Landscape fragmentation and resilience in seagrass meadows / Fragmentación del paisaje y resiliencia en praderas de fanerógamas marinas

Abstract

Overall, this thesis is an attempt at approaching seagrass ecosystems from a landscape perspective, with the aim of increasing knowledge about the processes generating patterns at large spatial scales, and understanding how these patterns interact with other drivers of seagrass function. Specifically, we focus on (i) understanding the dynamics of habitat fragmentation (Chapter 1), (ii) how this fragmentation interact with other seagrass disturbances, such as hydrodynamism, sedimentation and herbivory (Chapter 2 and 3) and which are the consequences of such fragmentation to biodiversity and ecosystem processes (Chapter 4). The thesis is structured in five chapters, the first four focus on the different objectives outlined above. Additionally we have included another chapter (Chapter 5) that relates seagrass quality with photosynthetic performances. All chapters originated from field observations (Chapter 1, 2, 4 and 5) or experimental manipulative experiments (Chapter 3) that posed individual questions and general hypothesis within the frame of the dynamics of habitat fragmentation at patch-size and landscape scale. For the two first chapters, we take advantage of extreme storm events and in one of them we exploit long-term data series (15-20 years).

Thus, we will first aim to integrate the different results related to the effects of habitat fragmentation and the interactions with external and internal disturbances. In this thesis we have observed how storms can be an important agent of fragmentation in shallow Posidonia oceanica meadows. One of the main causes is the limited capacity of this slowgrowing seagrass species to resist and recover from extreme storm events. This has been observed either by the response of the plant to a specific storm event (Chapter 1) than by the observed responses to storms in a long-term series (Chapter 2). Once the process of fragmentation was identified, we attempted to evaluate the resilience of the meadows to external and internal disturbances. In the case of the external disturbances we compared the storms effects on continuous and fragmented meadows in two congeneric species of Posidonia. The effect was much stronger in the fragmented meadows pointing out that fragmentation makes seagrass meadows less resilient and that the Mediterranean P. oceanica is particularly vulnerable due to its limited recovery potential. Evident limitation in this study was the low level of replication. This was not due to a design error, but to the rarity of the occurrence of extreme storm events and the limited availability of long data series. In spite of this, our results are consistent in stressing that fragmentation can be a critical factor for shallow seagrass maintenance. When we add to this disturbance the natural processes that ecosystems are facing, such as herbivory, fragmentation becomes even more critical as both factors are interacting in a combined way (Chapter 3). Consequently as both factors (fragmentation and herbivory) affect additively the same processes small fragmented patches tend to have lowest canopy height (i.e. less structure), less nutrient retention and lowest primary production. Interestingly, the influence of fragmentation on biodiversity and abundance of low mobility species, which was tested in chapter 4, showed opposite effects than expected. Specifically small fragments presented a much higher biodiversity of decapods, used as a model group, although it is not fully generalizable as the observed effect was site-specific. The apparently contradictory result could be in part explained by the low habitat use by predators observed in these small fragments. This final result points out the importance that small seagrass patches might have, as potential hotspots, for the maintenance of biodiversity. Finally we discuss the potential scenario that P. oceanica fragmented meadows might face under climate change. One of the clearest effects of climate change in several regions is that atmospheric and ocean regional conditions are expected to be modified, increasing the frequency of high-intensity cyclonic formations that can even strengthen the role of disturbances on these systems.

En general, la tesis pretende acercarse a los ecosistemas de fanerógamas marinas desde el punto de vista del paisaje, con el objetivo final de incrementar el conocimiento acerca de los procesos que generan los patrones a gran escala espacial, y entender como estos patrones interactúan con otros procesos funcionales de las fanerógamas marinas. Específicamente, a lo largo de la tesis nos hemos centrado en: (i) entender las dinámicas de la fragmentación del hábitat (capítulo 1), (ii) como esta fragmentación interactúa con otras perturbaciones, como el hidrodinamismo, la sedimentación y el herbivorismo (capítulos 2 y 3), y (iii) cual pueden ser las consecuencias de dicha fragmentación en la biodiversidad y los procesos del ecosistema (capítulo 4). Además de estos cuatro capítulos, se incluye un capítulo final (capítulo 5) que relaciona el estado de las praderas de fanerógamas marinas con las características fotosintéticas. Todos los capítulos se han redactado partiendo desde observaciones de campo (cap. 1, 2, 4 y 5) o experimentos manipulativos (cap. 3), y plantean tanto preguntas individuales como hipótesis generales en el marco de las dinámicas de fragmentación del hábitat a escala de manchas y de paisaje.