Biological control of phytoplankton by the subtropical submerged macrophytes Egeria densa and Potamogeton illinoensis: a mesocosm study

1. In temperate regions, submerged macrophytes can hamper phytoplankton blooms. Such an effect could arise directly, for instance via allelopathy, or indirectly, via competition for nutrients or the positive interaction between submerged macrophytes and zooplankton grazing. However, there is some evidence that the positive interaction between submerged macrophytes and zooplankton grazing is less marked in warmer regions, where the interaction is less well studied, and that negative effects of higher water plants on phytoplankton biomass are weaker. 2. We carried out two consecutive mesocosm experiments in Uruguay (subtropical South America) to study the effects of two common submerged macrophytes from this region (Egeria densa and Potamogeton illinoensis) on phytoplankton biomass, in the absence of zooplankton grazing. We compared phytoplankton development between different macrophyte treatments (no macrophytes, artificial macrophytes, real Egeria and real Potamogeton). We used artificial macrophytes to differentiate between physical effects (i.e. shading, sedimentation and competition with periphyton) and biological effects (i.e. nutrient competition and allelopathy). 3. In Experiment 1, we found no evidence for physical effects of macrophytes on phytoplankton biomass, but both macrophyte species seemed to exert strong biological effects on phytoplankton biomass. Only Egeria affected phytoplankton community structure, particularly tempering the dominance of Scenedesmus. Nutrient addition assays revealed that only Egeria suppressed phytoplankton through nutrient competition. 4. We performed a second mesocosm experiment with the same design, but applying saturating nutrient conditions as a way of excluding the effects of competition for nutrients. This experiment showed that both macrophytes were still able to suppress phytoplankton through biological mechanisms, providing evidence for allelopathic effects. Our results indicate that both common macrophytes are able to keep phytoplankton biomass low, even in the absence of zooplankton grazing.     

Aquatic macrophytes and phytoplankton of a quarry lake in the upper Vicenza range: preliminary study

Abstract

The work was carried out during 1995 in a quarry-lake and 13 samplings were performed to gather data on the aquatic flora. Sampling locations both along the shores and in the water provided data to define the position and the amount of covering by macrophytes. As regards the microphytes, considered as the sole phytoplankton component, the composition and the seasonal abundance were evaluated.     

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.     

Ambiguous climate impacts on competition between submerged macrophytes and phytoplankton in shallow lakes

1. Shallow lakes may switch from a state dominated by submerged macrophytes to a phytoplankton‐dominated state when a critical nutrient concentration is exceeded. We explore how climate change may affect this critical nutrient concentration by linking a graphical model to data from 83 lakes along a large climate gradient in South America. 2. The data indicate that in warmer climates, submerged macrophytes may tolerate more underwater shade than in cooler lakes. By contrast, the relationship between phytoplankton biomass [approximated by chlorophyll‐a (chl‐a) or biovolume] and nutrient concentrations did not change consistently along the climate gradient. In warmer climates, the correlation between phytoplankton biomass and nutrient concentrations was overall weak, especially at low total phosphorus (TP) concentrations where the chl‐a/ TP ratio could be either low or high. 3. Although the enhanced shade tolerance of submerged plants in warmer lakes might promote the stability of their dominance, the potentially high phytoplankton biomass at low nutrient concentrations suggests an overall low predictability of climate effects. 4. We found that near‐bottom oxygen concentrations are lower in warm lakes than in cooler lakes, implying that anoxic P release from eutrophic sediment in warm lakes likely causes higher TP concentrations in the water column. Subsequently, this may lead to a higher phytoplankton biomass in warmer lakes than in cooler lakes with similar external nutrient loadings. 5. Our results indicate that climate effects on the competitive balance between submerged macrophytes and phytoplankton are not straightforward.     

