Multi-scale spatial patterns of three seagrass species with different growth dynamics

Knowledge of landscape spatial patterns of seagrasses and their rates of loss and natural colonization is critical for understanding the ecology of this group of submerged aquatic plants. Seagrasses form extensive meadows that occupy sheltered coastal seas of the world. In this paper, we examine the multi-scale variability of three seagrass species over a large near-shore region (42 km²) in Western Australia. Geostatistical non-parametric methods were used to explore spatial variation in presence of Amphibolis griffithii , Posidonia coriacea and P. sinuosa , and to identify the spatial scales at which distinct patterns in the species distributions occur: <50, 50–610 and >610 m. Each species showed unique variance structure across local (<50 and 50–610 m) and regional scales (>610 m), suggesting differences in species biology, environmental requirements, inter-species interactions, and their ability to modify their environment. These observations reflect that 1) seagrass landscapes are created by processes that independently act on each seagrass species at different spatial scales; 2) the species' distributions differ in their hydrodynamic forcing, and; 3) seagrass species distributions reflect colonization history such that related species are separated in space because they have different places in the successional sequence. This cross-scale study demonstrates that shoot studies only partly address the spatial structure of seagrass landscapes and further large-scale spatially-explicit research is required before we can interpret the driving processes.     

Effects of pesticides on seagrass beds

In the German Wadden Sea there has been a remarkable decline in seagrass beds. It was the aim of this study to test whether herbicide contamination could be a reason for this. Concentrations of triazine herbicides such as atrazine, simazine and terbutylazine as well as phenylurea herbicides were measured in Wadden Sea sediments within or in the neighbourhood of seagrass meadows. Sediments were thus used as a marker for medium-term contamination of the Wadden Sea. The respective concentrations were examined in relation to the density and status of the seagrass meadows. Preliminary results show that there may be a connection between seagrass decline and herbicide contamination in the parts of the Wadden Sea sampled. A comparison with other contamination is also given. Received: 11 January 1999 / Received in revised form: 1 December 1999 / Accepted: 8 December 1999     

Microbial biomass and productivity in seagrass beds

This research focused on whether bacteria living in aerobic environments where Fe is often a limiting nutrient could access Fe associated with the clay mineral kaolinite. Kaolinite is one of the most abundant clays at the Earth's surface, and it often contains trace quantities of Fe as surface precipitates, accessory minerals, and structural substitutions. We hypothesized that aerobic bacteria may enhance kaolinite dissolution as a means of obtaining associated Fe. To test this hypothesis, we conducted microbial growth experiments in the presence of an aerobic Pseudomonas mendocina     

Distribution patterns of macrobenthic biodiversity in the intertidal seagrass beds of an estuarine system, and their conservation significance

Intertidal stands of seagrass are important elements in the ecology of many estuaries yet the manner in which their associated macrobenthic biodiversity is distributed throughout any single estuary has never been determined. This has now been attempted for the Knysna estuarine bay in the Garden Route National Park, South Africa, an important site for one vulnerable and declining seagrass, Cape dwarf-eelgrass (Nanozostera capensis), and for several associated animals. Although spanning a salinity range of <5–35, the seagrass beds of this estuary contained a unitary fauna with local variation in frequency and abundance of the various species. Faunal biodiversity was highest along the main estuarine channel in the marine-influenced region and declined both upstream and into the sheltered peripheral zones, overall faunal abundance being inversely correlated with species diversity (though not with species richness). This pattern results in the beds around the 4 km shoreline of a single island near the mouth supporting 91 % of the total macrobenthic invertebrate species present in the system. This situation is discussed in relation to such data as are available from other comparable systems and to the conservation of estuarine seagrass faunal biodiversity. It is concluded that in general and with caveats for some individual species of concern where conservation resources are limited attention would most profitably be focused on the seagrass meadows of downstream estuarine areas.     

