Amaya Álvarez-Ellacuria

Wave energy and the upper depth limit distribution of Posidonia oceanica

It is widely accepted that light availability sets the lower limit of seagrass bathymetric distribution, while the upper limit depends on the level of disturbance by currents and waves. The establishment of light requirements for seagrass growth has been a major focus of research in marine ecology, and different quantitative models provide predictions for seagrass lower depth limits. In contrast, the influence of energy levels on the establishment, growth, and maintenance of seagrasses has received less attention, and to date there are no quantitative models predicting the evolution of seagrasses as a function of hydrodynamics at a large scale level. Hence, it is not possible to predict either the upper depth limit of the distribution of seagrasses or the effects that different energy regimes will have on these limits. The aim of this work is to provide a comprehensible methodology for obtaining quantitative knowledge and predictive capacity for estimating the upper depth limit of seagrasses as a response to wave energy dissipated on the seafloor. The methodology has been applied using wave data from 1958 to 2001 in order to obtain the mean wave climate in deep water seaward from an open sandy beach in the Balearic Islands, western Mediterranean Sea where the seagrass Posidonia oceanica forms an extensive meadow. Mean wave conditions were propagated to the shore using a two-dimensional parabolic model over the detailed bathymetry. The resulting hydrodynamics were correlated with bottom type and the distribution of P. oceanica. Results showed a predicted near-bottom orbital velocity of between 38 and 42 cm s -1 as a determinant of the upper depth limit of P. oceanica. This work shows the importance of interdisciplinary effort in ecological modeling and, in particular, the need for hydrodynamical studies to elucidate the distribution of seagrasses in shallow depths. Moreover, the use of predictive models would permit evaluation of the effects of coastal activities (construction of ports, artificial reefs, beach nutrientinput, dredging) on benthic ecosystems.

Posidonia oceanica beach-cast litter in Mediterranean beaches: a coastal videomonitoring study

ABSTRACT Gómez-Pujol, L., Orfila, A., Álvarez-Ellacuría, A., Terrados, J. and Tintoré, J., 2013. Posidonia oceanica beach-caster litter in Mediterranean beaches: a coastal videomonitoring study. Mediterranean nearshore sandy and rocky bottoms are colonized by the endemic reef-building seagrass Posidonia oceanica. This species loses leaves in autumn that form large litter patches in the surf zone and huge litter banks on adjacent beaches, resulting in wedge and layered structures of few centimetres to several meters in thick (banquettes). Some authors pointed the importance of those banquettes for the protection of sandy beaches because they dissipate wave energy. By contrast other authors state that this effect is almost negligible. This work deals with the role of Posidonia oceanica accumulations in Mediterranean beach morphodynamics. By means of coastal video-monitoring and wave records we assess the marine conditions related to the formation and destruction of banquettes and evaluate their role in the protection of two sandy beaches (Cala Millor and Son Bou, Balearic Islands). Results indicate that banquettes are common beach features at the study sites although they are not persistent and experience complex construction and destruction dynamics throughout the year. Two different types of banquette construction can be differentiated over the year: one related to the reworking of older seagrass beach cast by alongshore currents and a second as a response to the incorporation of new volumes of dead leaves after energetic winter storms (Hs ~2 to 3 m). In both cases, seagrass cast accumulations are continuously built up and destroyed and rarely persist before the arrival of new sea storms. Therefore, at least for semi-enclosed sandy beaches, the protection role of seagrass banquettes should be reconsidered.

An Alert System for Beach Hazard Management in the Balearic Islands

A real-time beach hazard level associated with nearshore hydrodynamics is presented in this article. The suitability of the discussed alert system is illustrated via its application to fifteen beaches in the Balearic Islands (Western Mediterranean Sea) providing nearshore safety conditions for beach safety manager. The system provides daily forecasts of nearshore wave conditions using the deep water wave forecasts. The shallow water wave data (wave height, period, and direction) together with the morphology of the site (presence of bars, capes, beach type, etc.) are used to define a hazard level (low, medium, and high) associated with local conditions. The resulting hazard level is transmitted via SMS to lifeguards and local authorities for real-time beach management. The low computational cost of this system after the initial implementation and subsequent calibration results in a very suitable approach for beach management in order to mitigate risks related to local hydrodynamics.

The Impact of New Multi-platform Observing Systems in Science, Technology Development and Response to Society Needs; from Small to Large Scales…

New monitoring technologies are key components of ocean observatories, also called marine research infrastructures being implemented in the worlds oceans. As a result, new capabilities to characterise, in quasi-real time, the ocean state and its variability at small scales exist today. The challenge is the integration of theses multiplatform observing and forecasting systems to (a) monitor the variability at small scales (e.g. mesoscale/weeks) in order to (b) resolve the sub-basin/seasonal and inter-annual variability and by this (c) establish the decadal variability, understand the associated biases and correct them. The challenge is also to change focus and now monitor from small to large scales. SOCIB is leading this new small to large-scale multi-platform approach in ocean observation. Some examples are presented and discussed together with initial ideas on the optimal design of an observational network in the world oceans, responding to science priorities, technology development and response to strategic society needs.