浮游植物的化感作用

生物化感作用研究是近年来兴起的交叉学科,是化学生态学研究的重要领域。研究水域浮游植物化感作用对了解浮游植物之间、浮游植物与其他生物之间的相互作用及作用机理具有重要意义,对了解赤潮和水华的发生机制及其生态控制等具有非常重要的作用。综述了海洋和湖泊浮游植物化感作用和化感物质的内涵,讨论了水体浮游植物化感作用的特点、研究化感作用的基本方法、化感物质的种类以及影响化感物质作用的生物和非生物因素,详细介绍了浮游植物化感物质的作用机理以及逃避和拈抗化感作用的方式,同时对目前研究的热点问题及未来研究的方向做了简要概述。     

The phytoplankton community of a eutrophic reservoir

The dynamics of the phytoplankton community of a eutrophic reservoir are described for a two year period. Fifty-eight species were recorded, 25 of them common. Bacillariophyta dominated during the winter and early spring and Chlorophyta during late spring, to be replaced by a bloom of Cyanophyta. The mean and peak biomass of phytoplankton was 8.6 mg 1^–1 and 40.8 mg 1^–1 in 1981, and 8.3 mg 1^–1 and 37.6 mg 1^–1 in 1982. Temperature accounted for 67.3% and pH for 8% of the variation in total phytoplankton biomass over the two year period, using a multiple regression technique.Both horizontal and vertical patchiness, measured as an index of mean crowding, were recorded in the reservoir. Horizontal aggregations were associated with spring blooms of Chlorophyta and summer blooms of Cyanophyta, while vertical aggregations were most marked during the summer bloom of Cyanophyta. Concentrations of phytoplankton were influenced by wind, the prevailing southwesterly wind accumulating algae in the northeasterly arm of the reservoir during much of the year.     

淡水水生植物化感作用研究进展

水生植物的化感作用对淡水生态系统中水草的可持续管理和湖泊富营养化的生态控制具有非常重要的意义。本文综述了淡水水生植物化感作用的发展历史和研究现状,讨论了水生植物化感作用研究过程中所涉及的实验方法、生物测试方法、化感物质分离方法以及影响水生植物化感研究的环境因素。抑藻圈试验、藻类生长试验和浮萍生长试验是常用的生物测试方法;采用共生培养或单独培养的方法,从种植水中分离、鉴定化感物质,对证实复杂水环境中水生植物化感作用的存在,研究它们化感物质的释放途径和作用机理都具有十分重要的作用。     

沉水植物化感作用对西湖湿地浮游植物群落的影响

通过微宇宙实验,在控制光照和营养盐浓度的条件下分别研究了苦草(Vallisneria spiralis)、金鱼藻(Ceratophyllum demersum)和穗花狐尾藻(Myriophyllum spicatum)的化感作用对采集于杭州西湖湖西湿地的藻类密度、叶绿素a浓度、群落结构、多样性指数等的影响。其结果表明,3种沉水植物对微宇宙系统中的藻类都具有明显影响,藻类密度与叶绿素a浓度受到显著抑制,3个草-藻研究系统中藻类群落结构都发生了变化。在实验末期苦草组、金鱼藻组和穗花狐尾藻组中藻类总生物量(以细胞密度计)分别较初始值降低了37.06%、78.37%和83.40%。栅藻对3种沉水植物的化感作用敏感性较弱。藻类生物多样性方面,穗花狐尾藻系统中最高,其次是金鱼藻组,最后是苦草组,其Shannon-Wiener指数(H)分别为2.76、2.06和0.72,穗花狐尾藻组中H的显著高于苦草组(P0.05)。     

The influence of a dissolved inorganic nitrogen gradient on phytoplankton community dynamics in a chain of lakes

In a chain of lakes along which nutrient availability varies in a gradient, we performed factorial nutrient enrichment experiments to determine if nitrogen limitation was the principal factor controlling the differences in phytoplankton biomass, photosynthetic productivity, diversity, and species composition among two of the lakes in the chain. In the least productive lake, East Graham Lake, P and C enrichments (in the absence of N enrichment) had no effect on biomass and diversity, whereas within two weeks the N enrichments (alone or in any combination with P and/or C) increased the biomass and decreased the diversity of East Graham Lake phytoplankton to levels similar or identical to those in more productive Shoe Lake. Short-term ¹⁴C photosynthetic rates in East Graham Lake water also responded only to N in the third week. However, photosynthesis was stimulated by P in the first week, and a few species did increase in numbers with P enrichment, suggesting that some degree of P limitation remains in addition to the strong N limitation in East Graham Lake. A number of species responded individually to the enrichments in a manner similar to that of the overall community, and a strong overlapping of discriminant analysis scores for N-enriched East Graham Lake with those of Shoe Lake was consistent with our prediction that the community structure of N-enriched East Graham Lake water would shift toward that of Shoe Lake. However, many species did not respond consistently with these results, and the nutrients tested were clearly not a major factor in the differences in abundance of those species among the two lakes. The results support the argument that overall biomass production and diversity of the phytoplankton community in a lake can be a relatively simple function of a single most-limiting nutrient. However, many of the species responses also confirm that, while nutrient availability is an important factor in the control of the species composition of the community, other factors are likely to prevent reliable predictions of all species effects on the basis of nutrient availability alone.     

