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Seagrass berm deposition on a Mediterranean embayed beach
Estuarine, Coastal and Shelf Science 135 (2013) 171e181
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Seagrass berm deposition on a Mediterranean embayed beach Simone Simeone a, b, *, Sandro De Muro c, Giovanni De Falco a, ** a
Istituto per l’Ambiente Marino Costiero, CNR, U.O. Oristano, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy Fondazione IMC, Centro Marino Internazionale, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy c Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Via Trentino, 09100 Cagliari, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2013 Accepted 8 October 2013 Available online 16 October 2013
Seagrass leaf litter is commonly found along shores all around the world. The Mediterranean Sea is not an exception, and along the sandy shore wide and thick deposits of leaf litter can be found. The deposition of these structures has not yet been studied, our aim is to clarify the depositionaleerosive process of seagrass leaf litter on a Mediterranean beach. A video image system, consisting of a camera and a video camera, was used to investigate the dynamics of the deposition of seagrass litter on beaches. Timeexposure images proved useful in investigating the seagrass berm deposition over the monitored period by using the EOF analysis, while videos are used to describe every deposition and erosion cycle. The deposition of seagrass berms occurred from late October to early April, while during the rest of the year, the beach was free of litter. The first deposition occurred in October, when seagrass litter was available on the submerged bay domain, the seagrass berm erosion occurred after several peaks of wind speed, while the deposition started when the wind speed increases and the waves start to break near to the shoreline. The deposition of the leaf litter on the beach starts as a strandline at the landward edge of the wave action and proceeds seaward up to the shoreline. Litter residue, eroded away by the waves and floating in the inner surf zone can be redeposited in little patches at the end of an erosion cycle. In conclusion, leaf litter may be relevant to berm formation and litter floating in the inner surf zone and in the lower swash zone is part of the materials exchanged between submerged and emerged beach. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: banquette video monitoring beach morphology seagrass leaf litter foreshore
1. Introduction Seagrass wracks are commonly found on beaches all around the world (Kirkman and Kendrick, 1997; Ochieng and Erftmeijer, 1999; De Falco et al., 2003, 2008; Mateo et al., 2003), their deposition occurs on the beachface of the sandy shore (Simeone and De Falco, 2012). This deposits can influence the morphology and morphodynamics of the beaches (Short, 1999; De Falco et al., 2003). The seagrass litter wrack deposits could play a role in the sediment exchanges between the beach and the foredune, in fact wrack and seagrass litter stranded on beaches can influence the foredune development, trapping the sediment transported by the wind and favoring the development of a new foredune (Hemminga and Nieuwenhuize, 1990; Nordstrom et al., 2011). Seagrass wrack can be deposed as a steep berm. In this case, its shape may influence the
* Corresponding author. Istituto per l’Ambiente Marino Costiero, CNR, U.O. Oristano, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy. ** Corresponding author. E-mail addresses: [email protected] (S. Simeone), giovanni.defalco@ cnr.it (G. De Falco). 0272-7714/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2013.10.007
wind profile interfering with sediment wind resuspension and deposition on the foredune (Short and Hesp, 1982; Short, 1999). Furthermore, the Posidonia oceanica beach cast litter can provide nutrients for the incipient foredune in terms of leaves and parts of plant pushed by wind and trapped by pioneer plants (Cardona and Garcia, 2008). This trapped material may have a positive effect on the further colonization by plants and, consequently, contribute to the growth of the incipient foredune. Short (1999) defined the seagrass and litter accumulated along the sandy Australian beaches as “seagrass berms”, these deposits in some cases can have a thickness of about 2 m. In the Mediterranean sea, the seagrass Posidonia oceanica forms large meadows, mainly on sheltered coasts (De Falco et al., 2000; Infantes et al., 2009), the leaves, roots and rhizomes of this plant can be found along the Mediterranean beaches forming wide, thick deposits called “banquettes” by French authors (Boudouresque and Meisnesz, 1982; Jeudy de Grissac, 1984). In order to emphasize the role of the P. oceanica litter in beach morphology, the term “banquette” was abandoned because it doesn’t imply any relationship between seagrass litter and berm edification, and the term “seagrass berm” has been adopted in accordance with Short (1999). The occurrence of these deposits along the shores was mostly observed during the
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winter and autumn, and mainly involved the beachface (Mateo et al., 2003; Duarte, 2004; De Falco et al., 2008; Simeone and De Falco, 2012). Similar to sediment berms, seagrass berms can be considered features resulting from the accumulation of seagrass litter and sediments at the extreme landward edge of wave influence. Seagrass berms are not only composed of vegetal material (e.g. roots, leaves), since up to 100 kg m3 of sediment can be found trapped inside (Guala et al., 2006; De Falco et al., 2008; Simeone and De Falco, 2012). Previous knowledge of the depositional dynamics of seagrass berms on beaches was mainly derived from the morphology and composition of leaf litter deposited along the shores. In particular, a theoretical sequence of formation and destruction of banquettes was proposed by Mateo et al. (2003) and involved an initial stage of litter deposition, leading to a berm accretion up to the maximum height. Erosion due to wave action, acting at the base of the berm, leads to a scarp formation and a collapse of the banquette structures (Mateo et al., 2003). Differences in the morphology and composition of Posidonia oceanica seagrass berms were found to be influenced by beach exposure (exposed vs. sheltered beaches), on beaches of western Sardinia (western Mediterranean) (Simeone and De Falco, 2012). On exposed beaches, beach cast leaf litter was deposited in sub-aerial sections characterized by a strong elevation variability (berm area), whereas on sheltered beaches the berm edification/destruction was driven by the exchange of P. oceanica litter between the beachface and shoreface. This suggests that the deposition of seagrass berms can be influenced by swash morphodynamics, and starts landward where the swash cycle ends where the deposition of heavier material (mineral grain and rhizomes if present) occurs in correspondence to the swash limit (Simeone and De Falco, 2012). Observation on the complete depositional and erosive cycles of seagrass litter on beaches were not previously reported: video monitoring can be a useful tool for observing this process on a daily or seasonal basis. Several studies used the video monitoring systems to assess morphological variations in morphodynamic states of beaches: these systems are used widely in studying shoreline and beach cusp morphodynamics (Miller and Dean, 2007; Almar et al., 2008; Ojeda and Guillen, 2008), sandbar dynamics (Plant and Holman, 1997; van Enkevort and Ruessink, 2003; Armaroli and Ciavola, 2011), run-up measurements (Salmon et al., 2007) and the carrying capacity of beaches (Jimenez et al., 2007). Video monitoring has also been used to investigate the sedimentary and morphological processes involving the beachface and the swash zone (Osorio et al., 2012). Understanding of depositional dynamics of seagrass litter is fundamental from a beach management point of view, because seagrass litter removal from sandy shore, to increase the beach exploitation, is a widely diffused practice (Duarte, 2004; De Falco et al., 2008; Mossbauer et al., 2012; Simeone and De Falco, 2013) and is performed mainly with heavy machineries (De Falco et al., 2008). Consequently, seagrass litter removal could have a strong impact on the beach morphology, particularly on the beachface (Simeone and De Falco, 2013). The aim of this paper is to describe the depositional dynamics of the leaf litter of the seagrass Posidonia oceanica on a Mediterranean embayed beach. This process was described by observing cycles of seagrass wrack deposition, seagrass berm edification and erosion on a daily basis using a video monitoring system that collected a series of images each day. The study was done over a period of approximately one year between 2007 and 2008, the video-monitoring data were compared with wind data in order to track the relationship between depositional dynamics and wind speed peak. A statistical analysis was applied to the images collected daily in order to analyze the seasonal variability of
