The inter-annual changes in leaf formation and vertical growth rates and their correlation to the records available of environmental change (rainfall, mean sea level, water temperature, and transparency) were examined in 15 Posidonia oceanica meadows growing along the Spanish Mediterranean coast between 1967 and 1992. P. oceanica leaf production fluctuated interannually, but it did not exhibit any steady trend toward decline, indicative of nonhuman effects on changes in water quality in these areas. Conversely, the steady decline in vertical rhizome growth rate of P. oceanica observed in two sites suggests that shoreline erosion there could derive from human activities. In all meadows examined, interannual variability in vertical rhizome growth of P. oceanica showed clear oscillating trends, suggesting alternating episodes of sediment erosion and accretion every 7 yr and at least every 25 yr. Mean sea level and surface water temperature have been increasing for the last two decades, but water transparency has been declining. However, overall trends only accounted for 24-37% of the long-term climatic variance. Rainfall interannual changes were dominated by time scales of 8 and 28 yr, whereas water transparency, temperature, and sea level showed dominant time scales in the oscillations of 4 and 15 yr, 6 and 20 yr, and 11 and 27 yr, respectively. In addition, 33% of P. oceanica vertical growth variability in the southern Spanish Mediterranean coast derived from variability in rainfall, suggesting a rise of erosive coastal conditions during rainy years. The similarity in the interannual changes of seagrass growth over a wide spatial scale (I ,000 km), together with the significant coupling between seagrass growth and climate variability, points out climate change, and not widespread deterioration derived from anthropogenic pressure, as the main source of the observed changes in the Mcditcrrancan littoral zone. Anthropogenic pressure is causing widespread degradation of the Mediterranean coast, reflected in, among other negative symptoms, deteriorating water quality and shoreline erosion (UNEP 1989). High organic and inorganic nutrient discharge, both derived from exponentially growing coastal tourism and cities, is producing eutrophication in the Mediterranean coast (UNEP 1989). Coastal erosion has been attributed to interactive processes involving long-term (e.g. littoral subsidence, sea-level rise, increased storm frequency; Carter 1991; SCOR 1991) and short-term (e.g. enhanced river discharge; Meade and Emery 197 I ; high rainfall and low atmospheric pressure; Komar and Enfield 1987) phenomena and human interference with sediment budgets (Carter 199 1; UNEP 1989). Evaluation of the consequences of the widespread degradation of the Mediterranean coast is prevented by the scarcity of long-term records of water quality and sediment dynamics for the region (Duarte et al. 1992). Changes in the distribution, density, and biomass of seagrass communities may provide a reliable indication of changes in water quality (Dennison et al. 1993). Interannual changes in leaf production, which is highly sensitive to water-column nutrient enrichment (e.g. Neundorder and Kemp 1993) and associated reduction in underwater irradiance (e.g. I Present address: Centre for Estuarine and Coastal Ecology, N.I.O.O., Korringaweg 7, 4401 NT Yerseke, The Netherlands. Acknowledgments This work was funded by the Fundacion Ramon Areces and CICYT (project CLI951805). The Institute National de Metcorologia, Institut0 National de Occanografia, and Josep Pascual provided rainfall, sea level, and temperature and Secchi transparency data, respectively. We thank J. Cebrian, M. Gallegos, and B. Olesen for assistance in the field. K. Perry corrected the text. Dawes and Tomasko 1988), provide a biological tracer of coastal eutrophication. In addition, variability in elongation rate of vertical rhizomes (i.e. short shoots) of many seagrass species (e.g. Thalassia sp., Posidonia sp., Cymodocea sp., Halodule uninervis, Syringodium isoetifolium) closely reflects fluctuations. in sediment accretion (Patriquin 1975; Boudouresque et al. 1984; Marba and Duarte 1994). The vertical rhizornes (i.e. short shoots) of most seagrass species elongate at rates proportional to sediment burial and show the slowest growth under erosive conditions (Marba and Duarte 1994). The coupling between seagrass vertical growth and sediment dynamics is so close as to allow the use of many seagrass species as tracers of coastal sediment dynamics (Marba et al. 1994). The fluctuations in leaf production and vertical growth experienced during the lifespan of seagrass shoots can be reconstructed from marks imprinted on vertical rhizomes (Pergent and Pergent-Martini 1990; Duarte et al. 1994). Hence, examination of interannual fluctuations in leaf production and vertical rhizome growth of long-lived Mediterranean seagrasses should reflect changes in water quality and sediment dynamics in the Mediterranean littoral during seagrass lifespan. Posidonia oceanica (L.) Delile is the dominant seagrass species in the Mediterranean (den Hartog 1970), where it is experiencing widespread decline (e.g. Sanchez-Lizaso et al. 1990; Zavodnik and Jaklin 1990; Marba et al. 1996). P. oceanica ranks among the longest-lived seagrasses (Duarte 1991), with vertical rhizomes living more than 30 yr (Marba et al. 1996) and clones living over millennia (Picard 1965). The extended longevity of P. oceanica shoots renders this species an appropriate material to examine long-term (lo20 yr) changes in leaf production and vertical growth as proxies for long-term changes in water quality and sediment dynamics.