衰亡期黑藻与生长期菹草交替生长对水体磷迁移的影响

为利用冷暖种交替控制水体磷污染、抑制水体富营养化,揭示湖泊演化规律和机理。研究设置单季植物组(黑藻组、菹草组)和交替生长组(黑藻组+菹草组)进行实验。交替生长组在黑藻衰亡期种植菹草,监测各组上覆水和底泥中各形态磷含量的变化,计算黑藻衰亡释放磷及菹草生长吸收磷的总量,同时测定环境因子指标。分析沉水植物交替生长(黑藻+菹草)过程对衰亡期沉水植物(黑藻组)释放磷所带来的二次污染的消减作用,并分析环境因子变化与磷含量之间的关系。实验结果表明:黑藻+菹草组显著(P<0.05)降低了上覆水中总磷(TP)、溶解性总磷(DTP)和溶解性活性磷(SRP)的浓度;显著(P<0.05)降低了间隙水中DTP和SRP的浓度。底泥TP含量黑藻组呈上升趋势,黑藻+菹草和菹草组呈下降趋势。在采用菹草生物量期望2倍于衰亡植物黑藻生物量的模拟实验条件下,每实验组沉水植物黑藻衰亡分解所释放的磷为1.51 g,沉水植物菹草生长所富集吸收的磷为1.83 g。因此,菹草具备消减黑藻所释放磷的能力,可作为冷暖种交替控制水体富营养化的备选物种。实验组磷的迁移方向分别为:黑藻组磷迁移最终方向为底泥,黑藻+菹草组和菹草组磷的迁移方向为植物。黑藻+菹草组通过提高环境中DO和ORP,使得水相中磷向沉积物相中迁移,从而使得水相中各形态磷浓度保持在相对较低的水平。     

保安湖菹草种群的时空分布特征及环境效应分析

为了解近20年来保安湖菹草(Potamogeton crispus L.)种群的变化特征及其对水环境的影响, 研究以保安湖最大的湖区, 即主体湖为研究对象, 分季节对其沉水植物种类组成、生物量及水深(Z_M)、透明度(Z_SD)、水中总氮(TN)、总磷(TP)和浮游藻类叶绿素a(Chl. a)等环境指标进行了监测, 并分析了菹草在不同生活史阶段对环境影响的差异。结果表明: (1)2002年春季菹草生物量(B_P.c)均值为356 g/m2, 2012年上升至974 g/m2, 2019年为1901 g/m2, 菹草种群分布范围由中部扩展至整个湖区; (2)在春季菹草快速生长时, B_P.c与Z_M(Z_M≤3 m)呈显著正相关(r= 0.52, P<0.01), 与Chl. a呈显著负相关(r= –0.42, P<0.01), 与Z_SD、Z_SD/Z_M、TN、TP无相关关系; 有草区的Z_M(中位数为2.1 m)、Z_SD(中位数为0.93 m)和Z_SD/Z_M(中位数为0.48)显著高于无草区(1.8 m、0.45 m和0.28; P<0.05), 有草区的Chl. a(中位数为8.13 μg/L)显著低于无草区(14.10 μg/L; P<0.05); 有草区的Chl. a和TP的关系不明显, 无草区的Chl. a的含量随TP上升而增加, 且在相同TP条件下, 无草区Chl. a多数高于有草区; (3)在夏季菹草衰亡后, 夏季Z_SD/Z_M与B_P.c呈显著负相关(r= –0.47, P<0.01), Z_SD、TN、TP、Chl. a与B_P.c无相关关系; 无草区TN(中位数为1.30 mg/L)显著高于有草区(0.72 mg/L; P<0.05); 有草区和无草区的Chl. a均随TP的上升而增加, 在相同TP条件下, 无草区Chl. a和有草区差异不显著。以上结果表明近20年来保安湖菹草种群生物量呈上升趋势, 分布范围也在不断增加。在快速生长期(春季), 大量菹草的存在有利于水质的改善。在菹草衰亡(春末夏初)后, 其对水质产生的不利效应未持续整个夏季。     

