珠江口超微型浮游植物时空分布及其与环境因子的关系

对珠江口及近海区域进行了夏季和冬季超微型浮游植物(0.2-3μm)调查,分析了其时空分布及其与环境因子的关系.夏季,珠江口浮游植物密度与磷酸盐成显著正相关,且N/P远远高于30,表明浮游植物受到P限制.夏季超微型藻数量比冬季高一个数量级,其丰度与盐度成显著正相关而和营养盐(溶解性无机氮(DIN),PO4-P,SiO4-Si)显著负相关,表明珠江口超微型藻受到径流的负面影响,表现出其数量在虎门附近海域低,随着咸淡水混合程度的加剧逐渐增大的分布特征;超微型浮游植物叶绿素a在总叶绿素a中的比例也表现为河口上游低,到万山群岛附近海域达到最大,推测近海高光照、低营养盐更适宜超微型藻的生长,同时也说明超微型浮游植物适应贫营养环境的生态特点.     

广东大亚湾西南部海域营养盐结构的长期变化

分析了大亚湾西南部海域1982～2002年营养盐结构的长期变化。水域中溶解无机氮(DIN)的含量呈上升趋势,磷酸盐(PO₄^3-)含量从80年代初到90年代末逐年下降,2000年后有所恢复,硅酸盐(SiO₃^2-)的含量波动较大,但平均维持在20μmol·L^-1的较高水平上。各营养盐浓度比值的长期变化表现为氮磷比上升,硅氮比下降,硅磷比稳中有升。结合营养盐浓度及其比值的变化,可以认为大亚湾西南部海域中浮游植物生长的营养盐限制因子已经由过去的氮限制转变为磷限制;各营养盐的浓度及其比值在各季度的分布表明,浮游植物的生长春季和冬季受到氮限制的可能性较大,而夏季和秋季受磷限制的可能性较大;在营养盐的平面分布上,大鹏澳内DIN高于澳外水域,而PO₄^3-自1997年后低于澳外水域。     

北黄海獐子岛养殖海域营养水平与虾夷扇贝增殖渔获量评估

营养盐作为浮游植物生长的物质基础,对海洋生态系统的物质循环起着决定性作用。调查研究了獐子岛附近海域营养盐含量水平、空间分布及营养盐结构,并据此估算了现有营养水平可支持的潜在生物量,评估了该海域虾夷扇贝增殖渔获量。结果表明,獐子岛附近海域海水总体营养水平较低,底层水中的营养盐浓度是表层水的2—3倍,海水中的营养盐浓度基本高于浮游植物生长的最低阈值,且溶解无机氮与磷酸盐的比值(DIN/PO4-P)和硅酸盐与磷酸盐的比值(Si O3-Si/PO4-P)均22,全海域为磷营养限制。根据磷限制因子及食物链能流转移理论估算,该海域1000 km2现有的营养水平可支持一个生长周期内虾夷扇贝增殖的动态理论生产量为3.8—6.1万t,如人为增加5%—20%的水体磷,则虾夷扇贝增殖产量可增加0.25—1.00万t。     

汕头南澳-东山海域营养盐季节分布特征及其对浮游植物生长的潜在性限制

【摘要】营养盐是水体初级生产力和渔业资源产出的重要限制性因子。2015-2017年在汕头南澳和福建东山之间的海域设置了14个采样点, 分四个季节监测了该海域温盐和营养盐的空间分布特征。结果表明, 调查海域北部陆源径流输入是营养盐空间分布产生差异的最重要因子, 以溶解无机氮(DIN)和活性硅酸盐(DSi)受影响最为明显, 春季和夏季受径流营养盐输入的影响最大, 冬季受影响最小。南澳-东山海域的DIN、溶解无机磷(DIP)和DSi的浓度在表层周年平均分别为9.21、0.46和25.5 μmol·L–1, 底层分别为8.96、0.50和22.5 μmol·L–1。各营养盐的浓度基本表现为由西北近海向东南远海递减, 季节平均值表现为冬季>春季>夏季>秋季。冬季受闽浙沿岸流南下入侵的影响, 营养盐呈现出全年最高浓度, 表层平均DIN浓度是其他季节的三倍以上。春夏秋冬四个季节表层平均DIN/DIP比分别为24.3, 38.8, 6.7和24, 底层平均分别为15.4, 31.1, 9.6和23.6, 秋季水体的DIN/DIP比明显低于其他几个季节。总的来看, 南澳-东山海域的营养盐限制在不同季节变动较大: 春季近岸表现为相对磷限制, 远岸水体则表现为相对氮限制; 夏季和冬季都主要表现为磷的相对限制, 部分远海海域在夏季甚至出现磷的绝对限制; 秋季则主要表现为氮的相对限制。本研究系统阐述了南澳-东山海域营养盐限制的季节演替规律及变化机制, 为该海域生态环境管理和渔业增养殖规划提供基础数据。     

