INTERANNUAL CHANGES IN SEAGRASS (POSIDONIA-OCEANICA) GROWTH AND ENVIRONMENTAL-CHANGE IN THE SPANISH MEDITERRANEAN LITTORAL-ZONE

The interannual changes in leaf formation and vertical growth rates an d their correlation to the records available of environmental change ( rainfall, mean sea level, water temperature, and transparency) were ex amined in 15 Posidonia oceanica meadows growing along the Spanish Medi terranean coast between 1967 and 1992. P. oceanica leaf production flu ctuated 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 growt h rate of P. oceanica observed in two sites suggests that shoreline er osion there could derive from human activities. In all meadows examine d, interannual variability in vertical rhizome growth of P. oceanica s howed clear oscillating trends, suggesting alternating episodes of sed iment erosion and accretion every 7 yr and at least every 25 yr. Mean sea level and surface water temperature have been increasing for the l ast two decades, but water transparency has been declining. However, o verall trends only accounted for 24-37% of the long-term climatic vari ance. Rainfall interannual changes were dominated by time scales of 8 and 28 yr, whereas water transparency, temperature, and sea level show ed dominant time scales in the oscillations of 4 and 15 yr, 6 and 20 y r, and 11 and 27 yr, respectively. In addition, 33% of P. oceanica ver tical growth variability in the southern Spanish Mediterranean coast d erived from variability in rainfall, suggesting a rise of erosive coas tal conditions during rainy years. The similarity in the interannual c hanges of seagrass growth over a wide spatial scale (1,000 km), togeth er with the significant coupling between seagrass growth and climate v ariability, points out climate change, and not widespread deterioratio n derived from anthropogenic pressure, as the main source of the obser ved changes in the Mediterranean littoral zone.