Dinamica dei sistemi a Cymodocea nodosa,Zostera marina e Zostera noltii nel Mediterraneo

Abstract

Data on geographical and depth distribution, sediment granulometry, salinity, biomass variability, below/aboveground biomass ratio, and reproductive strategies of seagrass communities in the Mediterranean Sea were analysed to describe their dynamics patterns.

Notwithstanding their different latitudinal distribution, they have a similar seasonal biomass variability, deriving both from extrinsic forcing (e.g. light and temperature) and intrinsic species-specific components (e.g. reproduction pattern), these latter being responsible for their different structure and seasonal dynamics.     

Seagrass feeding choices and digestive strategies of the herbivorous fish Sarpa salpa

This is the first study investigating the plant–herbivore interaction between Sarpa salpa, which has overgrazed seagrass transplants in Portugal, and the seagrasses Cymodocea nodosa, Zostera marina and Zostera noltii, which have been considered for restoration. When offered the choice between the three seagrasses in outdoor tanks, adult S. salpa clearly preferred Z. noltii. Testing the seagrasses separately, mean ± s.d. feeding rates ranged from 21 ± 11 g seagrass fresh mass kg^?1 fish mass day^?1 for Z. marina to 32 ± 9 g seagrass fresh mass kg^?1 fish mass day^?1 for C. nodosa and 40 ± 11 g seagrass fresh mass kg^?1 fish mass day^?1 for Z. noltii (temperature = 16° C). Food‐processing rate in S. salpa did not differ between seagrasses, and there was no evidence of a regulation of processing rate according to food intake. Seagrasses differed substantially in nitrogen content and C:N, with C. nodosa containing the highest nitrogen content and lowest C:N (2·5 ± 0·1% and 14·0 ± 1·0), followed by Z. noltii (2·1 ± 0·1% and 17·0 ± 1·0) and Z. marina (1·4 ± 0·1% and 26·0 ± 2·0). Food‐processing rate in S. salpa and the nutritional value of the seagrasses were not correlated with the observed feeding preference and rate. The study suggests that C. nodosa and Z. marina are less at risk of overgrazing by S. salpa and might thus be preferable to Z. noltii for seagrass restoration in areas with noticeable abundances of this fish.     

Isolation and characterization of microsatellite markers for the seagrass Cymodocea nodosa

In order to study the spatial patterns of genetic diversity of a clonal marine angiosperm, the seagrass Cymodocea nodosa, microsatellite markers were obtained by screening a genomic library enriched for the (CT) dinucleotide motif. Of 38 primer pairs defined, 15 amplified polymorphic microsatellites and are described. These loci identified a number of alleles ranging from two to seven, and showed expected heterozygosity ranging from 0.35 to 0.76, when a group of 40 individuals from Cadiz Bay in Spain was analysed. Additionally, we describe here the multiplexing conditions for 12 of these loci.     

Effects of pesticides on seagrass beds

In the German Wadden Sea there has been a remarkable decline in seagrass beds. It was the aim of this study to test whether herbicide contamination could be a reason for this. Concentrations of triazine herbicides such as atrazine, simazine and terbutylazine as well as phenylurea herbicides were measured in Wadden Sea sediments within or in the neighbourhood of seagrass meadows. Sediments were thus used as a marker for medium-term contamination of the Wadden Sea. The respective concentrations were examined in relation to the density and status of the seagrass meadows. Preliminary results show that there may be a connection between seagrass decline and herbicide contamination in the parts of the Wadden Sea sampled. A comparison with other contamination is also given. Received: 11 January 1999 / Received in revised form: 1 December 1999 / Accepted: 8 December 1999     

Effects of herbivorous birds on intertidal seagrass beds in the northern Wadden Sea