Sphagnum growth and ecophysiology during mire succession

Sphagnum mosses are widespread in areas where mires exist and constitute a globally important carbon sink. Their ecophysiology is known to be related to the water level, but very little is currently known about the successional trend in Sphagnum. We hypothesized that moss species follow the known vascular plant growth strategy along the successional gradient (i.e., decrease in production and maximal photosynthesis while succession proceeds). To address this hypothesis, we studied links between the growth and related ecophysiological processes of Sphagnum mosses from a time-since-initiation chronosequence of five wetlands. We quantified the rates of increase in biomass and length of different Sphagnum species in relation to their CO₂ assimilation rates, their photosynthetic light reaction efficiencies, and their physiological states, as measured by the chlorophyll fluorescence method. In agreement with our hypothesis, increase in biomass and CO₂ exchange rate of Sphagnum mosses decreased along the successional gradient, following the tactics of more intensely studied vascular plants. Mosses at the young and old ends of the chronosequence showed indications of downregulation, measured as a low ratio between variable and maximum fluorescence (F _v/F _m). Our study divided the species into three groups; pioneer species, hollow species, and ombrotrophic hummock formers. The pioneer species S. fimbriatum is a ruderal plant that occurred at the first sites along the chronosequence, which were characterized by low stress but high disturbance. Hollow species are competitive plants that occurred at sites with low stress and low disturbance (i.e., in the wet depressions in the middle and at the old end of the chronosequence). Ombrotrophic hummock species are stress-tolerant plants that occurred at sites with high stress and low disturbance (i.e., at the old end of the chronosequence). The three groups along the mire successional gradient appeared to be somewhat analogous to the three primary strategies suggested by Grime.     

The growth patterns of some hydroids that are obligate epiphytes of seagrass leaves

The growth patterns of the hydroids Campanularia asymmetrica, Sertularia perpusilla and Algaophenia harpago on the Mediterranean seagrass Posidonia oceanica, and of Laomedea angulata on the temperate seagrass Zostera marina, were examined. The seagrass leaves grow from a basal meristem and have a short life which presents problems for these long-lived obligate epiphytes. Downward growth onto younger leaf tissue, shown by S. perpusilla and C. asymmetrica, reduces the risk of interspecific competition with other epiphytes, and places the hydroids on substrates that survive longer than older distal leaf tissue, close to short young leaves which they can colonize by stolon transfer. S. perpusilla was found only on the basal areas of leaves, possibly because of the progressive degeneration of their most apical tissue. C. asymmetrica showed no such degeneration and was found over most of the length of the leaves. The hydrorhizae of most small A. harpago and L. angulata grew both upward and downward along the leaves. Both species were most abundant on the apical halves of older leaves. This distal distribution is considered to be advantageous for these large branched hydroids which are suited for capturing planktonic prey, and to enhance their probability of colonizing neighbouring plants. Its disadvantages include the reduced probability of maintaining the hydroids on the same plant and an increased risk of being eaten by seagrass grazers.     

Diurnal changes in pore water sulfide concentrations in the seagrass Thalassia testudinum beds: the effects of seagrasses on sulfide dynamics

The dynamics of the seagrass-sulfide interaction were examined in relation to diel changes in sediment pore water sulfide concentrations in Thalassia testudinum beds and adjacent bare areas in Corpus Christi Bay and lower Laguna Madre, Texas, USA, during July 1996. Pore water sulfide concentrations in seagrass beds were significantly higher than in adjacent bare areas and showed strong diurnal variations; levels significantly decreased during mid-day at shallow sediment depths (0-10 cm) containing high below-ground tissue biomass and surface area. In contrast, diurnal variations in sediment sulfide concentrations were absent in adjacent bare patches, and at deeper (>10 cm) sediment depths characterized by low below-ground plant biomass or when the grasses were experimentally shaded. These observations suggest that the mid-day depressions in sulfide levels are linked to the transport of photosynthetically produced oxygen to seagrass below-ground tissues that fuels sediment sulfide oxidation. Lower sulfide concentrations in bare areas are likely a result of low sulfate reduction rates due to low organic matter available for remineralization. Further, high reoxidation rates due to rapid exchange between anoxic pore water and oxic overlying water are probably stimulated in bare areas by higher current velocity on the sediment surface than in seagrass beds. The dynamics of pore water sulfides in seagrass beds suggest no toxic sulfide intrusion into below-ground tissues during photosynthetic periods and demonstrate that the sediment chemical environment is considerably modified by seagrasses. The reduced sediment sulfide levels in seagrass beds during photosynthetic periods will enhance seagrass production through reduced sulfide toxicity to seagrasses and sediment microorganisms related to the nutrient cycling.     

