Flowering of the seagrass Posidonia oceanica in a north-western Mediterranean coastal area: temporal and spatial variations

Abstract

Sexual reproduction in Posidonia oceanica (L.) Delile is likely to be highly variable both in space and time. However, there has been little effort to identify at what spatial scale variability exists in the distribution and abundance of reproductive shoots or the magnitude of these variations. Such information is important for elucidating several aspects of the reproductive strategy of this species. Here, by using a reconstructing technique, I estimated the rate at which flowering events occurred over the last decades in three P. oceanica meadows along the coastline of the north-western Mediterranean (Italy). Then, variation in inflorescence frequency (i.e. the percentage of shoots that flowered in a particular year of the total number of shoots present in the sample in that year) was examined on a hierarchy of spatial scales, ranging from a few metres to meadows separated by tens of kilometres over a 3-year period. Separate analyses were also conducted to examine the patterns of spatial variability of several plant characteristics (i.e. shoot age and density of vegetative shoots) that may influence the timing of flowering. In total, ten reconstructed flowering events were recorded during the period 1983–1999. Despite the similar climatic conditions experienced in the three meadows surveyed, no inflorescence scars were found in one meadow. In the other two meadows, the flowering rate fluctuated considerably from year to year (0–26%), and simultaneous flowerings were detected only in 3 years (1993, 1994 and 1995). The pattern of spatial variability in the inflorescence frequency varied among years. An investigation of the flowering events recorded along the north-western Mediterranean coast reveals that in the studied period flowering also occurred across a broad range of meadows. Therefore, flowering could be a far more widespread phenomenon in this sector of the Mediterranean basin than previously thought. Analysis of variance showed significant variation in 1993 at the mean inflorescence frequency on very small scales (metres to tens of metres), whereas variation was not found at any of the scales investigated in 1994 and 1995. Overall, variation in the inflorescence frequency within individual meadows (65–93%) was at least an order of magnitude greater than that among separate meadows (0.5–19%). Patterns of spatial variation in the plant characteristics examined did not reflect those of flowering. It may be hypothesised that internal rhythms of populations, in conjunction with changes in environmental conditions, which are consistent over relatively large spatial (kilometres), are involved in the control of flowering induction. Environmental heterogeneity (in terms of nutrient availability) on a very local scale (metres to tens of metres) and intrinsic factors might affect the distribution of reproductive shoots within P. oceanica meadows.