seagrass depositionaleerosive cycles along the beach over the course of one year. 2. Study area The study area is located in the north of the Island of Sardinia in the western Mediterranean sea (Fig. 1a), the geology of Northern Sardinia is dominated by granitic formations with fields of basic dikes and Quaternary deposits present in the coastal areas and on alluvial plains. The geomorphology of the coastline is inherited from the geological setting of the area, with the main embayment following the principal fault lines. The coastline of the northeastern sector of Sardinia is characterized by a series of indentations with small embayed beaches lying between rocky headlands (Fig. 1b). Those embayments were deepened by rivers during the last glacial maximum (about 20,000 years ago) and the erosion caused by the rivers accentuated the indented morphology (De Muro et al., 2010). The studied beach, named Cala di Trana, is located in one of these indented bays, in the north of Sardinia (Fig. 1b and c), seafloor of the bay is colonized by a wide Posidonia oceanica meadow, which starts at a water depth of 5e10 m, and grows on a sandy bottom (Fig. 1c). The total length of Cala di Trana beach is about 250 m, the maximum subaerial cross-shore amplitude is about 25 m when measured from the toe of the dune up to the shoreline. The grain size of the sediment forming the beach, ranges from coarse sands and gravel in the foreshore to fine sands in the shoreface (De Muro et al., 2010). A single well-developed foredune characterizes the backshore area, this dune is interrupted by a little creek and by a seasonal pond on the eastern side of the beach. The wind regime in the north of Sardinia is dominated by two directions: (a) a westerly wind (about 50 percent of the wind regime) blows between 260 and 300 that is present most of the time during all seasons and (b) an easterly wind which blows between 60 and 100 , mainly during winter (about 30 percent of the wind regime) (Gerigny et al., 2011). Fig. 2c and d, highlighted the wind condition in Cala di Trana beach and showed that the more frequent events, during one year study, are calm and light breeze coming from south (Fig. 2d). The same pattern on wind speed and direction was confirmed by the wind rose recorded by the Porto Torres wind station (Fig. 2e) of the Rete Mareografica Nazionale (www.mareografico.it) located about 100 km west to the study site (Fig. 1a), for a longer period (Fig. 2e). The more intense wind comes from WeNW and EeNE, as shown in Fig. 2c, where the wind data with wind speed > 3.4 m s1 were reported. The analysis of two storms coming from the aforementioned directions (NW and NE) with a return period of one year showed that the wave height on the Cala di Trana bay is <1 m for both cases (De Falco et al., 2011). Furthermore we provide two images of the state of the sea inside the bay during the most intense wind event coming from NW and NE during the study year (Fig. 2f and g), which show that the waves, inside the bay, break in the beachface in both cases. In summary on Cala di Trana beach the wave height is low, the fetch is limited and the tide is negligible. 3. Methods From August 23rd 2007 to June 11th 2008 an SVMÒ video imaging System was installed on the top of the hill of Punta Sardegna, overlooking Cala di Trana beach (Fig. 1c; def); the system was equipped with a video camera which collected short videos (a few minutes long) three times a day, and a camera used to obtain the time-exposure image (Holland, 1998) (Fig. 1e,f). Furthermore the
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Fig. 1. Study area, a) location of study area; b) geomorphological overview of the study area; c) Cala di Trana beach and position of video station; d) time-exposure image; e) timex grayscale, masked image used to perform EOF analysis; f) timex RGB masked image of the beach.
video-monitoring station was equipped with sensors for wind direction and intensity (Fig. 1c). Video monitoring system used in this work was composed by a fotocamera Olimpus 6 mpixel (max resolution 3264 2488), with a reflex optic 38e118 mm and a video camera Super HAD CCD 1/200 , Lux 0.025, lens zoom 3.6e18 mm. The video camera was zoomed on the beach and the first frame of each video was recorded three times a day in order to observe the depositionaleerosive cycles of seagrass berms on a daily basis, the resolution of the this images was 320 240 pixel. The time-exposure images were zoomed to obtain a view of the whole bay, these images resulted from averaging multiple frames (1 frame per second for an interval of 10 min, total 600 images), the resolution of the timex images was 768 1024 pixel, the pixel resolution in the center of the beach was 0.6 m. The grayscale time exposure images are usually used to identify the position of the shoreline, the sandbar system, if present, and
other morphological features of the beaches (Holland et al., 1997; Haxel and Holman, 2004; Quartel et al., 2006). In our case, the time-exposure images, transformed in grayscale (Fig. 1e), were used to identify the litter deposits on the seafloor of the bay (submerged domain) and to analyze the variability of seagrass berm deposits on the sub-aerial beach during the monitored year. The spatial and temporal variability on the subaerial sector of the beach was analyzed by applying the empirical orthogonal function (EOF) analysis on the time-exposure images, the images were collected daily at the same time, between 10:00 and 10:30 am, in order to obtain comparable light conditions. The time-exposure images of the surf zone show patterns in which white areas correspond to the swash zone and, if present, to sandbar locations; this is because breaking waves produce foam in both areas while darker areas usually correspond to deeper water. Conversely, pixels of the dry beach are lighter than those of the water and darker than those of the swash zone and of the sandbars,
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Fig. 2. a) Position of the beach profiles; b) beach profiles and seagrass litter deposits; c) wind rose on Cala di Trana beach (wind speed > 3.4 m s1); d) wind rose on Cala di Trana beach; e) wind rose on Porto Torres; f) sea state on the bay during the most intense wind event from NW (09/11/2007); g) sea state on the bay during the most intense wind event from NE (18/12/2007).
while beach cast leaf litter results darker than the swash area. Following this approach based on light contrasts, different areas can be identified in each image. A mask was applied to the timeexposure images in order to focus the analysis only on the subaerial beach and on a limited area of the sea adjacent to the beach (Fig. 1e and f). The mask excluded the outer sector of the bay from the EOF analysis. This was done to exclude from the analysis the disturbance due to the presence of boats, reflection in the sea surface induced by little wind waves and other sources. The resulting resolution after the application of the mask was 196 82 pixel and in order to perform EOF these images were transformed in grayscale (Fig. 1e). We analyzed this domain to infer, by means of EOF, the relationship among the beachface and berm areas and wave breaking area, including the seagrass litter deposits. The aim of EOF is to describe the changes over the whole subaerial beach surface using the least number of functions, called eigenfunctions (Winant et al., 1975; Dean and Dalrymple, 2004). In this way, it is usually possible to account for a large percentage of the variance with a small number of terms (Winant et al., 1975; Dean and Dalrymple, 2004). To perform the EOF analysis, every time-exposure image was transformed into a 196 82 matrices in which every element nij represented the pixel grayscale intensity (linear: 0e255). The grayscale conversion of the images provides a standardization of the light intensity of each image, thus reducing the effect of any changes in light condition. Each matrix was then
vectorialized into a one-row vector with dimension p ¼ 196 82, then a new M matrix n columns and p rows were built. In this matrix, n is the number of observations and p is the location of each pixel for each image (Olita et al., 2011). A total of 209 time-exposure images were collected in a period of 294 days. EOF analysis was then performed on the M matrix. The spatial variability was then represented by reconverting the resulting eigenfunctions (called EOFs), into three matrices, finally, each EOF was transformed into a colored image in order to represent the variability of the subaerial beach. Wind data were used to relate the leaf litter depositionale erosive cycles to the wind speed trend, the latter is related to wave intensity in the bay. To confirm the presence and the thickness of the seagrass berm on the beach during the monitoring period, two beach profiles were collected in September 17th 2007 and in January 16th 2008 by using Differential Global Positioning System (Fig. 2a and b) (Morton et al., 1993). 4. Results 4.1. Depositionaleerosive cycles of seagrass litter Images were used to identify the presence and the residence time of seagrass litter deposits on the subaerial beach. Three conditions were observed (Fig. 3aec): (a) subaerial beach without
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seagrass litter deposits, named E0, E4b and E5; (b) well developed seagrass berm along the whole subaerial beach, named D1, D2, D3, D4, D5; (c) patchy seagrass litter deposits in the subaerial beach, named E1, E2, E3, E4a. The sequence of depositional and erosive events during the monitored year is showed in Fig. 3a and is compared to the average wind speed. Five depositionaleerosive events were recorded: the first deposition of seagrass berm occurred in October, the last in April, a residual presence of leaf litter on the backshore was recorded during intermediate erosive cycles (E1eE4a). Whereas seagrass berms were completely eroded and the beach resulted “clean” of leaf litter after the last cycle of depositioneerosion occurred between April 3rd 2008 and April 14th 2008. During the D3 depositional event the thickness of the seagrass berm, measured by means of DGPS, in the area in which they occur was about 1 m (Fig. 2b).
