Reverse engineering field-derived vertical distribution profiles to infer larval swimming behaviors

Temperature and nutrients alter the relative importance of stochastic and deterministic processes in the coastal macroinvertebrates biodiversity assembly on long-time scales

Macroinvertebrates play a vital role in coastal ecosystems and are an important indicator of ecosystem quality. Both anthropogenic activity and environmental changes may lead to significant changes in the marine macroinvertebrate community. However, the assembly process of benthic biodiversity and its mechanism driven by environmental factors at large scales remains unclear. Here, using the benthic field survey data of 15 years at large spatial and temporal scales from the Yellow Sea Large Marine Ecosystem, we investigated the relative importance of environmental selection, dispersal processes, random-deterministic processes of macroinvertebrates community diversity assembly, and the responses of this relative importance driven by temperature and nutrients. Results showed that the macroinvertebrates community diversity is mainly affected by dispersal. Nitrogen and phosphorus are the most important negative factors among environmental variables, while geographical distance is the main limiting factor of β diversity. Within the range of 0.35-0.70 mg/L of nutrients, increasing nutrient concentration can significantly facilitate the contribution of the decay effect to β diversity. Within the temperature range studied (15.0-18.0°C), both warming and cooling can lead to a greater tendency for species diversity assembly processes to be dominated by deterministic processes. The analysis contributes to a better understanding of the assembly process of the diversity of coastal marine macroinvertebrates communities and how they adapt to global biogeochemical processes.

Making Your Own Luck: Weak Vertical Swimming Improves Dispersal Success for Coastal Marine Larvae

Dispersive early life stages are common in nature. Although many dispersing organisms ("propagules") are passively moved by outside forces, some improve their chances of successful dispersal through weak movements that exploit the structure of the environment to great effect. The larvae of many coastal marine invertebrates, for instance, swim vertically through the water column to exploit depth-varying currents, food abundance, and predation risk. Several swimming behaviors and their effects on dispersal between habitats are characterized in the literature, yet it remains unclear when and why these behaviors are advantageous. We addressed this gap using a mathematical model of larval dispersal that scored how well behaviors allowed larvae to simultaneously locate habitats, avoid predators, and gather energy. We computed optimal larval behaviors through dynamic programming, and compared those optima against passive floating and three well documented behaviors from the literature. Optimal behaviors often (but not always) resembled the documented ones. However, our model predicted that the behaviors from the literature performed robustly well, if not optimally, across many conditions. Our results shed light on why some larval behaviors are widespread geographically and across species, and underscore the importance of carefully considering the weak movements of otherwise passive propagules when studying dispersal.

Modelled dispersal pathways of non-indigenous species in the Danish Wadden Sea

The introduction-rate of non-indigenous species (NIS) to coastal water bodies has accelerated over the last century. We present a model study assessing the fate of NIS released in likely point sources of the Danish Wadden Sea. We show that NIS-particles released in the deep North Sea are generally transported away from the Wadden Sea, while those released in the coastal North Sea and the Wadden Sea show large variability in track pattern and settlement location. Consequently, the introduction of NIS from ships entering the port of Esbjerg pose a threat to the Wadden Sea through primary and secondary spreading, while transport of species from sources in the south likely causes a slow and steady settling of NIS in the Wadden Sea and coastal North Sea. The study points to the importance of enforcing an efficient monitoring system to ensure early detection of changes to the species composition of the Wadden Sea.

