Mechanisms regulating abundance of submerged vegetation in shallow eutrophic lakes

Shallow eutrophic lakes tend to be either in a turbid state dominated by phytoplankton or in a clear-water state dominated by submerged macrovegetation. Recent studies suggest that the low water turbidity in the clear-water state is maintained through direct and indirect effects of the submerged vegetation. This study examined what mechanisms may cause a recession of the submerged vegetation in the clear-water state, and thereby a switch to the turbid state. The spatial distribution of submerged vegetation biomass was investigated in two shallow eutrophic lakes in the clear-water state in southern Sweden. Biomass of submerged vegetation was positively correlated with water depth and wave exposure, which also were mutually correlated, suggesting that mechanisms hampering submerged vegetation were strongest at shallow and/or sheltered locations. The growth of Myriophyllum spicatum, planted in the same substrate and at the same water depth, was compared between sheltered and wave exposed sites in two lakes. After 6 weeks the plants were significantly smaller at the sheltered sites, where periphyton production was about 5 times higher than at the exposed sites. Exclosure experiments were conducted to evaluate the effects of waterfowl grazing on macrophyte biomass. Potamogeton pectinatus growth was decreased by grazing, whereas M. spicatum was not affected. The effects were greater at a sheltered than at a wave-exposed site, and also negatively related to distance from the reed belt. These results suggest that competition from epiphytes and waterfowl grazing hamper the development of submerged vegetation at sheltered and/or shallow locations. An increased strength of these mechanisms may cause a recession of submerged vegetation in shallow eutrophic lakes in the clear-water state and thereby a switch to the turbid state. Received: 24 June 1996 / Accepted: 8 September 1996     

Predicting the hydraulic forces on submerged macrophytes from current velocity, biomass and morphology

Aquatic macrophytes are important in stabilising moderately eutrophic, shallow freshwater lakes in the clear-water state. The failure of macrophyte recovery in lakes with very soft, highly organic sediments that have been restored to clear water by biomanipulation (e.g. in the Norfolk Broads, UK) has suggested that the physical stability of the sediment may limit plant establishment. Hydraulic forces from water currents may be sufficient to break or remove plants. Our aim was to develop a simple model that could predict these forces from plant biomass, current velocity and plant form. We used an experimental flume to measure the hydraulic forces acting on shoots of 18 species of aquatic macrophyte of varying size and morphology. The hydraulic drag on the shoots was regressed on a theoretically derived predictor (shoot biomass × current velocity^1.5). Such linear regressions proved to be highly significant for most species. The slopes of these lines represent species-specific, hydraulic roughness factors that are analogous to classical drag coefficients. Shoot architecture parameters describing leaf and shoot shape had significant effects on the hydraulic roughness factor. Leaf width and shoot stiffness individually did not have a significant influence, but in combination with shoot shape they were significant. This hydraulic model was validated for a subset of species using measurements from an independent set of shoots. When measured and predicted hydraulic forces were compared, the fit was generally very good, except for two species with morphological variations. This simple model, together with the plant-specific factors, provides a basis for predicting the hydraulic forces acting on the root systems of macrophytes under field conditions. This information should allow prediction of the physical stability of individual plants, as an aid to shallow-lake management. Received: 11 March 1999 / Accepted: 18 January 2000     

