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.     

The effect of substrate media on the survival and growth of the eelgrass Zostera marina

We subjected transplants of eelgrass (Zostera marina) to different substrate media (high-silted soil, akadama soil, peat soil, humus soil, pond soil and natural sediment) for over 25 days under controlled laboratory conditions. Subsequently, an 80-day field transplantation experiment was conducted to assess the establishment success of transplants planted in high-silted soil, akadama soil and natural sediment. We measured plant response in terms of survivorship, morphology and productivity. Survival analysis combined with morphological and productivity assessment suggested that the optimum growing medium for the establishment of Z. marina transplants is high-silted soil. A redundancy analysis (RDA) revealed that the survival and growth of Z. marina transplants were positively related to the air-filled porosity of the substrate media and were negatively related to the organic matter content. This study will provide data that could prove helpful in successful eelgrass restoration and conservation.     

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.     

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.     

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.     

In situ phytoremediation of PAH- and PCB-contaminated marine sediments with eelgrass (Zostera marina)

In view of the fact that there are presently no cost-effective in situ treatment technologies for contaminated sediments, a 60-week-long phytoremediation feasibility study was conducted in seawater-supplied outdoor ponds to determine whether eelgrass (Zostera marina) is capable of removing polynuclear aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) from submerged marine sediments. It was determined that all PAHs and PCBs, independent of the number of aromatic rings and degree of chlorination, respectively, were removed to a much larger extent in planted sediments compared to unplanted controls. After 60 weeks of treatment, the concentration of total PAHs decreased by 73% in planted sediments but only 25% in unplanted controls. Similarly, total PCBs declined by 60% in the presence of plants while none were removed in the unplanted sediment. Overall, the apparent PAH and PCB biodegradation was greatest in the sediment layer that contained most of the eelgrass roots. Abiotic desorption tests conducted at week 32 confirmed that the phytoremediation process was not controlled by mass-transfer or bioavailability limitations since all PAHs and PCBs desorbed rapidly and to a large extent from the sediment. PAHs were detected in both roots and shoots, with root and shoot bioaccumulation factors for total PAHs amounting to approximately 3 and 1, respectively, after 60 weeks of phytoremediation treatment. Similarly, the root bioaccumulation factor for total PCBs was around 4, while no PCBs were detected in the eelgrass leaves at the end of the experiment. The total mass fraction of PAHs and PCBs absorbed and translocated by plant biomass during the 60-week period was insignificant, amounting to less than 0.5% of the total mass of PAHs and PCBs which was initially present in the sediment. Finally, the number of total heterotrophic bacteria and hydrocarbon degraders was slightly but not statistically significantly greater in planted sediments than in unplanted controls. After ruling out contaminant loss to the water column or absorption and transformation within plant cells, it is most likely that the presence of eelgrass stimulated the microbial biodegradation of PAHs and PCBs in the rhizosphere by releasing root exudates, plant enzymes, or even oxygen. Additional research is needed to further elucidate these potential phytoremediation mechanisms.     

Effects of the herbicide atrazine on adenine nucleotide levels in Zostera marina L. (eelgrass)

Response of adenine nucleotides (ATP, ADP, AMP) and adenylate energy charge (EC) to atrazine, a triazine herbicide, was evaluated as an indicator of metabolic state in Zostera marina L. (eelgrass), a submerged marine angiosperm. Short-term (6 h) atrazine stress reduced ATP and total adenylates (AT) at both 10 and 100 ppb, but EC remained constant. Net productivity decreased at 100, but not at 10 ppb atrazine over 6 h. Long-term (21 day) atrazine stress was evidenced by growth inhibition and 50% mortality near 100 ppb. EC was reduced at 0.1, 1.0 and 10 ppb atrazine, but ATP and EC increased with physiological response to severe stress (100 ppb) after 21 days. Apparently, ATP and AT decrease over the short-term but rebound over the long-term with severe atrazine stress, increasing beyond control levels before plant death results. Supplementing adenine nucleotide and EC results with more conventional quantitative analyses should afford greater knowledge of physiological response to environmental variation.     

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.     

