Production ecology of phyto- and zooplankton in a eutrophic pond dominated byChaoborus flavicans (Diptera: Chaobolidae)

Primary production of phytoplankton and secondary production of a daphnid and a chaoborid were studied in a small eutrophic pond. The gross primary production of phytoplankton was 290 gC m⁻² per 9 months during April–December. Regression analysis showed that the gross primary production was related to the incident solar radiation and the chlorophylla concentration and not to either total phosphorus or total inorganic nitrogen concentration. The mean chlorophylla concentration (14.2 mg m⁻³), however, was about half the expected value upon phosphorus loading of this pond. The mean zooplankton biomass was 1.60 g dry weight m⁻², of whichDaphnia rosea and cyclopoid copepods amounted to 0.69 g dry weight m⁻² and 0.61 g dry weight m⁻², respectively. The production ofD. rosea was high during May–July and October and the level for the whole 9 months was 22.6 g dry weight m⁻².Chaoborus flavicans produced 10 complete and one incomplete cohorts per year. Two consecutive cohorts overlapped during the growing season. The maximum density, the mean biomass, and the production were 19,100 m⁻², 0.81 g dry weight m⁻², and 11.7 g dry weight m⁻²yr⁻¹, respectively. As no fish was present in this pond, the emerging biomass amounted to 69% of larval production. The production ofC. flavicans larvae was high in comparison with zooplankton production during August–September, when the larvae possibly fed not only on zooplankton but also algae.     

Nitrogen and phosphorus economy of the riparian shrub <Emphasis Type="Italic">Salix gracilistyla</Emphasis> in western Japan

Salix gracilistyla is one of the dominant plants in the riparian vegetation of the upper-middle reaches of rivers in western Japan. This species colonizes mainly sandy habitats, where soil nutrient levels are low, but shows high potential for production. We hypothesized that S.␣gracilistyla uses nutrients conservatively within stands, showing a high resorption efficiency during leaf senescence. To test this hypothesis, we examined seasonal changes in nitrogen (N) and phosphorus (P) concentrations in aboveground organs of S. gracilistyla stands on a fluvial bar in the Ohtagawa River, western Japan. The concentrations in leaves decreased from April to May as leaves expanded. Thereafter, the concentrations showed little fluctuation until September. They declined considerably in autumn, possibly owing to nutrient resorption. We converted the nutrient concentrations in each organ to nutrient amounts per stand area on the basis of the biomass of each organ. The resorption efficiency of N and P in leaves during senescence were estimated to be 44 and 46%, respectively. Annual net increments of N and P in aboveground organs, calculated by adding the amounts in inflorescences and leaf litter to the annual increments in perennial organs, were estimated to be 9.9 g and 0.83 g m⁻² year⁻¹, respectively. The amounts released in leaf litter were 6.7 g N and 0.44 g P m⁻². These values are comparable to or larger than those of other deciduous trees. We conclude that S. gracilistyla stands acquire large amounts of nutrients and release a large proportion in leaf litter.     

Growth and biomass turnover ofHydrocharis dubia L. cultured under different nutrient conditions

Growth of a floating-leaved plant,Hydrocharis dubia L., was examined under varying nutrient conditions between 0.3 and 30 mgN l⁻¹ total inorganic nitrogen.H. dubia plants cultured under the most nutrient-rich condition showed the highest maximum ramet density (736 m⁻²), the highest maximum biomass (80.4 g dry weight m⁻²), and the highest total net production (185 g dry weight m⁻² in 82 days). Plants under nutrient-poor conditions had a relatively large proportion of root biomass and a small proportion of leaves with a long life span. Compared with other floating-leaved and terrestrial plants, the maximum biomass ofH. dubia was relatively small. This, and the rapid biomass turnover, was related to the short life span of leaves (13.2–18.7 days) and large biomass distribution to leaves.     

