Summary
This doctoral PhD Thesis was developed to explore interactions between benthic marine macrophytes, substratum type and fluid dynamics. Quantitative knowledge as well as a predictive capacity is obtained to estimate the presence of a seagrass meadow, two species of invasive macroalgae and two species of seagrass seedlings in response to near-bottom orbital velocities, drag forces, root anchoring capacity and substratum type. Additionally, the e fect of a seagrass meadow on wave propagation under natural conditions is also evaluated. In order to get a deeper knowledge on these processes, data from dif erent sources such as numerical models, aerial photographs, fi eld experiments and flume measurements are combined.
Light and temperature are considered as the main determinants for the spatial distribution of marine macrophytes but hydrodynamic conditions and substratum type are also key factors limiting the distribution of marine vegetation. Wave hydrodynamic ef ects on macrophytes have been studied in three ways: i) The near-bottom orbital velocities that set the upper limit of a Posidonia oceanica seagrass meadow are obtained by correlating hydrodynamics and the spatial distribution of the meadow. ii) The role of hydrodynamics in the establishment of P. oceanica and Cymodocea nodosa seagrass seedlings is evaluated. Drag forces and root anchoring capacity of seedlings are studied in a biological flume while seedling survival is addressed under natural conditions. iii) Substratum type plays an important role on the spatial settlement and distribution of marine macrophytes. Substratum cover of the invasive macroalgae Caulerpa taxifolia and Caulerpa racemosa indicates that these species are more abundant in rocks with photophilic algae and in the dead matte of seagrass P. oceanica than in sand or inside the P. oceanica meadow. Correlative evidence shows that C. taxifolia and C. racemosa tolerate near-bottom orbital velocities below 15 cm s-1 and that C. taxifolia cover declines at velocities above that value. iv) Wave damping induced by a seagrass meadow of P. oceanica is evaluated under natural conditions using data from bottom mounted acoustic doppler velocimeters.
The bottom roughness is calculated for the meadow as 0.42 m using flow velocities above the seagrass. Wave friction factor has been related to the drag coecient on the plant and obtained for two storms so as to compute the damping along a transect. Drag coecient values ranged from 0.1 to 0.4 during both storms. The expected wave decay coecient for di erent seagrass shoot densities and leaf lengths are also predicted.
A relation between fluid dynamics and benthic macrophytes is shown. High near-bottom orbital velocities have an e ect on the spatial distribution, growth and colonisation processes of seagrass and macroalgae species. The infuence of a seagrass meadow on wave propagation is also apparent, with potential impact on sediment stability and coastal erosion. Predicted and measured quantitative results are provided such as near-bottom orbital velocities and drag coecients that could be tested and compared to other benthic marine macrophytes species on other locations.
This Thesis is in the intersection between Ecology and Fluid dynamics a research area characterized by strong nonlinear interactions. To get a deep insight on the processes involved a bench of mathematical tools are used in order to apply physical principles to understand and predict the behaviour of marine macrophytes in the Mediterranean Sea. Besides, experiments to validate the theories have been developed under both controlled and natural conditions.
