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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Lake eutrophication associated with geographic location,lake morphology and climate in China

Lake eutrophication is influenced by both anthropogenic and natural factors. Few studies have examined relationships between eutrophication parameters and natural factors at a large spatial scale. This study explored these relationships using data from 103 lakes across China. Eutrophication parameters including total nitrogen (TN), total phosphorus (TP), TN:TP ratio, chemical oxygen demand (COD_Mn), chlorophyll-a (Chl-a), Secchi depth (SD), and trophic state index (TSI) were collected for the period 2001–2005. Sixteen natural factors included three of geographic location, five of lake morphology, and eight of climate variables. Pearson correlation analysis showed that TP and TSI were negatively related to elevation, lake depth, and lake volume, and positively related to longitude. All eutrophication parameters, except for COD_Mn and Chl-a, showed no significant correlation with climate variables. Multiple regression analyses indicated that natural factors together accounted for 13–58% of the variance in eutrophication parameters. When the 103 study lakes were classified into different groups based on longitude and elevation, regression analyses demonstrated that natural factors explained more variance in TN, TP, COD_Mn, Chl-a, and TSI in western lakes than in eastern lakes. Lake depth, volume, elevation, and mean annual precipitation were the main predictors of eutrophication parameters for different lake groups. Although anthropogenic impacts such as point- and nonpoint-source pollution are considered as the main determinants of lake eutrophication, our results suggest that some natural factors that reflect lake buffer capacity to nutrient inputs can also play important roles in explaining the eutrophication status of Chinese lakes.     

Addressing the trophic status in urban ponds: An evaluation of current trophic state indexes

Ponds are generally understudied. Quality problems in urban freshwaters can arise from eutrophication, and trophic status classification gives information related to their ecological situation. Ponds may strongly differ from larger lakes for which most current trophic status classification methods have been developed. This paper seeks to compare and contrast the suitability of six trophic status indexes (TSI) and OECD trophic classifications system currently used to evaluate the eutrophication level of urban ponds in the Subtropical region. The trophic status was evaluated based on total phosphorus (TP), soluble reactive phosphorus (SRP), total nitrogen (TN) and phytoplankton chlorophyll a (Chl a) in 12 ponds placed in different cities from the Pampean region (Argentina), in the warm and cold seasons. Our results demonstrate that there was no relationship between pond size (0.08–2.45 ha) and trophic status. TSIs estimated with Chl a showed significant differences between seasons and the range oligotrophic-hypertrophic of trophism was encountered. Conversely, TSIs estimated with nutrient concentrations (TP, SRP, TN) did not. The pond's classification in trophic levels fell in the eutrophic-hypertrophic extreme, and only considering TN/TP broadened the trophic range. No relationship was encountered between Chl a and P; however, Chl a and TN/TP were positively correlated during the warm season. Lower trophic levels regarding TSI (Chl a) were generally associated with a dense floating-macrophyte cover. We suggest that the TSI considered should correspond to the latitudinal region in which the ponds are located. Our results indicate that it appears restrictive to consider only P contents in ponds to assess eutrophication. Under the proposed framework, the key issues for the study of pond trophic classification will be to include not only P but N concentrations and TN/TP. Also, phytoplankton Chl a concentration jointly with floating macrophytes biomass/coverage should be considered as diagnosis parameters.     

Total inputs of phosphorus and nitrogen by wet deposition into Lake Taihu, China

Lake Taihu suffers from eutrophication caused by riverine nutrient inputs and air deposition. To characterize wet deposition of phosphorus (P) and nitrogen (N) to the lake, precipitation collection and measurements of total phosphorus (TP) and total nitrogen (TN) and other components at five cities around Lake Taihu were made from July 2002 to June 2003. TP and TN concentrations and deposition rates exhibited strong spatial variation in the whole catchment. An inverse correlation between station-averaged TP and TN concentrations and precipitation amount was found. Maximal TP concentration in rainfall was found in Suzhou, and maximal TN in Wuxi. However, highest wet deposition rates of TP and TN were found in Suzhou, which suggests that atmospheric nutrients are mostly from the east and northwest area of Lake Taihu. Mean TP and TN deposition rates were 0.03 and 2.0 t km⁻² year⁻¹ respectively in Lake Taihu, which are greater than reported values in other areas by comparision. Total N and P contributed to the lake by wet deposition were 75 and 4720 t per year, respectively, which represent about 7.3% and 16.5% of total annual N and P inputs via inflow rivers. Wet deposition, especially N, could have significant effects on eutrophication in the lake, which shows that air deposition should be taken into account while reducing the external nutrients in the lake.     