Phytoextraction of Cd and Zn from agricultural soils by Salix ssp. and intercropping of Salix caprea and Arabidopsis halleri

Contamination of agricultural topsoils with Cd above guideline values is of concern in many countries throughout the world. Extraction of metals from contaminated soils using high-biomass, metal-accumulating Salix sp. has been proposed as a low-cost, gentle remediation strategy, but reasonable phytoextraction rates remain to be demonstrated. In an outdoor pot experiment we assessed the phytoextraction potential for Cd and Zn of four willow species (Salix caprea, S. fragilis, S. × smithiana, S. × dasyclados) and intercropping of S. caprea with the hyperaccumulator Arabidopsis halleri on three moderately contaminated, agricultural soils. Large concentrations of Cd (250 mg kg⁻¹) and Zn (3,300 mg kg⁻¹) were determined in leaves of Salix × smithiana grown on a soil containing 13.4 mg kg⁻¹ Cd and 955 mg kg⁻¹ Zn, resulting in bioaccumulation factors of 27 (Cd) and 3 (Zn). Total removal of up to 20% Cd and 5% Zn after three vegetation periods were shown for Salix × smithiana closely followed by S. caprea, S. fragilis and S. × dasyclados. While total Cd concentrations in soils were reduced by up to 20%, 1 M NH₄NO₃-extractable metal concentrations did not significantly decrease within 3 years. Intercropping of S. caprea and A. halleri partly increased total removal of Zn, but did not enhance total Cd extraction compared to single plantings of S. caprea after two vegetation periods.     

Preliminary investigation of Okhotsk Sea ice algae; taxonomic composition and photosynthetic activity

Okhotsk Sea pack ice from Shiretoko in northern Hokkaido, sampled in March 2007, contained microalgal communities dominated by the centric diatoms Thalassiosira nordenskioeldi and T. punctigera. Domination by this genus is very unusual in sea ice. Communities from nearby fast ice at Saroma-ko lagoon were dominated by Detonula conferavea and Odontella aurita. Average microalgal biomass of the Okhotsk Sea pack ice (surface and bottom) was 1.59 ± 1.09 μg chla l⁻¹ and for fast ice (bottom only) at nearby Saroma-ko lagoon, 16.5 ± 3.2 μg l⁻¹ (=31.1 ± 5.0 mg chla m⁻²). Maximum quantum yield of the Shiretoko pack ice algal communities was 0.618 ± 0.056 with species-specific data ranging between 0.211 and 0.653. These community values are amongst the highest recorded for sea ice algae. Rapid light curves (RLC) on individual cells indicated maximum relative electron transfer rates (relETR) between 20.8 and 60.6, photosynthetic efficiency values (α) between 0.31 and 0.93 and onset of saturation values (E _k) between 33 and 91 μmol photons m⁻² s⁻¹. These data imply that the pack ice algal community at Shiretoko was healthy and actively photosynthesising. Maximum quantum yield of the Saroma-ko fast ice community was 0.401 ± 0.086, with values for different species between 0.361 and 0.560. RLC data from individual Saroma-ko fast ice algal cells indicated relETR between 55.3 and 60.6, α values between 0.609 and 0.816 and E _k values between 74 and 91 μmol photons m⁻² s⁻¹ which are consistent with measurements in previous years.     

Avoidance of willows from moderately polluted area by leaf beetle,Melasoma lapponica: effects of emission or induced resistance?

We studied preference for willows along a pollution gradient on the Kola Peninsula, Russia, by the leaf beetle, Melasoma lapponica. Multiple tests with leaf disks demonstrated low preference for Salix borealis, S. caprea and S. phylicifolia from the plot situated 14 km from the smelter, in comparison with conspecific plants from plots situated at 1 and 29 km distances. This pattern was observed when testing beetles orginating from any plot both in 1993 and 1994, using both young and mature leaves of S. borealis. Although fumigation of S. borealis with realistic SO₂ concentration (100 g/m³) increased plant palatability, preference for plants from our study plots did not correlate with plot-specific mean SO₂ concentrations. Furthermore, no correlation with foliar concentrations of the main metal pollutants (Ni and Cu) was found. Palatability of plants was negatively correlated with population density of M. lapponica, which peaked in the moderately polluted plot 14 km from the smelter. Within this plot, beetles clearly preferred non-damaged bushes of S. borealis to previously damaged bushes. We therefore conclude that low preference of S. borealis from the moderately polluted area was caused by plant resistance induced by severe damage from M. lapponica in previous years rather than by pollution impact. However, S. caprea and S. phylicifolia had little damage from M. lapponica, and low palatability of these species in the moderately polluted plot suggests changes in plant quality similar to changes in heavily damaged bushes of S. borealis.     

