Spatio-temporal variability in Posidonia oceanica seagrass meadows of the Western Mediterranean: shoot density and plant features

The spatial and temporal variability of shoot density and selected plant features of Posidonia oceanica seagrass were assessed in 2 different meadows off the Island of Ischia (Naples, Italy), subjected to different degrees of both human impact and hydrological conditions. A nested hierarchical sampling design was used (1) to examine patterns of distribution of shoot density and plant features at different spatial scales and (2) to analyze temporal patterns of variability along the 2 principal seasons, showing the minimum and maximum development of the plant canopy (summer and winter; 2 sampling dates for each season). The shoot density showed 2 main spatial scales of variability: between the 2 beds (a few km apart) and within each meadow (10s of m); how¬ever, there were no seasonal differences. Most morphometric features showed multiple spatial scales of variability, especially at the smallest scale (10s of cm), but none of these parameters var¬ied significantly between the meadows. Most of the morphometric parameters also displayed vari¬ations with season and date of sampling, likely related to the growth cycle of the plant. A permu¬tational analysis of variance identified several scales of variability for all features across the suite of scales from 10s to 100s of meters and for seasons and sampling dates. The present study pro¬vides evidence of the high spatio-temporal variability of P. oceanica plant features despite the dif¬ferent ecological status of the meadows. It is advisable to take this high variability into account during monitoring studies to avoid misinterpretation of natural variation.