沉水植物菹草对富营养化水体中TN 生态效应及模型研究

通过室内模拟构建水体营养盐和菹草生物量正交试验, 研究菹草从石芽萌发至菹草植株衰亡腐烂整个生命周期中, 对水体中TN 的生态效应, 并以此为依据建立菹草对富营养化水体生态效应模型。研究表明: 在不同生物量菹草作用下, 富营养化水体中TN 含量是先下降后上升的一个周期式过程; 在相同营养水平条件下, 菹草对TN 的吸收量随菹草生物量的增大而依次递增, 而在相同生物量的前提下, 菹草对TN 的修复力存在一个最佳营养水平范围, 即13.95-20.56 mg·L–1 之间; 菹草对富营养化水体生态效应模型分两部分: 菹草对富营养化水体中TN 与时间关系模型以及菹草生态修复水体TN 限度理论模型, 通过这两个模型的构建可以为生产实践中菹草在生态修复富营养化水体的投放量和菹草的最佳收割时间提供有力依据, 为工程实践提供有效保障。     

光强对菹草生长及抗氧化酶活性的影响

试验以菹草(Potamogeton crispus)为试验材料, 在7种不同光强(自然光的100%、60%、40%、20%、10%、5%和1%)条件下, 测定并分析菹草的形态指标(株高、叶片数)和生理指标(叶绿素、根系活力、抗氧化酶活性及丙二醛)对光强条件的响应, 为富营养化水体植物群落的构建提供理论依据。结果表明: (1)随着光照强度的增大, 菹草的株高、叶片数、生物量增长率显著上升, 而光强过高(CK组)和过低(1%组)菹草生长受到抑制; (2)菹草叶绿素含量随光照强度的增大, 呈先上升后下降趋势。1%组和CK组Chl. a、Chl. b、Chl. a+b和Car含量低, 其余各组含量较高; (3)随着试验时间的延长, 菹草根系活力呈先上升后下降趋势, 20%组、40%组和60%组的根系活力较大; (4)菹草SOD、POD和CAT活性整体表现为先升后降趋势, 试验后期1%和CK组SOD含量较高, POD、CAT含量较低。菹草叶片MDA含量在试验过程中不断上升, 抑制菹草的正常生长。自然光照的20%—60%, 菹草生长状况较好; 持续的弱光胁迫将导致植物活性氧代谢失调, 细胞受到伤害, 最终导致植物死亡。     

白洋淀菹草对富营养化水体总磷的净化

利用白洋淀区域鲥鯸淀、王家寨和小淀3个淀区的水体、底泥和菹草组成室内静态的模拟生长体系,研究白洋淀菹草在不同水体及底泥环境下的生长状况,以及不同菹草生长体系对水体总磷的净化效果.结果表明:鲥鯸淀菹草生长体系对水体总磷的净化效果最佳,最大去除率为87.9%,单位生物量对水体总磷的最大去除率为2.2%;王家寨和小淀菹草生长体系对水体总磷的最大去除率分别为47.4%和76.9%,单位生物量对水体总磷的最大去除率分别为0.9%和1.4%.3个菹草生长体系的底泥对水体总磷吸附的最大百分比分别为9.1%、7.4%和7.7%.菹草生长体系的TP-t和v-t拟合方程表明,水体中总磷浓度及其去除速率随时间的延长以负指数形式衰减.     

菹草茎叶微界面O2时空动态

采用高分辨率光纤氧微电极测定了富营养化水体中沉水植物菹草(Potamogeton crispus)茎叶微界面(0—2.0 mm)氧(O_2)。菹草叶微界面O_2浓度梯度具明显的时空变化。时间上,菹草叶微界面O_2浓度具有明显的生长阶段变化和昼夜变化。幼苗期和快速生长期微界面O_2浓度增加幅度较小,稳定期叶表O_2浓度梯度增加幅度最大,衰亡期叶微界面O_2浓度受附着物影响具明显的空间梯度。菹草叶微界面O_2表现为昼高夜低的单峰变化模式,主要受光照和温度的影响。空间上,越接近茎叶表面,O_2浓度越高。顶部幼叶微界面O_2浓度梯度增加较平缓,中部成熟叶微界面O_2浓度梯度变化最陡,波动幅度最大,中部茎和基部衰老叶微界面O_2浓度梯度由于受密集附着物的影响,在附着物表面达到最大值,进入附着层后略有下降。结果表明,菹草茎叶微界面O_2时空变化主要受附着物和植物光合放氧能力的影响。光纤微电极是一种分析植物叶微界面氧时空分布的理想工具,对深入研究植物微界面在富营养化水体中养分的迁移转化具有重要意义,可为水生植物生理生态研究提供有力工具。     

