Introduction

   Change and disturbance

  Change is inescapable. Natural communities are constantly changing at multiple spatial and temporal scales. Cycles, trends, cycles within trends, or even extinctions are commonplace. The earlier ideas of stability or constancy applied to communities have long ago been replaced by others like resilience or flexibility. But even these last ones, not to say the former, presuppose the existence of a certain average probabilistic state that tends to be maximised by a number of self-regulating mechanisms inherent to all natural systems (Levin 2005). Keeping an open mind and fighting the natural yearning of humans for predictability and against surprise, that tendency might be, at best, a theoretical truth.

Our knowledge of past climatic events provides a number of examples that demonstrate the disability of communities to recover between major climatic changes (e.g., Scheffer et al. 2001; Worm et al., 2006; Huges et al., 2007). In our world these events regularly change the communities, the ecosystems, and the landscapes. In such a scenario, the strategy of resisting that environmental variability seems not to be the most desirable one. Instead, there is abundant and growing evidence that adapting to environmental change provides the highest guaranty of success. Adapting means changing the ‘set of organisms’ and their interactions in a given environmental scenario by natural selection.

So climatic disturbance drives spatial and temporal heterogeneity and natural selection the evolution of life histories (Turner et al., 1998). Then, if disturbance and change are unavoidable and adapting to them seems to be something necessary and inherent to natural communities, why are we concerned about the effects of the storm of Sant Esteve 2008 on our coastal natural communities? We need to briefly reflect on two things to answer this question: 1. The concept of disturbance, and 2. Our dependence on the resources surrounding us.

In the mind of most of us, disturbance is pictured as something unusual, unpredictable and abrupt that forces ecosystems away from near-equilibrium conditions. In a context sufficiently broader, in a world where the examples of systems far from equilibrium are overwhelmingly more abundant than of those near static conditions, this notion can be meaningless. But locally, and for a given time frame, this can be true and certainly a matter of concern.

All forms of life base their survival upon maximizing the efficiency of the exploitation of the resources around them. In human societies, establishing what is efficient, what is necessary or what is right or wrong when talking about resources management, very easily turns into a metaphysic challenge. To simplify, humans seek all or some of the following benefits from nature: 1. protection, 2. food, or 3. pleasure. Although the edges separating these three benefits are seldom sharp and always specific of each society, there are many clear examples. The example par excellence is biodiversity. Preserving biodiversity is desirable because it ensures ecosystem resilience and constitutes and ‘arsenal’ of ways to cope with change. Concomitantly, it is also a source of energy (food) and beauty.

For the Catalan society of the beginning of the 21st Century, the storm that hit the coast the 26 December 2008, Sant Esteve´s Day was abrupt (sudden and unexpected) and extreme (unprecedented in the last 61 years). Several municipalities of the Costa Brava reported damage worth millions of euros embracing both public and private facilities and goods. The presumable impact of the storm on the natural coastal communities however, kept largely hidden below the sea surface. A detailed assessment of this impact is relevant for 1. determining possible major losses of ecosystem resilience and services, 2. understanding the mechanisms through which the different populations succumb or resist to extreme hydrodynamic forces, and 3. for setting the “zero point” as a reference to assessing the time needed by the various populations to recover from the perturbation or to consolidate a new state. Ultimately, all this information will augment our capability to predict the damage made by future storm events and therefore help the management of coastal resources.

State-of-the-art

Studies addressing the assessment of the impact of extreme storms on coastal natural populations and communities in temperate seas is surprisingly scarce. In the Mediterranean, despite if the wealth of information available about the genesis of marine storms and on the energy they release on the littoral zone, the literature on the effects on natural communities is virtually lacking. Most of the available studies address the effects of wave energy on intertidal marine organisms (see Denny, 1994, Denny, 1995, Denny, 1999, Denny, 2000, Denny and Blanchette, 2000, Martínez, 2001, Denny et al., 2003, Helmuth and Denny, 2003, Grémare et al., 2003, and Prowse and Pile, 2005) and shallow subtidal benthic macroalgae (Utter and Denny, 1996, Denny et al., 1997, Denny et al., 1998, Denny and Hale, 2003, Gaylord and Denny, 1997 and Kawamata, 2001), but not as associated to a storm and, certainly not to an extreme storm. The reasons are basically two: the low frequency of extreme storms and the scarcity of monitoring programmes able to provide the state of the communities before the event.