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The comparison of depositionaleerosive cycles with the average wind speed (Fig. 3a), showed that the seagrass berm, once deposed on the beach, was eroded after several events marked by peaks of wind speed (Fig. 3b and c). After the partial or total erosion, the deposition of the seagrass berm on the beach occurs after an event characterized by a high wind speed (events D2, D3, and D5). On the contrary, the depositional events D1 and D4, occurred in October and in February, were not related to the wind speed peak, but deposition occurred after a period characterized by low intensity wind speed at the end of the summer and from January to late February. Selected time-exposure images of the whole bay are reported in Fig. 4, the shooting dates are indicated by the black squares in the upper side of Fig. 3a: these images showed the presence and the position of the leaf litter in the seafloor of the bay. Fig. 4a highlighted a typical image of the beach during the summer period
Fig. 3. a) Wind speed recorded by the wind sensor of the video-monitoring system is showed as continuous line; deposition (D1eD5) and erosion (E1eE5) cycles of seagrass berm on the Cala di Trana beach, showed as a dashed line. Black squares identify the images shown in Fig. 4; b) particular on D1eE1 event; c) particular on D3eE3 event.
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Fig. 4. Time-exposure images taken on 27 August 2007, 22 October 2007, 13 November 2007, 14 December 2007, 29 December 2007, 04 February 2008, 05 April 2008, 14 May 2008. Seagrass leaf litter on the seafloor of the bay was highlighted by the arrows as well as the well-developed seagrass berm, the residual litter on the beach and the beach free of wrack litter.
before the deposition of seagrass berms: wrack litter are absent on the backshore and on the foreshore, while the seagrass litter deposited on the shoreface can be identified by the dark area at the center of the bay. The subsequent images were collected after depositional events (Fig. 4b, e and g) and after the partial erosion of seagrass berms (Fig. 4c, d and f). Finally, the last image (Fig. 4h), collected May 14th 2008, shows the beach without seagrass wrack, similar to the initial stage of Fig. 4a. The seagrass litter on the shoreface is distributed over most of the bay during the erosivee depositional cycles, but is confined to the middle of the bay before October and after April. Two daily sequences of video-frame images reporting an event of litter deposition and seagrass berm edification (D1) and an event of seagrass berm partial erosion (E4a) are reported in Figs. 5 and 6. The complete cycle of deposition started when the wind speed increases and the waves start to break near to the shoreline, the deposition occurs as a strandline at the upper limit of the swash zone, on the upper side of the beach (Fig. 5aec). During the wave decay, the deposition proceeded seaward and from the upper side of the bay toward the lower side (Fig. 5d and e). At the end of the event, the whole berm area of the beach was occupied by the seagrass berm (Fig. 5f). Conversely, the complete cycle of seagrass berm erosion started from the front of the seagrass berm deposited on the foreshore (Fig. 6a), and proceed landwards as the wave increased in intensity (Fig. 6bed) and run up increases. At the end of the wind event, the beachface resulted almost completely free of litter, with little residual deposits of the previous seagrass berm remaining in limited areas of the backshore (Fig. 6f).
4.2. EOF analysis of time-exposure images Results of EOF analysis show that the first three eigenfunctions (EOF1, EOF2 and EOF3) accounted for 83.6% of the variability in time-exposure image intensities (61.4%, 19.1% and 3.1% respectively). Each of the EOFs below the third explain less than 2.5% of the total variability, furthermore noise present in this EOFs can generate confusion in the interpretation, for this reason we analyze here only the first three EOFs. Those EOFs were taken into consideration in order to interpret the subaerial beach subdivision based on the variability of the pixel intensities. The resulting maps (Fig. 7aec) highlighted the areas of the beach that vary in concordance or in opposition of phases in terms of pixel intensity, whilst the temporal amplitude of each EOFs highlighted the trend of the described processes during the observation period (Fig. 7eeg). The variability of the first EOF (Fig. 7a) was mainly located alongshore in the subaerial beach, in particular the EOF1 identified the two different domains present on each images: a) the beach and b) the sea. The temporal amplitude of the EOF1 followed a temporal cycle (summerewinter) with negative values from August to October, positive values from October to March and, again, negative values from March to June (Fig. 7e). The spatial variability of the second EOF (Fig. 7b) emphasizes the alongshore subdivision of the beach, highlighted by the EOF1, however the pattern of the EOF2 identified a) the wave breaking zone and the b) berm deposition area, that in the majority of case was built up by Posidonia oceanica leaf litter (Fig. 3a). Temporal amplitude of the EOF2 showed a cyclic pattern, with positive value from October to early April and negative pattern for the
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Fig. 5. Depositional cycle of seagrass berm on the beach. In a, b) beginning of wave increase, the seagrass litter was deposited as a strandline; c) the first phase of seagrass edification; d, e) decreasing of wave height, seagrass berm development from the western side of the beach to the eastern side, and from the run-up’s edge to the foreshore; f) the seagrass deposits at the end of the cycle. The black arrows indicate the longshore direction of the seagrass berm deposition, while the blue arrow showed the seaward seagrass deposits accumulation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Erosive cycle of seagrass berm on the beach. a) Seagrass berm deposits, b, c, d) wave energy causes an erosion of seagrass berm from the eastern side of the beach toward the western side; e, f) decreasing of wave height, the beach is free of litter, only a little residue (leaves) was present on beach. The red arrows indicate the retreat in the longshore direction of the seagrass berm deposits, while the blue arrow showed the landward retreat of the front of the seagrass deposits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
rest of the year (Fig. 7f). In particular during the autumnelate winter period the temporal amplitude accounts for high positive values in comparison to the rest of the year, this period was the same in which the majority of deposition erosion cycles occurs. The temporal amplitude of EOF2 was positively correlated with the speed of winds coming from the NW and NE sectors (R ¼ 0.512, p < 0.05), this indicating that the pattern showed by the second EOF occurred mainly during the wind peak events (Fig. 8).
The spatial variability of the EOF3 (Fig. 7c) identified several sources of variability. Those sectors of variability were related to seagrass berm and to noise that was not related to specific processes (i.e shadow effects). Particularly one sector, located in the upper portion of the image (red strip occupying longshore about half of the shoreline in Fig. 7c) corresponds to the part of images in which the seagrass litter remains between two consecutive depositioneerosion events. The temporal amplitude of EOF3 shows a different pattern in comparison to the first two EOFs. In particular,
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Fig. 7. EOF analysis results: a) spatial distribution of EOF1; b) spatial distribution of EOF2; c) spatial distribution of EOF3; d) temporal variability of EOF1; e) temporal variability of EOF2; f) temporal variability of EOF3.
during the winter period, the temporal amplitude of EOF3 showed high negative values, whereas during the rest of the year the values were slightly positive (Fig. 7g). 5. Discussion Observations of depositionaleerosive cycles of seagrass wrack on an embayed beach over a period of one year highlighted the processes which control seagrass berm edification and erosion. The first depositional events, in October, were not related to a particular intense wind event: the meteorological data showed that wind events of similar intensity and direction were recorded during the months of August and September and from January to late February. The deposition erosion cycles occurred from late October up to early April, during this period the wind speed for each event was generally higher in comparison to the rest of the year (Fig. 3a): this intense winds, coming from NW and NE, generate waves that break very close to the shoreline (Fig. 2c, f and g).
The initial stage of Posidonia oceanica seagrass berm edification was probably related to the increase of litter availability on the seafloor of the bay during Autumn that can be due to the loss of leaves (Romero et al., 1992; Mateo and Romero, 1996) from the seagrass meadow adjacent to the bay. During the autumn a large number of leaves of P. oceanica are senescent and fall off the rhizomes (Mateo and Romero, 1996; Mateo et al., 2003): this process involves an increase of litter availability during the autumn and winter which can be transported onshore even under moderate wave action, triggering the initial stage of seagrass berm edification. Furthermore, the presence of headlands, reef and outcrops, as occurring in Cala di Trana bay, can limit the development of longshore, rip and feeder currents (Short, 1999), with a trapping of the leaf litter inside the bay. In this case the availability of seagrass litter in the inner sector of the bay could trigger a depositional event also in case of moderate wind speed (D4 event, Fig. 3a). On the contrary, low degree of embayment and higher wave height can favor the presence of rip, megarip and longshore circulation (Short, 1996), which can push out of the system the floating leaf litter.
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Fig. 8. Correlation between EOF2 temporal amplitude and wind speed (NW and NE direction).
