Resilience of Zostera marina habitats and response of the macroinvertebrate community to physical disturbance caused by clam harvesting

The consequences of physical disturbances to seagrasses depend on disturbance frequency relative to the capacity for recolonization and recovery following fragmentation. In a subtidal seagrass meadow of Zostera marina L., following a season of clam harvesting, we compared the temporal change of shoot density and biomass of this seagrass together with the community structure of the associated macroinvertebrates, at two sites differing in the intensity of the physical disturbance. The impacted site showed significantly lower shoot density and total biomass than the non-impacted site initially. The increase in above-ground biomass over four months (May to September) of this species was significantly higher (46%) at the impacted site than in the area not affected by the disturbance. Four months after cessation of the extraction activity, the biomass and density values of Z. marina reached similar values to those measured in the non-impacted site. The sexual reproductive effort of the seagrass population affected by the disturbance (4%) was significantly lower than at the non-impacted site (10%), which could influence genetic diversity and the seed bank. The community structure of molluscs showed 54% similarity between sites at the beginning of the study. Four months later, mollusc communities increased to a similarity of 74%. The current closure season (four months annually) established for the recovery of the exploited stocks of bivalves allowed the recovery of Z. marina density and biomass. Nevertheless, other population properties, such as those related to reproductive patterns, remained altered by the disturbance.     

Sulfur accumulation in eelgrass (Zostera marina) and effect of sulfur on eelgrass growth

Eelgrass (Zostera marina) was grown under exposure to high levels of sediment sulfides to examine their ability to reoxidize sulfides intruding into the plants. The plants were kept under full light (control and high sulfide level) and at 10% of light saturation (high sulfide level) for 3 weeks and growth and accumulation of elemental sulfur (S⁰) in the plants were examined. The growth rate was reduced with ∼75% in the low light treatment, whereas there was no significant difference between the rates at full light saturation. S⁰ was accumulating in the below-ground structures of the plants exposed to high sulfide concentrations with highest concentration in the youngest roots and oldest internodes. There was no accumulation of S⁰ in the leaves, suggesting that the intruding sulfides were reoxidized in the below-ground structures before reaching the leaves. The accumulation of S⁰ was higher in the roots of the low light treatment (up to two times) suggesting a larger intrusion of sulfides. These plants also appeared highly affected by the treatment with rotting meristems and increased mortality after the 3-week growth period. These results are the first to show an accumulation of sulfur compounds internally in seagrasses as a result of reoxidation of sulfides. The reoxidation is facilitated by the internal transport of oxygen and is an example of the advantage of the internal lacunae system in seagrasses.     

Food sources and trophic structure of fishes and benthic macroinvertebrates in a tropical seagrass meadow revealed by stable isotope analysis

Stable carbon and nitrogen isotope analysis was used to examine the food sources and trophic structure of 17 fish species and six groups of benthic macroinvertebrates in a seagrass meadow in North Sulawesi, Indonesia. The seagrass, their associated epiphytes, sediment organic matter (SOM) and particulate organic matter (POM) were identified to be the food sources, with δ¹³C values ranging from ?19.49 (POM) to ?9.66‰ (seagrass). The δ¹³C of the 23 fauna taxa were between ?18.57 (Arothron manilensis) and ?11.62‰ (Protoreaster sp.). For five of the six groups of benthic macroinvertebrates, seagrass and their epiphytes contributed more than 69.4%. For 14 of the 17 fish species, seagrass and their epiphytes are the main contributors. For 15 of the 17 fishes, the trophic levels inferred from SIA are lower than those from the previously reported diet composition analysis. These findings show that seagrass and their epiphytes are consumed by most of the fish and benthic macroinvertebrates, and are important for a large portion of the food web in seagrass meadows in the Coral Triangle area.     

