Marina Shoots Research Articles

Multiple paternity, fertilization success, and male quality: Mating system variation in the eelgrass, Zostera marina.

Genetic diversity can modulate a population's response to a changing environment and plays a critical role in its ecological function. While multiple processes act to maintain genetic diversity, sexual reproduction remains the primary driving force. Eelgrass (Zostera marina) is an important habitat-forming species found in temperate coastal ecosystems across the globe. Recent increases in sea surface temperatures have resulted in shifts to a mixed-annual life-history strategy (i.e., displaying characteristics of both annual and perennial meadows) at its southern edge-of-range. Given that mating systems are intimately linked to standing levels of genetic variation, understanding the scope of sexual reproduction can illuminate the processes that shape genetic diversity. To characterize edge-of-range eelgrass mating systems, developing seeds on flowering Z. marina shoots were genotyped from three meadows in Topsail, North Carolina. In all meadows, levels of multiple mating were high, with shoots pollinated by an average of eight sires (range: 3-16). The number of fertilized seeds (i.e., reproductive success) varied significantly across sires (range: 1-25) and was positively correlated with both individual heterozygosity and self-fertilization. Outcrossing rates were high (approx. 70%) and varied across spathes. No clones were detected, and kinship among sampled flowering shoots was low, supporting observed patterns of reproductive output. Given the role that genetic diversity plays in enhancing resistance to and resilience from ecological disturbance, disentangling the links between life history, sexual reproduction, and genetic variation will aid in informing the management and conservation of this key foundation species.

Invasive macroalgae in native seagrass beds: vectors of spread and impacts.

Worldwide, invasive species are spreading through marine systems at an unprecedented rate with both positive and negative consequences for ecosystems and the biological functioning of organisms. Human activities from shipping to habitat damage and modification are known vectors of spread, although biological interactions including epibiosis are increasingly recognized as potentially important to introduction into susceptible habitats. We assessed a novel mechanism of spread - limpets as transporters of an invasive alga, Sargassum muticum, into beds of the seagrass Zostera marina - and the physiological impact of its invasion. The association of S. muticum with three limpet species and other habitats was assessed using intertidal surveys on rocky shores and snorkelling at two seagrass sites in the UK. A 4-year field study tested the effect of S. muticum on Z. marina shoot density, dry weight and phenolic compounds (caffeic and tannic acid) content, and a laboratory experiment tested the impact of S. muticum on nutrient partitioning (C/H/N/P/Si), photosynthetic efficiency (Fv/Fm) and growth of Z. marina. On rocky shores 15 % of S. muticum occurrences were attached to the shells of live limpets. In seagrass beds 5 % of S. muticum occurrences were attached to the shells of dead limpets. The remainder were attached to rock, to cobblestones, to the seagrass matrix or embedded within the sand. Z. marina density and phenolics content was lower when S. muticum co-occurred with it. Over 3 years, photosynthetic responses of Z. marina to S. muticum were idiosyncratic, and S. muticum had no effect on nutrient partitioning in Z. marina. Our results show limpets support S. muticum as an epibiont and may act as a previously unreported transport mechanism introducing invaders into sensitive habitats. S. muticum reduced production of phenolics in Z. marina, which may weaken its defensive capabilities and facilitate proliferation of S. muticum. The effect of S. muticum on Z. marina photosynthesis requires further work but having no effect on the capacity of Z. marina to sequester nutrients suggests a degree of resilience to this invader.

The influence of increased seawater HCO3− concentration on the survival, growth and physiology of the eelgrass Zostera marina

Inorganic resources are a key determining factor in the growth process of seagrass; however, the process and mechanisms involved in the promoting effect of increased seawater HCO3− concentration on seagrass growth and propagation under controlled conditions are poorly understood. We subjected shoots of the eelgrass Zostera marina to different seawater HCO3− levels [2 (control), 5, 10, 15 and 20 mmol L−1] for 6 weeks under controlled, mesocosm conditions. We measured eelgrass response in terms of survivorship, growth, propagation, productivity, carbonic anhydrase, leaf pigmentation and carbohydrate concentrations. The ramet frequency of Z. marina reached 58.7% when exposed to 5 mmol L−1, which was 1.3 times higher than that of shoots under the control condition. Survival analysis combined with growth assessment suggested that the optimal range of seawater HCO3− concentration for the propagation of Z. marina shoots was 4.8–5.5 mmol L−1. The promotion of survival and propagation of Z. marina that stemmed from seawater HCO3− concentration mainly depended on the increase in chlorophyll content and accumulation of nonstructural carbohydrate. The total chlorophyll content of Z. marina leaves exposed to 5 mmol L−1 was 1.1 times higher than that of shoots under the control condition. The results provide valuable data that could prove helpful in the development of efficient artificial propagation technology for Z. marina shoots.