From October to April, a series of depositional and erosive events of beach cast leaf litter occurred; the area of deposition of Posidonia oceanica leaf litter corresponds to the berm area. During their maximum development, the seagrass berms occupies the whole beach longshore, whereas cross shore, seagrass berm occupies the beach from the landward limit of the wave action, up to the shoreline. The observation of daily images highlighted that the deposition starts at the upper limit of the run up, like the strandline, and proceeds seaward from the upper sector of the beach toward the lower sector of the beach, occupying the whole berm area as the waves decrease in height. On sandy beaches the material forming the berm was mainly derived from the inner surf zone (Russel et al., 2009), in our case, this material was also represented by leaf litter floating in the inner surf zone and in the lower swash area. Deposition of the seagrass berm occurred during the whole period in which the waves, generated by the wind peak, increasing and decay. The swash zone morphodynamics controls the process of deposition of the seagrass litter. In the swash zone, flow velocities, suspended sediment concentrations and suspended fluxes are greatest at the start of uprush, when the flow is most turbulent and the dominant mode of transport is expected to be a suspended load. The swash flow energetics decreases following the arrival of the swash front. The result is that the suspended material settles on the bed, leaving the water clear (Masselink and Puleo, 2006). In presence of seagrass litter the suspended material subjected to transport in the swash zone is composed by sediments, leaves and rhizomes (Simeone and De Falco, 2012) and is deposed at the limit of run up building up to the standline showed in Fig. 5. At the end of the deposition, the final shape of seagrass litter deposits on beach is comparable to a berm as showed in Fig. 2b. Conversely, during the erosive cycle, the removal of the seagrass berm by waves was faster and almost the entire seagrass berm was eroded during the phase in which waves increase in height (Fig. 6b and c). Residue of the seagrass berm in the form of floating litter, could remain in the swash zone and be deposited again when waves decay, however, the re-deposited litter accumulation was little in size and covered a very limited area of the beach. Once deposited, the wrack litter occupied the whole beachface from the run-up’s seaward edge to the shoreline, and tended to build up a single seagrass berm on the whole beach surface (Figs. 1b and 4b). These structures were also found in other beach systems (Short, 1999; De Falco et al., 2003; Simeone and De Falco, 2012) and could be responsible for an occasional migration of the shoreline several meters seaward (Basterretxea et al., 2007).
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On sandy estuarine beaches, the cast vegetation litter is generally high in low energy environments, due to the presence of vegetation growing in the intertidal zone, whilst in high energy environments the litter is generally accumulated on beaches as the results of an individual swash uprush and the amount of litter in each swash line is often small (Jackson et al., 2002). Conversely on Mediterranean beaches, the volume, thickness and cross-shore length of Posidonia oceanica litter deposits resulted higher on exposed beaches in comparison to sheltered ones (Mateo et al., 2003; Simeone and De Falco, 2012). The depositional dynamics observed in the Cala di Trana beach, is coherent with the depositional models proposed in previous studies (Mateo et al., 2003; Simeone and De Falco, 2012). The mentioned authors proposed that the depositions of the P. oceanica litter on beaches starts landward at the limit of run-up and proceeds seaward with a growing in wideness and thickness of the seagrass deposits, forming a seagrass berm, whilst the erosion of the seagrass deposits starts from the seaward front during a storm and proceeds landward. The daily observation of depositionaleerosive cycles obtained by the video-monitoring, supported the aforementioned models. EOF analysis on the time-exposure images highlighted different sources of variability on the emerged beach and on the submerged domain. EOF1 is related to the seasonal pattern of the sub-aerial beach: in particular by the EOF1 pattern the images can be divided into two domains: the beach and the inner surf zone (Fig. 7a). The presence of numerous peaks can be interpreted as the influence of the waves and consequently, of the correspondent runup, on the analyzed domain. The pattern of EOF2 highlighted the opposing brightness of the sub-aerial beach and the submerged domain, the former was the beachface that was occupied by seagrass berm, the latter was the wave breaking zone that appear brighter (Fig. 7b). During summer, the subaerial beach was bright due to the absence of seagrass litter, and the submerged domain was dark due to the lack of breaking waves. During winter, the berm area had lower pixel intensity compared to the submerged domain. This is because it was formed by beach cast leaf litter which appears darker than the other pixels in the images, and in particular to the pixel of the breaking and swash areas and to the pixel of the dry beach. Conversely, the submerged domain resulted brighter during winter, due to a more frequent occurrence of foam due to the presence of waves, and the maximum values of EOF2 evidence the sections of foam probably related to wave action. Temporal amplitude of EOF2, shown in Fig. 7f, was generally characterized by high positive values during the period of the year in which depositionaleerosive cycles of seagrass berm occurred, while low negative values characterized the rest of the year. This temporal pattern can be linked to the swash processes and to the berm formation that was built up by Posidonia oceanica leaf litter on this beach and which occurred various times, but only during the autumn and winter (Fig. 3a). This is because the litter was available and the high speed events, from NW and NE, occurred. The relationship between the temporal amplitude of the EOF2 and the wind speed (Fig. 8), suggests that the main forcing, which characterizes the subaerial beach, was the waves generated by winds, that pushed the litter landward, leading to a decrease in grayscale pixel intensity caused by deposition of the litter. The spatial variability of EOF3 is located in the sector of the subaerial beach where seagrass berms were always present from the first depositional cycle up to the complete seagrass berm erosion in spring. This area is located in the western sector of the beach and represents the initial stage of berm edification and the residual deposit after partial berm erosion. The observation of single depositional events showed that seagrass berm edification started
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in the western sector of the beach and continued toward the eastern sector, occupying nearly the whole emerged beach. As observed in microtidal sandy beaches, in the absence of swash overtopping, the berm tended to grow horizontally without discernible variations in height (Weir et al., 2006). This aspect could be relevant in the case of seagrass berm deposition, which can follow the same pattern of deposition on beaches. The vegetal litter accumulation is a diffuse phenomenon occurring on sandy beaches over different coastlines all around the world (De Falco et al., 2008; Mossbauer et al., 2012; Gomez Pujol et al., 2013; Simeone and De Falco, 2013). In the Mediterranean sea, specifically in Italy, Spain and France, this process involves several thousands of km of sandy coastline. (Duarte, 2004; De Falco et al., 2008). The yearly removal of several thousands of cubic meters of Posidonia oceanica seagrass litter from Mediterranean beaches has become a threat for the sandy shores (Simeone and De Falco, 2013), and the knowledge of the depositional dynamics could be useful to predict this kind of impact. Furthermore this practices, as well as cleaning operations carried out with mechanized machines, may be considered a potential stressor for dunes due to the role of the seagrass litter deposits in the foredune edification and dune fertilization (Cardona and Garcia, 2008; Nordstrom et al., 2012). Our study highlighted that in an embayed beach the litter availability and the wind peak, generating waves in the bay, can be considered as the main forcing that influencing the seagrass litter deposition on the beachface. Following this consideration, our study highlighted that the removal operation of the seagrass litter could not be done during the period in which exchange of litter between the shoreface and the beachface occurs. In fact, the litter removal during the wintereearly spring period does not prevent a further deposition, resulting in a useless practice; furthermore the litter removal during the period characterized by erosiveedepositional cycles, could affect the morphological variability of the beach and the exchange of material between the berm and the surf zone. Finally, this study confirms that leaf litter may be relevant to the berm formation and that the litter floating in the inner surf zone is an important part of the material exchanged between the submerged and emerged beach. 6. Conclusion The video-monitoring system is a useful tool for analyzing short-term processes acting upon a beach, in particular we used this tool to reveal the depositional dynamics of the leaf litter of the seagrass Posidonia oceanica on the Cala di Trana beach. This study highlighted that several cycles of depositionale erosive of seagrass berm were recorded over one year on an embayed beach. The complete cycle of deposition started when the wind speed increases and the waves start to break near to the shoreline, the deposition occurs as a strandline at the upper limit of the swash zone, on the upper side of the beach. Conversely, during the erosive cycle, the removal of the seagrass berm by waves was faster and almost the entire seagrass berm was eroded during the phase in which waves increase in height. Our study highlighted that in an embayed beach the litter availability and the wind speed, generating waves in the bay, can be considered as the main forcing that influencing the seagrass litter deposition on the beach. Acknowledgments This work was funded by the GERER project “Gestione ambientale integrata in località ad alto rischio d’erosione”, scientific coordinator Prof. Sandro De Muro, funded by the European Union (Interreg IIIA), and the RIAS project “Risposta e Adattamento dei sistemi costieri della Sardegna alle variazioni
