Sobre la matriz de análisis de los syllabus

The results of this study have shown that phytoplankton growth has a significant influence on air bubble residence time in seawater. The parameter that was directly influenced by phytoplankton photosynthesis and that was identified as a necessary precondition for major changes of BRT was the oxygen saturation of the water. Furthermore, during the course of growth experiments, BRT corresponded well with the photosynthesis-respiration (light- dark cycle) induced changes in oxygen saturation, for oxygen saturations below 150%, emphasising the dependency of BRT on the oxygen saturation of the water. The results of the saturation experiments with deionised water however indicate that oxygen

supersaturation on its own in a non biological, particle-free system does not show any significant influence on BRT. Changing the total gas saturation of the water by bubbling with air and by temperature variations had a greater influence on BRT than increasing the oxygen saturation alone, indicating that in non-biological systems, nitrogen saturation seems to be the more important factor influencing BRT. However, the importance of oxygen saturation for BRT in relation to phytoplankton growth becomes obvious considering the two growth experiments, when, despite a large increase in chlorophyll concentration, the water remained undersaturated with oxygen and consequently no significant increase in BRT occurred. Thus it can be concluded that the major factor that governs BRT is the dissolution of small air bubbles. Increasing supersaturation slows down the dissolution of small bubbles and the diffusion of oxygen from the supersaturated tank water, counter-balancing the pressure gradient and leading to the stabilisation and growth of small bubbles. The phytoplankton growth experiments have shown that a certain threshold in oxygen saturation has to be achieved for major changes to occur in BRT, so that small bubbles do not dissolve immediately after their formation but can remain in the water for longer periods of time. This threshold was found to be between 110-140% oxygen saturation and is considered to be specific with respect to the species composition of the phytoplankton.

However, the fact that oxygen saturation on its own showed no significant effect on BRT as indicated by the gas saturation experiments leads to the conclusion that BRT in

phytoplankton growth experiments is not only dependent on the saturation of the water with oxygen but is also determined by other phytoplankton-related parameters.

From the findings of several other investigations that were presented in the introductory part of this study it became apparent that phytoplankton produce surface active organic material, that is capable of reducing the surface tension of air-water interfaces and thus

should have significant influence on the size, surface structure, rise velocity and gas diffusion of air bubbles in water. However, it was not possible within the scope of this study to come to a definite conclusion about the production and influence of surface active material by phytoplankton and its effects on BRT. The results have shown that over the different growth phases of a phytoplankton experiment, the concentration of DOC in the water increased, indicating that the algae in the tank system produced significant amounts of DOM. It was not possible to detect greater surface activity in the samples with high DOC concentration, however, reasons for this are unclear and may well be linked to methodological limitations in comparison to the amounts and surface activity of material produced. However, it is also possible that the material produced by phytoplankton during senescence, when DOC concentrations increased was not surface active. The fact that

Nitzschia closterium had a significant effect on the surface shear viscosity demonstrates that at least some phytoplankton species have the ability to build up adsorption layers and thus alter the mechanical properties of an air-water interface. Nonetheless, resulting from the findings of other authors and indications in the results of the phytoplankton growth experiments (e.g. increasing DOC concentration and influence of Nitzschia closterium on the surface shear viscosity) it is very likely that during the phytoplankton growth

experiments, dissolved as well as particulate organic matter accumulated on the surfaces of the bubbles and thus decelerated their rise velocity, prevented their ability to coalesce and inhibited bubble dissolution. The assumption that other parameters in addition to oxygen saturation caused the increases in BRT during phytoplankton growth is reinforced by the fact that the regression models of oxygen saturation differed significantly for the

phytoplankton growth experiments. However, this part of the study remains uncertain as the design of the laboratory tank system and the acoustic determination of BRT

unfortunately provided no direct information on the rise velocity of bubbles, their size distributions and their size changes as they rose through the water column. But the fact that even in undersaturated deionised water, where bubble dissolution proceeds rapidly, the addition of large amounts of model substances, which are known to alter the mechanical properties of an interface, the viscosity and the surface tension resulted in longer BRT, demonstrates that changes of the interfacial properties of air bubbles have strong effects. Thus it is very likely that BRT was to some extent influenced by changes in interfacial properties during the phytoplankton growth experiments.