Optimizing Posidonia oceanica (L.) Delile shoot density: Lessons learned from a shallow meadow

Although shoot patchiness has long been studied in Posidonia oceanica meadows, small scale spatial structure of P. oceanica meadows is poorly known, as very few studies focused on this feature. In order to analyze spatial patterns within P. oceanica meadows that appear uniformly dense and undisturbed, we collected shoot density data at the Capo Rizzuto Marine Protected Area (NE Mediterranean, Italy). Intensive sampling was carried out within a square lattice at small spatial scale (i.e. in the 10−1-102 m2 range) and shoot counts were obtained from sample quadrats of different size (60, 40, 20 cm). Spatial data analysis highlighted high irregularity in shoot density from centimetric to larger spatial scales. Therefore the deviation between shoot density estimates obtained using conventional methods and the overall average of quadrat counts (assumed as the best estimate for true density) was never negligible even when larger counting quadrats or higher numbers of replicates were adopted. While shoot density is regarded as the most important property of a P. oceanica meadow and as an indicator of ecosystem health, uncertainty in density estimates and unknown expected errors impair the effectivity of this approach. However, we showed that error could be predicted based on sampling intensity and design in an apparently uniform meadow. Although results from a single case study cannot be generalized, our work is the first attempt at analyzing the problems related to density estimates obtained from shoot counts and it shows how sampling can be optimized to achieve any desired level of accuracy.