Toxicity of reduced nitrogen in eelgrass (Zostera marina) is highly dependent on shoot density and pH

In sheltered, eutrophicated estuaries, reduced nitrogen (NHx), and pH levels in the water layer can be greatly enhanced. In laboratory experiments, we studied the interactive effects of NHx, pH, and shoot density on the physiology and survival of eelgrass (Zostera marina). We tested long-term tolerance to NHx at pH 8 in a 5-week experiment. Short-term tolerance was tested for two shoot densities at both pH 8 and 9 in a 5-day experiment. At pH 8, eelgrass accumulated nitrogen as free amino acids when exposed to high loads of NHx, but showed no signs of necrosis. Low shoot density treatments became necrotic within days when exposed to NHx at pH 9. Increased NH3 intrusion and carbon limitation seemed to be the cause of this, as intracellular NHx could no longer be assimilated. Remarkably, experiments with high shoot densities at pH 9 showed hardly any necrosis, as the plants seemed to be able to alleviate the toxic effects of high NHx loads through joint NHx uptake. Our results suggest that NHx toxicity can be important in worldwide observed seagrass mass mortalities. We argue that the mitigating effect of high seagrass biomass on NHx toxicity is a positive feedback mechanism, potentially leading to alternative stable states in field conditions.     

Anthesis and seed production in Zostera marina L. (eelgrass) from the chesepeak by

Anthesis and seed production in Zostera marina L. were studied in three areas of the Chesapeake Bay from January to June 1980. Inflorescence primordia with distinguishable anthers and pistils were first observed in February when water temperature was 3°C. Development of the reproductive shoots in the field continued after February as water temperature rose, with the first evidence of pollen release in mid-April (water temperature 14.3°C). Stigmata loss was first observed in samples taken in late April at all locations by as water temperatures averaged above 16°C. Pollination was complete at all locations by 19 May and anthers were no longer present. Few reproduction shoots were found on 3–5 June and seed release was assumed to be complete by this time (water temperature 25°C). The density of flowering shoots ranged from 11 to 19% of the total number of shoots, producing an estimated 8127 seeds m^?2.Comparison of flowering events with other areas along a latitudinal gradient from North Carolina to Canada indicated that reproductive events occurred earlier in the most southern locations and at successively later dates with increasing latitude.     

Means of rapid eelgrass (Zostera marina L.) recolonisation in former dieback areas

Recolonisation of eelgrass (Zostera marina L.) was studied in a Danish estuary during summer 2001 following an anoxia event the previous summer. Leaf bundles had detached from the rhizomes while healthy-looking roots and rhizomes remained in the sediment. We hypothesise that the stabilising effect of remaining belowground biomass, the presence of rhizomes with buds, the presence of a large seed bank and the potential surviving shoots from the previous population may stimulate and speed up recovery in previously colonised areas compared to bare areas.A large seed bank containing more than 11,000 seeds m⁻² was found in the dieback area. Seeds were found in the upper 14 cm of the sediment but judging from the length of the hypocotyle of the seedlings, only seeds from the upper 5.5 cm of sediment germinated successfully. The upper 5.5 cm represented a seed pool of approximately 1000 seeds m⁻². Germination of these seeds was the primary mode of recolonisation in the estuary, since 96% of the plants in the investigated plot were seedlings. Only 4% of the plants were survivors from the previous year.Although densities of seedlings may exceed densities of surviving shoots, we argue that plants surviving oxygen depletion may still contribute considerably to the recolonisation of a former dieback area as these plants have faster elongation and branching rates and lower mortality rates relative to seedlings.There was no indication of recolonisation from dormant buds on rhizomes. This finding was confirmed in laboratory experiments where buds failed to germinate in the absence of the apical shoot. We examined the structure and ageing of buds and found general withering with age, indicating that buds should germinate shortly after the dieback if at all. Our results, therefore, suggest that rhizome buds are not dormant buds but simply side shoots that have failed to grow.     