Patterns of fish and sea urchin grazing on tropical Indo-Pacific seagrass beds

Thalassodendron ciliatum shoots were collected from natural populations of fished and unfished protected seagrass meadows to assess the herbivory of fish and sea urchins. Fish herbivory was restricted to unfished seagrass meadows, while sea urchin herbivory took place in fished as well as unfished areas where fish appeared not to be effective urchin predators. The results of this study confirm that grazing by sea urchins is important in tropical seagrass ecosystems and indicate that herbivorous fish graze, and probably consume, substantial amounts of seagrass production in fishing-protected habitats. The fact that much of the information on seagrass herbivory comes from heavily-fished meadows indicates that large-scale studies, which include unfished areas, are necessary to provide reliable spatial patterns of seagrass grazing distribution.     

A comparison of plant communities on the basis of their clonal growth patterns

This study was performed to analyse how a vegetative propagation pattern of plants affects the coexistence of species and subsequent species richness of the community. We compared community average clonal growth in the herbal communities of forests, wooded meadows, and open meadows in Laelatu, Estonia. The parameters used for the calculation of the community averages and measured for each species were ramet life span, rhizome branching, and clonal mobility. We also examined the intrinsic (i.e. independent of the environment) relationship between community clonal growth and plant species density. We found strong correlations between the environmental factors (productivity, light availability, and mowing regime) and community averages of clonal growth parameters, while species density was (negatively) correlated only with community average of rhizome increment. The community average of ramet life span decreased with the increasing biomass of the herb layer. No evidence was found to support the hypothesis that species-rich communities may consist of species with more contrasting mobility compared with species-poor communities. Independent of the effect of the environmental factors, species density was positively correlated with ramet density. There was intrinsic positive relationship between species density and community average of ramet life span at open meadow sites and intrinsic negative relationship between species density and community average of rhizome increment at wooded meadow sites. We conclude that in forest communities the capability of clonal plants to forage for light is favoured, while in unmown meadows a competitively strong phalanx growth form is advantageous. We established that ramet turnover increases and vegetative mobility decreases with increasing species diversity, although these two relationships depend strongly on the type of the studied community.Co-ordinating editor: J. Tuomi     

Evaluation of aboveground and belowground biomass recovery in physically disturbed seagrass beds

Several studies addressed aboveground biomass recovery in tropical and subtropical seagrass systems following physical disturbance. However, there are few studies documenting belowground biomass recovery despite the important functional and ecological role of roots and rhizomes for seagrass ecosystems. In this study, we compared the recovery of biomass (g dry weight m(-2)) as well as the biomass recovery rates in ten severely disturbed multi-species seagrass meadows, after the sediments were excavated and the seagrasses removed. Three sites were located in the tropics (Puerto Rico) and seven in the subtropics (Florida Keys), and all were originally dominated by Thalassia testudinum. Total aboveground biomass reached reference values at four out of ten sites studied, two in the Florida Keys and two in Puerto Rico. Total belowground biomass was lower at the disturbed locations compared to the references at all sites, apart from two sites in the Florida Keys where the compensatory effect of opportunistic species (Syringodium filiforme and Halodule wrightii) was observed. The results revealed large variation among sites in aboveground and belowground biomass for all species, with higher aboveground recovery than belowground for T. testudinum. Recovery rates for T. testudinum were highly variable across sites, but a general trend of faster aboveground than belowground recovery was observed. Equal rates between aboveground and belowground biomass were found for opportunistic species at several sites in the Florida Keys. These results indicate the importance of belowground biomass when assessing seagrass recovery and suggest that the appropriate metric to assess seagrass recovery should address belowground biomass as well as aboveground biomass in order to evaluate the full recovery of ecological services and functions performed by seagrasses. We point out regional differences in species composition and species shifts following severe disturbance events and discuss ecological implications of gap dynamics in multi-species seagrass meadows.     

