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Evidence of sediment gravity flows induced by trawling in the Palamós (Fonera) submarine canyon (northwestern Mediterranean)
ARTICLE IN PRESS
Deep-Sea Research I 53 (2006) 201–214 www.elsevier.com/locate/dsr
Evidence of sediment gravity flows induced by trawling in the Palamo´s (Fonera) submarine canyon (northwestern Mediterranean) A. Palanques, J. Martı´ n, P. Puig, J. Guille´n, J.B. Company, F. Sarda` Institut de Cie`ncies del Mar (CSIC), Passeig Maritim de la Barceloneta, 37-49, Barcelona 08003, Spain Received 18 October 2004; received in revised form 18 July 2005; accepted 20 October 2005 Available online 20 December 2005
Abstract Three mooring arrays were deployed in the Palamo´s Canyon axis with sediment traps, current meters and turbidimeters installed near the bottom and in intermediate waters. Frequent sharp and fast turbidity peaks along with current speed increases were recorded, particularly at 1200 m depth in spring and summer. During these events, near-bottom water turbidity increased by up to more than one order of magnitude, current velocity by two to four times and horizontal sediment fluxes by one to three orders of magnitude. When these events occurred, 9–11 days integrated downward particle fluxes collected by the near-bottom sediment trap increased by two to three times. These events were identified as sediment gravity flows triggered by trawling activities along the northern canyon wall. Sediment eroded by the trawling nets at 400–750 m depth on this wall seems to be channeled through a gully and transported downslope towards the canyon axis, where the 1200 m mooring was located. The sediment gravity flows recorded at the 1200 m site were not detected at deeper instrumented sites along the canyon axis, suggesting that they affect local areas of the canyon without traveling long distances downcanyon. These observations indicate that trawling can generate frequent sediment gravity flows and increase sediment fluxes locally in submarine canyons. Furthermore, in addition to the various natural processes currently causing sediment gravity flows and other sediment transport events, human activities such as trawling must be taken into account in modern submarine canyon sediment dynamics studies. r 2005 Elsevier Ltd. All rights reserved. Keywords: Submarine canyon; Sediment gravity flows; Trawling; Turbidity; Northwestern Mediterranean; Catalonia
1. Introduction Submarine canyons are preferential conduits for shelf–slope sediment transport and, although they were more active during low sea-level stands, they can still maintain a significant activity during high Corresponding author. Tel.: +34 93 2309517, +34 93 2309500; fax: +34 93 2309555. E-mail address: [email protected] (A. Palanques).
0967-0637/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2005.10.003
sea-level stands like the present one at least for fine sediment transport. For this reason, sedimentary activity in modern submarine canyons has often been studied in research projects devoted to analyzing shelf–slope transfer by measurement of downward particle fluxes or water turbidity with moored instruments (Gardner, 1989; Monaco et al., 1990; Heussner et al., 1999; Puig et al., 2000). In these studies, increases in near-bottom downward total mass fluxes and water turbidity were frequently
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associated with lateral inputs of suspended particulate matter coming from the shelf as a consequence of high energy events such as river floods and storms, and in some cases increases in turbidity were also associated with internal wave/tide activity. However, other unexpected increases in total mass flux without any clear relationships with storms or river flood events, and without any defined periodicity, could not be properly interpreted because of the lack of complementary data to explain their origin (Puig and Palanques, 1998; Heussner et al., 1999). In some cases, the influence of trawling has been hypothesized to interpret unexpected high particle fluxes occurring in summer with calm sea and low river discharge (Puig and Palanques, 1998; Palanques et al., 2005). In other cases it has been suggested just as a factor responsible for particle transfer (Stavrakakis et al., 2000). One reason to propose trawling as a source of resuspended sediment was that the moorings recording unexpected sediment flux increases tended to be those that had accidents with trawling boats. However, a direct record or a set of data that shows an evident relationship between trawling and sediment flux increases has never been obtained. Direct effects of bottom trawling are scraping and ploughing of the seabed and sediment resuspension. There are several types of trawl gears and one of the most common is the otter trawl. An otter trawl gear is a towed net with sweeplines (bridles), iron otterboards and warps (Fig. 1A). The mouth of the net is kept open by means of floats and weights, and is spread horizontally by forces exerted on the doors which are oriented obliquely to the trawl’s forward motion. The two otterboards of the benthic otter trawls are heavy (from hundreds of kilograms to tons) and imprint furrows in the sediment that are 0.2–2 m wide and up to 0.3 m deep, with the resulting sediment resuspension (Jones, 1992). The sweeplines and the footrope also cause sediment resuspension. The effect of trawling activities has been a topic of interest in ecology and fishery resource studies because of the impact of trawling gears on benthic fauna (de Groot, 1984; Hutchings, 1990; McAllister, 1991; de Groot and Lindeboom, 1994; Kaiser and Spencer, 1995, 1996; Auster et al., 1996), but recently it has also become a topic of interest because of its physical impact (Beon, 1990). Krost et al. (1990) observed that intensely fished areas had a high density of trawl tracks on the bottom, depending on the type of sediment. Churchill
(1989) suggested that large and highly turbid clouds of suspended sediment detected in the shelfbreak area of the mid-Atlantic Bight could be created in the wake of trawl doors. Schoellhamer (1996) observed highly turbid plumes near the seabed produced by resuspension by otter trawl gears on muddy sediment. Palanques et al. (2002) observed sediment erosion and an increase in water turbidity after inducing a trawling disturbance in an unfished shelf muddy area. Durrieu de Madron et al. (2005) monitored and modeled the dispersal of the plumes caused by trawling in shelf sediments. However, the physical effects of trawling in submarine canyons are still poorly known. In this paper, we show a direct relationship between trawling activities, turbidity peaks and sediment flux increases during the calm and dry season and discuss the mechanisms that cause these ‘‘unexpected’’ increases in sediment transport in submarine canyons, and the relevance that they have for present-day sediment dynamics in these environments. 2. Study area The study area is the Palamo´s Canyon, also known as the Fonera Canyon (Fig. 1B). This canyon is incised in the Catalan continental margin (northwestern Mediterranean Sea). The Palamo´s Canyon was selected for a multidisciplinary study (Palanques et al., 2005), because it is incised in the continental shelf and receives direct sediment inputs from the coastal zone and nearby rivers. The main river near the canyon head is the Ter River (Fig. 1B). The head of the Palamo´s Canyon is only about 4 km from the coastline, and the canyon is deeply incised in a continental shelf that is about 30 km wide. It is important to point out that at the shelfbreak of the study area this shelf-incised submarine canyon is deeper than 1200 m and that the canyons walls are traversed by gullies. The main characteristic of the regional circulation pattern in this margin is a slope current referred to as the Northern current (Millot, 1999), which is associated with a shelf–slope density front that in this area flows mainly towards the southwest (Font et al., 1988). This baroclinic current separates low-salinity shelf waters from denser open-sea saline waters (Castello´n et al., 1990; Garcı´ a et al., 1994). Regarding trawling activities, during the last decades deep-water fishery in the northwestern Mediterranean has targeted the red shrimp Aristeus
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Fig. 1. Schematic diagram of an otter trawl (A). Map of the Palamo´s Canyon showing the location of the mooring arrays and the fishing grounds (discontinuous black thick lines) in the study area (B). Note that the ‘‘Mongrı´ gully’’ (dark grey arrow) is just above the 1200 m depth canyon axis site.
