Seagrass systems under human pressureresponses and assesment tools (sistemas de fanerógamas marinas bajo presión humana: respuestas y herramientas de evaluación)

  1. Silvia Oliva Pérez
Supervised by:
  1. Javier Romero Martinengo Director
  2. Marta Pérez Vallmitjana Director

Defence university: Universitat de Barcelona

Year of defence: 2012

Type: Thesis

Teseo: 329535 DIALNET

Abstract

In the current context, given the increasing deterioration of coastal systems, it urges expanding the knowledge on the responses of these systems to the most prevalent human disturbances, as well as providing tools for their assessment. In this regard, engineer species mean a major goal, and seagrasses one of their principal exponents. On the other hand, among the cornucopia of existing human disturbances, eutrophication is the most widespread threat to coastal systems, and the main cause of seagrass beds decline. Thus, with the overall aim of exploring some of the detected lacks at this respect, we propose here four approaches, using at all times, as a target (chapter 1) or model (chapters 2-4) species, the seagrass Cymodocea nodosa. First, we assessed the bioindicator potential of such species by examining the response of a panoply of variables to a deterioration gradient, and, thus, to build a multi-bioindicator index based on C. nodosa. To this end, we chose the Ebro river delta bays as a geographical framework, due to the existence of an eutrophication gradient, and used, as a guide, the methodology developed by Romero et al. (2007) and Martínez-Crego et al. (2008). Thus, we conclude that the ecosystem structured by Cymodocea nodosa is a suitable one to obtain metrics from which deriving reliable bioindicators of water quality. Further, CYMOX, the index based on this species here constructed, adequately reflects the ecosystem health, and is useful in assessing the ecological status of coastal and transitional waters. Second, we studied the possible covariance between the variability in plant attributes commonly used biotic indices and the variability in sediment features. To that end we selected Gran Canaria Island as a geographical framework, and measured plant variables aiming to cover the different levels of biological organization. Thus, the sediment contributes to a substantial part of seagrass natural variability, explaining, in the assessed meadows, more than a half of the variability in plant attributes. This natural variability associated to the sediments features supports the need for and eco-regional approach for defining the reference conditions, especially in those species with a wide ecological range. Third, we tested the hypothesis of a differential response of seagrass to eutrophication modulated by sediment type. To this end, we chose three different sediments types, characterized by the carbonate and iron content and the particle size, and addressed our objective through an ex situ experimental approach. An eutrophication episode was simulated, and changes in key parameters of the plant and in the sediment and water column assessed. Effectively, sediment characteristics play an important role in modulating seagrass response to organic enrichment, modulation in which the relevance of the sediment aeration capacity should not be underestimated. Finally, we studied the variation in sexual reproductive effort along a gradient of eutrophication, as well as the possible effect of such variability in the genetic diversity and connectivity of these meadows. With that aim, samplings were conducted along the eutrophication gradient previously documented (chapter 1), the reproductive effort was measured, and microsatellite markers were used to assess the genetic diversity and connectivity of sampled meadows. Indeed, eutrophication promotes a dramatic reduction in the sexual reproductive effort and a reduction in the associated genotype richness. However, this latter was modest, indicating the existence of a cryptic sexual reproduction. Also, the isolation models here proposed (disturbance, clonality), which consider population-specific factors, adequately explain the genetic structure detected among the Cymodocea nodosa studied populations.