Detecting environmental impacts on the Mediterranean seagrass Posidonia oceanica (L.) Delile: the use of reconstructive methods in combination with 'beyond BACI' designs.

Interannual changes in leaf production and rhizome elongation rates of the seagrass Posidonia oceanica have been evaluated by means of reconstructive methods at the Travello meadow (Ligurian Sea, NW Mediterranean, Italy) to provide evidence of responses to the putative impact of a beach replenishment made with terrigenous materials in 1993. Two additional meadows (Genoa-Quinto and Noli) were sampled as controls. An asymmetrical sampling design ('beyond BACI': Before/After, Control/Impact) was thus used to detect the impact on the basis of a single sampling as dating methods obviated the lack of pre-impact data. At all three meadows investigated, leaf production and rhizome elongation rates were reconstructed for 12 previous years (from 1988 to 1999). A marked decrease in the leaf production rate (around 20%) was assessed only at Travello immediately after the putative disturbance. Control meadows, instead, did not display any significant variations in the pattern of change of this variable from before to after the putative impact. With regard to rhizome growth rates, no significant changes in space and time attributable to the putative impact have been detected. The present data also suggest that the impact studied may be considered as a pulse disturbance, since leaf production appeared to recover over a comparatively short-time scale (around 2-3 years) if compared to the low turn-over and high longevity of P. oceanica. The high potential of asymmetrical sampling designs in combination with dating methods is discussed in the light of the results presented here.