Direct and indirect effects of Gracilaria vermiculophylla on native Fucus vesiculosus

The perennial red macroalga Gracilaria vermiculophylla (Ohmi) Papenfuss has recently been introduced to the Baltic Sea and is a potential competitor to Fucus vesiculosus, the most common native perennial alga in large parts of the Baltic Sea. Gracilaria might interfere with Fucus through direct competition for resources. In addition, Gracilaria is a favoured refuge for mesograzers, which prefer to feed on Fucus. Mesocosm-experiments were conducted over one year in the Kiel Fjord in order to test the direct and indirect effects of Gracilaria on Fucus. Fucus was incubated with Gracilaria at three different densities and grazers in high or low abundances. High densities of Gracilaria inhibited the growth of Fucus adults and also reduced the half-life-time of Fucus germlings. Associated grazers also had a negative effect on Fucus adults. Our results suggest that Gracilaria is able to influence Fucus in the Baltic Sea through direct competition for resources and by exposing it to higher grazer pressur

Competitive ranks of three Fucus spp. (Phaeophyta) in laboratory experiments—testing of Keddy's competitive hierarchy model

Keddy's competitive hierarchy model describes species distribution patterns along gradients under equilibrium conditions and can potentially serve as an explanation for zonation patterns of intertidal seaweeds on rocky shores. One of the assumptions of the model is a competitive hierarchy with the top competitor occupying the benign end of the gradient. Another assumption is the consistency of competitive ranks of species in all environmental conditions included in the shared parts of species' fundamental niches. In laboratory replacement series experiments, the competitive ranks of pairs of Fucus species that occupy adjacent zones in the field were analysed and compared to ranks found in previous field experiments. Unattached thalli of Fucus serratus versus F. vesiculosus or F. vesiculosus versus F. spiralis, respectively, were held in aerated beakers to establish the competitive ranking of the three congeners. Each replacement series was conducted at three total densities. F. vesiculosus was clearly competitively dominant over F. serratus. In competition with F. spiralis, F. vesiculosus was only dominant at its lowest absolute input frequencies, but at higher frequencies dominance was reversed. At high densities, the total ranking was F. spiralis > F. vesiculosus > F. serratus, which is the opposite order to that which would be expected from Keddy's model. Although all three species thrived well under the laboratory conditions, the results did not reflect in situ competitive dominances, which may be an effect of nutrient competition in the laboratory. Keddy's assumption that competitive ranks are consistent over the whole range of fundamental niches cannot be supported for Fucus spp

Colonisation success of introduced oysters is driven by wave-related exposure

The Pacific oyster, Magallana gigas, is an extremely successful invader with established populations in marine and estuarine habitats almost all over the world. Ecological implications of the introduction of this species to indigenous communities are well documented. However, the processes by which this species successfully establishes in a recipient community is still insufficiently understood. The early detection of the oyster at the island of Helgoland (North Sea) provided the ideal opportunity to investigate whether physical mechanisms, such as wave exposure, influence their successful colonisation. We hypothesized that oyster colonisation benefits from wave-protected conditions. For this purpose, we evaluated colonisation success of M. gigas among wave-protected sites and wave-exposed sites along the island’s pier system. The densities of M. gigas were significantly higher at wave-protected sites than at wave-exposed sites, and the frequency distributions of oyster lengths indicated better growth and higher survival rates in the harbours. This higher colonisation success at wave-protected sites may be explained by the relative retention time of water masses in the harbours, probably resulting in both reduced larval drift and lower energy demands for secretion formation (i.e. firmer binding to the substrate). The fact that the density of M. gigas can vary greatly on small spatial scales depending on exposure corroborates a multiple exposure sampling approach to monitor oyster populations in order to avoid potential overestimations of population sizes in given areas

Surveying seaweeds from the Ulvales and Fucales in the world’s most frequently used artificial waterway, the Kiel Canal

The Kiel Canal is one of the world’s most frequently used inland waterways and connects the SW Baltic Sea with the Wadden Sea. At the same time, the canal is a highly eutrophicated environment that is characterized by salinities that range from 3 to 16. This brackish character could make the Kiel Canal an important stepping stone for the introductions of species into the inner Baltic Sea. It could also hinder the identification of native and introduced species, given the fact that salinity sometimes severely affects algal morphology. Here we report on a survey of introduced and native seaweed species in the canal, focusing on the dominant groups, which are Fucales and Ulvales. Of the Fucales, the introduced species Fucus evanescens was detected nearly exclusively inside the canal, while Fucus vesiculosus dominated rockweed communities directly outside the sluice gates. Morphological analysis and genetic barcoding distinguished three species of Ulvales, Ulva linza, Ulva intestinalis and an unknown and possibly introduced species of the genus Blidingia. Species distributions and – in the case of U. intestinalis – branching patterns were clearly affected by salinity, while thallus sizes appeared to be affected by the specific eutrophication status of sites within the canal

A tufA metabarcoding approach for Ulva and related seaweeds

Ulva-like green algae are notoriously difficult to distinguish due to their morphological variability and/or similarity. DNA barcoding approaches are therefore currently essential for their reliable identification. However, such approaches often fail when rare or inconspicuous species are to be detected in large mixed populations of Ulva species, for example, at early stages following the introduction of species into new habitats. We therefore developed a detection method based on next-generation DNA sequencing. The approach is suitable for the analysis of DNA traces in preserved water samples or in particles enriched by filtration from such samples. A new pair of primers was designed to amplify a 475 bp segment within the tufA marker gene. The primers were relatively group specific. 68.5% of all reads obtained after quality filtering represented the genus Ulva, 11.1% other Ulvophyceae, and only 20% other Chlorophyta, despite their relatively higher abundance in phytoplankton. The relatively short target amplicon still allows good differentiation of Ulvales and Ulothrichales at the species level. Using a database containing tufA sequences of 879 species - 281 of which were Ulvophyceae and 35 Ulva - we were able to detect mostly Ulvophyceae that had been previously detected in our study area in northern Germany using Sanger sequencing. However, the number of species detected at individual sites was generally higher than in previous studies, which could be due to drifting DNA: Analysis of samples collected at different distances from shore suggests that a sample collected at a given site may be influenced by Ulvophyceae within a radius of up to about 1 km in winter. In summer, this radius is reduced to less than 100 m, possibly due to the less frequent occurrence of strong wind events. Nonetheless, rare species may be detected with this new approach: At one site, an undescribed Blidingia species that was not previously known from our study area was repeatedly detected. Based on these findings, the species was searched for and found, and its identity confirmed by traditional tufA barcoding

Mapping and modeling eelgrass Zostera marina distribution in the western Baltic Sea

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