Catharina J.M. Philippart

Large-Scale Spatial Dynamics of Intertidal Mussel (Mytilus edulis L.) Bed Coverage in the German and Dutch Wadden Sea

Intertidal blue mussel beds are important for the functioning and community composition of coastal ecosystems. Modeling spatial dynamics of intertidal mussel beds is complicated because suitable habitat is spatially heterogeneously distributed and recruitment and loss are hard to predict. To get insight into the main determinants of dispersion, growth and loss of intertidal mussel beds, we analyzed spatial distributions and growth patterns in the German and Dutch Wadden Sea. We considered yearly distributions of adult intertidal mussel beds from 36 connected tidal basins between 1999 and 2010 and for the period 1968–1976. We found that in both periods the highest coverage of tidal flats by mussel beds occurs in the sheltered basins in the southern Wadden Sea. We used a stochastic growth model to investigate the effects of density dependence, winter temperature and storminess on changes in mussel bed coverage between 1999 and 2010. In contrast to expectation, we found no evidence that cold winters consistently induced events of synchronous population growth, nor did we find strong evidence for increased removal of adult mussel beds after stormy winter seasons. However, we did find synchronic growth within groups of proximate tidal basins and that synchrony between distant groups is mainly low or negative. Because the boundaries between synchronic groups are located near river mouths and in areas lacking suitable mussel bed habitat, we suggest that the metapopulation is under the control of larval dispersal conditions. Our study demonstrates the importance of moving from simple habitat suitability models to models that incorporate metapopulation processes to understand spatial dynamics of mussel beds. The spatio-dynamic structure revealed in this paper will be instrumental for that purpose.

Consensus forecasting of intertidal seagrass habitat in the Wadden Sea

1. After the dramatic eutrophication-induced decline of intertidal seagrasses in the 1970s, theWadden Sea has shown diverging developments. In the northern Wadden Sea, seagrass bedshave expanded and become denser, while in the southern Wadden Sea, only small beds withlow shoot densities are found. A lack of documentation of historical distributions hampersconservation management. Yet, the recovery in the northern Wadden Sea provides opportunityto construct robust habitat suitability models to support management.2. We tuned habitat distribution models based on 17 years of seagrass surveys in the northernWadden Sea and high-resolution hydrodynamics and geomorphology for the entire WaddenSea using five machine learning approaches. To obtain geographically transferablemodels, hyperparameters were tuned on the basis of prediction accuracy assessed by non-random,spatial cross-validation. The spatial cross-validation methodology was combined with aconsensus modelling approach.3. The predicted suitability scores correlated amongst each other and with the hold-out observationsin the training area indicating that the models converged and were transferable acrossspace. Prediction accuracy was improved by averaging the predictions of the best models.4. We graphically examined the relationship between the consensus suitability score andindependent presence-only data from outside the training area using the area-adjusted seagrassfrequency per suitability class (continuous Boyce index). The Boyce index was positivelycorrelated with the suitability score indicating the adequacy of the prediction methodology.5. We used the plot of the continuous Boyce index against habitat suitability score to demarcatethree habitat classes – unsuitable, marginal and suitable – for the entire international WaddenSea. This information is valuable for habitat conservation and restoration management.6. Divergence between predicted suitability and actual distributions from the recent past indicatesthat unaccounted factors limit seagrass development in the southern Wadden Sea.7. Synthesis and applications. Our methodology and data enabled us to produce a robust andvalidated consensus habitat suitability model. We identified highly suitable areas where intertidalseagrass meadows may establish and persist. Our work provides scientific underpinningfor effective conservation planning in a dynamic landscape and sets monitoring priorities.

Trophic relationship between the invasive parasitic copepod Mytilicola orientalis and its native blue mussel ( Mytilus edulis ) host

Invasive parasites can spill over to new hosts in invaded ecosystems with often unpredictable trophic relationships in the newly arising parasite-host interactions. In European seas, the intestinal copepod Mytilicola orientalis was co-introduced with Pacific oysters (Magallana gigas) and spilled over to native blue mussels (Mytilus edulis), with negative impacts on the condition of infected mussels. However, whether the parasite feeds on host tissue and/or stomach contents is yet unknown. To answer this question, we performed a stable isotope analysis in which we included mussel host tissue and the primary food sources of the mussels, microphytobenthos (MPB) and particulate organic matter (POM). The copepods were slightly enriched in δ15N (mean Δ15N ± s.d.; 1·22 ± 0·58‰) and δ13C (Δ13C 0·25 ± 0·32‰) with respect to their host. Stable isotope mixing models using a range of trophic fractionation factors indicated that host tissue was the main food resource with consistent additional contributions of MPB and POM. These results suggest that the trophic relationship of the invasive copepod with its mussel host is parasitic as well as commensalistic. Stable isotope studies such as this one may be a useful tool to unravel trophic relationships in new parasite-host associations in the course of invasions.

North Sea coastal ecology: Preface

Worldwide, the marine environment and especially coastal zones show strong spatial and temporal variability in physical conditions and ecology, and are at the same time affected by a variety of anthropogenic influences. These waters include the European North Sea, a semi-enclosed shelf sea surrounded by the United Kingdom, Belgium, the Netherlands, Germany, Denmark and Norway. Long-term (millennia) inhabitation of these coastal areas has resulted in a strong anthropogenic footprint such as polluted waters, deteriorated marine habitats and depleted fish stocks. Ongoing and future impacts include the combined effects of climate change (e.g., warming, acidification, deoxygenation), and modern contaminants (e.g. microplastics). The consequences of human impacts on the North Sea proper is determined, amongst others, by the interactions between the central basin and the surrounding intertidal basins, estuaries and tributaries at one hand and the Atlantic Ocean at the other. Over the years, symposia have been held and numerous papers were published focusing on various aspects of the North Sea dynamics. This volume of the Journal of Sea Research does not intend to summarize the present state of knowledge concerning North Sea ecology, but should be considered as a broad (though not exhaustive) overview of ongoing research especially in the coastal zone and with an emphasis on spatial and temporal variabilities, with the aim to sketch some lines of future research based on in total 19 contributions.