A Global Crisis for Seagrass Ecosystems

Seagrasses, marine flowering plants, have a long evolutionary history but are now challenged with rapid environmental changes as a result of coastal human population pressures. Seagrasses provide key ecological services, including organic carbon production and export, nutrient cycling, sediment stabilization, enhanced biodiversity, and trophic transfers to adjacent habitats in tropical and temperate regions. They also serve as “coastal canaries,” global biological sentinels of increasing anthropogenic influences in coastal ecosystems, with large-scale losses reported worldwide. Multiple stressors, including sediment and nutrient runoff, physical disturbance, invasive species, disease, commercial fishing practices, aquaculture, overgrazing, algal blooms, and global warming, cause seagrass declines at scales of square meters to hundreds of square kilometers. Reported seagrass losses have led to increased awareness of the need for seagrass protection, monitoring, management, and restoration. However, seagrass science, which has rapidly grown, is disconnected from public awareness of seagrasses, which has lagged behind awareness of other coastal ecosystems. There is a critical need for a targeted global conservation effort that includes a reduction of watershed nutrient and sediment inputs to seagrass habitats and a targeted educational program informing regulators and the public of the value of seagrass meadows.

Coupling of seagrass (Cymodocea nodosa) patch dynamics to subaqueous dune migratio

The coupling between patch dynamics - described by the patch growth (horizontal and vertical), patch mortality, and life-history of Cymodocea nodosa (Ucria) Aschers., and the disturbance caused by the migration of subaqueous dunes over the plants was examined in a shallow NW Mediterranean bay (Alfacs Bay) where this species maintains a patchy cover. C. nodosa shoots survived substantial burial rates (up to 2.4 mm/day) by growing vertically at rates proportional to, albeit four-fold slower than, burial rates. Patch death was caused by erosion as large subaqueous dunes migrated pass the plant patch. Patch growth was fastest over the progressing slope of the dunes ( similar to 2.5 m year super(-1)) and flowering was also stimulated by sand accretion. The time interval between the passage of consecutive dunes, which sets the time window available for patch development, ranged between 2 and 6 years. This time interval allowed C. nodosa to recolonize bare substrata, with patch formation occurring about half a year after the disturbance, and also allowed established shoots to complete their life-cycle and produce seeds and thus enable subsequent recolonization. The time windows available for patch development also set an upper limit to patch size of about 26 m. Significant cross correlations between dune topography and patch dynamics and plant flowering frequency provide evidence that the spatial heterogeneity in the vegetation is closely associated with the disturbance imposed by the migration of sand dunes. The migration of subaqueous dunes maintains C. nodosa in a continuous state of colonization involving spatially asynchronous patch growth and subsequent mortality, which is ultimately responsible for the characteristic patchy landscape of this Bay. 

vertical growth of thalassia testudinum seasonal and interannual variability

The vertical growth of shoots of the seagrass Thalassia testudinum Banks ex Konig in four meadows, along a range of exposure to waves, in the Mexican Caribbean was examined to elucidate its magnitude and its relationship to sediment dynamics. Average internodal length varied between 0.17 and 12.75 mm, and was greatest in the meadow which experienced the greatest burial by sand waves moved by Hurricane Gilbert (September 1988). Internodal length showed annual cycles, confirmed by the flower scars always preceding or coinciding with the annual minimum internodal length. These annual cycles on the shoot allowed estimation of annual leaf production, which varied, on average, between 14.2 and 19.3 leaves per shoot year-1. High vertical shoot growth was associated with long internodes and high leaf production rate, which increased with increasing vertical shoot growth to a maximum of approximately 25 leaves per shoot year-1, with vertical growth of about 30 mm year-1 or more. Average internodal length showed substantial interannual differences from perturbations derived from the passage of Hurricane Gilbert. The growth response of the plants surviving moderate burial and erosion after the hurricane involved enhanced vertical growth and increased leaf production, and reduced vertical growth, respectively, after 1988. The variability in shoot vertical growth of T testudinum can be separated into seasonal changes in plant growth, and long-term variability associated with episodic perturbations involving sediment redistribution by hurricanes.