灌河口邻近海域春季浮游植物的生态分布及其营养盐限制

2011年4月通过灌河口邻近海域的现场调查及营养加富培养实验,研究了春季灌河口邻近海域浮游植物生态分布特征以及硝酸盐、磷酸盐对浮游植物生长的限制作用,结果表明:共发现浮游植物68种,其中硅藻61种,优势度最高的为中肋骨条藻(Skeletonema costatum,Y=0.53),各个站位浮游植物的丰度介于0.84× 106-2.25×106个/L,均值为1.54×106个/L,种类范围为29-39种,均值为35种,叶绿素a浓度呈现近岸高外海低的特征,在2.66-6.67 μg/L变化,均值为3.89 μg/L,多样性指数介于2.60-3.79,均值为3.20,海域环境基本适宜浮游植物的生长;调查海域磷酸盐浓度的范围为0.35-0.90μmol/L,均值为0.58μmol/L,亚硝酸盐浓度范围为1.57-3.93 μmol/L,均值为3.08 μmol/L,两者分布均具有近岸高外海低的特征;铵盐浓度范围为3.145.43μmol/L,均值为3.95 μmol/L,其分布则是近岸低外海高;硝酸盐浓度严重偏高,在31.21-37.00μmol/L之间变化,均值为34.55 μmol/L,导致调查区域具有高N/P比(42-112),且浮游植物叶绿素a与磷酸盐浓度有显著的正相关(R2=0.80),而与无机氮线性关系不明显(R2=0.11);在P加富培养实验中,磷酸盐在3个培养组(对照,+P,++P)中的比吸收速率分别为0.36、0.43、0.51d-1,加P促进了P本身的吸收,硝酸盐和亚硝酸盐的吸收也得以促进,但没有磷酸盐那么显著,而铵盐浓度基本呈增加趋势,P的添加也促进了藻类的生长,培养结束后叶绿素a浓度最大值分别为77.24、90.57、96.49μg/L.在N加富培养实验中,硝酸盐的比吸收速率分别为0.39、0.049、0.025d-1,加N未促进硝酸盐本身的吸收,磷酸盐浓度在3个实验组变化曲线相似,其吸收也没有得到促进,亚硝酸盐在加N组中浓度是增加的,培养结速后加N组(+N,++N)叶绿素a浓度最大值分别为72.31、69.62μg/L,都小于对照组,N的添加也未促进藻类的生长.上述研究表明了春季灌河口邻近海域浮游植物的生长主要受到P的限制,而不是N限制.     

