Community Structure and Abundance of Archaea in a Zostera marina Meadow: A Comparison between Seagrass-Colonized and Bare Sediment Sites.

Seagrass colonization alters sediment physicochemical properties by depositing seagrass fibers and releasing organic carbon and oxygen from the roots. How this seagrass colonization-induced spatial heterogeneity affects archaeal community structure and abundance remains unclear. In this study, we investigated archaeal abundance, diversity, and composition in both vegetated and adjacent bare surface sediments of a Zostera marina meadow. High-throughput sequencing of 16S rDNA showed that Woesearchaeota, Bathyarchaeota, and Thaumarchaeota were the most abundant phyla across all samples, accounting for approximately 42%, 21%, and 17% of the total archaeal communities, respectively. In terms of relative abundance, Woesearchaeota and Bathyarchaeota were not significantly different between these two niches; however, specific subclades (Woese-3, Woese-21, Bathy-6, Bathy-18) were significantly enriched in vegetated sediments (P < 0.05), while Thaumarchaeota was favored in unvegetated sites (P = 0.02). The quantification of archaeal 16S rRNA genes showed that the absolute abundance of the whole archaeal community, Bathyarchaeota, and Woese-3, Woese-10, Woese-13, and Woese-21 was significantly more abundant in vegetated sediments than in bare sediments (P < 0.05). Our study expands the available knowledge of the distribution patterns and niche preferences of archaea in seagrass systems, especially for the different subclades of Woesearchaeota and Bathyarchaeota, in terms of both relative proportions and absolute quantities.

Structural complexity and fish body size interactively affect habitat optimality.

Predator-prey interactions are strongly influenced by habitat structure, particularly in coastal marine habitats such as seagrasses in which structural complexity (SC) may vary over small spatial scales. For seagrass mesopredators such as juvenile fishes, optimality models predict that fitness will be maximized at levels of SC that enhance foraging but minimize predation risk, both of which are functions of body size. We tested the hypothesis that in eelgrass (Zostera marina) habitat, optimal SC for juvenile giant kelpfish (Heterostichus rostratus), an abundant eelgrass mesopredator in southern California, changes through ontogeny. To do this, we quantified eelgrass SC effects on habitat associations, relative predation risk, and foraging efficiency for three size classes of juvenile giant kelpfish. We found that habitat selection differed with fish size: small fish selected dense eelgrass, whereas larger fish selected sparse eelgrass. Small kelpfish experienced the lowest relative predation risk in dense eelgrass but also had higher foraging efficiency in dense eelgrass, suggesting that dense eelgrass is selected by these fish because it minimizes risk and maximizes potential for growth. Surprisingly, larger kelpfish did not experience lower predation risk than small kelpfish. However, larger kelpfish experienced higher foraging efficiency in sparse eelgrass vs. dense eelgrass, suggesting that they select sparse eelgrass to maximize foraging efficiency. Our study highlights that trade-offs between predation risk and foraging can occur within a single habitat type, that studies should consider how habitat value changes through ontogeny, and that seagrass habitat value may be maximal when within-patch variability in SC is high.

The relative importance of trait vs. genetic differentiation for the outcome of interactions among plant genotypes.

Functional trait differences and genetic distance are increasingly used as metrics to predict the. outcome of species interactions and the maintenance of diversity. We apply these ideas to intraspecific diversity for the seagrass Zostera marina (eelgrass), by explicitly testing the influence of trait distance and genetic relatedness on the outcome of pairwise interactions among eelgrass genotypes. Increasing trait distance (but not relatedness) between eelgrass genotypes decreased the likelihood that both would persist over a year-long field experiment, contrary to our expectations based on niche partitioning. In plots in which one genotype excluded another, the biomass and growth of the remaining genotype increased with the trait distance and genetic relatedness of the initial pair, presumably due to a legacy of past interactions. Together these results suggest that sustained competition among functionally similar genotypes did not produce a clear winner, but rapid exclusion occurred among genotypes with distinct trait combinations. Borrowing from coexistence theory, we argue that fitness differences between genotypes with distinct traits overwhelmed any stabilizing effects of niche differentiation. Previously observed effects of eelgrass genetic diversity on performance may rely on nonadditive interactions among multiple genotypes or sufficient environmental heterogeneity to increase stabilizing forces and/or interactions.