Seasonal Patterns of Nitrogen and Phosphorus Limitation in Four German Lakes and the Predictability of Limitation Status from Ambient Nutrient Concentrations

To identify the seasonal pattern of nitrogen (N) and phosphorus (P) limitation of phytoplankton in four different lakes, biweekly experiments were conducted from the end of March to September 2011. Lake water samples were enriched with N, P or both nutrients and incubated under two different light intensities. Chlorophyll a fluorescence (Chla) was measured and a model selection procedure was used to assign bioassay outcomes to different limitation categories. N and P were both limiting at some point. For the shallow lakes there was a trend from P limitation in spring to N or light limitation later in the year, while the deep lake remained predominantly P limited. To determine the ability of in-lake N:P ratios to predict the relative strength of N vs. P limitation, three separate regression models were fit with the log-transformed ratio of Chla of the P and N treatments (Response ratio = RR) as the response variable and those of ambient total phosphorus:total nitrogen (TN:TP), dissolved inorganic nitrogen:soluble reactive phosphorus (DIN:SRP), TN:SRP and DIN:TP mass ratios as predictors. All four N:P ratios had significant positive relationships with RR, such that high N:P ratios were associated with P limitation and low N:P ratios with N limitation. The TN:TP and DIN:TP ratios performed better than the DIN:SRP and TN:SRP in terms of misclassification rate and the DIN:TP ratio had the highest R² value. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management. However, there is still uncertainty about the efficacy of N restriction to control populations of N₂ fixing cyanobacteria.     

Unexpected stasis in a changing world: Lake nutrient and chlorophyll trends since 1990

The United States (U.S.) has faced major environmental changes in recent decades, including agricultural intensification and urban expansion, as well as changes in atmospheric deposition and climate—all of which may influence eutrophication of freshwaters. However, it is unclear whether or how water quality in lakes across diverse ecological settings has responded to environmental change. We quantified water quality trends in 2913 lakes using nutrient and chlorophyll (Chl) observations from the Lake Multi‐Scaled Geospatial and Temporal Database of the Northeast U.S. (LAGOS‐NE), a collection of preexisting lake data mostly from state agencies. LAGOS‐NE was used to quantify whether lake water quality has changed from 1990 to 2013, and whether lake‐specific or regional geophysical factors were related to the observed changes. We modeled change through time using hierarchical linear models for total nitrogen (TN), total phosphorus (TP), stoichiometry (TN:TP), and Chl. Both the slopes (percent change per year) and intercepts (value in 1990) were allowed to vary by lake and region. Across all lakes, TN declined at a rate of 1.1% year^?1, while TP, TN:TP, and Chl did not change. A minority (7%–16%) of individual lakes had changing nutrients, stoichiometry, or Chl. Of those lakes that changed, we found differences in the geospatial variables that were most related to the observed change in the response variables. For example, TN and TN:TP trends were related to region‐level drivers associated with atmospheric deposition of N; TP trends were related to both lake and region‐level drivers associated with climate and land use; and Chl trends were found in regions with high air temperature at the beginning of the study period. We conclude that despite large environmental change and management efforts over recent decades, water quality of lakes in the Midwest and Northeast U.S. has not overwhelmingly degraded or improved.     

Lake trophic state and the limnological effects of omnivorous fish

Ecologists have hypothesized that planktivorous fish have greater effects on the plankton and water quality of oligotrophic lakes than eutrophic lakes. We tested this hypothesis in a tank-mesocosm experiment of factorial design in which five biomass levels of filter-feeding omnivorous gizzard shad (Dorosoma cepedianum) were cross-classified with two levels of lake trophic state achieved by filling tank-mesocosms with water and plankton transported by truck from two lakes with different trophic states. The presence of gizzard shad significantly increased total phosphorus, primary productivity, chlorophyll, and particulate phosphorus (PP) 2–20 and 20–200 μm and significantly decreased Secchi depth, cladocerans, copepods and PP > 200 μm. The effects of gizzard shad on chlorophyll, Secchi depth, cladocerans, copepods and PP 2–20 and > 200 μm were dependent on lake trophic state and most intense in the eutrophic lake system. This experiment suggests that filter-feeding omnivorous fish interact synergistically with trophic state so that the limnological effects of omnivorous fish become more intense with increased eutrophication.     

Nutrient–chlorophyll relationships in tropical–subtropical lakes: do temperate models fit?