The richest source of information to provide a framework for the results presented in this report come from tropical areas (the Caribbean in particular) and the effect of hurricanes on coral reef communities. Despite the species differ from those in the Mediterranean, the mechanisms determining damage on the communities may be perfectly comparable.

Lessons from hurricanes and coral reefs

Direct impact of hurricanes on corals translates into breakage or detachment of corals and other sessile organisms inhabiting the reef (algae, sponges, seagrasses, etc.). Depth, slope, degree of exposure, wind direction, and organism physiology and morphology have been identified as the most important factors explain the severity of the impact (Sousa 2001).

The life history of the community under study often explains an important part of the effects observed. The communities inhabiting an area subjected to regular visits of hurricanes are likely to be adapted to such disturbance regime. Therefore, the species present in the area will experience little change by the effect of the hurricane. The massive and small forms will be present in shallower areas and will be gradually replaced by more tree-like and delicate species with increasing depth or the gradient of shelter (Beltran-Torres et al 2003). 

In the case of coral reefs, the indirect damage caused by the instability of the substrate surrounding the organism (sand, boulders) is often realized by fragments of broken corals that impact intact colonies in what is known as the ‘domino effect’. This mechanism was described when documenting the effects of hurricane Hugo in 1989, with wind gusts up to 320 km/h (Edmunds and Witman, 1991). In this study, more than 33% of the massive round coral Monastrea annularis at 10m was killed. Despite of its shape, resistant to wave action, the fragments of the few colonies that were detached impacted the intact ones propagating the destruction by means of direct impact or abrasion. Similarly, a survey to evaluate the effects of 6 consecutive hurricanes that hit the French Polynesia between 1982 and 1983, showed that the most affected coral populations were those beyond 35m depth, with a 100% of destruction. In shallow waters, the destruction affected between 50 and 100% of the corals. Between the surface and 12m, the destructions was 60-80%. Between 12 and 30m, of 80%. It was concluded that the direct effect of the wave action affected the corals down to 18-20m, while in deeper areas the effect was exclusively caused by the massive debris ‘rain’ from upper layers (Harmelin-Vivien and Laboute, 1986).

A thorough study of the effects of hurricane Allen in Jamaican coral reefs provided many enlightening results (Woodley et al 1981). The communities were studied in the sheltered bay of Discovery Bay. Again, wave height, exposure to the wave direction, depth, slope, shape and size of the organisms were the major factors explaining differences in the effects. Gentler slopes received a stronger impact than vertical walls; 99% of tree-like shallow corals (Acropora spp.) were lost, while leaf-like corals (Agaricia agarices) lost 23% of their individuals and massive ones only 9% (like Montastrea annularis). Damage to gorgonians, corals, and sponges ranged from partial to complete mortality and was caused by abrasion, burial, and the tearing or fracture of tissue and skeleton.

Defining and characterizing a disturbance

In a recent review (Montefalcone et al, 2011) disturbance is defined as an unpredictable, episodic event, due to an external agent, which disrupts the state of the ecosystem causing abrupt mortality, and, hence, subtraction of biomass (from Latin dis, intensive + turbare, to throw into disorder). There are many other valid definitions but a discussion on the topic is out of the scope of this report.

A full understanding of the dynamics of populations within habitats subject to disturbance requires knowledge of the regime of disturbance and of the subsequent patterns of recolonization and succession in the disturbed patches. These patterns are a product of certain characteristics of the original disturbance and the life histories of the species available to reoccupy the disturbed site. Sousa (1984) compiled the main descriptors used to characterize a disturbance:

1. Areal extent - the size of the disturbed area.

2. 2.Magnitude - consists of the following two components:
   

a. Intensity a measure of the strength of the disturbing force (e.g. fire temperature,           wind speed, wave velocity).

2. b.Severity - a measure of the damage caused by the disturbing force.
   

Although both of these terms have often been used interchangeably, severity seems to denote better the amount of damage caused by a disturbance. 

3. 3.Frequency - the number of disturbances per unit time. Separate terms are used for the average frequency of disturbance at the local and the regional spatial scales:
   

a. Random point frequency - the mean number of disturbances per unit time at a random point within a region; this is often expressed as the recurrence or return interval (i.e. the average time between disturbances).

2. b.Regional frequency - the total number of disturbances that occur in a geographical area per unit time.
   