Fruit anatomy,seed germination and seedling development in the Japanese seagrass phyllospadix

Phyllospadix iwatensis Makino and phyllospadix japonicus Makino have similar frunt morphology and anatomy.The rhomboid fruit of Japanese phyllospadix is dark brown in colour and is characterized by two arms bearing stiff inflected bristles which can act as an anchoring system. The fruit covering consists of a thin cuticular seed coat and pericarp remains mainly fibrous endocarp. In the groove region of the fruit.the cuticular seed coat and endocarp are replaced by nucellus cells with wall in growths and crushed pigment strands with lignified walls.these tissues appera to control the transfer of nutrients to developing seed.the seed is oval with a small embryo and a large hypocotyl. the embryo is straight and simple,with the plumule containing three leaf primordia and a pair of root primordia surrounded by a cotyledon.the hypocotyl has large vontral lobe containing central provascular tissue and two small dorsal lobes.the hypocotyl contains starch.lipid and protein.and acts as a nutrient store.the seed of P.iwatensis has a dormancy period of 2-6 weeks and germination eventually reaches-65%.but is not synchronized.during germination the leaves emerge first.and then after at least three young leaves have formed and abseised.the roots emerge,usually?6 months after the commencement of germination.Utilizaton of the nutrient reserves is initially from the perihpery of the hypocotyl and then progressively towards its centre.

Seed germination and seedling morphology of the seagrass, Halophila engelmannii (Hydrocharitaceae)

Seeds of Halophila engelmannii Aschers., that were collected in Redfish Bay, Texas, at weekly intervals from mid-May to mid-June 1986, began to germinate 3–4 weeks after collection. Most of the collections subsequently showed an increase in the rate of germination under increased light intensity and all had a stoppage of germination after transfer to darkness, indicating a light requirement to break endogenous seed dormancy. During the 5 weeks after seeds germinated, seedlings in soil culture produced a rosette of six leaves before the appearance of a rhizome bud in the axil of the third leaf. The first node of the rhizome produced a root and an upright shoot with a pseudowhorl of three to five leaves.

Seed-bank development, germination and early seedling survival of two seagrass species from The Netherlands: Zostera marina L. and Zostera noltii hornem

Flowering and seed-bank development of annual Zostera marina L. and perennial Z. noltii hornem. were studied in the Zandkreek (S.W. Netherlands). Flowering of Z. noltii started at the end of June and continued until the end of September. A maximum of ca. 1000 flowering shoots (11% of the total amount of shoots per square metre) occurred in early August. Flowering of Z. marina started at the end of July and continued throughout October. Seed banks of both species appeared to be annual. Actual seed densities of Z. noltii were much lower than predicted on the basis of the amount of inflorescences.Germination was studied in the laboratory in relation to temperature (10, 20 and 30°C), salinity (1.0, 10.0, 20.0, 30.0 and 40.0‰) and stratification (at 4°C). Both species showed a maximal germination at 30°C and 1.0‰ salinity, decreasing with higher salinities and lower temperatures. Stratification stimulated germination only at salinities 20.0‰. Desiccation and anaerobia were lethal to Z. marina seeds. Seedlings of Z. marina survived best at 10°C and 10.0–20.0‰ salinity and those of Z. noltii survived best at 10°C and 1.0‰ salinity. Overall, seedlings of Z. marina survived better than those of Z. noltii.

Size and time of day affect the response of postlarvae and early juvenile grooved tiger prawns Penaeus semisulcatus De Haan (Decapoda:Penaeidae) to natural and artificial seagrass in the laboratory

Juvenile tiger prawns (Penaeus semisulcatus De Haan and P. esculentus Haswell) show a strong association with vegetated habitats and are rarely caught on non-vegetated areas. This pattern of distribution may be caused by postlarvae selecting vegetation when they settle, or to differences in post-settlement mortality in different habitats. In this study, we examined whether the postlarvae and early juvenile stages of P. semisulcatus would distinguish between seagrass (Zostera capricorni Aschers) without epiphytes, artificial seagrass and bare substratum in the laboratory. The responses of prawns reared from the egg to different stages of postlarval and juvenile development were tested to determine whether, and when, each size class showed a response to a particular habitat. Five size classes of postlarvae (average carapace lengths [CL] of 1.2, 1.4, 1.6, 1.7 and 2.0 mm) were offered a choice between Z. capricorni and bare sand. Small size classes of postlarvae either did not respond to Z. capricorni (1.2 and 1.6 mm CL), or were more abundant on bare substratum than Z. capricorni. In contrast, the largest size classes of postlarvae (1.7 and 2.0 mm CL) were more abundant on Z. capricorni during the day but not at night. The behaviour of postlarvae changed markedly at a size of 1.7 mm CL (22 days from the first nauplius): smaller postlarvae frequently swam in the water column; 1.7 and 2.0 mm CL postlarvae spent much more. time resting on the substrate and perched on seagrass leaves. This size at which postlarvae first respond to seagrass during the day, and show mainly benthic behaviour, is similar to the size at which they are found on shallow seagrass beds in northern Australia. Large postlarvae (2.7 mm CL) and juveniles (4.1 mm CL) both were more abundant on artificial seagrass than bare sand during the day but not at night, indicating that they respond to structured habitats. When large postlarvae (2.4 mm CL) and juveniles (3.5 mm CL) were offered a choice between Z. capricorni without epiphytes and artificial seagrass, they were more abundant on the Z. capricorni, which suggests that chemical cues from seagrass may explain some of the responses of P. semisulcatus to seagrass. (C) 1997 Elsevier Science B.V.