antennatus. The populations of this species show seasonal migrations, and its life cycle seems to be highly related to the geomorphology of the continental slope, which in this region is crossed by many submarine canyons (Sarda` et al., 1997). Specialized fishing boats follow these seasonal movements around the submarine canyons. In the
study area, the fishing gear used by these boats is a benthic otter trawl (Fig. 1A), which is large and is operated by fishing vessels with 700–1500 HP engines. These bottom trawls have two heavy otterboards (600–1200 kg), and the distance between them during trawling can be more than 100 m. The mouth of the net is 40–50 m wide, and the total
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length of the net is about 80–150 m. The sweeplines that connect the otterboards to the net have a total length of 60–200 m. It is important to note that these fishing gears are much larger than those used near the coast. In the area of the Palamo´s Canyon there are two fishing grounds, one on the northern canyon wall, which is called Sant Sebastia`, and one on the southern canyon wall, which is called Rostoll (Fig. 1B). 3. Observational and analytical work Moored instrument deployments and hydrographic surveys were carried out from March to November 2001 within and around the Palamo´s submarine canyon. In this study, we focus on the three moorings deployed in the canyon axis at 470 m depth (M2), 1200 m depth (M3) and 1700 m depth (M5) (Fig. 1B). On each of these moorings, a set of instruments consisting of an Aanderaa RCM 9 current meter equipped with temperature, pressure, conductivity and turbidity sensors and a Technicap PPS3/3 sediment trap was deployed near the bottom. Current meters were placed 12 m above bottom (mab) and sediment traps 22 mab. One set of these instruments was also deployed in intermediate waters of the canyon axis at the 1200 m depth, site (M3), at 400 m depth (800 mab). The study period was divided into two consecutive deployments. The time sampling of the current meters was set at 10 min. The sampling intervals of sediment traps were 9 and 11 days for the first and second deployment, respectively. Sediment traps worked during the entire deployment period. However, the current meters with turbidimeters installed near the bottom at 1200 m depth (M3) and 1700 m depth (M5) recorded only up to early August and early September, respectively. The temperature sensor worked well until mid-July. Samples of particulate matter collected by sediment traps were processed in the laboratory, and total mass fluxes (TMF) were estimated according to the method described by Heussner et al. (1990). The grain-size distribution of the sediment-trap samples was determined by a settling tube for the 450 mm fraction and by a Sedigraph for the o50 mm fraction, following the method described by Giro´ and Maldonado (1985). Current meter data were calibrated and processed following standard procedures for their interpretation. Turbidity data collected in NTU were converted into suspended
sediment concentration (SSC) following the methods described by Guille´n et al. (2000). Sediment fluxes were calculated at the mooring sites in the canyon. Instantaneous sediment fluxes were obtained by multiplying the current speed by the SSC at each sampling level and location. The Ter River discharge during the deployment period was supplied by the ‘‘Agencia Catalana de l’Aigua’’ (Catalan Autonomous Government) and the wave climate by ‘‘Puertos del Estado’’. Field data on fishing activity of the fleet operating around this canyon were obtained from the logbook given to the skipper of the boat that usually trawls in deeper waters (‘‘Gacela’’). Daily records of fishing depth and the precise time at which the otter benthic trawl crossed over the study area along the fishing ground were taken. As the fishing boats trawling around the Palamo´s Canyon all work together (i.e. they work in parallel covering different depths), these data are representative of the fleet fishing activity. The skipper was also asked to record other information in the logbook during the sampling period, such as the fishing ground that they visited each day, the weather and the periods of non-activity due to boat maintenance. 4. Results 4.1. TMF and SSC along the canyon axis Time series of near-bottom downward TMF and SSC recorded in the Palamo´s Canyon axis during the deployment period are shown in Fig. 2. At 470 m depth (M2), TMF ranged from 16.0 to 494.0 g m 2 d 1 and the mean TMF was 28.8 g m 2 d 1. During most of the deployment period TMF variability was not very high, but at the end of this period a severe storm took place and the last cup of the sediment trap was overfilled. Near-bottom SSC at 470 m depth ranged between 0.4 and 1.4 mg l 1 during most of the time, with some isolated spikes showing SSC increases of 3–24 mg l 1. During the storm, SSC increased to 10 mg l 1. At 1200 m depth (M3), near-bottom TMF ranged from 8.9 to 494 g m 2 d 1 and had a mean TMF of 44.3 g m 2 d 1. TMF at this site showed much higher values and much higher variability than at 470 m depth. This trend was also observed in the SSC temporal series, which showed background values of 0.4 to 1.4 mg l 1, but with frequent SSC spikes ranging from a few to more than 30 mg l 1. During the periods in which TMF increased, there
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Fig. 2. Time series of near-bottom downward particle fluxes (TMF) and water turbidity (SSC—suspended sediment concentration) at the sites of the moorings deployed along the canyon axis at 470, 1200 and 1700 m water depth (see locations in Fig. 1B). The November flux peak corresponds to a strong storm. Question mark indicates that the sediment trap was overfilled.
were always strong SSC spikes. At this site, the last near-bottom cup was also overfilled during the storm event and the last cup of the intermediate water trap increased drastically, indicating particle transfer in the entire water column. At 1700 m depth (M5), TMF were from three times to one order of magnitude lower than at 1200 m depth, and the mean TMF was 8.5 g m 2 d 1. However, at this site there were sporadic periods in August when TMF increased to 38.9 g m 2 d 1. The highest TMF, of 57.7 g m 2 d 1, corresponded to the November storm event. Near-bottom SSC at this site ranged from 0.1 to 0.8 mg l 1 during most of the recording period, but a few SSC spikes reaching up to 40 mg l 1 were also recorded. These spikes also occurred during the sampling periods in which TMF increased. 4.2. Increasing SSC, current speed, grain size and particle flux events A relevant feature recorded in the Palamo´s Canyon is the occurrence of high turbidity peaks, recorded mainly near the bottom at 1200 m depth in spring and summer, and associated with periods of increasing TMF (Fig. 2). These spring and summer turbidity peaks and increasing TMF periods were always detected near the bottom and never in
intermediate waters (Fig. 3), indicating that they were not related to vertical fluxes in the whole water column. Sporadically, some peaks in SSC and TMF were also recorded in the canyon axis near the bottom at 470 and at 1700 m depth, but they were much more isolated and scarce than at 1200 m depth, and none of the spring–summer events recorded at 1200 m depth was recorded at 470 or at 1700 m depth. During these events, SSC at 12 mab increased by more than one order of magnitude, reaching values higher than 30 mg l 1 and, simultaneously, nearbottom current speed increased by two to four times, reaching up to 24 cm s 1. Additionally, very small temperature increases (from 0.02 to 0.09 1C) were detected during these events. There were three welldefined current directions during increasing SSC events—two towards the S–SSW (1801 and 2251) down the northern canyon wall and one towards the East (1001) down the canyon axis (Fig. 4). Most of the events were down the canyon wall. Near-bottom (12 mab) instantaneous horizontal sediment fluxes at the 1200 m site increased by one to three orders of magnitude during these sediment transport events, reaching values of up to 7560 mg m 2 s 1 (Fig. 5A). The duration of these events ranged from 1 to 6 h, and it is also important to point out that at the 1200 m site most of them occurred on working days,
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Fig. 3. Time series of downward particle fluxes (TMF) and water turbidity (SSC—suspended sediment concentration) in intermediate and near-bottom waters of the canyon axis at 1200 m depth mooring site. The November flux peak corresponds to a strong storm. Question mark indicates that the sediment trap was overfilled.
during working hours and never under bad sea conditions (Fig. 6). The downward TMF recorded by the sediment traps increased by two to three times during the sampling periods in which these events occurred. The samples collected by the sediment traps during these sampling periods were coarser— the silt content increased from 42% to up to 55% (Fig. 5B), and the sand content increased from about 0.5% to up to 2.5%. Taking into account that these events lasted only a few hours and that the sediment trap periods were 9 and 11 days, the induced TMF and the grain size increases were significant. 4.3. Trawling During the period when complete data of TMF, turbidity and current were recorded, trawling activities in the Rostoll fishing ground took place mainly in mid- and late-March, in late-May and early-June and in late-July and early-August. Haul depths along the Rostoll fishing ground start close to 400 m and reach 450 m (Fig. 7). During the same period, trawling activities in the Sant Sebastia`
fishing ground were carried out for more days than in the Rostoll fishing ground. They took place in late-March, in early- and mid-April, in May, in midand late-June, in July and in early-August. In the Sant Sebastia` fishing ground, hauls were usually made deeper than in the Rostoll area. During most of the period (March–June), the hauls in this fishing ground were made along the northern canyon axis between 650 and 750 m depth, whereas in July they were made between 400 and 650 m depth (Fig. 8). Some of the days during which trawling activities did not take place in the Sant Sebastia` and Rostoll fishing grounds correspond to periods of maintenance of the ship that reported data and/or to days of bad weather. 5. Discussion The cause and the process that generate frequent events of increasing sediment fluxes oriented downward of the northern canyon wall and axis at 1200 m depth are discussed in this section. The recorded data showing peaks of SSC and current speed along
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Fig. 4. Time series of near-bottom water turbidity (SSC—suspended sediment concentration) and current velocity in the canyon axis at 1200 m depth. Scatter plot of current direction during periods of increased turbidity (45 mg l 1) (below).
