A comparison of irradiance and phosphorus effects on the growth of three submerged macrophytes

A fully factorial pond experiment was designed using two irradiance levels and two phosphorus concentrations to investigate irradiance and phosphorus effects on the growth of three submerged macrophytes: common waterweed (Elodea canadensis), Eurasian water milfoil (Myriophyllum spicatum), and water stargrass (Zosterella dubia). Results revealed that higher irradiance (230 μmol s⁻¹ m⁻² vs. 113 μmol s⁻¹ m⁻² at 2 m depth) had significant positive effects on submerged macrophyte growth: increasing the number of individuals (seven-fold), the number of species surviving (two-fold), aboveground biomass (11-fold), belowground biomass (10-fold), and total biomass (11-fold), whereas elevated sediment phosphorus (2.1–3.3 mg g⁻¹ vs. 0.7 mg g⁻¹ dry sediment) did not have any significant impact. However, responses to irradiance differ among macrophyte species due to their morphology and physiology. Waterweed increased in numbers of individuals and total biomass under high irradiance while biomass per individual remained the same (∼0.02 g). The other species increased both in numbers and biomass per individual. These results suggest that increased irradiance rather than decreased phosphorus loading is the main driver of changes in submerged macrophytes in North American temperate lake ecosystems.     

Epiphytes of the St. Lucia Estuary and their response to water level and salinity changes during a severe drought

St. Lucia is the largest estuary in South Africa with extensive areas of submerged macrophytes that fluctuate rapidly in response to water level and salinity changes. Epiphytes associated with submerged macrophytes were sampled during a severe drought between November 2004 and October 2005 when very low water level and high, variable salinity characterised the estuary. Potamogeton pectinatus and Ruppia cirrhosa were the dominant submerged macrophytes throughout the estuary, with P. pectinatus occurring at relatively low salinity (∼10 ppt) and R. cirrhosa at higher salinity (9–33 ppt). Zostera capensis, normally the other dominant submerged macrophyte, was conspicuously absent under the prevailing conditions. Epiphytic biomass, estimated as chlorophyll a, varied greatly between sites and over the 12 month sampling period, ranging from 10.9 to 71.7 mg Chl a m⁻² leaf area for P. pectinatus and 16.9–165.0 mg Chl a m⁻² leaf area for R. cirrhosa. Epiphytic biomass was twice as high in the Southern Lake where R. cirrhosa occurred, probably because the dominant epiphytes were macroalgae. An assessment of the diatom species composition of the epiphytic community indicated the dominance of only six species throughout the estuary. Neither epiphytic biomass nor diatom species composition showed strong statistical relationships with the environmental variables measured and it is believed that biological factors may be more important than the physico-chemical environment in determining epiphyte biomass distribution. Because epiphyte biomass is dependent on the presence of host surface area it will only contribute substantially to overall system biomass and productivity when submerged macrophyte area cover is high. In the St. Lucia Estuary this will occur when the water level is high and the upper level of the salinity gradient does not increase above that of seawater.     

Net aerial primary production (NAPP) of the marsh macrophyteScirpus maritimus estimated by a combination of destructive and non-destructive sampling methods

Net aerial primary production (NAPP) of marsh macrophytes is usually estimated either by destructive sampling techniques or by phenometric techniques. Destructive methods, however, are thought to be inaccurate while phenometric techniques are very labour intensive. In this study a new method is presented which allows an accurate and more efficient estimation of NAPP. The method combines destructive sampling to determine end-of-season biomass and phenometric techniques to estimate the mortality of biomass before the end of the season. NAPP is derived through summation of these two estimates. Techniques needed to calculate the precision of the NAPP estimate are provided. The so called hybrid technique was used to estimate NAPP ofScirpus maritimus L. in a brackish marsh along the Westerschelde estuary, the Netherlands. Estimated NAPP was 1372 g m^-2. End-of-season biomass accounted for 1106 g m^-2, while mortality contributed 266 g m^-2. Precision of the end-of-season biomass and the mortality estimates, expressed as coefficient of variation, was 18.2 and 26.0% respectively. The precision of the resultant, NAPP, was higher: 17.2%. These results indicate that NAPP could be estimated with a higher precision than end-of-season biomass. This contradicts the view that the accuracy of NAPP estimates can only be improved at the expense of its precision.     

