten Zellzahlen von über 3 × 10⁶/l am 28. und 30. August, bei nur 18–19µg Chlorophylla/l. Eine Million Gymnodinien produzierten 0,265 mg C in 6 Stunden im Inkubator. Etwa 3 × 10⁶ Gymnodinium sp. wurden in den obersten 16,5 m der Wassersäule in der Nähe Helgolands während des Blüte-Maximums gemessen (maximal 7,8 × 10⁶/l in 3 m). Die Gymnodinien waren tagsüber in den obersten Metern konzentriert. Eine solche Vertikalschichtung war besonders im Küstenwasser vor der Elbmündung ausgeprägt, wo bis zu 0,3 × 10⁶ Zellen/l gefunden wurden.Gymnodinium sp. bildete 96–99 % der Biomasse während der Blüte bei Helgoland. Es wurden ein Chlorophyll-a-Gehalt von nur 3,5µg für 10⁶ Zellen ermittelt und eine Extinktion von E=0,083/1 m für eine Suspension von 10⁶ Gymnodinien/l in Proben mit minimalem Gehalt an übrigem Plankton und Detritus gemessen. Der Nitrat- und Nitritgehalt im Wasser war zeitweise erschöpft, nicht aber das Phosphat. Wenn Wassermassen mitGymnodinium-Blüten nach Helgoland gedriftet wurden, fiel der Salzgehalt, und die Temperatur stieg an, was auf eine Entstehung der Wucherungen im brackwasser-beeinflußten Gebiet östlich Helgolands hindeutet. DieGymnodinium-Blüten entwickelten sich nach einer außergewöhnlich langen windarmen Periode und während eines minimalen Süßwasserzuflusses aus der Elbe. Vergiftungserscheinungen an Seetieren wurden bei Helgoland nicht festgestellt.

     

Ecological long-term research at Helgoland (German Bight,North Sea): retrospect and prospect—an introduction

This paper summarizes and evaluates ecological long-term observations at the island of Helgoland (German Bight, North Sea). It is an introduction to a series of seven contributions to an issue of Helgoland Marine Research (vol. 58, no. 4) dealing with observations on the physico-chemical environment and the local biota (pelagic bacteria, phytoplankton, zooplankton, macroalgae, macrozoobenthos). More than 150 years of research at Helgoland (at the Biologische Anstalt Helgoland, BAH, since 1892), and particularly the Helgoland Roads time-series which started in 1962, has provided a multitude of data which represents a valuable basis for analysing past changes, evaluating the present status and predicting future changes in ecosystem parameters. Predicting the consequences of the recent rapid ecological change on regional and global scales requires increased efforts of focus on long-term ecological observations. Future efforts in this field will rely on the application of innovative advanced technology and on the concerted activities of a large number of institutions which only collectively can establish the large-scale patterns of temporal and spatial change in ecosystem functions.     

Seasonal Dynamics and Modeling of a <Emphasis Type="Italic">Vibrio</Emphasis> Community in Coastal Waters of the North Sea

Vibrio species are ubiquitously distributed in marine waters all over the world. High genome plasticity due to frequent mutation, recombination, and lateral gene transfer enables Vibrio to adapt rapidly to environmental changes. The genus Vibrio comprises several human pathogens, which commonly cause outbreaks of severe diarrhea in tropical regions. In recent years, pathogenic Vibrio emerged also in coastal European waters. Little is known about factors driving the proliferation of Vibrio spp. in temperate waters such as the North Sea. In this study a quantification of Vibrio in the North Sea and their response to biotic and abiotic parameters were assessed. Between January and December 2009, Vibrio at Helgoland Roads (North Sea, Germany) were quantified using fluorescence in situ hybridization. Vibrio numbers up to 3.4 × 10⁴ cells × mL⁻¹ (2.2% of total microbial counts) were determined in summer, but their abundance was significantly lower in winter (5 × 10² cells × mL⁻¹). Correlations between Vibrio and nutrients (SiO₂, PO₄ ³⁻, DIN), Secchi depth, temperature, salinity, and chlorophyll a were calculated using Spearman rank analysis. Multiple stepwise regression analysis was carried out to analyze the additive influence of multiple factors on Vibrio. Based on these calculations, we found that high water temperature and low salinity best explained the increase of Vibrio cell numbers. Other environmental parameters, especially nutrients and chlorophyll a, also had an influence. All variables were shown to be subject to the overall seasonal dynamics at Helgoland Roads. Multiple regression models could represent an efficient and reliable tool to predict Vibrio abundances in response to the climate change in European waters.     