獐子岛及邻近海域秋季浮游植物的粒级结构及其影响因素

于2015年10月对獐子岛及邻近海域进行了航次调查,研究了獐子岛及邻近海域浮游植物粒级结构的空间分布特征及其环境影响因素。结果表明,秋季表层总叶绿素a、小型(20μm)、微型(2—20μm)和微微型(0.45—2μm)浮游植物叶绿素a浓度的范围分别为0.52—1.25、0.03—0.81、0.33—0.91、0—0.09μg/L,平均叶绿素a的浓度分别为0.76、0.19、0.53、0.03μg/L,对叶绿素a总量的贡献率分别为23.77%、72.26%和3.98%;底层总叶绿素a、小型(20μm)和微型(2—20μm)浮游植物叶绿素a浓度的范围分别为0.14—1.5、0.04—1.04、0.08—0.47μg/L,平均叶绿素a的浓度分别为0.46、0.22、0.24μg/L,对叶绿素a总量的贡献率分别为41.46%、58.50%。从垂直分布上来看,总叶绿素a浓度垂直变化为,表层底层;小型浮游植物垂直分布较为均匀;微型浮游植物垂直变化为,表层底层;微微型浮游植物垂直变化为,表层底层,且在表、底层均保持较低水平。秋季表层微型浮游植物(2—20μm)浓度与盐度呈正相关。底层总叶绿素a浓度与磷酸盐浓度呈正相关,小型浮游植物(20μm)浓度与磷酸盐和硅酸盐浓度均表现为正相关,微型浮游植物(2—20μm)浓度与温度呈正相关。统计分析结果表明,温度、盐度、磷酸盐及硅酸盐浓度是影响獐子岛及邻近海域秋季浮游植物粒级结构变动的重要因素。     

大亚湾大辣甲水域叶绿素a及营养盐的垂直分布与季节变化

为了较为系统地研究大亚湾水域叶绿素a和营养盐的垂直分层状况, 并为大亚湾海洋生态系统的数值模拟提供基础数据, 作者于大亚湾大辣甲和桑洲之间连线的中间点附近海域选点, 在2007年～2008年进行了4个季度的垂直采样监测。本次研究分析了大亚湾大辣甲水域叶绿素a及营养盐的垂直分布与季节变化, 并分析了叶绿素a与DIN、PO₄-P和SiO₃-Si的关系。结果表明:在整个监测时段内, 叶绿素a和DIN、PO₄-P、SiO₃-Si的浓度范围分别为0.50～5.80 mg·m^-3、0.023～0.159 mg·L^-1、1.606～19.69μg·L^-1和0.065～1.13 mg·L^-1, 营养盐和叶绿素a在4个季度里的垂直变化趋势各有不同;在季节变化上, SiO₃-Si与DIN比较相似, 总体呈现夏、冬、秋季高, 春季较低的特征, PO₄-P恰好相反, 夏季的浓度最低, 叶绿素a总体呈现夏、春、冬季高, 秋季较低的特征;在整个垂直水柱内, DIN和SiO₃-Si对叶绿素a的影响相对较小, PO₄-P和叶绿素a呈显著正相关关系。     

渤海中部营养盐赋存形态季节变化及其对营养盐库的影响

根据2013年7月(夏季),11月(秋季)和2014年5月(春季)渤海中部氮、磷、硅营养盐库各赋存形态数据以及温度、盐度等水文数据,分析海域不同季节各营养盐库以及相互间转化规律。结果表明:调查海域受陆源输入、吸收转化、扰动混合以及分解释放等因素影响,各营养盐库含量及其组分迁移转化呈现明显的季节特征,春夏高生产力季节为无机态向有机态和颗粒态转化期,秋季为有机态和颗粒态溶解分解释放期。夏季氮磷营养盐库含量分别为(37.43±10.09)μmol/L和(0.73±0.19)μmol/L,且以溶解有机态为主要赋存形态。秋季各营养盐库以及无机态组分含量受扰动以及分解释放影响均明显增加,而溶解有机态和颗粒态组分降低,其中颗粒氮(PN, particulate nitrogen)含量为(1.78±1.05)μmol/L,降幅66%;溶解有机磷(DOP, dissolved organic phosphorus)含量为(0.13±0.06)μmol/L,降幅66%;而溶解无机磷(DIP, dissolved inorganic phosphorus)为(0.54±0.20)μmol/L,升高10.8倍。春季各营养盐库组分变化明显,其中溶解无机氮(DIN, dissolved inorganic nitrogen)、DIP和活性硅酸盐(SiO_3, reactive silicate)受浮游植物吸收转化影响,含量较秋季分别降幅45%、89%和85%, DON受秋冬季分解影响,含量降低36%。而PN和DOP增加明显,分别增高27%和1.6倍。调查海域各赋存形态营养盐比值表明,无机态营养盐比值均远离Redfield比值,对海域内硅藻的优势种地位产生影响。而各有机态以及颗粒态组分比值显示相比较氮,DOP、颗粒态磷(PP, particulate phosphorus)和硅在秋冬季低生产力季节更易受到扰动和分解释放影响,对于缓解海域内磷硅限制状况具有重要作用。     