Community metabolism and buffering capacity of nitrogen in a ruppia cirrhosa meadow

Fluxes of oxygen, inorganic nitrogen (DIN) and denitrification (isotope pairing) were measured from January 1997 to February 1998 via intact cores incubation in a shallow brackish area within the eutrophic Valli di Comacchio (northern Adriatic coast, Italy). Rates were measured in the light and in the dark in sediments colonized by the rooted macrophyte Ruppia cirrhosa and in adjacent sediments with benthic microalgae. Ruppia biomass (25-414 g DW m^− 2) exhibited a seasonal evolution whilst that of microphytobenthos (12-66 mg chl a m^− 2) was more erratic. Net (NP) and gross (GP) primary productivity was 1.15 and 6.89 mol C m^− 2y^− 1 for bare and 25.4 and 51.7 mol C m^− 2y^− 1 for Ruppia vegetated sediments. Nitrogen pools in Ruppia standing stock varied from 43.6 to 631.4 (annual average 201.2) mmol N m^− 2; the macrophyte N content was correlated with DIN concentration in the water column. Estimated N pool in microphytobenthos was one order of magnitude lower (from 2.4 to 14.5 mmol N m^− 2, annual average 7.2). Theoretical DIN assimilation calculated from NP was 127.8 and 1112.6 mmol N m^− 2y^− 1 whilst that calculated from GP was 765 and 2282 mmol N m^− 2y^− 1 for microphytobenthos and Ruppia respectively. Measured annual fluxes of DIN were 974.6 and − 577 mmol N m^− 2y^− 1 in bare and Ruppia vegetated sediments meaning that the two sites were a source and sink for DIN and that from 25 to 50% of Ruppia annual DIN requirements came from the water column. During the period of this study total denitrification was lower in the macrophyte colonized (92.3 mmol N m^− 2y^− 1) compared to bare sediments (163.3 mmol N m^− 2y^− 1) as a probable consequence of higher competition between denitrifiers and phanerogams. At both sites the ratio between denitrification of water column nitrate (D_W) and denitrification coupled to nitrification (D_N) was >1.6 due to little oxygen penetration in reducing sediments (< 1.2 mm) and scarce nitrification activity. D_W (0-35 µmol N m^− 2h^− 1) was significantly correlated with water column NO₃⁻  (2-16 µM). Theoretical DIN assimilation to denitrification ratio varied from 12.0 to 24.8 for Ruppia vegetated and from 0.8 to 4.7 for unvegetated sediments.At Valle Smarlacca, Ruppia may influence nitrogen cycling by incorporating large DIN pools in biomass which is scattered in surrounding areas and fuels intense bacterial activity. With increasing anthropogenic nutrient input and insignificant organic matter export in the open sea the already severe eutrophic conditions are enhanced and may accelerate the decline of the macrophyte meadow.     

The effect of different submerged macrophyte species and biomass on sediment resuspension in a shallow freshwater lake

Our study aim was to elucidate the effects of different species of submerged macrophytes and biomass levels on sediment resuspension. For this purpose experiments were conducted in four different enclosures (Potamogeton maackianus enclosure-PE, Vallisneria spinulosa enclosure-VE, manipulated enclosure-ME and aquaculture enclosure-AE). A sediment trap method was employed and the experiments were conducted from summer to winter in a shallow freshwater lake located in central China. A total of 813, 1277, 613 and 693 g DW m⁻² of sediment was resuspended in VE, AE, ME and PE, respectively. Our results showed that P. maackianus was more effective than V. spinulosa in restraining sediment resuspension. Macrophytes reached their maximum effectiveness of reducing resuspension at a certain species-specific biomass threshold above which biomass effects on resuspension were negligible. The threshold biomass was estimated as 300 g m⁻² for P. maackianus. Accordingly, within a lake management and aquaculture aspect, we conclude that as long as biomass does not fall below this threshold its consumption will not influence sediment resuspension. In the mid-lower reaches of the Yangtze River macrophyte coverage protects the lake sediment against adverse effects of monsoon wind; if the vegetation is eroded aquaculture sediment resuspension increases significantly.     