Soluble polyamines in Salix myrsinifolia and S. myrsinites × S. myrsinifolia plantlets exposed to increased UV-B irradiation and decreased watering

Hybridisation between certain willow species is a common feature leading to novel genotypes varying in growth rate and stress tolerance. The objective of this 4-week study was to investigate the effects of decreased watering, enhanced ultraviolet-B irradiation (UV-B_BE, 280–315 nm, 7.2 kJ m⁻² day⁻¹) and combined decreased watering and enhanced UV-B irradiation on di- and polyamines in the leaves of Salix myrsinifolia and its hybrid with S. myrsinites. Control plantlets were well-watered and exposed to ambient UV-B irradiation (UV-B_BE, 3.6 kJ m⁻² day⁻¹). HPLC analyses showed that the constitutive concentrations of soluble di- and polyamines varied markedly between S. myrsinifolia and its hybrids. The degree of responses to treatments also varied: in S. myrsinifolia, concentrations of free putrescine were clearly increased by reduced watering, while in the hybrid willow, change in putrescine was less pronounced and not significant. Results also showed that the increase in putrescine in S. myrsinifolia by reduced watering was mitigated by concurrent enhancement of UV-B irradiation. There were no direct UV-B effects on the soluble polyamines.     

Effect of wastewater sludge on growth and heavy metal bioaccumulation of two Salix species

Fast-growing tree species, such as willows, can benefit from sludge application. While sludges are good fertilizers, they may contain heavy metals which could reduce productivity and cause environment risks. The aims of the present research were to: i) determine the biomass production of Salix discolor Mühl. and Salix viminalis L. when supplied with various amounts of dried and pelleted sludge and ii) assess the uptake and accumulation of heavy metals. Trials were carried out using unrooted cuttings that were planted in large plastic pots containing sandy soil and grown outdoors for a 20-week period. Five doses of sludge were applied: the equivalents of 0 (T0), 40 (T1), 80 (T2), 120 (T3), 160 (T4) and 200 (T5) kg “available” N ha^-1. Trees which received the highest dosage of sludge showed the best growth. Stem biomass was significantly greater for S. viminalis which had received sludge treatments. The relationship between the total biomass yield Y (g) and the rate of fertilization X (equivalent to kg of “available” nitrogen provided per hectare) is linear. Regression equations of predicted biomass production were established as follows: S. discolor, Y=28.36+0.56X and S. viminalis, Y=39.95+0.64X. For both species, the greatest stem biomass per g of N applied was produced with treatment 4 and 5. Amounts of nitrogen per leaf area (N/LA) and per dry leaf mass (N/DL) were higher for S. viminalis. The metal transfer coefficient did not vary between the species but was significantly different for Cd and Zn. Plants were able to absorb Cd and Zn, but were less able to absorb Ni, Hg, Cu, and Pb. It was concluded that the dried and pelleted sludge is a good fertilizer. S. discolor and particularly S. viminalis can be used as filters for the purification of wastewater sludge as well as for biomass production purposes. R F Huettl Section editor     

Nonvascular contribution to ecosystem NPP in a subarctic heath during early and late growing season

