Sulphide intrusion in eelgrass (Zostera marina L.)

Sudden events of seagrass die‐off have been suggested to be induced by invasion of the phytotoxin sulphide under environmental stress generating low oxygen supply in seagrass tissues. Laboratory experiments were conducted with eelgrass (Zostera marina L.) to measure intra‐plant changes in oxygen and sulphide by means of microelectrodes at different oxygen concentrations in the water column. The objectives were to examine whether sulphide intrusion into seagrass tissues can be induced, to determine the role of plant oxygen status for sulphide intrusion and to determine how fast internal sulphide pools are depleted after internal oxygen supplies have been restored. Under conditions with oxygen partial pressures (pO₂) above 7.4 kPa (> 35% of air saturation) within eelgrass rhizomes or meristematic tissues no intrusion of sulphide occurred in spite of high sediment concentrations of gaseous sulphide (> 1000 µm ). Lack of sulphide intrusion at high internal pO₂ suggested that oxygen release from the roots ensured complete re‐oxidation of sulphide in the rhizosphere. Under oxygen stress, however, the experiments clearly demonstrated intrusion of sulphide in eelgrass rhizomes and meristematic tissues. Rates of sulphide intrusion were controlled by internal pO₂, which in turn was controlled by water column oxygen concentrations. Maximum internal sulphide content reached 325 µm which by far exceeded the 1–10 µm known to inhibit mitochondrial activity in eukaryotic cells. Sulphide and low levels of oxygen could coexist in the eelgrass tissues reflecting fast internal transport of sulphide and slow rates of sulphide re‐oxidation. Upon re‐establishment of high internal oxygen concentrations the depletion of the sulphide pool was slow (half‐life = 20–30 min) indicating, that sulphide re‐oxidation within the eelgrass tissue was not bacterially or enzymatically facilitated but occurred by simple chemical oxidation. The results of this study are consistent with the proposed detrimental role of sulphide intrusion in events of sudden seagrass die‐off.     

Does the recruitment of a non‐native mussel in native eelgrass habitat explain their disjunct adult distributions?

We studied variability in the abundance of small individuals of an invasive mussel (Musculista senhousia) across the depth distribution of a native marine angiosperm, eelgrass (Zostera marina). Adult mussels and eelgrass have a disjunct local distribution, each limiting the other in complex ways. To assess whether eelgrass also influenced the distribution of juvenile mussels, we sampled inside and outside eelgrass beds in one site in Mission Bay and two in San Diego Bay, California, USA. We sampled mussels in size classes 0.26–0.50 mm, 0.51–1.00 mm, 1.10–2.00 mm and > 2.00 mm from September 1997 to April 1999. We also monitored gonad development in larger mussels and in situ growth of mussels ≤ 2 mm tagged with the chemical marker calcein. Spatial and temporal variations in mussel abundances were high but seasonal patterns were roughly similar at San Diego Bay sites; very few mussels were found in Mission Bay. Mussels with full gonads were found year‐round in San Diego Bay, as were mussels in the smallest size class (with a large peak in fall and a smaller secondary one in spring), suggesting that many of the smallest mussels represent recruitment. The observation that most, although not all, tagged mussels increased in size provides further support for recruitment. Some of the highest numbers of mussels in the smallest size class were found inside eelgrass beds, indicating that eelgrass does not restrict and may actually enhance the distribution of very small mussels. The disjunct distribution of adult mussels and eelgrass thus is apparently established primarily postrecruitment. M. senhousia is capable of year‐round reproduction, recruitment and growth, and thus is poised to preempt space from eelgrass following any disturbance that results in eelgrass declines, such as habitat fragmentation, eutrophication, or disease.     

