El temporal extremo de Sant Esteve 2008 y las comunidades litorales catalanas

Because of the unprecedented severity of the storm (the last similar one was recorded 61 years ago), it was soon made evident that this was the first opportunity to assess the effects of an extreme large scale meteorological perturbation on the structure and dynamics of coastal marine ecosystems using a wealth of human and technological resources. Besides, the initiative could benefit from the availability of a number of long-term data series of several benthic communities that are being monitored over the years by many researchers working in the area. This data series would allow for invaluable “before vs. after the storm” comparisons and for the correction for trends or cyclic patterns.

In view of the above, the uniqueness of the storm, and of the debate raised, the Research Group “Diversity and ecology of the marine benthos: from molecules to ecosystems” of the Centre for Advanced Studies of Blanes (CEAB-CSIC), elaborated an action plan to make a rigorous and multidisciplinary assessment of the impact caused by the storm on the various natural coastal communities. The plan included experts in benthic ecology of the Ecology Department of the University of Barcelona and of the Institute of Marine Sciences (ICM-CSIC), hydraulic engineers from the Polytechnic University of Barcelona (Laboratory of Marine Engineering) and from the University of Cantabria (Institute of Environmental Hydraulics), and experts of the Parc Natural del Montgrí, les Illes Medes i el Baix Ter (Generalitad de Catalunya). The initiative has been supported by around 50 participants (tenured, PhD students, and technicians), and has involved individuals and associations of professional fishermen and underwater sportsmen (spear fishermen and divers).

With very few exceptions, the extreme easterly storm that hit the Catalan coasts the 26 December 2008, did not have critical consequences from the ecological or economical points of view on the marine nearshore natural resources.

Only those populations with a particularly low turnover (Paramuricea clavata, Pinna nobilis, Posidonia oceanica), have not shown any significant recovery so far. Full recovery for these populations will take several decades.

The damages caused by this natural meteorological event are to be considered insignificant if compared to those derived from the anthropization of the coastal zone, namely, overfishing (species collapse), pollution and eutrophication (drastic diversity decrease, ecosystem state change), or trawling and coastal development (mechanical benthos destruction). This contrast, vividly evidences that human activities are far more unexpected and extreme for the natural evolutionary mechanisms than any storm recorded so far.

Overall conclusions

The storm of 26 December 2008 that hit the Catalan coasts was caused by a shallow depression over the Balearic Sea with a minimum pressure of 1012 hPa and a high pressure centre over Northern Europe of 1047hPa. This event was the largest ever recorded at the locations of Roses and Palamós (Costa Brava, Spain). The average shear stress was of 78.2 Nm-2with a maximum value of 233.5 Nm-2 estimated for the area of Portlligat Bay (to the North of the Bay of Roses) at 5 m of depth. The previous comparable storm in the Catalan coast was probably that of 1947, i.e., 61 years before the extreme event of Sant Esteve 2008. No temporal patterns or correlations with global atmospheric events have been found to be associated with these type of events, making them unpredictable.

Although the robustness and universality of the conclusions from some of the studies compiled in this project are affected by a number of limitations related to the timing of the sampling, the time range of the monitoring, or the methodological approach used, the information examined strongly suggests that the majority of nearshore natural communities of the Catalan coasts resisted well the effects of the extreme storm of 26 December 2008. Exceptions were some species and communities in the northern-most coasts which, owing to their delicate body structure, mobility of the surrounding substrate, and exposure to wave energy, suffered moderate to high impacts. This was the case for some algal communities (0% to 94% biomass loss from shallow environments to down to 24 m), and some populations of Posidonia oceanica(5% and 23% area loss below and above 10m depth, respectively), Paracentrotus lividus (up to 80% loss of individuals), Pinna nobilis (from 0% to 100% of the shallow populations), andParamuricea clavata (average loss of 13.4% of the individuals). Because of their low recruitment capacity and turnover, the loss of individuals of P. oceanica, P. nobilis, and P. clavata (all three protected species in the Mediterranean) is to be considered the most critical and potentially permanent.

Against intuition, the most important agent of damage was not a direct consequence of the hydrodynamic shear stress, but of the impact and abrasion caused by the relative movement of the substrate particles surrounding the organisms. Those growing on stable rocky substrates resisted quite well the impact, while sessile organisms growing among boulders or on the sand suffered the highest damage due to abrasion, burial, dislodgement or uprooting. The main factors modulating the damage were exposure to wave action (latitude, orientation, and depth), the type of surrounding substrate, and the morphological traits of the organism. Thus, the delicate algal community of the exposed shallow photophilic mobile environments underwent the most severe damage.

Observations and surveys carried on in 2010 and 2011, confirm the slow (algal cover, sea urchins), very slow or null recovery (seagrass meadows, gorgonians, fan mussels) predicted for those populations and communities that were severely impacted by the storm. The seagrass meadows and the algal communities are key for the spatial and trophic structuring of the ecosystems. While the first ones will take decades to slowly begin to recover, the algal cover lost from large areas of rocky bottom, have been largely recolonized by pioneering seasonal species, not by the original more climax ones. The recovery of this crucial mature algal community will not take place but in several years time.

Images:

Burial of Posidonia oceanica shoots in Medes Islands, Girona, as a consequence of the storm. Photo credit: Àlex Lorente.

Hundreds of fish were found stranded in L’Estartit, Girona, after the storm. Photo credit: Boris Weitzmann

Summary of the species, groups or communities studied and the changes experienced. Observed change % may refer to number of individuals, biomass, cover, observation frequency (upper infralittoral algal community), or catching efficiency (littoral fisheries resources). Impact scale used: positive, none; negative: low, moderate, high, extreme. ‘va’, ‘a’, and ‘r’ stand for zones where Cystoseira mediterranea was very abundant, abundant or rare, respectively. ‘ns’ stands for no significant. Notice that a ‘positive’ impact assessment refers to the effect of the storm on the concrete species. The effect of such change on the ecosystem/community functioning could be different.

Link to the Excutive Summary: http://www2.ceab.csic.es/GAME/Sant_Esteve_Storm/Executive_summary.html

Link to the project website: http://www2.ceab.csic.es/GAME/Sant_Esteve_Storm/HOME.html