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Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss
Seagrass berm deposition on a Mediterranean embayed beach Simone Simeone a, b, *, Sandro De Muro c, Giovanni De Falco a, ** a
Istituto per l’Ambiente Marino Costiero, CNR, U.O. Oristano, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy Fondazione IMC, Centro Marino Internazionale, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy c Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Via Trentino, 09100 Cagliari, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2013 Accepted 8 October 2013 Available online 16 October 2013
Seagrass leaf litter is commonly found along shores all around the world. The Mediterranean Sea is not an exception, and along the sandy shore wide and thick deposits of leaf litter can be found. The deposition of these structures has not yet been studied, our aim is to clarify the depositionaleerosive process of seagrass leaf litter on a Mediterranean beach. A video image system, consisting of a camera and a video camera, was used to investigate the dynamics of the deposition of seagrass litter on beaches. Timeexposure images proved useful in investigating the seagrass berm deposition over the monitored period by using the EOF analysis, while videos are used to describe every deposition and erosion cycle. The deposition of seagrass berms occurred from late October to early April, while during the rest of the year, the beach was free of litter. The first deposition occurred in October, when seagrass litter was available on the submerged bay domain, the seagrass berm erosion occurred after several peaks of wind speed, while the deposition started when the wind speed increases and the waves start to break near to the shoreline. The deposition of the leaf litter on the beach starts as a strandline at the landward edge of the wave action and proceeds seaward up to the shoreline. Litter residue, eroded away by the waves and floating in the inner surf zone can be redeposited in little patches at the end of an erosion cycle. In conclusion, leaf litter may be relevant to berm formation and litter floating in the inner surf zone and in the lower swash zone is part of the materials exchanged between submerged and emerged beach. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: banquette video monitoring beach morphology seagrass leaf litter foreshore
1. Introduction Seagrass wracks are commonly found on beaches all around the world (Kirkman and Kendrick, 1997; Ochieng and Erftmeijer, 1999; De Falco et al., 2003, 2008; Mateo et al., 2003), their deposition occurs on the beachface of the sandy shore (Simeone and De Falco, 2012). This deposits can influence the morphology and morphodynamics of the beaches (Short, 1999; De Falco et al., 2003). The seagrass litter wrack deposits could play a role in the sediment exchanges between the beach and the foredune, in fact wrack and seagrass litter stranded on beaches can influence the foredune development, trapping the sediment transported by the wind and favoring the development of a new foredune (Hemminga and Nieuwenhuize, 1990; Nordstrom et al., 2011). Seagrass wrack can be deposed as a steep berm. In this case, its shape may influence the
* Corresponding author. Istituto per l’Ambiente Marino Costiero, CNR, U.O. Oristano, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy. ** Corresponding author. E-mail addresses: [email protected] (S. Simeone), giovanni.defalco@ cnr.it (G. De Falco). 0272-7714/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2013.10.007
wind profile interfering with sediment wind resuspension and deposition on the foredune (Short and Hesp, 1982; Short, 1999). Furthermore, the Posidonia oceanica beach cast litter can provide nutrients for the incipient foredune in terms of leaves and parts of plant pushed by wind and trapped by pioneer plants (Cardona and Garcia, 2008). This trapped material may have a positive effect on the further colonization by plants and, consequently, contribute to the growth of the incipient foredune. Short (1999) defined the seagrass and litter accumulated along the sandy Australian beaches as “seagrass berms”, these deposits in some cases can have a thickness of about 2 m. In the Mediterranean sea, the seagrass Posidonia oceanica forms large meadows, mainly on sheltered coasts (De Falco et al., 2000; Infantes et al., 2009), the leaves, roots and rhizomes of this plant can be found along the Mediterranean beaches forming wide, thick deposits called “banquettes” by French authors (Boudouresque and Meisnesz, 1982; Jeudy de Grissac, 1984). In order to emphasize the role of the P. oceanica litter in beach morphology, the term “banquette” was abandoned because it doesn’t imply any relationship between seagrass litter and berm edification, and the term “seagrass berm” has been adopted in accordance with Short (1999). The occurrence of these deposits along the shores was mostly observed during the
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winter and autumn, and mainly involved the beachface (Mateo et al., 2003; Duarte, 2004; De Falco et al., 2008; Simeone and De Falco, 2012). Similar to sediment berms, seagrass berms can be considered features resulting from the accumulation of seagrass litter and sediments at the extreme landward edge of wave influence. Seagrass berms are not only composed of vegetal material (e.g. roots, leaves), since up to 100 kg m3 of sediment can be found trapped inside (Guala et al., 2006; De Falco et al., 2008; Simeone and De Falco, 2012). Previous knowledge of the depositional dynamics of seagrass berms on beaches was mainly derived from the morphology and composition of leaf litter deposited along the shores. In particular, a theoretical sequence of formation and destruction of banquettes was proposed by Mateo et al. (2003) and involved an initial stage of litter deposition, leading to a berm accretion up to the maximum height. Erosion due to wave action, acting at the base of the berm, leads to a scarp formation and a collapse of the banquette structures (Mateo et al., 2003). Differences in the morphology and composition of Posidonia oceanica seagrass berms were found to be influenced by beach exposure (exposed vs. sheltered beaches), on beaches of western Sardinia (western Mediterranean) (Simeone and De Falco, 2012). On exposed beaches, beach cast leaf litter was deposited in sub-aerial sections characterized by a strong elevation variability (berm area), whereas on sheltered beaches the berm edification/destruction was driven by the exchange of P. oceanica litter between the beachface and shoreface. This suggests that the deposition of seagrass berms can be influenced by swash morphodynamics, and starts landward where the swash cycle ends where the deposition of heavier material (mineral grain and rhizomes if present) occurs in correspondence to the swash limit (Simeone and De Falco, 2012). Observation on the complete depositional and erosive cycles of seagrass litter on beaches were not previously reported: video monitoring can be a useful tool for observing this process on a daily or seasonal basis. Several studies used the video monitoring systems to assess morphological variations in morphodynamic states of beaches: these systems are used widely in studying shoreline and beach cusp morphodynamics (Miller and Dean, 2007; Almar et al., 2008; Ojeda and Guillen, 2008), sandbar dynamics (Plant and Holman, 1997; van Enkevort and Ruessink, 2003; Armaroli and Ciavola, 2011), run-up measurements (Salmon et al., 2007) and the carrying capacity of beaches (Jimenez et al., 2007). Video monitoring has also been used to investigate the sedimentary and morphological processes involving the beachface and the swash zone (Osorio et al., 2012). Understanding of depositional dynamics of seagrass litter is fundamental from a beach management point of view, because seagrass litter removal from sandy shore, to increase the beach exploitation, is a widely diffused practice (Duarte, 2004; De Falco et al., 2008; Mossbauer et al., 2012; Simeone and De Falco, 2013) and is performed mainly with heavy machineries (De Falco et al., 2008). Consequently, seagrass litter removal could have a strong impact on the beach morphology, particularly on the beachface (Simeone and De Falco, 2013). The aim of this paper is to describe the depositional dynamics of the leaf litter of the seagrass Posidonia oceanica on a Mediterranean embayed beach. This process was described by observing cycles of seagrass wrack deposition, seagrass berm edification and erosion on a daily basis using a video monitoring system that collected a series of images each day. The study was done over a period of approximately one year between 2007 and 2008, the video-monitoring data were compared with wind data in order to track the relationship between depositional dynamics and wind speed peak. A statistical analysis was applied to the images collected daily in order to analyze the seasonal variability of