In tropical lakes relatively little is known about the general relationship between nutrient concentration and phytoplankton biomass. Using data from 192 lakes from tropical and subtropical regions we examine the relationship between total P (TP) and chlorophyll (Chl). The lakes are all located between 30° S to 31° N include systems in Asia, Africa, and North and South America but are dominated by Brazilian (n=79) and subtropical N. American (n=67) systems. The systems vary in morphometry (mean depth and lake area), trophic state as well total N (TN) to␣total P (TP) ratios and light extinction. Despite a nearly 500-fold range in TP concentrations (2–970 μg P l⁻¹), there was a poorer relationship between log TP and log Chl (r ²=0.42) than is generally observed for temperate systems from either narrow or broad geographic regions. N limitation is not a likely explanation for the relatively weak TP–Chl relationship in the tropical–subtropical systems. Systems had high average TN:TP ratios and neither a multiple regression with log TP and log TN nor separating systems with high TN:TP (>17 by weight) improved the predictive power of the log TP–log Chl relationship.     

太湖浮游植物优势种长期演化与富营养化进程的关系

利用1991年至2002年每月一次的监测资料,系统分析了浮游植物优势种和生物量的周年变化情况。同时,总氮、总磷和浮游植物叶绿素a含量等相关资料也被用于解释太湖富营养化演化与浮游植物的关系。结果显示,太湖总氮、总磷、叶绿素a和生物量均呈自梅梁湾底至湖心的逐步递减趋势。在20世纪80年代末太湖刚开始富营养化时,浮游植物优势种群从硅藻转变为蓝藻。之后,浮游植物优势种群一直是蓝藻,但各年的浮游植物总生物量有变化。总氮、总磷、叶绿素a和生物量的年均值持续增长至1996年,其后有逐步下降的趋势,究其原因可能和当地政府在太湖流域的控制排污行动有关。微囊藻在太湖的占优是太湖富营养化的标志之一。研究结果说明浮游植物在大型浅水湖泊中可以作为反映富营养化进程的生态指标。     

长江中游浅水湖泊水生植物氮磷含量与水柱营养的关系

水生植物组织内氮和磷(N和P)含量受到水体营养状况和植物生长状况影响。对长江中游江汉湖群不同营养水平湖泊中大型水生植物的N和P含量3个季度的研究表明,在不同生活型水生植物中,沉水植物主要分布在中营养到中富营养湖泊中,在富营养湖泊均无分布,浮叶和挺水植物在不同营养类型湖泊的沿岸带均有分布。N和P含量以沉水植物最高,浮叶植物次之,挺水植物最低。水生植物的N和P含量都达到或超过生长所需最低N和P阈值,代表性浮叶植物和沉水植物的N和P含量随着湖泊营养水平提高呈现规律性变化。湖泊5种常见的水生植物N和P含量与水柱中不同种类N和P浓度具有季节性相关:菱(TrapabispinosaRoxb.)春夏季P含量都与TP(总磷)和TDP(总溶解磷)明显相关,春季N含量与NH4—N(氨氮)明显相关;春季黄丝草(PotamogetonmaackianusA.Benn.)的P含量与TP明显相关,夏季与TDP明显相关,春季和夏季黄丝草和穗花狐尾藻(MyriophyllumspicatumL.)的N含量与TN(总氮)和TDN(总溶解氮)显著正相关,秋季成负相关;夏季芦苇(PhragmitescommunisTrin.)P含量与TP和TDP显著相关;春季芦苇和香蒲(TyphaorientalisPresl.)N含量与NH4N和NO2N(亚硝态氮)显著相关。       

淀山湖富营养化过程的统计学特征

湖泊营养物输入及响应指标的统计学规律正在受到越来越广泛的关注。对淀山湖在不同富营养化阶段和近期总磷TP、总氮TN和叶绿素Chl a的频率分布以及TP-Chl a关系的经验方程进行了分析,结果表明:(1)淀山湖TP、TN和Chl a的平均浓度和离散程度随着湖泊富营养化程度的加剧而增加,其中以Chl a的增幅最大;(2)在富营养化条件下,即使营养物TP得到一定程度的控制,Chl a大于15μg/L的概率继续增加了20%以上。仅仅削减营养物的峰值,对降低湖泊初级生产力水平的贡献有限;(3)TP-Chl a对数回归方程的斜率随湖泊富营养化程度的升高而增加,由20世纪80年代的0.54增加到目前的2.46。淀山湖营养物输入及响应指标的统计学特征,可以用来表征水体富营养化程度,评价湖泊生态恢复的进程和效果,为湖泊营养物基准和标准的制定提供最为实际的统计学支持。     

