Decomposition of duckweed (Lemna gibba) under axenic and microbial [-2pt] conditions: flux of nutrients between litter water and sediment, the impact of leaching and microbial degradation

The decomposition of axenic Lemna gibba has been studied over a 200 day period under laboratory conditions in the presence and absence of wastewater micro-organisms. The residual mass of plant litter in the decomposition vessels decreased three times more rapidly under biotic than abiotic conditions. The organic matter in the duckweed litter lost about half its weight within 67.9 days in the presence of micro-organisms while more than 200 days were required in axenic vessels. In the former case, AFDW loss followed an exponential pattern of decay. The rate constant was 0.0102 day ^–1 and the decay was virtually complete after 200 days. The C and K concentration of the remaining duckweed litter decreased; the N, Ca, Fe and B concentration increased in both treatments. The concentration of total N, P, K, Mg, and Mo increased in the receiving water in both treatments but was much higher under biotic than abiotic conditions. Mass balances of nutrients in the vessels and flux of these nutrients between compartments in the vessels (duckweed litter, water and sediment) have been determined. Under axenic conditions the release of elements was very slow. Only notably potassium leaching had occurred. Leaching of potassium, magnesium and organic carbon took place mainly during the first term of incubation and then slowed down. Under biotic decomposition the elemental content of the litter decreased by more than 50% over 43 days for K, 53 days for Mo, 64 days for C, 81 days for Mg, 101 days for S, 104 days for P, 108 days for Na, 111 days for N, 140 days for B. Calcium and iron immobilised in the litter. Most of the released N, S, P, K, Mg and Mo remained in the water, but B and Mn settled into the sediment. The result of the investigation demonstrated that the nutrient flux from decomposing duckweed litter is mainly a microbially mediated process.     

Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil

We studied C and N mineralisation patterns from a large number of plant materials (76 samples, covering 37 species and several plant parts), and quantified how these patterns related to biological origin and selected indicators of chemical composition. We determined C and N contents of whole plant material, in water soluble material and in fractions (neutral detergent soluble material, cellulose, hemicellulose and lignin) obtained by stepwise chemical digestion (modified van Soest method). Plant materials were incubated in a sandy soil under standardised conditions (15 °C, optimal water content, no N limitation) for 217days, and CO₂ evolution and soil mineral N contents were monitored regularly. The chemical composition of the plant materials was very diverse, as indicated by total N ranging from 2 to 59 mg N g^–1, (i.e. C/N-ratios between 7 and 227). Few materials were lignified (median lignin=4% of total C). A large proportion of plant N was found in the neutral detergent soluble (NDS) fraction (average 84%) but less of the plant C (average 46%). Over the entire incubation period, holocellulose C content was the single factor that best explained the variability of C mineralisation (r=–0.73 to –0.82). Overall, lignin C explained only a small proportion of the variability in C mineralisation (r=–0.44 to –0.51), but the higher the lignin content, the narrower the range of cumulative C mineralisation. Initial net N mineralisation rate was most closely correlated (r=0.76) to water soluble N content of the plant materials, but from Day 22, net N mineralisation was most closely correlated to total plant N and NDS-N contents (r varied between 0.90 and 0.94). The NDS-N content could thus be used to roughly categorise the net N mineralisation patterns into (i) sustained net N immobilisation for several months; (ii) initial net N immobilisation, followed by some re-mineralisation; and (iii) initially rapid and substantial net N mineralisation. Contrary to other studies, we did not find plant residue C/N or lignin/N-ratio to be closely correlated to decomposition and N mineralisation.     

