ALBERT

Description: Effects of global warming on marine ecosystems are far less understood than they are in terrestrial environments. Macrophyte-based coastal ecosystems are particularly vulnerable to global warming, because they often lack species redundancy. We tested whether summer heat waves have negative effects on an ecologically important ecosystem engineer, the eelgrass Zostera marina L., and whether high genotypic diversity may provide resilience in the face of climatic extremes. In a mesocosm experiment, we manipulated genotypic diversity of eelgrass patches fully crossed with water temperature (control vs. temperature stress) over 5 mo. We found a strong negative effect of warming and a positive effect of genotypic diversity on shoot densities of eelgrass. These results suggest that eelgrass meadows and associated ecosystem services will be negatively affected by predicted increases in summer temperature extremes. Genotypic diversity may provide critical response diversity for maintaining seagrass ecosystem functioning, and for adaptation to environmental change.

Description: Marine angiosperms, or seagrasses, continue to be a major focus of marine biologists because of their important ecological role in many coastal ecosystems. Seagrass population biology could benefit from a population genetic perspective because genetic data enable the extraction of useful demographic information such as isolation and gene flow between demes. Moreover, population genetic processes may contribute to the growing ecological risks of local population extinction. Progress in seagrass genetics is partly driven by novel genetic markers which detect variation at the DNA level and overcome the limited polymorphism of allozymes. Key results of studies in the past decade, mostly using RAPD and microsatellites, were (1) considerable genetic and genotypic (clonal) diversity is present in several species in contrast to earlier notions of low polymorphism detected at allozyme loci, and (2) genetic differentiation among populations seems to be the rule despite earlier reports of genetic uniformity. Pronounced genetic structure was detected between populations of 4 species examined thus far (Posidonia oceanica, P. australis, Zostera marina, Thalassia testudinum). The FST estimates varied widely and ranged from 0.01 to 0.623 across studies and species. Genetic differentiation at a systematic range of scales was only studied in eelgrass Zostera marina, where it was positively correlated with geographic distance. The high polymorphism of RAPD or microsatellite markers will allow the augmention of indirect estimates of gene flow by methods detecting individual immigration events through paternity analysis or assignment tests. Important conservation related issues such as the level of inbreeding and the effective population size have also been obtained from genetic marker data, but results are too scarce at the moment to allow generalizations. In Zostera marina and Posidonia australis, several population genetic attributes such as clonal diversity, mating system and effective population size varied among populations within species, highlighting that there is no Œtypical¹ population. An important gap in our knowledge is whether the effects of natural population fragmentation and patchiness enhance the genetic isolation of populations due to anthropogenic disturbances. It is also unclear whether genetic differentiation displayed at marker loci are correlated with fitness-related plant traits, and whether genetic or genotypic diversity is important for medium- to long-term meadow persistence. An assessment of the genetic and genotypic diversity at marker loci should be combined with experiments on the ecological plasticity and reaction norms of genotypes composing the populations in question. This way, the role of genetic diversity for seagrass population maintenance and growth in the face of changing environmental conditions can be evaluated.

Description: Within mono-specific meadows of clonal plants, genotypic diversity may functionally replace species diversity. Little is known about the variability in performance and plasticity of different genotypes towards anthropogenically induced stressors. In this field experiment we compared light-limitation stress responses and recovery of different eelgrass Zostera marina genotypes to assess the variability in phenotypic plasticity and gene expression between different genotypes. Replicated monoculture plots of 4 genotypes were subjected to a simulated turbidity period of 4 wk using shading screens, and their performance during light limitation and 4 wk of recovery was compared to non-shaded controls. In addition to growth and biomass, we investigated storage carbohydrates and quantified the expression of genes involved in carbohydrate metabolism, photosynthesis and control of oxidative stress. Plants showed remarkable plasticity in their stress responses and all phenotypic variables recovered to the control level within 4 wk. Depletion and subsequent restoration of sucrose levels differed among genotypes. In terms of gene expression, no consistent patterns were observed. Our study confirms that stress responses and recovery processes can vary substantially between genotypes and the results emphasize the importance of preserving regional genotypic diversity for immediate positive diversity effects and for adaptive evolution in response to global change.

Description: 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.

Description: 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.