不同基质对四种沉水植物生长的影响

通过模拟试验,研究了太湖五里湖主要3种基质类型(沙石、生土、湖泥)对4种沉水植物(苦草、马来眼子菜、金鱼藻、轮叶黑藻)生长的影响. 结果表明,生长于沙石、生土、湖泥上的苦草和马来眼子菜平均生物量分别为72.37、126.25、134.10 g和40.0、72.10、90.70 g,而金鱼藻和轮叶黑藻平均生物量分别为0.27、6.58、73.64 g和0.17、3.26、84.42 g,说明湖泥较适合这四种沉水植物生长. 苦草和马来眼子菜对相对贫瘠的生土有较强的适应性,而金鱼藻和轮叶黑藻不适宜在生土中生长. 生长在沙石上的4种沉水植物的生物量和株高最低,其中金鱼藻和轮叶黑藻于试验期间死亡.苦草的根系活力(TTC)低于马来眼子菜,生长在沙石、生土、湖泥中苦草的根系活力分别为0、(0.16±0.05) mg·g^-1·h^-1和(0.36±0.33) mg·g^-1·h^-1,而马来眼子菜则分别为(2.68±0.34) mg·g^-1·h^-1、(2.30±0.77) mg·g^-1·h^-1、(5.24±0.67) mg·g^-1·h^-1. 叶绿素、质膜透性和丙二醛(MDA)的测定结果进一步证明了以上结论.此外,苦草、马来眼子菜和轮叶黑藻对基质有较强的沁氧能力,其大小顺序为苦草＞马来眼子菜＞轮叶黑藻.     

沉水植物重建对富营养水体氮磷营养水平的影响

利用富营养浅水湖泊(武汉东湖)中所建立的大型实验围隔系统,研究了沉水植物对水体N、P营养水平的影响．结果表明,沉水植物重建后N、P营养水平显著降低．在研究期间,水生植物围隔总N和总P水平均显著低于对照围隔和大湖水体,而且水生植物围隔的总P含量一般维持在0．1mg·L^-1左右。季节性波动远低于对照围隔和大湖水体．水生植物围隔水体中氨态氮和亚硝态氮含量较低．而硝态氮含量与对照围隔和和大湖水体差别不大．由此可见。恢复以沉水植物为主的水生植被,可以有效地降低N、P营养循环速度,控制浮游植物过度增长,是重建富营养湖泊生态系统的重要措施．     

Effects of aquatic macrophytes on water quality and phytoplankton communities in shallow lakes

We investigated aquatic macrophytes, water quality, and phytoplankton biomass and species composition in three shallow lakes with different levels of vegetation cover and nutrient concentration in Kushiro Moor, during August 2000. Trapa japonica can live in a wide range of nutrient levels. This species forms an environment with a steeper extinction of light, higher concentrations of dissolved organic carbon (DOC), lower concentrations of dissolved oxygen (DO) near the bottom, and lower concentrations of nitrate+nitrite and soluble reactive phosphorus (SRP) than other vegetation types. The pH was much higher in a Polygonum amphibium community, and the DO near the bottom did not decrease compared to a T.japonica community in the summer. The relationship between chlorophyll a and the limiting nutrient (total phosphorus (TP) when total nitrogen (TN):TPis 10 and TN/10 when TN:TP is <10) significantly differed between lakes with and without submerged vegetation. The chlorophyll a concentrations at a given nutrient level were significantly lower in water with submerged macrophytes than in water without them. Correspondence analysis showed that the difference in phytoplankton community structure across sites was largely due to the presence or absence of submerged macrophytes, and the ordination of phytoplankton species in the lakes with submerged macrophytes is best explained by environmental gradients of TN, chlorophyll, pH and SRP.     

Photosynthesis of amphibious and obligately submerged plants in CO2-rich lowland streams

Small unshaded streams in lowland regions receive drainage water with high concentrations of free␣CO₂, and they support an abundant growth of amphibious and obligately submerged plants. Our first objective was to measure the CO₂ regime during summer in a wide range of small alkaline Danish streams subject to wide variation in temperature, O₂ and CO₂ during the day. The second objective was to determine the effect of these variations on daily changes in light-saturated photosynthesis in water of a homophyllous and a heterophyllous amphibious species that only used CO₂, and an obligately submerged species capable of using both HCO⁻ ₃ and CO₂. We found that the median CO₂ concentrations of the streams were 11 and 6 times above air saturation in the morning and the afternoon, respectively, but stream sites with dense plant growth had CO₂ concentrations approaching air saturation in the afternoon. In contrast, outlets from lakes had low CO₂ concentrations close to, or below, air saturation. The amphibious species showed a reduction of photosynthesis in water from morning to afternoon along with the decline in CO₂ concentrations, while increasing temperature and O₂ had little effect on photosynthesis. Photosynthesis of the obligately submerged species varied little with the change of CO₂ because of HCO₃ ⁻- use, and variations were mostly due to changes in O₂ concentration. Independent measurements showed that changes in temperature, O₂ and CO₂ could account for the daily variability of photosynthesis of all three species in water. The results imply that CO₂ supersaturation in small lowland streams is important for the rich representation of amphibious species and their contribution to system photosynthesis. Received: 2 March 1998 / Accepted: 24 July 1998     

