Seasonal and Interspecific Nutrient Mitigation Comparisons of Three Emergent Aquatic Macrophytes

The purpose of this study was to measure both summer and winter nutrient mitigation efficiencies of three aquatic plants found in agricultural drainage ditches in the lower Mississippi River Basin. Mesocosms (1.25 × 0.6 × 0.8 m) were filled with sediment and planted with monocultures of one of three obligate wetland plant species, Typha latifolia, Thalia dealbata, and Sagittaria latifolia, or left nonvegetated to serve as controls. Mesocosms were amended with nitrate, ammonium, and phosphate over a 4-h hydraulic retention time, followed by an 8-h flushing with nonamended water to assess residual nutrient leaching in both summer and winter exposures. Significant interactions between vegetation type and season were noted for both nitrate and total inorganic phosphorus concentrations and loads. Future research will focus on altering hydraulic retention time for improved efficiency, as well as the specific contribution of microbial activity to nutrient mitigation.     

Some Effects of Low-velocity Currents on the Metabolism of Aquatic Macrophytes

A prototype apparatus for making determinations of oxygen exchangesunder controlled conditions of water flow is briefly describedand some problems of technique are discussed in detail. Theresults include determinations of the photosynthesis and respirationof Ranunculus pseudofluitans and Potamogeton pectinatus in naturalwaters at velocities between 0.02 and 0.5 cm/sec, and some examinationof effects of changes in irradiance and oxygen concentration.Flow was laminar at all velocities. At low velocities photosynthesisincreased rapidly with velocity, but the rate of increase becameless at higher velocities. The size of the effects varied withthe metabolic capacity of the plant. For healthy shoots of R.pseudofluitans the maximum rate of photosynthesis was six timesthe probable static rate. These velocities are smaller thanthose in open water in streams, or even in the littoral of lakes,but may be comparable with the velocities within weed-beds.     

Mini-Review: Nutrient Cycling in Lakes and Streams: Insights from a Comparative Analysis

Understanding of general ecosystem principles may be improved by comparing disparate ecosystems. We compared nutrient cycling in lakes and streams to evaluate whether contrasts in hydrologic properties lead to different controls and different rates of internal nutrient cycling. Our primary focus was nutrient cycling that results in increased productivity, so we quantified nutrient cycling by defining the recycling ratio (ρ) as the number of times a nutrient molecule is sequestered by producers before export. An analytic model of nutrient cycling predicted that in lakes ρ is governed by the processes that promote the mineralization and retard the sedimentation of particulate-bound nutrients, whereas in streams, ρ is governed by processes that promote the uptake and retard the export of dissolved nutrients. These differences were the consequence of contrast between lakes and streams in the mass-specific export rates (mass exported · standing stock^-1· time^-1) of dissolved and particulate nutrients. Although ρ is calculated from readily measured ecosystem variables, we found very few published data sets that provided the necessary data for a given ecosystem. We calculated and compared ρ in two well-studied P-limited ecosystems, Peter Lake and West Fork Walker Branch (WFWB). When ecosystems were scaled so that water residence time was equal between these two ecosystems, ρ was three orders of magnitude greater in WFWB. However, when we scaled by P residence time, ρ was nearly equal between these two ecosystems. This suggests broad similarities in ρ across ecosystem types when ecosystem boundaries are defined so that turnover times of limiting nutrients are the same. Received 19 November 1998; accepted 6 October 1999.     

The Effect of Natural and Artificial Shade on the Macrophytes of Lowland Streams and the Use of Shade as a Management Technique

The reduction of the biomass of three main problem species of submerged aquatic macrophytes was proportional to the light at the stream surface both under artificially shaded sections of stream and in naturally shaded areas when compared to the biomass in adjacent open and unshaded sections of stream. The effect of marginal vegetation in varying the shading effect given to streams of differing width and orientation are described. It is recommended that light should be reduced to about half that presently available in the open, by shading from marginal vegetation but it is warned that too much shade is detrimental to the fish populations of the stream and leads to accentuated local accumulations of leaves. The long term effects are considered but it is expected that partial shading will increase the diversity of submerged plant species. It is suggested that the practice of stream realignment is discontinued and that the natural tendency of streams to create their own meandering channels is allowed but within some defined and generally acceptable framework.     