Potential respiration is a better respiratory predictor than biomass in young Artemia salina

These experiments test whether respiration can be predicted better from biomass or from potential respiration, a measurement of the mitochondrial and microsomal respiratory electron transport systems. For nearly a century Kleiber's law or a similar precursor have argued the importance of biomass in predicting respiration. In the last decade, a version of the Metabolic Theory of Ecology has elaborated on Kleiber's Law adding emphasis to the importance of biomass in predicting respiration. We argue that Kleiber's law works because biomass packages mitochondria and microsomal electron transport complexes. On a scale of five orders of magnitude we have shown previously that potential respiration predicts respiration as well as biomass in marine zooplankton. Here, using cultures of the branchiopod, Artemia salina and on a scale of less than 2 orders of magnitude, we investigated the power of biomass and potential respiration in predicting respiration. We measured biomass, respiration and potential respiration in Artemia grown in different ways and found that potential respiration (Ф) could predict respiration (R), both in µlO₂ h⁻¹ (R = 0.924Φ + 0.062, r² = 0.976), but biomass (as mg dry mass) could not (R = 27.02DM + 8.857, r² = 0.128). Furthermore the R/Ф ratio appeared independent of age and differences in the food source.     

Biomass accumulation and energy conversion efficiency in aromatic rice genotypes

A field experiment was conducted to evaluate photosynthetic efficiency along with different growth parameters of aromatic rice genotypes. Forty genotypes including three non-aromatic checks exhibited enormous variations for leaf area index (LAI), crop growth rate (CGR), relative growth rate (RGR), net assimilation rate (NAR), grain yield, total dry matter, harvest index and photosynthetic efficiency or energy use efficiency (Eμ) at panicle initiation and heading stages. Minimum LAI-value was 0.52 in Khazar at PI stage and maximum was 4.91 in Sakkor khora at heading stage. The CGR-value was in the range of 4.80−24.11 g m⁻² per day. The best yielder BR39 produced grain of 4.21 t ha⁻¹ and the worst yielder Khazar gave 1.42 t ha⁻¹. Total dry matter (TDM) yield varied from 4.04 to 12.26 t ha⁻¹ where genotypes proved their energy use efficiency a range between 0.58 to 1.65%. Eμ showed a significant positive relation with TDM (r = 0.80^**), CGR (r = 0.72^**) and grain yield (r = 0.66^**). A negative correlation was established between TDM and harvest index and LAI and RGR. Path analysis result showed that NAR at heading stage exerted highest positive direct effect (0.70) on Eμ.     

Factors controlling vegetation structure in peatland lakes—Two conceptual models of plant zonation

Two conceptual models of plant zonation in peatland lakes are given. The first represents vegetation on slightly sloping substrate (N < 0.2) in shallow and relatively large lakes. The vegetation is not diverse (H′ = 0.0 ± 0.01). The frequency and biomass of the dominant (Sphagnum denticulatum) correlate positively with lake size, and negatively with depth and substrate slope. They are also correlated with water transparency and water color (r = −0.53), concentrations of total organic carbon (r = −0.43), Ca²⁺ (r = 0.40) and humic acids (r = −0.46), and redox potential (r = 0.44). The second model represents vegetation on steep peat walls (N > 0.3) in deep, usually small lakes. Plants occur only on the upper part of the peat wall or form a multispecies curtain hanging from the lip of peat at the top. Species diversity in this scenario is higher (H′ = 0.18 ± 0.17). The curtains usually are composed of mosses such as Warnstorfia exannulata, S. cuspidatum and S. riparium, and vascular plants are rare. The frequency and biomass of bryophytes in this type of structure are related to substrate slope (r = 0.56), lake depth (r = 0.56), Ca²⁺ concentration (r = −0.69) and water color (r = −0.51). In both models, plant biomass is correlated with temperature (r = −0.78), irradiance (r = −0.64) and water oxygenation (r = −0.54).     