Recovery of a top predator mediates negative eutrophic effects on seagrass.

A fundamental goal of the study of ecology is to determine the drivers of habitat-forming vegetation, with much emphasis given to the relative importance to vegetation of "bottom-up" forces such as the role of nutrients and "top-down" forces such as the influence of herbivores and their predators. For coastal vegetation (e.g., kelp, seagrass, marsh, and mangroves) it has been well demonstrated that alterations to bottom-up forcing can cause major disturbances leading to loss of dominant vegetation. One such process is anthropogenic nutrient loading, which can lead to major changes in the abundance and species composition of primary producers, ultimately affecting important ecosystem services. In contrast, much less is known about the relative importance of apex predators on coastal vegetated ecosystems because most top predator populations have been depleted or lost completely. Here we provide evidence that an unusual four-level trophic cascade applies in one such system, whereby a top predator mitigates the bottom-up influences of nutrient loading. In a study of seagrass beds in an estuarine ecosystem exposed to extreme nutrient loading, we use a combination of a 50-y time series analysis, spatial comparisons, and mesocosm and field experiments to demonstrate that sea otters (Enhydra lutris) promote the growth and expansion of eelgrass (Zostera marina) through a trophic cascade, counteracting the negative effects of agriculturally induced nutrient loading. Our results add to a small but growing body of literature illustrating that significant interactions between bottom-up and top-down forces occur, in this case with consequences for the conservation of valued ecosystem services provided by seagrass.

Seagrass (Zostera marina) Colonization Promotes the Accumulation of Diazotrophic Bacteria and Alters the Relative Abundances of Specific Bacterial Lineages Involved in Benthic Carbon and Sulfur Cycling.

Seagrass colonization changes the chemistry and biogeochemical cycles mediated by microbes in coastal sediments. In this study, we molecularly characterized the diazotrophic assemblages and entire bacterial community in surface sediments of a Zostera marina-colonized coastal lagoon in northern China. Higher nitrogenase gene (nifH) copy numbers were detected in the sediments from the vegetated region than in the sediments from the unvegetated region nearby. The nifH phylotypes detected were mostly affiliated with the Geobacteraceae, Desulfobulbus, Desulfocapsa, and Pseudomonas. Redundancy analysis based on terminal restriction fragment length polymorphism analysis showed that the distribution of nifH genotypes was mostly shaped by the ratio of total organic carbon to total organic nitrogen, the concentration of cadmium in the sediments, and the pH of the overlying water. High-throughput sequencing and phylogenetic analyses of bacterial 16S rRNA genes also indicated the presence of Geobacteraceae and Desulfobulbaceae phylotypes in these samples. A comparison of these results with those of previous studies suggests the prevalence and predominance of iron(III)-reducing Geobacteraceae and sulfate-reducing Desulfobulbaceae diazotrophs in coastal sedimentary environments. Although the entire bacterial community structure was not significantly different between these two niches, Desulfococcus (Deltaproteobacteria) and Anaerolineae (Chloroflexi) presented with much higher proportions in the vegetated sediments, and Flavobacteriaceae (Bacteroidetes) occurred more frequently in the bare sediments. These data suggest that the high bioavailability of organic matter (indicated by relatively lower carbon-to-nitrogen ratios) and the less-reducing anaerobic condition in vegetated sediments may favor Desulfococcus and Anaerolineae lineages, which are potentially important populations in benthic carbon and sulfur cycling in the highly productive seagrass ecosystem.

The effect of parameter variability in the allometric projection of leaf growth rates for eelgrass (Zostera marina L.) II: the importance of data quality control procedures in bias reduction.