Salt Tolerance in Aquatic Macrophytes: Ionic Relation and Interaction

Effects of seawater salinity (SWS) and pure NaCl on the intracellular contents of Na⁺, K⁺, Mg²⁺, Ca²⁺, chlorophylls (Chl) and carotenoids (Car) were studied in three submerged aquatic macrophytes, Hydrilla verticillata, Najas indica and Najas gramenia, which differed in their tolerance to salinity. NaCl resulted in significant increase in Chl/Car ratio in the salt-sensitive H. verticillata and moderately salt-tolerant N. indica, but not in the salt-tolerant N. gramenia. SWS treatment did not result in any significant change in the ratio. The intracellular content of Na⁺ increased significantly in all the test plants upon exposure to both NaCl and SWS. The content of K⁺ decreased significantly in these plants upon salinity treatment, except in N. gramenia. The contents of Ca²⁺ and Mg²⁺ decreased significantly upon NaCl treatment and remained unchanged or increased upon SWS treatment. No relationship between salt tolerance and K⁺/Na⁺ ratio was observed. The maintenance of a minimal level of K⁺ was observed to be the most probable requirement of salt tolerance in aquatic macrophytes.     

Nutrient Constraints on Metabolism Affect the Temperature Regulation of Aquatic Bacterial Growth Efficiency

Inorganic nutrient availability and temperature are recognized as major regulators of organic carbon processing by aquatic bacteria, but little is known about how these two factors interact to control bacterial metabolic processes. We manipulated the temperature of boreal humic stream water samples within 0–25°C and measured bacterial production (BP) and respiration (BR) with and without inorganic nitrogen?+?phosphorus addition. Both BP and BR increased exponentially with temperature in all experiments, with Q ₁₀ values varying between 1.2 and 2.4. The bacterial growth efficiency (BGE) showed strong negative relationships with temperature in nutrient-enriched samples and in natural stream water where community-level BP and BR were not limited by nutrients. However, there were no relationships between BGE and temperature in samples where BP and BR were significantly constrained by the inorganic nutrient availability. The results suggest that metabolic responses of aquatic bacterial communities to temperature variations can be strongly dependent on whether the bacterial metabolism is limited by inorganic nutrients or not. Such responses can have consequences for both the carbon flux through aquatic food webs and for the flux of CO₂ from aquatic systems to the atmosphere.     

A Simple Model of Phosphorus Retention Evoked by Submerged Macrophytes in Lowland Rivers

This study was designed to quantify and model the effects of macrophytes on phosphorus retention in a lowland river. The seasonal course of phosphorus retention was calculated from the measured difference in TP between beginning and end of a 30-km river course and the estimated lateral P input. The coverage of submersed macrophytes was mapped and coincided with the difference between theoretical water level (without vegetation) and the observed one. Therefore, the increase in water level was used as measure of the macrophytes’ abundance. In years with rare vegetation (1991–1994), P was retained in winter and remobilized in summer. In years with dense stands of macrophytes (1995–2002), net P retention was highest in summer and amounted up to 20% of TP load, and was negative during winter. The annual P budget was close to zero in both periods. The found sinusoidal annual pattern of total phosphorus retention was used to create a retention model for vegetated lowland rivers.     

Glutamine Metabolism and Cycling in Neurospora crassa

[This corrects the article on p. toc in vol. 54.].     

Changes in Aquatic Macrophytes after Liming Thrush Lake, Minnesota

Thrush Lake, Minnesota, was treated with limestone in 1988 to evaluate the efficacy of protective base addition against the loss of sport fisheries in a sensitive, mildly acidic lake. Prior to treatment, the lake was stressed (pH 6.46, ANC 64 μeq/L) but not severely degraded by acidic deposition and had a macrophyte community typical of lakes in northeastern Minnesota with low acid-neutralizing capacity (ANC). This paper describes the changes observed in aquatic plant communities during the 5 years after treatment, as pH and ANC slowly returned to pretreatment levels. Sphagnum platyphyllum, intolerant of non-acid conditions, was completely eliminated from the lake. The charo-phyte, Nitella, that originally shared dominance in the deep littoral zone with S. platyphyllum, decreased in importance during the first 2 years after treatment. Two vascular plants, Potamogeton pusillus and Najas flexilis, were first found in the lake the year after treatment and were abundant for 2 years after liming, probably in response to a combination of more neutral pH and reduced cover of Nitella. As the ANC and pH slowly returned to pretreatment conditions, Nitella again increased in coverage and depth range, with a concomitant decrease in P. pusillus and N. flexilis. The moss, S. platyphyllum, had not reinvaded the lake by 1993, 2 years after its dramatic decline.     