The combined effect of seawater salinity and duration on the survival and growth of eelgrass Zostera marina

Abrupt changes in salinity are considered a key cause of seagrass bed degradation. In the study, we first quantified the effects of different combinations of salinity and duration on the responses of Zostera marina shoots in terms of survivorship, morphology, growth and physiology. The recovery potential (recovery of shoots from salinity stress to be returned to original salinity level) of Z. marina shoots was assessed. The LT50 (lethal time that caused an increase in mortality to 50 % of that of the control) and EC50 (effect concentration, i.e., salinity level that caused a decrease in growth to 50 % of that of the control) were calculated to reveal the quantitative relationship between salinity and duration that resulted in limiting effects on the survival and growth of Z. marina shoots. Z. marina shoots were exposed to different combinations of water salinity [20, 25, 30 (control), 35, 40, and 45 PSU] and duration (5, 10, 15 and 20 days), and then the shoots were transferred to the control condition for 30 days under laboratory conditions. The results showed that the survival rate of shoots at the end of recovery was significantly lower than those of shoots at the end of direct salinity stress at the hypersalinity levels of 40 and 45 PSU, indicating that short-term periods of increase in water salinity would lead to long-term effects on the survival of Z. marina shoots. The total leaf area of shoots at the end of recovery was significantly higher than those of shoots at the end of direct salinity stress at salinity levels of 25–35 PSU, indicating a significant recovery potential following the salinity stress. However, a poor recovery potential was found under combinations of hypersalinity (40–45 PSU) and duration of 20 days. Regression analysis revealed that the relationship between salinity and duration was a strong power function that could limit the growth of Z. marina. Pearson correlation analysis showed that the survival and growth of Z. marina shoots exposed to different salinity levels were significantly correlated with leaf soluble sugar contents when the duration was higher than 5 days. This study will further enhance our understanding of seagrass bed degeneration induced by abrupt changes in salinity.

Predator density, not structure, influences intraspecific competition in the mud crab Dyspanopeus sayi

Structured habitats can offer prey refuge by reducing the encounter rates between predators and prey. However, structured habitats can also reduce predator-predator encounters, thereby dampening intraspecific interactions and increasing overall prey consumption. In this study, we tested whether intraspecific competition among Sayi mud crabs Dyspanopeus sayi foraging on mussel Mytilus edulis prey was modified by the density of mimicked eelgrass Zostera marina shoots. Predation was measured in 3 predator densities (1, 3, or 6 predators) and 1 of 5 levels of shoot density. We found no impact of Z. marina on intraspecific competition, regardless of predator density. Intraspecific competition was weak; per capita crab consumption decreased as predator density increased and became significantly reduced when predator density increased from 1 to 6. However, there was no evidence of non-additive predator density effects when predator abundance doubled from 3 to 6 predators. Our results also indicate that eelgrass structure may not offer the sessile prey M. edulis refuge when one or more D. sayi predators are present. Further exploration is needed to understand whether the lack of a structure effect is due to our use of sessile rather than mobile prey as used in other studies. Predator density and not structural density influenced intraspecific competition among D. sayi predators when they were foraging for M. edulis in mimicked Z. marina shoots.

The combined effect of planting density and sediment fertilization on survival, growth and physiology of eelgrass Zostera marina.

We subjected shoots of eelgrass Zostera marina to different combinations of planting density [300 (control), 450, 600, 750, 900 shoots m-2] and sediment fertilization [0 (control), 35, 70, 105, 140gm-2] for 6weeks under controlled conditions. We measured eelgrass responses in terms of survivorship, growth, productivity, leaf pigmentation and carbohydrate concentrations. The ramet frequency of Z. marina reached 47.8% when exposed to the combination of 600 shoots m-2 and 70gm-2, which was 3.2 times higher than that of shoots under the control. Survival analysis combined with growth assessment suggested that the optimal ranges of planting density and sediment fertilization for the propagation of Z. marina shoots were 501 to 530 shoots m-2 and 51 to 60gm-2, respectively. The promotion of survival and propagation of Z. marina that stemmed from planting density and sediment fertilization mainly depended on the increase of chlorophyll content and accumulation of non-structural carbohydrate. The total chlorophyll content of Z. marina leaves exposed to the combination of 600 shoots m-2 and 70gm-2 was 2.1 times higher than that of shoots under the control. The results will provide data that could prove helpful in the development of efficient artificial propagation technology for Z. marina shoots.