The long-term effect of artificial destratification on phytoplankton species composition in a subtropical reservoir

1. The response of phytoplankton to the installation of an artificial destratification system in North Pine Dam, Brisbane (Australia) was investigated over an 18 year period (1984–2002); 11 years before and 7 years after installation. 2. An overall increase in phytoplankton abundance was revealed for some groups (in particular, diatoms, cyanobacteria and chlorophytes), but not for others (chlorophytes). Changes in the abundance of chlorophyte functional groups was attributed to eutrophication. 3. A strong spatial gradient in phytoplankton abundance and chlorophyll a was observed, with low abundance in the downstream regions affected by the destratification system which was likely because of light limitation induced by vertical mixing. The upstream region acted as a surrogate for the unaltered state of the reservoir, particularly as an indicator of eutrophication without direct influence from the destratification system. Despite the continuous trend in eutrophication of the reservoir, there has been a definite decrease in the rate of eutrophication (approximately 30%) since the installation of the destratification system at the downstream locations. 4. Correlations of the dominant cyanobacteria Cylindrospermopsis raciborskii with other genera changed after destratification, indicating that prior to destratification the dominance of Cylindrospermopsis was because of its ability to compete for phosphorus, whereas after destratification its dominance was because of its ability to compete for light.     

The influence of macrophytes on the spatial distribution of littoral rotifers

1. The effect of macrophytes on the spatial distribution of littoral rotifers was examined in Lake Rotomanuka, New Zealand (37°55'S, 175°19'E). Total rotifer abundances and those of abundant species, were compared between three macrophyte species, Myriophyllum propinquum , Eleocharis sphacelata and Egeria densa , and spatially across a littoral transect in relation to these species.
2. The abundances of many species, for example Euchlanis dilatata, Lecane closterocerca and L. lunaris, differed significantly between macrophyte species. More planktonic forms, Ascomorpha saltans , Keratella cochlearis and Synchaeta oblonga, however, showed no significant preference for macrophyte species.
3. Differences in rotifer abundances were evident even when different species of macrophyte grew in close proximity to one another, indicating that variations in physical and chemical conditions, which occur in the littoral of Lake Rotomanuka, could be largely discounted for much of the variation between macrophyte species.
4. Variation in rotifers between macrophytes was probably the result of a number of factors, including differences in macrophyte morphology, macrophyte age, epiphytic algal growths and the differential effects of predation by invertebrates and fish between macrophytes.
5. Variability of rotifer abundances spatially across the ecotone was less marked than between macrophyte species. The species of macrophyte occurring, and therefore the community composition and distribution of macrophyte species in the littoral, appears to be a major influence in the spatial structuring of rotifer communities in the littoral region of lakes.     

Allelopathic activity of volatile substance from submerged macrophytes on Microcystin aeruginosa

Considerable attention has been paid on the allelopathic potential of aquatic macrophytes against blue-green algae. This study compared inhibitory activity of volatile compounds that were contained in two submerged macrophytes, Ceratophyllum demersum and Vallisneria spiralis, on Microcystin aeruginosa. When the concentration of essential oils was 100 mg/L, the antialgal activities for fresh macrophytes were similar between C. demersum and V. spiralis but stronger than those for dry macrophytes. Essential oils from C. demersum in dry macrophytes showed stronger antialgal activity than those from V. spiralis. There exists a positive correlation between the concentration of the essential oils and the inhibitory activity. The essential oils were composed of fatty compounds, terpenoids, phenolic compounds, phthalates, and some unknown compounds. More than 40% of the essential oils from fresh macrophytes were composed of phthalates, whereas approximately 70% of those from dry macrophytes were composed of lipid compounds and terpenoids. However, approximately 20% of the essential oils from fresh macrophytes were composed of unknown compounds, which were unstable and undetectable in dry macrophytes. Therefore, their chemical structures and bioactivities need further study.     

