Dead Matte Research Articles

Invasion of alien macroalgae in different Mediterranean habitats

The study evaluated different macroalgal invasions in the main Mediterranean coastal habitats on hard bottom. Biodiversity, species composition and structure of macroalgal assemblages were compared among non-invaded areas and areas invaded by the Chlorophyta Caulerpa racemosa var. cylindracea and by the turf-forming Rhodophyta Womersleyella setacea in three different habitats: shallow rocky bottom, deep rocky bottom and dead matte of the seagrass Posidonia oceanica. Results showed that alien macroalgae constituted a relevant component of benthic assemblages in invaded areas of the Mediterranean Sea. Assemblages invaded by Womersleyella setacea and Caulerpa racemosa showed lower values of diversity and large differences in the structure and species composition related to non-nvaded assemblages. The species that mostly suffered from invasion were erect species reproducing sexually; moreover, the dominance of W. setacea led to low abundance of native filamentous algae, while C. racemosa colonization seemed particularly threatening for encrusting algae. All the studied habitats appeared highly invasible by alien macroalgae, even if W. setacea appeared more invasive in deeper habitats, while colonization of C. racemosa seemed more serious in shallower habitats; the dead matte of P. oceanica represented a suitable substrate for the spread of both species. Differences among assemblages in different habitats were reduced in invaded areas.

Substitution and phase shift within the Posidonia oceanica seagrass meadows of NW Mediterranean Sea

We investigated the conservation status and ecosystem substitution in six Posidonia oceanica (L.) Delile meadows in 2002 along 300 km of the Ligurian coast (NW Mediterranean Sea). We studied the meadows by scuba transects and we compared the meadows state of health through in situ shoot density counts and computing the Conservation Index (CI), the latter measuring the proportional amount of dead matte relative to live P. oceanica. Both measures evidenced a generalised state of regression for most of the six meadows. We also reported the occurrence, within the meadows, of the other common Mediterranean seagrass Cymodocea nodosa, the Mediterranean green alga Caulerpa prolifera, and the invasive green algae Caulerpa taxifolia and Caulerpa racemosa. We measured the replacement of P. oceanica by these species using the Substitution Index (SI), related to the proportional cover of each substitutes. These were ranked in order of their colonisation potential with respect to P. oceanica. We identified and measured a community “phase shift” occurring within the Mediterranean seagrass meadows using a combined index, the Phase-Shift Index (PSI). The analysis of the three environmental indices (CI, SI and PSI) at regional scale allowed to introduce three “regional” ordinal scales to classify the conservation status, the degree of substitution and the level of phase shift in P. oceanica meadows.

Anchoring damage on Posidonia oceanica meadow cover: A case study in Prelo cove (Ligurian Sea, NW Mediterranean)

Among the various types of human activities, the mechanical damages resulting from uncontrolled pleasure boats anchoring in shallow coastal waters would appear to be responsible for localized regressions of Posidonia oceanica meadows. This paper aims to describe and quantify the impacts of a large anchoring chains system on the structure of the P. oceanica meadow of Prelo cove (Ligurian Sea, NW Mediterranean). In this study, we provide evidence that this chains system had a negative effect on the meadow cover, generating dead ‘matte’ areas within the meadow. Meadow structure mapping, combined with the use of an environmental index (Conservation Index), which is linked to the proportional abundance of dead matte relative to living P. oceanica, underpins significant differences in the cover and in the conservation status of the meadow between areas characterized by the presence of the chains and areas without the chains. We also show that the chains affected the meadow in different ways according to P. oceanica cover. The approach proposed here, based on thematic mapping and a simple environmental index, provides relevant information for management actions on the conservation of Posidonia oceanica meadows.

Wanted dead or alive: high diversity of macroinvertebrates associated with living and ‘dead’ Posidonia oceanica matte

The Mediterranean endemic seagrass Posidonia oceanica forms beds characterised by a dense leaf canopy and a thick root-rhizome ‘matte’. Death of P. oceanica shoots leads to exposure of the underlying matte, which can persist for many years, and is termed ‘dead’ matte. Traditionally, dead matte has been regarded as a degraded habitat. To test whether this assumption was true, the motile macroinvertebrates of adjacent living (with shoots) and dead (without shoots) matte of P. oceanica were sampled in four different plots located at the same depth (5–6 m) in Mellieha Bay, Malta (central Mediterranean). The total number of species and abundance were significantly higher (ANOVA; P<0.05 and P<0.01, respectively) in the dead matte than in living P. oceanica matte, despite the presence of the foliar canopy in the latter. Multivariate analysis (MDS) clearly showed two main groups of assemblages, corresponding to the two matte types. The amphipods Leptocheirus guttatus and Maera grossimana, and the polychaete Nereis rava contributed most to the dissimilarity between the two different matte types. Several unique properties of the dead matte contributing to the unexpected higher number of species and abundance of motile macroinvertebrates associated with this habitat are discussed. The findings have important implications for the conservation of bare P. oceanica matte, which has been generally viewed as a habitat of low ecological value.