BACKGROUND: Eelgrass grants important ecological benefits including a nursery for waterfowl and fish species, shoreline stabilization, nutrient recycling and carbon sequestration. Upon the exacerbation of deleterious anthropogenic influences, re-establishment of eelgrass beds has mainly depended on transplantation. Productivity estimations provide valuable information for the appraisal of the restoration of ecological functions of natural populations. Assessments over early stages of transplants should preferably be nondestructive. Allometric scaling of eelgrass leaf biomass in terms of matching length provides a proxy that reduces leaf biomass and productivity estimations to simple measurements of leaf length and its elongation over a period. We examine how parameter variability impacts the accuracy of the considered proxy and the extent on what data quality and sample size influence the uncertainties of the involved allometric parameters. METHODS: We adapted a Median Absolute Deviation data quality control procedure to remove inconsistencies in the crude data. For evaluating the effect of parametric uncertainty we performed both a formal exploration and an analysis of the sensitivity of the allometric projection method to parameter changes. We used parameter estimates obtained by means of nonlinear regression from crude as well as processed data. RESULTS: We obtained reference leaf growth rates by allometric projection using parameter estimates produced by the crude data, and then considered changes in fitted parameters bounded by the modulus of the vector of the linked standard errors, we found absolute deviations up to 10% of reference values. After data quality control, the equivalent maximum deviation was under 7% of corresponding reference rates. Therefore, the addressed allometric method is robust. Even the smaller sized samples in the quality controlled dataset produced better accuracy levels than the whole set of crude data. CONCLUSIONS: We propose quality control of data as a highly recommended step in the overall procedure that leads to reliable allometric surrogates of eelgrass leaf growth rates. The proliferation of inconsistent replicates in the crude data points towards the importance of discarding incomplete leaves. We also recommend avoiding errors in estimating the biomass of small leaves for which precision of the used analytical scale might be an issue.

Optical sensor nanoparticles in artificial sediments--a new tool to visualize O2 dynamics around the rhizome and roots of seagrasses.

Seagrass communities provide important ecosystems services in coastal environments but are threatened by anthropogenic impacts. Especially the ability of seagrasses to aerate their below-ground tissue and immediate rhizosphere to prevent sulfide intrusion from the surrounding sediment is critical for their resilience to environmental disturbance. There is a need for chemical techniques that can map the O2 distribution and dynamics in the seagrass rhizosphere upon environmental changes and thereby identify critical stress thresholds of e.g. water flow, turbidity, and O2 conditions in the water phase. In a novel experimental approach, we incorporated optical O2 sensor nanoparticles into a transparent artificial sediment matrix consisting of pH-buffered deoxygenated sulfidic agar. Seagrass growth and photosynthesis was not inhibited in the experimental setup when the below-ground biomass was immobilized in the artificial sulfidic sediment with nanoparticles and showed root growth rates (∼ 5 mm day(-1)) and photosynthetic quantum yields (∼ 0.7) comparable to healthy seagrasses in their natural habitat. We mapped the real-time below ground O2 distribution and dynamics in the whole seagrass rhizosphere during experimental manipulation of light exposure and O2 content in the overlaying water. Those manipulations showed that oxygen release from the belowground tissue is much higher in light as compared to darkness and that water column hypoxia leads to diminished oxygen levels around the rhizome/roots. Oxygen release was visualized and analyzed on a whole rhizosphere level, which is a substantial improvement to existing methods relying on point measurements with O2 microsensors or partial mapping of the rhizosphere in close contact with a planar O2 optode. The combined use of optical nanoparticle-based sensors with artificial sediments enables imaging of chemical microenvironments in the rhizosphere of aquatic plants at high spatiotemporal resolution with a relatively simple experimental setup and thus represents a significant methodological advancement for studies of environmental impacts on aquatic plant ecophysiology.

Interactive effects of pesticide exposure and habitat structure on behavior and predation of a marine larval fish.

