A small-scale analysis of the spatial structure of a Posidonia oceanica meadow off the Island of Ischia (Gulf of Naples, Italy): Relationship with the seafloor morphology

Abstract

Posidonia oceanica L. (Delile), an endemic species of the Mediterranean, forms extensive meadows which are continuously endangered by anthropic impacts. The availability of up-to-date information about interannual changes in shoot density of meadows and the knowledge of its expansion capabilities are crucial elements for the development of effective protection plans. Conversely, spatial ecology is becoming an increasingly important component of resource management, and the use of quantitative data for constructing prognosis maps of the dynamics of ecosystem degradation and restoration by nonlinear simulation methods is a topical field of landscape ecology. Unfortunately, little is known on spatial patterns of shoot density of P. oceanica on a small scale, despite their increasing use as indicators of the status and/or trends of meadows. The spatial structure of a continuous P. oceanica meadow, extending from 1 to 33 m depth in Lacco Ameno (Gulf of Naples, Italy), is investigated here by the “kriging” technique, a method widely used for geostatistical purposes. The analysis detected peculiar spatial patterns of shoot density and facilitated a small-scale (square meters) model of the distribution of P. oceanica. The highest shoot densities were found at the shallow stand (430 shoots m −2, on the average, with a peak of 1000 shoots m −2 in a relatively small area, at 1 m depth) and the lowest at the deep stand (average density <300 shoots m −2 below 15 m depth). A high degree of patchiness was found in the shallow stand, down to 10 m depth. Nestlike patterns with a rounded shape, characterized by shoot density radially decreasing from the center, were demonstrated mainly in the shallow stand. An opposite trend was detected in one case, where the main nestlike pattern was characterized by a decrease of density towards the center. Nestlike patterns may be generated by the confluence and overlap of stolons expanding from proximal areas. The comparison of density and depth models indicated that the shape of the seafloor influences the density structure of meadows and the growth patterns of P. oceanica.