Genetic effects of eelgrass restoration efforts by fishers' seeding to recover seagrass beds as an important natural capital for coastal ecosystem services

The reestablishment of seagrass vegetation is a vital part of recovering coastal marine ecosystem services. Historically the Hinase area was a famous for the fishing by coastal pound netting in eelgrass beds, but this practice was progressively displaced with oyster farming due to an enormous decline in seagrass vegetation. For several decades, the local fishers' cooperative has worked to restore eelgrass beds by a seeding method. Through these efforts, seagrass vegetation in their fishing area has increased to about half of their previous area. This study examined the effect of long-term seeding by fishers on the recovery of eelgrass beds in the Hinase area, based on analysis of eelgrass genetic structure using microsatellite markers. Specimens for the DNA analysis were collected from each of all eelgrass meadows that the fishers conducted sowing eelgrass seeds as well as from the source sites where they collected the seeds. The results found that restored beds in the study area have high genetic diversity comparable to natural ones. The multiple regression analysis revealed that a combined model of seedling intensity and geographic distance (R² = .457) better explained genetic structure across our sampling sites than models of seedling intensity (R² = .092) or geographic distance only (R² = .344). This supports that the eelgrass seeds they sowed did not disturb the genetic structure but rather supplemented natural dispersal, suggesting that the fishers' seeding did not develop nonnatural seagrass meadows but certainly contributed to the recovery of natural seagrass meadows.     

Relative influence of habitat complexity and proximity to patch edges on seagrass epifaunal communities

Habitat structure at many scales influences faunal communities. Although habitat structure at different scales often covaries, studies rarely examine the relative effects of structure at multiple scales on faunal density and diversity. In shallow‐water seagrass systems, epifaunal density at local scales generally increases with increased habitat structural complexity (e.g. shoot density per unit area). In turn, structural complexity often varies with other aspects of habitat structure at patch scales, such as proximity to patch edges, which itself modifies ecological processes that structure epifaunal communities. We conducted surveys and a manipulative experiment in the eelgrass Zostera marina beds of San Diego Bay, California, USA, to determine (1) whether eelgrass structural complexity, epifaunal density and diversity, and fish (predator) density and diversity vary with proximity to patch edges, and (2) the relative influences of structural complexity, proximity to patch edges and predator presence on epifaunal distribution. Seagrass structural complexity generally increased from patch edges to patch interiors at all sites and in all sampling periods. However, patterns of epifaunal density, diversity, and biomass varied among sites and sampling periods, with density and biomass increasing from patch edges to interiors at some sites and decreasing at others. In the manipulative experiment, we allowed epifauna to colonize sparse or dense artificial seagrass habitat at both the edge and interior of a seagrass patch, and enclosed a subset of experimental units in predator exclusion cages. Overall, proximity to patch edges had a larger influence on epifaunal density and community structure than did structural complexity or predation, with the exception of some common taxa which responded more strongly to either complexity or predator exclusion. Our results emphasize the importance of addressing and evaluating habitat structure at multiple scales to better understand the distribution and interactions of organisms in a particular environment.     

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

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

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     

Seagrass bed patchiness: effects on epifaunal communities in San Diego Bay, USA

Seagrass habitat structure influences epifaunal density, diversity, community composition and survival, but covariation of structural elements at multiple scales (e.g., shoot density or biomass per unit area, patch size, and patch configuration) can confound studies attempting to correlate habitat structure with ecological patterns and processes. In this study, we standardized simulated seagrass shoot density and bed area among artificial seagrass beds in San Diego Bay, California, USA to evaluate the singular effect of seagrass bed configuration (“patchiness”) on the density and diversity of seagrass epifauna. Artificial seagrass beds all were 1 m², but were composed of a single large patch (“continuous” treatment), four smaller patches (“patchy” treatment), or 16 very small patches (“very patchy” treatment). We allowed epifauna to colonize beds for 1 month, and then sampled beds monthly over the next 3 months. Effects of seagrass bed patchiness on total epifaunal density and species-specific densities were highly variable among sampling dates, and there was no general trend for the effects of fragmentation on epifaunal densities to be positive or negative. Epifaunal diversity (measured as Simpson's index of diversity) was highest in very patchy or patchy beds on two out of the three sampling dates. Very patchy beds exhibited the highest dissimilarity in community composition in the first two sampling periods (August and September), but patchy beds exhibited the highest dissimilarity in the third sampling period (October). Our results indicate that seagrass patch configuration affects patterns of epifaunal density, diversity, and community composition in the absence of covarying bed area or structural complexity, and that patchy seagrass beds may be no less valuable as a habitat than are continuous seagrass beds. The spatial pattern employed when harvesting or planting seagrass may influence epifaunal habitat use and should be a key consideration in restoration plans.     