玄武湖菹草种群的发生原因及人工收割对水环境的影响

利用以沉水植物为主的水生植物进行水体生态修复是目前研究的热点问题, 为研究南京玄武湖2005-2006年的菹草(Potamogeton crispus)种群发生的原因及人工收割对水体的影响, 对玄武湖不同湖区定期监测其透明度、溶解氧、pH、TN、TP等水质指标, 并进行分析, 结果表明: 对湖泊蓝藻水华的应急处理, 使水体透明度提高179.5%, DO含量增高24.1%, TN、TP分别降低54.1%、74.5%, pH由9.1降至8.7, 水质改善是菹草种群萌发并能大规模生长的主要原因。而菹草生长阶段短期内对菹草进行大规模收割使水体DO含量降低42.1%, 透明度下降51.5%(P0.05), 收割虽从水体中携带走部分氮、磷营养盐, 但差异不显著(P0.05), 且收割后TP出现升高现象。故在对草藻型湖泊生态修复过程中, 可先期通过物理或化学手段改善水体透明度、调节pH、降低营养盐, 使其满足水生植物萌发及幼苗生长的需求, 为水生植物后期存活并生长打下基础, 在后期生态管理过程中, 应逐步收割植株, 缓慢从水体携带营养盐, 以达到改善水质, 恢复及重建水生生态系统的目的。       

透明度胁迫对菹草(Potamogeton crispus)生长的定量影响及其生长动力学模型

在实验室研究了不同水体透明度对沉水植物菹草生长的影响,结果表明,在实验条件下,当水体透明度低于12cm时,菹草由于水下光照不足而无法存活;透明度为15cm时菹草的生物量明显低于透明度为70cm(对照)、35cm和20cm 3种情况下的生物量,表明在透明度为15cm时,菹草的生长受到了明显的抑制.实验期间,菹草生物量变化的回归分析表明,不同透明度下菹草的生长均符合逻辑斯谛曲线.采用逻辑斯谛方程描述菹草的生长规律,结合实验结果,确定不同水体透明度下菹草的生长参数,结果显示:水体透明度对菹草生长的内禀增长率rm影响较大,而对菹草生长的环境容纳量k无明显影响.综合考虑水体透明度对菹草生长的影响,建立考虑水深和透明度因素的菹草生长动力学模型,该模型可预测出在外部水环境因素作用下菹草的动态生长过程.     

五种沉水植物对富营养化水体的净化效果

为达到净化水质的目的,采用移栽沉水植物控制富营养化水体中营养盐含量和浮游藻类生物量。本研究在夏季藻类密度较高的富营养化水体中移栽5种长江中下游流域常见沉水植物,比较不同沉水植物去除营养盐和控制藻类总量的能力。研究结果:5种沉水植物对水体总氮含量去除率的大小顺序为:竹叶眼子菜>黑藻>苦草>微齿眼子菜>菹草,对总磷去除率大小顺序为:竹叶眼子菜>黑藻>微齿眼子菜>苦草>菹草;竹叶眼子菜控制水体中藻类总量的效果最佳,苦草、微齿眼子菜及菹草次之,黑藻对水体总磷和浮游植物的去除效果均极显著(p≤0.01),而对总氮含量的作用影响不明显(p=0.209)。综合营养盐吸收作用和藻类控制效果来看,竹叶眼子菜在整个实验过程中生长状态良好并达到较为稳定的净化作用,是夏季治理浅型富营养化静水水体的理想物种之一。     

几种生态因子对菹草光合作用的影响

本文研究了光照、pH、温度对菹草光合作用的影响。在一定温度条件下,菹草的净产氧量与一定范围的光照强度呈直线相关。菹草的光补偿点随温度的上升而上升。在菹草自然生活的环境中,温度低于30℃时,升温有利于菹草的光合作用。高pH(PH>10.0)下碳源缺乏对菹草的光合作用影响较大。高pH与强光照射的协同作用严重影响菹草的光合作用。水温与氮、磷营养盐不足并非夏季自然水体中菹草死亡的主要原因。而不良光照(水表层光抑制,中、下层光饥饿)和高pH下缺乏光合碳源的协同作用便可能导致菹草夏季死亡。     

