Rocky Coasts Research Articles

Effects of artificial light at night on the mobility of the sea urchin Paracentrotus lividus

Light pollution poses a significant global threat to biodiversity, driven by the increasing coastal urbanization and the resulting growth of artificial light at night (ALAN). However, to date, the scientific community has focused mainly on studying its ecological effects within the terrestrial environment. It is only recently that attention has turned to coastal marine systems which are crucial due to their essential contribution at the ecosystem level. These environments, characterized by their high productivity, also play a crucial role in protecting coasts against erosion. The aim of this case study was to investigate the possible effects of ALAN on the sea urchin species Paracentrotus lividus in four areas of an Italian rocky coast, selected according to a gradient of light intensity (0, 0.4, 3 and 25 lux), from April 2022 to February 2023. Effects of ALAN were examined by measuring the density and size of sea urchins and also their reactivity to a stress condition through an innovative technique of overturning sea urchins to study their physiological response in the presence or absence of artificial light. In addition, the permanence of sea urchins in the four areas was evaluated through an efficient tagging test. Results show how these organisms, typically nocturnal, suffer negative effects of ALAN in terms of minor density and mobility, expressed as the speed of response to an adverse event, compared to a dark area.

Null models to explain highly diverse community of intertidal decapods on a temperate rocky coast in Antofagasta, northern Chile (23º27ʹS)

Abstract The decapod fauna on intertidal shores in northern Chile is characterized by its relatively high diversity and abundance due to the presence of continuous upwelling by the Humboldt Current, which results in a high productivity. The decapod fauna on intertidal rocky shores is characterized by coexisting species that inhabits in natural shelters such as cracks and eroded rocks, as well as among macroalgae. We aimed to monitor the brachyuran and anomuran (porcellanid) crabs in three intertidal sites during two periods of time in September-December 2019 in the Antofagasta region, one site without human intervention and two sites in the city of Antofagasta, for which we applied null models in ecology. We assumed that the communities are random, a viewpoint that is more robust because it reduces the risk of error type I. The results of the application of null models on the co-occurrence species revealed that species associations are mainly structured for all sites, whereas the niche-sharing null model revealed that the reported species did not share ecological niches and in consequence there is not interspecific competition. The results would probably indicate that the species of intertidal decapods have a micro-specific ecological-niche differentiation, which would explain the existence of structured pattern in species associations. The results were compared with information for other sites along the continental Chilean coast.

Storm or tsunamis: Boulder deposits on the rocky coasts of the Balearic Islands (Spain)

The origin of large imbricate boulders at the edge of coastal cliffs, by storm or tsunamis, is usually controversial. The Balearic Islands are a perfect place to elucidate this origin because the surrounding marine basins have low storm energy compared to oceans, and they are located 250 km N of the main tsunamigenic area of the Western Mediterranean. Thus, there have been recent and historical tsunamis affecting the Balearic Islands, especially in its southern and eastern parts. Tsunami simulations from submarine earthquakes on the North African coast predict the impact of tsunami waves on the Balearic Islands. Large coastal boulders of the Balearic Islands are located in the exact places numerical models predict they will be. Despite the fact that the height and energy of storm waves are much larger from the north and west, these boulders are located in the eastern and southern parts of the islands. Documentary sources confirm a very large tsunami affecting the southeast coast of Majorca in 1756, with a run-in up to 2 km and a run-up up to 45 m. Hydrodynamic equations applied to these boulders results in storm wave height values never recorded in 60 years. In February 2020, the highest storm wave ever recorded (14.33 m) was detected at Minorca buoy, related to storm Gloria. As predicted by hydrodynamic equations, none of the boulders studied moved significantly, and only three new boulders, located at sea level or at the cliff edge, were quarried. According to radiocarbon data and rate of growth of dissolution pans, the year of emplacement of boulders in the studied localities ranges between 1591 CE and 1894 CE. Thus, the distribution of the boulder sites along the islands, the historical record of a large tsunami, the run-up necessary for their placement, and the insignificant boulder movement produced by the highest storm recorded indicates that the cliff-top boulders of the Balearic Islands were quarried by North African tsunami waves.