Nutrient-phytoplankton relationships in a tropical meromictic soda lake

Seasonal variation through one year in total nitrogen (TN), total phosphorus (TP), phytoplankton biomass, phytoplankton species composition and other environmental factors were examined in Lake Sonachi, a tropical meromictic soda lake. Mean concentrations of TN and TP were 11 000 µg N l^-1 and 100 µg P l^-1, respectively. Maximum concentrations of TN and TP occurred in the monimolimnion. Phytoplankton biomass ranged from 350 to 1260 mg m^-3. Synechococcus bacillaris, a small coccoid cyanophyte, dominated the phytoplankton. The mean chlorophyll a concentration of 37 mg · m^-3 was a modest value when compared with those of other tropical soda lakes. High TN:TP ratios indicated phosphorus limitation in the lake.     

Climate-driven changes in energy and mass inputs systematically alter nutrient concentration and stoichiometry in deep and shallow regions of Lake Champlain

Concentrations of nitrogen (N) and phosphorus (P) in lakes may be differentially impacted by climate-driven changes in nutrient loading and by direct impacts of temperature and wind speed on internal nutrient cycling. Such changes may result in systematic shifts in lake N:P under future climate warming. We used 21 years of monitoring data to compare long-term and intra-annual trends in total N (TN), total P (TP) and TN:TP at 15 sites in Lake Champlain to concurrent measurements of watershed nutrient inputs and meteorological drivers. TN:TP declined sharply lake-wide, particularly in the past decade, yet the drivers of this trend varied based on site depth. In deep sites, declines were driven by changes in watershed loading of dissolved P and N and (in some cases) by decreases in hypolimnetic dissolved oxygen. In shallow sites, declines in TN:TP were primarily driven by long-term increases in temperature and decreases in wind speed, and exhibited systematic seasonal variability in TN:TP due to the timing of sediment P loading, N removal processes, and external nutrient inputs. We developed a conceptual model to explain the observed trends, and suggest that while climate drivers have affected nutrient dynamics in shallow and deep sites differently, both deep and shallow sites are likely to experience further declines in N:P and increases in cyanobacteria dominance if recent climate trends continue.     

Using aquatic macrophyte community indices to define the ecological status of European lakes

Defining the overall ecological status of lakes according to the Water Framework Directive (WFD) is to be partially based on the species composition of the aquatic macrophyte community. We tested three assessment methods to define the ecological status of the macrophyte community in response to a eutrophication pressure as reflected by total phosphorus concentrations in lake water. An absolute species richness, a trophic index (TI) and a lake trophic ranking (LTR) method were tested at Europe-wide, regional and national scales as well as by alkalinity category, using data from 1,147 lakes from 12 European states. Total phosphorus data were used to represent the trophic status of individual samples and were plotted against the calculated TI and LTR values. Additionally, the LTR method was tested in some individual lakes with a relatively long time series of monitoring data. The TI correlated well with total P in the Northern European lake types, whereas the relationship in the Central European lake types was less clear. The relationship between total P and light extinction is often very good in the Northern European lake types compared to the Central European lake types. This can be one of the reasons for a better agreement between the indices and eutrophication pressure in the Northern European lake types. The response of individual lakes to changes in the abiotic environment was sometimes represented incorrectly by the indices used, which is a cause of concern for the use of single indices in status assessments in practice.     

Performance of a new phytoplankton composition metric along a eutrophication gradient in Nordic lakes

A new phytoplankton metric is presented, which is developed from a large dataset of Norwegian lakes (>2,000 samples from >400 lakes). In contrast to previous metrics, this index is not built on selected ‘indicative’ taxa, but uses all available taxonomic information at genus and species level. Taxa optima with respect to lake trophic status (derived from total phosphorus concentrations) are used to calculate a phytoplankton trophic index (TI) for each sample. Analysis of the TI shows that phytoplankton communities exhibit highly non-linear responses to eutrophication in Norwegian lakes. Reference lakes are characterized by very similar TIs despite having considerable variation in total phosphorus and chlorophyll a concentrations. TI exhibits a non-linear distribution along the eutrophication gradient which separates unimpacted from impacted sites in the study area. We further show that TI exhibits smaller seasonal variations than chlorophyll a, making it a more reliable indicator for lake monitoring. Implications for its applicability within the WFD are discussed.     