Microsatellite loci in eelgrass Zostera marina reveal marked polymorphism within and among populations

Using an enriched genomic library, we developed seven (CT)n/(GA)n microsatellite loci for eelgrass Zostera marina L. Enrichment is described and highly recommended for genomes in which microsatellites are rare, such as in many plants. A test for polymorphism was performed on individuals from three geographically separated populations (N = 15/population) and revealed considerable genetic variation. The number of alleles per locus varied between five and 11 and the observed heterozygosities for single loci ranged from 0.16 to 0.81 within populations. Mean allele lengths were markedly different among populations, indicating that the identified loci will be useful in studying population structure in Z. marina. As the frequency of the most abundant multilocus genotype within populations was always < 1%, these loci have sufficient resolving power to address clone size in predominantly vegetatively reproducing populations.     

Characterization of single nucleotide polymorphism markers for eelgrass (Zostera marina)

We characterized 37 single nucleotide polymorphism (SNP) makers for eelgrass Zostera marina. SNP markers were developed using existing EST (expressed sequence tag)-libraries to locate polymorphic loci and develop primers from the functional expressed genes that are deposited in The ZOSTERA database (V1.2.1). SNP loci were genotyped using a single-base-extension approach which facilitated high-throughput genotyping with minimal optimization time. These markers show a wide range of variability among 25 eelgrass populations and will be useful for population genetic studies including evaluation of population structure, historical demography, and phylogeography. Potential applications include haplotype inference of physically linked SNPs and identification of genes under selection for temperature and desiccation stress.     

Seed- versus transplant-based eelgrass (Zostera marina L.) restoration success in a temperate marine lake

Despite active seagrass restoration gaining traction as a tool to halt and reverse worldwide seagrass losses, overall success remains limited. Restoration strategies, through seeding or transplantation, face different environmental bottlenecks that limit success. Choosing the most appropriate strategy of the two for a specific location, however, is hampered by lack of direct practical comparisons between strategies within a single system. To investigate potential life stage dependent bottlenecks, we compared seed-based and transplant-based restoration of Zostera marina in the subtidal saltwater Lake Grevelingen. Our results demonstrate that seedling recruitment was negatively impacted by bioturbation from the lugworm Arenicola marina and sediment movement due to hydrodynamic exposure. Transplant-based restoration was clearly more successful but surprisingly best predicted by leaf gluing by the ragworm Platynereis dumerilii. This previously undescribed interaction caused seagrass leaves to clump and reduce effective photosynthetic surface and leaf movement. We suggest that the observed behavior of these worms may result from a lack of foodweb interactions, illustrating the importance of  trophic control for seagrass restoration. Thus, in addition to recognizing life stage dependent environmental bottlenecks for restoration strategy selection, seagrass restoration may also require the active recovery of their associated food webs.     

Identification and characterization of 14 polymorphic EST‐derived microsatellites in eelgrass (Zostera marina)

Zostera marina, the dominant seagrass on the Northern Hemisphere, forms the basis of important but threatened marine ecosystems. Here, we report 14 microsatellite DNA markers derived from an expressed sequence tag library corresponding to a wide range of genes. All loci were moderately to highly polymorphic, with allele numbers ranging from three to eight in a single Wadden Sea population of 48 individuals. Observed heterozygosities ranged from 0.082 to 0.837. Reaction conditions for five pooled polymerase chain reactions are given. The markers will advance the population genetics of seagrasses because they allow indirect tests of selection on closely linked genes.     

Policy plans and management measures to restore eelgrass (Zostera marina L.) in the Dutch Wadden Sea

The Dutch Wadden Sea has been changed dramatically over the last centuries by human activities like land reclamation and different forms of fishery. This has, amongst other things, led to changes in the number of biological communities. One of the changes was the near extinction of eelgrass (Zostera marina L.) in the Dutch Wadden Sea. The deterioration of the area led to policy plans in the late 1980s that aimed at restoring the original natural communities of which the eelgrass community was one. This paper presents a restoration strategy which contains a selection procedure for suitable transplantation sites. The selection procedure is based on factors such as sediment composition, exposure time, current velocity and wave action. These were combined in a GIS-based map integrating these factors. One important action in the restoration process is to increase the number of freshwater discharge points to meet the requirements of the brackish water community in general and the growing conditions for eelgrass in particular. Received: 27 October 1999 / Received in revised form: 3 February 2000 / Accepted: 3 February 2000     

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.     