罗非鱼对附着藻类和浮游植物的影响

鱼类通过牧食和营养盐排泄可以对水体生态系统产生影响,杂食性鱼类由于可摄食不同生境中的食物,可使生境之间的耦合作用发生变化。罗非鱼是我国南方很多水体的优势种,食物包括敞水生境的浮游植物和基质表层生境的附着藻类等。为了解罗非鱼对浮游植物和附着藻类的影响,实验在室外模拟条件下,分别设置罗非鱼组和无鱼对照组的两组处理,分析了罗非鱼对附着藻类及浮游植物生物量(叶绿素a)等的影响。结果表明:(1)罗非鱼显著地降低了附着藻类生物量,罗非鱼组中的附着藻类叶绿素a的平均值为0.15 mg·cm^-2,显著低于对照组中的1.26mg·cm^-2;(2)罗非鱼显著地增加了浮游植物的生物量,罗非鱼组中的浮游植物叶绿素a平均值为31.99μg·L^-1,显著高于对照组中的14.99μg·L^-1。研究结果显示,杂食性的罗非鱼可以促进系统的附着藻类向浮游植物转化。从控制浮游植物生物量的角度看,湖泊等水体的管理应该对罗非鱼密度加以有效控制。     

The impact of petroleum hydrocarbons (diesel) on periphyton in an impacted tropical estuary based on in situ microcosms

The distribution of petroleum hydrocarbons and their effects on the periphytic algal biomass using in situ microcosms were investigated in Ponggol estuary located on the northeastern coast of Singapore. Dissolved or dispersed petroleum hydrocarbon (DDPH) concentrations in the surface and bottom waters and absorbed or adsorbed petroleum hydrocarbon (AAPH) concentrations in sediments were monitored from July 1999 to June 2000. Results showed concentrations ranging from 4.42 to 248.94 μg l⁻¹, from 0.35 to 1099.65 μg l⁻¹, and from 20.55 to 541.01 mg kg⁻¹ for DDPH in surface and bottom waters and AAPH in sediments, respectively. Accidental spillages of fuel from dredgers operating in the estuary, fuel and engine oil from recreational boats, shipping operations in the adjacent strait, and runoff monsoon drains in the vicinity were some of the possible sources of petroleum hydrocarbons in the estuary. An assessment of environmentally realistic concentrations of petroleum hydrocarbons on periphytic algal biomass using in situ microcosms revealed signs of acute toxicity. A reduction in periphytic algal biomass (with respect to controls) of 68-93% was observed for various treatments exposed to diesel.     

Algae of periphyton of Lake Teletskoye

The generalized data are given on the composition and structure of phytoperiphyton of Lake Teletskoye on different substrates (rocks, macrophytes and their residues). It is noted that in spite of the differences in the taxonomic structure and level of development the algocenoses of periphyton of various substrates have similar ecological range. This gives a possibility of using the algocenoses of periphyton on different substrates for assessing the quality of the lake water.     

Effects of macrograzers and light on periphyton stoichiometry

Ecological stoichiometry describes the biochemical constraints of trophic interactions emerging from the different nutrient content and nutrient demand of producers and consumers, respectively. Most research on this topic originates from well-mixed pelagic food webs, whereas the idea has received far less attention in spatially structured habitats. Here, we test how light as well as grazing and nutrient regeneration by consumers affects growth and biomass of benthic primary producers. In the first laboratory experiment, we manipulated grazer presence (two different snail species plus ungrazed control), in the second experiment we factorially combined manipulation of grazer presence and light intensity. We monitored snail and periphyton biomass as well as dissolved and particulate nutrients (nitrogen and phosphorus) over time. Grazers significantly reduced algal biomass in both experiments. Grazers affected periphyton nutrient content depending on the prevailing nutrient limitation and their own body stoichiometry. In the nitrogen (N-) limited first experiment, grazers increased N both in the periphyton and in the water column. The effect was stronger for grazers with lower N-content. In the phosphorus (P-) limited second experiment, grazers increased the P-content of the periphyton, but the grazer with lower N-content had additionally positive effects on algal N. Light reduction did not affect periphyton biomass, but increased chlorophyll-, N- and P-content of the periphyton. These experiments revealed that the indirect effects of grazers on periphyton were bound by stoichiometric constraints of nutrient incorporation and excretion.     

Bacterial periphyton of the Vltava River

Bacterial periphyton formed during 48 hours was studied by glass slide method and direct counting in Vltava River in Praha, Czechoslovakia. At water temperatures 8–11°C the numbers of rods ranged between 24,000 and 336,000 per 1 cm² and those of cocci between 30,000 and 228,000. The relation rods: cocci ranged between 0,9 and 2,4 with an average value of 1,7, whereas in a fishpond this average was 0,5. Among the periphyton 81,3% bacterial cells were active. The rods: cocci relation seems to be a good indicator of water pollution by organic matter, but numbers distinguishing the individual saprobic levels cannot be given yet.