The influence of decreased salinity levels on the survival, growth and physiology of eelgrass Zostera marina

Low salinity generally promotes the growth and propagation of temperate seagrasses, but the appropriate range is unclear. We subjected shoots of eelgrass Zostera marina to different salinity levels [10, 15, 20, 25, 30 PSU (control)] for 6 weeks under controlled laboratory conditions. We measured eelgrass responses in terms of survivorship, growth, productivity, leaf pigmentation and carbohydrate concentrations. Survival analysis combined with growth assessment suggested that the optimal salinity range for the propagation of Z. marina shoots was 18–21 PSU. Structural equation model (SEM) analysis indicated that the promotion effect of decreased salinity levels on the survival and growth of Z. marina shoots mainly depended on the increase in chlorophyll content and the accumulation and synthesis of nonstructural carbohydrates. The carotenoid content and soluble sugar content of the aboveground tissues of Z. marina shoots exposed to 20 PSU were 1.1 and 1.6 times higher than those of shoots under the control, respectively. The results will provide valuable data that could prove helpful in the development of efficient artificial propagation technology for Z. marina shoots.

Carbon limitation in response to nutrient loading in an eelgrass mesocosm: influence of water residence time.

Altered primary productivity associated with eutrophication impacts not only ecosystem structure but also the biogeochemical cycling of oxygen and carbon. We conducted laboratory experiments to empirically determine how residence time (1, 3, 10 d) influences eutrophication responses in a simplified Pacific Northwest Zostera marina-green macroalgal community. We expected long-residence time (RT) systems to exhibit eutrophication impairments. Instead, we observed an accumulation of nutrients at all RTs and a shift in the dissolved inorganic carbon speciation away from CO2 (aq) with unexpected consequences for eel grass plant condition, including shoot mortality. Most metrics responded more strongly to temperature treatments than to RT treatments. No dramatic shifts in the relative abundance of Z. marina and green macro algae were detected. Z. marina shoot density proliferated in cool temperatures (12°C) with a modest decline at 20°C. Eelgrass loss was associated with high total scale pH (pHT) and CO2 (aq) concentrations of <10 μmol kg-1 CO2 (aq), but not with high nutrients. Z. marina δ13C values support the hypo thesis that carbon availability was greater at short RT. Further, very low leaf sugar concentrations are consistent with extreme photosynthetic CO2 (aq) limitation. We suggest that the effects of extremely low environ mental car bon concentrations (CO2 (aq)) and increased respiration at warm temperatures (20°C) and other physiological processes can lead to internal carbon limitation and shoot mortality. Eutrophication responses to nutrient loading are more nuanced than just light limitation of eelgrass and require additional research on the interaction of the biogeochemical environment and plant physiology to better understand estuarine ecosystem disruption.

Comparative study among Avicennia marina, Phragmites australis, and Moringa oleifera based ethanolic-extracts for their antimicrobial, antioxidant, and cytotoxic activities.