2006年夏季南黄海浮游植物叶绿素a分布特征及其环境调控机制

根据2006年夏季南黄海生态调查资料,分析了浮游植物现存量(叶绿素a)的空间分布特征及其主要环境调控机制.结果表明:南黄海浮游植物叶绿素a浓度范围为0.07～12.17 mg/m3,平均值为1.42 mg/m3;浮游植物叶绿素a平面分布呈近岸混合区高于外海层化区的特征,总体上随水体盐度和营养盐梯度的变化而变化,其中浅海陆架锋区对浮游植物有明显的聚集作用,垂直结构中层化区叶绿素次表层最大值现象明显,且最大值的深度和量值具有明显的区域差异;叶绿素a浓度与主要环境因子的相关性分析表明,在较大空间尺度上营养盐是浮游植物的主要调控因子,温度、光照和浮游动物摄食等也在一定空间和程度上对浮游植物的生长产生影响.     

小型生活污水受纳水体浮游植物增长的氮、磷限制研究

为研究生活污水处理后其受纳水体中浮游植物增长的氮磷限制,选取某生活污水处理系统的受纳水体为研究对象,依据我国《城镇污水处理厂污染物排放标准》(GB189182002)一级A标准(氨氮5 mg/L和磷0.5 mg/L)进行氮磷营养盐最高浓度和浓度梯度添加微宇宙实验模拟实验。最高浓度添加实验结果显示N、P双添加的实验组中3d后叶绿素a的浓度显著(P0.05)高于单独添加氮和单独添加磷实验组。因此,氮和磷是被研究水体浮游植物生长的共同限制因子。同时结果还暗示受纳水体接纳处理后的生活污水仍可能会造成浮游植物在短期内剧烈增长。浓度梯度添加实验结果显示,将磷控制在0.27 mg/L或者将氮控制在1.0 mg/L以下,可以有效降低被研究水体浮游植物的增长。据此可以进一步严格生活污水处理后的排放标准以降低受纳水体水华的风险。     

Phosphorus and nitrogen limitation of phytoplankton growth in eutrophic Lake Inba, Japan

Lake Inba is one of the most eutrophic lakes in Japan. In this study, field sampling and nutrient enrichment bioassays were conducted to determine the seasonal patterns of nutrient limitation for phytoplankton growth in this lake. Phytoplankton biomass increased significantly with the additions of phosphorus (P) on almost all sampling dates, indicating P limitation of phytoplankton growth from spring to autumn. However, nitrogen (N) limitation was also observed during summer (i.e., 19 August). On 10 August, a typhoon struck Lake Inba. After this event, dissolved inorganic nitrogen (DIN) and phosphorus concentrations increased, probably because of increased river discharge. At the same time, phytoplankton growth in the control treatment became relatively high, with the addition of neither P nor N stimulating the growth. However, 10 days after the typhoon, the phytoplankton growth rate in the control treatment decreased, with only the addition of N having a significant positive effect on phytoplankton growth. N limitation during summer is caused by the low concentrations of DIN, as well as changes in the N:P ratio due to allochthonous nutrient loads. These results indicate that a reduction of both P and N input is necessary to control phytoplankton blooms in Lake Inba.     

Lake‐type‐specific seasonal patterns of nutrient limitation in German lakes,with target nitrogen and phosphorus concentrations for good ecological status

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SEASONAL CYCLING OF ALGAL NUTRIENT LIMITATION IN CHAUTAUQUA LAKE,NEW YORK1

