Author: uset

THE UNDERESTIMATION OF SEAGRASS
BIOLOGICAL CYCLE IN THE BIOMONITORING
OF COASTAL TRACE ELEMENT POLLUTION
J. Richir1*, S. Sartoretto2, S. Gobert1 (*jonathan.richir@ulg.ac.be)
1Laboratory of Oceanology, University of Liège, Sart-Tilman, B6c, 4000 Liège, Belgium
2IFREMER, Zone Portuaire de Brégaillon, 83500 La Seyne-sur-mer, France

Introduction
The seagrass Posidonia oceanica (L.) Delile forms monospecific meadows along Mediterranean coasts from the surface to depths of about 40 m (Fig.1). It has been largely used for the biomonitoring of pollutions in Cr, Ni, Cu, Zn, Cd, Pb and/or Fe. In contrast, other trace elements (TEs) like As, V, Ag, Be, Al, Mn, Co, Se, Mo, Sn, Sb or Bi have been subject to nearly no ecotoxicological survey with that species. Moreover, the understanding of the seasonal variation of TE levels according to the seagrass biological cycle, a prerequisite to the intercomparison of sites sampled at different times of the year, has been little investigated.

Material and methods
● TE levels were biomonitored in April 2007 along the French Mediterranean littoral (Fig.2). ● TE bioaccumulation was seasonally studied (March, June and November) for 3 years (2008-2010) in P. oceanica sampled in the pristine Calvi Bay site and in the impacted Plateau des Chèvres site located near the Cortiou outlet discharging wastewaters from Marseille city (Fig.2). ● P. oceanica biometry was measured according to the method of Giraud. ● TEs were analysed by DRC-ICP-MS.

Fig.2. 18 sites along Provence-Alpes-Côte d’Azur (PACA; 1-11) and Corsican (I-VII) coasts (France, NW Mediterranean) sampled in April 2007 for P. oceanica. Colored Calvi and P. Chèvres sites were further monitored for 3 years (2008-2010).

Results and discussion
French Mediterranean coasts are submitted to local, diffuse and/or chronic contaminations both by TEs broadly or little biomonitored with P. oceanica (Fig.3, left). High TE levels could be linked to specific anthropic activities: agriculture (Mo), mining (Cr, Sb, Zn), industries (As), storage and refinement of oil products (V, Pb), presence of major ports and urban centres (Sn, Bi, Ag). In P. oceanica from the pristine Calvi Bay site, TE concentrations further evolved seasonally according to the ecophysiological cycle of that perennial, deciduous plant (Fig.3, middle). In the impacted Plateau des Chèvres site, this seasonal evolution could be disrupted (Fig.3, right).

Fig.1. Above - 3D schematic view of a portion of a P. oceanica meadow, drawing of (A) a shoot of leaves fixed on a plagiotropic rhizome, and detailed drawings of (B, C) adult, (D) intermediate and (E) juvenile leaves. Left - Pictures of a P. oceanica meadow (above), of the matte (middle) and of an accumulation of dead leaves hosting the litter monster (below).

Fig.3. Spatial variations of As, Ag and Bi concentrations (µg gDW-1) in P. oceanica sampled in April 2007 in 18 sites along Provence-Alpes-Côte d’Azur (PACA; 1-11) and Corsican (I-VII) coasts (France, NW Mediterranean; left graphs; Calvi site colored in blue, Plateau des Chèvres site colored in red) and temporal variations of As, Ag and Bi concentrations (µg gDW-1) in P. oceanica seasonally sampled for 3 years (2008-2010) in the pristine Calvi Bay site (middle graphs) and in the impacted Plateau des Chèvres site (right graphs). The foliar surface of shoots (FSShoot; mean ± SD, in mm2) is an indicator of the cyclic growth of leaves; the ratio between the foliar surface of adult leaves and the foliar surface of intermediate leaves (FSAL/FSIL; mean ± SD) is an indicator of the seasonal aging of shoots.

Conclusion
These complementary studies allowed highlighting that roles played by both the seagrass biological cycle and its environment are of the same order of magnitude in the bioaccumulation process of TEs in P. oceanica. The natural cyclic evolution of seagrass TE concentrations should therefore be systematically quantified in regional reference sites in order to properly intercompare results from biomonitoring surveys conducted at different times of the year.

References - Boudouresque, C.F., Meinesz, A., 1982. Cahiers du Parc national de Port-Cros 4, 79 pp. Libes, M., Boudouresque, C.F., 1987. Marine Ecology-Progress Series 38 (2), 177–186. Giraud, G., 1979. Bulletin du Muséum d’histoire naturelle de Marseille 39, 33–39. Luy, N. et al., 2012. Ecological Indicators 18 (1), 269–277. Richir, J. et al., 2013. Aquatic Toxicology 140–141, 157– 173.

● As concentrations increased from November to June, as did the shoot foliar surface (an indicator of the cyclic growth of leaves). The more leaves grew, the more their exposure duration to As increased, the more the latter was bioaccumulated in shoots, and this until the next major leaf fall period. ● The preferential accumulation of Ag in below ground organs, its efficient basipetal translocation from leaves towards rhizomes and the dilution of bioaccumulated Ag during the entire shoot growth could explain the decrease of Ag concentrations from November to June, whereas the shoot foliar surface increased. ● Bi concentrations increased from March to November, as did the ratio between adult and intermediate leaf foliar surfaces (an indicator of the cyclic aging of shoots). The preferential bioaccumulation of Bi in adult leaves could result in both a longer exposure duration of old adult leaves to Bi and a dilution effect in actively growing young intermediate leaves.

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Libes, M., Boudouresque, C.F., 1987. Uptake and long-distance transport of carbon in the marine phanerogam Posidonia oceanica. Marine Ecology-Progress Series 38 (2), 177–186.
Giraud, G., 1979. Sur une méthode de mesure et de comptage des structures foliaires de Posidonia oceanica (Linnaeus) Delile. Bulletin du Muséum d’histoire naturelle de Marseille 39, 33–39.
Luy, N., Gobert, S., Sartoretto, S., Biondo, R., Bouquegneau, J.M., Richir, J., 2012. Chemical contamination along the Mediterranean French coast using Posidonia oceanica (L.) Delile above-ground tissues: a multiple trace element study. Ecological Indicators 18 (1), 269–277.
Richir, J., Luy, N., Lepoint, G., Rozet, E., Alvera Azcarate, A., Gobert, S., 2013. Experimental in situ exposure of the seagrass Posidonia oceanica (L.) Delile to 15 trace elements. Aquatic Toxicology 140–141, 157– 173.