Response of native Egeria najas Planch. and invasive Hydrilla verticillata (L.f.) Royle to altered hydroecological regime in a subtropical river

Egeria najas Planch. is the dominant native submersed macrophyte of the Upper Paraná River in Brazil, while Hydrilla verticillata (L.f.) Royle has recently invaded this area. From January 2006 to December 2007, comprising two annual flood cycles, we conducted monthly surveys at two river stations and two lakes connected to the river within this stretch of the Paraná River, aiming to understand how the hydrological regime influences the distribution and abundance of these native and invasive Hydrocharitaceae species. Hydrilla did not develop in the lakes, possibly due to the elevated proportion of organic matter in the sediment (∼10% DW). However, the exotic species dominated the river sites apparently suppressing E. najas. In the lakes E. najas reached a maximum biomass of 628 ± 82 g DW m⁻² but did not surpass 333 ± 83 g DW m⁻² in the river, where H. verticillata peaked at 1415 ± 255 g DW m⁻². Macrophyte biomass development was greatest during low-water periods, with transparent water and high temperatures. Floods probably affected submersed macrophytes (especially in 2007, when an extreme flood caused by an El Niño Southern Oscillation (ENSO) event occurred) via sediment movement and plant scouring (uprooting) effects, coupled with reduced water transparency. Macrophyte recovery started soon after the (less intense) 2006 flood but was delayed in 2007. In the river recovery started five months after the major flood, but in the lakes no significant plant regeneration was found even nine months after the disturbance. E. najas and H. verticillata started regeneration practically at the same time but H. verticillata had much higher rates of biomass increase.     

Tolerance and remedial function of rooted submersed macrophyte Vallisneria spiralis to phenanthrene in freshwater sediments

Tolerance and remedial function of submersed macrophyte Vallisneria spiralis to phenanthrene in freshwater sediments were investigated by manipulating initial phenanthrene concentrations in sediments from 8 to 80 mg kg⁻¹ dry sediment. The biomass growth of V. spiralis on phenanthrene-spiked sediments was not adversely affected until initial phenanthrene concentrations in sediments increased to 80 mg kg⁻¹ dry sediment. V. spiralis might evolve adaptive mechanisms to toxic contaminants in sediment, and then could change the growth patterns in order to decrease the toxicity on its growth. The removal efficiencies of phenanthrene from the planted sediments were 18% higher than those from the sediments without plant even under an initial phenanthrene concentration of 80 mg kg⁻¹ dry sediments. The enhanced removal of phenanthrene in sediments by the plant might be achieved mainly by the synergism between plant roots and microbes in the rhizosphere.     

Effects of abiotic conditions on Phragmites australis along geographic gradients in Lake Burullus, Egypt

Stand structure and biomass production of Phragmites australis (Cav.) Trin. ex Steud. were analyzed along north-south and east-west transects in the Burullus coastal lagoon (N Egypt, 410 km²) at monthly intervals over a period of 1 year (February 2003 until January 2004). For this purpose, young and old stands were selected at eight different locations in the lagoon. It was found that the north-south transect mainly represented a fertility gradient (207-286 mg l⁻¹ TN, 30-106 mg l⁻¹ TP), while the east-west transect was associated with significantly decreasing salinity (7-4 ppt). All morphological and biomass variables of P. australis were significantly different between young and old stands. On average, the old (7.3 ± 0.2 kg DW m⁻²) accumulated three times more total above-ground biomass than the young stands (2.5 ± 0.1 kg DW m⁻²). Shoot height, diameter and shoot dry weight significantly increased by 25-50% with increasing fertility along the north-south transect. Shoot density significantly decreased from north to south, while it almost doubled in the north sites from 109 ± 6 to 216 ± 7 shoots m⁻² along the west-east transect. In separate stepwise multiple regressions, variation in water quality explained 34-63% of the variation in morphology and total above-ground biomass in the old stands (salinity and water level were most important for biomass, transparency also for height and density) while it explained 16-42% of variation in young stands (mainly transparency).     

