First record of the invasive alga Caulerpa racemosa ( Caulerpales , Chlorophyta ) in the Gulf of Arzew ( western Algeria )

The first invasive record of the green alga Caulerpa racemosa (Forsskål) J. Agardh in the Mediterranean Sea was in 1990 in Tripoli (Libya) (J. Agardh) and since that period, this Chlorophyta has been rapidly spreading to all Mediterranean countries. However, very few assessment studies have been carried out along the Algerian coast. In this paper, we present, for the first time new observation data concerning an indicated proliferation of this invasive species in Salamandre and Stidia (Gulf of Arzew, western Algeria); both these sites are very important for sea fishing activities and traffic.


Introduction
Biological invasions are a widespread and significant component of human-caused global environmental change (Rejmánek 2008).In recent decades, marine and coastal bioinvasions represent a serious threat to ecosystems and native biodiversity and cause profound economic impacts (Ribera and Boudouresque 1995;Grosholz 2002;Ruesink et al. 2006;Occhipinti-Ambrogi 2007;Antolić et al. 2008).
The Mediterranean sea, particularly harbour areas, are considered to be the most severely affected regions for marine species invasions, where around 600 flora and fauna species are considered as introduced species (Boudouresque and Verlaque 2005;Boudouresque et al. 2005;Zenetos et al. 2005) and from which 100 macrophyte species (Ribera Siguan 2002) can be counted.
In the Algerian coast, the presence of C. racemosa was recently and for the first time reported in Algiers Bay (Ould-Ahmed and Meinesz 2007) (Figure 1; Table 1).The present paper reports the first occurrence of the C. racemosa "invasive variety" in the gulf of Arzew (western Algeria) in the framework of the macrophyte diversity assessment and marine pollution biomonitoring (Taleb et al. 2009) in   this coastal marine area, and will contribute to fill the data gaps of this algal proliferation in the south west of the Mediterranean basin.

Materials and methods
Sampling was focused on the Stidia and Salamandre sites located in the west of the industrial and fishery harbour of Mostaganem City (Figure 1; Table 1), where a large population of C. racemosa was observed during November 2009, the period corresponding to the annual maximum development of these species in the northwest of the Mediterranean sea (Capiomont et al. 2005;Ruitton et al. 2005b).
In order to record the scale of distribution and the density of C. racemosa as well as the associated native macroalgae and seagrass at each site, four 1 m 2 quadrats were laid at random on the bottom of four stations spread out along a 500 m transect.The abundance of the seaweed and seagrass species occurring within the quadrats was estimated in percentage cover.

Results and discussion
Caulerpa racemosa' (Figure 2) observed percentage cover in all stations varied between 92 to 100% and 72 to 100% respectively in Salamandre and Stidia monitored sites (Figure 3).
The absence of the phaeophyceae Cystoseira spp., indicators of high ecological status was also noted, while on the other hand the presence of both Corallina elongata J. Ellis and Solander and Ulva lactuca Linnaeus (Figure 3) were recorded, indicating poor to bad coastal water quality (Arévalo et al. 2007).
The recent appearance of the invasive alga C. racemosa in this study area can be attributed to the importance of sea traffic in this Mediterranean zone, because our observation stations are very close to the large petrochemical harbour of Arzew and the industrial and fishery harbour of Mostaganem.However, the increased occurrence of C. racemosa at the proximity of large cities, industrial, cargo, passenger, fishery and recreational boating harbours does not necessarily demonstrate an affinity for polluted areas but may be an artefact due to secondary dispersal mechanisms via ship traffic and fishing activities (Klein and Verlaque 2008).At least, it attests to the tolerance of C. racemosa to high levels of pollution.
Posidonia oceanica was recorded only at two stations at the Salamandre site (Figure 3).The percentage cover rate ranges from 26 to 62%.Several studies conducted on the impact of C. racemosa on macrophyte assemblages (rocky substrate, dead Posidonia oceanica mats, coralligenous and detritic assemblages) indicate a decrease in the total number of species and in the total macrophyte cover in the presence of C. racemosa (Ceccherelli et al. 2001;Piazzi et al. 2001a, b;2005;Piazzi and Cinelli 2003;Balata et al. 2004;Cinelli et al. 2007;Klein and Verlaque 2007;Antolić et al. 2008).The exposure of the Salamandre and Stidia colonised area to winds, strong waves and important hydrodynamic flow, resulting from prevailing West to South-West winds (43%) from October to February and East to the North-East winds (40%) from May to September (ONM 2009), in addition to the flow of the coastal cyclonic currents in the Gulf of Arzew (Mega 2002;Mega and Lensari 2006) could reinforce the hypothesis of the dispersion of C. racemosa to this coastal zone.
The Stidia site represents a small fishery harbour where fishermen clean their nets, sometimes charged with algae, in particular C. racemosa.On this subject, the economic impact of C. racemosa has never been quantified (Klein and Verlaque 2008).However, there has been some speculation on the basis of observations by fisherman in Italy who found their fishing nets clogged with C. racemosa (Magri et al. 2001).

Conclusion
The data of this preliminary survey reflect the possible consequences of the C. racemosa invasion event in this area including modifications of physical and chemical conditions (water movement, sediment deposition, substrate characteristics), as well as profound modifications of benthic assemblages.The study emphasises the need to develop more monitoring research program and policies aimed at prevention and mitigation of the impacts of aquatic invasive alien species on the biological diversity of the Algerian coast.

Figure 1 .
Figure 1.Geographical location of all records of Caulerpa racemosa in Algeria and our study area (yellow rectangle) (adapted from Google Earth, 2010).

Table 1 .
Characteristics of the sampling areas colonized by Caulerpa racemosa.
* Positions were recorded using differencial Garmin 12 GPS