4. 4.Predictability - measured by the variance in the mean time between disturbances.
   

5. Turnover rate or rotation period - the mean time required to disturb the entire area in question.

We will show how the storm of Sant Esteve’s 2008 affected a long stretch of the Catalan coast with very variable intensity and severity. We will also show that a storm of such magnitude is believed to be very infrequent and unpredictable. Fortunately, a consequence of that low frequency is that the rotation period cannot be but very long (see further).

Damage: mechanisms and factors

Water motion (wave action) is the agent of the damage caused by storms to marine organisms. The primary mechanism is the force that water exerts on them just as wind does in terrestrial environments. In addition, suspended particulate matter (e.g. sediment) or larger objects (e.g. logs, cobbles, fragments of calcareous organisms) abrade the substratum over which the water flows and may hit on organism structures that protrude the surface (e.g. Yoshioka 1989). Aquatic organisms may also be killed either by burial under sediments that have been displaced and redeposited by moving water (e.g., Cabaco et al., 2008), by dislodgement or uprooting after sediment removal, or by exposure to air when water motion changes drainage regimes (Hughes and Connell 1999). For this reason, the substrate where the organism is located and/or surrounded by, plays a critical role and, consequently the stability of the substratum directly determines the rates of disturbance. In streams and on the seashore, strong water motion overturns loose rocks, damaging the attached organisms. The frequency of this kind of disturbance declines with increasing rock size.

The relative position of the organism respect to the direction of the wave energy is obviously also crucial. Depending on the degree of exposure (or degree of sheltering) to the wave action, individuals of the same population may be totally devastated while others, only a few meters away, may remain intact. 

The morphological and physiological characteristics of the organisms strongly determine the severity of the damage caused by wave action. The risk of damage or death by a given wave force varies among species that differ in shape, flexibility, or internal structure, among other factors. For example, tree or bush-like corals or algae (e.g. gorgonians, Acropora spp., Cystoseira spp., etc.) are more susceptible to wave damage than mound or sheet-like or encrusting types. Increasing size and/or age decreases the risk of damage or death from some forces (e.g. of woody vegetation from ground fires; intertidal organisms from desiccation stress; coral colonies from sediment burial) but increases vulnerability to others (e.g., large, old trees to windthrow; older stands of trees and bushes to crown fires; larger intertidal organisms and branched corals to wave forces).

The compilation of studies presented in this report provide unedited information of most of the mechanism referred to above and the actual damage they caused in various communities and populations.

After the disturbance: recovery or change

Another major issue when evaluating the impact on a population is the time required by the affected population to recover, in other words, the ability of the organisms to repopulate the gaps created. This is an important information required to assess the effective severity of the damage, i.e., if the damage is going to be temporary or permanent. In the first case, it is important to know the rate of recovery and, in the second case, which will be the implications of the loss (or the dramatic reduction) of a population for the community or the ecosystem.

It might be interesting to summarize here the factors that would be determining how fast the ‘gaps’ opened by the storm in the benthos (and, in the case of some fish populations, also in the water column), will be repopulated by resident or allochthonous species. The rate and pattern of reestablishment following a disturbance may depend on the following elements (modified from Sousa 1984):

1. 1.The morphological and reproductive traits of species that are present on the site when the disturbance occurs. Such traits determine, in part, the likelihood that these species will survive the event and rapidly reoccupy the site.
   

2. 2.The reproductive and dispersal biology of species that were not present on the site when it is disturbed but that live within dispersal distance of it.
   

3. The characteristics of the disturbed patch including:

a. the prevalence of the effects of the disturbance that created it,

b. its size and shape,

c. its location and degree of isolation from sources of potential colonists,

     d. the time it was created.

At one extreme are large clearings with no survivors and no dormant propagules. These areas will initially be recolonized by species producing widely dispersed propagules at the time the patch is opened. Germination or metamorphosis of the propagules soon after dispersal and rapid early growth are advantageous for successful establishment. If they are not re-disturbed subsequently, such patches often undergo a long period of succession as other species slowly invade and gradually replace earlier colonists. At the other extreme, the smallest disturbances are filled almost exclusively, and relatively quickly, by the vegetative growth of survivors living either within the clearing or on its edge. Little or no successional replacement takes place in this case.