Bryophytes and lichens abound in many arctic ecosystems and can contribute substantially to the ecosystem net primary production (NPP). Because of their growth seasonality and their potential for growth out of the growing season peak, bryophyte and lichen contribution to NPP may be particularly significant when vascular plants are less active and ecosystems act as a source of carbon (C). To clarify these dynamics, nonvascular and vascular aboveground NPP was compared for a subarctic heath during two contrasting periods of the growing season, viz. early-mid summer and late summer-early autumn. Nonvascular NPP was determined by assessing shoot biomass increment of three moss species (Hylocomium splendens, Pleurozium schreberi and Dicranum elongatum) and by scaling to ecosystem level using average standing crop. For D. elongatum, these estimates were compared with production estimates obtained from measurements of shoot length increase. Vascular NPP was determined by harvesting shrub and herb apical growth and considering production due to stem secondary growth of shrubs. Hylocomium splendens and Pleurozium schreberi showed highest biomass growth in late summer, whereas for D. elongatum this occurred in early summer. Maximum relative growth rates were ca. 0.003–0.007 g g⁻¹ d⁻¹. For D. elongatum, production estimates from length growth differed from estimations from biomass growth, likely because of an uncoupling between length growth and biomass shoot growth. Nonvascular NPP was 0.37 and 0.46 g dry weight m⁻² d⁻¹, in early and late summer, respectively, whereas in the same periods vascular NPP was 3.6 and 1.1 g dry weight m⁻² d⁻¹. The contribution of nonvascular NPP to total aboveground NPP was therefore minor in early summer but substantial in late summer, when 25% of the C accumulated by the vegetation was incorporated into nonvascular plant tissue. The expected global change-induced reduction of nonvascular plant biomass in subarctic heath is likely therefore to enhance C release during the late part of the growing season.     

Variation in the uptake of Arsenic, Cadmium, Lead, and Zinc by different species of willows Salix spp. grown in contaminated soils

The experiment assessed the variability of in seven clones of willow plants of high biomass production (Salix smithiana S-218, Salix smithiana S-150, Salix viminalis S-519, Salix alba S-464, Salix ’Pyramidalis’ S-141, Salix dasyclados S-406, Salix rubens S-391). They were planted in a pots for three vegetation periods in three soils differing in the total content of risk elements. Comparing the calculated relative decrease of total metal contents in soils, the phytoextraction potential of willows was obtained for cadmium (Cd) and zinc (Zn), moderately contaminated Cambisol and uncontaminated Chernozem, where aboveground biomass removed about 30% Cd and 5% Zn of the total element content, respectively. The clones showed variability in removing Cd and Zn, depending on soil type and contamination level: S. smithiana (S-150) and S. rubens (S-391) demonstrated the highest phytoextraction effect for Cd and Zn. For lead (Pb) and arsenic (As), the ability to accumulate the aboveground biomass of willows was found to be negligible in both soils. The results confirmed that willow plants show promising results for several elements, mainly for mobile ones like cadmium and zinc in moderate levels of contamination. The differences in accumulation among the clones seemed to be affected more by the properties of clones, not by the soil element concentrations or soil properties. However, confirmation and verification of the results in field conditions as well as more detailed investigation of the mechanisms of cadmium uptake in rhizosphere of willow plants will be determined by further research.     

Nomenclature of the promising biomass coppice willows,Salix x sericans Tausch ex Kern., Salix dasyclados Wimm. and Salix ‘Aquatica Gigantea’

Summary

Some promising biomass clones traded in Great Britain as Salix x smithiana Willd. (S. viminalis x S. cinerea) are in fact S. x sericans Tausch ex Kern. (S. viminalis x S. caprea) but the major problem discussed is that two distinct willows are masquerading under the same name, S. dasyclados. To avoid confusion it is suggested that S. x dasyclados Wimmer is retained for the hybrid (2n = 57), a female clone and formerly a notable Dutch and English basket willow, whereas for S. dasyclados Skvortsov non Wimmer, a species ranging from the Baltic to Siberia with 2n = 76, the name S. burjatica Nasarov, as suggested by Chmelar, is preferred. The willows distributed and designated Salix aquatica Gigantea by the Danish willows specialist Jensen, which formed the basis of many early biomass trials, are regarded at present as selections of, or very similar to, S. burjatica Nasarov (S. dasyclados sensu Skvortsov non Wimmer). In addition to a higher level of ploidy, they are visually distinguishable from S. x dasyclados Wimm. by their wider and longer leaves, persistent stipules and the existence of male forms.     

