Hydrobiologia 503: 59–67, 2003. ´ M.B. Jones, A. Ing´olfsson, E. Olafsson, G.V. Helgason, K. Gunnarsson & J. Svavarsson (eds), Migrations and Dispersal of Marine Organisms. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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Hitch-hiking on floating marine debris: macrobenthic species in the Western Mediterranean Sea Stefano Aliani1 & Anne Molcard1,2 1 CNR,
Istituto Scienze Marine - Sezione La Spezia, Forte Santa Teresa, 19036 Pozzuolo di Lerici (SP), Italy RSMAS-MPO University of Miami, 4600 Rickenbacker Causeway Miami, FL, 33149-1098, U.S.A. E-mail:
[email protected] 2
Key words: dispersal, rafting, flotsam, benthos, Lagrangian model
Abstract Marine litter has been defined as solid materials of human origin discarded at sea, or reaching the sea through waterways. The effect of marine debris on wildlife, tourism and human health is well documented and there is considerable scientific literature about plastic litter in the sea and over the seabed, mostly highlighting the possible impact on marine mammals and tourism. Dispersal of marine and terrestrial organisms on floating objects has biogeographical and ecological interest. For some species, extension of their geographical range is more likely to be related to transport of mature individuals on floating rafts than to the active or passive dispersal of reproductive propagules. Variability and variety of rafting materials has increased dramatically in recent years and marine litter has been used widely as a raft by ‘hitch-hiking’ species. This paper reports on the benthic invertebrates living on marine debris transported by wind and surface currents over the western Mediterranean Sea. Plastics accounted for the major item of debris because of poor degradability, however glass, cans, fishing nets and polyurethane containers, were also found. Macro-benthos living on raft material comprised mainly molluscs, polychaetes and bryozoans. Large fish were found commonly below large plastic bags. Estimations of the distances that may be covered by hitch-hiking species and the contribution of rafting to the theoretical dispersal of species is provided.
Introduction Typical marine invertebrates have life histories that include at least one dispersal stage. Tropical and warm-temperate species (e.g. >150 species of prosobranch gastropods) have a very long planktonic development and their teleplanic larvae provide evidence of very long range dispersal potential (Scheltema, 1988). However, there are exceptions to the traditional hypothesis ‘longer larval period – wider distribution’ (Thorson, 1950; Mileikovski, 1971) and alternative modes of dispersal have been considered (Zibrowius, 1983; Scheltema, 1986). For example, for littoral gastropods along the east coast of North America, there is no clear relationship between the length of development reported from laboratory experiments and geographical range. Apparently, dispersal along continental coastlines may be accomplished by stepwise
progression as long as there are no ecological barriers such as ocean basins (Scheltema, 1989). Brooding species, or species with a short-lived larval phase, may also be widely distributed and, for some of them (mainly living on hard substrata), extension of geographical range may be achieved through transport of mature individuals by floating rafts, rather than through the active or passive dispersal of reproductive propagules (Highsmith, 1985). The so-called ‘Rockall paradox’ is one example (Johannesson, 1988). Evidence from hydroids showed that wide oceanic distances are covered more by rafting hydroids than by long-distance dispersal of planulae (Cornelius, 1992; Boero & Bouillon, 1993). Colonies of the ascidian Botrylloides sp., attached to Zostera leaves in California, were found 200 times further away, and had greater recruitment success, compared with swimming larvae (Worchester, 1994).
