Effects of ultraviolet-B radiation on phytoplankton - zooplankton interactions = [Effecten van ultraviolet-B straling op interacties tussen fytoplankton en zooplankton]

The decrease in stratospheric ozone concentration has received wide attention because the ozone layer protects the earth from harmful ultraviolet-B radiation (UVB, 280-320 nm). UVB radiation is harmful for organisms, and therefore scientific research into how UVB radiation affects organisms and ecosystems receives great interest. This thesis describes the effects of UVB radiation on interactions between phytoplankton (algae) and zooplankton (waterfleas) in freshwater ecosystems.The underlying hypothesis in this thesis is that phytoplankton is directly affected by UVB radiation because it needs visible light (PAR, 400-700 nm) for photosynthesis, and is consequently also exposed to UVB. Zooplankton on the other hand is not dependent on light, and is able to move actively through the water. This capability of vertical migration, and the possible ability to detect UV radiation may allow zooplankters to regulate their exposure to UVB. Therefore, indirect effects on zooplankton through changes in phytoplankton (its food) may play a more important role than direct UVB effects.UVB, UVA (320-400 nm) and PAR radiation were measured in 19 aquatic systems in the Netherlands. In most systems the penetration of UVB radiation was limited to the upper decimetres. High phytoplankton biomass or high concentrations of humic acids caused the limited light penetration. Lake Maarsseveen was the clearest system in this study with a vertical attenuation coefficient (K d ) for UVB of 9.1 (m -1). This corresponded to a 1% UVB penetration depth of 51 cm. The effect of UVB radiation on a system will be a combination of penetration depth, mixing processes, and depth of the system.There is a number of UV-mediated qualitative changes in phytoplankton that may affect herbivorous zooplankton. Changes in phytoplankton species composition, increase in cell size and increase in cell wall thickness will negatively affect ingestion and digestion by zooplankton. Phytoplankton cell biochemistry is also affected by UVB radiation, namely a decrease in lipids and proteins, and an increase in carbohydrates. This will strongly influence the cells nutrient quality. The potential negative effect of UV on fatty acids (FA) in phytoplankton call for special attention. FA play a major role in nutrition for most animals, and as such UV-mediated lipid peroxidation or reduced biosynthesis of essential FA could be a major determinant of food quality for aquatic herbivores.Experiments with zooplankton grazing on UVB-stressed phytoplankton showed varying effects on grazing rates. Grazing rates of the large waterflea Daphnia were not affected by UVB-stressed phytoplankton. The small waterflea Bosmina and small rotifer Brachionus had both increased and decreased grazing rates on UVB-stressed phytoplankton. These experiments suggest that possible effects of enhanced UVB radiation on phytoplankton-zooplankton interactions are not straightforward predictable from grazing experiments. Moreover, extrapolation of data to predict ecosystem response to UVB stress seems unjustified.Life history experiments with Daphnia feeding on several species of UVB-stressed phytoplankton showed that life history traits were negatively affected. Effects of UVB-stressed phytoplankton on the population growth rate of Daphnia were not significant. Effects on clutch sizes and quality of offspring were significant. In general, a smaller number of juveniles of poorer quality was produced in the UVB treatments. This may have implications for the food web functioning. The UVB effect was dependent on the phytoplankton species. The UVB effects may be caused by a change in food quality combined with possible reduced digestibility of UVB stressed phytoplankton.No UVB effects were found in the experiments with indoor model ecosystems. UVB radiation had no significant effect on the phytoplankton, zooplankton, periphyton or macro-invertebrate communities in these model ecosystems. A bio-assay with Daphnia feeding on phytoplankton from the model ecosystems showed that phytoplankton from the UVB treatments had a negative effect on Daphnia growth and survival, and to a lesser extent on fecundity. These results indicate that the transfer of energy from phytoplankton to zooplankton can be negatively influenced by UVB radiation. Overall, the model ecosystems were not affected by the UVB stress. From these results it can be concluded that a natural ecosystem with similar penetration of UVB can be resistant to UVB radiation presently occurring at temperate latitude.Field experiments at 3 different latitudes (in the Netherlands, Norway and Spitsbergen) showed that present levels of UVB radiation may negatively affect phytoplankton and zooplankton. The different phytoplankton species showed different responses in growth rate to UVB, UVA and PAR radiation. This implies that increased levels of UVB radiation could lead to shifts in phytoplankton community structure. Chlamydomonas (a green algae with flagella) responded to UVB radiation with a loss of flagella, whereas the growth rate was not affected.Daphnia in Lake Zwemlust (the Netherlands) responded to UVB with a decreased grazing rate and smaller body size, whereas the survival was not affected. Daphnia grazing rates were affected in all three locations, though variance was large and differences marginal. This shows that solar UVB radiation in potential can reduce the transfer of energy from phytoplankton to zooplankton. UVB effects were comparable between the three locations. The magnitude of the UVB effects was quite different, due to very different weather conditions.A field study was done in the province of Saskatchewan, Canada, comparing seston (suspended live and dead particles) quantity and quality of ponds and lakes with different light penetration properties. Multivariate analysis suggested that the phytoplankton species composition was influenced by the light climate of the studied system. A standardized laboratory experiment with Daphnia showed that the phospholipid content of the seston was the best predictor of Daphnia growth, because of the high nutritional value of phospholipids. It was hypothesized that in the studied systems, light climate and especially the penetration of ultraviolet radiation was important in determining the phospholipid content of the seston. The proposed relationships between light climate, phospholipid content of the seston, and Daphnia growth need experimental confirmation.Summarizing the different experiments described in this thesis, the effects of UVB on the phytoplankton-zooplankton interactions were present, detectable, and mostly negative. The magnitude of the UVB effects was not large. It was not possible to make generalizations because of the species-specific reactions to UVB. The experiments were all short-term, and it may be erroneous to make long-term predictions based on these results. However, the subtle differences found in this study may be important in determining ecosystem functioning. UVB radiation interacts with other environmental variables (such as temperature, nutrient concentrations, and vertical mixing), and is already playing a role in the functioning of an ecosystem.See also:project descriptionandc.v. of the author.