The environmental change in coastal ecosystems during the late holocene as recorded in seagrass sedimentary archives

Abstract

Coastal ecosystems, especially the vegetated areas, are among the most threatened ecosystems in the world, undergoing a fast and constant decline. Their losses are of serious concern due to their elevated production, providing many ecosystem services essential to the well-being of our societies. Behind the regressive trends of the coastal ecosystems, there is a plethora of adverse human pressures, going from local and regional impacts, including anthropogenic activities in and outside the coastal regions, to large-scale drivers of change, such as the global warming. Nevertheless, there is a critical lack of long-term information about the vegetated coastal ecosystems, information that can provide baseline ecological data of their natural dynamics and vulnerability. Seagrasses are marine plants, engineering species that form underwater meadows, which, among many other services, provide essential habitat for many other organisms. Seagrasses meadows are experiencing a widespread decline since the early 20th century. This regression is accelerated for the Mediterranean endemic seagrass species Posidonia oceanica. Long-term studies are of particular interest in P. oceanica meadows because this species is a large-slow growing and long-lived seagrass, which substantial changes and responses manifest over time scales of decades to centuries. A deeper understanding of seagrass long-term dynamics can help managers to apply meadow-specific actions and act at the appropriate temporal scales. The discipline of paleoecology allows the study of long-term ecosystem dynamics on time scales of centuries to millennia, and it can be used in seagrass meadows thanks to the organic deposits accumulated below P. oceanica meadows. Paleoreconstructions using seagrass deposits are still scarce and have mainly focused on allogenic (externally controlled) processes. In this dissertation, a paleoecological approach at a regional spatial-scale was used to explore the long-term dynamics of the autogenic and biotic ecological components of Mediterranean seagrass meadows, mainly P. oceanica meadows. Initially, we investigated the usefulness of several biogeochemical proxies and a technique (FTIR-ATR spectroscopy) so far unexplored in seagrass deposits, as well as which were the main biogeochemical processes recorded by them. We described the long-term dynamics of the seagrass ecosystem, the main drivers of change, and their relative importance. The results indicated that seagrass long-term dynamics are oscillating. Even though most meadows showed regressive trends during the last 150 years, seagrass trends varied spatially, with the main spatial differences occurring at the inter-regional level. Differences in long-term dynamics between local sites seemed mostly dependent on the environmental background of each site, which also affected seagrass long- term resilience. The major factors responsible for long-term variability of seagrass ecosystem dynamics were multiple and at both, local and large spatial scales. However, the balance between the contribution of local and large-scale drivers varied spatially. The influence of climate seemed especially crucial in meadows surrounded by more turbid waters, under the influence of higher fluvial discharges. These meadows showed lower long-term ecosystemic resilience. In summary, this research showed that seagrass long-term dynamics can be studied through their paleoecological record, providing a valuable frame of reference for evaluating the magnitude of current changes and consequences of combined diverse impacts on these marine ecosystems. The results of this thesis indicated that despite some spatial variability of the long-term dynamics, the major changes occurred over the last century, predominating trends of seagrass decline or community compositional changes. Moreover, our results point to a more acute negative impact of present climate change in meadows where light availability is compromised due to local factors. The overall spatial variability regarding seagrass long-term dynamics highlights the need for meadow-specific local management with background information, information that can be obtained from paleoecological studies.