seagrass depositionaleerosive cycles along the beach over the course of one year. 2. Study area The study area is located in the north of the Island of Sardinia in the western Mediterranean sea (Fig. 1a), the geology of Northern Sardinia is dominated by granitic formations with fields of basic dikes and Quaternary deposits present in the coastal areas and on alluvial plains. The geomorphology of the coastline is inherited from the geological setting of the area, with the main embayment following the principal fault lines. The coastline of the northeastern sector of Sardinia is characterized by a series of indentations with small embayed beaches lying between rocky headlands (Fig. 1b). Those embayments were deepened by rivers during the last glacial maximum (about 20,000 years ago) and the erosion caused by the rivers accentuated the indented morphology (De Muro et al., 2010). The studied beach, named Cala di Trana, is located in one of these indented bays, in the north of Sardinia (Fig. 1b and c), seafloor of the bay is colonized by a wide Posidonia oceanica meadow, which starts at a water depth of 5e10 m, and grows on a sandy bottom (Fig. 1c). The total length of Cala di Trana beach is about 250 m, the maximum subaerial cross-shore amplitude is about 25 m when measured from the toe of the dune up to the shoreline. The grain size of the sediment forming the beach, ranges from coarse sands and gravel in the foreshore to fine sands in the shoreface (De Muro et al., 2010). A single well-developed foredune characterizes the backshore area, this dune is interrupted by a little creek and by a seasonal pond on the eastern side of the beach. The wind regime in the north of Sardinia is dominated by two directions: (a) a westerly wind (about 50 percent of the wind regime) blows between 260 and 300 that is present most of the time during all seasons and (b) an easterly wind which blows between 60 and 100 , mainly during winter (about 30 percent of the wind regime) (Gerigny et al., 2011). Fig. 2c and d, highlighted the wind condition in Cala di Trana beach and showed that the more frequent events, during one year study, are calm and light breeze coming from south (Fig. 2d). The same pattern on wind speed and direction was confirmed by the wind rose recorded by the Porto Torres wind station (Fig. 2e) of the Rete Mareografica Nazionale (www.mareografico.it) located about 100 km west to the study site (Fig. 1a), for a longer period (Fig. 2e). The more intense wind comes from WeNW and EeNE, as shown in Fig. 2c, where the wind data with wind speed > 3.4 m s1 were reported. The analysis of two storms coming from the aforementioned directions (NW and NE) with a return period of one year showed that the wave height on the Cala di Trana bay is <1 m for both cases (De Falco et al., 2011). Furthermore we provide two images of the state of the sea inside the bay during the most intense wind event coming from NW and NE during the study year (Fig. 2f and g), which show that the waves, inside the bay, break in the beachface in both cases. In summary on Cala di Trana beach the wave height is low, the fetch is limited and the tide is negligible. 3. Methods From August 23rd 2007 to June 11th 2008 an SVMÒ video imaging System was installed on the top of the hill of Punta Sardegna, overlooking Cala di Trana beach (Fig. 1c; def); the system was equipped with a video camera which collected short videos (a few minutes long) three times a day, and a camera used to obtain the time-exposure image (Holland, 1998) (Fig. 1e,f). Furthermore the
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Fig. 1. Study area, a) location of study area; b) geomorphological overview of the study area; c) Cala di Trana beach and position of video station; d) time-exposure image; e) timex grayscale, masked image used to perform EOF analysis; f) timex RGB masked image of the beach.
video-monitoring station was equipped with sensors for wind direction and intensity (Fig. 1c). Video monitoring system used in this work was composed by a fotocamera Olimpus 6 mpixel (max resolution 3264 2488), with a reflex optic 38e118 mm and a video camera Super HAD CCD 1/200 , Lux 0.025, lens zoom 3.6e18 mm. The video camera was zoomed on the beach and the first frame of each video was recorded three times a day in order to observe the depositionaleerosive cycles of seagrass berms on a daily basis, the resolution of the this images was 320 240 pixel. The time-exposure images were zoomed to obtain a view of the whole bay, these images resulted from averaging multiple frames (1 frame per second for an interval of 10 min, total 600 images), the resolution of the timex images was 768 1024 pixel, the pixel resolution in the center of the beach was 0.6 m. The grayscale time exposure images are usually used to identify the position of the shoreline, the sandbar system, if present, and
other morphological features of the beaches (Holland et al., 1997; Haxel and Holman, 2004; Quartel et al., 2006). In our case, the time-exposure images, transformed in grayscale (Fig. 1e), were used to identify the litter deposits on the seafloor of the bay (submerged domain) and to analyze the variability of seagrass berm deposits on the sub-aerial beach during the monitored year. The spatial and temporal variability on the subaerial sector of the beach was analyzed by applying the empirical orthogonal function (EOF) analysis on the time-exposure images, the images were collected daily at the same time, between 10:00 and 10:30 am, in order to obtain comparable light conditions. The time-exposure images of the surf zone show patterns in which white areas correspond to the swash zone and, if present, to sandbar locations; this is because breaking waves produce foam in both areas while darker areas usually correspond to deeper water. Conversely, pixels of the dry beach are lighter than those of the water and darker than those of the swash zone and of the sandbars,
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Fig. 2. a) Position of the beach profiles; b) beach profiles and seagrass litter deposits; c) wind rose on Cala di Trana beach (wind speed > 3.4 m s1); d) wind rose on Cala di Trana beach; e) wind rose on Porto Torres; f) sea state on the bay during the most intense wind event from NW (09/11/2007); g) sea state on the bay during the most intense wind event from NE (18/12/2007).
while beach cast leaf litter results darker than the swash area. Following this approach based on light contrasts, different areas can be identified in each image. A mask was applied to the timeexposure images in order to focus the analysis only on the subaerial beach and on a limited area of the sea adjacent to the beach (Fig. 1e and f). The mask excluded the outer sector of the bay from the EOF analysis. This was done to exclude from the analysis the disturbance due to the presence of boats, reflection in the sea surface induced by little wind waves and other sources. The resulting resolution after the application of the mask was 196 82 pixel and in order to perform EOF these images were transformed in grayscale (Fig. 1e). We analyzed this domain to infer, by means of EOF, the relationship among the beachface and berm areas and wave breaking area, including the seagrass litter deposits. The aim of EOF is to describe the changes over the whole subaerial beach surface using the least number of functions, called eigenfunctions (Winant et al., 1975; Dean and Dalrymple, 2004). In this way, it is usually possible to account for a large percentage of the variance with a small number of terms (Winant et al., 1975; Dean and Dalrymple, 2004). To perform the EOF analysis, every time-exposure image was transformed into a 196 82 matrices in which every element nij represented the pixel grayscale intensity (linear: 0e255). The grayscale conversion of the images provides a standardization of the light intensity of each image, thus reducing the effect of any changes in light condition. Each matrix was then
vectorialized into a one-row vector with dimension p ¼ 196 82, then a new M matrix n columns and p rows were built. In this matrix, n is the number of observations and p is the location of each pixel for each image (Olita et al., 2011). A total of 209 time-exposure images were collected in a period of 294 days. EOF analysis was then performed on the M matrix. The spatial variability was then represented by reconverting the resulting eigenfunctions (called EOFs), into three matrices, finally, each EOF was transformed into a colored image in order to represent the variability of the subaerial beach. Wind data were used to relate the leaf litter depositionale erosive cycles to the wind speed trend, the latter is related to wave intensity in the bay. To confirm the presence and the thickness of the seagrass berm on the beach during the monitoring period, two beach profiles were collected in September 17th 2007 and in January 16th 2008 by using Differential Global Positioning System (Fig. 2a and b) (Morton et al., 1993). 4. Results 4.1. Depositionaleerosive cycles of seagrass litter Images were used to identify the presence and the residence time of seagrass litter deposits on the subaerial beach. Three conditions were observed (Fig. 3aec): (a) subaerial beach without
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seagrass litter deposits, named E0, E4b and E5; (b) well developed seagrass berm along the whole subaerial beach, named D1, D2, D3, D4, D5; (c) patchy seagrass litter deposits in the subaerial beach, named E1, E2, E3, E4a. The sequence of depositional and erosive events during the monitored year is showed in Fig. 3a and is compared to the average wind speed. Five depositionaleerosive events were recorded: the first deposition of seagrass berm occurred in October, the last in April, a residual presence of leaf litter on the backshore was recorded during intermediate erosive cycles (E1eE4a). Whereas seagrass berms were completely eroded and the beach resulted “clean” of leaf litter after the last cycle of depositioneerosion occurred between April 3rd 2008 and April 14th 2008. During the D3 depositional event the thickness of the seagrass berm, measured by means of DGPS, in the area in which they occur was about 1 m (Fig. 2b).