Life in a Gilbert-type delta system: Ichnoassociations of the Ventimiglia palaeovalley and their sequence stratigraphic significance (Pliocene, NW Italy)

Gilbert-type deltas are inherently associated with stressful conditions for the benthic infauna, i.e. highly shifting substrates, coarse-grained sediments, traction sedimentation, supercritical flows, and salinity fluctuations. Nevertheless, trace fossils are locally abundant in the Gilbert-type delta system of Ventimiglia (NW Italy), representing the Pliocene infill of an incised valley that developed during the Messinian sea-level fall. The seismic-scale outcrops of the Ventimiglia delta system allow to link ichnological features (ichnotaxa, bioturbation intensity) to the major depositional environments of the deltaic system. Accordingly, rhizoliths characterize the delta plain, whereas delta top deposits are virtually unbioturbated. The delta slope is characterized by Planolites, Thalassinoides, Scolicia-like burrows, and Ophiomorpha. Proximal prodelta deposits show a characteristic Skolithos-Chondrites ichnoassociation, whereas Chondrites and sharp-walled burrows (Thalassinoides?) dominate the distal prodelta. Reworked beach deposits occasionally present bioerosional ichnotaxa (Entobia, Gastrochaenolites, and Maeandropolydora), which suggest the presence of a nearby rocky coast. Overall, bioturbation intensity increases with increasing distance from the delta-top setting, whereas ichnodiversity has its maximum in delta-toe and prodelta units. Moreover, a clear sequence-stratigraphic signature is observed for the Ventimiglia ichnoassociations. The ichnological evolution of the Ventimiglia delta system, with bioturbation intensity increasing in forced-regressive systems tracts of successive sequences, is here hypothesized as a signature of overfilled incised valleys. The Skolithos-Chondrites ichnoassociation is especially common in the highstand systems tract, possibly reflecting higher amounts of suspended matter during the sedimentation of the highstand systems tract itself. The basal surface of forced regression usually preserves passively filled burrows with a sharp unlined margin, which fits well with a sediment-starved seafloor.

A landmark-based geometric morphometric approach to quantify deviations from bilateral symmetry in polyplacophorans

The class Polyplacophora (chitons) represents a dorsoventrally flattened mollusk group that has an oval-shaped body covered with eight overlapping sclerites providing bilateral symmetry. Chitons show abnormalities (teratologies) that are characterized by symmetry deviations between the right and left sides of their bodies. As these deviations do not result in damage that affects vital functions, chitons are able to reach adult stages. In this study, we quantify the asymmetric condition of the species Chiton articulatus using a landmark-based geometric morphometric approach to assess variation in shape and bilateral symmetry. A geometric configuration of 22 landmarks and 50 semi-landmarks was created to evaluate shape variation in abnormal and deformed specimens compared to normal Chiton articulatus specimens. Vectors of change in the body shape configurations of chitons indicate that the greatest change occurs in the anterior part of the body, with less change in the middle and posterior parts. This gives chitons a widened appearance and provides anatomic compensation to restore the bilateral symmetry of the body scleritome. The diverse abnormalities and deformities had little impact on shape variations and confirmed that the coalescence condition is an intermediate step between a normal condition and the abnormal conditions of hypomerism or hypermerism. The low levels of fluctuating asymmetry expressed in C. articulatus indicate that despite living in areas of high stress, such as the rocky intertidal coast, this species maintains stability in its development and shape. Our results can serve as a model for studying bilateral symmetry deviation in polyplacophorans.