大叶藻对氮磷营养盐的吸收动力学特征

营养盐是影响海草生长的关键因子, 目前有关海草不同组织对不同形式氮和磷的吸收特征尚不明确。该研究通过利用海草地上和地下组织分隔培养装置, 设置不同的氨态氮、硝态氮和磷酸盐浓度, 探究了大叶藻(Zostera marina)植株及其地上和地下组织对氮磷营养盐的吸收动力学特征。结果显示: (1)大叶藻对氮磷的吸收符合饱和吸收动力学特征, 吸收速率和水体氮磷浓度可用米式方程描述; (2)大叶藻植株对NH₄⁺-N的最大吸收速率(V_max, 52 μmol·g^-1·h^-1)显著高于其对NO₃^--N的V_max (39 μmol·g^-1·h^-1); (3)大叶藻地上组织和地下组织均可吸收氮磷, 但地上组织对氨态氮、硝态氮、磷酸盐的V_max分别为43.1、30.5和15.6 μmol·g^-1·h^-1, 为地下组织的2.6、1.2和6倍。结果表明, 大叶藻对氨态氮的吸收能力高于硝态氮, 且对氮磷的吸收可能主要依赖地上组织(叶片)。结果为查明大叶藻对氮磷的吸收利用机制及评估大叶藻的海洋生态效应提供了理论依据。     

Decline of Canada geese (Branta canadensis) and common goldeneye (Bucephala clangula) associated with a collapse of eelgrass (Zostera marina) in a Nova Scotia estuary

Mean numbers of migrant Canada geese (Branta canadensis) in Antigonish Harbour in the southern Gulf of St. Lawrence (Canada) during October to December were similar (approx. 450–500 birds) for the period 1998–2000. Similarly, during this period, geese used two foraging sites. However, in 2001, the average number of birds decreased by half and the primary foraging sites were used only rarely. This coincided with a decline of about 95% in the biomass of roots and rhizomes of eelgrass (Zostera marina) that occurred between October 2000 and 2001. Eelgrass is the principal food of geese in this estuary. In addition, there was a reduction of around 50% in the numbers of common goldeneye (Bucephala clangula), which feed on invertebrates associated with eelgrass. Lower than usual weekly abundances of geese and goldeneye are probably the result of an unusually short residence time in the estuary, rather than a decline in the total number of visiting migrants. We attribute these changes in the distribution and abundance of geese and goldeneyes to the dramatic decline in eelgrass. Electronic Publication     

Range-edge genetic diversity: locally poor extant southern patches maintain a regionally diverse hotspot in the seagrass Zostera marina

Refugial populations at the rear edge are predicted to contain higher genetic diversity than those resulting from expansion, such as in post-glacial recolonizations. However, peripheral populations are also predicted to have decreased diversity compared to the centre of a species' distribution. We aim to test these predictions by comparing genetic diversity in populations at the limits of distribution of the seagrass Zostera marina, with populations in the species' previously described central diversity 'hotspot'. Zostera marina populations show decreased allelic richness, heterozygosity and genotypic richness in both the 'rear' edge and the 'leading' edge compared to the diversity 'hotspot' in the North Sea/Baltic region. However, when populations are pooled, genetic diversity at the southern range is as high as in the North Sea/Baltic region while the 'leading edge' remains low in genetic diversity. The decreased genetic diversity in these southern Iberian populations compared to more central populations is possibly the effect of drift because of small effective population size, as a result of reduced habitat, low sexual reproduction and low gene flow. However, when considering the whole southern edge of distribution rather than per population, diversity is as high as in the central 'hotspot' in the North Sea/Baltic region. We conclude that diversity patterns assessed per population can mask the real regional richness that is typical of rear edge populations, which have played a key role in the species biogeographical history and as marginal diversity hotspots have very high conservation value.     

