Los efectos de factores naturales y antropogénicos en fanerógamas marinas

Résumé

Seagrasses are considered one of the most important organisms in the world, since they form extensive meadows which provide high biodiversity and a wide variety of ecological services. Despite its importance, seagrass meadows are globally declining, especially due to anthropogenic pressures and climate change. The sexual reproduction of the seagrasses is responsible of the genetic variability of the species and therefore, it is the principal way to develop characters to resist future environmental perturbations. However, due to the difficulties of monitoring the dispersion of the fruits and seeds of seagrasses in the field, the sexual reproduction is poorly understood. This limits the effective conservation of the seagrass meadows. In addition, seagrasses have numerous bottlenecks during the sexual recruitment, especially during the development of the seedlings. These bottlenecks affect its rates of colonization and also lead in the uprooting of the transplants in environmental restoration projects. To increase our understanding in these areas, the general aim of this thesis is to shed light on sexual recruitment strategies, and also on the effects of natural and anthropogenic drivers on seagrasses. Different approaches, species and development stages of seagrasses are used to achieve this aim, resulting in five different chapters whose methodology, results and conclusions are exposed below. In the first chapter of the thesis is evaluated the sexual recruitment of the seagrass Posidonia oceanica. To do so, the morphology of fruits and seeds collected in the beaches of the Murcia Region are studied. Subsequently, mesocosm experiments to test the influence of the light and substrata type in the seed development are tested. The results show that the seeds of this seagrass go through three essential stages from its dispersion in the interior of fruits over the sea surface until its settlement in the sediment. In the Chapter 2 are tested the effects of the substrate hardness and seed burial in P. oceanica seedlings. To do so, seeds are cultured in aquaria by using hard and unconsolidated substrata and thus, root and leaf development are compared between treatments. Subsequently, these seedlings are transplanted to sand with three levels of seed burial. The results show that the seedlings which are previously cultured in sand have a greater development than the cultured in hard substrata. In addition, seed burial do not affect the development of P. oceanica. In the Chapter 3 are evaluated the responses of P. oceanica seedlings to the increase of temperatures due to heat waves. The results show that the current and forecasted temperatures expected for the end of the century in the Mediterranean will compromise the colonization capacity of this seagrass. In the Chapter 4 also is evaluated the effects of heat waves in P. oceanica seedlings but in combination of other environmental stressors: seed burial and grazing. The results show that the combination of severe environmental stressors limits the development of P. oceanica seedlings in a more intensive level than when the factors separately occur. In addition, the effects of the heat waves override the effects of both environmental stressors. In the last chapter is studied the effects of ocean acidification in homospecific and heterospecific beds of H. wrightii and R. maritima. The results show that, contrary to expectations, the increase in CO2 availability will not benefit the seagrasses of the estuarine system of the Gulf of Mexico. The information compiled in this thesis about the sexual recruitment and influence of natural and anthropogenic drivers on seagrasses is essential to improve the management and conservation of these important habitat-forming species.