A major issue facing nuclear power stations is how to effectively deal with radioactive waste. This waste, as it comes from a reactor, is emitting large quantities of ionizing radiation which is usually confined. Another form of radioactive wastes is the mill tailings from uranium processing plants. These tailings are sites characterized by low-level chronic radiation. The mill tailings of the Vitro Chemical Plant, in Salt Lake City, Utah, have been a point of radionuclide concentration and environmental contamination for 20 years. These tailings may adversely affect both surrounding ecosystems and any biological systems seeking to become established on the site. To test the potential hazard of this site to the succession of plant species I examined the interphase chromosome volume and relative amounts of DNA per chromosome from plants growing on this site and those on a control site. These nuclear parameters indicate the relative radio-sensitivity of a species and would demonstrate the total effectiveness of the low-level chronic radiation in altering plant succession. The radiosensitive plant Tradescantia clone 02 was also grown in five soil samples from the mill tailings which represented a progressive increase in radioactivity. The purpose was to determine how effective these radiation levels are in altering reproductive integrity, fecundity, and somatic mutation rates in radiosensitive plant species. There was a difference in species composition between plant communities growing on the mill tailings as compared to the controls as determined by coefficient of community. However, there was no difference in interphase chromosome volume or relative amounts of DNA per chromosome between plants growing on these two sites. The difference in species composition is attributed to the length of time each site has been undergoing succession, with the control site in a more advanced stage. Tradescantia grown in soil with a radiation dose greater than 0.10 mR/hr had significantly reduced reproductive integrity and fecundity, as measured by the number of stunted hairs on a stamen and poll en viability, and increased numbers of somatic mutations. Based on these data the radioactive mill tailings from the Vitro Chemical Plant have the potential to alter plant successional patterns due to their detrimental effect on any species that is relatively radiosensitive.

Future climate scenarios in the Baltic Sea project increasing sea surface temperature, as well as increasing precipitation and river runoff resulting in decreased salinity. These changes can severely impact the dynamics and function of brackish water communities, specifically phytoplankton. Phytoplankton are a significant source of organic matter to other trophic levels, and some species can be toxic. Their response to future climate conditions is of great relevance for the health of humans and aquatic ecosystems. The aim of this thesis was to assess the potential for climate-induced changes, such as decreasing salinity, to affect phytoplankton dynamics, physiology and chemical profiles in the Baltic Sea.      Phytoplankton successional patterns in the Baltic Proper consist of a spring bloom where diatoms and dinoflagellates co-occur and a summer bloom dominated by filamentous/colonial cyanobacteria. The consensus is that future warmer conditions will promote filamentous/colonial cyanobacteria blooms. This thesis shows that phytoplankton biomass in the spring bloom was lower in years with milder winters compared with cold winters. This suggests that in terms of annual carbon export to higher trophic levels, loss of biomass from the spring bloom is unlikely to be compensated by summer cyanobacteria. High frequency sampling of phytoplankton performed in this thesis revealed a strong relationship between the dynamics of pico- and filamentous cyanobacteria. Large genetic diversity was found in cyanobacterial populations with high niche differentiation among the same species. At community level, high temperature and low salinity were the main factors shaping the summer cyanobacterial composition. These conditions may promote the predominance of opportunistic filamentous cyanobacteria, e.g. Nodularia spumigena. This species produces various bioactive compounds, including non-ribosomal peptides such as the hepatotoxin nodularin. In this work, N. spumigena subpopulations evolved different physiological strategies, including chemical profiles, to cope with salinity stress. This high phenotypic plasticity ensures survival in future climate conditions. Under salinity stress, some subpopulations displayed shorter filaments as a trade-off. This indicates that the future freshening of the Baltic Sea may promote grazing on filamentous cyanobacteria and modify carbon flows in the ecosystem. In this thesis, Baltic N. spumigena chemotypes and genotypes grouped into two main clusters without influence of geographical origin. Thus, chemical profiling can be used to explore conspecific diversity in closely genetically related N. spumigena subpopulations.      Overall, this thesis has significantly expanded the knowledge on phytoplankton community and population responses to short- and long-term environmental changes, relevant to project the impacts of future climate conditions in the Baltic Sea.