Coastal development has generated multiple stressors in marine and estuarine ecosystems, including habitat degradation and pollutant exposure, but the effects of these stressors on the ecology of fishes remain poorly understood. We studied the separate and combined effects of an acute 4 h sublethal exposure of the pyrethroid pesticide esfenvalerate and structural habitat complexity on behavior and predation risk of larval topsmelt (Atherinops affinis). Larvae were exposed to four nominal esfenvalerate concentrations (control, 0.12, 0.59, 1.18 µg/L), before placement into 12 L mesocosms with a three-spine stickleback (Gasterosteus aculeatus) predator. Five treatments of artificial eelgrass included a (1) uniform and (2) patchy distribution of eelgrass at a low density (500 shoots per m(2)), a (3) uniform and (4) patchy distribution of eelgrass at a high density (1,000 shoots per m(2)), and (5) the absence of eelgrass. The capture success of predators and aggregative behavior of prey were observed in each mesocosm for 10 min of each trial, and mortality of prey was recorded after 60 min. Exposure to esfenvalerate increased the proportion of larvae with swimming abnormalities. Surprisingly, prey mortality did not increase linearly with pesticide exposure but increased with habitat structure (density of eelgrass), which may have been a consequence of compensating predator behavior. The degree of prey aggregation decreased with both habitat structure and pesticide exposure, suggesting that anti-predator behaviors by prey may have been hampered by the interactive effects of both of these factors.

Habitat-former effects on prey behaviour increase predation and non-predation mortality.

Habitat-forming species can influence mortality on associated species via altering structural and non-structural abiotic conditions. Importantly, these effects can occur simultaneously and in opposite directions, although how they contribute to the net outcomes for predator-prey interactions remain unexplored. Seagrasses often have positive effects on associated fauna because their structure directly reduces predator encounter rates. However, we identified a 'risky' behaviour (shallower burial) in an infaunal bivalve at a high seagrass cover--likely induced by non-structural abiotic change--suggesting positive effects may be outweighed by risky behaviours. We determined whether the physical structure of the seagrass interacted with burial behaviour of clams to determine the predation and non-predation mortality and whether these interactions were mediated by the cover of the seagrass. Surveys on an intertidal sand flat in Tasmania, Australia showed that the highest densities of a dominant bivalve, Katelysia scalarina, occurred at low (33%) seagrass cover, but the lowest densities and the highest proportion of unburied clams occurred at high (100%) cover. A field experiment manipulating burial depth, seagrass cover and predator access demonstrated that unburied clams suffered very high predation and non-predation mortality compared to buried clams (~4x higher), which outweighed any positive effects of the seagrass structure in reducing predator access. Being unburied also had non-lethal consequences with surviving unburied clams having a reduced tissue biomass compared to buried clams. In this system, predation was driven by the availability of prey when they undertake a risky behaviour (shallow burial). However, significant changes in behaviour may only occur once a threshold of habitat-former density is reached. In this instance, changes in behaviour were likely due to seagrass effects on sediment redox potential, which decreased significantly above 33% seagrass cover. Our findings demonstrate that the negative effects of a habitat-former on the behaviour of associated species, via alteration of non-structural abiotic conditions, can outweigh any positive effects provided by increasing habitat structure as is commonly reported for habitat-formers.

Seagrasses are negatively affected by organic matter loading and Arenicola marina activity in a laboratory experiment.

When two ecosystem engineers share the same natural environment, the outcome of their interaction will be unclear if they have contrasting habitat-modifying effects (e.g., sediment stabilization vs. sediment destabilization). The outcome of the interaction may depend on local environmental conditions such as season or sediment type, which may affect the extent and type of habitat modification by the ecosystem engineers involved. We mechanistically studied the interaction between the sediment-stabilizing seagrass Zostera noltii and the bioturbating and sediment-destabilizing lugworm Arenicola marina, which sometimes co-occur for prolonged periods. We investigated (1) if the negative sediment destabilization effect of A. marina on Z. noltii might be counteracted by positive biogeochemical effects of bioirrigation (burrow flushing) by A. marina in sulfide-rich sediments, and (2) if previously observed nutrient release by A. marina bioirrigation could affect seagrasses. We tested the individual and combined effects of A. marina presence and high porewater sulfide concentrations (induced by organic matter addition) on seagrass biomass in a full factorial lab experiment. Contrary to our expectations, we did not find an effect of A. marina on porewater sulfide concentrations. A. marina activities affected the seagrass physically as well as by pumping nutrients, mainly ammonium and phosphate, from the porewater to the surface water, which promoted epiphyte growth on seagrass leaves in our experimental set-up. We conclude that A. marina bioirrigation did not alleviate sulfide stress to seagrasses. Instead, we found synergistic negative effects of the presence of A. marina and high sediment sulfide levels on seagrass biomass.