Effect of the filling season on aquatic plants in Mediterranean temporary ponds

Aims Pond environmental conditions may differ among years with regards to the season in which ponds begin to fill. We experimentally evaluated how seedling emergence, plant growth and phenology differed among years in which filling occurred in winter, autumn or spring.Methods We collected sediments from a natural temporary pond and located them in aquariums. They were placed in a climatic chamber that simulated annual variation in field environmental temperatures and light conditions. Aquariums were assigned to one of three treatments, which differed in the date on which they were filled with water (autumn, winter and spring). We counted the number of seedlings of different species emerged and recorded data about the presence of flowers, seeds or spores every week. The experiment was finished in June, when we harvested the plants and estimated their biomass.Important findings In most species, seedling emergences were primarily related to time after filling, and thus synchronized their life cycles with the unpredictably timed wet phase of the ponds. Autumn filling resulted in the highest numbers of seeds/spores. However, winter filling promoted plant growth the most. In the spring filling treatment, more terrestrial plant seedlings emerged and fewer seeds/spores were produced. When ponds are flooded earlier, plants may produce a higher number of propagules. However, in years when inundation is delayed to spring and hydroperiods are short, seedling emergence deplete the seed bank and there is little to no seed production, while terrestrial monocots are able to colonize pond basin.     

Effects of water velocity on photosynthesis and dark respiration in submerged stream macrophytes

The effects of flow velocities on dark respiration and net photosynthesis of eight submerged stream macrophytes were examined in a laboratory oxygen chamber. The shoots/leaves were exposed to saturating free-CO₂ concentrations and were attached basally so that they could move in the flowing water. Net photosynthesis declined by 34–61% as flow velocity increased from 1 to 8.6cm s^?1, while dark respiration increased 2.4-fold over the same range. The increase in dark respiration could only account for between 19 and 67% of the decrease in net photosynthesis. The relationship between flow velocity (U) and net photosynthesis (P) was described by: P=b×U^a. The exponent, a, varied from -0.20 to –0.48 and showed a negative correlation to the surface: volume (SA: V) ratio of the plants, i.e. species with high SA: V ratio were more sensitive to flow. In contrast, net photosynthesis of plants firmly attached to a supporting frame was not significantly affected by increasing flow velocity. This result indicates that the physical stress imposed on the plants by agitation or stretching in the flowing water is a key factor for the observed response.     

An experimental system to study ecophysiological responses of submerged macrophytes to temperature and light

An experimental growth system was devised to study the ecophysiological responses of submerged macrophytes to temperature and light. Using the system, a pilot study was conducted to compare responses to light of three representative species common to the littoral zone of Lake Biwa: Vallisneria asiatica Miki var. biwaensis Miki, which is endemic to Japan, Potamogeton maackianus A. Benn., a dominant submerged macrophyte in Lake Biwa, and Elodea nuttallii (Planch.) St. John, one of the most influential submerged exotic macrophytes naturalized in Japan. Different relative growth rate responses and other growth parameters among the species were observed. We ascertained the experimental system to be useful for the comparison of ecophysiological responses of submerged macrophytes.     