How do macrophyte distribution patterns affect hydraulic resistances?

In eutrophic river systems, macrophytes attain high biomass with reduced drainage and increased flooding risk. To avoid these problems, water managers remove vegetation. Total removal, however, increases wash out of macro-invertebrate communities reducing the ecological value of rivers. Partial vegetation removal reduces this washout and prevents an increase in hydraulic resistance. In this, study the hydraulic performance of three partial vegetation removal patterns was tested. From the results it was seen that hydraulic resistance, expressed as Manning's n, was varying between 0.025 m^−1/3 s and 0.050 m^−1/3 s. Compared with the empty situation, the different distribution patterns increased resistance between 14 and 23%. Hydraulic resistance of these patterns was also significantly influenced by the species present in the vegetation patches. Three groups of macrophyte plants (emerged, floating leaved and submerged) with significantly different hydraulic resistances were determined. The emerged species Sparganium erectum generated the least resistance with an average friction of 0.03 m^−1/3 s. Stuckenia pectinata and Potamogeton natans had slightly higher friction values around 0.4 m^−1/3 s. Ranunculus penicillatus and Callitriche platycarpa had average friction values around 0.05 m^−1/3 s.The proposed vegetation removal patterns are good alternatives to create a management system, which minimally increases hydraulic resistance but still guarantees the ecological functions.     

Overyielding and the role of complementary use of nitrogen in wetland plant communities

Biodiversity and ecosystem functioning experiments have demonstrated that plant biomass of species grown in mixtures is often greater than plant biomass of monocultures (i.e., mixtures over yield). While we understand that plant species utilize resources differently, how a combination of species increases resource use and productivity is not well known, especially in wetland ecosystems. Here, we used a mesocosm experiment to explore diversity effects on plant biomass production and to examine the role of N partitioning as a mechanism for overyielding in wetland ecosystems. Plant functional groups (FGs) represented the unit of diversity, and we included five levels of diversity (0-4 FGs). To test for N partitioning, we used a stable isotope technique to determine niche breadth and proportion similarity of inorganic N use (NO₃⁻ and NH₄⁺) for individual FGs as well as mixtures containing 3 and 4 FGs. We found that total plant biomass increased in the first season from an average of 290 ± 60 SE g ash-free dry mass (AFDM) m⁻² at the 1 FG level to 490 ± 70 g AFDM m⁻² at the 4 FG level and in the second season from an average of 560 ± 80 g AFDM m⁻² at the 1 FG level to 1000 ± 90 g AFDM m⁻² at the 4 FG level indicating overyielding. Plant species comprising the majority of mesocosm biomass demonstrated preferential uptake of ¹⁵NO₃⁻, while species with relatively less biomass (e.g., Acorus calamus and Carex crinita) preferred ¹⁵NH₄⁺. Concentrations of ¹⁵N in biomass increased with FG richness, but only in the ¹⁵NO₃⁻ treatment. Niche breadth did not vary among levels of FG richness. We observed a greater niche overlap with an increase of FGs, with species taking up greater proportion of ¹⁵NO₃⁻ than ¹⁵NH₄⁺. Our results indicate that plant overyielding in wetland mesocosms is not the result of niche partitioning of N chemical forms, but is associated with greater uptake of NO₃.     

Forest biomass and volume estimation using airborne LiDAR in a cool-temperate forest of northern Hokkaido,Japan

