Changes In Temperature Stratification Research Articles

Reconciling single-chamber Mg / Ca with whole-shell δ<sup>18</sup>O in surface to deep-dwelling planktonic foraminifera from the Mozambique Channel

Abstract. Most planktonic foraminifera migrate vertically through the water column during life, meeting a range of depth-related conditions as they grow and calcify. For reconstructing past ocean conditions from geochemical signals recorded in their shells, it is therefore necessary to know vertical habitat preferences. Species with a shallow habitat and limited vertical migration will reflect conditions of the surface mixed layer and short-term and mesoscale (i.e. seasonal) perturbations therein. Species spanning a wider range of depth habitats, however, will contain a more heterogeneous, intra-specimen variability (e.g. Mg / Ca and δ18O), which is less for species calcifying below the thermocline. Obtained single-chamber Mg / Ca ratios are combined with single-specimen δ18O and δ13C of the surface-water inhabitant Globigerinoides ruber, the thermocline-dwelling Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, and the deep dweller Globorotalia scitula from the Mozambique Channel. Species-specific Mg / Ca, δ13C and δ18O data combined with a depth-resolved mass balance model confirm distinctive migration and calcification patterns for each species as a function of hydrography. Whereas single-specimen δ18O rarely reflects changes in depth habitat related to hydrography (e.g. temperature), measured Mg / Ca of the last chambers can only be explained by active migration in response to changes in temperature stratification. Foraminiferal geochemistry and modelled depth habitats shows that the single-chamber Mg / Ca and single shell δ18O are in agreement with each other and in line with the changes in hydrography induced by eddies.

Modelling the distribution of Anabaena and Melosira in a stratified river weir pool

We have developed a model to predict the stratification dynamics and its influence on the distribution of algal species in a river with strong diurnal changes in temperature stratification. While the phytoplankton concentrations vary in response to buoyancy and light limited growth, the populations are also redistributed vertically due to diurnal mixing caused by surface evaporation and night time cooling. The parameterization of the vertical eddy diffusivity coefficient within the model takes into account the generation of mixing by heat losses at the surface and the suppression of mixing by thermal stratification. The model is successfully tested against extensive temperature and algal concentration data in Maude weir pool on the Murrumbidgee River, Australia. Given literature values of floating/sinking rates and maximum growth rates for the cyanobacterium Anabaena and the diatom Melosira, we were able to reproduce extremely well the time evolution of concentrations of each species over long periods of low flow.

Experimental and observational studies of radiolarian physiological ecology: 4. Factors determining the distribution and survival of Didymocyrtis tetrathalamus tetrathalamus with implications for paleoecological interpretations

The mean growth, longevity and vitality (based on axopodial morphology and cytoplasmic flow) of Didymocyrtis tetrathalamus tetrathalamus were assessed in laboratory culture in relation to variations in temperature, salinity and light. Maximum longevity and vitality occurred in a temperature range of 21°C to 27°C. However, some individuals survived for extended periods (up to 18 days) with normal axopodia and cytoplasmic flow at temperatures as low as 10°C. Though abundant in tropical and subtropical locations and often thought to be exclusively warmer-water species, these data suggest D. tetrathalamus may have a lower temperature tolerance than indicated by it geographical location. This is in contrast to Spongaster tetras (a species found in the same locale) that is decidedly disabled by temperatures below approximately 20°C. Moreover, abundance of D. tetrathalamus in the natural environment near Barbados increased substantially during periods of cooler surface water temperatures associated with changes in temperature stratification of the upper water column. These data suggests that although D. tetrathalamus is typically found in low to mid latitude tropical and subtropical locations, they may not be restricted to warmer water masses assuming other environmental and trophic factors are favorable. Food vacuole contents, based on transmission electron microscopic evidence, include large proportions of diatoms, other algae and small masses of cytoplasm that lack plastids and appear to be microheterotrophs. Microflagellates (Kinetoplastida) are observed within the peripheral cytoplasm and may be consumed as prey. Eubacteria are only occasionally observed in digestive vacuoles. Partially dissolved fragments of diatom frustules are incorporated into the skeleton of D. tetrathalamus and may be used as a source of skeletal material in low silicate environments. The composite data from this research suggests that D. tetrathalamus is highly adaptive and may be capable of surviving in a broader range of trophic and physical environmental conditions than previously assumed.