Antifeedant, antibacterial, and antilarval compounds from the South China Sea seagrass Enhalus acoroides

We investigated chemical constituents and the antifeedant, antibacterial, and antilarval activities of EtOH (ethanol) extracts of the South China Sea seagrass Enhalus acoroides. Eleven pure compounds including four flavonoids and five steroids were obtained. Among these compounds, three flavonoids were antifeedant against second-instar larvae of Spodoptera litura, two flavonoids had antibacterial activity towards several marine bacteria, and one flavonoid showed strong antilarval activity against Bugula neritina larvae. This is the first description of isolation and bioactivity of secondary metabolites from E. acoroides.

Comparative study on CO2 emissions from different types of alpine meadows during grass exuberance period

Potentilla fruticosa scrub, Kobresia humilis meadow and Kobresia tibetica meadow are widely distributed on the Qinghai-Tibet Plateau. During the grass exuberance period from 3 July to 4September, based on close chamber-GC method, a study on CO2 emissions from different treatments was conducted in these meadows at Haibei research station, CAS. Results indicated that mean CO2emission rates from various treatments were 672.09+152.37 mgm-2h-1 for FC (grass treatment); 425.41+191.99 mgrn-2h-1 for FJ (grass exclusion treatment); 280.36+174.83 mgrn-2h-1 for FL (grass and roots exclusion treatment); 838.95+237.02 mgm-2h-1 for GG (scrub+grass treatment); 528.48+205.67 mgm-2h-1for GC (grass treatment); 268.97 ±99.72 mgm-2h-1 for GL (grass and roots exclusion treatment); and 659.20±94.83 mgm-2h-1 for LC (grass treatment), respectively (FC, FJ, FL, GG, GC, GL, LC were the Chinese abbreviation for various treatments). Furthermore, Kobresia humilis meadow, Potentilla fruticosa scrub meadow and Kobresia tibetica meadow differed greatly in average CO2 emission rate of soil-plant system, in the order of GG＞FC＞LC＞GC. Moreover, in Kobresia humilis meadow,heterotrophic and autotrophic respiration accounted for 42% and 58% of the total respiration of soil-plant system respectively, whereas, in Potentilla fruticosa scrub meadow, heterotrophic and autotrophic respiration accounted for 32% and 68% of total system respiration from G-G; 49% and 51%from GC. In addition, root respiration from Kobresia humilis meadow approximated 145 mgCO2m-2h-1,contributed 34% to soil respiration. During the experiment period, Kobresia humilis meadow and Potentilla fruticosa scrub meadow had a net carbon fixation of 111.11 grn-2 and 243.89 grn-2,respectively. Results also showed that soil temperature was the main factor which influenced CO2 emission from alpine meadow ecosystem, significant correlations were found between soil temperature at 5 cm depth and CO2 emission from GG, GC, FC and FJ treatments. In addition, soil moisture may be the inhibitory factor of CO2 emission from Kobresia tibetica meadow, and more detailed analyses should be done in further research.

Nitrous oxide emissions from two alpine meadows in the Qinghai-Tibetan Plateau

Nitrous oxide (N2O) emission was measured in a Kobresia humilis meadow and a Potentilla fruticosa meadow in the Qinghai-Tibet Plateau from June 2003 to July 2006. Five treatments were setup in the two alpine meadows. Two bare soil treatments were setup in the K. humilis meadow (BSK) and in the P. fruticosa meadow (BSP) by removing the above- and belowground plant biomass. Three plant community treatments were setup with one in the K. humilis meadow (herbaceous community in the K. humilis meadow-HCK) and two in the P. fruticosa meadow (herbaceous community in the P. fruticosa meadow-HCP, and shrub community in the P. fruticosa meadow-SCP). Nitrous oxide emission from BSP was estimated to be 38.1 +/- 3.6 mu g m(-2) h(-1), significantly higher than from BSK (30.2 +/- 2.8 mu g m(-2) h(-1)) during the whole experiment period. Rates from the two herbaceous blocks (HCK and HCP) were close to 39.5 mu g m(-2) stop h(-1) during the whole experimental period whereas shrub community (SCP) showed significant high emission rates of N2O. Annual rate of N2O emission was estimated to be 356.7 +/- 8.3 and 295.0 +/- 11.6 mg m(-2) year(-1) from the alpine P. fruticosa meadow and from the alpine K. humilis meadow, respectively. These results suggest that alpine meadows in the Qinghai-Tibetan Plateau are an important source of N2O, contributing an average of 0.3 Tg N2O year(-1). We concluded that N2O emission will decrease, due to a predicted vegetation shift from shrubs to grasses imposed by overgrazing.