Genetic population structure of Tectura paleacea: implications for the mechanisms regulating population structure in patchy coastal habitats

The seagrass limpet Tectura paleacea (Gastropoda; Patellogastropoda) belongs to a seagrass obligate lineage that has shifted from the Caribbean in the late Miocene, across the Isthmus of Panama prior to the closing of the Panamanian seaway, and then northward to its modern Baja California - Oregon distribution. To address whether larval entrainment by seagrass beds contributes to population structuring, populations were sampled at six California/Oregon localities approximately 2 degrees latitude apart during two post-settlement periods in July 2002 and June 2003. Partial cytochrome oxidase b (Cytb) sequences were obtained from 20 individuals (10 per year) from each population in order to determine the levels of population subdivision/connectivity. From the 120 individuals sequenced, there were eighty-one unique haplotypes, with the greatest haplotype diversity occurring in southern populations. The only significant genetic break detected was consistent with a peri-Point Conception (PPC) biogeographic boundary while populations north and south of Point Conception were each panmictic. The data further indicate that populations found south of the PPC biogeographic boundary originated from northern populations. This pattern of population structure suggests that seagrass patches are not entraining the larvae of T. paleacea by altering flow regimes within their environment; a process hypothesized to produce extensive genetic subdivision on fine geographic scales. In contrast to the haplotype data, morphological patterns vary significantly over very fine geographic scales that are inconsistent with the observed patterns of genetic population structure, indicating that morphological variation in T. paleacea might be attributed to differential ecophenotypic expression in response to local habitat variability throughout its distribution. These results suggest that highly localized conservation efforts may not be as effective as large-scale conservation efforts in near shore marine environments.     

Plant genotype identity and diversity interact with mesograzer species diversity to influence detrital consumption in eelgrass meadows

Seagrass meadows are among the world's most productive ecosystems, and as in many other systems, genetic diversity is correlated with increased production. However, only a small fraction of seagrass production is directly consumed, and instead much of the secondary production is fueled by the detrital food web. Here, we study the roles of plant genetic diversity and grazer species diversity on detrital consumption in California eelgrass Zostera marina meadows. We used three common mesograzers—an amphipod, Ampithoe lacertosa, an isopod, Idotea resecata, and a polychaete, Platynereis bicanaliculata. Each grazer consumed eelgrass detritus at rates greater than live eelgrass or macroalgae. This detrital consumption, however, was not spread evenly over leaves shed from different eelgrass clones. Palatability and consumption varied because of genotype specific differences in leaf texture, secondary metabolites (phenolics), and nutritional quality (nitrogen). Further, detritus derived from some eelgrass genotypes was palatable to all grazers, while detritus from other genotypes was preferentially consumed by only one grazer species. Under monospecific grazer assemblages, plant genetic identity but not diversity influenced detritus consumption. However, more realistic, diverse mesoconsumer communities combined with high plant‐detrital genotypic diversity resulted in greater consumption and grazer survival. These results provide a mechanism for field observations of increased mesograzer density and diversity in genetically diverse seagrass assemblages and offer a potential explanation for variation in results of resource diversity– detrital processing experiments in the literature, which often exclude macroinvertebrate taxa. More broadly, our findings support the emerging principle that biodiversity effects are strongest when diversity in both consumer and resource taxa are present.     