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Zusammenfassung Es wurde der bakterielle Bewuchs von Glassplatten im Vltava-Fluss in Praha untersucht. Zweitägige Exposition erwies sich als am meisten geeignet, um den Bewuchs mikroskopisch durch direkte Zählung quantitativ zu erfassen. In Abhängigkeit von der Tiefe, Wassertemperatur, Verunreinigungsgrad, Sonnenstrahlung u.a. entwickelte sich der Bewuchs quantitativ unterschiedlich und zeigte auch eine unterschiedliche Relation von Stäbchen zu Kokken. Im beta-mesosaproben Bereich wurden im Herbst (Wassertemperaturen 8–11 °C) 24.000 bis 336.000 Stäbchen und 30.000 bis 228.000 Kokken per 1 cm² gefunden, mit den Mittelwerten 163.100 Stäbchen und 104.100 Kokken. Die Vergleichswerte von einem Fischteich in Motol lagen für Stäbchen im gleichen Bereich, während die Kokken, bis 254.000 erreichten.Die Relation Stäbchen: Kokken variierte im Vltava-Fluss zwischen 0,9 (nur einmal) und 2,4 mit dem Mittelwert 1,7, im Fischteich zwischen 0,4 ufnd 0,7 mit dem Mittelwert 0,5. Es hat sich erwiesen, dass diese Relation brauchbare Angaben über die organische Verunreinigung gibt. Leider liegen bisher zu wenige Ergebnisse vor, um eine Skale gegenüber den Saprobitätsstufen zu errichten.Unter den Bewuchsbakterien gab es in der Vltava durchschnittlich 81,3% aktive Zellen, wie durch Fluoreszenzanalyse festgestellt wurde.Fast alle Daten indizieren eine schwächere Verunreinigung des Wassers am linken Ufer als am rechten.

     

Multifractal growth in periphyton communities

Periphyton is an aquatic community composed by algae, bacteria, fungi, and other microorganisms that can develop a complex architecture comparable to tropical forests. We analyzed the spatial pattern of a periphyton community along a succession developed in experimental tanks. Our aim was to identify regularities that may help us to explain the patchiness of this community. Therefore, we estimated the spatial pattern of periphyton biomass using a non‐destructive image analysis technique to obtain a temporal series of the spatial distribution. These were analyzed using multifractal techniques. Multifractals are analogous to fractals but they look at the geometry of quantities instead of the geometry of pattern. To use these techniques the object of study must show scale invariance and then can be characterized by a spectra of fractal dimensions. Self‐organization describes the evolution of complex structures that emerge spontaneously driven internally by variations of the system itself. The spatial distribution of biomass showed scale invariance at all stages of succession and as the periphyton developed in a homogeneous landscape, in a demonstration of self‐organized behavior. Self‐organization to a critical state (SOC) is presented in the complex systems literature as a general explanation for scale invariance in nature. SOC requires a mechanism where the history of past events in a place influence the actual dynamics, this was termed ecological memory. The scale invariance was found from the very beginning of the succession thus self‐organized criticality is a very improbable explanation for the pattern because there would be not enough time for the build‐up of ecological memory. Positive interactions between algae and bacteria, and the existence of different spatial scales of colonization and growth are the likely causes of this pattern. Our work is a demonstration of how large scale patterns emerge from local biotic interactions.     

Water chemistry and periphyton in an alpine wetland

Remote high elevation sites are thought to be good sites to monitor global change and anthropogenic effects on ecosystems. This study was conducted during 1987–1990 in a high elevation wetland (3593 m) located in the Green Lakes Valley, Front Range, Colorado (USA). Salix spp. was the dominant riparian species in this 2 ha. wetland. Small shallow pools (<0.5 m depth) constituted a water area of 236 m³. The major source of water during the study period was snowmelt. The wetland had a well defined outlet and inlet, although an undetermined amount of water entered as groundwater from the snow patch above. Outlet discharge was 424–460 m³ during the month of July and declined thereafter as water input from the snowpatch declined. Inlet discharge was 67% of outlet discharge. Water temperatures in the outlet were always less than 6.8°C, pH 6.0–6.3, and mean conductivity 30.8 µS cm^–1. Both NO _inf3 ^sup– and SO _inf4 ^sup–2 were higher in the inlet thanin the outlet. Dominant cations in the inlet and outlet waters were Ca⁺² Mg⁺² > K⁺ + Na⁺; dominant anions were SO _inf4 ^sup–2 HCO _inf3 ^sup– > NO _inf3 ^sup– Cl^–. Nutrient limitation by P was demonstrated once using nutrient diffusing substrata. No limitation could be shown for NO _inf3 ^sup– , HCO _inf3 ^sup– , or Fe+EDTA. Slow colonization rates of periphyton on tiles were attributed to low temperatures and/or ultraviolet radiation. However, interannual differences in biomass on tiles were as much as 300% after 35 days. A minimum of 16–54 samples would be needed to detect a significant interannual change in biomass on tiles after 35 days assuming that the extreme case for periphyton patchiness. Global climate change is likely to affect discharge and water temperature in this wetland which hill have direct and indirect affects on population dynamics and ecosystem function.     