Trees are recognized as a carbon reservoir, and precise and convenient methods for forest biomass estimation are required for adequate carbon management. Airborne light detection and ranging (LiDAR) is considered to be one of the solutions for large-scale forest biomass evaluation. To clarify the relationship between mean canopy height determined by airborne LiDAR and forest timber volume and biomass of cool-temperate forests in northern Hokkaido, Japan, we conducted LiDAR observations covering the total area of the Teshio Experimental Forest (225 km²) of Hokkaido University and compared the results with ground surveys and previous studies. Timber volume and aboveground tree carbon content of the studied forest stands ranged from 101.43 to 480.40 m³ ha^–1 and from 30.78 to 180.54 MgC ha^–1, respectively. The LiDAR mean canopy height explained the variation among stands well (volume: r² = 0.80, RMSE = 55.04 m³ ha^–1; aboveground tree carbon content: r² = 0.78, RMSE = 19.10 MgC ha^–1) when one simple linear regression equation was used for all types (hardwood, coniferous, and mixed) of forest stands. The determination of a regression equation for each forest type did not improve the prediction power for hardwood (volume: r² = 0.84, RMSE = 62.66 m³ ha^–1; aboveground tree carbon content: r² = 0.76, RMSE = 27.05 MgC ha^–1) or coniferous forests (volume: r² = 0.75, RMSE = 51.07 m³ ha^–1; aboveground tree carbon content: r² = 0.58, RMSE = 19.00 MgC ha^–1). Thus, the combined regression equation that includes three forest types appears to be adequate for practical application to large-scale forest biomass estimation.     

Productivity, carbon dioxide uptake and net energy return of microalgal bubble column photobioreactors

This work examined the energy return of Chlorella vulgaris and Dunaliella tertiolecta cultivated in a gas-sparged photobioreactor design where the power input for sparging was manipulated (10, 20, and 50 W m⁻³). Dry weight, organic carbon and heating values of the biomass were measured, plus a suite of variables including Fv/Fm and dissolved oxygen. A model for predicting the higher heating value of microalgal biomass was developed and used to measure the energetic performance of batch cultivations. High power inputs enhanced maximum biomass yields, but did not improve the energy return. Cultivation in 10 W m⁻³ showed up to a 39% higher cumulative net energy return than 50 W m⁻³, and increased the cumulative net energy ratio up to fourfold. The highest net energy ratio for power input was 19.3 (D. tertiolecta, 12% CO₂, 10 W m⁻³). These systems may be a sustainable method of biomass production, but their effectiveness is sensitive to operational parameters.     

Growth, secondary production and gonad development of two co-existing amphioxus species (Branchiostoma belcheri and B. malayanum) in subtropical Hong Kong

The present study investigated the growth, secondary production and gonad development of two co-existing amphioxus species Branchiostoma belcheri and B. malayanum in subtropical Hong Kong from June 2005 to June 2006. Based on the modal progression analysis, amphioxus populations were decomposed into separate cohorts. The von Bertalanffy growth models were also estimated according to the size incremental data. From the growth models, the size ranges of one, two and three-year-old B. belcheri were estimated to be 5-28 mm, 28-38 mm and 38-45 mm BL, respectively; while the one and two-year-old B. malayanum were estimated to be 7-30 mm and 30-35 mm BL, respectively. The secondary production was calculated at 1.15 g m^− 2 yr^− 1 DW or 0.63 g m^− 2 yr^− 1 AFDW for B. belcheri with density 424 ind m^− 2, and 0.51 g m^− 2 yr^− 1 DW or 0.40 g m^− 2 yr^− 1 AFDW for B. malayanum with density 121 ind m^− 2. The production to biomass ratio (P/B) was 1.13 for B. belcheri and 0.98 for B. malayanum. Changes in the gonad length index indicated that B. belcheri spawned mainly in June and July, while B. malayanum mainly in April and August. As compared with B. belcheri, B. malayanum was characterized by rapid growth, shorter life span, early maturity and lower population density. Such differences in population dynamics may allow both species to share a similar habitat and co-exist in subtropical waters of Hong Kong.     

Is the biomass of water hyacinth lost through herbivory in native areas important?

