Plant and Meadow Structure Characterisation of Posidonia oceanica in Its Westernmost Distribution Range

Posidonia oceanica is an endemic seagrass species from the Mediterranean Sea that provides critical ecological services to coastal environments. This species is distributed from the Turkish to the Spanish coast, where its westernmost record was documented in Punta Chullera, Malaga (36 degrees 18 ' 36.45 '' N, 5 degrees 14 ' 54.31 '' W). Nevertheless, previous studies suggested that its distribution was even further west, although these populations were never described. In this study, we documented and characterised the only known P. oceanica population on the coast of Cadiz, in Cala Sardina (36 degrees 18 ' 38.80 '' N, 5 degrees 15 ' 15.13 '' W). The newly documented population of P. oceanica presented a fragmented structure, consisting of nine patches found in a rocky shallow area surrounded by the invasive algae Rugulopteryx okamurae, with a total size of 61.14 m(2). Shoots had a relatively small size (21.0 +/- 2.9 cm) in comparison with centrally-distributed populations. The relatively small size of the plants, alongside the observed low shoot density (437 +/- 42 shoots m(-2)) and leaf area index (4.8 +/- 0.7 m(2) m(-2)), may indicate that this meadow could be exposed to sub-optimal environmental conditions for plant development. By contrast, the meadow showed relatively high production rates (0.03 +/- 0.01 leaf day(-1) shoot(-1)) in comparison with other Mediterranean populations. The percentage of carbon in plant leaves was 38.73 +/- 1.38%, while the nitrogen and C/N were 1.38 +/- 0.37% and 29.93 +/- 6.57, respectively. The documentation of this meadow extends the distribution of this species to the Mediterranean coast of Cadiz, making this region the place with the highest seagrass biodiversity (four species) in the Iberian Peninsula, and potentially in Europe. This exploratory study provides a baseline to examine the potential effects of climate change, anthropogenic disturbances or the presence of invasive species.