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The comparison of depositionaleerosive cycles with the average wind speed (Fig. 3a), showed that the seagrass berm, once deposed on the beach, was eroded after several events marked by peaks of wind speed (Fig. 3b and c). After the partial or total erosion, the deposition of the seagrass berm on the beach occurs after an event characterized by a high wind speed (events D2, D3, and D5). On the contrary, the depositional events D1 and D4, occurred in October and in February, were not related to the wind speed peak, but deposition occurred after a period characterized by low intensity wind speed at the end of the summer and from January to late February. Selected time-exposure images of the whole bay are reported in Fig. 4, the shooting dates are indicated by the black squares in the upper side of Fig. 3a: these images showed the presence and the position of the leaf litter in the seafloor of the bay. Fig. 4a highlighted a typical image of the beach during the summer period
Fig. 3. a) Wind speed recorded by the wind sensor of the video-monitoring system is showed as continuous line; deposition (D1eD5) and erosion (E1eE5) cycles of seagrass berm on the Cala di Trana beach, showed as a dashed line. Black squares identify the images shown in Fig. 4; b) particular on D1eE1 event; c) particular on D3eE3 event.
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Fig. 4. Time-exposure images taken on 27 August 2007, 22 October 2007, 13 November 2007, 14 December 2007, 29 December 2007, 04 February 2008, 05 April 2008, 14 May 2008. Seagrass leaf litter on the seafloor of the bay was highlighted by the arrows as well as the well-developed seagrass berm, the residual litter on the beach and the beach free of wrack litter.
before the deposition of seagrass berms: wrack litter are absent on the backshore and on the foreshore, while the seagrass litter deposited on the shoreface can be identified by the dark area at the center of the bay. The subsequent images were collected after depositional events (Fig. 4b, e and g) and after the partial erosion of seagrass berms (Fig. 4c, d and f). Finally, the last image (Fig. 4h), collected May 14th 2008, shows the beach without seagrass wrack, similar to the initial stage of Fig. 4a. The seagrass litter on the shoreface is distributed over most of the bay during the erosivee depositional cycles, but is confined to the middle of the bay before October and after April. Two daily sequences of video-frame images reporting an event of litter deposition and seagrass berm edification (D1) and an event of seagrass berm partial erosion (E4a) are reported in Figs. 5 and 6. The complete cycle of deposition started when the wind speed increases and the waves start to break near to the shoreline, the deposition occurs as a strandline at the upper limit of the swash zone, on the upper side of the beach (Fig. 5aec). During the wave decay, the deposition proceeded seaward and from the upper side of the bay toward the lower side (Fig. 5d and e). At the end of the event, the whole berm area of the beach was occupied by the seagrass berm (Fig. 5f). Conversely, the complete cycle of seagrass berm erosion started from the front of the seagrass berm deposited on the foreshore (Fig. 6a), and proceed landwards as the wave increased in intensity (Fig. 6bed) and run up increases. At the end of the wind event, the beachface resulted almost completely free of litter, with little residual deposits of the previous seagrass berm remaining in limited areas of the backshore (Fig. 6f).
4.2. EOF analysis of time-exposure images Results of EOF analysis show that the first three eigenfunctions (EOF1, EOF2 and EOF3) accounted for 83.6% of the variability in time-exposure image intensities (61.4%, 19.1% and 3.1% respectively). Each of the EOFs below the third explain less than 2.5% of the total variability, furthermore noise present in this EOFs can generate confusion in the interpretation, for this reason we analyze here only the first three EOFs. Those EOFs were taken into consideration in order to interpret the subaerial beach subdivision based on the variability of the pixel intensities. The resulting maps (Fig. 7aec) highlighted the areas of the beach that vary in concordance or in opposition of phases in terms of pixel intensity, whilst the temporal amplitude of each EOFs highlighted the trend of the described processes during the observation period (Fig. 7eeg). The variability of the first EOF (Fig. 7a) was mainly located alongshore in the subaerial beach, in particular the EOF1 identified the two different domains present on each images: a) the beach and b) the sea. The temporal amplitude of the EOF1 followed a temporal cycle (summerewinter) with negative values from August to October, positive values from October to March and, again, negative values from March to June (Fig. 7e). The spatial variability of the second EOF (Fig. 7b) emphasizes the alongshore subdivision of the beach, highlighted by the EOF1, however the pattern of the EOF2 identified a) the wave breaking zone and the b) berm deposition area, that in the majority of case was built up by Posidonia oceanica leaf litter (Fig. 3a). Temporal amplitude of the EOF2 showed a cyclic pattern, with positive value from October to early April and negative pattern for the
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Fig. 5. Depositional cycle of seagrass berm on the beach. In a, b) beginning of wave increase, the seagrass litter was deposited as a strandline; c) the first phase of seagrass edification; d, e) decreasing of wave height, seagrass berm development from the western side of the beach to the eastern side, and from the run-up’s edge to the foreshore; f) the seagrass deposits at the end of the cycle. The black arrows indicate the longshore direction of the seagrass berm deposition, while the blue arrow showed the seaward seagrass deposits accumulation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Erosive cycle of seagrass berm on the beach. a) Seagrass berm deposits, b, c, d) wave energy causes an erosion of seagrass berm from the eastern side of the beach toward the western side; e, f) decreasing of wave height, the beach is free of litter, only a little residue (leaves) was present on beach. The red arrows indicate the retreat in the longshore direction of the seagrass berm deposits, while the blue arrow showed the landward retreat of the front of the seagrass deposits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
rest of the year (Fig. 7f). In particular during the autumnelate winter period the temporal amplitude accounts for high positive values in comparison to the rest of the year, this period was the same in which the majority of deposition erosion cycles occurs. The temporal amplitude of EOF2 was positively correlated with the speed of winds coming from the NW and NE sectors (R ¼ 0.512, p < 0.05), this indicating that the pattern showed by the second EOF occurred mainly during the wind peak events (Fig. 8).
The spatial variability of the EOF3 (Fig. 7c) identified several sources of variability. Those sectors of variability were related to seagrass berm and to noise that was not related to specific processes (i.e shadow effects). Particularly one sector, located in the upper portion of the image (red strip occupying longshore about half of the shoreline in Fig. 7c) corresponds to the part of images in which the seagrass litter remains between two consecutive depositioneerosion events. The temporal amplitude of EOF3 shows a different pattern in comparison to the first two EOFs. In particular,
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Fig. 7. EOF analysis results: a) spatial distribution of EOF1; b) spatial distribution of EOF2; c) spatial distribution of EOF3; d) temporal variability of EOF1; e) temporal variability of EOF2; f) temporal variability of EOF3.
during the winter period, the temporal amplitude of EOF3 showed high negative values, whereas during the rest of the year the values were slightly positive (Fig. 7g). 5. Discussion Observations of depositionaleerosive cycles of seagrass wrack on an embayed beach over a period of one year highlighted the processes which control seagrass berm edification and erosion. The first depositional events, in October, were not related to a particular intense wind event: the meteorological data showed that wind events of similar intensity and direction were recorded during the months of August and September and from January to late February. The deposition erosion cycles occurred from late October up to early April, during this period the wind speed for each event was generally higher in comparison to the rest of the year (Fig. 3a): this intense winds, coming from NW and NE, generate waves that break very close to the shoreline (Fig. 2c, f and g).
The initial stage of Posidonia oceanica seagrass berm edification was probably related to the increase of litter availability on the seafloor of the bay during Autumn that can be due to the loss of leaves (Romero et al., 1992; Mateo and Romero, 1996) from the seagrass meadow adjacent to the bay. During the autumn a large number of leaves of P. oceanica are senescent and fall off the rhizomes (Mateo and Romero, 1996; Mateo et al., 2003): this process involves an increase of litter availability during the autumn and winter which can be transported onshore even under moderate wave action, triggering the initial stage of seagrass berm edification. Furthermore, the presence of headlands, reef and outcrops, as occurring in Cala di Trana bay, can limit the development of longshore, rip and feeder currents (Short, 1999), with a trapping of the leaf litter inside the bay. In this case the availability of seagrass litter in the inner sector of the bay could trigger a depositional event also in case of moderate wind speed (D4 event, Fig. 3a). On the contrary, low degree of embayment and higher wave height can favor the presence of rip, megarip and longshore circulation (Short, 1996), which can push out of the system the floating leaf litter.
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Fig. 8. Correlation between EOF2 temporal amplitude and wind speed (NW and NE direction).