Algal nutrient enrichment bioassays were conducted between May 1975 and August 1978 using water samples collected from Chautauqua Lake, New York. Photosynthetic fixation rates of natural phytoplankton assemblages were enhanced by additions of phosphorus and nitrogen, although enrichment with other nutrients had no significant stimulatory effect on algal photosynthesis. Whereas phosphorus stimulated in spring and early summer, both nitrogen and phosphorus enhanced photosynthesis in midsummer and fall. Relative to the effect of phosphorus enrichment, enhancement of photosynthesis by nitrogen during the summer and fall was highest in the northern part of the lake. During the period of ice cover, photosynthesis did not appear to be limited by nutrients in that nutrient additions (P, N, Si, C, Fe, trace metals) did not enhance fixation rates. Observed temporal fluctuations in the response of the algae to P and N correlated with changes in the lake water N:P ratio as well as with temporal changes in dissolved orthophosphate and nitrate-nitrite nitrogen. The N:P ratio decreased drastically in the summer and remained at ca. 10 or less through mid-fall, suggesting that N concentrations were inadequate for the non-N-fixing phytoplankton. Studies over 3 yr indicate that states of P and N limitation undergo time-space fluctuations that occur in a cyclic pattern in the surface waters of Chautauqua Lake.     

Long-term response of a shallow, moderately flushed lake to reduced external phosphorus and nitrogen loading

1. The responses of nutrient concentrations, plankton, macrophytes and macrozoobenthos to a reduction in external nutrient loading and to contemporary climatic change were studied in the shallow, moderately flushed Lake Müggelsee (Berlin, Germany). Weekly to biweekly data from 1979 to 2003 were compared with less frequently collected historical data. 2. A reduction of more than 50% in both total phosphorus (TP) and total nitrogen (TN) loading from the hypertrophic (1979–90) to the eutrophic period (1997–2003) was followed by an immediate decline in TN concentrations in the lake. TP concentrations only declined during winter and spring. During summer, phosphorus (P) release from the sediments was favoured by a drastic reduction in nitrate import. Therefore, Müggelsee acted as a net P source for 6 years after the external load reduction despite a mean water retention time of only 0.1–0.16 years. 3. Because of the likely limitation by P in spring and nitrogen (N) in summer, phytoplankton biovolume declined immediately after nutrient loading was reduced. The formerly dominant cyanobacteria (Oscillatoriales) Limnothrix redekei and Planktothrix agardhii disappeared, but the mean biovolume of the N₂‐fixing species Aphanizomenon flos‐aquae remained constant. 4. The abundance of Daphnia spp. in summer decreased by half, while that of cyclopoid copepod species increased. Abundances of benthic macroinvertebrates (mainly chironomids) decreased by about 80%. A resource control of both phytoplankton and zooplankton is indicated by significant positive correlations between nutrient concentrations and phytoplankton biovolume and between phytoplankton and zooplankton biomass. 5. Water transparency in spring increased after nutrient reduction and resulted in re‐colonisation of the lake by Potamogeton pectinatus. However, this process was severely hampered by periphyton shading and grazing by waterfowl and fish. 6. Water temperatures in Müggelsee have increased in winter, early spring and summer since 1979. The earlier development of the phytoplankton spring bloom was associated with shorter periods with ice cover, while direct temperature effects were responsible for the earlier development of the daphnid maximum in spring.     

Resource Ratios and Phytoplankton Species Composition in a Strongly Stratified Lake

The epilimnetic phytoplankton and its relations to nutrient content in Lake Verevi through the whole vegetation period in 2000 were studied. Lake Verevi (surface 12.6 ha, mean depth 3.6 m, maximum depth 11 m) is a hypertrophic hard-water lake, where the so-called spring meromixis occurs due to an extremely warm spring. Most dissolved nutrients in the epilimnion were low already in spring, and their concentrations were quite stable during the study period. The concentration of total silicon was very low in spring but increased rapidly in summer. Total phosphorus followed the pattern for stratified eutrophic lakes, and total nitrogen was quite high. The stoichiometric N:P ratio fluctuated between 25 and 81. The dynamics of phytoplankton biomass with a spring peak from April to May and a late summer peak from July to August is typical of Estonian eutrophic lakes. Green algae and chrysophytes occurred in the phytoplankton throughout the vegetation period. The spring peak was dominated by diatoms (Synedra ulna and Synedra acus var. angustissima) and the summer peak was caused by Aphanizomenon klebahnii and Ceratium hirundinella. The study showed that in physically stratified systems, the total concentration of limiting resources and plain physical factors (light and temperature) may be more important in the determination of phytoplankton dominants than different resource ratios. A combination of light and temperature optimum, along with nutrient utilization and transport capacity, effectively segregates phytoplankton species and can be used for the explanation of seasonal succession pattern.     