North Atlantic phylogeography and large-scale population differentiation of the seagrass Zostera marina L

As the most widespread seagrass in temperate waters of the Northern Hemisphere, Zostera marina provides a unique opportunity to investigate the extent to which the historical legacy of the last glacial maximum (LGM18 000-10 000 years bp) is detectable in modern population genetic structure. We used sequences from the nuclear rDNA-internal transcribed spacer (ITS) and chloroplast matK-intron, and nine microsatellite loci to survey 49 populations (> 2000 individuals) from throughout the species' range. Minimal sequence variation between Pacific and Atlantic populations combined with biogeographical groupings derived from the microsatellite data, suggest that the trans-Arctic connection is currently open. The east Pacific and west Atlantic are more connected than either is to the east Atlantic. Allelic richness was almost two-fold higher in the Pacific. Populations from putative Atlantic refugia now represent the southern edges of the distribution and are not genetically diverse. Unexpectedly, the highest allelic diversity was observed in the North Sea-Wadden Sea-southwest Baltic region. Except for the Mediterranean and Black Seas, significant isolation-by-distance was found from ~150 to 5000 km. A transition from weak to strong isolation-by-distance occurred at ~150 km among northern European populations suggesting this scale as the natural limit for dispersal within the metapopulation. Links between historical and contemporary processes are discussed in terms of the projected effects of climate change on coastal marine plants. The identification of a high genetic diversity hotspot in Northern Europe provides a basis for restoration decisions.     

A microsatellite-based estimation of clonal diversity and population subdivision in Zostera marina, a marine flowering plant

We examined the genetic population structure in eelgrass (Zostera marina L.), the dominant seagrass species of the northern hemisphere, over spatial scales from 12 km to 10 000 km using the polymorphism of DNA microsatellites. Twelve populations were genotyped for six loci representing a total of 67 alleles. Populations sampled included the North Sea (four), the Baltic Sea (three), the western Atlantic (two), the eastern Atlantic (one), the Mediterranean Sea (one) and the eastern Pacific (one). Microsatellites revealed substantial genetic variation in a plant group with low allozyme diversity. Average expected heterozygosities per population (monoclonal populations excluded) ranged from 0.32 to 0.61 (mean = 0. 48) and allele numbers varied between 3.3 and 6.7 (mean = 4.7). Using the expected frequency of multilocus genotypes within populations, we distinguished ramets from genetic individuals (i.e. equivalent to clones). Differences in clonal diversity among populations varied widely and ranged from maximal diversity (i.e. all ramets with different genotype) to near or total monoclonality (two populations). All multiple sampled ramets were excluded from further analysis of genetic differentiation within and between populations. All but one population were in Hardy-Weinberg equilibrium, indicating that Zostera marina is predominantly outcrossing. From a regression of the pairwise population differentiation with distance, we obtained an effective population size Ne of 2440-5000. The overall genetic differentiation among eelgrass populations, assessed as rho (a standardized estimate of Slatkin's RST) was 0.384 (95% CI 0.34-0.44, P < 0.001). Genetic differentiation was weak among three North Sea populations situated 12-42 km distant from one another, suggesting that tidal currents result in an efficient exchange of propagules. In the Baltic and in Nova Scotia, a small but statistically significant fraction of the genetic variance was distributed between populations (rho = 0.029-0. 053) at scales of 15-35 km. Pairwise genetic differentiation between European populations were correlated with distance between populations up to a distance of 4500 km (linear differentiation-by-distance model, R2 = 0.67). In contrast, both Nova Scotian populations were genetically much closer to North Sea and Baltic populations than expected from their geographical distance (pairwise rho = 0.03-0.08, P < 0.01). A biogeographical cluster of Canadian with Baltic/North Sea populations was also supported using a neighbour-joining tree based on Cavalli-Sforza's chord distance. Relatedness between populations may be very different from predictions based on geographical vicinity.     