Multi-decadal Changes in Water Table Levels Alter Peatland Carbon Cycling

Globally, peatlands store a large quantity of soil carbon that can be subsequently modified by hydrologic alterations from land-use change and climate change. However, there are many uncertainties in predicting how carbon cycling and greenhouse gas emissions are altered by long-term changes in hydrology. Therefore, the goal of this study was to quantify how multi-decadal manipulations of water table (WT) levels affected carbon cycling (plant production and net ecosystem exchange from three eddy covariance towers) in a peatland complex modified by levee construction, which created a wetter area up-gradient of the levee (mean WT was 12.1 cm below the surface), a dry area below the levee (36.8 cm), and an adjacent reference site not affected by the levee (21.6 cm). We found that mean total plant production was greatest in the reference site (311.9 g C m^?2 y^?1), followed by the dry site (290.5 g C m^?2 y^?1), and lowest in the wet site (227.1 g C m^?2 y^?1). Net ecosystem exchange during the growing season was negative for all sites (sink), with the wet site having the greatest sink and the dry site having the lowest sink. Ecosystem respiration increased and CH₄ emissions decreased with a decreasing WT level. This research demonstrates that human alteration of peatland WT levels can have long-term (>50 years) consequences on peatland carbon cycling.     

Differential Response of Benthos to Natural and Anthropogenic Disturbances in 3 Lowland Streams

The macroinvertebrate faunal assemblages of 8 sites on three small streams in SE England were examined annually in spring between 1987 and 1993. Considerable ‘natural’ variability was observed in the sites superimposed over a background of low-intensity anthropogenic disturbance such as farming, quarrying, and urban influences. Landscape changes and activities in the catchment (saline drainage, removal of topsoil, pipeline crossings of streams) associated with the construction of the UK terminal for the Channel Tunnel, resulted in further temporary disturbance at some of the sites. Most changes in faunal composition were related to natural and anthropogenically induced modifications of the stream substrate. The response and recovery time of sites to disturbances was very variable and was related to the hydraulic and substrate characteristics of the stream bed, with least change and quickest recovery at sites with coarse substrates and high slope. Variations between years in the occurrence of taxa were generally more apparent with species data than with family data except where environmental change was great. Despite the observed differences in faunal composition the main elements of the fauna were fairly constant throughout the study period. Analyses of the data with biotic indices, the predictive RIVPACS system and ordination analyses have all shown clearly the annual variability of faunal parameters used to measure environmental quality. The study has shown that very basic rapid assessment techniques can provide insights into faunal responses to disturbance, especially if the work is long-term and that before authoritative statements concerning environmental impact can be made it is essential to have knowledge of the natural variation to be expected in streams of differing characteristics.     

Coupling Nutrient Uptake and Energy Flow in Headwater Streams

Nutrient cycling and energy flow in ecosystems are tightly linked through the metabolic processes of organisms. Greater uptake of inorganic nutrients is expected to be associated with higher rates of metabolism [gross primary production (GPP) and respiration (R)], due to assimilatory demand of both autotrophs and heterotrophs. However, relationships between uptake and metabolism should vary with the relative contribution of autochthonous and allochthonous sources of organic matter. To investigate the relationship between metabolism and nutrient uptake, we used whole-stream and benthic chamber methods to measure rates of nitrate–nitrogen (NO₃–N) uptake and metabolism in four headwater streams chosen to span a range of light availability and therefore differing rates of GPP and contributions of autochthonous carbon. We coupled whole-stream metabolism with measures of NO₃–N uptake conducted repeatedly over the same stream reach during both day and night, as well as incubating benthic sediments under both light and dark conditions. NO₃–N uptake was generally greater in daylight compared to dark conditions, and although day-night differences in whole-stream uptake were not significant, light–dark differences in benthic chambers were significant at three of the four sites. Estimates of N demand indicated that assimilation by photoautotrophs could account for the majority of NO₃–N uptake at the two sites with relatively open canopies. Contrary to expectations, photoautotrophs contributed substantially to NO₃–N uptake even at the two closed-canopy sites, which had low values of GPP/R and relied heavily on allochthonous carbon to fuel R.     

