Executive Summary

    The presence of a shallow depression over the Balearic Sea with a minimum pressure of 1012 hPa and a high pressure centre over northern Europe (1047hPa) around the 26 December 2008 (Sant Esteve’s Day in Catalonia), originated an extreme storm that hit the Catalan coast with maximum wave heights of 14.4 m coming from the east. This event is the largest ever recorded in the area and probably the most virulent one in the last 61 years. The unprecedented swell originated caused three casualties and extensive damage in harbors, waterfront esplanades, strands, and other coastal infrastructures worth millions of Euros. No attention, however, was paid to the potential damage caused to the nearshore marine resources.

Many conspicuous evidences pointed to a possible relevant damage to the coastal marine biota (e.g., hundreds of fish stranded in the Costa Brava, the displacement of heavy boulders at depths down to 20m, or totally denuded photophilic algal communities). Owing to these evidences and to some rash talk to the press by some scientists, a debate was raised about possible catastrophic consequences to the natural heritage and to the aesthetic value of some submarine seascapes of the Catalan Coast. In response to this uncertainty, 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 assessment of the impact caused by the storm on the natural coastal populations and 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 (Generalitat de Catalunya)”. The overall initiative was supported by around 50 participants (tenured, PhD students, technicians, and naturalists), and involved associations of professional fishermen and underwater sportsmen (spear fishermen and divers).

The physical analysis of the storm allowed to establish three different areas: (i) the northernmost part of the Catalan coast (Costa Brava) – characterized by the Roses and Palamós buoys – where the greatest wave power, the largest wave heights, and the longest storm duration were recorded, and where the storm first began. Following the storm classification of Mendoza and Jiménez (2008), the storm in this area can be classified as “extreme”. This storm was also the largest ever recorded by those two buoys. Maximum shear stress was above 70 Nm-2 at 5 m depth; (ii) the central coast – characterised by the Tordera and Llobregat buoys – where the wave power decreased down to about half of that in the northern area, with smaller wave heights (although still large), a slightly shorter duration and slightly later impact. Maximum shear stress was between ca. 50 and 70 Nm-2 at 5 m depth; and (iii) the southernmost area (the rest of the Catalan coast), where the wave power was only one third of that of the Costa Brava, with relatively small wave heights. Maximum shear stress below ca. 50 Nm-2 at 5 m depth. The average shear stress was of 78.2 Nm-2 for the whole coast, with 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 (see Figure 1).

Against intuition, the most important agent of damage was not the direct effect of the hydrodynamic shear stress, but the impact and abrasion caused by the relative movement of the substrate particles surrounding the organisms. An appropriate analogy is that represented by a firearm or a bomb without or with bullets or shrapnel. A shot or a moderate explosion generates a shock wave that  may case some damage, but normally only in the very vicinity of the explosion source. However, when that wave carries hard particles with it, the effect can be highly destructive. Those organisms growing on stable rocky substrates resulted virtually unaffected by the storm (algae, sponges) 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 (continuous rock, boulders, sand), and the morphological traits of the organism. The abundance of highly targeted fish species (including those accessible to artisanal nets and bottom long-lines) was not altered by the storm or the changes observed were small and/or not conclusive. For a number of commercial species the catch rate patterns changed significantly. For those for which catch rate increased (e.g., Palinurus elephas, Phycis phycis), an increase in the catchability, not in the abundance, seemed to be the most plausible reason. For these species, a higher vulnerability is predicted and a medium- to long-term negative effect hypothesised.

Among the hundreds of species monitored in the ca. 200 stations along the Catalan coast, only around 20 species and 1 community in the northern-most area suffered moderate to high impacts owing to their delicate body structure, mobility of the surrounding substrate, and exposure to wave energy. This was the case for some algal communities (0% to 94% biomass loss from shallow environments to down to 24 m depth), and some populations of the seagrass Posidonia oceanica (5% and 23% area loss below and above 10m depth, respectively), the sea urchin Paracentrotus lividus (up to 80% loss of individuals), the fan mussel Pinna nobilis (from 0% to 100% of the shallow populations), and the soft coral Paramuricea 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 one (Table 1).

Observations and surveys conducted in 2010 and 2011, confirmed the slow (algal cover, sea urchins) or 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. The recovery of the original mature algal community, however, will not take place but in several years time.

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 before and after the storm, or the methodological approach used. In spite of this, the information examined provided sound evidence supporting that the majority of nearshore natural populations and communities of the Catalan coasts resisted well the effects of the extreme storm of 26 December 2008. With very few exceptions, no critical ecological or economical damage has been caused to the coastal ecosystem. It remains unclear, however, to which extent may have the overall resilience of the coastal ecosystem been eroded. The monitoring programmes may be able to answer this question in the coming years.

In any case, 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.

Figure 1. Preliminary representation of the distribution of the shear stress applied, during Sant Esteve’s Day 2008 (26 December) in some of the stations sampled in this project. The image encompasses all depths, from 3 to 25 m. The higher intensity of the storm and the larger sampling effort made in the Costa Brava (in the area of Medes Islands and Montgrí coasts, in particular) is made evident (circle diameter is proportional to the shear stress). See Chapter 2 for details on the estimates of the shear stress.

Table 1. 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.