Mangrove rehabilitation on Carey Island,Malaysia: an evaluation of replanting techniques and sediment properties

We assess the suitability of conventional replanting techniques of Rhizophora mucronata and the relationship with soil properties, and compare the differences between rehabilitation and non-rehabilitation sites, on Carey Island, Malaysia. The average survival rate of planted seedlings at the rehabilitation site was 46% in the first six months, gradually reducing to complete mortality after one year, while no survival was recorded at the non-rehabilitation site from the beginning. Over the short period, survival of the clumped planting technique yielded the highest survival rate (75%) compared with random (33%) and uniform (30%) techniques; with a height increment of 2.48?±?1.87?cm/month, diameter increment of 0.49?±?0.81?mm/month, leaf increment of 2.05?±?2.80 and chlorophyll content of 32.08?±?5.80 SPAD (= Soil Plant Analysis Development values). Fifteen new recruits of Avicennia spp. with a 60% survival rate were recorded in the rehabilitation site at the end of the study. Soil texture changed significantly at the rehabilitation site from sandy loam (clay, 2.85%; silt 33.93%; sand, 63.21%) to silty loam (clay, 1.88%; silt 60.74%; sand, 37.38%). Soil nutrients: carbon (C), nitrogen (N) and potassium (K) were significantly higher in the top 40?cm at the rehabilitation site while phosphorus (P) was lower. We argue that nutrient differences between the rehabilitation and non-rehabilitation sites affected the health of both planted and wild seedlings and ultimately the success of the rehabilitation effort. As the soil condition improves at the rehabilitation site, natural recruitment would be the best available option in this restoration process.     

Effects of copper enrichment on survival,growth and photosynthetic pigment of seedlings and young plants of the eelgrass Zostera marina

Copper (Cu²⁺) is an essential nutrient for plants but toxic at high concentrations. We subjected seedlings and young plants of eelgrass Zostera marina to different seawater Cu concentrations (3, 4, 5, 10, 30 and 50?µg?l^?1) for over 30 days under controlled laboratory conditions. Natural seawater without added Cu (3?µg?l^?1) was used as reference seawater. We measured plant response in terms of survivorship, morphology, growth, productivity and leaf pigment concentration. Survival analysis combined with morphological, dynamic and productive assessment suggested that the optimum seawater Cu concentration for the establishment of Z. marina seedlings and young plants is 4?μg?l^?1. The photosynthetic response of young plants to copper enrichment, including an increase in chlorophyll content under low Cu concentration treatment but significant decrease when treated with high concentrations of Cu, is similar to those reported for other seagrass species. NOEC (no observed effect concentration), LOEC (lowest observed effect concentration) and LC₅₀ (lethal concentration that caused an increase in mortality to 50% of that of the control) values of seedlings were significantly lower than those of young plants, implying a reduced Cu tolerance to high concentrations (>10?μg?l^?1). This study provides data that could prove helpful in the development of successful eelgrass restoration and conservation.     

Heavy metals in eelgrass (Zostera marina L.) during growth and decomposition

The distributions of cadmium, chromium, lead, and zinc in eelgrass were studied in samples collected from the field, and the loss/accumulation of the metals during decomposition of eelgrass leaves was studied in laboratory experiments.Concentrations of heavy metals in the below grounds parts were greater in the roots than in the different age groups of the rhizomes. In the rhizomes, the highest concentrations of lead were recorded in the oldest parts, whereas highest chromium and zinc concentrations were found in the youngest parts. The concentration of cadmium did not vary. In the above ground parts, the concentrations of cadmium, lead, and zinc increased with age of the leaves, and concentrations in the leaves were greater than in the stem fraction. The concentrations of chromium decreased with age of the leaves.In the laboratory study of decomposition of leaf material, the concentrations of chromium, lead and zinc increased significantly and a net absorption from the surrounding water was recorded. Cadmium concentrations were relatively constant and a loss of cadmium was proportional to the release of soluble organic compounds indicating an association of cadmium with the soluble phase.The investigation demonstrated the utility of compositional analyses and decomposition experiments in assessing the significance of eelgrass in the heavy metal cycling in coastal areas. Furthermore, significant differences in the fate of heavy metals associated with eelgrass detritus are discussed.     

Decomposition processes of eelgrass,Zostera marina L.