with slight temperature increases suggest the occurrence of events during which water and sediment moved downslope, presumably as a sediment gravity flow. Conceptually, the term ‘‘sediment gravity flow’’ includes any flow by which sediment moves because of its contribution to the density of the fluid, creating negative buoyancy (Middleton and Hampton, 1973, 1976). It can be used to describe a whole suite of processes from landslides (almost entirely regulated by particle–particle collisions) to dilute turbidity currents (which are
regulated almost entirely by turbulence of the interstitial fluid). The recorded data do not allow the specific type of gravity flow to be identified as it does not indicate whether or not the flows are turbulent, the vertical distribution of sediment concentration in the bottom boundary layer or the potential concentration at the starting point. However, the flow directions (always downslope) and the magnitude of the SSC and current velocity peaks in comparison with the regular values (i.e. more than one order of magnitude and two to four times
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Fig. 5. Near-bottom instantaneous horizontal sediment fluxes at the 1200 m site (A). Temporal series of downward particle fluxes (TMF) and silt content in near-bottom waters of the canyon axis at 1200 m depth (B). The November flux peak corresponds to a strong storm. Question mark indicates that the sediment trap was overfilled.
higher, respectively), as well as the increase in TMF and grain size of the samples collected in the sediment trap at 22 mab, are characteristics that can be clearly attributed to sediment gravity flows. Presumably, particle concentrations and current velocities closer to the bottom were much higher than at 12 mab, and suspended sediment was coarser than at 22 mab, as the sediment gravity flows at the instrumented levels could be diluted. Up to now, direct evidence of sediment gravity flow activity in submarine canyons have been very scarce because of the extreme difficulty of monitor-
ing them in the sea. Some evidence of modern sediment gravity flow activity was recorded in the Var Canyon (Crassous et al., 1991), in the Monterrey Canyon (Paull et al., 2003), in the Zayre submarine valley (Khripounoff et al., 2003) and in the Eel Canyon (Puig et al., 2003, 2004). Very recent turbidite deposits were also sampled in the Capbreton Canyon, indicating modern gravity flows activity (Mulder et al., 2001). Little is known about the mechanisms that trigger sediment gravity flow events, their frequency or intensity. Several studies have shown that large
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Fig. 6. Ter River water discharge (Ds), significant wave height (Hs) and near-bottom turbidity (SSC—suspended sediment concentration) in spring and summer 2001. Dashed lines and dates correspond to Sundays in order to distinguish the working days.
volumes of sediment can be transported down the axis of some submarine canyons by the action of different mechanisms such as internal waves, tidal motions, high-energy storms affecting the upper portion of submarine canyons, hyperpycnal river plumes formed during river floods, earthquakes or failure on canyon walls (Shepard et al., 1979; Shepard and Dill, 1966; Okey, 1997; Mulder and Syvitski, 1995; Johnson et al., 2001). However, in the Palamo´s Canyon, sediment gravity flows took place during the calm and dry season without relevant storm and river flood events (Fig. 6), and there were no seismic movements during this period of time. In addition, there is a lack of periodicity in the SSC peaks, which means that they were not generated by resuspension due to oscillatory mechanisms such as internal waves. The fact that sediment gravity flows in the Palamo´s Canyon always occurred on working days, in working hours, and never under rough sea conditions (Fig. 6) suggests that they are associated with the trawling activities in the study area where big otter trawls are used and could be the triggering mechanism. To demonstrate this, we analyzed the
relationship between the turbidity peaks and the number of hauls per day and ship and the depths of hauls over the 1200 m canyon mooring site during the deployment period. Fig. 7 shows that most of the hauls on the Rostoll fishing ground were made when no major sediment gravity-flow events occurred, indicating that these hauls were not related to these events. However, in the Sant Sebastia` fishing ground (Fig. 8), almost all the sediment gravity flow events occurred when fishing boats were trawling there, indicating a relationship between these events and the hauls made in this fishing ground. This fits with the observation that the near-bottom currents during the sediment gravity flow events flowed from the northern canyon wall and not from the southern wall (Fig. 4). It is important to stress that the Sant Sebastia` fishing ground traverses a gully incised in the northern canyon wall—the Mongrı´ gully—that ends at the canyon axis just where the 1200 m depth mooring was located (Fig. 1B). Checking in detail the sediment gravity flow events at 1200 m depth, we observed that sudden turbidity and current velocity increases occurred about 2 h after the fishing fleet
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Fig. 7. Number of hauls per day and trawler recorded by the ‘‘Gacela’’ skipper in the Rostoll fishing ground in spring and summer 2001 (above). Water turbidity (SSC—suspended sediment concentration) and haul depth in the Rostoll fishing ground in spring and summer 2001 (below).
traversed the Mongrı´ gully at about 500–600 m above the 1200 m mooring site (Fig. 9). Considering that the distance between the mooring site and the San Sebastia` fishing ground (i.e. the axis of the Mongrı´ gully) is about 2800 m, the sediment gravity flows had to move downslope at about 40 cm s 1 to travel this distance. This value is within the same order of magnitude as the velocities measured at the 1200 m site during these events, although it is slightly higher because it represents an integrated current velocity (i.e. from the source to the mooring) and the current meter records were obtained at 12 mab, where the sediment gravity flow was presumably diluted. This indicates that trawling-induced sediment gravity flows were probably channeled downslope through the gully and towards the canyon axis, and constitutes direct evidence of the relationship between these events and trawling activities in the northern canyon wall fishing ground. Not all the hauls produced sediment gravity flows, but practically whenever an event took place at the 1200 m site, the trawling ships were working in the Mongrı´ gully area, and these events were practically never recorded at this site when the
ships remained in the harbor because of bad weather or for maintenance. We did not record any sediment gravity flow from the Rostoll fishing ground because the hauls there are shallower and do not traverse any major gully ending around the mooring site. As a consequence, trawling activity in the San Sebastia` fishing ground significantly increased the downward particle fluxes at the 1200 m site. It is difficult to estimate the natural particle fluxes at this site, but using as a reference the downward particle fluxes at the 470 m depth site, the mean near-bottom TMF at the 1200 m depth site (43.3 g m 2 d 1) increased by 53% in comparison with that at the 470 m depth site (28.8 g m 2 d 1). Considering that natural TMF at 1200 m should be lower than at 470 m, as is indicated by the TMF trend in most of the submarine canyons studied, the contribution of trawling to the TMF at the 1200 m site must be higher than 53%. The fact that the current direction induced during some sediment gravity flows was downcanyon (towards 1001) (Fig. 4) suggests that some of these events could have been generated outside the
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Fig. 8. Number of hauls per day and trawler recorded by the ‘‘Gacela’’ skipper in the Sant Sebastia` fishing ground in spring and summer 2001 (above). Water turbidity (SSC—suspended sediment concentration) and haul depth in the Sant Sebastia` fishing ground in spring and summer 2001 (below).