Phytoplankton biomass,P-I relationships and primary production in the Weddell Sea,Antarctica, during the austral autumn

An investigation into the changing phytoplankton biomass and total water column production during autumn sea ice formation in the eastern Weddell Sea, Antarctica showed reduced biomass concentrations and extremely low daily primary production. Mean chlorophyll-a concentration for the entire study period was extremely low, 0.15±0.01 mg.m⁻³ with a maximum of 0.35 mg.m⁻³ found along the first transect to the east of the grid. Areas of low biomass were identified as those either associated with heavy grazing or with deep mixing and corresponding low light levels. In most cases phytoplankton in the <20-μm size classes dominated. Integrated biomass to 100 m ranged from 7.1 to 28.0 mg.m⁻² and correlated positively with surface chlorophyll-a concentrations. Mean P^Bmax (photosynthetic capacity) and α^B (initial slope of the photosynthesis-irradiance curve) were 1.25±0.19 mgC. mgChla ⁻¹.h⁻¹ and 0.042±0.009 mgC.mgChla ⁻¹.h⁻¹.(μmol.m⁻².s⁻¹)⁻¹ respectively. The mean index of photoadaptation,I _k, was 32.2±4.0 μmol.m⁻².s⁻¹ and photoinhibition was found in all cases. Primary production was integrated to the critical depth (Z _cr) at each production station and ranged from 15.6 to 41.5 mgC.m⁻².d⁻¹. It appears that, other than grazing intensity, the relationship between the critical depth and the mixing depth (Z _mix) is an important factor as, ultimately, light availability due both to the late season and growing sea ice cover severely limits production during the austral autumn.     

Effect of light and temperature on the cyanobacterium <Emphasis Type="Italic">Arthronema africanum</Emphasis> - a prospective phycobiliprotein-producing strain

The effect of light intensity (50–300 μmol photons m⁻² s⁻¹) and temperature (15–50°C) on chlorophyll a, carotenoid and phycobiliprotein content in Arthronema africanum biomass was studied. Maximum growth rate was measured at 300 μmol photons m⁻² s⁻¹ and 36°C after 96 h of cultivation. The chlorophyll a content increased along with the increase in light intensity and temperature and reached 2.4% of dry weight at 150 μmol photons m⁻² s⁻¹ and 36°C, but it decreased at higher temperatures. The level of carotenoids did not change significantly under temperature changes at illumination of 50 and 100 μmol photons m⁻² s⁻¹. Carotenoids were about 1% of the dry weight at higher light intensities: 150 and 300 μmol photons m⁻² s⁻¹. Arthronema africanum contained C-phycocyanin and allophycocyanin but no phycoerythrin. The total phycobiliprotein content was extremely high, more than 30% of the dry algal biomass, thus the cyanobacterium could be deemed an alternative producer of C-phycocyanin. A highest total of phycobiliproteins was reached at light intensity of 150 μmol photons m⁻² s⁻¹ and temperature of 36°C, C-phycocyanin and allophycocyanin amounting, respectively, to 23% and 12% of the dry algal biomass. Extremely low (<15°C) and high temperatures (>47°C) decreased phycobiliprotein content regardless of light intensity.     

Primary production dynamics of dominant hydrophytes in Lake Provala (Serbia)

The objective of this investigation was to analyze the primary production of the dominant hydrophytes by monitoring levels of organic matter and organic carbon and estimating photosynthetic potential via the total chlorophyll content. The survey was conducted in Lake Provala (Serbia) throughout the peak vegetation period of the year 2000. The contents of organic matter and organic carbon for Myriophyllum spicatum L. were 105.11 g m⁻² and 73.66 g m⁻², Nymphoides peltata (Gmel.) Kunt. were 95.51 g m⁻² and 45.26 g m⁻² and Ceratophyllum demersum L. were 52.17 g m⁻² and 29.75 g m⁻². Chlorophyll A (Chl a) and chlorophyll A+B (Chl a+b) pigments ranged from 1.54 mg g⁻¹(Chl a) and 2.1 mg g⁻¹(Chl a+b) in M. spicatum to 5.27 mg g⁻¹(Chl a) and 7.53 mg g⁻¹(Chl a+b) in C. demersum. At full leaf out, the latter aquatic plants exceeded 50% cover of the open water surface. All species achieved maximum growth in June, but significant differences in growth dynamics were observed. At the end of the vegetation period, these plants sink to the bottom and decompose     

Biomass, carbonate standing stock and production of the mediterranean coralCladocora caespitosa (L.)