Effects of artificial grassland establishment on soil nutrients and carbon properties in a black-soil-type degraded grassland

Despite a growing knowledge of nutrient limitation for mangrove species and how mangroves adapt to low nutrients, there is scant information about the relative importance of N:P ratio and leaf phenolics variability in determining nutrient conservation. In this study, we evaluated possible nutrient conservation strategies of a mangrove Rhizophora stylosa under nutrient limitation. 1. The leaf nutrient concentrations of R. stylosa changed with season, with the highest N concentration in winter and the highest P concentration in spring for both mature and senescent leaves. Leaf N and P concentrations decreased significantly during leaf senescence. Based on N:P ratios R. stylosa forest was N-limited. Accordingly, the nitrogen resorption efficiency (NRE) was significantly higher than phosphorus resorption efficiency (PRE) for the R. stylosa leaves during leaf senescence. The NRE and PRE both reached the highest in the autumn. Average N and P concentrations in the senescent leaves were 0.15% and 0.06% for R. stylosa, respectively, indicating a complete resorption of N and an incomplete resorption of P. There was a significant negative correlation between nitrogen resorption proficiency (NRP) and NRE, meanwhile phosphorus resorption proficiency (PRP) and PRE correlation was also highly significantly. 2. R. stylosa leaves contained relatively high tannin level. Total phenolics, extractable condensed tannins and total condensed tannins contents increased during leaf senescence, and changed between seasons. The lowest concentrations of total phenolics, extractable condensed tannins and total condensed tannins occurred in summer, total phenolics concentrations were inversely related to nitrogen or phosphorus concentrations. 3. Our results confirmed that resorption efficiency during leaf senescence depends on the type of nutrient limitation, and NRE was much higher than PRE under N-limited conditions. R. stylosa forest developed several nutrient conservation strategies in the intertidal coastline surroundings, including high nitrogen resorption efficiency, low nutrient losses and high tannins level.     

Effects of modified nutrient concentrations and ratios on the structure and function of the native phytoplankton community in the Neuse River Estuary, North Carolina, USA

A variety of analyses were used to assess the structure (community composition) and function (assimilation number, nitrogen fixation) of phytoplankton in the Neuse River Estuary (NRE), NC under ambient and modified nutrient concentrations. Dilution bioassays were employed to reduce the concentration of nitrogen (N) or both N and phosphorus (P) and thus compare varied DIN:DIP ratios. Experimental manipulations created conditions that may result from mandated N load reductions to the estuary. We hypothesized that unilateral reduction of N loading to the NRE would increase the activity, abundance and diversity of N₂ fixing cyanobacteria. Changes in phytoplankton primary productivity, N₂ fixation (nitrogenase activity), genetic potential for N₂ fixation (presence of nifH), phytoplankton taxonomic composition (diagnostic photopigment concentration) and abundances of N₂ fixing cyanobacteria (microscopy) were determined. Decreasing ambient DIN:DIP ratios in NRE samples resulted in increased rates of N₂ fixation when seed populations were present and environmental conditions were amenable. Decreasing the DIN:DIP ratio did not lead to an increase in the abundance or diversity of N₂ fixing cyanobacteria. Because N₂ fixing cyanobacteria were only actively fixing nitrogen during periods of low riverine N discharge (summer and early autumn), lowering nutrient ratios may not have a major impact on the NRE. However, the maximum potential amount of N from N₂ fixation was calculated using rates from this study and was found to be approximately 3% of total riverine loading of N to the NRE. Because N₂ fixation occurs farther downstream and later in the year than riverine N loading to the NRE, there is potential for N₂ fixation to modify N dynamics. Analyses of the phytoplankton community as a whole in these relatively short term experiments indicated that reduced DIN:DIP may not have a major impact on their structure and function.     

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.