From October to April, a series of depositional and erosive events of beach cast leaf litter occurred; the area of deposition of Posidonia oceanica leaf litter corresponds to the berm area. During their maximum development, the seagrass berms occupies the whole beach longshore, whereas cross shore, seagrass berm occupies the beach from the landward limit of the wave action, up to the shoreline. The observation of daily images highlighted that the deposition starts at the upper limit of the run up, like the strandline, and proceeds seaward from the upper sector of the beach toward the lower sector of the beach, occupying the whole berm area as the waves decrease in height. On sandy beaches the material forming the berm was mainly derived from the inner surf zone (Russel et al., 2009), in our case, this material was also represented by leaf litter floating in the inner surf zone and in the lower swash area. Deposition of the seagrass berm occurred during the whole period in which the waves, generated by the wind peak, increasing and decay. The swash zone morphodynamics controls the process of deposition of the seagrass litter. In the swash zone, flow velocities, suspended sediment concentrations and suspended fluxes are greatest at the start of uprush, when the flow is most turbulent and the dominant mode of transport is expected to be a suspended load. The swash flow energetics decreases following the arrival of the swash front. The result is that the suspended material settles on the bed, leaving the water clear (Masselink and Puleo, 2006). In presence of seagrass litter the suspended material subjected to transport in the swash zone is composed by sediments, leaves and rhizomes (Simeone and De Falco, 2012) and is deposed at the limit of run up building up to the standline showed in Fig. 5. At the end of the deposition, the final shape of seagrass litter deposits on beach is comparable to a berm as showed in Fig. 2b. Conversely, during the erosive cycle, the removal of the seagrass berm by waves was faster and almost the entire seagrass berm was eroded during the phase in which waves increase in height (Fig. 6b and c). Residue of the seagrass berm in the form of floating litter, could remain in the swash zone and be deposited again when waves decay, however, the re-deposited litter accumulation was little in size and covered a very limited area of the beach. Once deposited, the wrack litter occupied the whole beachface from the run-up’s seaward edge to the shoreline, and tended to build up a single seagrass berm on the whole beach surface (Figs. 1b and 4b). These structures were also found in other beach systems (Short, 1999; De Falco et al., 2003; Simeone and De Falco, 2012) and could be responsible for an occasional migration of the shoreline several meters seaward (Basterretxea et al., 2007).
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On sandy estuarine beaches, the cast vegetation litter is generally high in low energy environments, due to the presence of vegetation growing in the intertidal zone, whilst in high energy environments the litter is generally accumulated on beaches as the results of an individual swash uprush and the amount of litter in each swash line is often small (Jackson et al., 2002). Conversely on Mediterranean beaches, the volume, thickness and cross-shore length of Posidonia oceanica litter deposits resulted higher on exposed beaches in comparison to sheltered ones (Mateo et al., 2003; Simeone and De Falco, 2012). The depositional dynamics observed in the Cala di Trana beach, is coherent with the depositional models proposed in previous studies (Mateo et al., 2003; Simeone and De Falco, 2012). The mentioned authors proposed that the depositions of the P. oceanica litter on beaches starts landward at the limit of run-up and proceeds seaward with a growing in wideness and thickness of the seagrass deposits, forming a seagrass berm, whilst the erosion of the seagrass deposits starts from the seaward front during a storm and proceeds landward. The daily observation of depositionaleerosive cycles obtained by the video-monitoring, supported the aforementioned models. EOF analysis on the time-exposure images highlighted different sources of variability on the emerged beach and on the submerged domain. EOF1 is related to the seasonal pattern of the sub-aerial beach: in particular by the EOF1 pattern the images can be divided into two domains: the beach and the inner surf zone (Fig. 7a). The presence of numerous peaks can be interpreted as the influence of the waves and consequently, of the correspondent runup, on the analyzed domain. The pattern of EOF2 highlighted the opposing brightness of the sub-aerial beach and the submerged domain, the former was the beachface that was occupied by seagrass berm, the latter was the wave breaking zone that appear brighter (Fig. 7b). During summer, the subaerial beach was bright due to the absence of seagrass litter, and the submerged domain was dark due to the lack of breaking waves. During winter, the berm area had lower pixel intensity compared to the submerged domain. This is because it was formed by beach cast leaf litter which appears darker than the other pixels in the images, and in particular to the pixel of the breaking and swash areas and to the pixel of the dry beach. Conversely, the submerged domain resulted brighter during winter, due to a more frequent occurrence of foam due to the presence of waves, and the maximum values of EOF2 evidence the sections of foam probably related to wave action. Temporal amplitude of EOF2, shown in Fig. 7f, was generally characterized by high positive values during the period of the year in which depositionaleerosive cycles of seagrass berm occurred, while low negative values characterized the rest of the year. This temporal pattern can be linked to the swash processes and to the berm formation that was built up by Posidonia oceanica leaf litter on this beach and which occurred various times, but only during the autumn and winter (Fig. 3a). This is because the litter was available and the high speed events, from NW and NE, occurred. The relationship between the temporal amplitude of the EOF2 and the wind speed (Fig. 8), suggests that the main forcing, which characterizes the subaerial beach, was the waves generated by winds, that pushed the litter landward, leading to a decrease in grayscale pixel intensity caused by deposition of the litter. The spatial variability of EOF3 is located in the sector of the subaerial beach where seagrass berms were always present from the first depositional cycle up to the complete seagrass berm erosion in spring. This area is located in the western sector of the beach and represents the initial stage of berm edification and the residual deposit after partial berm erosion. The observation of single depositional events showed that seagrass berm edification started
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in the western sector of the beach and continued toward the eastern sector, occupying nearly the whole emerged beach. As observed in microtidal sandy beaches, in the absence of swash overtopping, the berm tended to grow horizontally without discernible variations in height (Weir et al., 2006). This aspect could be relevant in the case of seagrass berm deposition, which can follow the same pattern of deposition on beaches. The vegetal litter accumulation is a diffuse phenomenon occurring on sandy beaches over different coastlines all around the world (De Falco et al., 2008; Mossbauer et al., 2012; Gomez Pujol et al., 2013; Simeone and De Falco, 2013). In the Mediterranean sea, specifically in Italy, Spain and France, this process involves several thousands of km of sandy coastline. (Duarte, 2004; De Falco et al., 2008). The yearly removal of several thousands of cubic meters of Posidonia oceanica seagrass litter from Mediterranean beaches has become a threat for the sandy shores (Simeone and De Falco, 2013), and the knowledge of the depositional dynamics could be useful to predict this kind of impact. Furthermore this practices, as well as cleaning operations carried out with mechanized machines, may be considered a potential stressor for dunes due to the role of the seagrass litter deposits in the foredune edification and dune fertilization (Cardona and Garcia, 2008; Nordstrom et al., 2012). Our study highlighted that in an embayed beach the litter availability and the wind peak, generating waves in the bay, can be considered as the main forcing that influencing the seagrass litter deposition on the beachface. Following this consideration, our study highlighted that the removal operation of the seagrass litter could not be done during the period in which exchange of litter between the shoreface and the beachface occurs. In fact, the litter removal during the wintereearly spring period does not prevent a further deposition, resulting in a useless practice; furthermore the litter removal during the period characterized by erosiveedepositional cycles, could affect the morphological variability of the beach and the exchange of material between the berm and the surf zone. Finally, this study confirms that leaf litter may be relevant to the berm formation and that the litter floating in the inner surf zone is an important part of the material exchanged between the submerged and emerged beach. 6. Conclusion The video-monitoring system is a useful tool for analyzing short-term processes acting upon a beach, in particular we used this tool to reveal the depositional dynamics of the leaf litter of the seagrass Posidonia oceanica on the Cala di Trana beach. This study highlighted that several cycles of depositionale erosive of seagrass berm were recorded over one year on an embayed beach. The complete cycle of deposition started when the wind speed increases and the waves start to break near to the shoreline, the deposition occurs as a strandline at the upper limit of the swash zone, on the upper side of the beach. Conversely, during the erosive cycle, the removal of the seagrass berm by waves was faster and almost the entire seagrass berm was eroded during the phase in which waves increase in height. Our study highlighted that in an embayed beach the litter availability and the wind speed, generating waves in the bay, can be considered as the main forcing that influencing the seagrass litter deposition on the beach. Acknowledgments This work was funded by the GERER project “Gestione ambientale integrata in località ad alto rischio d’erosione”, scientific coordinator Prof. Sandro De Muro, funded by the European Union (Interreg IIIA), and the RIAS project “Risposta e Adattamento dei sistemi costieri della Sardegna alle variazioni
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