Summary The Mediterranean coralCladocora caespitosa often occurs in large beds, i.e. populations of hemispherical clonies with stock densities varying between 1.9 and 4 coloneis ·m⁻². Laboratory measurements of volume, skeleton weight, surface and number of corallites per colony, coupled with mean annual growth rates evaluated through sclerochronology, allowed for the estimation of biomass, skeleton bulk density, calcimass (carbonate standing stock) and secondary production (both organic and inorganic) of twoC. caespitosa beds at 4 and 9 m depth. The mean colony biomass varied between 0.73 and 0.99 kg dw ·m⁻², corresponding to a calcimass between 2 and 5 kg CaCO₃·m⁻². Organic secondary production was 215.5–305.4 g dw of polyps ·m⁻²·y⁻¹, while the potential (mineral) production was 1.1–1.7 kg CaCO₃·m⁻²·y⁻¹, for the year 1996–1997. These values show thatC. caespitosa is one of the major carbonate producers within the Mediterranean and one of the major epibenthic species originating stable carbonate frameworks both in recent and past times.     

The structure,diversity and somatic production of the fish community in an estuarine coastal lagoon,Ria de Aveiro (Portugal)

The present study aims to determine biological fish production of a lagoon and relate this to the commercial fisheries yield. The fish community of an estuarine lagoon in the west coast of Portugal was sampled between November 1998 and November 2000 to estimate the production ecology of the community, including somatic production, population size, species richness, species diversity, and biomass. Using the Allen curve method of determination, the total annual fish production of all fish species in the lagoon was calculated at 90.3 tonnes or 2.1 g m⁻² year⁻¹ in the first year and 106.7 tonnes or 2.5 g m⁻² year⁻¹ in the second year. The marine seasonal migrant species, sardine, Sardina pilchardus, which colonises the lagoon during the juvenile period of its life stages, produced more than 35 tonnes in each year and accounted for >39 and >33%, in the first and second year respectively, of the total fish production in this lagoon. Sardine was numerically more abundant (18,217 specimens) but due to their small size contributed only 13% to the total biomass. Sardine was thus the most important fish species in terms of the consumption and production processes of the whole fish community in this system. Commercial fisheries’ records indicate that approximately 300 tonnes per annum of fish are taken from the lagoon, which corresponds to three times more than the estimated production in the lagoon. Thus, if it exists, the sustainability of the fishery appears to depend on the immigration of fish from the adjacent coastal area and it is questioned whether the fishery is sustainable in the long-term. The findings indicate that careful and effective management of the lagoon is required to ensure a long-term healthy aquatic environment and sustainable catches in the future.     

Enhanced iturin A production by <Emphasis Type="Italic">Bacillus subtilis</Emphasis> and its effect on suppression of the plant pathogen <Emphasis Type="Italic">Rhizoctonia solani</Emphasis>

The behavior of Streptomyces peucetius var. caesius N47 was studied in a glucose limited chemostat with a complex cultivation medium. The steady-state study yielded the characteristic constants μ _max over 0.10 h⁻¹, Y _XS 0.536 g g⁻¹, and m_S 0.54 mg g⁻¹ h⁻¹. The product of secondary metabolism, ɛ-rhodomycinone, was produced with characteristics Y _PX 12.99 mg g⁻¹ and m _P 1.20 mg g⁻¹ h⁻¹. Significant correlations were found for phosphate and glucose consumption with biomass and ɛ-rhodomycinone production. Metabolic flux analysis was conducted to estimate intracellular fluxes at different dilution rates. TCA, PPP, and shikimate pathway fluxes exhibited bigger values with production than with growth. Environmental perturbation experiments with temperature, airflow, and pH changes on a steady-state chemostat implied that an elevation of pH could be the most effective way to shift the cells from growing to producing, as the pH change induced the biggest transient increase to the calculated ɛ-rhodomycinone flux.     