Summary In Lake Grevelingen decomposition of eelgrass was studied in the field with the litter bag method from July 1977 till February 1978. After 6 months only 6% refractory matter remained in the bags. Under aerobic conditions the decomposition of eelgrass is completed within one year. The organic fraction decreased from 80 to 55%. Chlorophyll a was always present in the detritus, but with this parameter no decomposition stages could be distinguished. Fragmentation was mainly physical, and a particle size spectrum showed a maximum towards the small pieces.The POC content was fairly constant, and the N and P content changed during the decomposition. The C/N and C/P ratios increased the first 10 weeks (leaching) and decreased after 10 weeks (microbial colonization). This did not correspond with the three decomposition stages, based on the dry weight loss per day. The C/N ratio does not seem to be a simple index for the decomposition stage in eelgrass.Communication no. 235 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

玻璃化法超低温保存对大叶藻种子活力的影响

通过设置玻璃化法超低温保存的3个关键要素,即冰冻保护液处理、玻璃化溶液脱水时间和冷冻处理,研究了玻璃化法超低温保存对大叶藻种子活力的影响.结果表明:冰冻保护液处理和玻璃化溶液脱水时间对大叶藻种子活力无显著影响;冷冻处理组种子活力显著低于对照组,而冰冻保护液和玻璃化脱水处理对冷冻后种子活力的影响不显著,表明冰冻保护液和玻璃化溶液未对种子起到明显的保护作用;未冰冻保护液处理组玻璃化溶液脱水4h时种子活力显著低于其他脱水时间,而冰冻保护液处理组脱水4h时与其他脱水时间无明显不同,表明较长时间的玻璃化溶液脱水处理需使用冰冻保护液进行预处理.研究结果为超低温保存大叶藻种子提供了参考.     

Seed germination and seedling growth of Zostera marina L. (eelgrass) in the chesapeake bay

Seed germination and seedling growth of Zostera marina L. were monitored in the Chesapeake Bay in 1979 and 1980. Harvested seeds were placed in small acrylic tubes at several sites representing the salinity range of Z. marina distribution. Seed germination was observed first in late September and continued through May, with peaks in the fall and spring. The majority of seeds that germinated (66%) did so between December and March when water temperatures ranged from 0–10°C. There was no correlation between sites (different salinity regimes) and frequency of germination rates, indicating that salinity was not a major factor in the germination process in this study. Additional information on seed germination was available for seeds collected in 1977 and 1980 and subsequently monitored for germination at only one site. These data were similar to germination frequency recorded in 1979–1980.Seedling growth was measured from individuals collected from an existing Zostera marina bed. Seedlings were collected from November through May, at which time we could no longer distinguish seedlings from existing vegetative stock. Growth was characterized by the increased length of the primary shoot, number of leaves per shoot and numbers of shoots per plant. Seedling growth was slow during the winter months (water temperature ? 10°C) but rapidly increased in the spring (temperatures > 10°C). The size range of the harvested seedlings indicated that seed germination in the field probably occurred from October through April, corroborating evidence from the seed germination experiments.     

Photoadaptation and growth of Zostera marina L. (eelgrass) transplants along a depth gradient

Photosynthetic and growth responses were assessed in Zostera marina L. transplants within and beyond the natural extent of an eelgrass meadow in Great Harbor, Woods Hole, MA. Transplant survival and rapid growth inshore of the shallow edge of the meadow (0.5 and 0.8 m water depth) indicated a periodic disturbance factor maintaining the shallow edge of the meadow. Transplant mortality, reduced growth, and a negative carbon balance of eelgrass transplanted offshore the deep edge of the meadow (7 and 10m) supported the hypothesis of light-limited eelgrass growth in the deep regions of the Great Harbor meadow. Photoadaptive responses occurred along the water depth gradient, and both photosynthesis and growth responses were used to assess the genetic vs. phenotypic components of eelgrass response to the water depth gradient. Reciprocal transplants between shallow (1.3 m) and deep (5.5 m) areas within the eelgrass meadow indicated photosynthetic and growth responses were primarily a result of growth habitat rather than genetic differentiation within the eelgrass meadow.     

Control of microbial growth on eelgrass (Zostera marina L.: Spermatophyta) by leaf-derived metabolites

Curie point pyrolysis-mass spectrometry is a powerful method for fast characterization of complex, nonvolatile materials. Fast, reproducible heating of the material results in a characteristic mixture of volatile fragment products, which is analyzed on-line by mass spectrometry. The method can be used for various purposes ranging from classification and identification to quality control and biochemical analysis and has already proven to be a versatile tool in the fields of (micro-) biology, biochemistry, soil science and geochemistry. Our fully automated Py-MS system for batch-wise analysis of series of samples will be presented, together with computer methods for multivariate analysis of the spectral data. Some results obtained within the application-fields mentioned above will also be given.     