Phosphorus concentration in sediment, water and tissues of␣three submerged macrophytes of Myall Lake, Australia

Phosphorus content in sediment, water and tissues of three macrophyte species growing in Myall Lake, Australia were studied from January to November, 2004. The sites investigated were North–West (NW), North–East (NE), South–West (SW) bays and Central deep area of the lake (CL). The objective of this study was to investigate how total phosphorus (TP) in plant tissues relate to phosphorus pools and the role played by the aquatic macrophyte species under investigation in phosphorus recycling in the lake. Of the four investigated sites of the lake, TP in plant tissues were significantly higher in North–West and South–West bays compared to the rest. Najas marina had significantly higher TP content (e.g., 1.55 and 1.44 mg/g dw.; P < 0.05) for NW and SW respectively, than the other two species. N. marina is a rooted macrophyte while charophytes (C. fibrosa and Nitella hyalina) are pseudo-rooted macrophytes. Total phosphorus in the sediment and water column were significantly higher in Central deep area of the lake compared to the other three bays (P < 0.05, n 5). Soluble reactive phosphorus (SRP) and total dissolved phosphorus (TDP) in sediment pore water correlated significantly with phosphorus content in the tissue of N. marina ( ; ) as well as TP in sediment (␣ and ). Using the two-compartmental uptake model, it was observed that, sediment was the main compartment through which Ni. hyalina obtained phosphorus while for the other two species, water column was the uptake route for the phosphorus. These correlations suggest that, water column and sediments are important pathways for phosphorus uptake in plants.     

Decomposition of macroalgae,vascular plants and sediment detritus in seawater: Use of stepwise thermogravimetry

The applicability of a recently presented method (Stepwise Thermogravimetry, STG) to characterize biogenic organic matter (Kristensen 1990) was tested in comparative decomposition experiments. The initial microbial decay of pre-dried, fresh detritus from 6 different plant materials (2 macroalgae, 2 seagrasses, and 2 tree leaves) was examined for 70 days in aerobic seawater slurries. In addition, slurries of sediment detritus of low reactivity, representing the late stage of plant decay, were allowed to decompose aerobically and anaerobically for 200 days. Macroalgae lost 40–44% carbon over 70 days, whereas seagrasses lost 29–33% and tree leaves lost 0–8%. After a 3–5 days leaching phase, the temporal pattern of POC and PON loss from the plant detritus was exponential with higher rates for the former resulting in a 5–28% reduced C:N ratio. The Rp index decreased (<20%) during the initial leaching phase followed by a 30–40% increase to the end. Initial Rp was directly proportional to decay rate. POC loss in the anaerobic sediment slurry was 10% over the 200 day period (the aerobic was hampered by low pH). Preferential loss of PON caused a 30% increase in C:N ratio. The Rp index of sediment detritus also increased with 30%. Although the present laboratory experiments not fully simulate the natural environment, the Rp-C:N relationship obtained from the two slurry experiments can illustrate the general pattern of plant decay from fresh to refractory (humic) detritus. During initial aerobic decay, rapid leaching and microbial growth causes a decrease in both Rp and C:N (Rapid growth, phase 1). When all labile substrates have been consumed and the slower decay is controlled by enzymatic attack on particles with an associated production of humic compounds and accumulation of nitrogen rich bacterial cell remains, Rp increases and C:N decreases (Slow growth, phase 2). Later, when condensed humic compounds have accumulated, decay ceases. Most carbon is now bound in forms which are of low availability to bacteria and a preferential mineralization of nitrogen occurs; both Rp and C:N increases (Condenzation, phase 3).     

Diversity of catechol 2,3-dioxygenase genes of bacteria responding to dissolved organic matter derived from different sources in a eutrophic lake

Catechol 2,3-dioxygenase (C23O) is an extradiol dioxygenase that plays an important role in degrading aromatic compounds such as those found at polluted sites. However, little is known about the diversity of C23O genes in unpolluted environments. In such environments, various factors, including the quality and quantity of dissolved organic matter (DOM), could influence the composition and behaviour of bacterial community possessing C230 genes. We investigated C23O genes in bacteria responding to DOM from various sources in a eutrophic lake by PCR and cloning. Six microcosms filled with lake water containing indigenous bacteria and DOM from different sources were incubated for 10 days. After 1 or 2 days of incubation, C23O genes were detected in the microcosms enriched with DOM recovered from inflow river water and humus from reed grass. The sequences were very diverse but had features conserved in extradiol dioxygenases. The clone libraries generated on day 2 showed distinctive compositions among microcosms, indicating that bacteria possessing a variety of C23O genes responded differently to DOM from different sources. After 10 days of incubation, C23O genes in a previously unidentified gene cluster, 'Cluster X', became dominant in the libraries.     