Plankton time — space dynamics in German Bight — a systems approach

Summary Long term continuous plankton measurements at Helgoland (North Sea) have provided a set of data which could be used for ecological functional analysis with respect to prey predator theory. Local dynamics display trophic feedback between selected populations. Phytoplankton, small copepods, Noctiluca miliaris, Pleurobrachia pileus and Beroe gracilis conform with theoretical assumptions. The functional relationships are less significant in averaged population dynamics compared with single ear processes. The local prey-predator cycles are to be understood as population waves travelling through German Bight. Such population waves over an area of 18 000 km² have been investigated and are displayed for the above given zooplankton populations as computer graphics and analysed in their progression for the population of P. pileus.     

Seasonal occurrence and abundance of caridean shrimp larvae at Helgoland,German Bight

Plankton samples were collected from January 1985 to January 1986 three times per week at Helgoland to study seasonal occurrence and abundance of caridean shrimp larvae. A total of eleven species were obtained. Ninety-one % of all larvae collected during the sample period belonged toCrangon crangon L. andCrangon allmanni Kinahan, 6% toPhilocheras trispinosus Hailstone and 3% to the remaining eight species. Collections were generally dominated byC. crangon larvae. However,C. allmanni larvae were most abundant in June coinciding with hatching activities of the population near Helgoland.C. allmanni was observed to have the highest density of all species with approximately 8 larvae per m³. Larvae ofEualus occultus (Lebour),Eualus pusiolus (Kroyer),Hippolyte varians Leach andAthanas nitescens Leach were most likely released by populations inhabiting the rocky intertidal zone around Helgoland. The presence ofProcessa modica Williamson & Rochanaburanon andProcessa nouveli holthuisi Al-Adhub & Williamson in the German Bight was verified by observations of a series of different developmental stages. Larvae of the rare speciesCaridion steveni Lebour were also recorded. The observed shrimp species were placed into three different groups with respect to their seasonal occurrence. Possible advantages of the timing of larval dispersal relative to predation and food availability are given. The results on seasonal occurrence and relative abundance are discussed in relation to environmental factors (temperature, salinity) as well as to the geographical distribution of the species.     

Comparison of different DNA-extraction techniques to investigate the bacterial community of marine copepods

Marine zooplanktic organisms, such as copepods, are usually associated with large numbers of bacteria. Some of these bacteria live attached to copepods’ exoskeleton, while others prevail in their intestine and faecal pellets. Until now, general conclusions concerning the identity of these bacteria are problematic since the majority of previous studies focused on cultivable bacteria only. Hence, to date little is known on whether copepod genera or species harbour distinct bacterial populations and about the nature of this association. To shed more light on these copepod/bacteria consortia, the focus of this study was the development and evaluation of a suitable approach to extract bacterial DNA from different North Sea copepod genera. Furthermore, the bacterial DNA was analysed by PCR-DGGE and subsequent sequencing of excised bands. The result of this work was an appropriate extraction method for batches of ten to one copepod specimens and offered first insights as to which bacteria are attached to the copepods Acartia sp. and Temora sp. from Helgoland Roads (German Bight) and a laboratory-grown Acartia tonsa culture. It revealed the prevalence of Alphaproteobacteria.     

Effects of temperature and salinity on larval development ofElminius modestus (Crustacea,Cirripedia) from Helgoland (North Sea) and New Zealand

Larvae ofElminius modestus (Darwin) from four different populations (Portobello, Leigh, Doubtless Bay [New Zealand] and Helgoland [North Sea]) were reared at different salinity and temperature combinations. The larvae ofE. modestus from Helgoland developed successfully at a wide range of temperature (6° to 24 °C) and salinity (20 to 50 S). Mortality was highest at 10 S; only at 12° and 18 °C did a small percentage develop to the cypris. The larvae from New Zealand were reared at a temperature range of 12°–24 °C at 20, 30 and 40 S; mortality increased in all populations at all salinities with decreasing temperature and was extremely high at 12 °C and 40 S. The temperature influence on larval duration could be described in all cases by a power function. No significant differences in temperature influences on developmental times between the tested salinities were found, except for the Portobello population at 20 S. Significant differences were found in the temperature influence on larval development between the populations from Helgoland and the North Island of New Zealand (Leigh, Doubtless Bay). No differences were found between the Helgoland and Portobello population. The pooled data for the temperature influence on the larval development of the three tested New Zealand populations at 20, 30 and 40 S and the pooled Helgoland data at 20, 30 and 40 S show highly significant differences.Larval size (stage VI) was influenced by experimental conditions. The larvae grew bigger at low temperatures and attained their maximum size at 30 S (Helgoland). There was a strong reduction in larval size at temperatures from 18° to 24 °C. The larvae of the New Zealand populations were smaller than those from Helgoland. The greatest difference in size existed between the larvae from Portobello and Helgoland.     