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.     

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.     

Reproduction of annual eelgrass: Variation among habitats and comparison with perennial eelgrass (Zostera marina L.)

The variation in reproductive potential of annual eelgrass was examined along a continuous gradient on an extensive mudflat bordering on and sloping down from the shore (Eastern River) and in a habitat mosaic in a salt marsh (Petpeswick Inlet) in Nova Scotia. Spathe and flower production as well as plant density were compared among habitats. Of the four habitats investigated in Petpeswick Inlet, the largest numbers of spathes and flowers per spathe were produced by plants in ponds on raised flats of Spartina alterniflora Loisel. The highest number of seeds per unit area was produced by plants in depressions on these flats which drained with each low tide. Seed production of annual eelgrass in drained depressions (4 889 seeds per 625 cm²) was seven times that of perennial shoots in creeks and higher than any records in the literature for perennial eelgrass. On average, seed production of annual eelgrass in this study was higher than values reported for other locations. Along the gradient, both annual and perennial eelgrass showed peaks in reproductive potential, but the annual peaked further up the gradient where there was greater exposure to air at low tide. Transplanting studies indicated that the among-habitat differences in reproductive potential were largely controlled by environmental as opposed to genetic factors. The possible effect of inter- and intraspecific competition on the reproductive potential of annual eelgrass was investigated experimentally in two habitats where co-existing species were abundant. In creeks the presence of perennial eelgrass significantly reduced the reproductive potential of annual eelgrass, but in a drained depression, the removal of Ruppia maritima L. s.l. had no effect. The upper distribution limit of annual eelgrass is likely determined by desiccation while the lower limit is probably determined by a combination of light availability (to some extent affected by perennial eelgrass) and exposure to spring rains which would significantly enhance seed germination.     

Seasonal and year-to-year variations in the growth of Zostera marina L. (eelgrass) in the lower Chesapeake Bay

Seasonal and year-to-year variations in the growth of Zostera marina L. were measured at three sites in two locations in the lower Chesapeake Bay between 1978 and 1980. The maximum values for the 1979 above- and belowground standing crop ranged from 161–336 g dry wt m⁻² and 61–155 g dry wt m⁻², respectively, leaf length was 19.6–59.7 cm and shoot density 1418–2576 shoot m⁻². Values for 1980 tended to be greater and may be related to climatical differences between the two years. Maximum values were usually recorded in the months of June and July when water temperatures were between 20 and 25°C. Significant loss of leaves occurred in July and August, when water temperatures ranged between 25 and 30°C, while new shoots began to appear more rapidly in late September as water temperatures dropped below 20°C. The greatest increase in all growth parameters occurred from April to June during which time reproductive shoots were present, and accounted for up to 25% of the total number of shoots.     

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.     

Enumeration and characterization of nitrogen-fixing bacteria in an eelgrass (Zostera marina) bed

Marine nitrogen-fixing bacteria distributed in the eelgrass bed and seawater of Aburatsubo Inlet, Kanagawa, Japan were investigated using anaerobic and microaerobic enrichment culture methods. The present enrichment culture methods are simple and efficient for enumeration and isolation of nitrogen-fixing bacteria from marine environments. Mostprobable-number (MPN) values obtained for nitrogen-fixing bacteria ranged from 1.1×10² to 4.6×10²/ml for seawater, 4.0×10⁴ to 4.3×10⁵/g wet wt for eelgrass-bed sediment, and 2.1 × 10⁵ to 1.2 × 10⁷/g wet wt for eelgrass-root samples. More than 100 strains of halophilic, nitrogen-fixing bacteria belonging to the family Vibrionaceae were isolated from the MPN tubes. These isolates were roughly classified into seven groups on the basis of their physiological and biochemical characteristics. The majority of the isolates were assigned to the genusVibrio and one group to the genusPhotobacterium. However, there was also a group that could not be identified to the generic level. All isolates expressed nitrogen fixation activities under anaerobic conditions, and no organic growth factors were required for their activities.     