沉水植物光合作用的特点与研究进展

沉水植物属于高等植物,由陆生被子植物演化而来,它们在形态、光合生态生理方面对水下生活环境发生了一系列适应性变化。沉水植物的光合作用受水体中光、温度、pH和无机碳等影响,本文对此进行了综述。水中低CO2扩散率以及细胞外较厚的扩散层阻碍了沉水植物净碳的吸收,因此,沉水植物光合作用速率受到无机碳供应的限制。为获得无机碳,沉水植物在形态结构和生理生化上表现一定的特性,包括薄的叶片层并含有叶绿体以及对HCO3-利用的能力,拟C4型和CAM型光合代谢途径的选择。这些是沉水植物碳浓缩机制的具体体现。     

保安湖沉水植物分布的GIS模拟

以GPS为辅助,在保安湖主体湖和桥墩湖进行沉水植物调查。将野外调查数据构建GIS数据库,基于GIS软件平台,运用Kringing插值法对保安湖沉水植物分布进行模拟。模拟结果清楚地显示出调查时保安湖沉水植物及4优势种(金鱼藻Ceratophyllum demersum L.、穗状狐尾藻Myriophyllum spicatum L.、微齿眼子菜Potamogeton maackianus A．Benn．及苦草Vallisneria spiralis L．)分别在湖中的分布情况。统计后结果表明沉水植物覆盖面积占全湖面积的92．9％,总生物量(湿重)为143682．4t。比较历史数据,发现湖中水生植被组成特征已由微齿眼子菜绝对优势转变为4种植物的共同优势。     

Determination of tipping points for aquatic plants and water quality parameters in fish pond systems: A multi-year approach

High levels of nutrients in fish ponds by fish farming may cause significant eutrophication leading to a loss in species richness and a decrease of cover of aquatic plants to phytoplankton dominance. This shift can be represented by a tipping point where a significant change in the state of the ecosystem is observed such as a change from high to low aquatic plants species richness and cover. A total of 100 fish ponds were studied during five years in the Dombes region, France, to determine tipping points in aquatic plant richness and cover using chlorophyll α (CHL), water transparency, Total N (TN) and Total P (TP) gradients with two statistical methods. The relationships between tipping points, nutrient loads and yearly variations in weather conditions were also evaluated. Looking at the five years data, tipping points were observed in aquatic plant richness at 6 and 60 μg/L for CHL, and at 3.90 mg/L for TN concentration; as well as at 70 cm for water transparency, but no tipping point was found with TP. For aquatic plant cover, tipping points were observed at 11 μg/L for CHL, 2.42 mg/L for TN, 0.05 mg/L for TP, and at 62 cm for water transparency. These tipping points showed a significant decrease of aquatic plant species richness and cover, linked to the nutrient concentrations which drive the competition between the primary producers phytoplankton and aquatic plants. However, tipping points could vary significantly between years. The inter-annual variability may be due to an early occurrence of phytoplankton blooms in some ponds in a year preventing the establishment of aquatic plants, and thus influencing the value of tipping points. Weather conditions influence the competition between primary producers by impacting chlorophyll α and nutrients concentrations. When weather conditions supported increased nutrient concentrations, the development of phytoplankton and aquatic plants was facilitated and tipping points in aquatic plant richness and cover occurred with relatively high values. Thus, a significant decrease of plant cover and richness occurred at higher level of nutrients compared to the other years. In these cases, aquatic plants dominated over phytoplankton for the spring period, and also often during summer. In conclusion, tipping points observed are mainly linked to the competition between aquatic plants and phytoplankton. In shallow and eutrophic systems like fish ponds where nutrients are not a limiting resource, weather conditions act temporarily during spring as the main regulator of this competition.     

Long‐term dynamics of submerged macrophytes and algae in a small and shallow,eutrophic lake: implications for the stability of macrophyte‐dominance