Growing season ecosystem respirations and associated component fluxes in two alpine meadows on the Tibetan Plateau

From 30 June to 24 September in 2003 ecosystem respiration (Re) in two alpine meadows on the Tibetan Plateau were measured using static chamber- and gas chromatography- (GC) based techniques. Simultaneously, plant removal treatments were set to partition Re into plant autotrophic respiration (Ra) and microbial heterotrophic respiration (Rh). Results indicated that Re had clear diurnal and seasonal variation patterns in both of the meadows. The seasonal variability of Re at both meadow sites was caused mainly by changes in Ra, rather than Rh. Moreover, at the Kobresia humilis meadow site (K_site), Ra and Rh accounted for 54% and 46% of Re, respectively. While at the Potentilla fruticosa scrub meadow (P_site), the counterparts accounted for 61% and 39%, respectively. T test showed that there was significant difference in Re rates between the two meadows (t = 2.387, P = 0.022). However, no significant difference was found in Rh rates, whereas a significant difference was observed in Ra rates between the two meadows. Thus, the difference in Re rate between the two meadows was mainly attributed to plant autotrophic respirations. During the growing season, the two meadows showed relatively low Q(10) values, suggesting that Re, especially Rh was not sensitive to temperature variation in the growing season. Additionally, Re and Rh at the K_site, as well as Rh at the P_site was negatively correlated with soil moisture, indicating that soil moisture would also play an important role in respirations.

Changes in plant biomass and species composition of alpine Kobresia meadows along altitudinal gradient on the Qinghai-Tibetan Plateau

Alpine Kobresia meadows are major vegetation types on the Qinghai-Tibetan Plateau. There is growing concern over their relationships among biodiversity, productivity and environments. Despite the importance of species composition, species richness, the type of different growth forms, and plant biomass structure for Kobresia meadow ecosystems, few studies have been focused on the relationship between biomass and environmental gradient in the Kobresia meadow plant communities, particularly in relation to soil moisture and edaphic gradients. We measured the plant species composition, herbaceous litter, aboveground and belowground biomass in three Kobresia meadow plant communities in Haibei Alpine Meadow Ecosystem Research Station from 2001 to 2004. Community differences in plant species composition were reflected in biomass distribution. The total biomass showed a decrease from 13196.96 +/- 719.69 g/m(2) in the sedge-dominated K. tibetica swamp to 2869.58 +/- 147.52 g/m(2) in the forb and sedge dominated K. pygmaea meadow, and to 2153.08 +/- 141.95 g/m(2) in the forbs and grasses dominated K. humilis along with the increase of altitude. The vertical distribution of belowground biomass is distinct in the three meadow communities, and the belowground biomass at the depth of 0-10 cm in K. tibetica swamp meadow was significantly higher than that in K. humilis and K. pygmaea meadows (P < 0.01). The herbaceous litter in K. tibetica swamp was significantly higher than those in K. pygnaeca and K. humilis meadows. The effects of plant litter are enhanced when ground water and soil moisture levels are raised. The relative importance of litter and vegetation may vary with soil water availability. In the K. tibetica swamp, total biomass was negatively correlated to species richness (P < 0.05); aboveground biomass was positively correlated to soil organic matter, soil moisture, and plant cover (P < 0.05); belowground biomass was positively correlated with soil moisture (P < 0.05). However, in the K. pygnaeca and K. humilis meadow communities, aboveground biomass was positively correlated to soil organic matter and soil total nitrogen (P < 0.05). This suggests that the distribution of biomass coincided with soil moisture and edaphic gradient in alpine meadows.

Effect of imitated global warming on Delta C-13 values in seven plant species growing in Tibet alpine meadows

Open-top chambers were used to estimate the possible effects of global warming on delta C-13 of seven plant species grown in alpine meadow ecosystem. The delta C-13 values of plant species were lower after long-term growth in open-top chambers. In the course of experiment, temperature significantly increased inside the chambers by 4 degrees C. Plant species grown at a lower elevation above sea level had higher delta C-13 values as compared to those grown at a higher elevation. This was in accordance with the effect of open-top chamber on delta C-13 values in plants. Greater availability of CO2 and lower water vapor as indicated by an increase in discrimination against (CO2)-C-13, probably result in more negative delta C-13 values of plants because higher stomatal conductance increases availability of CO2 and causes greater discrimination against (CO2)-C-13. The plant species studied could be the indicator species for testing global warming by the change in carbon isotope ratios at the two growth temperatures.