Genetic relatedness influences plant biomass accumulation in eelgrass (Zostera marina).

In multispecies assemblages, phylogenetic relatedness often predicts total community biomass. In assemblages dominated by a single species, increasing the number of genotypes increases total production, but the role of genetic relatedness is unknown. We used data from three published experiments and a field survey of eelgrass (Zostera marina), a habitat-forming marine angiosperm, to examine the strength and direction of the relationship between genetic relatedness and plant biomass. The genetic relatedness of an assemblage strongly predicted its biomass, more so than the number of genotypes. However, contrary to the pattern observed in multispecies assemblages, maximum biomass occurred in assemblages of more closely related individuals. The mechanisms underlying this pattern remain unclear; however, our data support a role for both trait differentiation and cooperation among kin. Many habitat-forming species interact intensely with conspecifics of varying relatedness; thus, genetic relatedness could influence the functioning of ecosystems dominated by such species.

Using allometric procedures to substantiate the plastochrone method for eelgrass leaf growth assessments.

Estimation of leaf productivity in eelgrass (Zostera marina L.) is crucial for evaluating the ecological role of this important seagrass species. Although leaf marking techniques are widely used to obtain estimates of leaf productivity, the accuracy of these assessments, has been questioned mainly because these fail to account for leaf growth below the reference mark and also because they apparently disregard the contribution of mature leaf tissues to the growth rate of leaves. On the other hand, the plastochrone method is a simpler technique that has been considered to effectively capture growth in a more realistic way, thereby providing more accurate assessments of both above- and below-ground productivities. But since the actual values of eelgrass growth rates are difficult to obtain, the worth of the plastochrone method has been largely vindicated because it produces assessments that overestimate productivity as compared to estimates obtained by leaf marking. Additionally, whenever eelgrass leaf biomass can be allometrically scaled in terms of matching leaf length in a consistent way, the associated leaf growth rates can be also projected allometrically. In this contribution, we used that approach to derive an authentication of the plastochrone method and formally demonstrate that, as has been claimed to occur for leaf marking approaches, the plastochrone method itself underestimates actual values of eelgrass leaf growth rates. We also show that this unavoidable bias is mainly due to the inadequacy of single-leaf biomass assessments in providing a proxy for the growth of all leaf tissue in a shoot over a given interval. Moreover, the derived formulae give conditions under which assessments of leaf growth rates using the plastochrone method would systematically underestimate matching values obtained by leaf marking procedures. And, assessments of leaf growth rates obtained by using the present data show that plastochrone method estimations underestimated corresponding proxies obtained allometrically (27%), or through leaf marking (35%). Allometric projection is recommended as a simpler and more effective procedure to reduce the bias in eelgrass leaf productivity estimations that associates to the use of plastochrone methods.

Top-down control of epifauna by fishes enhances seagrass production.

Predators can influence the structure and function of ecosystems by altering the composition or behavior of herbivore communities. Overexploitation of predators, therefore, may lead to habitat loss by altering important top-down interactions that facilitate habitat-forming species. In seagrass beds, top-down control of algal growth by mesograzers appears to facilitate seagrass production. The indirect consequences of higher-order trophic interactions, however, remain unclear. Although predators may limit the beneficial effects of algal mesograzers, it is also possible that they limit the abundance of invertebrates that consume and foul seagrasses. We used experimental enclosure and exclosure cages to explore the direct and indirect effects of microcarnivorous fishes on epifaunal invertebrates, epiphytic loads, and seagrass growth in a natural eelgrass (Zostera marina) bed in San Diego Bay, California, USA. Contrary to expectations, when fishes were excluded, invertebrate abundance increased by 300-1000%, fouling on eelgrass leaves increased by 600%, and eelgrass production declined by 50%. Despite high densities of predators in enclosures, subsequent effects did not differ from ambient conditions. When predators were excluded, however, abundances of epifauna (including tube-building crustaceans and an eelgrass-grazing limpet) increased dramatically, resulting in reduced seagrass production. Our results are supported by several studies of eelgrass communities in the northeastern Pacific, characterized by coastal upwelling, inverse estuaries, and a voracious seagrass-consuming limpet. These strong, positive, indirect effects of microcarnivores on seagrass production contrast with the beneficial mesograzer paradigm, highlighting the need for hypotheses to be tested across a variety of ecosystems with varying biophysical characteristics.