1. Submerged macrophyte and phytoplankton components of eutrophic, shallow lakes have frequently undergone dynamic changes in composition and abundance with important consequences for lake functioning and stability. However, because of a paucity of long‐term survey data, we know little regarding the nature, direction and sequencing of such changes over decadal–centennial or longer timescales. 2. To circumvent this problem, we analysed multiple (n = 5) chronologically correlated sediment cores for plant macro‐remains and a single core for pollen and diatoms from one small, shallow, English lake (Felbrigg Hall Lake, Norfolk, U.K.), documenting 250 years of change to macrophyte and algal communities. 3. All five cores showed broadly similar stratigraphic changes in macrophyte remains with three distinct phases of macrophyte development: Myriophyllum–Chara–Potamogeton (c. pre‐1900), to Ceratophyllum–Chara–Potamogeton (c. 1900–1960) and finally to Zannichellia–Potamogeton (c. post‐1960). Macrophyte species richness declined from at least 10 species pre‐1900 to just four species at the present day. Additionally, in the final Zannichellia–Potamogeton phase, a directional shift between epi‐benthic and phytoplankton‐based primary production was indicated by the diatom data. 4. Based on macrophyte–seasonality relationships established for the region, concomitant with the final shift to Zannichellia–Potamogeton, we infer a reduction in the seasonal duration of plant dominance (plant‐covered period). Furthermore, we hypothesise that this change in species composition resulted in a situation whereby macrophyte populations were seasonally ‘sandwiched’ between two phytoplankton peaks in spring and late summer as observed in the contemporary lake. 5. We suggest that eutrophication‐induced reductions in macrophyte species richness, especially if the number of plant‐seasonal strategies is reduced, may constrict the plant growing season. In turn, this may render a shallow lake increasingly vulnerable to seasonal invasions of phytoplankton resulting in further species losses in the plant community. Thus, as part of a slow (over perhaps 10–100s of years) and self‐perpetuating process, macrophytes may be gradually pushed out by phytoplankton without the need for a perturbation as required in the alternative stable states model of plant loss.     

Community collocation of four submerged macrophytes on two kinds of sediments in Lake Taihu, China

To explore a method for rapid restoration and artificial regulation of submerged macrophytes in large-scale restoration of eutrophic lakes, the succession and the biodiversity changes of four communities composed of four native, common submerged macrophytes, Hydrilla verticillata, Potamogeton malaianus, Vallisneria spiralis and Najas marina, on two kinds of sediments were investigated. Under low light intensity (reduced by 99%), the plant biomass changed with seasonal changes, plant competition, and environmental stress. The competitive capability for light differed in the four species due to different shoot height and tiller number. After 405 days of transplantation, H. verticillata became dominant in all communities. The biomass of H. verticillata, with strong ability to endure low water light environment, accounted for more than 90% of the total community biomass, and P. malaianus had only weak growth, while V. spiralis and N. marina almost disappeared. Based on livability and biomass of submerged macrophytes on two sediment types, brown clay sediment appeared to be more favorable for the settlement of the plants, while fertile sludge sediment was suitable for vegetative growth. In conclusion, the improvement of habitats and the selection of appropriate plant species are of the greatest importance for ecological restoration of the aquatic ecosystem.     

Submerged vascular plants and water chemistry in the coastal marsh Albufera de Mallorca (Balearic Islands)

Ten species of aquatic macrophytes have been analyzed for seven environmental variables by means of a significance test. A synthetic view has been obtained through a cluster analysis for species and a principal component analysis for 17 variables which form the multidimensional space where we project the species habitats. Myriophyllum spicatum L., Najas marina L., Potamogeton crispus L., Potamogeton pectinatus L. and Zannichellia pedunculata Reichenb. are widely distributed in the Albufera. Ceratophyllum submersum L. and Ruppia maritima L. var. brevirostris Ag. are considered stenoic. The tolerance of Ceratophyllum submersum to salts is significantly low and that of Ruppia cirrhosa (Petagna) Grande and Ruppia maritima var. brevirostris significantly high. Ceratophyllum submersum has a significantly negative distribution with regard to chlorophyll a and phosphate concentrations. Ceratophyllum demersum L. and C. submersum primarily occur in nitrate-rich waters whereas Ruppia cirrhosa primarily occurs in low nitrate waters.     