Fauna of Natural Seagrass and Transplanted Halodule wrightii (Shoalgrass) Beds in Galveston Bay, Texas

Abstract We compared nekton and benthos densities and community compositions in a natural mixed seagrass bed dominated by Halodule wrightii (shoalgrass) with those found in three shoalgrass transplant sites and adjoining sand habitats in western Galveston Bay, Texas, U.S.A. Quantitative drop traps and cores were used to compare communities up to seven times over 36 months post‐transplant where transplant beds survived. Total densities of fishes, decapods, annelids, benthic crustaceans, and most dominant species were significantly higher in natural seagrass than in transplanted shoalgrass or sand habitats during most sampling periods. On occasion, fish and decapod densities were significantly higher in transplanted shoalgrass than in adjoining sand habitats. No consistent faunal differences were found among transplant sites before two of three sites failed. Taxonomic comparison of community compositions indicated that nekton and benthos communities in natural seagrass beds were usually distinct from those in transplanted beds or sand habitats, which were similar. We conclude that reestablishing a shoalgrass bed that resembles a natural seagrass bed and its faunal communities in the Galveston Bay system will take longer than 3 years, provided that transplants persist.     

Interspecies mating in sympatric species of Syngnathus pipefish

Hybridization is thought to be an important source of novel genetic variation, and interspecific hybridization may increase the adaptive potential of wild populations. While hybridization has not been previously reported in syngnathid fishes (seahorses and pipefish), the sympatric occurrence of closely related species at high densities increases the probability of interspecies mating in this group. Southern California is home to five species of Syngnathus pipefish, and these species frequently co-occur in near-shore eelgrass beds along the California coast. Recent work has identified exceptionally high levels of genetic diversity in southern populations of Syngnathus leptorhynchus, a widespread species which ranges from Mexico to Alaska. Microsatellite genotyping and mitochondrial sequence data are used here to study the population genetics of S. leptorhynchus and S. auliscus at a site in San Diego Bay where they are found to co-occur at high densities. While no adult hybrids were detected in the study population, analysis of male broods indicates that interspecies mating is occurring between the two species. The lack of premating isolating mechanisms between these two relatives suggests that hybridization may be common in sympatric species of Syngnathus.     

Management of natural <Emphasis Type="Italic">Ulva</Emphasis> spp. blooms in San Quintin Bay,Baja California: Is it justified?

According to Zertuche-González et al. (2009), Ulva spp. blooms, favored by oyster cultivation, are likely displacing subtidal meadows of Zostera marina in San Quintin Bay, Baja California. The authors propose a partial removal of the seaweed to reduce the risk of eutrophication and eelgrass displacement in the bay. We warn about the removal of Ulva spp. biomass by raising six arguments that emphasize the necessity of a historical and ecosystem-based management for San Quintin Bay. First, processes other than competitive exclusion by Ulva spp. blooms more likely explain changes of Z. marina subtidal meadows in the past decade. Second, there is no consistent evidence that oyster cultivation is promoting blooms of Ulva spp. and the loss of eelgrass. Third, the removal of Ulva spp. biomass is based on experiences of heavily anthropogenically eutrophic systems, while San Quintin Bay is not. Fourth, the proposed course of actions to restore eelgrass meadows ignores general historical baselines of estuarine and coastal systems by confusing what it means to be “pristine.” Fifth, despite the important experimental evidence indicating strong top–down control in temperate seagrass meadows, Zertuche-González et al. (2009) underestimated the capacity of consumers in structuring dynamics of vegetated soft-bottom communities in San Quintin Bay. Sixth, Ulva expansa may exert positive effects on seagrass ecosystem properties and functions. Instead, we propose protection against the propagation of unsustainable practices in the bay, and the reintroduction of large consumers that are now absent in this ecosystem. An ecosystem-based analysis of the role of Ulva spp. on eelgrass dynamics is needed.     