During autumn migration (September to December), brent geese (Branta b. bernicla) and wigeon (Anas penelope) feed on the seagrass Zostera noltii in the nearshore, upper tidal zone leeward of the island of Sylt (eastern North Sea). To graze on leaves and shoots above the sediment and on rhizomes and roots below, these birds reworked the entire upper 1 cm layer of sediment eight times within this 3-month period. In addition, brent geese excavated pits 3–10 cm deep by trampling in order to feed on below-ground phytomass. About 12% of the seagrass beds became pitted to an average depth of 4.5 cm. Using net exclosures, it was estimated that birds removed 34 g dry weight m^–2 of above-ground and 28 g of below-ground phytomass. This corresponds to 45% of the phytomass in September. Of the overall loss of phytomass from September to December, 63% was caused by birds. Roughly half of the leaves fell off anyway until December and the other half were taken by the birds. Below the ground, phytomass remained almost constant where birds were excluded, while with birds phytomass of rhizomes and roots was halved. In spite of this strong effect, in the next vegetation period the blade density was lower at former exclosure sites compared to the ambient seagrass bed. The underlying process seems to be a self-inhibition of dense overwintering seagrass by mud accretion. Assuming our experimental results can be scaled up to the entire seagrass bed, we hypothesize that in the sheltered upper intertidal zone, seasonal erosion caused by herbivorous geese and ducks is necessary for the persistence of Z. noltii. Received: 7 January 1999 / Received in revised form: 23 August 1999 / Accepted: 25 August 1999     

Within-population spatial genetic structure, neighbourhood size and clonal subrange in the seagrass Cymodocea nodosa

Abstract The extent of clonality within populations strongly influences their spatial genetic structure (SGS), yet this is hardly ever thoroughly analysed. We employed spatial autocorrelation analysis to study effects of sexual and clonal reproduction on dispersal of the dioecious seagrass Cymodocea nodosa. Analyses were performed both at genet level (i.e. excluding clonal repeats) and at ramet level. Clonal structure was characterized by the clonal subrange, a spatial measure of the linear limits where clonality still affects SGS. We show that the clonal subrange is equivalent to the distance where the probability of clonal identity approaches zero. This combined approach was applied to two meadows with different levels of disturbance, Cadiz (stable) and Alfacs (disturbed). Genotypic richness, the proportion of the sample representing distinct genotypes, was moderate (0.38 Cadiz, 0.46 Alfacs) mostly due to dominance of a few clones. Expected heterozygosities were comparable to those found in other clonal plants. SGS analyses at the genet level revealed extremely restricted gene dispersal in Cadiz (Sp = 0.052, a statistic reflecting the decrease of pairwise kinship with distance), the strongest SGS found for seagrass species, comparable only to values for selfing herbaceous land plants. At Cadiz the clonal subrange extended across shorter distances (20-25 m) than in Alfacs (30-35 m). Comparisons of sexual and vegetative components of gene dispersal suggest that, as a dispersal vector within meadows, clonal spread is at least as important as sexual reproduction. The restricted dispersal and SGS pattern in both meadows indicates that the species follows a repeated seedling recruitment strategy.     

Is there a future for elephants in West Africa?

The elephants of West Africa have experienced a long history of human disturbance. Before 1800 they were much affected by the precolonial empires of the savanna and Sahelian zones, the trans-Saharan trade routes, and the coastal trade established by the Europeans. During the 19th century, the increasing demand for ivory from Europe and North America, the European penetration of the region, and the evolution of breech-loading rifles devastated the remaining elephants. The elephant population of West Africa collapsed before the outbreak of World War I because of intense hunting for ivory. This collapse pre-empted the decline that would have occurred anyway due to the rapid growth of the human population and consequent loss of habitat. Elephants now find themselves in about 70 small isolated populations that cover only 5% of the region. These fragments are very vulnerable – whether in the arid lands or the humid forests – to poaching and general human disturbance. There are few data on numbers; most of the population estimates are guesses. Two-thirds of the populations are thought to consist of fewer than 200 animals and therefore have a low probability of surviving the next century. As more habitat is lost to human activities, West African elephants will soon remain only in protected areas. But many parks and reserves are managed poorly and cannot offer effective protection; they do not guarantee a future for elephants. In addition, their crop-raiding habit makes elephants unpopular in rural communities surrounding protected areas. Human populations are expected to continue growing and resources for conservation are scarce. The future of West African elephants lies in a small network of well-protected areas.     