Microbial resistance and other emerging health risk problems related to the side effects of synthetic drugs are the major factors that result in the research regarding natural products. Fruits, leaves, seeds, and oils-based phyto-constituents are the most important source of pharmaceutical products. Plant extract chemistry depends largely on species, plant components, solvent utilized, and extraction technique. This study was aimed to compare the ethanolic extracts of a mangrove plant, i.e., Avicennia marina (1E: Lower half of A. marina‘s pneumatophores, 2E: A. marina‘s leaves, 3E: Upper half of A. marina‘s pneumatophores, and 4E: A. marina‘s shoots), with non-mangrove plants, i.e., Phragmites australis (5E: P. australis‘s shoot), and Moringa oleifera (6E: M. oleifera‘s leaves) for their antimicrobial activities, total phenolic contents, antioxidant activity, and cytotoxicity potential. The antimicrobial activity assays were performed on gram-positive bacteria (i.e., Bacillus subtilis and Staphylococcus aureus), gram-negative bacteria (i.e., Escherichia coli, and Pseudomonas aeruginosa), and fungi (i.e., Aspergillus niger, Candida albicans, and Rhizopus spp.). We estimated antioxidant activity by TAC, DPPH, and FRAP assays, and the cytotoxicity was evaluated by MTT assay. The results of antimicrobial activities revealed that B. subtilis was the most sensitive to the tested plant extracts compared to S. aureus, while it only showed sensitivity to 6E and Imipenem. 5E and 6E showed statistically similar results against P. aeruginosa as compared to Ceftazidime. E. coli was the most resistant bacteria against tested plant extracts. Among the tested plant extracts, maximum inhibition activity was observed by 6E against A. niger (22 ± 0.57 mm), which was statistically similar to the response of 6E against C. albicans and 3E against Rhizopus spp. 2E did not show any activity against tested fungi. We found that 6E (208.54 ± 1.92 mg g−1) contains maximum phenolic contents followed by 1E (159.42 ± 3.22 mg g−1), 5E (131.08 ± 3.10 mg g−1), 4E (i.e., 72.41 ± 2.96 mg g−1), 3E (67.41 ± 1.68 mg g−1), and 2E (48.72 ± 1.71 mg g−1). The results depict a significant positive correlation between the phenolic contents and the antioxidant activities. As a result, phenolic content may be a natural antioxidant source.

Effect of artificial seagrass on hydrodynamic thresholds for the early establishment of Zostera marina

Seagrass meadows have disappeared on many coastal sections due to anthropogenic disturbances, diseases, and/or eutrophication. To facilitate informed seagrass restoration, we i) quantified the hydrodynamic dislodgement thresholds for newly transplanted Z. marina shoots, and ii) tested the effect of artificial seagrass (ASG) as a hydrodynamic protection measure. Experiments were carried out by planting Z. marina rhizomes with living shoots into a sediment bed and exposing them to a range of wave and current conditions in a flume. The use of ASG significantly reduced wave height, as well as current velocity. The applied waves led to the development of ripples whereas currents led to erosion of the sediment bed. The number of shoots that were uprooted and dislodged increased with increasing bed shear stress and erosion. By reducing bed shear stress, the ASG raised the input current velocity threshold, which the transplanted shoots were able to withstand. The present study offers insight into the effect of artificial seagrass (ASG) on wave and current attenuation, as well as sediment erosion and shoot dislodgement. Our results help to inform the setting of hydrodynamic thresholds for the early establishment of Z. marina and to define the improvement of hydrodynamic conditions by ASG.

In situ Responses of the Eelgrass Zostera marina L. to Water Depth and Light Availability in the Context of Increasing Coastal Water Turbidity: Implications for Conservation and Restoration.

Accelerating losses of seagrass meadows has led to efforts to restore these highly productive and beneficial ecosystems globally. Depth and light availability are critical determinants of seagrass restoration success. Eelgrass (Zostera marina L.) is the dominant seagrass species in the temperate northern hemisphere, but its global distribution has reduced dramatically. The main aims of this study were to determine: (1) the depth limit for Z. marina survival in Ailian Bay, north China, and (2) how light availability affects the growth and recruitment of Z. marina as a basis for identifying a suitable depth range for successful restoration. To achieve these aims, Z. marina shoots were transplanted from a nearby donor site, Swan Lake, to an experimental site, Ailian Bay, and the temporal responses of Z. marina shoots to light availability at water depths ranging from 1 to 8 m were investigated using in situ suspended cultures. Four suspended shoot transplantation experiments were conducted in 4 years. The results showed that the transplanted Z. marina shoots could survive and branch during an annual growth cycle, permanently underwater, at a depth ≤3 m. Due to the local turbidity of the waters in Ailian Bay, a depth of 4 m led to sufficient light deprivation (reduced to 6.48–10.08% of surface irradiance) to negatively affect seagrass shoot density and clonal reproduction. In addition, reproductive shoot density also tended to decline with water depth and light deprivation. Our results indicated that Z. marina population recruitment, through sexual and asexual (clonal growth) reproduction, were negatively affected by increasing water depth and light deprivation. These findings may provide a suitable depth range for the successful restoration of Z. marina in local coastal waters. They may also be applied to the management and restoration of Z. marina globally.