Autochthonous Versus Allochthonous Resources in a Tropical Rocky Shore Trophic Web Adjacent to a Marine Riparian Area

Marine riparian areas and coastal vegetation are essential and important to the coastal marine ecosystem, although their interactions and functions are still unknown and ignored in marine ecological studies and integrated management planning. In southeastern Brazil, allochthonous resources derived from riparian Atlantic rainforests bordering rocky shores have been observed in abundance together with the shallow subtidal rocky reef benthos. In this study, we used stable isotopes (δ13C and δ15N) to characterize the main components in a benthic trophic web on a shallow tropical rocky shore, to identify the proportional contributions of allochthonous (marine riparian vegetation—MRV) to autochthonous (phytoplankton and algae) inputs and to test which basal food resources contributed most to the marine community on the Atlantic Forest–rocky coast interface. We found eight major food resources and seventeen consumers that we classified into different groups according to their feeding habits and biology. Although the main source of basal resources in the benthic trophic web in the present study remained autochthonous, the allochthonous resources were assimilated by all consumers. MRV is thus an important resource for some primary consumers and it should be included as a potential source of basal resources in marine ecosystems adjacent to marine riparian areas.

26,000 years of environmental evolution of an incised valley in a rocky coast (La Janda wetland, SW Iberia).

The La Janda wetland record allows to expand the knowledge of the evolution of a restricted estuary since the Last Glacial Maximum. Multiproxy analysis of the sediments (facies, geochemistry, pollen, fossils) reveals that this system exhibits a classic incised valley sequence but differences with the adjacent estuaries can be established. The La Janda sedimentary infill can be split in four main sequences: (1) the Falling Stage Systems Tract (>20.6 ka BP) and (2) the Lowstand Systems Tract (16.7–20.6 ka BP) deposits record fluvial sedimentation in a narrow and incised valley, here seminally identified in the Gulf of Cádiz estuarine system; and (3) the Transgressive Systems Tract (7–16.7 ka BP), ending with the Holocene marine flooding surface at 7 ka BP, is composed of estuarine and marine basin deposits infilling a still steep and narrow topography coupled with the fast retreat of the river mouths. The (4) Highstand Systems Tract (0–7 ka BP) is composed of estuarine basin deposits with fluvial and tidal currents deposits in a wide a gentle sloped estuarine valley and it records the transformation of the basin into a terrestrial one. Comparison to other areas in the Gulf of Cádiz allows to correlate the different estuaries and to distinguish between open and restricted ones by the development of spit systems during the Highstand Systems Tract. But further work is needed to clarify the stratigraphy of the present eustatic cycle in the Gulf of Cádiz as the scarcity of data does not allow to determine the evolution of its estuaries during the last fall and rise of sea level. Such knowledge is crucial to disentangle the role of climate and tectonics in the future evolution of our coasts and design measures to adapt to sea level rise.

Seascape characterization of a Mediterranean vermetid reef: a structural complexity assessment

In the Mediterranean Sea, vermetid reefs can modify coastal seascapes forming platforms in the intertidal zone of rocky coasts. With their three-dimensional and seaward-expanding structure, these bioconstructions support high biodiversity levels and provide important ecological functions and ecosystem services. In this study, we perform a first structural characterization of a vermetid reef seascape (hereafter, vermetid reefscape) and quantitatively assess the degree of their contribution to the structural complexity of the coastal seascape. Aerial images of a vermetid reef coast were acquired in the Marine Protected Area of Capo Gallo-Isola delle Femmine (Southern Tyrrhenian Sea) by means of unmanned aerial vehicle technology. In the seascape, the outer reef, platform, and coast classes were identified and digitized in GIS environment. The resulting vermetid reefscape was analysed by means of FRAGSTATS software using metrics belonging to the “area-edge”, “shape”, and “aggregation” categories. To quantitatively assess the structural complexity, the edge density, area-weighted perimeter area ratio, and landscape shape index metrics were applied to the seascapes with and without the vermetid reefs to simulate a sea level rise scenario. In addition, the effect of their presence in terms of coast length (i.e., total edge) was statistically tested using the Wilcoxon signed rank test. The outer reef had the highest total edge value (5,785.6 m) and, at the same time, the lowest class area (1,040 m2). It was also the patchiest, and the most fragmented and geometrically complex class in the seascape. Overall, the bioconstruction positively contributed to the structural complexity of the seascape with higher values of coastal area (12%), edge density (139%), area-weighted perimeter-area ratio (90%), and landscape shape index (66%). The Wilcoxon test revealed a statistically significant effect of the vermetid reefs presence on the coastal total edge (z = 3.9, p < 0.005), with a large effect size (r = 0.74). The results showed an overall higher structural complexity of the vermetid reefscape, indicating that its loss would lead to a significantly less complex seascape, entailing detrimental effects on the supported biodiversity levels and the valuable ecosystem services provided.