Disturbance decreases genotypic diversity by reducing colonization: Implications for disturbance-diversity feedbacks.

One objective of eco-evolutionary dynamics is to understand how the interplay between ecology and evolution on contemporary timescales contributes to the maintenance of biodiversity. Disturbance is an ecological process that can alter species diversity through both ecological and evolutionary effects on colonization and extinction dynamics. While analogous mechanisms likely operate among genotypes within a population, empirical evidence demonstrating the relationship between disturbance and genotypic diversity remains limited. We experimentally tested how disturbance altered the colonization (gain) and extinction (loss) of genets within a population of the marine angiosperm Zostera marina (eelgrass). In a 2-year field experiment conducted in northern California, we mimicked grazing disturbance by migratory geese by clipping leaves at varying frequencies during the winter months. Surprisingly, we found the greatest rates of new colonization in the absence of disturbance and that clipping had negligible effects on extinction. We hypothesize that genet extinction was not driven by selective mortality from clipping or from any stochastic loss resulting from the reduced shoot densities in clipped plots. We also hypothesize that increased flowering effort and facilitation within and among clones drove the increased colonization of new genets in the undisturbed treatment. This balance between colonization and extinction resulted in a negative relationship between clipping frequency and net changes in genotypic richness. We interpret our results in light of prior work showing that genotypic diversity increased resistance to grazing disturbance. We suggest that both directions of a feedback between disturbance and diversity occur in this system with consequences for the maintenance of eelgrass genotypic diversity.

The amount of light reaching the leaves in seagrass (Zostera marina) meadows.

Seagrass meadows, and other submerged vegetated habitats, support a wide range of essential ecological services, but the true extents of these services are in many ways still not quantified. One important tool needed to assess and model many of these services is accurate estimations of the systems´ primary productivity. Such productivity estimations require an understanding of the underwater light field, especially regarding the amount of light that actually reaches the plants' photosynthetic tissue. In this study, we tested a simple practical approach to estimate leaf light exposure, relative to incoming light at the canopy, by attaching light sensitive film at different positions on leaves of Zostera marina, eelgrass, in four seagrass meadows composed of different shoot density and at two different depths. We found that the light reaching the leaves decreased linearly down through the canopy. While the upper parts of the leaves received approximately the same level of light (photosynthetic photon flux density, PPFD) as recorded with a PAR meter at the canopy top, the average light that the seagrass leaves were exposed to varied between 40 and 60% of the light on top of the canopy, with an overall average of 48%. We recommend that actual light interception is measured when assessing or modelling light depending processes in submerged vegetation, but if this is not achievable a rough estimation for vegetation similar to Z. marina would be to use a correction factor of 0.5 to compensate for the reduced light due to leaf orientation and internal shading.

Response of eelgrass (Zostera marina) to an adjacent Olympia oyster restoration project.

Recent restoration efforts for the native Olympia oyster, Ostrea lurida, are commonly motivated by potential return of oyster-associated ecosystem services, including increased water filtration. The potential impact of such restoration on another species of ecological concern, eelgrass, Zostera marina, is unclear, but has been hypothesized to be positive if oyster filter feeding increases light penetration to eelgrass. For two years after construction of an oyster restoration project, we assessed the response of adjacent eelgrass (impact) compared to control and reference eelgrass beds by monitoring changes in light intensity, eelgrass shoot density, biomass, leaf morphometrics, and epiphyte load. We observed lower light intensity consistently over time, including prior to restoration, near the constructed oyster bed relative to the control and one of the reference locations. We also observed minor variations between control and impact eelgrass morphology and density. However, the changes observed were not outside the range of natural variation expected in this system, based upon comparisons to reference eelgrass beds, nor were they detrimental. This limited impact to eelgrass may be due in part to the incorporation of a buffer distance between the restored oyster bed and the existing eelgrass bed, which may have dampened both positive and negative impacts. These findings provide evidence that Olympia oyster restoration and eelgrass conservation goals can be compatible and occur simultaneously.