Restoration experiment of Zostera marina L. in a subtropical coastal lagoon

Zostera marina is the dominant seagrass species in coastal lagoons on the western coast of Baja California Peninsula, and due to its coastal location it is threatened by natural and anthropogenic factors, as is happening in Puerto San Carlos, B.C.S., where a fish cannery unloads its wastewater to the beach. Apparently an extensive intertidal meadow replacement was established by great amounts of green macroalgae. We evaluated the possibility to mitigate the impacts of this cannery with transplants of Z. marina meadow using adult plants. The transplant experiment was made in two different seasons for which two undisturbed donor meadows were chosen: El Cuervo and San Carlitos. The winter one obtained a 30% and in San Carlitos 90% after 13 months and the autumn transplant in San Carlos obtained a 0% of survival after 3 months. The results of these transplant activities were reflected in the shoot density at the end of the experiment (San Carlos was of 482 shoots/m² and San Carlitos of 818 shoots/m²s and agree with the density of the natural meadows. This experiment shows that it is possible to develop a small-scale seagrass restoration as mitigation for Baja California coastal lagoons which are under severe threat for coastal development.     

Spatially explicit and multi-sourced genetic information is critical for conservation of an endangered plant species,San Diego thornmint (<Emphasis Type="Italic">Acanthomintha ilicifolia</Emphasis>)

San Diego thornmint Acanthomintha ilicifolia (Gray) Gray (Lamiaceae) is a winter herb restricted to San Diego county in the United States and Baja California Norte in Mexico. Historic records document 80 occurrences of this species, with 55 extant occurrences in San Diego County currently known. We compared three measures of genetic variation to inform ongoing conservation efforts: putatively neutral genetic structure revealed from isozyme markers, apparent cytogenetic variation confirmed using flow cytometry, and potentially adaptive morphological variation quantified in a common-garden study. Together, these data indicated that this rare endemic is genetically complex, revealing significant differentiation of neutral and potentially adaptive genetic variation among populations, and possessing at least two cytotypes, sometimes even within the same population. While additional study is required to resolve the extent of potential local adaptation in this species, conservation plans should limit the movement of germplasm among occurrences and monitor populations in order to limit potential long-term impacts to population viability. Given that these findings challenge the canonical model of genetic structure in rare plants (low genetic variation and limited genetic structure), we recommend guidelines to apply genetic information to conservation strategies.     

The role of anthropogenic influences in a mixed-conifer forest mortality episode

Abstract. Anthropogenic influences on recent tree mortality in mid-montane mixed conifer forests of southern California, USA, and northern Baja California, Mexico, were investigated. The Pinus jeffreyi-Abies concolor phase of the mixed-conifer montane community was sampled at three sites, characterized by: (1) low levels of air pollution and long-term fire suppression (Holcomb Valley in the San Bernardino Mountains, California); (2) chronic, severe air pollution and long-term fire suppression (Barton Flats in the San Bernardino Mountains, California); and (3) little air pollution and no fire suppression (La Corona Arriba in the Sierra San Pedro Martir in Baja California, Mexico). Similar percentages of dead standing trees, around 14 %, were found at both San Bernardino sites, but a comparatively low level, 4 %, was found in the Mexican forest. Anthropogenic effects, in particular fire suppression, may play an important role in enhancing the impact of natural stresses on the dynamics of mixed conifer forests.     

Relationships between bed age,bed size,and genetic structure in Chesapeake Bay (Virginia,USA) eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis> L.)

Genetic structure and diversity can reveal the demographic and selective forces to which populations have been exposed, elucidate genetic connections among populations, and inform conservation strategies. Beds of the clonal marine angiosperm Zostera marinaL. (eelgrass) in Chesapeake Bay (Virginia, USA) display significant morphological and genetic variation; abundance has fluctuated widely in recent decades, and eelgrass conservation is a major concern, raising questions about how genetic diversity is distributed and structured within this metapopulation. This study examined the influence of bed age (<65years versus<6years) and size (>100ha versus<10ha) on morphological and genetic (allozyme) structure and diversity within Chesapeake Bay eelgrass beds. Although both morphology and genetic diversity varied significantly among individual beds (F _ST=0.198), neither varied consistently with bed age or size. The Chesapeake eelgrass beds studied were significantly inbred (mean F _IS=0.680 over all beds), with inbreeding in old, small beds significantly lower than in other bed types. Genetic and geographic distances within and among beds were uncorrelated, providing no clear evidence of isolation by distance at the scale of 10's of km. These results suggest that local environmental conditions have a greater influence on plant morphology than do bed age or size. They support the hypotheses that eelgrass beds are established by multiple founder genotypes but experience little gene flow thereafter, and that beds are maintained with little loss of genetic diversity for up to 65 years. Since phenotypic and genotypic variation is partitioned among beds of multiple ages and sizes, eelgrass conservation efforts should maximize preservation of diversity by minimizing losses of all beds.     