Mongrı´ gully and closer to the canyon head, and once they reached the canyon axis they were reoriented, flowing for some distance along the canyon axis. However, the sediment gravity flows recorded at the 1200 m canyon axis site were not recorded at the 1700 m canyon axis site, indicating that they did not travel too far along the canyon axis, where the slope is less steep than on the canyon wall, and that they affected only a limited part of the canyon. The location of the Sant Sebastia` fishing ground (Fig. 1B) also explains why the events recorded at 1200 m depth site were not recorded at the 470 and 1700 m depth sites. Sporadically, some isolated gravity flow events were also recorded near the bottom at the canyon head at 400 m depth, and in the canyon axis at 1700 m depth. These isolated gravity flow events occurred on days without recorded trawling activities in the area. This suggests that other causes could also have triggered sediment gravity flows in the Palamo´s Canyon, although more sporadically than those generated by trawling activities at 1200 m depth. Based on the observations collected in this study, we can infer that the sediment remobilization
generated by fishing trawlers in sloping areas, such as the Palamo´s northern canyon wall, can trigger sediment gravity flows. Thus, besides the action of several natural mechanisms, the impact of trawling must also be considered in studies of modern sediment dynamics in areas close to fishing grounds. In the case of submarine canyons, the effect of the erosion caused by trawlers working on the canyon walls can reach depths well beyond the ones at which they work. In this study, sediment eroded at 600–700 m reached the canyon axis at 1200 m depth in about 2 h. This process delivers resuspended sediment to the canyon floor and alters near-bottom hydrodynamics in pulses with an artificial frequency, which may induce some indirect impact on deep ecosystems that are distant from where the trawling activity occurs. One indirect impact of trawling on deep ecosystems could be sediment suffocation of some unique ecosystems, such as deep-sea corals, which have been recently discovered in the Mediterranean (Tursi et al., 2004). In the specific case of the Palamo´s Canyon, it is worth mentioning that this human activity has been conducted on a daily basis for at least the last 50 yr, and that it could have increased canyon
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sediment fluxes during that period of time. As trawling on canyon walls is relatively common, other submarine canyons could be affected in a similar way. Nowadays, besides the several natural mechanisms that can cause sediment gravity flows and other sediment transport events, anthropogenic activities such as trawling must also be taken into account in studies of modern sediment dynamics in environments near fishing grounds.
officers and crew of the R.V. Garcia del Cid for their help and support during the surveys. We also thank the ‘‘Cofradia de Pescadors de Palamo´s’’ for their collaboration and especially Conrad Massague´r, skipper of the ‘‘Gacela’’ fishing boat. It also received additional support from an INTAS project (INTAS-460).
6. Conclusions
Auster, P.J., Malatesta, J., Langton, R.W., Watling, L., Valentine, P.C., Donaldson, L.S., Langton, E.W., Shepard, A.N., Babb, I.G., 1996. The impacts of mobile fishing gear on seafloor habitats in the Gulf of Maine (Northwest Atlantic): implications for conservation of fish populations. Review of Fisheries Science 4 (2), 185–202. BEON, 1990. Effects of beamtrawl fishery on the bottom fauna of the North Sea. Beleidsgericht Ecologisch Onderzoek Noordzee. BEON Report No. 8, 58pp. Castello´n, A., Font, J., Garcı´ a, E., 1990. The Liguro–Provencal–Catalan current (NW Mediterranean) observed by Doppler profiling in the Balearic Sea. Scientia Marina 54, 269–276. Churchil, J., 1989. The effect of commercial trawling on sediment resuspension and transport over the Middle Atlantic Bight continental shelf. Continental Shelf Research 9, 841–864. Crassous, P., Khripounoff, A., La Rosa, J., Miquel, J.C., 1991. Remises en suspension se´dimentaires observe´es en Me´diterrane´e par 2000 m de profondeur a` l’aide de pie`ges a` particules. Oceanologica Acta 14 (2), 115–121. de Groot, S.J., 1984. The impact of bottom trawling on benthic fauna of the North Sea. Ocean Management 9, 177–190. de Groot, S.J., Lindeboom, H.J., 1994. Environmental impact of bottom gears on benthic fauna in relation to natural resources management and protection of the North Sea. Netherland Institute for Sea Research (NIOZ). Report 199-11, Netherlands Institute for Fisheries Research (RIVO-DLO) Report CO26/94. Durrieu de Madron, X., Ferre´, B., Le Corre, G., Grenz, C., Conan, P., Pujo-Pay, M., Buscail, R., Bodiot, O., 2005. Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements. Continental Shelf Research 25 (19–20), 2387–2409. Font, J., Salat, J., Tintore´, J., 1988. Permanent features of the circulation in the Catalan Sea. Oceanologica Acta 9, 51–57. Garcı´ a, E., Tintore´, J., Pinot, J.M., Font, J., Manrı´ quez, M., 1994. Surface circulation and dynamics of the Balearic Sea. In: La Violette, P. (Ed.), Seasonal and Interannual Variability of the western Mediterranean Sea, vol. 46. American Geophysical Union, Coastal and Estuarine Studies, pp. 73–91. Gardner, W.D., 1989. Baltimore Canyon as a modern conduit of sediment to the deep sea. Deep-Sea Research 36, 323–358. Giro´, S., Maldonado, A., 1985. Ana´lisis granulome´trico por me´todos automa´ticos: tubo de sedimentacio´n y Sedigraph. Acta Geolo´gica Hispa´nica 20, 95–102. Guille´n, J., Palanques, A., Puig, P., Durrieu de Madron, X., Nyffeler, F., 2000. Field calibration of optical sensors for
Study of time series data collected in the Palamo´s submarine canyon and presented in this paper support the following conclusions: (1) During spring and summer 2001, sharp and relatively frequent near-bottom turbidity peaks associated with increases in near-bottom currents oriented downslope indicate the existence of present sediment gravity flow activity in the Palamo´s Canyon. (2) Most of the observed sediment gravity flows were triggered by the erosive action of trawling activities developed at the fishing ground of the northern canyon wall, where a deeply incised gully channeled them towards the canyon axis around 1200 m depth. These events affect mainly this canyon area without reaching much deeper downcanyon. (3) The sediment gravity flows recorded in the canyon axis at 1200 m depth increased one to three orders of magnitude the near-bottom horizontal sediment fluxes, and two to three times the TMF recorded with sediment traps, generating also a coarsening of the trapped sediment. (4) Trawling could have increased sediment fluxes in the mid-Palamo´s canyon During the last decades. Trawling in other canyons could induce similar effects. Acknowledgements This work was carried out in the framework of the CANYON project funded by the ‘‘Direccio´n General de Ensen˜anza Superior e Investigacio´n Cientı´ fica’’ Ref MAR99-1060-CO3-01, 02 and 03. The authors gratefully thank Agustı´ Julia`, Maribel Lloret and Benjamı´ n Casas for their help during the instrument deployments and retrievals, and the
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Deep-Sea Research I 53 (2006) 201–214 www.elsevier.com/locate/dsr
Evidence of sediment gravity flows induced by trawling in the Palamo´s (Fonera) submarine canyon (northwestern Mediterranean) A. Palanques, J. Martı´ n, P. Puig, J. Guille´n, J.B. Company, F. Sarda` Institut de Cie`ncies del Mar (CSIC), Passeig Maritim de la Barceloneta, 37-49, Barcelona 08003, Spain Received 18 October 2004; received in revised form 18 July 2005; accepted 20 October 2005 Available online 20 December 2005
Abstract Three mooring arrays were deployed in the Palamo´s Canyon axis with sediment traps, current meters and turbidimeters installed near the bottom and in intermediate waters. Frequent sharp and fast turbidity peaks along with current speed increases were recorded, particularly at 1200 m depth in spring and summer. During these events, near-bottom water turbidity increased by up to more than one order of magnitude, current velocity by two to four times and horizontal sediment fluxes by one to three orders of magnitude. When these events occurred, 9–11 days integrated downward particle fluxes collected by the near-bottom sediment trap increased by two to three times. These events were identified as sediment gravity flows triggered by trawling activities along the northern canyon wall. Sediment eroded by the trawling nets at 400–750 m depth on this wall seems to be channeled through a gully and transported downslope towards the canyon axis, where the 1200 m mooring was located. The sediment gravity flows recorded at the 1200 m site were not detected at deeper instrumented sites along the canyon axis, suggesting that they affect local areas of the canyon without traveling long distances downcanyon. These observations indicate that trawling can generate frequent sediment gravity flows and increase sediment fluxes locally in submarine canyons. Furthermore, in addition to the various natural processes currently causing sediment gravity flows and other sediment transport events, human activities such as trawling must be taken into account in modern submarine canyon sediment dynamics studies. r 2005 Elsevier Ltd. All rights reserved. Keywords: Submarine canyon; Sediment gravity flows; Trawling; Turbidity; Northwestern Mediterranean; Catalonia
1. Introduction Submarine canyons are preferential conduits for shelf–slope sediment transport and, although they were more active during low sea-level stands, they can still maintain a significant activity during high Corresponding author. Tel.: +34 93 2309517, +34 93 2309500; fax: +34 93 2309555. E-mail address: [email protected] (A. Palanques).
