Natural physical factors influencing the distribution, bed type and architectural characteristics of Posidonio oceanica meadows

Abstract

Water movement influences seagrass ecosystems at every spatial scale, from the shoot level up to the landscape level. However, the influence of water movement on the landscape spatial arrangement and architectural complexity of Mediterranean seagrasses, in particular relatively deep water meadows of the endemic seagrass Posidonia oceanica (L.) Delile, have not been well studied. The aims of the present study were to establish whether water movement influences seagrass landscape spatial configuration and architectural attributes. A hydrodynamic model, WEMo (Wave Exposure Model), was used to model estimates of energy due to wind-generated wave energy at the surface and shear energy at the seabed. Hourly wind speed and direction data for the years 2009-2011 were used to assess the exposure of five coastal sites on the northeastern coast of the Maltese Islands: San Blas, Dallhlet Qorrot, Ramla Bay, Selmun and Bahar ic-Caghaq, which supported three different bed types of Posidonia oceanica: Patchy, reticulate and continuous, which were mapped to a depth of circa 15m using a combination of colour vertical aerial photographs and SCUBA diving surveys. Shoot density counts and shoots for the laboratory analysis of seagrass meadow architectural attributes were collected from a station located on each of the three bed types at each site. The mapping results indicated that there was a clear transition from continuous meadows to increased patchiness, lower seagrass cover, and smaller patches with more complex shapes along a wave exposure gradient from low energy to high energy sites. The results of assessment of meadow architectural attributes indicated a decrease in architectural complexity at sites exposed to strong wave action; however, local factors other than water movement were implied to have a greater determining influence on seagrass architectural characteristics. The results also indicated that bottom shear stress better represents the energy experienced in deep meadows than surface wave energy and this hydrodynamic variable may be applicable for future studies exploring the influence of wind-generated wave energy on seagrass habitats within this approximate depth range. Overall, the results derived from the present study corroborate those from other research studies on the relationship between seagrass habitat and the hydrodynamic setting but expand the state of knowledge for both Posidonia oceanica and meadows of this seagrass in deep (down to 15 m) waters.