The mechanisms for recolonizing the gaps vary widely depending on the species characteristics. These mechanisms will range from (i) germination from locally stored propagules or seeds of the damaged species, (ii) regrowth of damaged tissues, (iii) vegetative growth from neighbouring individuals, or (iv) arrival and establishment of propagules or individuals of non-resident species.

This study

Approach

This report is mainly based on the data and results that were obtained in several scientific studies that have been carried out, or are still en course, by scientific personnel belonging to the CEAB (CSIC), ICM (CSIC) and the Department of Ecology of the University of Barcelona. Only in some cases, new studies have been specifically devoted or adapted (e.g., sampling date moved to right after the storm) to assess the effects of the storm. So the sampling design and strategy of most of the studies collected here cannot be considered ideal designs for the so-called ‘Before and After Control Impact’ (BACI) or the most suitable “beyond BACI” designs (e.g., Underwood AJ. 1991, Smith et al. 1993). That uses to happen in most studies dealing with catastrophic events due to the intrinsic unpredictability of suchlike phenomena. In the present case, the impossibility of implementing such a design is reflected in the lack of true controls (i. e., comparable zones not impacted by the storm). However, the wealth of sampling stations with all the range of degrees of exposure, and the extensive experience of the researchers involved, has allowed to unequivocally assess the effects of the storm for many communities and populations. This study is, therefore, an unplanned, uncontrolled, but highly valuable and unique natural experiment.

The project was conceived in two steps: one to assess the immediate effects of the storm, and a second one to follow the evolution of the areas affected. The first corresponds to the present project (PIEC-CSIC), and the second was submitted for funding to the MICINN “Acciones Complementarias” scheme (2009) without success.

In this report, therefore, references to the recovery of the impacted ecosystems will not be done but very occasionally in those cases where the ecosystem has been revisited in the years following the storm.

Populations/communities studied

In this study we compile the assessment of the impact of the storm on rocky and soft bottom sessile populations and communities as well as on littoral fish populations and some deep sea fauna. The results obtained for these last ones, in particular, represent a specially unique achievement of this project. With some notable exceptions, ecologists have largely overlooked the significant role of physical disturbance in the biology of mobile animals. This is mainly because the direct effects of disturbance on mobile organisms are not as easy to observe and measure as for sessile organisms. Severe climatic conditions can indirectly exert a strong negative impact on populations of mobile organisms by eliminating or producing shortages in vital resources, damaging nursery areas and habitats, or hitting vulnerable forms of the species.

Specific studies are devoted to sea urchins, sponges, soft corals, bivalves, seagrasses, highly targeted fish species, or canyon deep sea swimmers. More general studies addressed communities as a whole, such as upper sub-littoral, rocky bottom or soft bottom communities. The area of study embraces the whole Catalan coast but is mainly focused in the Costa Brava and, most specifically in its northern part (Medes-Montgrí).

Objectives and justification

We compile the results of a multidisciplinary group created ad hoc to uncover the impact of the storm. We present data on the types and magnitudes of damage caused by Sant Esteve’s storm 2008 along the Catalan coast (NW Spanish Mediterranean). More concretely, the study aims at assessing:

1. The exceptional meteorological setting that ended in this extreme storm,

2. How the energy generated by the storm was translated into tangential forces applied to the studied communities,

3. The impact of these forces on a number of different populations and communities inhabiting on rocky and soft bottoms,

4. The effects of the storm on four different types of fish assemblages, 

5. How hydrodynamic and biological patterns correlate, and

6. 6.The overall effect of this extreme climatic event on the natural nearshore marine resources of the Catalan coast.
   

As far as we are aware, the timeliness and comprehensiveness of the approach taken in this initiative have no precedent in the Mediterranean. In fact, with the exception of  a few works on the fan mussel Pinna nobilis, studies addressing the effect of a storm in natural coastal populations in the Mediterranean are virtually lacking. We believe that the data and observations presented here are pioneering and of an unusual value for the following reasons:

First: The magnitude of the storm had no precedent in the last 61 years. It is the first time that a ‘natural large-scale in situ experiment’ is possible;

Second: We benefitted from the also unique availability of a wealth of monitoring programs and observations before the storm;

Third: Owing to the increasing human pressure in the coastal zone, the Spanish Mediterranean nearshore natural ecosystems are experiencing an also growing deterioration. Assessing the damage and establishing starting reference points after an extreme event, are invaluable information to assess ecosystem resilience.

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