Restoring Eelgrass (Zostera marina L.) Habitats Using a Simple and Effective Transplanting Technique

Eelgrass beds in coastal waters of China have declined substantially over the past 30 years. In this study, a simple new transplanting technique was developed for eelgrass (Zostera marina L.) restoration. To assist in anchoring single shoots, several rhizomes of rooted shoots were bound to a small elongate stone (50–150 g) with biodegradable thread (cotton or hemp), and then the bound packet was buried at an angle in the sediments at a depth of 2–4 cm. This stone anchoring method was used to transplant eelgrass in early November 2009 and late May 2010 in Huiquan Bay, Qingdao. The method led to high success. Three month survivorship of the transplanted shoots at the two transplant sites was >95%. From April 20 to November 19, 2012, the following characteristics of the 2009 and 2010 transplanted eelgrass beds were monitored: morphological changes, shoot density, shoot height, leaf biomass, and sediment particle size. Results showed that the sexual reproduction period of the planted eelgrass was from April to August, and vegetative reproduction reached its peak in autumn. Maximum shoot height and biomass were observed in June and July. After becoming established, the transplanted eelgrass beds were statistically equal to natural eelgrass beds nearby in terms of shoot height, biomass, and seasonal variations. This indicates that the transplant technique is effective for eelgrass restoration in coastal waters.     

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.     

Evidence of Eelgrass (Zostera marina) Seed Dispersal by Northern Diamondback Terrapin (Malaclemys terrapin terrapin) in Lower Chesapeake Bay

The initial discovery in May 2009 of eelgrass (Zostera marina) seeds in fecal samples of wild-caught northern diamondback terrapins (Malaclemys terrapin terrapin) was the first field evidence of eelgrass seed ingestion in this species. This finding suggested the potential of terrapins as seed dispersers in eelgrass beds, which we sampled for two additional years (2010 and 2011). Seeds were only found in feces of terrapins captured prior to June 8 in all three years, coinciding with eelgrass seed maturation and release. Numbers of seeds in terrapin feces varied annually and decreased greatly in 2011 after an eelgrass die off in late 2010. The condition of seeds in terrapin feces was viable-mature, germinated, damaged, or immature. Of terrapins captured during time of seed release, 97% were males and juvenile females, both of which had head widths <30 mm. The fraction of individuals with ingested seeds was 33% for males, 35% for small females, and only 6% for large (mature) females. Probability of seed ingestion decreased exponentially with increasing terrapin head width; only males and small females (head width <30 mm) were likely to be vectors of seed dispersal. The characteristic that diamondback terrapins have well-defined home ranges allowed us to estimate the number of terrapins potentially dispersing eelgrass seeds annually. In seagrass beds of the Goodwin Islands region (lower York River, Virginia), there were 559 to 799 terrapins, which could disperse between 1,341 and 1,677 eelgrass seeds annually. These would represent a small proportion of total seed production within a single seagrass bed. However, based on probable home range distances, terrapins can easily traverse eelgrass meadow boundaries, thereby dispersing seeds beyond the bed of origin. Given the relatively short dispersion distance of eelgrass seeds, the diamondback terrapin may be a major source of inter-bed seed dispersal and genetic diversity.     

