<p>The introduction of non-indigenous species and environmental changes are both important threats to marine ecosystems. Environmental changes occur simultaneously and might impact marine organisms synergically or antagonistly. The success of invasive species has been attributed in part to their greater capacity to acclimate to changing conditions. However, the effect of environmental factors on marine invasive species has been little studied. This thesis studied the tolerance of different life stages of the invasive brown seaweed Undaria pinnatifida to UVB and temperature. Also, the possible interactive effect of an increase of temperature and UVB on U. pinnatifida was evaluated. The tolerance of motile zoospores U. pinnatifida to increasing UVB irradiances was studied in laboratory experiments, and a strong negative effect of UVB on motile zoospores of U. pinnatifida was observed. However, zoospores can recover from UVB stress and the degree of recovery depended on UVB irradiances and exposure time. Their ability to recover could increase the opportunity of zoospores to survive and succeed in the invasion process and shows that U. pinnatifida can survive after UVB stress when environmental conditions improve. The effect of light treatments combined with temperature was also investigated in early life stages of U. pinnatifida. Both treatments affected early life stages independently. Early life stages were particularly sensitive to UVB; more so than the other light treatments and temperature. The tolerance of early life stages to a wide range of light and temperature conditions might allow this species to maintain viable populations where they already exist, but also might permit it to invade other areas if predicted environmental changes occur in the future. The effect of consecutive exposures to PAR and UV treatments at different temperatures on sporophytes of U. pinnatifida and the possible photoprotective role of phlorotannins were investigated. There was an interactive effect of light treatments, temperature during the exposure of sporophytes. Sporophytes were highly sensitive to UVB but not to the other light treatments nor to an increase of water temperature. There was no evidence of induction of phlorotannins by UVB and the other light treatments in U. pinnatifida. The sensitivity of sporophytes of U. pinnatifida to UVB and the lack of photoprotective role of phlorotannins suggests this species might have other strategies for success in the intertidal and might direct its energy mainly to growth and reproduction rather than to photoprotection and repair. The response of the photosynthetic capacity and phlorotannins content to seasonal variations of light and UVB of U. pinnatifida was investigated. A clear seasonal trend in the photosynthetic capacity was observed in sporophytes that were correlated to PAR and UVB irradiances measured in the field. Phlorotannins were variable throughout the sampling period where soluble phlorotannins had seasonal and interannual variation, while there was no clear seasonal variation in cell wall phlorotannins concentration. No correlation between both phlorotannins and PAR and UVB was detected. The response of the photosynthetic apparatus to variations of light illustrates the capacity of this species to acclimate to ambient light conditions. Variation of phlorotannins throughout the year could be attributed to a combination of factors such as grazer and wound healing rather than only to the light conditions. This research contributed to a better understanding of the tolerance of U. pinnatifida to environmental factors. This study showed that the tolerance and response to environmental factors is life stage specific. The major factor affecting U. pinnatifida negatively was UVB, while temperature had little impact on this species. The capacity of U. pinnatifida to inhabit a wide range of light and temperature conditions permits this species to succeed in coastal ecosystems, and these characteristics could permit this species to succeed under future climate change scenarios.</p>
Immigration and early life stages recruitment of the European flounder (Platichthys flesus) to an estuarine nursery: The influence of environmental factors