60 Theories about the role of rafting in species distribution have become refined for several taxa living on hard substrata. Following Guppy’s (1917) monograph on trans-Atlantic flotsam, rafting has been invoked as a main way of increasing the range of hydroids, (Ralph, 1961) and remote populations of cnidarians (Cornelius, 1992). Jackson (1986) proposed that rafting is the only reasonable explanation for the existence of the vast majority of (benthic) clonal species on IndoPacific oceanic islands and argued that, as medusal life was too short to disperse, species used flotsam (i.e. by rafting). Analysis of surface currents and reported drift patterns of pumice were used to estimate rafting of tropical corals (Jokiel, 1989); floating corals, due to air trapped, wander the sea for months (DeVantier, 1992). The variety and availability of rafting material has increased dramatically in recent years with the spread of human population (Barnes, 2002). Ship hulls, glass, plastic bottles and even items of footwear can be used as rafts by hitch-hiking species. Prior to the spread of humans, available rafts were volcanic pumices, macroalgae, seagrasses, trees and seeds. Today marine litter is also available, i.e. solid material of human origin that is discarded at sea, or reach the sea through waterways or domestic and industrial outfalls (National Academy of Science, 1975 cited in Rees & Pond, 1995). Monitoring marine debris has been undertaken for several years (Rees & Pond, 1995), including assessment of the distribution and abundance of floating plastic in the Gulf of Mexico by aerial surveys (LeckeMitchell & Mullin, 1992), and by ship observers in the Eastern Mediterranean Sea (McCoy, 1988). Most papers focused mainly on the impact of debris on the marine environment in terms of pollution, aesthetic damage to tourism, or risks for turtles, cetaceans and fish. In this paper, we identify the floating objects in the Ligurian Sea from Corsica to the Ligurian Coast (Western Mediterranean) and we asses their potential as ways of extending dispersal range of the benthic invertebrates living on them. Macrobenthic species were identified and the time needed for these benthic species to cover the distance from Corsica to Ligurian Coast by hitch hiking on flotsam was inferred from a numerical simulation of a transport model.
Materials and methods Study area The Ligurian Sea is situated at the north eastern border of the Western Mediterranean and is connected to the southern basin (Tyrrhenian Sea) across the Corsica Channel. The eastern and northern borders are the Tuscan and Ligurian coasts, and the western border is open toward the Provençal region and the Gulf of Lions. The major large-scale feature of the deep and surface layers of the Ligurian Sea is a cyclonic circulation active all year round, more intense in winter than in summer. Climatic forcing can greatly change the intensity of fluxes but the general pattern can be considered permanent (Astraldi & Gasparini, 1992). Southern waters, filling the Ligurian Sea, occur in two main currents running along each side of northern Corsica. The West Corsica Current runs along the western side of Corsica, and the warm and salty Tyrrhenian Current passes trough the Corsica Channel between Capraia and Corsica (Artale et al., 1994). The two waters merge to the north of Corsica and flow along the Ligurian coast toward the Gulf of Lions (Astraldi et al., 1995). Species living offshore in the Tyrrhenian basin can be carried northward by the Tyrrhenian current across the Corsica Channel, providing a forced passage for the fauna. Changes in the water fluxes due to climatic variability at this point can control faunal exchanges from the warm Tyrrhenian basin to the colder Ligurian Sea (Astraldi et al., 1995; Aliani & Meloni, 1999).
The oceanographic cruise Visual sightings of floating objects were made in the Ligurian Sea from 10 to 15 July 1997 from the top deck of the R/V Urania. The type and GPS position of all objects on the sea surface during favourable light and sea conditions while underway and at each station were recorded. Sightings were made close to the Ligurian Coast (areas A,B,C, in Fig. 1), and close to Corsica and the coast of Tuscany (areas D,E,F, in Fig. 1). Samples of floating objects were collected after 15 min of searching by the ship’s tender. Immediately after retrieval, samples were fixed in a buffered formaldehyde-seawater solution (final concentration: 5%) and the macrobenthic species identified in the laboratory. Cluster analysis was performed on the presence/absence species × samples matrix.
61
Figure 1. Map of the explored part of the Ligurian Sea. Small open circles indicate sightings and filled circles indicate sampling stations. Multiple sightings and replicates of samples are indicated with a single circles. Coordinates are expressed in degrees and decimals.