Birth seasonality in cotton-top tamarins (saguinus oedipus) despite constant food supply and body weight

Goldizen et al. (1988) reported that wild saddle-back tamarins (Saguinus fuscicollis, Callitrichidae) show birth seasonality that is correlated with food supply and body weight. They suggested a sequence of ultimate causality in which shortage of food leads to reduced body weight which leads to timing of weaning and lactation when resources are more abundant. Cotton-top tamarins in captivity show birth seasonality despite constant food supply and body weight. Although natural availability of fruit and insects (which are key foods for tamarins) is related to rainfall, birth seasonality and body weight in captive cotton-top tamarins are unrelated to rainfall. The most likely proximate mechanism for seasonality of births in tamarins is photo-period, given existing data on populations living in natural and artificial lighting.     

Increasing prevalence of the marine cladoceran Penilia avirostris (Dana, 1852) in the North Sea

Rising sea surface temperatures in the North Sea have had consequential effects on not only indigenous plankton species, but also on the possibility of successful colonisation of the area by invasive plankton species. Previous studies have noted the introduction and integration into the plankton community of various phytoplankton species, but establishment of zooplankton organisms in the North Sea is less well-documented. Examining continuous plankton recorder (CPR) survey data and zooplankton results from the Helgoland Roads study, the autumn of 1999 witnessed the occurrence of the marine cladoceran Penilia avirostris in large numbers in the North Sea. The rapid appearance of the species corresponded with exceptionally warm sea surface temperatures (SSTs). Since 1999, the species has become a regular feature of the autumnal zooplankton community of the North Sea. In 2002 and 2003, the species occurred in greater abundance than recorded before. It is suggested that increased autumn SSTs have proved favourable to P. avirostris, with warmer conditions contributing to the success of the species resting eggs and aiding colonisation.Communicated by H.-D. Franke     

The human impact in the German Bight: Eutrophication during three decades (1962–1991)

The human impact in the German Bight, in the form of anthropogenic eutrophication, has been documented by a 30-year time-series measurement near the island of Helgoland. Since 1962, the Biologische Anstalt Helgoland has measured inorganic nutrients and phytoplankton abundance from daily samples at Helgoland Roads, a position 60 km off the main source of eutrophication, the River Elbe. Since the early sixties, phosphate concentrations rose for about a decade, levelling off to about twice the former concentrations for another decade, and then decreasing (since 1982) as a result of phosphate-reducing measures. Nitrate concentrations, however, have only increased since 1980/81, following Elbe river flood events. In 1987, three times the former concentrations were reached. A decrease has been observed only since 1991. This different development of phosphorus and nitrogen eutrophication led to a shift of inorganic N/P-ratios in the German Bight. The phosphate increase was more pronounced in the late summer “regeneration mode” conditions, the nitrate increase in the winter months. The eutrophication is not restricted to the inner German Bight and coastal waters of a salinity of <33, but has also occurred in more saline waters at S>33 psu (practical salinity unit), as characteristic for the outer German Bight. In this more saline water, phosphate and nitrate maximum levels occurred three years later, compared with the average Helgoland data, which are more representative of the inner German Bight. It is suggested that suspended particulate organic matter, as a long-distance carrier of nutrients, might have caused this delayed eutrophication in the outer German Bight waters. While the human impact is obvious as to nutrient concentrations, it is less obvious in phytoplankton stock enhancement. A general increase in phytoplankton biomass (about 3–4 times) was found, but this was mainly due to unidentified nanoflagellates of unknown trophic state, and subject to methodological errors. The causal relationships of phytoplankton stocks and eutrophication are not clearly understood, as natural variability is large and hydrographical factors possibly dominate. Additional nutrient input by Elbe river floods did not always result in elevated phytoplankton stocks near Helgoland, while extended periods of vertical density stratification of the German Bight water caused large plankton blooms.     

