A Note On Tidal Asymmetry In The Severn Estuary

A one-dimensional, non-linear numerical model is used to investigate the tidally averaged frictional stress and set-up of water level due to tidal asymmetry in the Severn Estuary; these quantities depend on the overtides in the region. A linearized model of the overtides is applied to calculations of the M4 currents in order to delineate the mechanisms responsible for their generation. The relative importance of individual non-linear mechanisms to the tidally averaged stress and set-up is determined; these mechanisms are interactions between tidal flow and changes in depth or breadth over a cross-section, frictional interaction between the tidal flow and Stokes drift, interaction between the tidal fluctuations in water depth and frictional retardation and non-linear advection.

High dispersal potential has maintained long-term population stability in the North Atlantic copepod Calanus finmarchicus

The cool-water copepod Calanus finmarchicus is a key species in North Atlantic marine ecosystems since it represents an important food resource for the developmental stages of several fish of major economic value. Over the last 40 years, however, data from the Continuous Plankton Recorder survey have highlighted a 70 per cent reduction in C. finmarchicus biomass, coupled with a gradual northward shift in the species's distribution, which have both been linked with climate change. To determine the potential for C. finmarchicus to track changes in habitat availability and maintain stable effective population sizes, we have assessed levels of gene flow and dispersal in current populations, as well as using a coalescent approach together with palaeodistribution modelling to elucidate the historical population demography of the species over previous changes in Earth's climate. Our findings indicate high levels of dispersal and a constant effective population size over the period 359 000–566 000 BP and suggest that C. finmarchicus possesses the capacity to track changes in available habitat, a feature that may be of crucial importance to the species's ability to cope with the current period of global climate change.

The dispersal of phytoplankton populations by enhanced turbulent mixing in a shallow coastal sea

A single tidal cycle survey in a Lagrangian reference frame was conducted in autumn 2010 to evaluate the impact of short-term, episodic and enhanced turbulent mixing on large chain-forming phytoplankton. Observations of turbulence using a free-falling microstructure profiler were undertaken, along with near-simultaneous profiles with an in-line digital holographic camera at station L4 (50° 15′ N 4° 13′ W, depth 50 m) in the Western English Channel. Profiles from each instrument were collected hourly whilst following a drogued drifter. Results from an ADCP attached to the drifter showed pronounced vertical shear, indicating that the water column structure consisted of two layers, restricting interpretation of the Lagrangian experiment to the upper ~ 25 m. Atmospheric conditions deteriorated during the mid-point of the survey, resulting in values of turbulent dissipation reaching a maximum of 10− 4 W kg− 1 toward the surface in the upper 10 m. Chain-forming phytoplankton > 200 μm were counted using the data from the holographic camera for the two periods, before and after the enhanced mixing event. As mixing increased phytoplankton underwent chain breakage, were dispersed by advection through their removal from the upper to lower layer and subjected to aggregation with other suspended material. Depth averaged counts of phytoplankton were reduced from a maximum of around 2050 L− 1 before the increased turbulence, to 1070 L− 1 after, with each of these mechanisms contributing to this reduction. These results demonstrate the sensitivity of phytoplantkon populations to moderate increases in turbulent activity, yielding consequences for accurate forecasting of the role played by phytoplankton in climate studies and also for the ecosystem in general in their role as primary producers.

Historical data reveal power-law dispersal patterns of invasive aquatic species.

Understanding how invasive species spread is of particular concern in the current era of globalisation and rapid environmental change. The occurrence of super-diffusive movements within the context of Lévy flights has been discussed with respect to particle physics, human movements, microzooplankton, disease spread in global epidemiology and animal foraging behaviour. Super-diffusive movements provide a theoretical explanation for the rapid spread of organisms and disease, but their applicability to empirical data on the historic spread of organisms has rarely been tested. This study focuses on the role of long-distance dispersal in the invasion dynamics of aquatic invasive species across three contrasting areas and spatial scales: open ocean (north-east Atlantic), enclosed sea (Mediterranean) and an island environment (Ireland). Study species included five freshwater plant species, Azolla filiculoides, Elodea canadensis, Lagarosiphon major, Elodea nuttallii and Lemna minuta; and ten species of marine algae, Asparagopsis armata, Antithamnionella elegans, Antithamnionella ternifolia, Codium fragile, Colpomenia peregrina, Caulerpa taxifolia, Dasysiphonia sp., Sargassum muticum, Undaria pinnatifida and Womersleyella setacea. A simulation model is constructed to show the validity of using historical data to reconstruct dispersal kernels. Lévy movement patterns similar to those previously observed in humans and wild animals are evident in the re-constructed dispersal pattern of invasive aquatic species. Such patterns may be widespread among invasive species and could be exacerbated by further development of trade networks, human travel and environmental change. These findings have implications for our ability to predict and manage future invasions, and improve our understanding of the potential for spread of organisms including infectious diseases, plant pests and genetically modified organisms.

Biophysical models of larval dispersal in the Benguela Current ecosystem

We synthesise and update results from the suite of biophysical, larval-dispersal models developed in the Benguela Current ecosystem. Biophysical models of larval dispersal use outputs of physical hydrodynamic models as inputs to individual-based models in which biological processes acting during the larval life are included. In the Benguela, such models were first applied to simulate the dispersal of anchovy Engraulis encrasicolus and sardine Sardinops sagax ichthyoplankton, and more recently of the early life stages of chokka-squid Loligo reynaudii and Cape hakes Merluccius spp. We identify how the models have helped advance understanding of key processes for these species. We then discuss which aspects of the early life of marine species in the Benguela Current ecosystem are still not well understood and could benefit from new modelling studies.