Transport models A Lagrangian model (Falco et al., 2000) simulating the motion of independent particles in a turbulent flow was implemented to study dispersion of passive tracers by simulating a high number of particles. The model assumed that the velocity field can be decomposed in two distinct components: a large-scale mean flow U and a turbulent mesoscale field u . The tracer particles are advected through two separate processes, the advection by U and the turbulent transport by u , which is characterised by some simple transport parameters such as the variance σ 2 and the turbulent decorrelation timescale TL . dx = (U + u )dt, du = −(1/TL )u dt + (σ 2 /TL )1/2 dw,
where dw is a random increment from normal distribution with zero mean and second order moment. This equation states that turbulent velocity following particles is a linear Gaussian-Markov process, characterised by an exponential autocovariance with e-folding timescale TL (Risken, 1989). For the mean flow, we used the output of a general circulation model applied to the Mediterranean Sea (Demirov & Pinardi, 2002) forced by a perpetual (monthly mean) surface forcing for a 7-year period, and averaged in a unique climatological surface mean current field (Fig. 2a). The spatial resolution of the model is 1/8 deg corresponding to 12 km grid resolution, that does not allow the resolution of eddy dynamics due to the high energetic mesoscale variability present in the Ligurian Sea. The mean flow can be underestimated and these small mesoscale effects
62 ent objects, mostly plastic bags (at different stages of degradation) and plastic debris (hard substrata) with some Styrofoam, bottles, wood and fishing gear. All were equally distributed both in the northern (Liguria, area A+B+C) and southern part of the basin (Corsica, area D+E+F). For stations close to the Ligurian coast, dolphins, sea turtles and large fish were sighted below the larger objects. Samples
Figure 2. (a, above) Surface mean velocity obtained from a OGCM at 1/8deg resolution, units cm/s; (b, below) Typical trajectories obtained from a Lagrangian model.
were taken into account in the turbulent flow by the variance. The computational domain, where we apply the Lagrangian model, shown in Figure 1b, extends between 8.5◦ E and 12.5◦ E in longitude and between 41.25◦ N and 44.5◦ N in latitude. Values for σ 2 and TL are taken from observations (Rupolo, 1993), and are set to 50 cm2 /s2 and 1.5 d, respectively. Reflection boundary conditions were used at the closed boundaries, which represent the simplest way to guarantee the well-mixed conditions (Thompson, 1987). At the southern and northern boundaries, particles were free to exit following the current.
Results Sightings We recorded 169 sightings of different objects from Corsica to the Ligurian coast (Fig. 3). The average distance between sightings (on the transect direction) was 0.7 km and debris density was estimated to be of the order of 14–25 items per km2 (Aliani et al., 2003). At these sites, we were able to identify 260 differ-
The type of objects found in the samples was similar amongst the sightings, and included mainly man-made objects such as plastic litter, Styrofoam and a small percentage of natural floats such as pieces of wood, leaves and algae. All samples were colonised and a total of 22 macrobenthic species was found in 14 samples (Table 1). Some species were common to most flotsam. The most frequent species (>5 findings) were the lepadomorph barnacle Lepas pectinata, found at 12 out of 14 stations and the isopod Idotea metallica (9 stations). In some cases, juvenile or early life stages were the only fauna on the flotsam. In other cases, a complex population with individuals of different size classes was found. Idotea metallica was found preferentially on larger samples. The polychaetes Spirobranchus polytrema and Nereis falsa (7 findings each) occurred as adults and juveniles. Three records of Phtisica marina were found in stations with natural debris (e.g. seagrass leaves, algae) in Corsica and Liguria. Small specimens of the nudibranch mollusc Doto sp. were found at three stations and their distribution was limited to the southern stations close to Corsica (west and north), none was found in the Ligurian Sea. The hydroid Obelia dichotoma had a similar pattern. Hydroids (and bryozoans) accounted for a high number of species (5 and 4 species, respectively). Different unidentified egg masses were also found in some samples and some species had reproductive structures. The plot of the cumulative number of species in all samples showed that the species-samples curve reached an asymptote after 10 samples, indicating exhaustive sampling and adequate evaluation of total species richness (Fig. 4). Curve for the Ligurian Sea, reached an asymptote after 4 samples and the curve for Corsica reached the same after 6 samples, suggesting difference in species richness between the two sites. These curves also suggested that little contribution to total richness from new samples from either Liguria or Corsica can be expected.