Plant biodiversity patterns along a climatic gradient and across protected areas in West Africa

Knowledge of spatial patterns of biological diversity is fundamental for ecological and biogeographical analyses and for priority setting in nature conservation, particularly in West Africa where the existing high biodiversity is increasingly threatened by human activities. The maximum entropy approach was used to model the geographic distribution of 3,393 vascular plant species at a spatial resolution of 0.0833°. Species richness decreases along temperature and precipitation gradients with high species numbers in the south and lower numbers towards the north of the transect. All centres of plant species diversity are confined to humid areas in concordance with the high positive correlation between species richness and rainfall which appears to be the most important delimiter for the distribution ranges of many species in the area. The effectiveness of the existing protected areas at regional and national levels is investigated based on the proportion of species covered. Considering the whole study area, 95% of all species are covered by protected areas according to their distribution ranges. However, the proportion of species covered is considerably lower for some countries such as Benin and Togo. Our results could provide guidance for essential land use management interventions to decision‐makers and conservationists in the region.     

Genetic effects of eelgrass restoration efforts by fishers' seeding to recover seagrass beds as an important natural capital for coastal ecosystem services

The reestablishment of seagrass vegetation is a vital part of recovering coastal marine ecosystem services. Historically the Hinase area was a famous for the fishing by coastal pound netting in eelgrass beds, but this practice was progressively displaced with oyster farming due to an enormous decline in seagrass vegetation. For several decades, the local fishers' cooperative has worked to restore eelgrass beds by a seeding method. Through these efforts, seagrass vegetation in their fishing area has increased to about half of their previous area. This study examined the effect of long-term seeding by fishers on the recovery of eelgrass beds in the Hinase area, based on analysis of eelgrass genetic structure using microsatellite markers. Specimens for the DNA analysis were collected from each of all eelgrass meadows that the fishers conducted sowing eelgrass seeds as well as from the source sites where they collected the seeds. The results found that restored beds in the study area have high genetic diversity comparable to natural ones. The multiple regression analysis revealed that a combined model of seedling intensity and geographic distance (R² = .457) better explained genetic structure across our sampling sites than models of seedling intensity (R² = .092) or geographic distance only (R² = .344). This supports that the eelgrass seeds they sowed did not disturb the genetic structure but rather supplemented natural dispersal, suggesting that the fishers' seeding did not develop nonnatural seagrass meadows but certainly contributed to the recovery of natural seagrass meadows.     

Vertical distribution and seasonal variation of eelgrass beds in Iwachi Bay,Izu Peninsula,Japan

The vertical distribution and population structure of eelgrass beds were surveyed in Iwachi Bay, along the Pacific coast of central Japan. Samplings were conducted from May through November 1977 by SCUBA. Eelgrass was distributed between 3 and 11 m in depth. The relative light intensity at 12 m depth was 11% at the lower range. The highest population density was 290 shoots/m² in September and the fresh weight of biomass was 888 g/m² in July at 7 m depth. The maximum mean leaf area index was about 3 at 10 m depth in July. The ratio of reproductive shoots to the total shoots was about 36% at 7 m depth in June. Eelgrass showed good growth at 7–10 m depth, which is comparatively deeper than other eelgrass habitations. The high values of water transparency and sunshine duration, as well as solar radiation compared with other localities was believed to contribute to the growth of eelgrass in deeper waters in Iwachi Bay.     

Genetic differentiation and secondary contact zone in the seagrass Cymodocea nodosa across the Mediterranean–Atlantic transition region

Aim A central question in evolutionary ecology is the nature of environmental barriers that can limit gene flow and induce population genetic divergence, a first step towards speciation. Here we study the geographical barrier constituted by the transition zone between the Atlantic Ocean and the Mediterranean Sea, using as our model Cymodocea nodosa, a seagrass distributed throughout the Mediterranean and in the Atlantic, from central Portugal to Mauritania. We also test predictions about the genetic footprints of Pleistocene glaciations. Location The Atlantic–Mediterranean transition region and adjacent areas in the Atlantic (Mauritania to south‐west Portugal) and the Mediterranean. Methods We used eight microsatellite markers to compare 20 seagrass meadows in the Atlantic and 27 meadows in the Mediterranean, focusing on the transition between these basins. Results Populations from these two regions form coherent groups containing several unique, high‐frequency alleles for the Atlantic and for the Mediterranean, with some admixture west of the Almeria–Oran Front (Portugal, south‐west Spain and Morocco). These are populations where only one or a few genotypes were found, for all but Cadiz, but remarkably still show the footprint of a contact zone. This extremely low genotypic richness at the Atlantic northern edge contrasts with the high values (low clonality) at the Atlantic southern edge and in most of the Mediterranean. The most divergent populations are those at the higher temperature range limits: the southernmost Atlantic populations and the easternmost Mediterranean, both potential footprints of vicariance. Main conclusions A biogeographical transition region occurs close to the Almeria–Oran front. A secondary contact zone in Atlantic Iberia and Morocco results from two distinct dispersal sources: the Mediterranean and southernmost Atlantic populations, possibly during warmer interglacial or post‐glacial periods. The presence of high‐frequency diagnostic alleles in present‐day disjunct populations from the southernmost Atlantic region indicates that their separation from all remaining populations is ancient, and suggests an old, stable rear edge.     