Morphological and Chemical Changes Induced by Herbivory in Three Common Aquatic Macrophytes

The Dry Matter Content (DMC), the total phenolic content, the production of new branches and the plant fragmentation were compared in three macrophyte species (Elodea canadensis, Elodea nuttallii and Myriophyllum spicatum) exposed or not to snail herbivory. Grazing significantly reduced the DMC of M. spicatum and E. canadensis, but had no effect on the DMC of E. nuttallii. The phenolic contents of Elodea species were not modified by snail herbivory, whereas that of M. spicatum significantly increased when exposed to grazers. The number of new branches produced by M. spicatum and E. canadensis plants, and the fragmentation of E. canadensis also increased in response to herbivory. Chemical defences are therefore probably constitutive in Elodea and induced in M. spicatum, and morphological changes can be related to species growth form and synthesis of phenolic compounds. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies of Diurnal Acid Fluctuations in British Isoetid-type Submerged Aquatic Macrophytes

British isoetid species are examined for the presence of diurnalfluctuations in tritratable acidity (to pH 6·4), in plantscollected directly from a small lake and in plants grown inconstant conditions in the laboratory. Wide diurnal fluctuationsare present in Isoetes lacustris and in both submerged and terrestrialpopulations of Littorella uniflora. They are absent in Lobeliadortmanna, Subularia aquatica, Eriocaulon septangulare, Ranunculusflammula and Pilularia globulifera. The significance of submerged CAM is discussed in relation toother carbon accumulating mechanisms in isoetids and in considerationof their general ecology. Crassulacean acid metabolism, photosynthesis, isoetid, oligotrophic lakes     

Bioaccumulation Potential and Physiological Responses of Aquatic Macrophytes to Pb Pollution

In view of their potential bioaccumulation of heavy metals, Ceratophyllum demersum and Myriophyllum spicatum was studied under hydroponic cultures enriched by different Pb concentrations (25, 50, 75 mg/l) for 1–7 days. Both species exerted remarkable capabilities to concentrate Pb in their tissues as compared to control. The highest accumulation value of Pb (164.26 mg/g.dw) was recorded in C. demersum and the most of metal (91.72 mg/g dw) accumulated after 1 d. Significant reduction in photosynthetic pigments and appearance of morphological symptoms such as chlorosis and fragmentation of leaves were evident after 7d at 75 mg/l. The activity of POX and APX, carotenoids and proline showed induction at lower concentration and duration followed by decline. Major re-shuffle in protein patterns appeared as a tolerant mechanism, which both species developed under Pb toxicity. Results suggest that both species responded positively to Pb concentration and accumulated high amount of metal. Due to metal accumulation coupled with detoxification potential, both species appear to have potential for use as phytoremediators and the developed responses can be used as reliable biomarkers for Pb water pollution.     

Ecosystem Consequences of Tree Monodominance for Nitrogen Cycling in Lowland Tropical Forest

Understanding how plant functional traits shape nutrient limitation and cycling on land is a major challenge in ecology. This is especially true for lowland forest ecosystems of the tropics which can be taxonomically and functionally diverse and rich in bioavailable nitrogen (N). In many tropical regions, however, diverse forests occur side-by-side with monodominant forest (one species >60% of canopy); the long-term biogeochemical consequences of tree monodominance are unclear. Particularly uncertain is whether the monodominant plant-soil system modifies nutrient balance at the ecosystem level. Here, we use chemical and stable isotope techniques to examine N cycling in old-growth Mora excelsa and diverse watershed rainforests on the island of Trinidad. Across 26 small watershed forests and 4 years, we show that Mora monodominance reduces bioavailable nitrate in the plant-soil system to exceedingly low levels which, in turn, results in small hydrologic and gaseous N losses at the watershed-level relative to adjacent N-rich diverse forests. Bioavailable N in soils and streams remained low and remarkably stable through time in Mora forests; N levels in diverse forests, on the other hand, showed high sensitivity to seasonal and inter-annual rainfall variation. Total mineral N losses from diverse forests exceeded inputs from atmospheric deposition, consistent with N saturation, while losses from Mora forests did not, suggesting N limitation. Our measures suggest that this difference cannot be explained by environmental factors but instead by low internal production and efficient retention of bioavailable N in the Mora plant-soil system. These results demonstrate ecosystem-level consequences of a tree species on the N cycle opposite to cases where trees enhance ecosystem N supply via N₂ fixation and suggest that, over time, Mora monodominance may generate progressive N draw-down in the plant-soil system.     

农业生态系统养分循环研究概况

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