Effects of herbivorous birds on intertidal seagrass beds in the northern Wadden Sea

During autumn migration (September to December), brent geese (Branta b. bernicla) and wigeon (Anas penelope) feed on the seagrass Zostera noltii in the nearshore, upper tidal zone leeward of the island of Sylt (eastern North Sea). To graze on leaves and shoots above the sediment and on rhizomes and roots below, these birds reworked the entire upper 1 cm layer of sediment eight times within this 3-month period. In addition, brent geese excavated pits 3–10 cm deep by trampling in order to feed on below-ground phytomass. About 12% of the seagrass beds became pitted to an average depth of 4.5 cm. Using net exclosures, it was estimated that birds removed 34 g dry weight m^–2 of above-ground and 28 g of below-ground phytomass. This corresponds to 45% of the phytomass in September. Of the overall loss of phytomass from September to December, 63% was caused by birds. Roughly half of the leaves fell off anyway until December and the other half were taken by the birds. Below the ground, phytomass remained almost constant where birds were excluded, while with birds phytomass of rhizomes and roots was halved. In spite of this strong effect, in the next vegetation period the blade density was lower at former exclosure sites compared to the ambient seagrass bed. The underlying process seems to be a self-inhibition of dense overwintering seagrass by mud accretion. Assuming our experimental results can be scaled up to the entire seagrass bed, we hypothesize that in the sheltered upper intertidal zone, seasonal erosion caused by herbivorous geese and ducks is necessary for the persistence of Z. noltii. Received: 7 January 1999 / Received in revised form: 23 August 1999 / Accepted: 25 August 1999     

海草床生态系统固氮微生物研究现状与展望

海草床生态系统是三大典型海洋生态系统之一,具有较高的生产力水平和十分重要的生态意义,其海草附着物中包括许多固氮微生物,通过生物固氮提供该生态系统"新"氮源,在一定程度上缓解了氮对初级生产力的限制.通过对近些年来固氮活性的测定方法、固氮群落组成以及影响因素等的研究进展进行了综合分析,较全面地分析了海草床生态系统的固氮微生物研究现状,并在次基础上对海草床固氮微生物的系统研究进行了展望.     

Influences of biological interactions on community structure within seagrass beds and sandprawn-dominated sandflats

The faunal composition of high-shore beds of eelgrass Zostera capensis versus unvegetated sandflats dominated by the sandprawn Callianassa kraussi was investigated by experimentally transplanting plots of eelgrass into sandflats where the sandprawn was either left unmanipulated or eliminated by prior faunal suffocation. After 12 months, multidimensional scaling defined five faunal clusters that were dictated primarily by the presence or absence of eelgrass rather than sandprawns. By 18 months, eelgrass and sandprawns played equal roles in defining faunal clusters. After 30 months, sandprawns had displaced the eelgrass transplants and faunal composition converged on that of sandflat control plots.Most species were either ‘eelgrass-associated’ or ‘sandflat-associated’, but three had unique responses to the treatments: two high-shore species (Assiminea globulus and Hydrobia sp.) that were abundant in eelgrass beds never appeared in the eelgrass transplants, whereas another eelgrass-occupant, Siphonaria compressa, became superabundant on the transplants for 18 months.Unexpectedly, the eelgrass fauna was never more rich or diverse than that of unvegetated sandflats, although abundance was greater in the eelgrass. As hypothesised, the sandflats were disproportionally populated by burrowers and eelgrass by non-burrowers, but a second hypothesis that these habitats should support mainly hard-bodied and soft-bodied animals respectively was rejected.The distinctively different faunas of eelgrass and unvegetated sandflats are seemingly respectively maintained by sediment stabilisation by eelgrass versus bioturbation by sandprawns. The transplants of eelgrass into sandflats developed an eelgrass-associated fauna provided sandprawns were excluded, but ultimately reverted to a sandflat fauna once sandprawns reinvaded.     