63
Figure 3. Percentage of major types of floating debris sighted in Liguria and Corsica. Table 1. List of sessile macrofauna found on rafting objects. Stations are labelled with a letter and number code (e.g. L1, F3. . .); multiple samples from the same station with additional letter or numbers in brackets Area
Algae ind. Arbacia lixula (L.) Bowerbankia gracilis Leidy Callopora lineata (L.) Clytia hemisphaerica (L.) Cymodocea nodosa (Ucria) Asch. Cystoseira sp. Doto sp. Electra posidoniae Gautier Eudendrium sp. Fiona pinnata (Eschscholtz) Fosliella farinosa (Lam.) Gonothyrea loveni Allm. Idotea metallica Bosc Laomedea angulata Hincks Lepas pectinata (Spengler) Membranipora membranacea (L.) Nereis falsa Qautrefages Obelia dichotoma (L.,) Phtisica marina Slabber Posidonia oceanica Delile Spirobranchus polytrema (Phil.)
A L1(A) L1(C) + + + + -
+ + + +
B L2(C1) L2(C2) + + + + + + + +
Cluster Analysis of the species × station matrix separated 4 groups (similarity 57%) (Fig. 5). They were accounted by two single samples (F4 and
+ + + +
C L3(B) L3(C) + -
+ + + -
F E D F1 F2 F3(A) F3(C) F3(B) F4 F5 A7 - - - + - - - - - - - - - - - - + + - - - - - -
+ + + + + + + + +
+ + + + +
+ + + + + + + + +
+ + + + -
+ + -
+ + + + +
L1a) with species living on rafting plants (Cymodocea and Cystoseira) and two larger groups. One group (F1,F2,F5,L3b), included samples with few species
64
Figure 4. Cumulative number of species plotted against the number of samples in Liguria (square), Corsica (triangle) and against total number of samples (diamond). Table 2. Travel days and percentage of particles reaching region F3, and reaching the Ligurian coast when launched from F3
Figure 5. Cluster analysis on the species x stations matrix. The dotted line indicates the 57% similarity.
and the other (F3,A7,L1,L2,L3) had higher richness. Samples of these groups were spread over the basin without any apparent gradient and geographical pattern. Numerical simulation In the first experiment, 1000 particles were released in the western part of the Corsica channel (42.5◦ N 9.625◦ E), corresponding to the starting point of our observation of floating objects in the 1997 cruise. Figure 2b shows some typical trajectories obtained from the transport model and the pattern compare favourably with real drifter data (see web site
Travel days
Reaching F3
Reaching coast
10 20 30 40 50
1% 5% 11%
5% 19% 33% 40% 45%
doga.ogs.trieste.it and follow the drifter data links, Poulain). Figure 6 shows the diffusion clouds at 3, 30 and 60 d. The higher concentration can be found in the channel even though the clear northern advection mainly due to Tyrrhenian Current along the eastern side of Corsica allows some particles to reach the northern part of the Ligurian Sea. Figure 6 also depicts the time evolution of the percentage of particles able to reach the Ligurian Coast (all the particles that are crossing the 44◦ N parallel are considered landed in the Ligurian Coast). We first assumed that particles in the first experiment were hypothetical dispersal larvae and we set their life time to 30 d. Our model shows that only 3% of the total number of particles released were able to reach the Ligurian coast within this time. Later on, we propose that some individuals use marine debris for transportation, and live and reproduce on these rafts. Length of larval life is not a limiting factor for adults settled on rafts, so we con-
65
Figure 6. Dispersion of simulated particles and time evolution (lower right panel) of percentage that reach the Ligurian coast.
sidered their life time as infinite. In this case, we used the curve of Figure 6 for longer periods (>30 d), taking into account the probability for the benthic species passing through the Corsica Channel of finding a suitable object to use as a raft. In the second experiment, we focused on this probability. Area F is the closer region to the launching point where some floating objects were observed. Table 2 assumed the percentage of marine debris that had reached that region starting from the first point of release. After 30 d, 11% reached this region. We simulated 1000 new particles (rafting objects) starting at position F3 (43.33◦ N 9.64◦ E), and determined the percentage arriving in the Ligurian coast. The probability of reaching the coast increased, because the starting point was closer and the life time of the animal was almost infinite. After 30 d (Table 2), 33% of the animals that had taken the raft were found near the coast; after 50 d 45%.