0967-0637/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2005.10.003
sea-level stands like the present one at least for fine sediment transport. For this reason, sedimentary activity in modern submarine canyons has often been studied in research projects devoted to analyzing shelf–slope transfer by measurement of downward particle fluxes or water turbidity with moored instruments (Gardner, 1989; Monaco et al., 1990; Heussner et al., 1999; Puig et al., 2000). In these studies, increases in near-bottom downward total mass fluxes and water turbidity were frequently
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associated with lateral inputs of suspended particulate matter coming from the shelf as a consequence of high energy events such as river floods and storms, and in some cases increases in turbidity were also associated with internal wave/tide activity. However, other unexpected increases in total mass flux without any clear relationships with storms or river flood events, and without any defined periodicity, could not be properly interpreted because of the lack of complementary data to explain their origin (Puig and Palanques, 1998; Heussner et al., 1999). In some cases, the influence of trawling has been hypothesized to interpret unexpected high particle fluxes occurring in summer with calm sea and low river discharge (Puig and Palanques, 1998; Palanques et al., 2005). In other cases it has been suggested just as a factor responsible for particle transfer (Stavrakakis et al., 2000). One reason to propose trawling as a source of resuspended sediment was that the moorings recording unexpected sediment flux increases tended to be those that had accidents with trawling boats. However, a direct record or a set of data that shows an evident relationship between trawling and sediment flux increases has never been obtained. Direct effects of bottom trawling are scraping and ploughing of the seabed and sediment resuspension. There are several types of trawl gears and one of the most common is the otter trawl. An otter trawl gear is a towed net with sweeplines (bridles), iron otterboards and warps (Fig. 1A). The mouth of the net is kept open by means of floats and weights, and is spread horizontally by forces exerted on the doors which are oriented obliquely to the trawl’s forward motion. The two otterboards of the benthic otter trawls are heavy (from hundreds of kilograms to tons) and imprint furrows in the sediment that are 0.2–2 m wide and up to 0.3 m deep, with the resulting sediment resuspension (Jones, 1992). The sweeplines and the footrope also cause sediment resuspension. The effect of trawling activities has been a topic of interest in ecology and fishery resource studies because of the impact of trawling gears on benthic fauna (de Groot, 1984; Hutchings, 1990; McAllister, 1991; de Groot and Lindeboom, 1994; Kaiser and Spencer, 1995, 1996; Auster et al., 1996), but recently it has also become a topic of interest because of its physical impact (Beon, 1990). Krost et al. (1990) observed that intensely fished areas had a high density of trawl tracks on the bottom, depending on the type of sediment. Churchill
(1989) suggested that large and highly turbid clouds of suspended sediment detected in the shelfbreak area of the mid-Atlantic Bight could be created in the wake of trawl doors. Schoellhamer (1996) observed highly turbid plumes near the seabed produced by resuspension by otter trawl gears on muddy sediment. Palanques et al. (2002) observed sediment erosion and an increase in water turbidity after inducing a trawling disturbance in an unfished shelf muddy area. Durrieu de Madron et al. (2005) monitored and modeled the dispersal of the plumes caused by trawling in shelf sediments. However, the physical effects of trawling in submarine canyons are still poorly known. In this paper, we show a direct relationship between trawling activities, turbidity peaks and sediment flux increases during the calm and dry season and discuss the mechanisms that cause these ‘‘unexpected’’ increases in sediment transport in submarine canyons, and the relevance that they have for present-day sediment dynamics in these environments. 2. Study area The study area is the Palamo´s Canyon, also known as the Fonera Canyon (Fig. 1B). This canyon is incised in the Catalan continental margin (northwestern Mediterranean Sea). The Palamo´s Canyon was selected for a multidisciplinary study (Palanques et al., 2005), because it is incised in the continental shelf and receives direct sediment inputs from the coastal zone and nearby rivers. The main river near the canyon head is the Ter River (Fig. 1B). The head of the Palamo´s Canyon is only about 4 km from the coastline, and the canyon is deeply incised in a continental shelf that is about 30 km wide. It is important to point out that at the shelfbreak of the study area this shelf-incised submarine canyon is deeper than 1200 m and that the canyons walls are traversed by gullies. The main characteristic of the regional circulation pattern in this margin is a slope current referred to as the Northern current (Millot, 1999), which is associated with a shelf–slope density front that in this area flows mainly towards the southwest (Font et al., 1988). This baroclinic current separates low-salinity shelf waters from denser open-sea saline waters (Castello´n et al., 1990; Garcı´ a et al., 1994). Regarding trawling activities, during the last decades deep-water fishery in the northwestern Mediterranean has targeted the red shrimp Aristeus
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Fig. 1. Schematic diagram of an otter trawl (A). Map of the Palamo´s Canyon showing the location of the mooring arrays and the fishing grounds (discontinuous black thick lines) in the study area (B). Note that the ‘‘Mongrı´ gully’’ (dark grey arrow) is just above the 1200 m depth canyon axis site.
antennatus. The populations of this species show seasonal migrations, and its life cycle seems to be highly related to the geomorphology of the continental slope, which in this region is crossed by many submarine canyons (Sarda` et al., 1997). Specialized fishing boats follow these seasonal movements around the submarine canyons. In the
study area, the fishing gear used by these boats is a benthic otter trawl (Fig. 1A), which is large and is operated by fishing vessels with 700–1500 HP engines. These bottom trawls have two heavy otterboards (600–1200 kg), and the distance between them during trawling can be more than 100 m. The mouth of the net is 40–50 m wide, and the total
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length of the net is about 80–150 m. The sweeplines that connect the otterboards to the net have a total length of 60–200 m. It is important to note that these fishing gears are much larger than those used near the coast. In the area of the Palamo´s Canyon there are two fishing grounds, one on the northern canyon wall, which is called Sant Sebastia`, and one on the southern canyon wall, which is called Rostoll (Fig. 1B). 3. Observational and analytical work Moored instrument deployments and hydrographic surveys were carried out from March to November 2001 within and around the Palamo´s submarine canyon. In this study, we focus on the three moorings deployed in the canyon axis at 470 m depth (M2), 1200 m depth (M3) and 1700 m depth (M5) (Fig. 1B). On each of these moorings, a set of instruments consisting of an Aanderaa RCM 9 current meter equipped with temperature, pressure, conductivity and turbidity sensors and a Technicap PPS3/3 sediment trap was deployed near the bottom. Current meters were placed 12 m above bottom (mab) and sediment traps 22 mab. One set of these instruments was also deployed in intermediate waters of the canyon axis at the 1200 m depth, site (M3), at 400 m depth (800 mab). The study period was divided into two consecutive deployments. The time sampling of the current meters was set at 10 min. The sampling intervals of sediment traps were 9 and 11 days for the first and second deployment, respectively. Sediment traps worked during the entire deployment period. However, the current meters with turbidimeters installed near the bottom at 1200 m depth (M3) and 1700 m depth (M5) recorded only up to early August and early September, respectively. The temperature sensor worked well until mid-July. Samples of particulate matter collected by sediment traps were processed in the laboratory, and total mass fluxes (TMF) were estimated according to the method described by Heussner et al. (1990). The grain-size distribution of the sediment-trap samples was determined by a settling tube for the 450 mm fraction and by a Sedigraph for the o50 mm fraction, following the method described by Giro´ and Maldonado (1985). Current meter data were calibrated and processed following standard procedures for their interpretation. Turbidity data collected in NTU were converted into suspended
sediment concentration (SSC) following the methods described by Guille´n et al. (2000). Sediment fluxes were calculated at the mooring sites in the canyon. Instantaneous sediment fluxes were obtained by multiplying the current speed by the SSC at each sampling level and location. The Ter River discharge during the deployment period was supplied by the ‘‘Agencia Catalana de l’Aigua’’ (Catalan Autonomous Government) and the wave climate by ‘‘Puertos del Estado’’. Field data on fishing activity of the fleet operating around this canyon were obtained from the logbook given to the skipper of the boat that usually trawls in deeper waters (‘‘Gacela’’). Daily records of fishing depth and the precise time at which the otter benthic trawl crossed over the study area along the fishing ground were taken. As the fishing boats trawling around the Palamo´s Canyon all work together (i.e. they work in parallel covering different depths), these data are representative of the fleet fishing activity. The skipper was also asked to record other information in the logbook during the sampling period, such as the fishing ground that they visited each day, the weather and the periods of non-activity due to boat maintenance. 4. Results 4.1. TMF and SSC along the canyon axis Time series of near-bottom downward TMF and SSC recorded in the Palamo´s Canyon axis during the deployment period are shown in Fig. 2. At 470 m depth (M2), TMF ranged from 16.0 to 494.0 g m 2 d 1 and the mean TMF was 28.8 g m 2 d 1. During most of the deployment period TMF variability was not very high, but at the end of this period a severe storm took place and the last cup of the sediment trap was overfilled. Near-bottom SSC at 470 m depth ranged between 0.4 and 1.4 mg l 1 during most of the time, with some isolated spikes showing SSC increases of 3–24 mg l 1. During the storm, SSC increased to 10 mg l 1. At 1200 m depth (M3), near-bottom TMF ranged from 8.9 to 494 g m 2 d 1 and had a mean TMF of 44.3 g m 2 d 1. TMF at this site showed much higher values and much higher variability than at 470 m depth. This trend was also observed in the SSC temporal series, which showed background values of 0.4 to 1.4 mg l 1, but with frequent SSC spikes ranging from a few to more than 30 mg l 1. During the periods in which TMF increased, there
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30 20 10 0
Fig. 2. Time series of near-bottom downward particle fluxes (TMF) and water turbidity (SSC—suspended sediment concentration) at the sites of the moorings deployed along the canyon axis at 470, 1200 and 1700 m water depth (see locations in Fig. 1B). The November flux peak corresponds to a strong storm. Question mark indicates that the sediment trap was overfilled.