Heterogeneity of periphyton in French Mediterranean rivers

The ecological niche of periphytic algae can be characterized by a long list of environmental variables: hydrology, substratum, light, water chemistry, temperature, other biota (Rott, 1991). In the french mediterranean area, all the watercourse types are represented; we investigated permanent or intermittent, natural or perturbed streams, large rivers or brooks and tricklets flowing out on limestone or crystalline massifs. Most of studies are based on samples of periphyton from natural substrata (e. g. pebbles, cobbles, sand, silt and clay and macrophytes), but also some from artificial substrata. We also sampled simultaneously measurements of water quality and other relevant environmental variables. All investigations in mediterranean area show the high sensibility of periphyton reacting quickly to environmental parameters; it is shown by a heterogeneous pattern of colonization at different levels.

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Résumé Selon Rott (1991), la niche écologique des algues périphytiques peut être caractérisée par une longue liste de paramètres environnementaux: hydrologie, substratum, lumière, chimie des eaux, température et autres facteurs biotiques. Les peuplements d'algues des rivières du sud-est de la France ont été très longtemps délaissés ou très peu étudiés. Cependant de nombreux types de cours d'eau sont représentés (permanents ou temporaires, naturels ou perturbés, larges rivières ou petits cours d'eau s'écoulant sur des massifs karstiques ou cristallins). La morphométrie de ces cours d'eau est fréquemment et profondément remaniée par les stress hydriques liés aux caractéristiques climatiques de la région.Des études menées depuis plus de dix ans sur des substrats naturels (cailloux, galets, sables, argiles, limons et macrophytes) permettent de constater que, du fait de sa très grande sensibilité, le periphyton réagit rapidement aux variations des conditions expérimentales ce qui se traduit par une très forte hétérogénéité de colonisation.

     

Nutrient limitation of phytoplankton and periphyton growth in upland lakes

SUMMARY 1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were visited three times between April and August 2000. On each occasion water chemistry was measured and phytoplankton bioassays were performed in the laboratory to assess growth‐rate and yield limitation by phosphorus and nitrogen. In addition, yield limitation of periphyton growth was investigated twice, in situ, using nutrient‐diffusing substrata. 2. Over the whole season the percentage frequency of P, N and co‐limitation was 24, 13 and 63%, respectively, for phytoplankton rate limitation and 20, 22 and 58%, respectively, for phytoplankton yield limitation. 3. A clear response of periphyton yield to nutrient additions was found in 75% of all cases and of these, co‐limitation was most common (54%). Average percentage frequency for P and N limitation was 26 and 20%, respectively. 4. Phytoplankton and periphyton showed seasonal changes in nutrient limitation within sites. In particular, co‐limitation became progressively more common as the season progressed. 5. The response of phytoplankton growth rate to ammonium and nitrate addition was identical, but ammonium was a slightly better source of nitrogen than nitrate for phytoplankton yield on 7% and for periphyton yield on 10% of the occasions. However, the magnitude of the effect was small. 6. The concentration of dissolved inorganic nitrogen (DIN) and the molar ratio of DIN to total dissolved phosphorus (TDP), appeared to be the main environmental factors controlling the extent of nitrogen or phosphorus limitation at a given site. Nitrogen limitation was more likely than phosphorus limitation where the DIN was <6.5 mmol m^?3 and the ratio of DIN : TDP was <53. Co‐limitation was the most likely outcome at a DIN concentration <13 mmol m^?3 and at a DIN : TDP molar ratio <250. Above these values phosphorus limitation was most likely. 7. The relatively high frequency of nitrogen limitation and co‐limitation at higher N : P ratios than previously reported, may result from the inability of nitrogen‐fixing cyanobacteria to thrive in these upland lakes where pH and the concentration of phosphorus tended to be low and where flushing rates tended to be high.