Discussion Our results indicated that the same type of floating object is spread over the Ligurian sea from Corsica to the eastern Ligurian coast. The amount of debris
is probably higher as large-sized objects are more likely to be observed (McCoy, 1988; Dufault & Whitehead, 1994). Mainly man-made (plastic, fishing tools, Styrofoam) objects were found. Prior to the spread of humans, volcanic pumices, trees and seeds were the most common rafts (Guppy, 1917; DeVantier, 1992); however new types of rafts are now available, and these are very abundant, largely indestructible and almost continuously distributed (Barnes, 2002). Some of this flotsam is completely submerged and remains just below the surface where it is transported by currents. Others (Styrofoam, bottles) have a ‘free board’ and are transported more by the wind than by currents. This has important implications for separating two different types of rafts whose directions and velocities of dispersal are not necessarily the same but share the same sea surface. Most of our flotsam was plastic bags or plastic debris, and was completely submerged. Samples included all types of objects and the benthic species on them were distributed without any marked difference in species composition between sites. Despite active discussion about the introduction of southern species in the Ligurian sea (Morri & Bianchi, 2001), no alien species was found on our rafts. The list included species reported previously in the
66 Mediterranean Sea as offshore fouling species (Bellan Santini,1970; Relini 1976; Relini et al., 1977; Aliani & Meloni, 1999; Relini et al., 2000). Most rafted species may be included in the ‘list of superwanderers’ (Cornelius, 1992; Aliani et al., 1998) that use rafts as their major means of dispersion (Cornelius, 1992). As Barnes (2002) pointed out, the distance from the mainland does not seem to influence the proportion of debris colonised and these species are widely distributed offshore in the Mediterranean Sea. In contrast to these common widespread species, we encountered also single individuals. For these species, hitch hiking on rafts may offer a final opportunity to survive for a larvae close to the end of its planktonic period (Toonen & Pawlik, 1994) or just an alternative artificial substratum (Tursi et al., 1985). Their contribution to global biodiversity is not well understood. Another group of species, separated by Cluster Analysis, included those found rarely on plastic objects but commonly on seagrass leaves and on wood. They are typically reported as epibionts of seagrass and we found them as reproductive adults much further than the distance covered by short-living larvae. Rafting on leaves is a very effective means of dispersal for these species. Species can jump to new rafts in the open sea as some unidentified eggs found on the rafts were too young to suggest a long dispersal history. Accumulation of floating objects in strips by Langmuir circulation (Nimmo Smith & Thorpe, 1999) may help in transferring to a new raft or exchanges of gametes between close rafts. A more complex community will probably evolve from the unidentified eggs and seasonal fluctuations in community composition may also occur. Our numerical experiments showed that there is a limited chance for benthic invertebrates to travel long distances alone without mortality due to ecological constraints (Cowen et al., 2000). Using floating objects as a means of transportation (where they can also reproduce), increases their probability of survival. As plastic litter is everywhere, the probability of finding a raft is higher than hypothezised. In conclusion, a continuum of species and rafts exist in the Ligurian sea. Rafts have different ages according to the dispersal processes that brought them offshore or according to recruitment processes occurring in far or nearby rafts. For these individuals, rafting objects extend their potential dispersal range beyond their theoretical extension.
Acknowledgements We thank Annalisa Griffa for useful discussion and suggestions, and Encho Demirov for supplying the OGCM output. We are also grateful to S. Geraci, coordinator of the cruise, and to M. Faimali, F. Garaventa, M. Andrenacci, E. Canepa, E. Schiano for valuable help in the field work. Special thanks to the Captain and the crew of the R/V Urania. This paper forms part of the framework of the Italian Research Project SINAPSI. We thank two anonymous referees for greatly improving the manuscript with their suggestions.
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