First assessment of the Caulerpa racemosa (Caulerpales, Chlorophyta) invasion along the French Mediterranean coast

The introduced green alga Caulerpa racemosa var. cylindracea has been rapidly spreading in the Mediterranean Sea since 1990. It was first observed in France in 1997 (Marseilles). In early 2004, the stretch of the French Mediterranean coastline and the surface area affected by the invasion were estimated at about 83 km and 4014 ha, respectively. The depth range of colonized areas was usually 10–35 m depth. Shallow (0–10 m) and deep (down to 40 m) dense meadows were rarely observed. In contrast to the dead matte of Posidonia oceanica, which constituted the most widely colonized substratum, dense P. oceanica meadows and fine sand with large ripple-marks were not invaded. Few rocky areas were colonized and coarse sand bottoms were usually colonized below 20 m depth. All the colonized areas were exposed to human activities and more than 40% were fishing areas. Mild climate, suitable substrata, presence of vectors of dispersal and absence of efficient biological control make the French Mediterranean coast particularly vulnerable to the further spread of the alga.

A new synthetic index and a protocol for monitoring the status ofPosidonia oceanica meadows: a case study at Sanremo (Ligurian Sea, NW Mediterranean)

1. The status of a Posidonia oceanica meadow in front of the town of Sanremo, Italy, was studied through a combined use of benthic mapping and synthetic indices. 2. Mapping was accomplished by integrating side scan sonar imagery and data collected by scuba diving along transects placed perpendicularly to the coastline. A thematic map (scale 1:5000) was produced. Extent of the meadow, occurrence of dead matte areas, and morphology of the lower limit (with new definition) are all described. 3. Two synthetic environmental indices were applied to transect data in order to quantify the status of the meadow: the Conservation Index and the Substitution Index. The former is related to the proportion of dead matte; the latter is a novel index measuring the amount of replacement of the ‘constructional’ seagrass P. oceanica by the ‘non-constructional’ seagrass Cymodocea nodosa. The potential of a ‘phase shift’ in Ligurian Sea seagrass meadows is discussed. 4. The approach here proposed, based on detailed mapping plus synthetic indices, may provide immediate information to evaluate the state of Mediterranean Posidonia oceanica for monitoring and management. Copyright © 2006 John Wiley & Sons, Ltd.

Distribution and Dominance of Two Introduced Turf-forming Macroalgae on the Coast of Tuscany, Italy, Northwestern Mediterranean Sea in Relation to Different Habitats and Sedimentation

The occurrence of filamentous turf-forming macroalgae assemblages has been widely reported in tropical and temperate oceans. In the Mediterranean Sea the development of dense and persistent turfs has recently started and has been linked to the spread of the two introduced filamentous Rhodophyta Acrothamnion preissii and Womersleyella setacea. In this work algal assemblages on rocky bottoms and rhizomes of Posidonia oceanica were studied during one year in three localities of the northwestern Mediterranean Sea (Leghorn, Vada Shoals and Gorgona Island) characterised by different sedimentation rate and composition of particulate matter. Dead matte of P. oceanica was also studied on Vada Shoals. Results showed that turfs are able to develop on all habitats investigated, but different patterns are found. Turfs were abundant on rhizomes of P. oceanica in all localities studied while on rocks they reached high cover at Leghorn and Vada but not on Gorgona Island. The dead matte of P. oceanica showed patterns similar to those found on rocky bottoms. Turfs seem positively influenced by sedimentation while neither the nature of particulate matter nor other polluting substances had any important result on their spread. The development of turfs negatively affects species number and diversity of invaded communities, while cover did not show significant variations. Womersleyella setacea replaced Acrothamnion preissii on rocky bottoms and on dead matte, but not on Posidonia oceanica rhizomes where Acrothamnion preissii was dominant everywhere. In spite of the occurrence of competition between the two algae, it did not seem to limit the development of turfs as they reached similar cover values independently of the dominant species.

In situ survival and development of Posidonia oceanica (L.) Delile seedlings

The sexual reproductive success of the marine phanerogam Posidonia oceanica (L.) Delile was evaluated in two different beds along the Tuscan coast, Italy. Seed germination and the development of seedlings was followed in situ over a period of 2 years. In addition, a pilot study was set up on the effect of depth and substratum on seedlings survival and development. During the study period the density of seedlings decreased from initial values of about three seedlings m −2 to 1.9 in the two beds. The rhizome length increased from about 2 cm at the beginning to about 5 cm at the end of the observation period. The average length of the longest leaves reached maximum values during summer and a minimum in autumn, while the number of leaves did not show a clear seasonal trend. The analysis of variance performed showed significant differences in survival, linked both to substratum (dead matte > rock > gravel) and to depth (10 m > 2 m). The analysis also showed significant differences in rhizome growth linked to depth (10 m > 2 m). The results showed the importance of the post-germination phases in determining the reproductive success of P. oceanica particularly during the first year of life. These phases seem to be closely linked to the environmental characteristic of the settlement area; in particular we observed that seedling establishment, survival and growth strongly depended on depth and on substratum characteristics.