Woody plant phenology in the West Africa savanna

In the savanna of West Africa the seasonality of rainfall, with a drought period of at least four months, strongly influences the vegetation. Rainfall is a very critical abiotic variable and therefore plant species must be well adapted to survive in this habitat.
In our research, phenological patterns of 120 woody plant species have been examined based on the presence of green leaves. According to the patterns found, these species can be classified in phenological groups, which represent different strategies for survival. Two extreme strategies are found to resist drought: (1) by using the waterstorage in the deeper soil layers and river beds and by restricting drought-damage through scleromorphic features, and (2) by avoiding the drought through foliage shedding in the dry period.
The first strategy is represented by the riparian and upland evergreens , and the semi-evergreens . The evergreens bear leaves the whole year, gradually replacing old leaves by new ones. The riparian evergreens are strictly bound to riverbeds and grow in or immediately adjacent to them. The semi-evergreens shed their leaves and start sprouting during a short period (one-two weeks) once a year. Because the evergreens and the semi-evergreens are in leaf in the dry period they have to protect themselves to drought damage by scleromorphic features.
Contrary to these species are the deciduous species which are bare for at least some months per year. When the dry season starts their leaves dry out and are subsequently shed. They start sprouting before or at the beginning of the first rains. Although much less in number, some deciduous trees also have scleromorphic features to resist drought-damage. The strategy of sprouting just before the rainy season begins indicates that certain water resources remain available to these deep-rooting woody plants throughout the year, providing them with a fully operating photosynthetic apparatus when favourable conditions arrive.     

An examination of observer bias as a source of error in surveys of seagrass shoots

Abstract Mensurative experiments investigated the effects of different observers on estimates of the density of shoots of two species of seagrass: Posidonia australis Hook and Zostera capricorni Aschers. Balanced programmes of sampling were used to examine variation in counts of seagrass shoots attributable to different observers, sizes of quadrats, depths and locations within large beds of each species of seagrass. A separate experiment examined differences between novice observers and a more experienced observer, when an ‘optimal’ size of sampling unit was used. Estimated densities of Zostera shoots varied inconsistently among observers, quadrats, depths and locations. Differences between observers were not affected by the size of quadrat used to count Posidonia shoots, but varied between locations in the seagrass bed. Experience had only a minor impact on biases. Only two of 12 novices produced counts that were different from the experienced observer. These results emphasize the importance of considering both accuracy and precision in the design of field studies of seagrasses.     

Circular pools in the seagrass beds of the Banc d’Arguin,Mauritania, and their possible origin

The tidal flats of the Banc d’Arguin, Mauritania, are covered by vast beds of Zostera noltii. At low tide these seagrass beds appear to be interspersed with partly vegetated, circular pools of 5–25 m diameter. Between February and May 2001 we described these pools and studied their possible origin. Several hypotheses regarding the origin have been developed. The first group of hypotheses assumes that the pools result from erosion activity. Since human disturbance of seagrass beds at the Banc d’Arguin is virtually non-existent, causes should be found in natural bed disturbances and/or tide or wave action. Therefore, small gaps, simulating holes dug by the crab Callinectes marginatus, were made to see if they would further erode by tidal currents or waves. The experiments showed no erosion. Neither we found support for other hypotheses assuming erosion to be the cause of circular pools. The alternative group of hypotheses stated that sedimentation on the flats would be responsible. We conclude that accretion of creek remnants is the most likely process behind the development of the pools; this conclusion is based on both mapping of the pattern of pools, the sediment profile in and around the pools and the distribution of seagrass biomass. Also the disturbance experiments showed bed accretion rather than bed erosion and support this hypothesis.     