Description: Seagrasses worldwide are commonly infected by endophytic protists of the genus Labyrinthula. To date, the nature of interaction of endophyte and host is not well understood. In eelgrass (Zostera marina) Labyrinthula zosterae may become virulent (pathogenic) and lead to the loss of entire sea grass beds. One of the best known examples of any marine epidemic were outbreaks of the ´wasting disease´ on both sides of the Atlantic in the 1930s, but smaller infestations have been reported until recently. Up to now, detection of infection by Labyrinthula was based on the wasting index, i.e. the relative area of leaf lesions or microscopy, while genetic data are virtually absent. We characterized a ~1400 base pair portion of the 18S small subunit rDNA in L. zosterae isolates (N=41) from six northern European sites and one southern location (Adriatic Sea) in order to assess identity and potential diversity of endophytic protists. Because there are indications that low salinity impedes Labyrinthula growth, sampling sites included a wide range of salinities from 5-34 psu. A search against the non-redundant GENBANK data base revealed that most isolates are 99% similar to the only L. zosterae 18S sequence available from the data base at all but the Finish site (salinity values 5-7 psu). At the latter site, a different Labyrinthula species occurred, which was also found in fully marine Wadden Sea cultures. A third species was detected in Skagerrak, south-western Baltic and North Sea samples (20-25 psu). We conclude that L. zosterae is widespread among northern European eelgrass sites across wide ranges of salinity.

Description: The nature of many microbe-host interactions is not static, but may shift along a continuum from mutualistic to harmful depending on the environmental conditions. In this study, we assessed the interaction between the foundation plant eelgrass Zostera marina and the frequently associated protist Labyrinthula zosterae. We tested how an important environmental factor, nutrient availability, would modulate their interaction. We experimentally infected naive eelgrass plants in combination with 2 nutrient levels (fertilized and non-fertilized). We followed L. zosterae infection, eelgrass growth parameters and host defense gene expression over 3 wk in large 600 l tanks. Inoculation with L. zosterae and nutrient limitation both reduced eelgrass growth. These effects were additive, whereas no interaction of nutrient treatment and L. zosterae inoculation was detected. Gene expression levels of 15 candidate genes revealed a reduced expression of photosynthesis-related genes but an increased expression of classical stress genes such as Hsp80 in inoculated plants 2 d post-inoculation. However, we found no effects on plant mortality, and plants were able to clear high infection levels within 3 wk to ambient background levels of infection as assessed via specific RT-qPCR designed to quantify endophytic L. zosterae. Thus, we found no evidence that L. zosterae is a facultative mutualist that facilitates eelgrass growth under nutrient-limiting conditions. We suggest that the interaction between contemporary L. zosterae genotypes and Z. marina represents a mild form of parasitism in northern Europe because the damage to the plant is moderate even under nutrient limitation stress.

Description: In the northern hemisphere, eelgrass Zostera marina L. is the most important and widespread seagrass species. Despite its ecological importance, baseline data on eelgrass distribution and abundance are mostly absent, particularly in subtidal areas with relatively turbid waters. Here, we report a combined approach of vegetation mapping in the Baltic Sea coupled to a species distribution model (SDM). Eelgrass cover was mapped continuously in the summers of 2010 and 2011 with an underwater towed camera along ~400 km of seafloor. Eelgrass populated 80% of the study region and occurred at water depths between 0.6 and 7.6 m at sheltered to moderately exposed coasts. Mean patch length was 128.6 m but was higher at sheltered locations, with a maximum of 〉2000 m. The video observations (n = 7824) were used as empiric input to the SDMs. Using generalized additive models, 3 predictor variables (depth, wave exposure, and slope), which were selected based on Akaike’s information criterion, were sufficient to predict eelgrass presence/absence. Along with a very good overall discriminative ability (area under the receiver-operating characteristic curve ROC/AUC = 0.82), depth (as a proxy for light), wave exposure, and slope contributed 66, 29, and 5%, respectively, to the final model. The estimated total areal extent of eelgrass in the study region amounts to 140.5 km2 and comprises about 11.5% of all known Baltic seagrass beds. The present work is, to the best of our knowledge, the largest study undertaken to date on vegetation mapping and the first to assess distribution of eelgrass quantitatively in the western Baltic Sea.