Rapid anaerobic mineralization of pyridine in a subsurface sediment inoculated with a pyridine-degrading Alcaligenes sp.

Summary A denitrifying bacterium capable of pyridine mineralization under anaerobic conditions was isolated from polluted soil. The bacterium, identified as Alcaligenes sp., was used in inoculation experiments. A subsurface sediment from a polluted site was amended with 10 g/g ¹⁴C-labeled pyridine, and 250 g/g nitrate, and then inoculated with the bacterium at an inoculum size of 4.5 × 10⁷ cells/g. After 44 h incubation at 28° C under anaerobic conditions, 67% of the radioactivity was recovered as ¹⁴CO₂: 2% was extracted with 50% methanol, and 24% was recovered by combustion of the sediment. Analysis of the methanol extract revealed that no pyridine could be detected in the inoculated sediment. In contrast, mineralization of pyridine by the native microflora in the sediment occurred much more slowly: after 7 days of incubation only 10% of the added radioactivity was recovered as ¹⁴CO₂. At an inoculum size of 2 × 10³ cells/g pyridine mineralization was not as effective as at an inoculum size of 2 × 10⁷ cells/g. It is presumed that suppression of the introduced bacteria by the native microflora of the sediment prevents degradation at a low inoculum size. Amending the sediment with nitrate and phosphate improved pyridine mineralization by the introduced bacterium. These findings demonstrate the feasibility of using soil inoculation anaerobically for the bioremediation of pyridine-polluted soils.     

Paleolimnological evaluation of historical trophic state conditions in hypereutrophic Lake Thonotosassa, Florida, USA

We used paleolimnological methods to evaluate historical water quality in Lake Thonotosassa, Hillsborough County, Florida, USA. Sediment mapping shows that organic deposits are unevenly distributed in the lake. Two short (<130 cm) sediment cores from the depositional zone were analyzed for radioisotopes (²¹⁰Pb, ²²⁶Ra, and ¹³⁷Cs), bulk density, organic matter concentration, nutrients (C,N,P), and diatoms. ²¹⁰Pb results indicate that the profiles represent > 100 years of sediment accumulation. There is an abrupt change in sediment composition at about the turn of the century (80 cm depth), above which bulk density decreases and concentrations of organic matter, total C, total N, total P, and ²²⁶Ra activity increase. Diatom-based reconstructions of historical water-column trophic conditions indicate progressive nutrient enrichment in the lake during the past 100 years. Stratigraphic changes in diatom assemblages suggest that anthropogenic nutrient loading converted Lake Thonotosassa from a naturally eutrophic system to a hypereutrophic waterbody after 1900. Given the edaphic setting of Lake Thonotosassa, efforts to mitigate recent anthropogenic impacts will, at best, yield the eutrophic conditions that characterized the lake prior to human disturbance. This study illustrates the importance of paleolimnological data for targeting realistic water quality conditions when lake restoration is contemplated.Journal Series No. R-05019 of the Florida Agricultural Experiment Station     

Nutrient relationships in shallow water in an African Lake,Lake Naivasha

Summary In the littoral zone of a shallow, tropical lake (Lake Naivasha, Kenya), average nutrient composition of emergent macrophytes along a permanent transect (0–2m depth) on a dry weight basis was: P 0.23%; N 0.96%; and S 0.11%. In the hydrosoil the average composition was much lower, sediments were: P 0.03%; N 0.24%; and S 0.05%. The water depth varied, with lake edge being exposed during the annual drawdown for a part of the year and subsequently being inundated. Water quality varied considerably during the year (temperature 19–28°C; pH 7.0–8.0; conductivity 282–975 Scm^-1).Of the three nutrients in the water of the littoral zone, N had the highest mean concentration (4.25 mg·1^-1) while P was intermediate (1.90) and sulphur had the least (0.99). The distribution of nutrients followed a decreasing gradient from shore to open water. High levels of nutrients were recorded in September following the inundation of drawdown soil and plant material.The large stock of nutrients generated in the littoral zone helps to replenish nutrients in the open lake where low concentrations are typical.     