Factors controlling the vertical zonation of the intertidal seagrass, Zostera japonica in its native range in the northwestern Pacific

The desiccation tolerance of the intertidal seagrass Zostera japonica has been demonstrated in a number of studies; however, the factors limiting expansion of intertidal seagrass species into subtidal zones remain controversial. We transplanted Z. japonica shoots from the intermediate intertidal zone into the plots with and without Z. marina shoots in both the lower intertidal and shallow subtidal zones to investigate the factors controlling Z. japonica growth in these zones. Daily photon flux density at the Z. japonica canopy level was attenuated by both water depth and coexisting Z. marina shoots but more strongly by Z. marina shoots than water depth in the transplant plots. The shoot density and biomass of Z. japonica transplants were significantly lower in transplant plots in the subtidal zone than in the lower intertidal zone. Although the photon flux density was significantly lower in transplant plots containing Z. marina shoots, the growth of Z. japonica transplants did not differ significantly between plots with and those without Z. marina shoots. Z. japonica transplants exhibited photoacclimatory responses such as increased shoot height and chlorophyll content under the lower-light conditions, offsetting the reduced light availability so that no significant differences in transplant growth occurred between plots with and those without Z. marina shoots. As the growth of Z. japonica transplants decreased significantly in the subtidal zone, the interactive effects of environmental stresses associated with tidal inundation and reduced light availability may restrict penetration of the intertidal seagrass Z. japonica into the subtidal zone. The persistence of high photosynthetic performance after air exposure and a regular arrangement of the densely overlapped leaves atop wet sediments may be desiccation tolerance mechanisms for Z. japonica in the intertidal zone.

Heavy metal accumulation and phytoremediation potential by transplants of the seagrass Zostera marina in the polluted bay systems.

Although seagrasses can incorporate heavy metals from the marine environment, few studies have been conducted on heavy metal uptake and phytoremediation potential by seagrass transplants in the heavy metal contaminated sediments. Zostera marina shoots were transplanted in two polluted bay systems on Korean coasts to evaluate the heavy metal contaminations in sediments and the possibility of using Z. marina transplants as a bioindicator and phytoremediation agent. The major concentrated metals in sediments were As, Cu, Fe, and Pb in Jaran Bay, and Cd, Co, Zn, and Hg in Onsan Bay. The Co, Zn, Pb, and Hg concentrations in Z. marina tissues reflected the sediment heavy metal concentrations, and thus the tissue heavy metal concentrations may be used as bio-indicators of the metal contaminations. Since Z. marina transplants accumulated a great amount of heavy metals in their tissues, they may have the phytoremediation potential for the heavy metal contaminated sediments.

Influence of Water Temperature Anomalies on the Growth of Zostera marina Plants Held Under High and Low Irradiance Levels

Coastal water temperature anomalies are expected to intensify during the current period of global climate change. Sudden water temperature elevation (e.g., heat waves) or decrease (e.g., summer upwelling) can have a far-reaching influence on seagrasses in temperate regions. We examined the influence of anomalously elevated and decreased water temperatures on the growth of Zostera marina plants held under two different levels of irradiance. Core samples of wild Z. marina shoots were cultured under either suddenly elevated (from the in situ level of 20 to 27 °C during the fall experiment) or decreased (from the in situ level of 27 to 20 °C during the summer experiment) temperature. We included light as an experimental factor because the light requirements of seagrasses increase when plants are under thermal stress. The growth of Z. marina shoots under anomalously elevated water temperature conditions, which simulated marine heat waves, was significantly declined, whereas the plant growth under suddenly decreased water temperature conditions, which simulated summer upwelling events, was maintained or slightly increased. In the fall 2014 experimental period, Z. marina shoots under the suddenly elevated water temperature condition (27 °C) showed significant decreases in shoot density, biomass, and leaf productivity by ca. 40–80%, whereas plants held under the ambient water temperature condition (20 °C) maintained or slightly increased their growths. In the summer 2015 experimental period, Z. marina under the sudden temperature reduction conditions (20 °C) maintained relatively high shoot density and leaf productivity, whereas plants under the in situ high water temperature condition (27 °C) showed significantly reduced growths. The growth of Z. marina shoots, which were cultured in the optimal temperature condition (20 °C), was enhanced by increasing underwater irradiance. However, the adverse effects of high water temperature on Z. marina growth could not be reversed by increasing irradiance levels at the end of the experiments. According to the results of this study, the intensified sea surface water temperature anomalies induced by global climate change will alter the growth and distribution of temperate seagrasses and, consequently, the structure and function of the coastal seagrass ecosystems in the temperate region.