The microbiome of North Sea copepods

Copepods can be associated with different kinds and different numbers of bacteria. This was already shown in the past with culture-dependent microbial methods or microscopy and more recently by using molecular tools. In our present study, we investigated the bacterial community of four frequently occurring copepod species, Acartia sp., Temora longicornis, Centropages sp. and Calanus helgolandicus from Helgoland Roads (North Sea) over a period of 2 years using DGGE (denaturing gradient gel electrophoresis) and subsequent sequencing of 16S-rDNA fragments. To complement the PCR-DGGE analyses, clone libraries of copepod samples from June 2007 to 208 were generated. Based on the DGGE banding patterns of the two years survey, we found no significant differences between the communities of distinct copepod species, nor did we find any seasonality. Overall, we identified 67 phylotypes (>97 % similarity) falling into the bacterial phyla of Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria. The most abundant phylotypes were affiliated to the Alphaproteobacteria. In comparison with PCR-DGGE and clone libraries, phylotypes of the Gammaproteobacteria dominated the clone libraries, whereas Alphaproteobacteria were most abundant in the PCR-DGGE analyses.     

Phenological shifts of three interacting zooplankton groups in relation to climate change

Over the past several decades, global warming has been linked to shifts in the distributions and abundances of species. In the southern North Sea, temperatures have increased in the last three decades and this will likely have consequences on the seasonality of marine organisms living in the area. Ctenophores such as Beroe gracilis and Pleurobrachia pileus could be particularly affected by changes in their own phenology and that of their prey, thus causing shifts in ecosystem function. Despite their global relevance and their potentially deleterious effect on the fishing industry, only a few long‐term records of ctenophore abundance exist, and most of these records are semiquantitative in nature. Therefore, our knowledge of the influence of environmental factors on their population development is presently very limited. In this study, the long‐term abundance dynamics of B. gracilis, P. pileus and their food calanoid copepods were analysed for a highly temporally resolved time series in the German Bight at Helgoland Roads. Special attention was focused on the response of these organisms to climate warming. Bayesian statistics showed that the phenology of the two ctenophores shifted in a step‐like mode in the year 1987/1988 to permanent earlier appearances. The seasonal change in the population blooms of P. pileus and B. gracilis correlated remarkably well with a step‐like increase in winter and spring sea surface temperatures of the southern North Sea. Possible explanations for the changes observed in these organisms include higher reproductive rates, increased winter survival rates or both. Interannual variations in ctenophore abundances correlated best with the interannual changes in spring temperatures, although the impact of temperature on B. gracilis appeared less pronounced. The changes in copepods abundance were not consistent with changes in P. pileus and B. gracilis. P. pileus showed longer periods of high abundance after the permanent seasonal advancement. These longer periods were correlated with a decline in the average autumn abundance of copepods. Changes in the phenology of these organisms raise the concerns on the declining state of fish stocks, which could potentially be exacerbated by gelatinous zooplankton outbreaks. These conditions may ultimately lead to trophic dead ends by channelling the flow of energy away from higher trophic levels.     

Heat stress responses and population genetics of the kelp Laminaria digitata (Phaeophyceae) across latitudes reveal differentiation among North Atlantic populations

To understand the thermal plasticity of a coastal foundation species across its latitudinal distribution, we assess physiological responses to high temperature stress in the kelp Laminaria digitata in combination with population genetic characteristics and relate heat resilience to genetic features and phylogeography. We hypothesize that populations from Arctic and cold‐temperate locations are less heat resilient than populations from warm distributional edges. Using meristems of natural L. digitata populations from six locations ranging between Kongsfjorden, Spitsbergen (79°N), and Quiberon, France (47°N), we performed a common‐garden heat stress experiment applying 15°C to 23°C over eight days. We assessed growth, photosynthetic quantum yield, carbon and nitrogen storage, and xanthophyll pigment contents as response traits. Population connectivity and genetic diversity were analyzed with microsatellite markers. Results from the heat stress experiment suggest that the upper temperature limit of L. digitata is nearly identical across its distribution range, but subtle differences in growth and stress responses were revealed for three populations from the species’ ecological range margins. Two populations at the species’ warm distribution limit showed higher temperature tolerance compared to other populations in growth at 19°C and recovery from 21°C (Quiberon, France), and photosynthetic quantum yield and xanthophyll pigment responses at 23°C (Helgoland, Germany). In L. digitata from the northernmost population (Spitsbergen, Norway), quantum yield indicated the highest heat sensitivity. Microsatellite genotyping revealed all sampled populations to be genetically distinct, with a strong hierarchical structure between southern and northern clades. Genetic diversity was lowest in the isolated population of the North Sea island of Helgoland and highest in Roscoff in the English Channel. All together, these results support the hypothesis of moderate local differentiation across L. digitata's European distribution, whereas effects are likely too weak to ameliorate the species’ capacity to withstand ocean warming and marine heatwaves at the southern range edge.