沉水植物中挥发性物质对铜绿微囊藻的化感作用

比较了两种淡水沉水植物金鱼藻和苦草中挥发性物质对铜绿微囊藻的化感抑制作用,在100mg／L浓度下,新鲜植物挥发油的抑藻作用非常显著,两种植物抑藻活性相近,但是在干粉材料中金鱼藻挥发油的抑藻活性明显强于苦草,且挥发油浓度与抑藻活性呈正相关。不同植物之间以及新鲜植物和植物干粉之间挥发油的成分和相对含量差别较大,新鲜植物挥发油中含有40％的邻苯二甲酸酯,而在于粉挥发油中70％为脂肪族化合物和萜类物质。在新鲜植物挥发油中约占20％的是不稳定的未知物,其化学结构和生物活性有待进一步研究。     

Effects of barley straw (Hordeum vulgare) on freshwater and brackish phytoplankton and cyanobacteria

A short-term laboratory study was conductedto investigate the effect of barley strawin controlling several common phytoplanktonand cyanobacterial species. Following aone-month incubation of barley straw incoarsely filtered fresh Potomac River andbrackish Patuxent River waters, the growthof six autotrophic taxa was followed inculture. Barley straw slurry reduced theyield of three taxa (Ankistrodesmusfalcatus, Chlorella capsulata, Isochrysis sp.) in comparison withcultures not receiving the slurry. Although no significant changes in growthwere detected with three other taxa (Cyclotella sp., Prorocentrumminimum, freshwater Pseudanabaenasp.), some patterns indicated potentialimpacts of the barley straw. First, ahigher addition of straw to Cyclotella sp. resulted in a lower biomassaccumulation than in cultures receivinglower levels. Second, the bloom-formingdinoflagellate Prorcentrum minimumwas apparently stimulated at low barleystraw levels, perhaps suggesting conditionsassociated with the straw(metals-chelation, bacterial-producednutrients) might stimulate dinoflagellategrowth. Third, species shifts wereobserved in two of the cultures, withbarley straw favoring shifts from Isochrysis to a Cyclotella sp. –Thalassiosira sp. mixture and shiftsfrom Pseudanabaena to a Pseudanabaena – Scenedesmus mixture. These results provide new records for thesusceptibility of freshwater and brackishphytoplankton taxa to barley strawexposure, including species-specificresponses and shifts in species dominancein mixed assemblages.     

The influence of ventilation systems on water depth penetration of emergent macrophytes

1. Roots of emergent macrophytes depend on oxygen transported from aerial parts by diffusion and, in some species, by pressurised ventilation. The aim of this study was to determine if species with pressurised ventilation grow in deeper water in situ than species lacking this ability.
2. Species with pressurised ventilation grew significantly deeper than species lacking pressurised ventilation, and this was true in substrates with low as well as high redox potential.
3. Two species with low or non-detectable flow rates ( Schoenoplectus lacustris and Equisetum fluviatile , respectively) were found in unexpectedly deep water. Thus, pressurised ventilation is not a prerequisite for growth in deep water. It is, however, suggested that species with pressurised ventilation have a competitive advantage in deep water resulting in long-term competitive exclusion of species lacking pressurised ventilation.     

大亚湾浮游植物群落特征

于2002年冬、春、夏和秋季对大亚湾浮游植物进行采样调查,分析了浮游植物的种类组成、丰度、优势种、多样性及群落结构的季节变化特征和平面分布特征。并讨论了浮游植物与营养盐、水温及环流等环境因子之间的关系。2002年大亚湾浮游植物共鉴定出48属114种(包括变型和变种),丰度范围在5.79×104~5.37×106cells/m3之间,平均值为1.14×106cells/m3。其中硅藻共37属84种,其种数和细胞丰度都占绝对优势,平均丰度为1.08×106cells/m3,其次为甲藻,9属23种,平均丰度为9.91×104cells/m3。此外还鉴定出蓝藻和金藻。大亚湾浮游植物丰度变化呈单一周期型,春夏季高,秋冬季节低。虽然硅藻的丰度占优势,但秋季硅藻丰度降低(占总丰度75.8%)使甲藻和蓝藻所占比例上升。研究得出春夏季大亚湾浮游植物主要以沿岸暖水性种类为主,秋季和冬季除沿岸暖水种之外,广布种和大洋种也较多,尤其在冬季后者占优势。大亚湾浮游植物优势种类多,不同季节既有交叉又有演替。与以往调查资料相比,部分优势种发生变化,优势程度顺序和细胞丰度发生了一定改变,个体较大的细胞丰度优势逐渐增加。另外,受季风、潮流、地理位置及人类活动影响,大亚湾浮游植物丰度和群落结构有一定的季节和平面分布特征。大亚湾浮游植物的多样性在夏季偏低,尤其在大亚湾核电站和大鹏澳养殖区附近表现明显。大亚湾浮游植物的丰度、种数、优势种演替及群落结构等其它群落特征与营养盐尤其是氮、磷和N/P、水温、环流等环境因子密切相关。     