Variable responses of native eelgrass Zostera marina to a non-indigenous bivalve Musculista senhousia

The transport and establishment of non-indigenous species in coastal marine environments are increasing worldwide, yet few studies have experimentally addressed the interactions between potentially dominant non-native species and native organisms. We studied the effects of the introduced mussel Musculista senhousia on leaf and rhizome growth and shoot density of eelgrass Zostera marina in San Diego Bay, California. We added M. senhousia over a natural range in biomass (0–1200 g dry mass/m²) to eelgrass in transplanted and established beds. The effects of the non-indigenous mussel varied from facilitation to interference depending on time, the abundance of M. senhousia, and the response variable considered. Consistent results were that mussel additions linearly inhibited eelgrass rhizome elongation rates. With 800 g dry mass/m² of M. senhousia, eelgrass rhizomes grew 40% less than controls in two eelgrass transplantations and in one established eelgrass bed. These results indicate that M. senhousia, could both impair the success of transplantations of eelgrass, which spread vegetatively by rhizomes, and the spread of established Z. marina beds to areas inhabited by M. senhousia. Although effects on leaf growth were not always significant, in August in both eelgrass transplantations and established meadows leaf growth was fertilized by mussels, and showed a saturation-type relationship to sediment ammonium concentrations. Ammonium concentrations and sediment organic content were linear functions of mussel biomass. We found only small, non-consistent effects of M. senhousia on shoot density of eelgrass over 6-month periods. In established eelgrass beds, but not in transplanted eelgrass patches (≈0.8 m in diameter), added mussels suffered large declines. Hence, eelgrass is likely to be affected by M. senhousia primarily where Z. marina beds are patchy and sparse. Our study has management and conservation implications for eelgrass because many beds are already seriously degraded and limited in southern California where the mussel is very abundant. Received: 31 May 1997 / Accepted: 4 September 1997     

Production and decomposition of eelgrass (Zostera marina L.) in saline Lake Grevelingen

Summary During five 28-hours measurements in 1981, the oxygen production and consumption in an eelgrass community in saline Lake Grevelingen were investigated using light plexiglass enclosures. Applying a conversion factor of 0.29 the amount of carbon fixed and the amount of organic carbon mineralized were estimated. Gross and net production were estimated over 24-hours periods.There appeared to be a good correlation between production and insolation on the water surface. For every measurement period the production as a function of light and aboveground eelgrass biomass in the enclosure were calculated. This showed a maximum of 5.10^–6 mg C.J.^–1 g dry weight^–1 in April and minimum of 1.4.10^–6 mg C.J.^–1 g^–1 in August.Using the calculated production coefficients, the insolation and the eelgrass biomass the gross production, net production and consumption during the growing season of 1976 were calculated. Gross production amounted to 340 gC.m^–2, and net production came to 130 g C.m^–2. Approximately 60 gC.m^–2 was respired by the eelgrass plants while the remaining 150 gC.m^–2 was consumed or mineralized by other organisms on the sampling spot. Approximately 120 g C.m^–2.yr^–1 was transported by wind and wave action towards the eastern part of the lake where it became anaerobically degraded. This resulted in the formation of sulfide and methane.Communication no. 236 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

Identification of rosmarinic acid and sulfated flavonoids as inhibitors of microfouling on the surface of eelgrass Zostera marina

A bioassay-guided approach was used to identify defense compounds that are present on the surface of Zostera marina and which inhibit settlement of microfoulers at natural concentrations. Moderately polar eelgrass surface extracts inhibited the settlement of seven marine bacteria and one yeast that originated from non-living substrata. In contrast, five other bacterial strains that had been directly isolated from eelgrass surfaces were all insensitive, which suggested a selective effect of surface metabolites on the microbial communities present on eelgrass. Bioassay-guided isolation of active compounds from the extracts in combination with UPLC-MS and ¹H-NMR spectroscopy resulted in the identification of rosmarinic acid, luteolin-7-sulfate and diosmetin-7-sulfate or its isomer chrysoeriol-7-sulfate. All three compounds are nontoxic repellents, as they did not inhibit bacterial growth, but prevented bacterial settlement in a dose-dependent manner. Between 15.6 and 106.8 μg ml^?1 of rosmarinic acid were present on the eelgrass surface, enough for half maximal settlement inhibition of bacteria.