From the Rocky Coast and Across the Mountaintops: Maine's Collaborative Early Intervention Program for Children Who Are D/deaf or Hard of Hearing

Purpose: The purpose of this article is to share the old and new process of early intervention services in Maine for children who are D/deaf or hard of hearing (DHH), the Exploring Language and Communication Opportunities (ELCO) process and resources available to families of children who are DHH living in Maine. Conclusions: Prior to 2016, families in Maine with children who are DHH were referred to multiple agencies for services, causing confusion and increased stress. In addition, when gathering information to better support their child, families were receiving conflicting information across their service providers. With a new referral process and ELCO, a unified message related to communication and language opportunities is shared, as well as the resources available in the community.

Life History Parameters and Fishing Aspects of the Alien Nimble Spray Crab Percnon gibbesi in a Native Area of the Central-East Atlantic.

Percnon gibbesi is a native crab species characteristic of intertidal and subtidal zones of the Atlantic coast of the European Macaronesian archipelagos (Azores, Madeira, and Canary Islands), and probably also in the neighbouring rocky coasts of northwest Africa. P. gibbesi is considered an invasive alien species in almost all of the Mediterranean, with expanding populations from Spain to Turkey, including Libya; However, its biology and ecology are highly unknown, despite all its range of distribution. In the intertidal zones of Gran Canaria Island, this crab, in the intertidal zones of Gran Canaria Island, shows a carapace length range between 4.1 and 22.7 mm (4.1-22.7 in males and 5.7-22.3 in females), where females showed higher weights and lengths than males on average; However, males predominated in all samples, with a sex ratio of 1:0.57. The L∞ for this crab was estimated to be 27 ± 3 mm (23 ± 4 mm for females and 25 ± 4 mm for males). The growth coefficient (K) was 0.24 year-1, the total mortality (Z) was Z = 1.71 year-1, and the natural mortality (M) was 0.47 year-1. Although females grow faster than males, males are more abundant in the larger length classes. Although the presence of ovigerous females indicated that reproduction takes place twice a year, from March to April and from August to September, the number of cohorts detected by the modal progression analysis showed that reproduction takes place all year.

Prey predictability and preparation versus recovery breathing strategies in European shags Gulosus aristotelis (L.) diving in different habitats

AbstractA recent study on the diving behaviour of European shags (Gulosus aristotelis (L.)) foraging in kelp forests off rocky coasts of Norway suggests surface durations are related only to the duration of the preceding dive, and hence are being used for respiratory recovery. These results contrast with earlier reports concerning shags foraging in highly tidal estuarine waters off the coast of Lundy Island, SW England, where there was a stronger relationship between dive durations and preceding pre‐dive surface durations, suggesting the use of preparatory variation in oxygen loading. These two datasets were collected using different methods, and statistically analysed in quite different ways, so the contrasting results here could be due to different methodologies rather than the ecological differences between the two foraging environments. Here, we re‐analyse the two datasets using similar statistical methods, and we confirm the contrasting results produced by the two datasets. We, therefore, conclude that shag breathing strategies do differ between these two marine environments, presumably reflecting adaptive facultatively plastic responses to differences in predictability of foraging dive durations. Off the Norwegian coast, unpredictable variation in the depth and availability of pelagic prey in complex environments may require more responsive post‐dive respiratory recovery on the surface after each dive. In the more uniform English near‐shore environment, however, pre‐dive preparatory oxygen loading customised to match predictable dive‐to‐dive variation in benthic prey depths and foraging durations, may be more time and energy efficient.