Sedimentation in a large lake: the importance of fluctuations in water level

1. A sediment trap study was conducted in the central basin of Lower Lough Erne, Northern Ireland.
2. The material caught was low in organic matter, ash weight was 77% of dry weight on average.
3. Mass deposition of material was shown to be correlated with lake water level. When lake water level fell or rose above the middle range, the amount of material caught by the trap increased markedly. The effect was exaggerated by an unusually dry summer in 1995 which caused abnormally low water levels in the lake.
4. Comparison of Erne data with Windermere shows similar deposition rate of many substances, but the deposition rate of iron is thirteen times higher in Erne than in Windermere, and iron made up 39% of the ash weight in the Erne trap data.
5. Only 17% of the silica removed from the water column during diatom growth was recovered in the trap over the same time period.
6. Trap data are compared with historical data for sediment deposition and the implications of sediment focusing are discussed.     

The role of bacteria in the nutrient exchange between sediment and water in a flow-through system

The contribution of bacteria to phosphorus (P) and nitrogen (N ) release from, or retention in, sediment was studied in a flow-through system. Live and formaldehyde-killed sediment communities were incubated in 25-liter bottles with a continuous flow of P- or P + N-enriched water. Sediment bacteria in the killed communities were inhibited by adding formaldehyde (final concentration 0.04% v/v) to the sediment before the start of the experiment. Bacterial activity in the live sediments measured with [³H]thymidine and [¹⁴C]leucine incorporation techniques did not change essentially during the experiment period (7–8 days). Chemical mechanisms were found to be of principal importance in PO₄-P retention in the sediment. In the live samples, the net retention of PO₄-P was lower than in the killed samples, which was likely due to the reduced O₂ conditions in the sediment as a consequence of bacterial mineralization. In total P exchange, however, bacteria increased the retention rate by recycling dissolved organic P in the sediment. In the live communities the retention of N was very efficient, and all the introduced NH₄ -N and NO₃-N was immobilized by sediment bacteria. Nitrogen enrichment, however, did not alter the P exchange rates. The gradual emergence of bacterial activity (and grazing) in the killed communities, subsequent to the dilution of formaldehyde concentration, enhanced the release of PO₄-P and NH₄-N from sediment.     

The sediment column as a record of trophic status: examples from Bosherston Lakes,SW Wales

Bosherston Lakes are a series of interconnected, mesotrophic to hypereutrophic, artificially-created coastal marl lakes in Dyfed, South West Wales. Progressive eutrophication of the lake system has been produced by a high external phosphorus loading which includes phosphorus-rich effluent from a sewage treatment works (STW) in the catchment of the Lakes.Cores were taken from four sites of varying eutrophic status within the Lakes. In the surface sediment layer, organic C, N and P concentrations generally correlate directly with trophic status and reflect distance from the source of P input. At one site, sediment stratigraphy records a clear transition at 20–15 cm depth, marked by a sharp upward increase in porosity, organic C, N, and P, and iron-associated-P; decreases in organic matter C/N, C/P and N/P ratios; a sharp decrease in carbonate, and a change in the subfossil diatom assemblage. Lead-210 dating indicates that this change occurred in the period 1919 to 1938.The diatom stratigraphy and sediment geochemistry suggest that this transition reflects an increase in trophic status at this site, probably as a result of the influx of nutrient-rich water. This took place when the management of the Stackpole estate surrounding the lake system, fell into decline during the period 1919–1938.     