were always strong SSC spikes. At this site, the last near-bottom cup was also overfilled during the storm event and the last cup of the intermediate water trap increased drastically, indicating particle transfer in the entire water column. At 1700 m depth (M5), TMF were from three times to one order of magnitude lower than at 1200 m depth, and the mean TMF was 8.5 g m 2 d 1. However, at this site there were sporadic periods in August when TMF increased to 38.9 g m 2 d 1. The highest TMF, of 57.7 g m 2 d 1, corresponded to the November storm event. Near-bottom SSC at this site ranged from 0.1 to 0.8 mg l 1 during most of the recording period, but a few SSC spikes reaching up to 40 mg l 1 were also recorded. These spikes also occurred during the sampling periods in which TMF increased. 4.2. Increasing SSC, current speed, grain size and particle flux events A relevant feature recorded in the Palamo´s Canyon is the occurrence of high turbidity peaks, recorded mainly near the bottom at 1200 m depth in spring and summer, and associated with periods of increasing TMF (Fig. 2). These spring and summer turbidity peaks and increasing TMF periods were always detected near the bottom and never in
intermediate waters (Fig. 3), indicating that they were not related to vertical fluxes in the whole water column. Sporadically, some peaks in SSC and TMF were also recorded in the canyon axis near the bottom at 470 and at 1700 m depth, but they were much more isolated and scarce than at 1200 m depth, and none of the spring–summer events recorded at 1200 m depth was recorded at 470 or at 1700 m depth. During these events, SSC at 12 mab increased by more than one order of magnitude, reaching values higher than 30 mg l 1 and, simultaneously, nearbottom current speed increased by two to four times, reaching up to 24 cm s 1. Additionally, very small temperature increases (from 0.02 to 0.09 1C) were detected during these events. There were three welldefined current directions during increasing SSC events—two towards the S–SSW (1801 and 2251) down the northern canyon wall and one towards the East (1001) down the canyon axis (Fig. 4). Most of the events were down the canyon wall. Near-bottom (12 mab) instantaneous horizontal sediment fluxes at the 1200 m site increased by one to three orders of magnitude during these sediment transport events, reaching values of up to 7560 mg m 2 s 1 (Fig. 5A). The duration of these events ranged from 1 to 6 h, and it is also important to point out that at the 1200 m site most of them occurred on working days,
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M3. Intermediate water (1200 m, 822 mab)
100 80
30 SSC (mg l-1)
TMF (g m-2d-1)
M3. Intermediate water (1200 m, 812mab)
40
60 40
20
10
20
0
0 M3. Near-bottom (1200 m, 22 mab)
M3. Near-bottom (1200 m,12 mab)
100
40
30 SSC (mg l-1)
TMF (g m-2d-1)
80 60 40 20 0
20
10
0 Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Fig. 3. Time series of downward particle fluxes (TMF) and water turbidity (SSC—suspended sediment concentration) in intermediate and near-bottom waters of the canyon axis at 1200 m depth mooring site. The November flux peak corresponds to a strong storm. Question mark indicates that the sediment trap was overfilled.
during working hours and never under bad sea conditions (Fig. 6). The downward TMF recorded by the sediment traps increased by two to three times during the sampling periods in which these events occurred. The samples collected by the sediment traps during these sampling periods were coarser— the silt content increased from 42% to up to 55% (Fig. 5B), and the sand content increased from about 0.5% to up to 2.5%. Taking into account that these events lasted only a few hours and that the sediment trap periods were 9 and 11 days, the induced TMF and the grain size increases were significant. 4.3. Trawling During the period when complete data of TMF, turbidity and current were recorded, trawling activities in the Rostoll fishing ground took place mainly in mid- and late-March, in late-May and early-June and in late-July and early-August. Haul depths along the Rostoll fishing ground start close to 400 m and reach 450 m (Fig. 7). During the same period, trawling activities in the Sant Sebastia`
fishing ground were carried out for more days than in the Rostoll fishing ground. They took place in late-March, in early- and mid-April, in May, in midand late-June, in July and in early-August. In the Sant Sebastia` fishing ground, hauls were usually made deeper than in the Rostoll area. During most of the period (March–June), the hauls in this fishing ground were made along the northern canyon axis between 650 and 750 m depth, whereas in July they were made between 400 and 650 m depth (Fig. 8). Some of the days during which trawling activities did not take place in the Sant Sebastia` and Rostoll fishing grounds correspond to periods of maintenance of the ship that reported data and/or to days of bad weather. 5. Discussion The cause and the process that generate frequent events of increasing sediment fluxes oriented downward of the northern canyon wall and axis at 1200 m depth are discussed in this section. The recorded data showing peaks of SSC and current speed along
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40
207
M3. Canyon Axis 1200 m
30 20 10 0
Current Veloc. (cms-1)
25 20 15 10 5 0 March
April
May
June
July
0 45
315 Current direction º for SSC peaks > 5 mg l-1
0
5
C.V. cm s-1 10 15 20
25 90
270
225
135
180
Fig. 4. Time series of near-bottom water turbidity (SSC—suspended sediment concentration) and current velocity in the canyon axis at 1200 m depth. Scatter plot of current direction during periods of increased turbidity (45 mg l 1) (below).
with slight temperature increases suggest the occurrence of events during which water and sediment moved downslope, presumably as a sediment gravity flow. Conceptually, the term ‘‘sediment gravity flow’’ includes any flow by which sediment moves because of its contribution to the density of the fluid, creating negative buoyancy (Middleton and Hampton, 1973, 1976). It can be used to describe a whole suite of processes from landslides (almost entirely regulated by particle–particle collisions) to dilute turbidity currents (which are
regulated almost entirely by turbulence of the interstitial fluid). The recorded data do not allow the specific type of gravity flow to be identified as it does not indicate whether or not the flows are turbulent, the vertical distribution of sediment concentration in the bottom boundary layer or the potential concentration at the starting point. However, the flow directions (always downslope) and the magnitude of the SSC and current velocity peaks in comparison with the regular values (i.e. more than one order of magnitude and two to four times
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Inst. horizontal Sed. Flux (mg m-2s-1)
8000
(A)
6000
4000
2000
0 100
?
