A multivariate index based on the seagrass Posidonia oceanica (POMI) to assess ecological status of coastal waters under the water framework directive (WFD)

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Abstract

We propose here a multivariate index based on structural and functional attributes of the Posidonia oceanica ecosystem (Posidonia oceanica multivariate index: POMI) to assess the ecological status of coastal waters following WFD requirements. POMI is based on the combination, through principal component analysis, of physiological, morphological, structural and community level seagrass descriptors (or metrics), univocally related to environmental quality. Scores on the first axis are normalised to a 0–1 scale (EQR) using reference and worst sites. The index was tested by sampling 22 seagrass beds in the Catalan coast (ca. 500 km, NW Mediterranean). The results show a clustering of variables on both sides of the first axis of the PCA, indicating a common relation of all metrics with ecological status. Moreover, the values (EQR) obtained reflect human pressure levels. The proposed index allows a practical and relatively simple assessment of the ecological status of coastal waters, and contains a considerable amount of ecological information, which can be useful for managing purposes.

Introduction

Aquatic systems worldwide are under increasing anthropogenic pressure, which is causing a significant water quality decline. This has led to the development of national or trans-national strategies aimed at conserve and/or recover the ecological quality of the aquatic environment. One sound initiative at this respect was the publication, by the European Union, of the water framework directive (WFD 2000/60/EC), which establishes the basis of policies for the monitoring, protection and enhancement of the status of aquatic systems in the Member States. The main goal of the WFD is to achieve (or maintain at least) a ‘good water status’ for all the European waters by 2015. This directive introduces some novel views in the field of water management, such as the acknowledgement of the link between water quality and the status of the supporting/supported ecosystem, and the relevance of organisms in the definition and evaluation of water quality. Consequently, the directive defines the concept of ecological status as the quality of the structure and functioning of aquatic ecosystems associated with surface waters. A key issue is the mandatory use of biological indicators for monitoring and assessment of the ecological status of all water bodies. Some organisms or groups of organisms sensitive to anthropogenic pressures (biological quality elements, BQE) are identified as the most appropriate for this purpose. In particular, for coastal waters, phytoplankton, macroalgae and angiosperms, and benthic invertebrate fauna are the BQEs to be considered. According to the WFD, biological variables indicative of the status of these BQEs (hereafter termed metrics) should be used for evaluation and monitoring purposes.

Indeed, the implementation of the WFD needs a strong scientific basis, and the scientific community has responded vigorously to this challenge, as reflected in an increasing number of papers addressing issues related to WFD. For example, in the marine field, biotic indexes based on invertebrates of soft-bottom benthos (biotic index/biotic coefficient, BI/BC Borja et al., 2000, Borja et al., 2003; benthic quality index, BQI Rosenberg et al., 2004; benthic index, BENTIX Simboura and Zenetos, 2002), macroalgal communities (ecological evaluation index, EEI Orfanidis et al., 2001, Orfanidis et al., 2003, Panayotidis et al., 2004), phytoplankton abundance (Vila et al., 2005) or depth limit of the marine angiosperm Zostera marina (Krause-Jensen et al., 2005) have been developed.

Seagrasses are very sensitive to changes in their environment, and, specifically, to human impacts, to the point that a worldwide decline seems to be taking place (Short and Wyllie-Echeverria, 1996). To our knowledge, little effort has been invested to adapt seagrass bioindicators to WFD requirements, and this has been achieved only in the Atlantic species Zostera marina (Krause-Jensen et al., 2005), very scarce in Mediterranean waters. A recent approach to monitoring and management of Mediterranean species, though not related to the WFD, has been proposed (Montefalcone et al., 2006). Yet, the long-lived species Posidonia oceanica, Mediterranean endemics, can be efficiently used as a BQE due to, at least, three reasons: (i) its very high sensitivity to disturbances (Francour et al., 1999, Delgado et al., 1999, Ruiz et al., 2001, Ruiz and Romero, 2003); (ii) its wide distribution along the Mediterranean coasts (Procaccini et al., 2003), and (iii) the large amount of knowledge on the biology and ecology of the species (Romero, 2004), and on specific responses of the plant and its associated ecosystem to specific anthropogenic disturbances (e.g. Campanella et al., 2001, Cancemi et al., 2003, Ruiz et al., 2001, Pergent et al., 1999, Vizzini and Mazzola, 2004, Francour et al., 1999). Furthermore, some P. oceanica descriptors have already been used to assess the general environmental quality of coastal zones (Boudouresque et al., 2000, Pergent-Martini and Pergent, 2000, Moreno et al., 2001, Pergent-Martini et al., 2005).