Carbonate standing stock and carbonate production of the bryozoanPentapora fascialis in the North-Western Mediterranean

Summary Pentapora fascialis, one of the largest living bryozoan, is often a predominant part of the benthos on hard subtidal bottoms in the Mediterranean Sea. Conversion factors calculated from laboratory measurements of colony size, biomass and skeleton weight, combined with density of colonies and mean annual growth rate allowed the estimation of carbonate standing stock, biomass and carbonate production ofPentapora fascialis in five sites in the Ligurian Sea. Carbonate standing stock ranged from 281 to 2490 g·m⁻², colony biomass varied from 8.82 to 78.01 g·m⁻², with a ratio biomass to carbonate standing stock of about 3%. Carbonate production of the bryozoan ranged in the five sites from 358 to 1214 g·m⁻²·y⁻¹. If compared with the few data available on carbonate production of bryozoans and other sublittoral benthic bioconstructors in the temperate regions,Pentapora fascialis has to be considered one of the major contributors to the carbonatebudget.     

Salinity effect on plant growth and leaf demography of the mangrove, Avicennia germinans L.

We assessed the effect of salinity on plant growth and leaf expansion rates, as well as the leaf life span and the dynamics of leaf production and mortality in seedlings of Avicennia germinans L. grown at 0, 170, 430, 680, and 940 mol m⁻³ NaCl. The relative growth rates (RGR) after 27 weeks reached a maximum (10.4 mg g⁻¹ d⁻¹) in 170 mol m⁻³ NaCl and decreased by 47 and 44% in plants grown at 680 and 940 mol m⁻³ NaCl. The relative leaf expansion rate (RLER) was maximal at 170 mol m⁻³ NaCl (120 cm m⁻² d⁻¹) and decreased by 57 and 52% in plants grown at 680 and 940 mol m⁻³ NaCl, respectively. In the same manner as RGR and RLER, the leaf production (P) and leaf death (D) decreased in 81 and 67% when salinity increased from 170 to 940 mol m⁻³ NaCl, respectively. Since the decrease in P with salinity was more pronounced than the decrease in D, the net accumulation of leaves per plant decreased with salinity. Additionally, an evident increase in annual mortality rates (λ) and death probability was observed with salinity. Leaf half-life (t _0.5) was 425 days in plants grown at 0 mol m⁻³ NaCl, and decreased to 75 days at 940 mol m⁻³ NaCl. Thus, increasing salinity caused an increase in mortality rate whereas production of new leaves and leaf longevity decreased and, finally, the leaf area was reduced.     

Invasion of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> Enhanced Ecosystem Carbon and Nitrogen Stocks in the Yangtze Estuary,China

Whether plant invasion increases ecosystem carbon (C) stocks is controversial largely due to the lack of knowledge about differences in ecophysiological properties between invasive and native species. We conducted a field experiment in which we measured ecophysiological properties to explore the response of the ecosystem C stocks to the invasion of Spartina alterniflora (Spartina) in wetlands dominated by native Scirpus mariqueter (Scirpus) and Phragmites australis (Phragmites) in the Yangtze Estuary, China. We measured growing season length, leaf area index (LAI), net photosynthetic rate (Pn), root biomass, net primary production (NPP), litter quality and litter decomposition, plant and soil C and nitrogen (N) stocks in ecosystems dominated by the three species. Our results showed that Spartina had a longer growing season, higher LAI, higher Pn, and greater root biomass than Scirpus and Phragmites. Net primary production (NPP) was 2.16 kg C m⁻² y⁻¹ in Spartina ecosystems, which was, on average, 1.44 and 0.47 kg C m⁻² y⁻¹ greater than that in Scirpus and Phragmites ecosystems, respectively. The litter decomposition rate, particularly the belowground decomposition rate, was lower for Spartina than Scirpus and Phragmites due to the lower litter quality of Spartina. The ecosystem C stock (20.94 kg m⁻²) for Spartina was greater than that for Scirpus (17.07 kg m⁻²), Phragmites (19.51 kg m⁻²) and the mudflats (15.12 kg m⁻²). Additionally, Spartina ecosystems had a significantly greater N stock (698.8 g m⁻²) than Scirpus (597.1 g m⁻²), Phragmites ecosystems (578.2 g m⁻²) and the mudflats (375.1 g m⁻²). Our results suggest that Spartina invasion altered ecophysiological processes, resulted in changes in NPP and litter decomposition, and ultimately led to enhanced ecosystem C and N stocks in the invaded ecosystems in comparison to the ecosystems with native species.