Effects of light intensity on the invasive macroalgae Halimeda incrassata

Abstract

[eng] Biological invasions are a major driver of ecosystem change worldwide. The Mediterranean Sea is a hotspot for invasive species, with many of those being of tropical origin, and thus expected to be further favoured by warming. Halimeda incrassata is a tropical seaweed that was detected in the Mediterranean for the first time in 2011 in Mallorca, where it has expanded significantly in the last 10 years, having impacts on native communities. Yet, little is still known about its ecology, thus, in this study, I examined the role of light limitation in influencing the invasive potential of this species. Specifically, an in situ manipulative experiment was conducted in a shallow (5m) H. incrassata bed to determine the effects of light intensity on its abundance, growth and photophysiology. Four different light treatments (control, moderate reduction, severe reduction and total light reduction) were maintained (using shading with different mesh) for 10 weeks. Significant negative effects of light reduction were detected on algal abundance (decreasing ca 55% between control and total reduction in situ). On the other hand, total plot biomass (g DW) did not differ amongst treatments, although there was a tendency towards the loss of larger individuals as light decreased. Thalli growth was significantly affected by light reduction, being ca 75% lower under total reduction compared to controls. Regarding chlorophyll measurements, Fv/Fm rate, photosynthetic efficiency and ETRmax were significantly affected by different light intensities after 5 weeks of experiment, but at the end of the experiment, all plants completely recovered photosynthetically. Pigment content (Chla, Chlb and carotenoids) did not exhibit any significant differences between light treatments, although plants under total darkness appeared as white. Such evidences could indicate a mechanism of acclimatization involving photosynthesis and pigmentation in response to light limitation. Overall, the results of this thesis highlight that H. incrassata is very resistant to low light conditions, which has important implications when considering its invasive potential and dispersal capacities.