Chemical evolution of the Salton Sea, California: nutrient and selenium dynamics

The Salton Sea is a 1000-km² terminal lake located in the desert area of southeastern California. This saline (44000 mg l^–1 dissolved solids) lake started as fresh water in 1905–07 by accidental flooding of the Colorado River, and it is maintained by agricultural runoff of irrigation water diverted from the Colorado River. The Salton Sea and surrounding wetlands have recently acquired substantial ecological importance because of the death of large numbers of birds and fish, and the establishment of a program to restore the health of the Sea. In this report, we present new data on the salinity and concentration of selected chemicals in the Salton Sea water, porewater and sediments, emphasizing the constituents of concern: nutrients (N and P), Se and salinity. Chemical profiles from a Salton Sea core estimated to have a sedimentation rate of 2.3 mm yr^–1 show increasing concentrations of OC, N, and P in younger sediment that are believed to reflect increasing eutrophication of the lake. Porewater profiles from two locations in the Sea show that diffusion from bottom sediment is only a minor source of nutrients to the overlying water as compared to irrigation water inputs. Although loss of N and Se by microbial-mediated volatilization is possible, comparison of selected element concentrations in river inputs and water and sediments from the Salton Sea indicates that most of the N (from fertilizer) and virtually all of the Se (delivered in irrigation water from the Colorado River) discharged to the Sea still reside within its bottom sediment. Laboratory simulation on mixtures of sediment and water from the Salton Sea suggest that sediment is a potential source of N and Se to the water column under aerobic conditions. Hence, it is important that any engineered changes made to the Salton Sea for remediation or for transfer of water out of the basin do not result in remobilization of nutrients and Se from the bottom sediment into the overlying water.     

An integrated approach to relate Holocene climatic, hydrological, morphological and vegetation changes in the southeastern Amazon region

The geomorphological analysis of the river Tocantins alluvial area was carried out by remote sensing techniques, jointly with studies of sedimentary facies, mineralogy and geochemistry, pollen and spores, C and N isotopes data and radiocarbon dating of a sediment core from a floodplain lake, near the river Tocantins in the southeastern Amazon region. The aim was to identify and discuss the relationship between the main geomorphological processes and climatic factors, as well as their influence on vegetation patterns. The data indicate three phases of sediment and organic matter accumulation associated with vegetation changes during the Holocene: in Phase 1 (>7760 ± 80 cal yrs b.p.), the channel was gradually being disconnected from its main course, accumulating organic matter from terrestrial woody plants; in Phase 2 (>7760 ± 80 to 5145 ± 175 cal yrs b.p.), the very low energy flows allowed the deposition of autochthonous organic mud from suspension. Herbaceous vegetation and a Mauritia palm-swamp dominated the study area, followed by an increase in the contribution of freshwater dissolved organic carbon (DOC) to the lake; Phase 3 (5145 ± 175 cal yrs b.p. until present) records the expansion of ferns and arboreal plants, and greater freshwater organic matter input into the lake with anoxic water conditions. The development of the studied lake occurred by geomorphological changes and the filling process of an abandoned channel according to hydrodynamics and vegetation changes which were probably influenced by climatic changes during the Holocene.     

A comparison of ammonia-oxidiser populations in eutrophic and oligotrophic basins of a large freshwater lake

A combination of PCR amplification and oligonucleotide probing was used to investigate the populations of ammonia-oxidisers of the -Proteobacteria in the eutrophic and oligotrophic basins of Lake Windermere, a large temperate lake in the English Lake District. Numbers of ammonia-oxidisers (MPN) in the Windermere lakewater were low (< 100 cells ml^–1) throughout the year with the exception of peaks in August, which coincided with stratification, and November in the South Basin where overturn may have introduced ammonia-oxidising bacteria into the water column. Sediment samples contained larger populations of ammonia oxidisers, usually ca. 10⁴ per g. dry weight, which remained relatively constant throughout the seasonal cycle in both Basins. DNA was recovered from lakewater and sediment samples and Nitrosospiraand N. europaea-eutrophalineage16S rRNA genes amplified in a nested PCR reaction, with confirmation of identity by oligonucleotide hybridisation. Nitrosospira 16S rDNA was readily detected in all samples and therefore found to be ubiquitous. In contrast, nitrosomonad DNA of the N. europaea-eutropha lineage could only be detected in the oligotrophic North Basin. Enrichment cultures of lakewater samples only exhibited nitrification at low (0.67 mM) and medium (5 mM) ammonium concentrations, whilst sediment enrichments nitrified at all concentrations tested including high (12.5 mM) ammonium medium. These data suggest that ammonia-oxidiser populations may be physiologically distinguished between lakewater and sediment, and that species distribution in a single lake is non-uniform.     

Leaf litter of a dominant cushion plant shifts nitrogen mineralization to immobilization at high but not low temperature in an alpine meadow

Aims

We evaluated the effects of temperature and addition of leaf litter of Androsace tapete MaximWe–a dominant cushion plant species of alpine meadows on the Tibetan Plateau–on carbon (C) and nitrogen (N) mineralization, microbial biomass C (MBC) and N (MBN).