Population structure and genetic diversity among eelgrass (Zostera marina) beds and depths in San Francisco Bay

The seagrass Zostera marina is widely distributed in coastal regions throughout much of the northern hemisphere, forms the foundation of an important ecological habitat, and is suffering population declines. Studies in the Atlantic and Pacific oceans indicate that the degree of population genetic differentiation is location dependent. San Francisco Bay, California, USA, is a high-current, high-wind environment where rafting of seed-bearing shoots has the potential to enhance genetic connectivity among Z. marina populations. We tested Z. marina from six locations, including one annual population, within the bay to assess population differentiation and to compare levels of within-population genetic diversity. Using 7 microsatellite loci, we found significant differentiation among all populations. The annual population had significantly higher clonal diversity than the others but showed no detectible differences in heterozygosity or allelic richness. There appears to be sufficient input of genetic variation through sexual reproduction or immigration into the perennial populations to prevent significant declines in the number and frequency of alleles. In additional depth comparisons, we found differentiation among deep and shallow portions in 1 of 3 beds evaluated. Genetic drift, sweepstakes recruitment, dispersal limitation, and possibly natural selection may have combined to produce genetic differentiation over a spatial scale of 3-30 km in Z. marina. This implies that the scale of genetic differentiation may be smaller than expected for seagrasses in other locations too. We suggest that populations in close proximity may not be interchangeable for use as restoration material.     

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.     

Top-down impact through a bottom-up mechanism: the effect of limpet grazing on growth,productivity and carbon allocation of Zostera marina L. (eelgrass)

The unusual appearance of a commensal eelgrass limpet [Tectura depicta (Berry)] from southern California at high density (up to 10 shoot^–1) has coincided with the catastrophic decline of a subtidal Zostera marina L. meadow in Monterey Bay, California. Some commensal limpets graze the chloroplast-rich epidermis of eelgrass leaves, but were not known to affect seagrass growth or productivity. We evaluated the effect on eelgrass productivity of grazing by limpets maintained at natural densities (8±2 shoot^–1) in a natural light mesocosm for 45 days. Growth rates, carbon reserves, root proliferation and net photosynthesis of grazed plants were 50–80% below those of ungrazed plants, but biomass-specific respiration was unaffected. The daily period of irradiance-saturated photosynthesis (H _sat) needed to maintain positive carbon balance in grazed plants approached 13.5 h, compared with 5–6 h for ungrazed plants. The amount of carbon allocated to roots of ungrazed plants was 800% higher than for grazed plants. By grazing the chlorophyll-rich epidermis, T. depicta induced carbon limitation in eelgrass growing in an other-wise light-replete environment. Continued northward movement of T. depicta, may have significant impacts on eelgrass production and population dynamics in the northeast Pacific, even thought this limpet consumes very little plant biomass. This interaction is a dramatic example of top-down control (grazing/predation) of eelgrass productivity and survival operating via a bottom-up mechanism (photosynthesis limitation).     

Seed production from the mixed mating system of Chesapeake Bay (USA) eelgrass (Zostera marina; Zosteraceae)

In monoecious plants, gametes can be exchanged in three ways: among unrelated genets (outbreeding), with close relatives (inbreeding), or within individuals (geitonogamous selfing). These different mating systems may have consequences for population demography and fitness. The experiment presented herein used artificial crosses to examine the mating system of Chesapeake Bay, Virginia, USA eelgrass (Zostera marina L; Zosteraceae), a bisexual submerged aquatic plant that can outbreed, inbreed, and self. Genetic data indicate severe heterozygosity deficiencies and patchy genotype distribution in these beds, suggesting that plants therein reproduce primarily by vegetative propagation, autogamy, or geitonogamy. To clarify eelgrass reproductive strategies, flowers from three genetically and geographically distinct beds were hand-pollinated in outbred, inbred, and selfed matings. Fertilization success and seed production, life history stages which contribute greatly to the numeric maintenance of populations, were monitored. We found no evidence that inbreeding had negative consequences for seed production. On the contrary, selfed matings produced seeds significantly more frequently than outcrossed matings and produced significantly larger numbers of seeds than either inbred or outbred matings. These results contrast with patterns for eelgrass in other regions but might be expected for similar populations in which pollen limitation or a short reproductive season renders selfing advantageous.     