Effects of Zostera marina rhizosphere and leaf detritus on the concentration and distribution of pore-water sulfide in marine sediments

The relationship between seagrasses and pore-water hydrogen sulfide in the sediment is complex due to the opposing effects of the seagrass root system, which can reduce sulfide concentration by promoting its oxidation to sulfate, and the burial of organic matter from the plant itself, which can increase sulfide concentration by stimulating anaerobic sulfate reduction. Here, field samples of pore-water sulfide were collected in areas with and without Zostera marina shoots using Diffusive Gradients in Thin-Films (DGTs) to obtain two-dimensional visualizations of sulfide concentrations within the sediment in relation to location of eelgrass detritus and the rhizosphere. To decouple the effects of leaf detritus and the eelgrass rhizosphere on sulfide, manipulative laboratory studies were conducted over 4 weeks using 10 aquaria with or without eelgrass and different quantities of eelgrass detritus planted at 4 cm and 11 cm depths. Spatial distribution of pore-water sulfide was again measured via DGT. We found high spatial variation in pore-water sulfide concentration depending on the location analyzed relative to the rhizosphere or detritus. In the field study, pore-water sulfide concentration was greater in samples with eelgrass present, and the presence of leaf detritus accounted for higher than average sulfide concentrations in the sediment. In the laboratory experiment, sulfide concentrations localized around the leaf detritus increased with higher mass of added detritus, and sediment within the rhizosphere exhibited reduced sulfide concentrations. These results suggest that on the large scale the presence of eelgrass increases sulfide concentration, but on the smaller scale the pattern varies, with higher concentrations near buried detritus and lower concentrations near root tips.

Effects of habitat fragmentation on Zostera marina seed distribution

Habitat fragmentation is a process which can alter the spatial configuration and reduce the overall area of a habitat. This generally results in a degradation of habitat functioning. Fragmentation of seagrass (Zostera marina) beds has become increasingly common, and it may threaten the valuable ecosystem services they provide. Sexual reproduction through flowering and seed dispersal could contribute to the species’ potential resiliency by reducing its vulnerability to fragmentation. We investigated whether the proportion and density of flowering Z. marina shoots, and subsequently the density and distribution of seeds, differed between fragmented and continuous beds. Our results revealed that while flowering effort did not differ between the two bed types, seed density was significantly reduced in fragmented versus continuous beds. Further, seed distributions were altered in fragmented beds when compared to continuous beds, both within and directly outside the bed’s boundaries. Seagrass patch size positively influenced seed density, with lower seed densities in small patches. Fragmented beds consistently contained fewer seeds per-unit-area than continuous beds, regardless of bed seagrass area and flowering effort. Collectively, these results emphasize the vulnerability of Z. marina to habitat fragmentation by demonstrating a negative effect on seed density and an impact on seed distribution, which likely reduces the potential advantages of sexual reproduction for bed growth and resiliency to perturbations.

Seagrass wasting disease varies with salinity and depth in natural Zostera marina populations

In the 1930s the wasting disease pathogen Labyrinthula zosterae is believed to have killed 90% of the temperate seagrass Zostera marina in the Atlantic Ocean. Despite the devastating impact of this disease the host–pathogen interaction is still poorly understood, and few field studies have investigated factors correlating with the prevalence and abundance of L. zosterae. This study measures wasting disease in natural populations of Z. marina, showing a strong correlation between the disease and both salinity and water depth. No infection was detected in Z. marina shoots from low salinity (13–25 PSU) meadows, whereas most shoots carried the disease in high salinity (25–29 PSU). Shallow (1 m) living Z. marina shoots were also more infected compared to shoots in deeper (5 m) meadows. In addition, infection and transplantation experiments showed that Z. marina shoots from low salinity meadows with low pathogen pressure were more susceptible to L. zosterae infection. The higher susceptibility could not be explained by lower content of inhibitory defense compounds in the shoots. Instead, extracts from all Z. marina shoots significantly reduced pathogen growth, suggesting that Z. marina contains inhibitory compounds that function as a constitutive defense. Overall, the results show that seagrass wasting disease is common in natural Z. marina populations in the study area and that it increases with salinity and decreases with depth. Our findings also suggest that low salinity areas can act as a refuge against seagrass wasting disease.