Differences in composition of small,motile invertebrate assemblages from seagrass and unvegetated habitats in a southern Australian estuary

Assemblages of small, motile invertebrates (epifauna) from eelgrass(Zostera muelleri) and unvegetated habitats in a shallow, marine-dominatedestuary were compared at five sampling periods over one year. Assemblagesbased on abundance and biomass of 21 taxa from the two habitats groupedseparately in multivariate analyses (MDS ordination), and these groupingswere shown to be significant using an analysis of similarities (ANOSIM)randomisation routine. Secondarily to habitat differences, weak influencesof water temperature and distance to open water, but not of salinity, weredetected at some periods. Abundance and biomass of key taxa and all speciescombined were higha in eelgrass than in unvegetated habitat. Cumaceans wereexceptional in being collected predominantly from unvegetated habitat. Totalepifaunal production and crustacean production estimated using twovariables, (1) the biomass of individuals of each size class, and (2) watertemperature, were also higher in eelgrass than in unvegetated habitat. Thehigher abundance in eelgrass of taxa such as amphipods, harpacticoidcopepods and polychaetes that are major components of the diets of smallfish is consistent with a model explaining higher fish numbers in eelgrassin terms of prey availability.     

Conservation, Small Mammals, and the Future of Sacred Groves in West Africa

This article considers the conservation of relict natural habitat in West Africa, especially habitat preserved in traditional sacred groves. Government-sanctioned conservation is contrasted with local grassroots efforts of conservation. Evidence for the ecological value of sacred groves is based on results of a field study of small mammal communities conducted on the Accra Plains of Ghana and on published sources on the conservation and use of sacred groves from various countries. The study employed standard mark-and-recapture techniques for the sampling for terrestrial small mammals, and mist netting for the sampling of bat communities. Pragmatic approaches combining conservation and sustainable use are considered, as are traditional values that have preserved the sacred groves in the past for up to several hundred years in some cases. In part because these groves shelter unique small mammal and plant communities, traditional values and protection mechanisms should be integrated into the newly emerging cultural and religious contexts. The issues encountered during this study reveal that effective conservation involving local peoples requires a concerted interdisciplinary effort. This revised version was published online in November 2006 with corrections to the Cover Date.     

Variability in host plant resistance of Sorghum to Striga Hermonthica infestation in West Africa

Fourteen elite sorghum lines were evaluated for their resistance to Striga hermonthica at three locations in Nigeria and Mali. Results showed that many of the lines especially MALISOR 84-1, SAMSORG 41, 97-SB-F5DT-64 (Keninkédié) and the check SRN 39 remained resistant to Striga in all locations with low emerged Striga counts, while SAMSORG 14 had the highest Striga infestation in all locations. Considerable variation in reaction to Striga infestation was observed on Séguètana, 97-SB-F5DT-63 (Wasa), 97-SB-F5DT-65, CMDT 38, CMDT 39 and CMDT 45 which were susceptible to Striga at Samaru, Nigeria but were resistant to Striga at both locations in Mali. Based on low Striga resistance and high grain yield, lines MALISOR 84-1, SAMSORG 41, 97-SB-F5DT-64, 97-SB-F5DT-65, CMDT 39 and SAMSORT 14 have been nominated for wider evaluation across more West African countries.     

Isolation and characterization of novel microsatellite markers for Cymodocea serrulata (Cymodoceaceae), a seagrass distributed widely in the Indo‐Pacific region

Cymodocea serrulata is a tropical seagrass species distributed widely in the Indo‐Pacific region. We developed 16 novel microsatellite (simple sequence repeat) markers for C. serrulata using next‐generation sequencing for use in genetic studies. The applicability of these markers was attested by genotyping of 40 individuals collected from a natural population in the Philippines. Of the 16 loci, 15 showed polymorphism. For the 15 polymorphic markers, the number of alleles per locus ranged from two to seven, and the observed and expected heterozygosities ranged from 0.131–1.000 and 0.124–0.788, respectively. These markers are useful tools for elucidating genetic diversity, connectivity, and structure in this foundational coastal species.