Biodiversity mediates top–down control in eelgrass ecosystems: a global comparative‐experimental approach

Nutrient pollution and reduced grazing each can stimulate algal blooms as shown by numerous experiments. But because experiments rarely incorporate natural variation in environmental factors and biodiversity, conditions determining the relative strength of bottom–up and top–down forcing remain unresolved. We factorially added nutrients and reduced grazing at 15 sites across the range of the marine foundation species eelgrass (Zostera marina) to quantify how top–down and bottom–up control interact with natural gradients in biodiversity and environmental forcing. Experiments confirmed modest top–down control of algae, whereas fertilisation had no general effect. Unexpectedly, grazer and algal biomass were better predicted by cross‐site variation in grazer and eelgrass diversity than by global environmental gradients. Moreover, these large‐scale patterns corresponded strikingly with prior small‐scale experiments. Our results link global and local evidence that biodiversity and top–down control strongly influence functioning of threatened seagrass ecosystems, and suggest that biodiversity is comparably important to global change stressors.     

Genetic and historical evidence disagree on likely sources of the Atlantic amethyst gem clam Gemma gemma (Totten, 1834) in California

Aim Historical information about source populations of invasive species is often limited; therefore, genetic analyses are used. We compared inference about source populations from historical and genetic data for the oyster‐associated clam, Gemma gemma that invaded California from the USA Atlantic coast. Location Mid‐Atlantic (North Carolina, Maryland), Northeastern (New Jersey, New York, Massachusetts) and the California coasts (Elkhorn Slough, San Francisco Bay, Bolinas Lagoon, Tomales Bay, Bodega Harbor). Methods The documented history of transplantation of Eastern oysters to California was reviewed. Cytochrome c oxidase subunit I (COI) sequences from recent and archived clams were examined in a haplotype network. We used AMOVA to detect geographic genetic structure and a permutation test for significant reductions in diversity. Results Chesapeake Bay oysters were transplanted to New York prior to shipment to San Francisco Bay and from there to peripheral bays. Gemma in the Northeastern and Mid‐Atlantic regions were genetically differentiated. In California, populations in Bodega Harbor and Tomales Bay were genetically similar to those in the Mid‐Atlantic area while clams in San Francisco Bay, Elkhorn Slough and Bolinas Lagoon resembled populations in the Northeastern region. In California, genetic variation was not highest in San Francisco Bay despite greater magnitude of oyster plantings. Haplotypes varied over time in native and introduced populations. Main Conclusions Historical records and inferences from genetics agree that both Northeastern and Mid‐Atlantic regions were sources for Gemma in California. Only complex genetic hypotheses reconcile the strong segregation of haplotypes in California to the historical evidence of mixing in their proximate source (New York). These hypotheses include sorting of mixtures of haplotypes or selection in non‐native areas. Haplotype turnover in San Francisco and Massachusetts samples over time suggests that the sorting hypothesis is plausible. We suggest, however, that Gemma was introduced independently and recently to Tomales Bay and Bodega Harbor.     

Effects of sediment nutrients on seagrasses: Literature review and mesocosm experiment

A review of the literature shows that seagrass growth, abundance and morphology are strongly linked to available nutrient resources. In north temperate climates and in habitats with terrigenous sediments, nitrogen limitation occurs in both intertidal and subtidal beds. Typically, seagrasses growing in terrigenous sediments have ample phosphorus but lack nitrogen, and the plants' chemical composition is depleted in nitrogen. However, seagrasses occurring in tropical environments and carbonate sediments appear to experience phosphorus limitation due to binding of phosphate in the sediments. Thus, it is the sediment geochemistry in seagrass beds that is important in determining the limiting nutrient to seagrass growth.Examination of the literature indicates that field research on seagrass relationships involves too many interactive factors to be able to say certainly that any one plant characteristics is caused by any one environmental factor. Using mesocosms (partially enclosed outdoor experimental set-ups) one environmental factor can be changed between the treatment tank and the control. Therefore, we can determine experimentally that a plant characteristic is affected by a particular altered environmental factor. Experimental mesocosms used to grow eelgrass, Zostera marina L., in substrata of varied nitrogen composition showed the dramatic effect of insufficient nitrogen on eelgrass growth, abundance and leaf morphology. Additionally, eelgrass leaf tissue from low ammonium sand substratum is significantly depleted in nitrogen, demonstrating the supposition that nitrogen is limiting in terrigenous environments.