Methods

A laboratory incubation experiment with and without cushion plant litter addition was conducted for 112 days at three temperature regimes (?1, 5 and 11 °C). C and net N mineralization were simultaneously measured during the incubation period.

Results

C and N mineralization were affected by interactions between litter addition and temperature. Litter addition increased C mineralization and MBN but shifted N mineralization to immobilization at higher temperature. The positive relationship between net N mineralization and MBC and MBN was shifted to a negative one through cushion plant litter addition. Cushion plant litter also changed the relationship between C mineralization and net N mineralization from insignificantly positive to significantly negative.

Conclusions

These findings indicate that low temperature in winter could be important for alpine plants because low temperature can increase net N mineralization and supply plants with available N for their growth in the early growing season. During the growing season, climate warming–either directly through a temperature effect or indirectly through triggering increased cushion plant litter production–might lead to stronger competition for N between alpine plants and microorganisms.     

Multiple stressors trigger ecological changes in tropical Lake La Tembladera (Ecuador)

We examined the relationship between cladoceran, limnological, geological and biological variables in a recent sediment sequence to assess the cladoceran community response to flood conditions and human impacts during the last century in shallow Lake La Tembladera, on the southern coast of Ecuador. We recovered three sediment cores from central and littoral locations to analyze cladoceran subfossils and plant macrofossils. Redundancy analysis identified three environmental variables controlling the assemblage composition: the presence of non-aquatic plants, As and Be concentrations in the sediment. Before AD 1925, the surface area of the lake was smaller than it is now and the most representative cladocerans were Euryalona at lower lake levels and Leydigiopsis at relatively higher lake levels. After ca. AD 1925, anthropogenic works increased the lake level and the expanse of the littoral zone, favoring phytophilous cladocerans such as Kurzia. Around ca. AD 1990, the disposal of mining tailings contaminated with arsenic increased cladoceran sexual reproduction (total chydorid ephippia) and favored certain taxa. Our study shows the influence of anthropogenic activities on hydrologic regime, lake stage and heavy metal contamination and our results indicate the environmental trajectory of this lake, as it shifted from more natural to more impacted conditions in the last century.     

Sedimentary photosynthetic pigments of algae and phototrophic bacteria in Lake Hamana,Japan: temporal changes of anoxia in its five basins

We analyzed photosynthetic pigments of algae and bacteria (phototrophic sulfur bacteria: Chromatium and brown Chlorobium) in sediment cores and water samples obtained from five basins of Lake Hamana, a brackish, eutrophic, holomictic lake in Japan, and discussed our findings in relation to the distribution of the phototrophs. The four outer basins are connected to the central basin by narrow inlets. The prevalence of anoxia in Lake Hamana was demonstrated by the widespread presence of bacterial pigments in each core. The construction of training walls in 1954–1956 to direct tidal currents into the lake via Imagire-guchi Channel, the sole inlet for seawater, increased the lake water circulation, suppressed the development of anoxia, and caused Chromatium to disappear. Strong correlations (r ² 0.7) between total algal carotenoid (TAC) and total bacterial carotenoid (TBC) contents in each core were found in four basins. We ascribe this to the induction of anoxia by water stratification and algal proliferation, which precede the growth of phototrophic sulfur bacteria in the deeper layers of the water column. The slopes of the TBC–TAC correlations in the sediment cores, indicating the extent and stability of anoxia at each site, differed among basins (0.23–0.67) and were inversely related to the exchange rate of water by seawater intrusion in each basin.     

