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 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 (1,000 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 Mediterranean littoral zone.