A Comparative Test of Mechanized and Manual Transplanting of Eelgrass, Zostera marina, in Chesapeake Bay

Abstract The laborious process of manual seagrass transplanting has often limited the size of seagrass restoration efforts. This study tested the efficiency of a mechanized planting boat, previously used for transplanting Halodule wrightii, relative to manual transplanting methods for establishing Zostera marina in Chesapeake Bay. Eelgrass planting was conducted at two sites, one each in the Rappahannock and James rivers, in October 2001. The methods were evaluated by three criteria: (1) initial planting success = proportion of attempted planting units (PUs) initially established (number confirmed in sediment by divers/number attempted); (2) survival = proportion of the initially established PUs persisting over 1, 4, and 24 weeks; and (3) efficiency = labor (in person·seconds) invested in each surviving PU. Initial planting success was significantly lower for the planting boat (24 and 56% at the Rappahannock and James sites, respectively) than for manual transplanting (100% at both sites). At the Rappahannock site, survival of initially established PUs declined over time for both methods, but while mean survival was always higher for manually planted rows, differences in survival between methods were not statistically significant. At the James site, survival to 1 and 4 weeks was significantly lower for the machine than for the manual method, but survival to 24 weeks was not significantly different. While the machine was able to attempt PUs faster than the manual method (2.2 s/PU vs. 5.8 s/PU, respectively), this speed was offset by poorer planting success rates, resulting in a much greater total labor investment for each machine‐planted PU that persisted to 24 weeks than for each similarly persisting manually planted PU (40.6 person·seconds/PU and 22.4 person·seconds/PU, respectively, averaged across sites). In summary, those PUs successfully planted by the machine survived similarly to PUs planted by hand, but as a result of poorer initial planting success, the machine required a greater investment of labor and plant donor stock for each PU surviving to 24 weeks. Therefore, in its tested configuration this planting boat is not a significant improvement over the manual method for transplanting eelgrass.     

Effectiveness of pectin extracted from the eelgrass Zostera marina for alleviating lead-induced liver injury

The effects of pectin from the eelgrass Zostera marina on toxic liver injury induced by enteral administration of lead acetate are examined in experiments on rats. The results show that pectin helps to rapidly reduce lead concentration in liver, to decrease lipid peroxidation, and to normalize the levels of total cholesterol and triglycerides in blood serum and liver.     

Habitat suitability of the Wadden Sea for restoration of Zostera marina beds

A conceptual model is proposed, describing potential Zostera marina habitats in the Wadden Sea, based on reported data from laboratory, mesocosm and field studies. Controlling factors in the model are dynamics, degree of desiccation, turbidity, nutrients and salinity. A distinction has been made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. The model relates the decline of Z. marina in the Wadden Sea to increased sediment and water dynamics, turbidity, drainage of sediments (resulting in increased degree of desiccation) and total nutrient loads during the twentieth century. The upper and lower delineation of both the higher and the lower zone of potential Z. marina habitats appear to be determined by one or a combination of several of these factors. Environmental changes in one of these factors will therefore influence the borderlines of the zones. The lower zone of Z. marina will be mainly affected by increased turbidity, sediment dynamics, degree of desiccation during low tide and nutrient load. The higher zone will be affected by increases in water and sediment dynamics, desiccation rates and nutrient loads. Potential Z. marina habitats are located above approx. –0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s) in sheltered, undisturbed locations, and preferably where some freshwater influence is present. At locations with a high, near-marine, salinity, the nutrient load has to be low to allow the growth of Z. marina. The sediment should retain enough water during low tide to keep the plants moist. Our results suggest that the return of Z. marina beds within a reasonable time-scale will require not only suitable habitat conditions, but also revegetation measures, as the changes in the environment resulting from the disappearance of Z. marina may impede its recovery, and the natural import of propagules will be unlikely. Furthermore, the lower zone of Z. marina may require a genotype that is no longer found in the Wadden Sea. Received: 26 April 1999 / Received in revised form: 15 October 1999 / Accepted: 16 October 1999     