Influence of area,habitat and water chemistry on richness and composition of macrophyte assemblages in southern Brazilian wetlands

Questions: Two hypotheses were tested: (1) physical features, such as wetland surface area and habitat diversity, together with water chemistry, are important determinants of species richness and composition of macrophyte assemblages and (2) species richness and composition of macrophyte assemblages differ between wetlands of different types (i.e., palustrine versus lacustrine) and between wetlands of different hydrologies (i.e. permanent versus intermittent). Location: A subtropical coastal plain segment (2500 km²) of southern Brazil. Methods: Quarterly collections were carried out in 15 wetlands (2004–2005) in southern Brazil. Differences in richness over time were tested using repeated measures ANOVA. Stepwise multiple regression was performed to investigate relationships between total richness and environmental variables. Significance of differences between wetland types and hydroperiods on species composition was verified by MRPP (Multi‐Response Permutation Procedure). The influence of the environmental variables on species composition was assessed using CCA (Canonical Correspondence Analysis). Results: Macrophyte species richness changed with time, was not significantly different between wetland types, but was higher in permanent wetlands than in intermittent ones. Area, habitat diversity and soluble reactive phosphorus concentration explained 76% of the variation in species richness. Species composition was different between permanent and intermittent wetlands, although it was not significantly different between wetland types. Area, habitat diversity and water chemistry explained 50.1% of species composition. Conclusions: Species richness and composition of wetland macrophytes were mainly determined by area, habitat diversity and hydroperiod. These results can be used for the development of conservation and management programs in southern Brazil.     

The species composition,occurrence and temporal stability of submerged aquatic macrophyte patches along the main channel border of pool 5A,Upper Mississippi River

Species composition, relative abundance, distribution and physical habitat associations of submerged aquatic macrophytes in the main channel border (MCB) habitat of Pool 5A, Upper Mississippi River (UMR) were investigated during the summers of 1980 and 1983. The submerged aquatic macrophytes in Pool .5A MCB were a small and stable component of the river ecosystem. Submerged plants occurred primarily in small, monospecific clumps. Clumps in close proximity to each other formed plant patches. Plant patches were stable in location and number between 1980 and 1983; 82.5% of the patches first observed in 1980 were present in 1983. Submerged macrophytes covered about 10–12 ha of the 201 ha MCB in Pool 5A. Submerged plants were most common in the lower two-thirds of the pool. Ten species of aquatic macrophytes occurred on rock channel-training structures and eleven occurred on non-rock substrates in the MCB. The most common submerged plants, in order of abundance, were Vallisneria americana Michx., Heteranthra dubia Jacq., Potamogeton pectinatus L., Ceratophyllum demersum L. and Potamogeton americanus C. & S.     

Reduction of benthic macroinvertebrates due to waterfowl foraging on submerged vegetation during autumn migration

If present in large numbers, as during migration, herbivorous waterfowlmay reduce the amount of submerged vegetation. Because the vegetation is a keyfactor in shallow eutrophic lakes, removal of the green biomass can be expectedto affect also other biota that depend on the vegetation. We conducted anexperiment to determine how the abundance of chironomids andPisidium sp. were affected by intense foraging ofwaterfowlon the submerged plant Potamogeton pectinatus. This wasdone in Lake Ringsjön in southern Sweden, during the autumn migration ofthe birds. Three treatments, replicated six times, were used: (i) closed cagesthat excluded all waterfowl, (ii) semi-open cages that excluded only largewaterfowl (geese and swans), and (iii) open plots where all waterfowl couldfreely enter. Waterfowl densities were monitored during the experiment. Theresults suggest that the foraging of large waterfowl (swans) had a clearlynegative effect on macroinvertebrate abundance and aboveground biomass ofP. pectinatus. At the end of the experiment, the densityofchironomids was about 46% lower in the open than in the closed cages. Ingeneral, the density of Pisidium sp. tended to be lower inthe open plots. Small waterfowl alone did not seem to affect either thevegetation or macroinvertebrates. We suggest that thePisidium sp. was influenced at an early stage of grazing,when waterfowl foraged on aboveground biomass, whereas chironomids wereaffectedat a later stage, when swans were digging for below-ground tubers.