Defence chemistry modulation by light and temperature (results in 4 excel files, zipped)

The goals of this study were (1) to investigate whether Fucus vesiculosus regulates the production of its antifouling defence chemicals against microfoulers in response to light limitation and temperature shifts and (2) to investigate if different surface concentrations of defence compounds shape epibacterial communities. F. vesiculosus was incubated in indoor mesocosms at five different temperature conditions (5 to 25°C) and in outdoor mesocosms under six differently reduced sunlight conditions (0 to 100%), respectively. Algal surface concentrations of previously identified antifouling compounds - dimethylsulphopropionate (DMSP), fucoxanthin and proline - were determined and the bacterial community composition was characterized by in-depth sequencing of the 16S-rRNA gene. Altogether, the effect of different treatment levels upon defence compound concentrations was limited. Under all conditions DMSP alone appeared to be sufficiently concentrated to warrant for at least a partial inhibitory action against epibiotic bacteria of F. vesiculosus. In contrast, proline and fucoxanthin rarely reached the necessary concentration ranges for self-contained inhibition. Nonetheless, in both experiments along with the direct influence of temperature and light, all three compounds apparently affected (and thereby shaped) the overall bacterial community composition associated with F. vesiculosus since tendencies for insensitivity towards all three compounds were observed among bacterial taxa that typically dominate those communities. Given that the concentrations of at least one of the compounds (in most cases DMSP) were always high enough to inhibit bacterial settlement, we conclude that the capacity of F. vesiculosus for such defence will hardly be compromised by shading or warming to temperatures up to 25°C.

Raw data of SCADCP (self-contained Acoustic Doppler Current Profiler) from three moorings in the Eastern Weddell Sea, 2005-2008

The success of any efforts to determine the effects of climate change on marine ecosystems depends on understanding in the first instance the natural variations, which contemporarily occur on the interannual and shorter time scales. Here we present results on the environmental controls of zooplankton distribution patterns and behaviour in the eastern Weddell Sea, Southern Ocean. Zooplankton abundance and vertical migration are derived from the mean volume backscattering strength (MVBS) and the vertical velocity measured by moored acoustic Doppler current profilers (ADCPs), which were deployed simultaneously at 64°S, 66.5°S and 69°S along the Greenwich Meridian from February, 2005, until March, 2008. While these time series span a period of full three years they resolve hourly changes. A highly persistent behavioural pattern found at all three mooring locations is the synchronous diel vertical migration (DVM) of two distinct groups of zooplankton that migrate between a deep residence depth during daytime and a shallow depth during nighttime. The DVM was closely coupled to the astronomical daylight cycles. However, while the DVM was symmetric around local noon, the annual modulation of the DVM was clearly asymmetric around winter solstice or summer solstice, respectively, at all three mooring sites. DVM at our observation sites persisted throughout winter, even at the highest latitude exposed to the polar night. Since the magnitude as well as the relative rate of change of illumination is minimal at this time, we propose that the ultimate causes of DVM separated from the light-mediated proximal cue that coordinates it. In all three years, a marked change in the migration behaviour occurred in late spring (late October/early November), when DVM ceased. The complete suspension of DVM after early November is possibly caused by the combination of two factors: (1) increased availability of food in the surface mixed layer provided by the phytoplankton spring bloom, and (2) vanishing diurnal enhancement of the threat from visually oriented predators when the illumination is quasi-continuous during the polar and subpolar summer. Zooplankton abundance in the water column, estimated as the mean MVBS in the depth range 50-300 m, was highest end of summer and lowest mid to end winter on the average annual cycle. However, zooplankton abundance varied several-fold between years and between locations. Based on satellite and in situ data of chlorophyll and sea ice as well as on hydrographic measurements, the interannual and spatial variations of zooplankton mean abundance can be explained by differences in the magnitude of the phytoplankton spring bloom, which develops during the seasonal sea ice retreat. Whereas the vernal ice melt appears necessary to stimulate the blooming of phytoplankton, it is not the determinator of the blooms magnitude, its areal extent and duration. A possible explanation for the limitation of the phytoplankton bloom in some years is top-down control. We hypothesise that the phytoplankton spring development can be curbed by grazing when the zooplankton had attained high abundance by growth during the preceding summer.