Epibionts provide their basibionts with associational resistance to predation but at a cost

Epibiosis is increasingly considered a survival strategy in space-limited environments. However, epibionts can create a new interface between its host, environment and potential predators which may alter predator-prey relationships and biological functioning. Ex-situ experiments investigated the potential costs and benefits of epibiont barnacles on mortality and feeding rate of the mussel, Mytilus edulis, and its predator, the whelk Nucella lapillus. Mussels with living epibiont barnacles suffered no mortality from whelk predation, but when barnacles were absent, mortality was ∼21% over 48 days. Further comparisons revealed the structural complexity of barnacles provided mussels with protection from whelk predation, while the presence of living barnacles increased predator-prey encounters but led to predators targeting barnacles over mussels. Feeding trials revealed feeding rate increased by ∼24% in mussels with living epibionts over mussels with dead or without epibionts, indicating potential costs of hosting epibionts. Our results show that epibionts provide important associational resistance for mussels against whelk predation but a potential cost to the mussel of hosting epibionts requiring increased energy acquisition. These findings advance our understanding of associational resistance derived from epibionts and serve to highlight the potential trade-offs affecting basibiont functioning while showing the importance of positive ecological interactions in ecosystem structure and functioning.

The role of wind in controlling the connectivity of blue mussels (Mytilus edulis L.) populations

BACKGROUND

Larval connectivity between distinct benthic populations is essential for their persistence. Although connectivity is difficult to measure in situ, it can be predicted via models that simulate biophysical interactions between larval behaviour and ocean currents. The blue mussel (Mytilus Edulis L.) is widespread throughout the northern hemisphere and extensively commercialised worldwide. In the Irish Sea, this industry represents ~ 50% of Welsh shellfisheries, where cultivation is mainly based on wild spat. However, the main sources and amount of spat varied interannually (1100 tonnes harvest in 2014 against zero in 2018). The aim of this study is to characterise the structure and dynamics of the blue mussel metapopulation within the northern part of the Irish Sea.

METHODS

We develop a Lagrangian particle tracking model, driven by a high-resolution (from 30 to 5000 m) validated unstructured coastal hydrodynamic model of the Irish Sea, to simulate spatial and temporal variability of larval dispersal and connectivity between distinct mussel populations and potential settlement areas.

RESULTS

Our results showed that: (1) larvae positioned near the surface were strongly influenced by wind-driven currents suggesting that connectivity networks had the potential to span hundreds of kilometres; (2) in contrast, larvae positioned deeper in the water column were driven by tidal currents, producing intricate spatial patterns of connectivity between mussel beds over tens of kilometres that were consistent over time.

CONCLUSIONS

Dispersal of mussel larvae in the tidally energetic Irish Sea during the April-May spawning season is potentially driven by wind-driven surface currents, as confirmed by fisherman observations of inter-annual variability in wild spat collection. These results have important implications for metapopulation dynamics within the context of climate change and sustainable shellfisheries management (i.e. gain and loss of populations and harvest areas according to wind conditions).

Unaided dispersal risk of Magallana gigas into and around the UK: combining particle tracking modelling and environmental suitability scoring

Marine non-indigenous species are a significant threat to marine ecosystems with prevention of introduction and early detection considered to be the only effective management strategy. Knowledge of the unaided pathway has received relatively little attention, despite being integral to the implementation of robust monitoring and surveillance. Here, particle tracking modelling is combined with spatial analysis of environmental suitability, to highlight UK coastal areas at risk of introduction and spread of Magallana gigas by the unaided pathway. ‘Introduction into UK’ scenarios were based on spawning from the continental coast, Republic of Ireland, Channel Islands and Isle of Man and ‘spread within UK’ scenarios were based on spawning from known UK wild populations and aquaculture sites. Artificial structures were included as spawning sites in an introduction scenario. The UK coast was scored, based on parameters influencing larval settlement, to reflect environmental suitability. Risk maps were produced to highlight areas of the UK coast at elevated risk of introduction and spread of M. gigas by the unaided pathway. This study highlights that introduction of M. gigas into UK waters via the unaided pathway is possible, with offshore structures increasing the potential geographical extent of introduction. Further, there is potential for substantial secondary spread from aquaculture sites and wild populations in the UK. The results of the study are considered in the context of national M. gigas management, whilst the approach is contextualised more broadly as a tool to further understanding of a little-known, yet significant pathway.