The effects of disturbance events on phytoplankton community structure in a small temperate reservoir

1 The effects of disturbances, in the form of storm events, on phytoplankton community structure were examined over the course of four years in Eau Galle Reservoir, Wisconsin, USA.
2 Disturbances consistently brought about significant, but highly transient, increases in apparent phytoplankton species richness. It is likely that these resulted from temporary increases in the biomass of previously undetected rare species.
3 Substantial shifts in community dominance were confined to large, early season events, and were seldom long-lived. Later 'climax' communities were highly resistant to any changes in dominance, even when increases in species richness occurred.
4 Regardless of when they occurred, disturbances tended to favour species from a narrow range of the successional sequence.     

Hyperthermal effluent Effects on heleoplanktonic Cladocera and the influence of submerged macrophytes

In July and August, 1974, measurements were made of the standing crops of Cladocera in the littoral zone of Par Pond (Savannah River Plant, Aiken, South Carolina, U.S.A.), which receives hyperthermal effluent from a nuclear reactor. Crops of Ceriodaphnia spp. and Diaphanosoma brachyurum were greater in the heated than in the ambient area, while Bosmina longirostris maintained higher standing crops in the ambient area than in the area receiving hyperthermal effluent. In August, 1974, exclosures were placed in the effluent-affected area to test the hypothesis that the high density of rooted aquatic macrophytes in the effluent-affected area influences the standing crop of these Cladocera. The effects of changes in reactor effluent temperature were also determined in the exclosure experiments. The results of the exclosure study support two generalizations: 1) the presence of dense rooted vegetation allows higher standing crops of Ceriodaphnia spp. and D. brachyurum; and 2) lower temperatures than those usually found at the heated station would favor B. longirostris standing crops, while the higher effluent temperatures favor Ceriodaphnia spp. and D. brachyurum.     

Phytoplankton and submerged macrophytes in the aquatic ecosystems of the Danube Delta during the last decade

The dynamics are presented of the main primary producers in seven representative Danube Delta lakes. Generally, the primary productivity developed from the macrophyte-epiphyte complex towards the phytoplankton. Species changes occurred in the two main compartments with increasing relative abundance of colonial blue-green algae in the phytoplankton and in submerged macrophytes of species with a vertical growth strategy. These changes are linked to accelerated eutrophication of the lakes, with increased phosphorus loading and a reduction in N:P ratio.     

The sensitivity of phytoplankton in Loch Leven (U.K.) to changes in nutrient load and water temperature

1. Loch Leven is a shallow, eutrophic lake in Scotland, U.K. It has experienced much change over the 30 years that it has been studied; this has primarily been due to reduced nutrient loads to the lake through active catchment management. Its recovery has been slow and, therefore, we used a phytoplankton community model (PROTECH) to test its sensitivity to changing nutrient loads and water temperature.
2. PROTECH was initialized to simulate the observed phytoplankton community in 1995 and was then repeatedly run through a combination of step-wise changes in water temperature and nutrient load (two treatments were simulated for nutrient load: one changing both nitrate and phosphorus, and one changing just phosphorus). The effect on total chlorophyll- a concentration, cyanobacteria abundance and phytoplankton diversity was examined.
3. Whilst changes in temperature had little effect, variations in the nutrient load produced a range of responses. Increasing only the phosphorus load caused a large increase in Anabaena abundance and total chlorophyll- a concentration. However, the opposite response was recorded when nitrate load was changed as well, with Anabaena increasing its biomass under reduced nutrient load scenarios.
4. The key factor determining the type of response appeared to be nitrogen availability. Anabaena , a nitrogen fixer, could exploit the phosphorus resource of Loch Leven under limiting nitrogen conditions, allowing it to dominate under most of the scenarios tested apart from those supplying extra nitrogen to the lake. The model predictions agree with the observed data, which show that Anabaena continues to dominate the summer phytoplankton bloom in Loch Leven despite the considerable reduction in phosphorus supply from the catchment. This research provides a possible explanation for this.