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 role of advection in the distribution of plankton populations at a moored 1-D coastal observatory

The degree to which advection modulates the distribution of plankton populations at a 1-D coastal observatory was assessed at station L4 in the western English Channel (50°15′N 4°13′W, depth 50 m), part of the Western Channel Observatory (WCO). Five tidal-cycle surveys were conducted, three in spring and two in summer 2010. Observations of the physical characteristics of L4 were obtained by using a moored acoustic doppler current profiler (ADCP) and a free-falling microstructure sensor (MSS). The moored ADCP highlighted the presence of vertical shear, with typical values of U during spring tides of ∼0.5 m s−1 at the surface and ∼0.2 m s−1 at the bed. The distribution of phyto- and zooplankton populations above a size threshold of 200 μm were examined using an in-line holographic imaging system, the Holocam. Variability in time as well as depth is a common feature throughout each of the surveys, with examples of recorded numbers of phytoplankton that ranged between 1300 L−1 and 2300 L−1 at the same depth but at different points within the tidal cycle. Further, at the same points in the tidal cycle the number of recorded zooplankton was also seen to vary, specifically with the identification of gelatinous planula in spring that increased the observed number to maximums of between 140 L−1 and 220 L−1 in the upper layer, considerably higher that the corresponding WP-2 net counts for a similar period. Specific aspects of the movement and transfer of plankton relating to advection and interaction with the pycnocline are identified, both across tidal cycles and seasons.