The effects of dispersal mode on the spatial distribution patterns of intertidal molluscs.

1. As many species of marine benthic invertebrates have a limited capacity for movement as adults, dispersal mode is often considered as a determinant of geographical ranges, genetic structure and evolutionary history. Species that reproduce without a larval stage can only disperse by floating or rafting. It is proposed that the colonization processes associated with such direct developing species result in spatial distributions that show relatively greater fine scale patchiness than the distributions of species with a larval dispersal stage. This hypothesis was tested by collecting molluscs at different spatial scales in the Isle of Man. 2. Spatial distribution patterns supported the predictions based on dispersal mode. Estimated variance components for species with larval dispersal suggested that the majority of the spatial variation was associated with variation between shores. In comparison, there was relatively more variability within shores for abundance counts of species with direct development. 3. Multivariate analyses reflected the univariate results. An assemblage of direct developers provided a better discrimination between sites (100 m separation) but the group of species with larval dispersal gave a clearer separation of shores (separated by several km). 4. The fine scale spatial structure of direct developing species was reflected in higher average species diversity within quadrats. Species richness also reflected dispersal mode, with a higher fraction of the regional species pool present for direct developers in comparison to species with larval dispersal. This may reflect the improved local persistence of taxa that avoid the larval dispersal stage.

Dispersal mode and assessments of recovery on the shores of Gruinard, the 'anthrax island'

Formaldehyde run-off was an unintended impact of the anthrax decontamination procedure on the island of Gruinard. The death of intertidal organisms was observed where formaldehyde reached the shore during 1986. The extent to which shores on Gruinard have recovered was assessed with survey work in 2000. Recovery estimates were based on the hypothesis that the process of recolonization is partly dependent on species' dispersal capability. Underdevelopment of the assemblage of species lacking planktonic dispersal stages (direct developers) is therefore evidence that the process of recolonization is ongoing, rather than complete. A novel multivariate test showed that, when comparing quadrats from Gruinard and nearby mainland shores, assemblages of direct developing molluscs were significantly more distinct than assemblages of molluscs with planktonic dispersal stages. The average densities of species with direct development were generally lower on Gruinard than on mainland shores. While some species with direct development have similar densities on Gruinard and on the surrounding shores, the recovery of the overall assemblage was still incomplete after 14 years. In contrast, the harvested species, Littorina littorea, appeared to benefit from the absence of humans visiting Gruinard's shores. Populations of L. littorea on Gruinard contained significantly higher proportions of large individuals. Depending on the dispersal capabilities of different species, Gruinard is either still in recovery or acts as a reserve.

Widespread dispersal of the microsporidian Nosema ceranae, an emergent pathogen of the western honey bee, Apis mellifera

The economically most important honey bee species, Apis mellifera, was formerly considered to be parasitized by one microsporidian, Nosema apis. Recently, [Higes, M., Martin, R., Meana, A., 2006. Nosema ceranae, a new microsporidian parasite in honeybees in Europe, J. Invertebr. Pathol. 92, 93-95] and [Huang, W.-F., Jiang, J.-H., Chen, Y.-W., Wang, C.-H., 2007. A Nosema ceranae isolate from the honeybee Apis mellifera. Apidologie 38, 30-37] used 16S (SSU) rRNA gene sequences to demonstrate the presence of Nosema ceranae in A. mellifera from Spain and Taiwan, respectively. We developed a rapid method to differentiate between N. apis and N. ceranae based on PCR-RFLPs of partial SSU rRNA. The reliability of the method was confirmed by sequencing 29 isolates from across the world (N = 9 isolates gave N. apis RFLPs and sequences, N = 20 isolates gave N. ceranae RFLPs and sequences; 100%, correct classification). We then employed the method to analyze N = 115 isolates from across the world. Our data, combined with N = 36 additional published sequences demonstrate that (i) N. ceranae most likely jumped host to A. mellifera, probably within the last decade, (ii) that host colonies and individuals may be co-infected by both microsporidia species, and that (iii) N. ceranae is now a parasite of A. mellifera across most of the world. The rapid, long-distance dispersal of N. ceranae is likely due to transport of infected honey bees by commercial or hobbyist beekeepers. We discuss the implications of this emergent pathogen for worldwide beekeeping. (c) 2007 Elsevier Inc. All rights reserved.

High dispersal potential has maintained long-term population stability in the North Atlantic copepod Calanus finmarchicus

The cool-water copepod Calanus finmarchicus is a key species in North Atlantic marine ecosystems since it represents an important food resource for the developmental stages of several fish of major economic value. Over the last 40 years, however, data from the Continuous Plankton Recorder survey have highlighted a 70 per cent reduction in C. finmarchicus biomass, coupled with a gradual northward shift in the species's distribution, which have both been linked with climate change. To determine the potential for C. finmarchicus to track changes in habitat availability and maintain stable effective population sizes, we have assessed levels of gene flow and dispersal in current populations, as well as using a coalescent approach together with palaeodistribution modelling to elucidate the historical population demography of the species over previous changes in Earth's climate. Our findings indicate high levels of dispersal and a constant effective population size over the period 359 000-566 000 BP and suggest that C. finmarchicus possesses the capacity to track changes in available habitat, a feature that may be of crucial importance to the species's ability to cope with the current period of global climate change.