Conclusions

Main conclusion

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.

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-2 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. 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), and 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.

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.

Specific conclusions

The populations of the sea urchin Paracentrotus lividus experienced the highest massive mortality ever recorded in the Mediterranean with a loss of up to 80% of the individuals and 90% of the biomass in the area Medes-Montgrí. In the various Posidonia oceanica meadows studied, the average loss recorded was of 46% of the individuals in meadows growing in Fenals Cove at 8 m. A continuation of the monitoring programme of this population is needed to follow its evolution and the interactions with its predators and with the algal cover. It will provide invaluable and unprecedented ecological information on complex interactions in these benthic populations in the Mediterranean.

Wave action was responsible of the overall loss of 13.4% of the Paramuricea clavata colonies in the Medes-Montgrí area. This value was found to be 8 times greater than the natural mortality rate due to detachment (1.5% y-1). The effects of the storm in this species has to be considered as very severe because in this long-lived organisms, with a very low recruitment rate, even a small increase in the mortality rate may produce unsustainable long-term effects on the population.

The overall mortality of tagged sponges populations in the Costa Brava was low (6.6%). Collectively, the results suggest that most of the sponge species studied have evolved effective anchoring mechanisms, shapes, and textures to withstand the forces generated during heavy, periodic, and even extreme storm events. The effects of the storm have been found to be potentially beneficial to the bioeroding sponge Cliona viridis. The elimination of a major part of the algal cover may have increased light availability for these photosymbionts-bearing sponges.

The deep-water, slow-growing Fucal Cystoseira zosteroides lost up to 80% of its biomass in some of the studied locations, being the highest ever recorded. One of these populations was found to be quickly recovered owing to a very high rate of recruitment. Differential traits of the two sites studied has allowed to hypothesise that catastrophic events such an exceptional storm can be determinant for the structure and long-term dynamics of C. zosteroides populations. However, the combination of these events and smaller scale, short-term factors, make it almost impossible to predict the distribution and size structure of this stands.

A thorough study before and after the storm using detailed cartography along the Catalan coast showed that the upper sub-littoral macroalgal communities were not significantly affected. The emblematic macroalga Cystoseira Mediterranea, in particular, far from being affected seemed to have experienced a certain progression in some areas, probably at the expense of a decline in Corallina elongata. In some areas of Medes-Montgrí, the results are suggestive of small changes in the organization of the shallow benthic communities, where the benthic per cent cover of bare rock and hydrozoans significantly increased after the storm at the expense of foliose algae. In contrast, some deeper algal communities, from 5 m to up to 25 m, have been devastated by the storm. While exposure was not determinant for upper sub-littoral communities, it was critical for those algal communities living on a mobile substrate. Wave action combined with lose medium to big-sized boulders had a high destructive effect on the photophilic algal community of Medes-Montgrí, resulting in cover loss from 0% to 94%.

Around 23% and 5% of the area of the Posidonia oceanica meadows above and below 10 m surveyed in this project, respectively, were found covered with more than 10 cm of sediment. As deleterious effects on this seagrass have been observed starting with only 4 cm of sediment, a much higher mortality is suspected. Moreover, as the effect of uprooting was not addressed in this project, an even larger mortality for P. oceanica meadows is hypothesised. Owing to the extent of the disturbance, the low recruitment rate of this species, and its key role as ecosystem services provider, the impact on P. oceanica meadows is categorized as severe.
Other emblematic sandy bottom dwellers such as the endangered fan mussel Pinna nobilis resulted untouched or devastated depending on the degree of exposure. In the leeward zone of the Meda Gran in Medes Islands, all the population censed before the storm was found intact. Conversely, all the individuals of the population growing in Salpatxot area, largely exposed to the storm, disappeared. An intermediate impact was observed in Giverola Cove, where the population slightly sheltered by the north end of the cove was reduced in 16%, contrasting with the loss of 33% of the individuals in the more exposed area. As for the seagrass P. oceanica, the main mechanism of damage was burial.

In contrast, the macroinvertebrates of soft-bottom environments (Mediterranean well calibrated fine sand community) from 8 to 14 m depth resulted unaffected.

A significant decrease in the mean species richness, abundance and total biomass of littoral rocky reef fish assemblages was observed when compared to counts from previous years (1999, 2002, 2005). The most conspicuous effects were: 1) a decrease of the abundance and biomass of Chromis chromis, Coris Julis and Diplodus sargus (to, at most, 50%) and 2) a decrease of biomass, but not of the total abundance, which was counterbalanced by a very high recruitment after the storm in Serranus cabrilla, Symphodus tinca and small Symphodus spp. (to, at most, 50%). The rest of the species studied, including the populations of cryptobenthic fishes from the upper sublittoral zone, resulted unaffected by the storm.

Dusky groupers (Epinephelus marginatus) were undoubtedly affected by the storm as evidenced by the 30 specimens found on the beach of l’Estartit the day after the storm. However, from the results obtained here it cannot be concluded that the storm had a significant impact on them and other highly targeted species in Medes-Montgrí. There are, at best, some indications of possible effects, such as the reduction in dusky groupers in the totally protected area of Medes Islands, along with the increase in this species in the partially protected area of El Molinet. Such a reduction, however, falls within the range of normal interannual variations. The sea bass Dicentrarchus labrax showed a sharp decline from 2008 to 2009 that may have been caused by the storm, and which led to the minimum values for this species since 1992. However, the poor statistical quality of the density and biomass data of for this species, prevents from obtaining a sound conclusion.

This project has provided unprecedented information suggesting that the abundance of most of the species accessible to artisanal nets and bottom long-lines was not altered by the extreme storm. However, the catch rate patterns of a significant number of species (up to 18) changed substantially. Most of these species showed an increase in catch rates. This phenomenon might be due to an increase in the spatial aggregation of the species, which increased their fishing availability. If this proved true, the observed changes would be suggestive of an increase in species vulnerability and, therefore of a negative effect on the populations in the medium term.

Results suggest that the storm had a stronger effect on the Blanes submarine Canyon than at the open slope as showed by the collapse of swimmers abundance after the extreme event. Inside the canyon significant differences were recorded on taxa composition of swimmers between samples collected before and after the storm. The results also highlight the role of the Blanes Canyon as preferential conduit for matter and energy exchange between shelf and deep basin. Although the effect has to be considered occasional, further studies are needed in order to understand its potential impact for the ecosystem.

Of the 42 individuals, including artisanal fishermen, spear fishermen, and SCUBA divers, inquired in this project about their perception on the effects of the storm on the marine biota, around 70% reported moderate to severe effects in some populations of both the benthos and the fish assemblages in the Costa Brava. The high degree of detail and skill in their observations was remarkable,  showing a good agreement with the results obtained by the research groups of this project. Factors like latitude, type of substrate, or depth were neatly identified by underwater sportsmen. It has to be concluded that these three groups have to be regarded as a valuable source of information of changes in the marine benthos both for monitoring purposes and for assessing impacts after rare extreme events.