56
80
60 48 40
Silt content (%)
TMF (g m-2d-1)
52
44 20
40
0
(B)
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Fig. 5. Near-bottom instantaneous horizontal sediment fluxes at the 1200 m site (A). Temporal series of downward particle fluxes (TMF) and silt content in near-bottom waters of the canyon axis at 1200 m depth (B). The November flux peak corresponds to a strong storm. Question mark indicates that the sediment trap was overfilled.
higher, respectively), as well as the increase in TMF and grain size of the samples collected in the sediment trap at 22 mab, are characteristics that can be clearly attributed to sediment gravity flows. Presumably, particle concentrations and current velocities closer to the bottom were much higher than at 12 mab, and suspended sediment was coarser than at 22 mab, as the sediment gravity flows at the instrumented levels could be diluted. Up to now, direct evidence of sediment gravity flow activity in submarine canyons have been very scarce because of the extreme difficulty of monitor-
ing them in the sea. Some evidence of modern sediment gravity flow activity was recorded in the Var Canyon (Crassous et al., 1991), in the Monterrey Canyon (Paull et al., 2003), in the Zayre submarine valley (Khripounoff et al., 2003) and in the Eel Canyon (Puig et al., 2003, 2004). Very recent turbidite deposits were also sampled in the Capbreton Canyon, indicating modern gravity flows activity (Mulder et al., 2001). Little is known about the mechanisms that trigger sediment gravity flow events, their frequency or intensity. Several studies have shown that large
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40
209
Ter River discharge
30 20 10 0 4
Wave height
Hs (m)
3 2 1 0 M3. Near-bottom (1200 m, 12 mab)
SSC (mg l -1)
40 30 20 10
29-Jul
22-Jul
15-Jul
8-Jul
1-Jul
24-Jun
17-Jun
10-Jun
3-Jun
27-May
20-May
13-May
6-May
29-Apr
22-Apr
15-Apr
8-Apr
1-Apr
25-Mar
18-Mar
0
Time (days)
Fig. 6. Ter River water discharge (Ds), significant wave height (Hs) and near-bottom turbidity (SSC—suspended sediment concentration) in spring and summer 2001. Dashed lines and dates correspond to Sundays in order to distinguish the working days.
volumes of sediment can be transported down the axis of some submarine canyons by the action of different mechanisms such as internal waves, tidal motions, high-energy storms affecting the upper portion of submarine canyons, hyperpycnal river plumes formed during river floods, earthquakes or failure on canyon walls (Shepard et al., 1979; Shepard and Dill, 1966; Okey, 1997; Mulder and Syvitski, 1995; Johnson et al., 2001). However, in the Palamo´s Canyon, sediment gravity flows took place during the calm and dry season without relevant storm and river flood events (Fig. 6), and there were no seismic movements during this period of time. In addition, there is a lack of periodicity in the SSC peaks, which means that they were not generated by resuspension due to oscillatory mechanisms such as internal waves. The fact that sediment gravity flows in the Palamo´s Canyon always occurred on working days, in working hours, and never under rough sea conditions (Fig. 6) suggests that they are associated with the trawling activities in the study area where big otter trawls are used and could be the triggering mechanism. To demonstrate this, we analyzed the
relationship between the turbidity peaks and the number of hauls per day and ship and the depths of hauls over the 1200 m canyon mooring site during the deployment period. Fig. 7 shows that most of the hauls on the Rostoll fishing ground were made when no major sediment gravity-flow events occurred, indicating that these hauls were not related to these events. However, in the Sant Sebastia` fishing ground (Fig. 8), almost all the sediment gravity flow events occurred when fishing boats were trawling there, indicating a relationship between these events and the hauls made in this fishing ground. This fits with the observation that the near-bottom currents during the sediment gravity flow events flowed from the northern canyon wall and not from the southern wall (Fig. 4). It is important to stress that the Sant Sebastia` fishing ground traverses a gully incised in the northern canyon wall—the Mongrı´ gully—that ends at the canyon axis just where the 1200 m depth mooring was located (Fig. 1B). Checking in detail the sediment gravity flow events at 1200 m depth, we observed that sudden turbidity and current velocity increases occurred about 2 h after the fishing fleet
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5 4 3 2 1 0
SSC (mg l-1)
50
800
M3. Near-bottom (1200 m, 12 mab)
40
700
30
600
20
500
10
400
0
300 1
/0
/3
18
01
4/
1/
1
/0
/4
15
1
/0
/4
29
1
1 /0 /5 27 Time (days)
/0
/5
13
1
/0
/6
10
1
/0
/6
24
01
7/
8/
1
/0
/7
22
Haul depth (m)
Hauls day-1 trawler-1
Rostoll fishing ground
Fig. 7. Number of hauls per day and trawler recorded by the ‘‘Gacela’’ skipper in the Rostoll fishing ground in spring and summer 2001 (above). Water turbidity (SSC—suspended sediment concentration) and haul depth in the Rostoll fishing ground in spring and summer 2001 (below).
traversed the Mongrı´ gully at about 500–600 m above the 1200 m mooring site (Fig. 9). Considering that the distance between the mooring site and the San Sebastia` fishing ground (i.e. the axis of the Mongrı´ gully) is about 2800 m, the sediment gravity flows had to move downslope at about 40 cm s 1 to travel this distance. This value is within the same order of magnitude as the velocities measured at the 1200 m site during these events, although it is slightly higher because it represents an integrated current velocity (i.e. from the source to the mooring) and the current meter records were obtained at 12 mab, where the sediment gravity flow was presumably diluted. This indicates that trawling-induced sediment gravity flows were probably channeled downslope through the gully and towards the canyon axis, and constitutes direct evidence of the relationship between these events and trawling activities in the northern canyon wall fishing ground. Not all the hauls produced sediment gravity flows, but practically whenever an event took place at the 1200 m site, the trawling ships were working in the Mongrı´ gully area, and these events were practically never recorded at this site when the
ships remained in the harbor because of bad weather or for maintenance. We did not record any sediment gravity flow from the Rostoll fishing ground because the hauls there are shallower and do not traverse any major gully ending around the mooring site. As a consequence, trawling activity in the San Sebastia` fishing ground significantly increased the downward particle fluxes at the 1200 m site. It is difficult to estimate the natural particle fluxes at this site, but using as a reference the downward particle fluxes at the 470 m depth site, the mean near-bottom TMF at the 1200 m depth site (43.3 g m 2 d 1) increased by 53% in comparison with that at the 470 m depth site (28.8 g m 2 d 1). Considering that natural TMF at 1200 m should be lower than at 470 m, as is indicated by the TMF trend in most of the submarine canyons studied, the contribution of trawling to the TMF at the 1200 m site must be higher than 53%. The fact that the current direction induced during some sediment gravity flows was downcanyon (towards 1001) (Fig. 4) suggests that some of these events could have been generated outside the
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4 3 2 1 0
SSC (mg l -1)
50
M3. Near-bottom (1200 m, 12 mab)
800
40
700
30
600
20
500
10
400
0
300 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /0 /3 1/4 8/4 5/4 2/4 9/4 6/5 3/5 0/5 7/5 3/6 0/6 7/6 4/6 1/7 8/7 5/7 2/7 9/7 2 2 1 2 1 1 2 1 2 1 2 2
1
/0
/3
18
Haul depth (m)
Hauls day -1 trawler -1
Sant Sebastià fishing ground 5
25
Time (days)
Fig. 8. Number of hauls per day and trawler recorded by the ‘‘Gacela’’ skipper in the Sant Sebastia` fishing ground in spring and summer 2001 (above). Water turbidity (SSC—suspended sediment concentration) and haul depth in the Sant Sebastia` fishing ground in spring and summer 2001 (below).