洱海底泥特性对七种沉水植物生长的影响

为研究洱海底泥特性对沉水植物生长的影响,采用不同比例洱海底泥与湖岸土壤掺混形成五种基质,并分别移栽苦草、黑藻、微齿眼子菜、马来眼子菜、光叶眼子菜、穿叶眼子菜和狐尾藻,进行为期70d的室外生长实验,结果表明不同基质对几种植物的影响具种间差异。（1）在基质为50%深层底泥+50%湖岸土壤（碳、氮、磷含量分别为31.59、0.334和0.095 mg/g）时,苦草、马来眼子菜和光叶眼子菜的株高最大;基质为100%深层底泥（碳、氮、磷含量分别为37.88、0.803和0.149 mg/g）时,黑藻、微齿眼子菜、穿叶眼子菜和狐尾藻的株高最大;（2）基质为100%深层底泥时,苦草、黑藻、微齿眼子菜、马来眼子菜和光叶眼子菜生物量增加最多且相对生长速率最大;基质为100%浅层底泥（碳、氮、磷含量分别为77.37、5.691和0.136 mg/g）时,穿叶眼子菜生物量增加最多,相对生长速率最大;狐尾藻在基质为50%浅层底泥+50%深层底泥（碳、氮、磷含量分别为49.27、2.005和0.131 mg/g）时生物量增加最多,相对生长速率最大;（3）基质为100%湖岸土壤（碳、氮、磷含量分别为22.06、0.327和0.231 mg/g）时,7种沉水植物均生长缓慢,生物量增加较少。综上所述,中营养底泥（碳、氮、磷含量分别为31.59-49.27、0.334-2.005和0.095-0.131 mg/g）更适合沉水植物生长,底泥中过高或过低营养都不利于沉水植物生长。     

Bacterial secondary production on vascular plant detritus: relationships to detritus composition and degradation rate.

Bacterial production at the expense of vascular plant detritus was measured for three emergent plant species (Juncus effusus, Panicum hemitomon, and Typha latifolia) degrading in the littoral zone of a thermally impacted lake. Bacterial secondary production, measured as tritiated thymidine incorporation into DNA, ranged from 0.01 to 0.81 microgram of bacterial C mg of detritus-1 day-1. The three plant species differed with respect to the amount of bacterial productivity they supported per milligram of detritus, in accordance with the predicted biodegradability of the plant material based on initial nitrogen content, lignin content, and C/N ratio. Bacterial production also varied throughout the 22 weeks of in situ decomposition and was positively related to the nitrogen content and lignin content of the remaining detritus, as well as to the temperature of the lake water. Over time, production was negatively related to the C/N ratio and cellulose content of the degrading plant material. Bacterial production on degrading plant material was also calculated on the basis of plant surface area and ranged from 0.17 to 1.98 micrograms of bacterial C cm-2 day-1. Surface area-based calculations did not correlate well with either initial plant composition or changing composition of the remaining detritus during decomposition. The rate of bacterial detritus degradation, calculated from measured production of surface-attached bacteria, was much lower than the actual rate of weight loss of plant material. This discrepancy may be attributable to the importance of nonbacterial organisms in the degradation and loss of plant material from litterbags or to the microbially mediated solubilization of particulate material prior to bacterial utilization, or both.     

Effects of zooplankton on sedimentation in pelagic ecosystems: Theory and test in two lakes of the Canadian shield

A simple heuristic theory based on conservation of matter and describing the fate of autotrophic production in pelagic ecosystems was developed to assist in conceptualizing how zooplankton affect sedimentation processes in lakes and oceans. The theory predicted that effects of zooplankton on the fraction of autotrophic incorporation that sediments (the export ratio) will be a function of factors related both to zooplankton digestion and egestion and to tendencies of particulate matter to sediment directly prior to mineralization. As a result, effects of zooplankton grazing on the export ratio were predicted to be site-dependent, a function of physical conditions and zooplankton communities characterizing an ecosystem. The theory was tested by monitoring autotrophic production of C, N, and P, sedimentation of C, N, and P, and zooplankton biomass in two lakes of the Canadian shield characterized by contrasting morphometry and food-web structure. In Lake 110, a small elongate lake protected from wind exposure, export ratios of C, N, and P declined strongly with zooplankton biomass. In contrast, in L240, a larger lake with considerably greater wind exposure, export ratios increased with zooplankton biomass. These results were consistent with predictions of our theory that effects of zooplankton on sedimentation processes will depend on the tendency of particulate matter to directly sediment relative to the tendency of egested materials to sediment. However, no significant differences in relationships between export ratios for C, N, and P and zooplankton biomass were found.