The lamina area damaged and biomass per leaves removed by invertebrate herbivores were measured across seasons on water hyacinth, Eichhornia crassipes (Mart.) Solms (Pontederiaceae). The amount of the leaf biomass per meter square lost through herbivory was also assessed in different sampling dates in the plant population. Ten leaves of water hyacinth were sampled in each of 18 site-habitat-date combinations. Sampling dates were chosen to follow the plant phenology. The lamina area damaged (surface abrasions and holes) was measured with the visual estimation method; biomass removed by herbivores (surface abrasions and holes) was calculated indirectly from the damaged lamina area. Significant differences in total damaged area and removed biomass per lamina were found between sampling dates at each site, with highest values in March (end of growth period). Total damaged area per lamina (surface abrasions + holes) varied between 11% in March and 6% in July (decay period). Total removed biomass (surface abrasions + holes) varied between 27% in March and 13% in July. Significant differences in biomass removed by herbivory were found between sampling dates at each site. Biomass of lamina removed by herbivores per m⁻² varied between 26 and 13% in different seasons. The herbivore damage of discrete samples and the indirect method to calculate the biomass removed is useful in sites with aquatic free floating plants, where experimental exclusion of insects may be difficult to carry out.     

Estimating Arundo donax shoot biomass

We developed an equation for estimating Arundo donax shoot dry weight from shoot length. The equation, shoot dry weight (g) = 14.254 (standard error = ±0.275) × shoot height² (m), was as effective at explaining a high proportion of total variation in shoot dry weight (R² = 0.90) as more complicated equations containing additional morphometric parameters. Tested against two independent datasets, the equation provided accurate estimates of dry weight for shoots ranging from 0.3 to 7.06 m height (dataset 1, P < 0.0001, R² = 0.87, N = 29; dataset 2, P < 0.0001, R² = 0.82, N = 192). The equation provides aboveground biomass estimates from stem counts and heights more rapidly than harvest methods.     

Photosynthetic and growth responses of Egeria densa to photosynthetic active radiation

Egeria densa, a submerged aquatic macrophyte native to South America, has successfully invaded many reservoirs in Brazil and elsewhere. Ecophysiological responses of E. densa to light availability were assessed in microcosm experiments. Under low light conditions, we found that apical shoots expanded more rapidly than those under higher light exposure, allowing the plant to reach the higher light conditions of the surface. E. densa showed low k_m (15.6-34.8 μmol m⁻² s⁻¹ PAR) and light compensation point values (7.5-16.2 μmol m⁻² s⁻¹ PAR), indicating that it is able to effectively exploit the low radiation levels available at high depths and turbid waters. This may represent a competitive advantage over other submerged species, and it helps to explain the successful spread of E. densa in Brazilian reservoirs.     

Early growth responses of mangroves to different rates of nitrogen and phosphorus supply

Experiments were conducted in an outdoor facility to quantify growth responses of six mangrove species to rates of dissolved inorganic nitrogen and phosphorus supply mimicking the range of N and P mineralization rates in natural soils. Growth of all six species on nitrogen was nonlinear. Stem extension rates of Rhizophora apiculata and Xylocarpus granatum were enhanced to the highest rate of N supply (50 mmol m^− 2 d^− 1); Bruguiera gymnorrhiza, Avicennia marina, and Xylocarpus moluccensis stem growth leveled off by 10 mmol m^− 2 d^− 1. Stem growth of Ceriops tagal peaked at 24-26 mmol N m^− 2 d^− 1. Except for A. marina and C. tagal, rates of biomass increase declined at the highest supply rate, indicating NH₄⁺ toxicity. At different rates of P supply, stem extension rates and rates of biomass increase of R. apiculata and C. tagal best-fit Gaussian curves and B. gymnorrhiza stem growth and biomass increase best-fit sigmoidal and Gaussian curves, respectively; X. moluccensis stem and biomass growth increased linearly, but stem and biomass growth rates of A. marina did not vary in relation to P supply. Stem growth of X. granatum was Gaussian but rates of biomass increase best-fit a quadratic equation. Changes in leaf and root N and P content mirrored the growth responses. As rates of N and P mineralization in natural mangrove soils overlap with the lowest rates of N and P supplied in these experiments, the growth responses imply that mangroves are intrinsically nutrient-limited at mineralization rates often encountered in nature. Such species specificity may have significant implications for recruitment success and the establishment of species gradients within mangrove forests.