Microtopography promotes coexistence of an invasive seagrass and its native congener

Environmental heterogeneity can promote coexistence, and is therefore predicted to increase invasibility of communities, but decrease invasion impacts. We examined the role of local-scale environmental heterogeneity in promoting the coexistence of the invasive seagrass, Zostera japonica, and its native congener Zostera marina in a patch mosaic. In its introduced range, Z. japonica often co-occurs with the native Z. marina in a patch mosaic associated with intertidal microtopography (centimeter to decimeter relief over meter to decameter distances). Here, Z. marina inhabits depressions that retain water during low tides, and Z. japonica inhabits well-drained mounds. Transplant experiments revealed that Z. marina suppressed Z. japonica shoot densities, more so in pools than on mounds. Z. marina suppressed Z. japonica above-ground and below-ground biomass by 47 and 19 % respectively, on mounds, and by over 60 % in pools. Z. marina shoot densities and biomass were 40 and 95 % lower, respectively, on mounds, regardless of Z. japonica presence. Topographic context remained the most influential predictor of Z. marina responses, even when we transplanted Z. marina into higher densities of Z. japonica. These results indicate that the native Z. marina is physiologically restricted from mounds and competitively excludes the introduced Z. japonica from pools. We provide empirical evidence of local-scale heterogeneity promoting coexistence of an invasive and a native macrophyte, supporting the hypothesis that environmental heterogeneity increases invasibility and decreases invasion impacts. Furthermore, active control of the invader in mixed beds is unlikely to benefit the native.

Assessing establishment success of Zostera marina transplants through measurements of shoot morphology and growth

Since significant seagrass declines have been reported worldwide, numerous seagrass restoration projects through transplantation have been attempted in recent decades. In this study, Zostera marina shoots were transplanted into Jindong Bay on the southern coast of Korea in November 2006 to assess establishment success of the transplants to a new transplant environment. Shoot density, individual shoot weight, productivity, and morphological characteristics of transplants and reference plants in the vicinity of the planting site were monitored monthly for 13 months. Although shoot size of transplants was smaller than that of reference plants at the start of transplantation, individual shoot weight, leaf width, shoot height, and rhizome diameter of transplants increased rapidly, reaching even higher values than those of reference plants 5 months after transplantation. These results suggest that eelgrass transplants established morphologically 5 months after transplantation. Shoot productivity of transplants was lower than that of reference population during the first 5–6 months following transplantation, but became higher than that of reference population 6 months after transplantation. The higher transplant productivity was likely due to the lower shoot density at the transplant site than that at the reference population. Rapid changes in shoot morphology and growth of transplants indicated that eelgrass transplants had great morphological plasticity and established successfully in the new environment within 5–6 months. In addition to survival rates of transplants, monitoring of shoot morphology and growth appeared to be an effective approach for accurate assessment of the establishment success of eelgrass transplant.

Experimental manipulation of sediment organic content and water column aeration reduces Zostera marina (eelgrass) growth and survival

The effect of synergy between sediment organic enrichment and lack of night oxygen renewal in the water column on growth and survival of Zostera marina, and how it is reflected in the sulfur parameters in the plants ( δ 34 S, TS and S 0) was studied experimentally. An experiment consisting of Z. marina mesocosms with different levels of organic enrichment and water column aeration was established, and the effects on sediment conditions, sulfide invasion and growth and survival of Z. marina were examined over a 4 week period. Shoots growing in Ambient Organic matter–sediments showed signs of sulfide invasion, as TS increased in all plant compartments and δ 34 S of the plant tissues decreased during the experiment, but neither growth rate nor survival were significantly affected. The lack of night oxygen renewal had no evident effects in non-enriched sediments as porewater sulfide concentrations, AVS- and CRS-pools were not different from the corresponding 24 h aeration treatment. Plant growth rate and survival were neither different from the corresponding 24 h aeration treatment. On the contrary, shoots growing on High Organic matter-sediments suffered a massive sulfide invasion and it was directly correlated to the observed decrease in growth rates. Even though the lack of night oxygen renewal had no evident effects on sediment variables there were, however, strong indications that the different aeration levels affected plant performance, suggesting a lower sulfide oxidation capacity and confirming that low water column oxygen concentrations reduces the defense capacity of the shoots against sulfide invasion. Although δ 34 S, TS and S 0 concentrations together provided a powerful set of indicators to detect the invasion of sulfide in Z. marina shoots, this study enlightens the need for a deeper investigation of sulfide intrusion in seagrasses and the relation between plant sulfur parameters and sediment conditions.