Analysis of the Rockfall Phenomena Contributing to the Evolution of a Pocket Beach Area Using Traditional and Remotely Acquired Data (Lo Zingaro Nature Reserve, Southern Italy)

The coastal domain of central western Sicily is characterized by the presence of rocky coasts, which mainly consist of pocket beaches situated between bedrock headlands that constitute ecological niches of great touristic and economic value. In this peculiar morphodynamic system, the sedimentary contributions are mainly derived from the rockfall that affects the back of nearby cliffs or the sediment supply of small streams that flow into it. In this study, we investigated the geomorphological processes and related landforms that contribute to the evolution of a pocket beach area located in a coastal sector of NW Sicily Island. The cliffs in this are affected by several rockfalls, and deposits from these rockfalls also add to the rate of sedimentary contribution. The analysis was conducted through the application of traditional approaches and contemporary methods that have previously been used to forecast the collection of input data in the field, often under difficult conditions due to the accessibility of the sites, and which have been supported by UAV surveys. Through the analysis of the digital models of terrain and orthophotos, geometrical and multitemporal analyses of landforms were carried out. A dedicated software was utilized for the detection of rockfall runout zones and block trajectories and for defining the automatic extraction of rock mass discontinuities. The data were compared with those derived from traditional geomechanical surveys. The availability of the existing and acquired remote sensing data proved essential for this study for both defining the reference geological model and for performing the site-specific analysis of rockfall.

Geological Controls on Marine Cavernous Landforms along Japanese Pacific-side Rocky Coasts

Marine cavernous landforms, including notches, caves, arches and tunnels, characterize the rocky coast landscape and are strongly influenced by the geology (e.g., rock strength and structure) of sea cliffs. Geological controls on the development and shape of marine cavernous landforms are evaluated based on field investigations along the Pacific coastlines of Honshu Island, Japan. Morphological parameters, width (w), depth (d), and height (h), of cavernous landforms were measured directly or through images taken with an unmanned aerial vehicle, together with geological factors, such as rock types, strength (Schmidt hammer rebound values: R), dips and strikes of bedding, major joints, and fault planes. In total, 76 caves are investigated in six coastal areas: Sanriku Coast, Joban Coast, Boso Peninsula, Miura Peninsula, Izu Peninsula, and Kii Peninsula. According to shape index, d/w, and presence of the open end, cavernous landforms are classified into notches (d/w < 1, closed end), caves (d/w ≥ 1, closed end), arches (d/w < 1, open end), and tunnels (d/w ≥ 1, open end). An analysis shows that the major geological controls differ between notches and the other three forms. Low rock strength (R < 40) and sub-horizontal bedding are, respectively, the primary and secondary controls on the formation of notches, whereas weak rock structures (joints, faults and bedding) with a dip steeper than 30° (vertical or steeply inclined structures) and a strike trending perpendicular to the cliff face are the major controls on the formation of caves, arches and tunnels. Rock strength also affects planar forms defined by the shape index, promoting a deepening of cavernous forms, particularly when the cliff has a medium rock strength (R = 30-50) that provides an optimal balance between erosion force and resisting force. In general, vertical or steep structures contribute to the deepening of holes, whereas horizontal or gentle structures favor widening.

Transgressive rocky coasts in the geological record: Insights from Miocene granitic rocky shorelines and modern examples

Rocky shorelines are commonly punctuated, with alternating high relief cliffs and incised embayments which host “pocket beaches”. While multiple cases of ancient rocky shorelines associated with low relief ravinement surfaces have been documented in the geological record, deposits formed in pocket beaches and joint-cut coves are more rarely described. This poses the question “are high relief rocky coastlines and their associated deposits not preserved or have they been previously overlooked? Here we document exhumed examples of ancient granitic rocky shoreline systems of diverse morphologies from the Early Miocene of northern Austria, and compare them with modern systems in UK, Corsica, Spain and Norway. The preserved ancient examples offer a good opportunity to characterize these sedimentary systems, provide diagnostic criteria for their recognition and discuss the main controls on their occurrence and preservation in the rock record.From their stratigraphic and sedimentological analysis, and its comparison with modern examples, we interpret that these rocky shorelines form and get preserved during rapid rates of combined tectonic and eustatic sea-level rise, along lithological, structural and weathered “weakness” zones affected by normal low wave energy conditions interrupted by storms. These results provide a mechanism for predicting their potential occurrence and distribution during transgression of rocky coasts, with implications for exploration around structural highs and coastal management.