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.

The distribution of large floating seagrass (Zostera marina) aggregations in northern temperate zones of Bohai Bay in the Bohai Sea, China.

Seagrass meadows (Zostera marina) are important coastal ecosystems with high levels of productivity and biodiversity. They often have high biomass turnover and are susceptible to dislodgment, leading to export of biomass from seagrass beds. In September 2016, two cruises covering a total area of 52 km x 15 km (38°57'1.14"-39° 0'41.28" N, 118°45'23.22"-118°47'6.96" E) found floating Z. marina aggregations along 13 km of the transect in the northernmost area of Bohai Bay, in the Bohai Sea, China. This floating seagrass was 6.3-13.4 km northeast (offshore) of the Caofeidian seagrass bed, which is a large (10 km2) seagrass bed discovered in 2015 in the Bohai Sea, China. The modal length of floating intact shoots (from meristem to longest leaf tip) matched samples from the Caofeidian seagrass bed. The dominant individuals lengths were 40-50 cm, with less than 5% of the total number of individuals found in larger size categories (80-90 and 90-100 cm). We concluded that they originated from the nearby Caofeidian seagrass meadows.

Local patchiness of macrobenthic faunal abundance displays homogeneity across the disparate seagrass systems of an estuarine bay.

Spatial variation in the degree of local patchiness of macrobenthic assemblage abundance was assessed across the 16 km2 warm-temperate Knysna estuarine bay (South Africa) where the seagrass Zostera (Zosterella) capensis grows under a broad spectrum of environmental conditions and supports invertebrate assemblages at a wide range of local density (<2000->320000 ind. m-2). Macrobenthic assemblage abundance at all 27 representative sites examined displayed low-level but highly-significant spatial patchiness (mean Lloyd's index, IP = 1.148). Except at high tidal levels, however, the magnitude of this local patchiness did not vary statistically across the system (CV 4.3%) regardless of assemblage abundance, location or species composition. Patchinesses well within ±1 standard deviation of Knysna's value also characterise an equivalent Z. (Zosterella) capricorni assemblage in subtropical Queensland (IP 1.169) and another, Z. (Zosterella) noltei, assemblage in cool-temperate England (IP 1.135), suggesting that at local scales intertidal dwarf-eelgrass macrobenthic abundance displays a characteristic level of patchiness.

A unique meadow of the marine angiosperm Zostera japonica, covering a large area in the turbid intertidal Yellow River Delta, China.

Marine submerged aquatic angiosperms (seagrasses) are declining globally. The species Zostera japonica Asch. & Graebn. is endangered in its native range in Asia, but has been successfully introduced to North America. A large area (1031.8 ha) of Z. japonica meadow has recently been discovered in the intertidal zone of Yellow River Delta, China. This seagrass occurs along both sides of the river mouth, forming dense meadows in turbid water conditions. Seasonal investigations over two years were conducted to examine the distribution, biomass, seed reproduction, seed bank, and population recruitment of the seagrass meadows at three sites in the intertidal zone. The meadows generally showed relatively high coverage, biomass, reproductive effort, and seed production in August. The seed bank was found to be large and contributed to population recruitment. There were significant inter-annual variations overall, and at individual sites. These variations are likely due to winter temperatures, which determine the abundance of overwintering shoots and seedling success. Differences in micro-topography may also play a role in producing variations in seedling success between sites. Microsatellite analysis revealed a high genetic exchange between the two sides of the river mouth. The results indicate that the seagrass bed in the Yellow River Delta shallow waters is in good condition, which can be attributed to its location within a national nature reserve. Establishment of protected areas might act as an effective way to mitigate the anthropogenic disturbance, conserve the seagrass meadows, and then enhance critical ecosystem functions.