Desiccation,Moisture Content and Germination of Zostera marina L. Seed

Zostera marina is the only seagrass species whose seeds have been successfully used in large‐scale restoration. Although progress has been made in refining Z. marina restoration protocols, additional information on Z. marina seed physiology is necessary as the science of seagrass restoration evolves. We tested the germination rates of Z. marina seeds under different relative humidities and temperatures for different periods of time. Z. marina seed moisture content (MC) and germination rates were also tested when seeds were exposed to a temperature of 25°C and relative humidity of 50%. Z. marina seeds suffered higher mortality when exposed to lower relative humidity and higher temperature for longer period of exposure time. A significant negative correlation was detected between seed germination rate and MC. Z. marina seeds are sensitive to desiccation exposure and long periods of exposure to air should be prevented to minimize seed mortality when seeds are used in restoration projects.     

Fitness‐consequences of geitonogamous selfing in a clonal marine angiosperm (Zostera marina)

Plant mating systems have received considerable attention because the proportion of selfed vs. outcrossed progeny is an important evolutionary factor. In clonally reproducing plants, geitonogamous selfing between distant ramets belonging to the same genet is expected to be widespread, yet empirical data are sparse. Nothing is known about between‐ramet selfing in aquatic flowering plants with subaqueous pollen transfer, most of which display pronounced clonal reproduction. From two locations in the western Baltic Sea, I present data on the effects of patch isolation and clonal diversity on the outcrossing rate of eelgrass, Zostera marina L., based on the genotypes of maternal plants and recently fertilized ovules scored at eight microsatellite loci. There were no differences in outcrossing rates between vegetation patches and continuous meadow although patches were nearly always composed of single genets. Quantitative effects of clonal diversity were present in the continuous vegetation where a significant positive correlation between genet diversity and the proportion of outcrossed offspring was detected (Kendall’s τ=0.82, P=0.0017). On a population‐scale as well, the genotypic diversity was positively correlated with outcrossing. The relative fitness of selfed offspring was low (ω ± 95% confidence interval=0.56 ± 0.032 and 0.322 ± 0.15) indicating that geitonogamy incurred substantial fitness costs. Selfing rates in Z. marina may not be in evolutionary equilibrium because of spatial and temporal heterogeneity of clonal size and diversity. The high prevalence of dioecy in seagrasses may have evolved to avoid the fitness costs associated with geitonogamy.     

Genome scans detect consistent divergent selection among subtidal vs. intertidal populations of the marine angiosperm Zostera marina

Genome scans are a powerful tool to detect natural selection in natural populations among a larger sample of marker loci. We used replicated habitat comparisons to search for consistent signals of selection among contrasting populations of the seagrass Zostera marina, a marine flowering plant with important ecological functions. We compared two different habitat types in the North Frisian Wadden Sea, either permanently submerged (subtidal) or subjected to aerial exposure (intertidal). In three independent population pairs, each consisting of one tidal creek and one tidal flat population each, we carried out a genome scan with 14 expressed sequence tag (EST)-derived microsatellites situated in 5'- or 3'-untranslated regions of putative genes, in addition to 11 anonymous genomic microsatellites. By using two approaches for outlier identification, one anonymous and two EST-derived microsatellites showed population differentiation patterns not consistent with neutrality. These microsatellites were detected in several parallel population comparisons, suggesting that they are under diverging selection. One of these loci is linked to a putative nodulin gene, which is responsible for water channelling across cellular membranes, suggesting a functional link of the observed genetic divergence with habitat characteristics.