Mongrı´ gully and closer to the canyon head, and once they reached the canyon axis they were reoriented, flowing for some distance along the canyon axis. However, the sediment gravity flows recorded at the 1200 m canyon axis site were not recorded at the 1700 m canyon axis site, indicating that they did not travel too far along the canyon axis, where the slope is less steep than on the canyon wall, and that they affected only a limited part of the canyon. The location of the Sant Sebastia` fishing ground (Fig. 1B) also explains why the events recorded at 1200 m depth site were not recorded at the 470 and 1700 m depth sites. Sporadically, some isolated gravity flow events were also recorded near the bottom at the canyon head at 400 m depth, and in the canyon axis at 1700 m depth. These isolated gravity flow events occurred on days without recorded trawling activities in the area. This suggests that other causes could also have triggered sediment gravity flows in the Palamo´s Canyon, although more sporadically than those generated by trawling activities at 1200 m depth. Based on the observations collected in this study, we can infer that the sediment remobilization
generated by fishing trawlers in sloping areas, such as the Palamo´s northern canyon wall, can trigger sediment gravity flows. Thus, besides the action of several natural mechanisms, the impact of trawling must also be considered in studies of modern sediment dynamics in areas close to fishing grounds. In the case of submarine canyons, the effect of the erosion caused by trawlers working on the canyon walls can reach depths well beyond the ones at which they work. In this study, sediment eroded at 600–700 m reached the canyon axis at 1200 m depth in about 2 h. This process delivers resuspended sediment to the canyon floor and alters near-bottom hydrodynamics in pulses with an artificial frequency, which may induce some indirect impact on deep ecosystems that are distant from where the trawling activity occurs. One indirect impact of trawling on deep ecosystems could be sediment suffocation of some unique ecosystems, such as deep-sea corals, which have been recently discovered in the Mediterranean (Tursi et al., 2004). In the specific case of the Palamo´s Canyon, it is worth mentioning that this human activity has been conducted on a daily basis for at least the last 50 yr, and that it could have increased canyon
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March 29 peak
May 11 peak
June 21peak
SSC (mg l-1)
30 24 18 12 6 0
C. Speed (cm s-1)
25 20 15 10 5 0 8:00
9:00
10:00 11:00 Time (hours)
12:00
8:00
9:00
10:00 11:00 12:00 13:00
0
Time (hours)
0
315
90
135
225
315
45 C. vel. (cm/s)
0 5 10 15 20 25
270
0
315
45 C. vel. (cm/s)
36
13:00 14:00 15:00 16:00 17:00 18:00 19:00
Time (hours)
0 5 10 15 20 25
270
0 5 10 15 20 25
90
270
90
225
135
225
45 C. vel. (cm/s)
135
180
180
180
Current direction º
Current direction º
Current direction º
July 9 peak
July 23 peak
August 1 peak
SSC (mg l-1)
30 24 18 12 6 0
C. Speed (cm s-1)
25 20 15 10 5 0 7:00
8:00
9:00
10:00 11:00 12:00
8:00 9:00 10:00 11:00 12:00 13:00 14:00 14:00 15:00 16:00 17:00 18:00 19:00
Time (hours)
Time (hours)
0 315
0 45
315
C. vel. (cm/s) 90
135
0 45
315
C. vel. (cm/s)
0 5 10 15 20 25
270
225
Time (hours)
0 5 10 15 20 25
270
90
225
135
45 C. vel. (cm/s) 0 5 10 15 20 25
270
90
225
135
180
180
180
Current direction º
Current direction º
Current direction º
Fig. 9. Detail of the SSC peaks, current speed and of current direction (in scatter plot) during the main gravity flow events. Turbidity and downslope current speed increases occurred about 2 h after the fishing fleet crossed the Mongrı´ gully along the Sant Sebastia` fishing ground.
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sediment fluxes during that period of time. As trawling on canyon walls is relatively common, other submarine canyons could be affected in a similar way. Nowadays, besides the several natural mechanisms that can cause sediment gravity flows and other sediment transport events, anthropogenic activities such as trawling must also be taken into account in studies of modern sediment dynamics in environments near fishing grounds.
officers and crew of the R.V. Garcia del Cid for their help and support during the surveys. We also thank the ‘‘Cofradia de Pescadors de Palamo´s’’ for their collaboration and especially Conrad Massague´r, skipper of the ‘‘Gacela’’ fishing boat. It also received additional support from an INTAS project (INTAS-460).
6. Conclusions
Auster, P.J., Malatesta, J., Langton, R.W., Watling, L., Valentine, P.C., Donaldson, L.S., Langton, E.W., Shepard, A.N., Babb, I.G., 1996. The impacts of mobile fishing gear on seafloor habitats in the Gulf of Maine (Northwest Atlantic): implications for conservation of fish populations. Review of Fisheries Science 4 (2), 185–202. BEON, 1990. Effects of beamtrawl fishery on the bottom fauna of the North Sea. Beleidsgericht Ecologisch Onderzoek Noordzee. BEON Report No. 8, 58pp. Castello´n, A., Font, J., Garcı´ a, E., 1990. The Liguro–Provencal–Catalan current (NW Mediterranean) observed by Doppler profiling in the Balearic Sea. Scientia Marina 54, 269–276. Churchil, J., 1989. The effect of commercial trawling on sediment resuspension and transport over the Middle Atlantic Bight continental shelf. Continental Shelf Research 9, 841–864. Crassous, P., Khripounoff, A., La Rosa, J., Miquel, J.C., 1991. Remises en suspension se´dimentaires observe´es en Me´diterrane´e par 2000 m de profondeur a` l’aide de pie`ges a` particules. Oceanologica Acta 14 (2), 115–121. de Groot, S.J., 1984. The impact of bottom trawling on benthic fauna of the North Sea. Ocean Management 9, 177–190. de Groot, S.J., Lindeboom, H.J., 1994. Environmental impact of bottom gears on benthic fauna in relation to natural resources management and protection of the North Sea. Netherland Institute for Sea Research (NIOZ). Report 199-11, Netherlands Institute for Fisheries Research (RIVO-DLO) Report CO26/94. Durrieu de Madron, X., Ferre´, B., Le Corre, G., Grenz, C., Conan, P., Pujo-Pay, M., Buscail, R., Bodiot, O., 2005. Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements. Continental Shelf Research 25 (19–20), 2387–2409. Font, J., Salat, J., Tintore´, J., 1988. Permanent features of the circulation in the Catalan Sea. Oceanologica Acta 9, 51–57. Garcı´ a, E., Tintore´, J., Pinot, J.M., Font, J., Manrı´ quez, M., 1994. Surface circulation and dynamics of the Balearic Sea. In: La Violette, P. (Ed.), Seasonal and Interannual Variability of the western Mediterranean Sea, vol. 46. American Geophysical Union, Coastal and Estuarine Studies, pp. 73–91. Gardner, W.D., 1989. Baltimore Canyon as a modern conduit of sediment to the deep sea. Deep-Sea Research 36, 323–358. Giro´, S., Maldonado, A., 1985. Ana´lisis granulome´trico por me´todos automa´ticos: tubo de sedimentacio´n y Sedigraph. Acta Geolo´gica Hispa´nica 20, 95–102. Guille´n, J., Palanques, A., Puig, P., Durrieu de Madron, X., Nyffeler, F., 2000. Field calibration of optical sensors for
Study of time series data collected in the Palamo´s submarine canyon and presented in this paper support the following conclusions: (1) During spring and summer 2001, sharp and relatively frequent near-bottom turbidity peaks associated with increases in near-bottom currents oriented downslope indicate the existence of present sediment gravity flow activity in the Palamo´s Canyon. (2) Most of the observed sediment gravity flows were triggered by the erosive action of trawling activities developed at the fishing ground of the northern canyon wall, where a deeply incised gully channeled them towards the canyon axis around 1200 m depth. These events affect mainly this canyon area without reaching much deeper downcanyon. (3) The sediment gravity flows recorded in the canyon axis at 1200 m depth increased one to three orders of magnitude the near-bottom horizontal sediment fluxes, and two to three times the TMF recorded with sediment traps, generating also a coarsening of the trapped sediment. (4) Trawling could have increased sediment fluxes in the mid-Palamo´s canyon During the last decades. Trawling in other canyons could induce similar effects. Acknowledgements This work was carried out in the framework of the CANYON project funded by the ‘‘Direccio´n General de Ensen˜anza Superior e Investigacio´n Cientı´ fica’’ Ref MAR99-1060-CO3-01, 02 and 03. The authors gratefully thank Agustı´ Julia`, Maribel Lloret and Benjamı´ n Casas for their help during the instrument deployments and retrievals, and the
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