The Marine Otter Lontra felina (Molina, 1782) (Mammalia: Carnivora: Mustelidae) along the marine protected areas in Peru

Lontra felina (Molina, 1782) or Marine Otter is an endangered species according to the IUCN Red List and lives mainly on the rocky coast of Peru and Chile. In Peru it inhabits some marine protected areas, ports and some rivers. To determine the current state of conservation, monitoring data of National Reserves of Paracas, San Fernando &amp; the Guano´s Islands, Islets &amp; Capes National Reserve System were analysed as well as the management plans of each marine protected area. According to the results, the Marine Otter is only a target species in the Paracas Reserve. The Guano´s Islands, Islets &amp; Capes National Reserve System is a network of mini-reserves that maintains the largest population of Marine Otters in Peru in at least 14 of its 33 protected sites. The effectiveness of management for L. felina is only verified in the case of the National Reserve of Paracas.

Reproductive aspects of Brachidontes rodriguezii in the high intertidal; extreme environmental effects

In this study we tried to reveal if under conditions of high-stress mussels presented some trade off about reproductive aspects like size/frequency of oocytes and the reproductive cycle of the mussel Brachidontes rodriguezii. This species dominates the rocky intertidal coast of Buenos Aires province, Argentine and is a prey item of several exploited species. Mussels were collected from December 2011 to May 2013 in the upper level of it distribution in the high intertidal coast. Gonad tissue samples were processed according to standard histological procedures. Four gonadal development stages were described for males and females all year round. According to the significant decrease in the frequency of mature oocytes, the spawning events were in February 2012/2013. This study also reports hermaphrodite and parasite mussels in very low frequency. We concluded that the frequency and duration of the post-spawning gonadal development stage were the most notable difference along the vertical distribution, suggesting that the high-stress condition of high intertidal impacts negatively on gonad restoration.

High-energy, microtidal nearshore deposits and their provenance (Lower Miocene, Burdigalian/Eggenburgian, Alpine-Carpathian Foredeep, Lower Austria)

During the Early Miocene (Early Burdigalian/Eggenburgian) marine transgression at the southeastern margin of the Bohemian Massif, gradual flooding occurred along a rocky coast on granitic bedrock of the Thaya Batholith under high-energy, wave-dominated, microtidal and mixed fair-weather and storm conditions. Deposits of the Burgschleinitz Formation overlie a basal unconformity above a subaerial weathered basement surface (transgressive erosional surface) and are interpreted as a transgressive systems tract. The deposits can be divided into four facies associations/depositional environments, i.e., upper-shoreface, foreshore, gravelly beach and backshore/lagoon. Two stages of transgression and successive overtopping of the basement, with different coastal physiographies, were documented. During the initial stage of transgression a barrier island system developed with relatively fine-grained deposits, reflecting the flooding of the distant parts of the Thaya Batholith with a relative flat basement morphology. The subsequent continuation of the transgression led to the flooding of the more proximal parts of the Thaya Batholith with a steeper relief and formation of a rocky shoreline with deposition of gravelly sediments along palaeo-sea cliffs or wave-cut platforms. While gravel clasts of the deposits investigated originate directly from the underlying granites of the Thaya Batholith, provenance studies show that metamorphic rocks of the Moravian Superunit in the hinterland were the main source of sands. This distant source material was probably delivered mainly by small creeks and alluvial fans to the nearshore. Significant differences in heavy mineral composition of the same formation in the wider vicinity indicate primarily local sources and rapid deposition with subordinate longshore transport, which may reflect a complex coastal palaeogeography. The Lower Miocene deposits of the Burgschleinitz Formation investigated are a rare example of ancient rocky shore deposits, which generally have low preservation potential in the geological record.