The response of interstitial ammonium in eelgrass (Zostera marina L.) beds to environmental perturbations

Natural and human-induced perturbations of eelgrass (Zostera marina L.) beds were used to examine the interaction between the sediment interstitial ammonium pool and nitrogen uptake by the plants. Eelgrass colonization of unvegetated areas was accompanied by a substantial decrease in the interstitial ammonium pool over a 4-yr period. The changes in interstitial ammonium and shoot density during colonization support an already determined relationship between shoot density and ammonium pool measurements. In field perturbation experiments, removing eelgrass leaves and sealing the sediment surface altered the flux of ammonium from the interstitial ammonium pool, and resulted in a rapid increase in interstitial ammonium concentrations. Measurements of ammonium accumulation under the various perturbation conditions and a control permitted calculation of the sediment ammonium flux. These estimates include uptake by eelgrass roots, regeneration in the root zone, and diffusion from the sediments. Nitrogen limitation was observed in some eelgrass beds.     

A simulation model of production,seasonal changes in biomass and distribution of eelgrass (Zostera marina) in Lake Grevelingen

Summary A simulation model has been described, based on data from Lake Grevelingen, The Netherlands, as a predictive tool for lake management. The model has been developed as part of a large-scale aquatic modelling effort in Lake Grevelingen, carried out in close cooperation with the Delft Hydraulics Laboratory and the Delta Department, Environmental Research Division of the Ministry of Transport and Public Works. Available data on growth rates per unit eelgrass biomass, obtained with the leaf-marking technique, and on above- en below ground biomass and shoot density changes per unit area have been used. A space limitation depending on density of the above ground biomass and a growth limitation due to shortage in below ground biomass have been introduced. The seasonal changes in eelgrass production, both above and below ground, have been simulated as functions of the external forcing variables light, water temperature, wind generated water movements and of the internal control variables due to aging of the plant material. The vertical distribution of eelgrass can be partly explained from the modelling results on space, light and below ground biomass limitations. From the shore down to about 1 m waterdepth the above ground eelgrass biomass suffers from space limitation. Between 1 and 2 m production and biomass reach maximum values. Between 2 and 3 m waterdepth the above ground eelgrass growth is limited by the availability of below ground biomass. Between 3 and 5 m waterdepth both below ground biomass and light are the growth-limiting factors. Below 5 m waterdepth light is not sufficient to sustain net growth of eelgrass from rhizomes. Together with additional data — not used in the model — on seed production and growth of eelgrass shoots from seeds the vertical and horizontal distribution of the dominant macrophyte in the lake can be explained.     

Glutamine synthetase activity and free amino acid pools of eelgrass (Zostera marina L.) roots

Activity of the enzyme glutamine synthetase (GS, EC 6.3.1.2) was determined in vitro for roots of the marine angiosperm Zostera marina L. (eelgrass) collected from a population in Great Harbor, Woods Hole, Massachusetts, U.S.A. The GS synthetase activity was lowest in roots of plants collected from the shallow region of the eelgrass bed (12.0 μmol·g⁻¹ (fresh wt)· h⁻¹) and increased in the mid (3.0 m, 40.3 μmol·g⁻¹ (fresh wt)·h⁻¹) and deep (5.0 m, 72.3 μmol·g⁻¹ (fresh wt)·h⁻¹) plant collection depths. GS transferase activity increased with collection depth in a similar manner: shallow, 28.6 μmol·g⁻¹ (fresh wt)·h⁻¹; mid, 52.0 μmol·g⁻¹ (fresh wt)·h⁻¹; deep, 92.8 μmol·g⁻¹ (fresh wt)·h⁻¹. When sediment-embedded plants were held in continuous darkness for 2 days to create extended root anoxia, root GS activities nearly doubled. In contrast, in vivo incorporation of ¹⁴C-glutamate into glutamine and protein residue remained constant or declined under short-term hypoxia and anoxia. During aerobic recovery from anoxia, root labelling of glutamine and protein increased markedly. Free amino acid patterns of eelgrass roots growing in situ were determined over a diurnal cycle. Total free amino acid content was maximal at dawn and decreased 50% by noon. In contrast, the proportion of glutamine was lowest at dawn and maximal at noon for both shallow and deep-growing plants. Despite differences in depth-specific plant sizes, root/rhizome/shoot ratios, and relative growth rates, the daily whole plant nitrogen demand of shallow and deep growing plants were equivalent. When corrected for assay temperature response, the enzyme synthetase activities measured in vitro suggest that all of the plant nitrogen assimilation requirements can be met within daylight hours during the period of peak summer biomass.     

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