In the present study we take advantage of these previous knowledge and experiences to design a biotic index, fully compliant with WFD requirements, based on P. oceanica ecosystem and aimed at assessing the ecological status of coastal waters. In general, biological or ecological indicators are measures based on the presence, abundance or state of health of organisms (or assemblages) that provide simple and efficient methods to quantify the degree of ecosystem integrity (health, exposure to stress, etc.). In this respect, a strategy to cope with the inherent biological complexity and natural variability that provides optimal tools and also a considerable amount of ecological information is the use of metrics encompassing multiple organization levels in the biological hierarchy, from the sub-individual to the ecosystem level (Harding, 1992, Dale and Beyeler, 2001, Adams, 2005). Therefore, we have chosen a multivariate approach, by measuring structural and functional attributes at different organisation levels (physiological, individual, population, etc.) in the P. oceanica ecosystem to develop a biotic index (P. oceanica multivariate index: POMI).

We evaluate the usefulness and validity of this index by using it to assess the ecological status of the Catalan coastal waters (ca. 500 km of coastline, NW Mediterranean).

Section snippets

Selection of metrics

To select the variables (metrics) of P. oceanica that better reflect the ecological status, a pilot study was carried out in 2002 (Martínez-Crego, 2005). During this work, ca. 60 metrics, ‘a priori’ sensitive to disturbances, were chosen, and tested in seagrass meadows of known and different ecological status. From these results, and considering also practical reasons of their possible application (feasibility, cost, etc.) a total of 14 metrics were finally selected.

Among them, five are

Classification of the ecological status

The classification of the ecological status should be based on the deviation of the status of the biological quality element from its potential status under pristine (i.e. undisturbed or nearly undisturbed) conditions, named reference conditions. This ecological status should be expressed using a scale from 1 (corresponding to the reference conditions) to 0 (severe deterioration, a great amount of the BQE is badly damaged or missing). The ratio between the actual status of a given BQE in a

Results

The PCA applied to the field data allowed to extract a first component explaining 57% of the total variability, while the second explained less than 10%, indicating a main source of variation common to all metrics. The metrics positively correlated to component I (Fig. 2) are clearly indicative of bad status (e.g. higher percentage of leaves with necrosis corresponds to worse status), while those negatively correlated are indicative of good status (e.g. higher shoot density corresponds to,

Discussion

P. oceanica multivariate index (POMI) seems to adequately reflect ecosystem status and coastal water quality, as required by the WFD. We base this assertion on two arguments:

  • (a)

    All the metrics used have been repeatedly reported to reflect seagrass health (or deterioration) and widely used (Delgado et al., 1999, Longstaff and Dennison, 1999, Prange and Dennison, 2000, Jones et al., 2001, Udy et al., 1999, Ruiz and Romero, 2003, Invers et al., 2004, Marín-Guirao et al., 2005). The large amount of

Acknowledgements

This study was performed at the request and with the financial support of the Agència Catalana de l’Aigua (Departament de Medi Ambient i Habitatge, Generalitat de Catalunya), which strongly encouraged our work. We are specially indebted to Mariona de Torres and Marta Manzanera for their collaboration and support. We also would like to thank Simone Mariani for his collaboration in field works, and Susana Pinedo and Xavi Torras for advice. Nutrient and isotopic determinations were performed at

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