The phenology of submerged macrophytes from mediterranean wetlands as a sentinel of climate change

Abstract

The study of phenology, i.e. the timing of recurrent biological events, is being used to track climate-change effects on organisms. Although submerged macrophytes play a key role in structuring shallow ecosystems, little information about their phenology is available. This thesis presents the first data (from 2014 to 2016) of a desirable long-term study. The main aims were: (i) to establish a methodological basis for the phenological study of submerged macrophytes, (ii) to describe the reproductive phenology of six species (Chara hispida, C. aspera, C. canescens, Nitella hyalina, Stuckenia pectinata and Utricularia australis) in two Mediterranean brackish ponds; and (iii) to unravel if their phenology might act as a tracker of climate change. A huge sampling effort and thorough observation, plant by plant, were performed to detect the presence and production intensity of gametangia/oospores and flowers/fruits. The size of gametangia from each charophyte species was conscientiously measured to design a quantitative cost-effective method to differentiate phenophases. Different physical and chemical features of water (temperature, light, water level, conductivity, pH, etc.) were also monitored where the plants grew. Several approaches and analysis methods were used to describe and compare the data. The thermal time model, using the growing degree-days parameter, was applied to understand the reproductive onsets, peaks and periods. Circular statistics, a developing, powerful tool in phenological studies, was applied to test the species-specific seasonality in reproduction. Aquatic angiosperms reproduced mainly vegetatively, while charophytes showed a high intensity of sexual reproduction and a long fertile period with a strong peak in spring-summer and occasionally a weak one in autumn, depending on the rainfall maximum. Each species of submerged macrophytes showed distinct phenological patterns, depending on its life-history strategies and the species-specific responses to interacting environmental factors. Even variations within a same ecosystem, e.g. related to water depth, affected the reproductive timing of individuals from the same population. Underwater temperature, water level and salinity were the factors altered by climate change that most affected the ripening of gametangia/oospores and the flowering/fruiting of submerged macrophytes. Charophytes, particularly, have shown themselves to be good candidates for tracking variations in climate change factors. Accurate forecasts on shallow ecosystems require more detailed knowledge of the responses of submerged macrophytes throughout their whole life cycles and across latitudes. Phenological variables can be applied to the conservation and management of declining populations and habitats. This thesis has laid the foundation for the establishment of a realistic design of a long-term data series and will hopefully inspire more long-term, inter-site and multi-latitude studies of macrophyte phenology.