Morphological and molecular investigation of some xanthid crabs from the Egyptian coast of the Red Sea

Family Xanthidae comprises 15 subfamilies and over 600 accepted species; they are represented well in the foreshore marine environments. Members of family Xanthidae are multi-colored crabs, usually inhabiting rocky coasts, coral reefs, and mud flats, all of which are well represented along the Egyptian coast of the Red Sea. Here, we utilized cytochrome oxidase subunit I (COI) sequences combined with morphology to provide information on some xanthid specimens collected from the Egyptian coast of the Red Sea. Six species within four genera (Leptodius, Etisus, Cyclodius, Chlorodiella) were collected. Genetic distances combined with morphological analyses showed intraspecific variations between two morphotypes of Leptodius exaratus. Two Etisus species were examined, E. laevimanus and E. sp. The latter Etisus sp. was close to E. frontalis, especially with regard to frontal lobe morphology, but different in male's first gonopod, with interspecific genetic distances. We also identified Chlorodiella nigra and C. laevissima. Obtained genetic distances between two morphotypes of Cyclodius granulatus revealed that these morphotypes are also likely cases of intraspecific variation. The results of this study should provide a basis for future work on family Xanthidae along the coasts of the Red Sea, which is needed as data remain scant.

Cliff-top boulder morphodynamics on the high–energy volcanic rocky coast of the Reykjanes Peninsula (SW Iceland)

Cliff-top boulder deposits (CBDs) are morphological indicators of high-energy conditions. Since 2014, a monitoring of CBDs dynamics has been undertaken on the south-western coast of Iceland (Reykjanes Peninsula) to monitor their long-term activation (quarrying, transport and deposition) as a proxy of the inter-annual winter storminess variations and basaltic cliff erosion processes in a context of rocky coast progradation. Annual topomorphological surveys of four study sites were conducted and Structure-from-Motion photogrammetry was performed to quantify CBDs displacements. Hydrodynamic conditions were analyzed based on offshore waves and water level. Results show that CBDs activation occurs every winter, regardless of the variability of hydrodynamic conditions. Depending on the site and the year, >2% and 17% of the CBDs accumulated above 8 m to 10 m asl at the top of the cliffs are regularly mobilized. While inland movements represent the main mode of transport of blocks (between 50% to 60%), seaward and longshore movements are also well represented (10% to 20%). Longshore displacement is favored by the wide tabular morpho-structural setting of the wave-scour cliff-top platforms, which is explained by the structure of pāhoehoe lava flows. The activation of CBDs –measured from the volumes of displaced boulders–, shows a good correspondence with the frequency and duration of storms. However, as was the case during the winter of 2018–2019, it was rather the intensity of two highly morphogenic episodes combining storm waves and especially very high spring tide water levels, that generated the largest boulders displacements. Substantial interannual activation of the CBDs confirms that they constitute an important and still understudied proxy of the morphogenic system of high-energy rocky coasts, whose the dynamic in terms of carrying, transport, and deposition, could significantly increase with rising sea level.

Monitoring Coastal Erosion Using Remote Images: Comparison between Physically and Remotely Acquired Data on a Limestone Coast

Boulder-sized clasts on rocky coasts are considered as erosional signatures of extreme wave events and boulder attributes are often used in numerical models to estimate wave characteristics. The use of unmanned aerial vehicle (UAV) technology and related software has facilitated the monitoring of coastal areas, by generating models from which 2D and 3D measurements can be derived. However, the reliability and preciseness of such measurements is still to be determined. This study seeks to analyse the accuracy of boulder measurements by comparing the dimension data obtained through in-situ measurements with ex situ data generated from digital models, based on UAV images. The study area is a bouldered sloping coast located on the southeast coast of Malta (Central Mediterranean) that has developed into multiple limestone dipped strata with a fractured and heavily jointed morphology. The dimensions of c. 200 boulders in different morphological settings, such as clusters or ridges, have been statistically compared. The results show a very strong correlation between the two datasets, both in 2D and 3D; however some notable differences were observed at the individual boulder level. For the majority of boulders analysed, the A and B axes dimensions varied by ±10% to 20%. The C axis proved to be harder to measure accurately and showed a wider range of difference. Boulder volume results in the majority of cases varied from 0% to ±40%. Some tested methods of volume calculation may be more accurate and realistic than others depending on the boulder position in relation to other clasts and shore morphology. An automated digital analysis of the terrain surface to identify the boulder extents may offer possibilities for a more accurate estimation of boulder attributes.