Linking hyphal growth to colony dynamics: Spatially explicit models of mycelia

The processes responsible for the growth of fungal mycelia act over a vast range of spatial scales; while nutrient uptake occurs at the molecular level, the fungal colony can expand by the order of centimetres each day. Although experiments can provide exceptionally detailed information on processes at specific scales, it can be difficult to relate those processes between different spatial scales. In this article a series of mathematical models are described that link the different spatial scales found within a mycelium. The models are all closely related to each other and are applied to a range of growth environments, and the advantages and limitations of each modelling approach are discussed.     

Fitness and evolution in clonal plants: the impact of clonal growth

Seeds have often been emphasized in estimates of plant fitness because they are the units that carry genes to the next generation, disperse, and found new populations. We contend that clonal growth also needs to be considered when estimating fitness in clonal plants, regardless of whether fitness is measured from a genet or ramet perspective. Clonal growth affects genet fitness through both genet persistence and seed production. It affects ramet fitness through new ramet production, because both seeds and clonal propagants are considered offspring. The differential production of clonal propagants will contribute to fitness differences among individuals which may result in population-level changes in allele frequencies (i.e. microevolution). We describe a form of selection unique to clonal organisms, genotypic selection, that can result in evolution. Genotypic selection occurs when genotypically based traits are associated with differences in the rate of ramet production. It can lead to evolutionary change in quantitative trait means both directly and indirectly. It leads directly to change in the ramet population by increasing the proportion of ramets with more advantageous trait values. From the genet perspective, it leads indirectly to evolution within and among populations whenever significant portions of the genetic effect on a trait are inherited through seed. We argue that under most conditions, clonal growth will play a major role in the microevolution of clonal plants.     

重金属在海草中累积及其对海草生长的影响

海草是生长在近海浅水中的单子叶高等植物,对维持近海生态平衡起着重要作用,而重金属污染对海草有重要影响.研究重金属对海草的影响过程与机理对海洋生态环境保护具有重要意义.本文总结了国内外近年来在海草受重金属污染影响方面的研究进展,包括海草重金属累积的特征及其主要影响因素,重金属对海草生理生态特别是光合效率的影响,以及海草对重金属胁迫的内在生理生化适应机制等.提出了今后的研究方向,通过同位素示踪的方法比较海草通过叶片吸收与通过根吸收重金属的差异,进行多种重金属在毒性协同和拮抗方面的研究,比较重金属对海草急性毒性和慢性毒性的差异,加强重金属对海草毒性机理的研究等.     

Diversity of European seagrass indicators: patterns within and across regions

Seagrasses are key components of coastal marine ecosystems and many monitoring programmes worldwide assess seagrass health and apply seagrasses as indicators of environmental status. This study aims at identifying the diversity and characteristics of seagrass indicators in use within and across European ecoregions in order to provide an overview of seagrass monitoring effort in Europe. We identified 49 seagrass indicators used in 42 monitoring programmes and including a total of 51 metrics. The seagrass metrics represented 6 broad categories covering different seagrass organizational levels and spatial scales. The large diversity is particularly striking considering that the pan-European Water Framework Directive sets common demands for the presence and abundance of seagrasses and related disturbance-sensitive species. The diversity of indicators reduces the possibility to provide pan-European overviews of the status of seagrass ecosystems. The diversity can be partially justified by differences in species, differences in habitat conditions and associated communities but also seems to be determined by tradition. Within each European region, we strongly encourage the evaluation of seagrass indicator–pressure responses and quantification of the uncertainty of classification associated to the indicator in order to identify the most effective seagrass indicators for assessing ecological quality of coastal and transitional water bodies.