Assessing lake eutrophication using chironomids: understanding the nature of community response in different lake types

1. Total phosphorus (TP) and chlorophyll a (Chl a) chironomid inference models ( Brodersen & Lindegaard, 1999 ; Brooks, Bennion & Birks, 2001 ) were used in an attempt to reconstruct changes in nutrients from three very different lake types. Both training sets were expanded, particularly at the low end of the nutrient gradient, using contemporary chironomid assemblages and environmental parameters from 12 British lakes, although this had little improvement on the model performances. 2. Dissimilarity analyses showed that the historic chironomid assemblages did not have good analogues in the original calibration or extended datasets. However, since the transfer functions are based on weighted averages of the trophic optima for the taxa present and not on community similarities, reasonable downcore inferences were produced. Ordination analyses also showed that the lakes retain their ‘identity’ over time, as the sample dissimilarities within lakes were less than the dissimilarities between lakes. 3. Analysis of the three historic lake profiles showed a range of chironomid community responses to lake development. Chironomids from a shallow lake, Slapton Ley, responded indirectly to nutrient enrichment (TP), probably through altered substrate, macrophyte and fish conditions, rather than directly to primary productivity (Chl a). A stratified lake, Old Mill Reservoir, showed a loss of the profundal chironomid fauna due to increasing primary productivity (Chl a) coupled with increasing hypoxia. A response to nutrients (TP or total nitrogen (TN)) at this site is also indirect, and the TP reconstruction therefore cannot be reliably interpreted. The third lake, March Ghyll Reservoir has little change in historic chironomid communities, suggesting that this well mixed, relatively unproductive lake has changed less than the other lakes. 4. Using chironomids to reconstruct nutrient histories does not follow a simple scheme. The response to changes in nutrients may be direct, but mediated through other ecosystem components. As alternative stable states are possible at a given level of TP it is also likely that alternative chironomid communities exist under similar nutrient conditions. Changes in biological communities can thus occur over thresholds, and it is only biological proxies that can reflect such ecosystem switches within palaeoenvironmental investigations.     

Retention of nitrogen, phosphorus and silicon in a large semi-arid riverine lake system

Lakes and reservoirs (impoundments) are often viewed as a sink for nutrients within the river continuum. To date, most studies on nutrient retention within impoundments are derived from the temperate climate zones of Europe and North America, only consider one nutrient, and are often short-term (1–2 years). Here, we present a long-term (17 year) data set and nutrient (nitrogen, phosphorus and silica) budget for two connected semi-arid lakes (the Lower Lakes) at the terminus of the River Murray, Australia. Most of the filterable reactive phosphorus and nitrate entering the lakes were retained (77 and 92%, respectively). Total phosphorus (TP) was also strongly retained (55% of the annual TP load on average) and the annual TP retention rates could be predicted as a function of the areal hydraulic loading rate (annual lake outflow/lake surface area). On average, there was a slight net retention (7%) of the annual total nitrogen (TN) load but a slight net export (6% of the load) of organic N. TN retention as function of the areal hydraulic loading rate was lower than expected from existing models, possibly because of high nitrogen fixation rates in the Lower Lakes. Silica was retained (39%) at similar rates to those observed in previous studies. There was also a marked increase in the TN:TP and TN:Si ratios within the lake (TN:TP~30 and TN:Si~0.67) compared to those entering (TN:TP~15, TN:Si~0.45), as a consequence of the relatively low net retention of nitrogen.     

Does high nitrogen loading prevent clear‐water conditions in shallow lakes at moderately high phosphorus concentrations?

1. The effect of total nitrogen (TN) and phosphorus (TP) loading on trophic structure and water clarity was studied during summer in 24 field enclosures fixed in, and kept open to, the sediment in a shallow lake. The experiment involved a control treatment and five treatments to which nutrients were added: (i) high phosphorus, (ii) moderate nitrogen, (iii) high nitrogen, (iv) high phosphorus and moderate nitrogen and (v) high phosphorus and high nitrogen. To reduce zooplankton grazers, 1⁺ fish (Perca fluviatilis L.) were stocked in all enclosures at a density of 3.7 individuals m^?2. 2. With the addition of phosphorus, chlorophyll a and the total biovolume of phytoplankton rose significantly at moderate and high nitrogen. Cyanobacteria or chlorophytes dominated in all enclosures to which we added phosphorus as well as in the high nitrogen treatment, while cryptophytes dominated in the moderate nitrogen enclosures and the controls. 3. At the end of the experiment, the biomass of the submerged macrophytes Elodea canadensis and Potamogeton sp. was significantly lower in the dual treatments (TN, TP) than in single nutrient treatments and controls and the water clarity declined. The shift to a turbid state with low plant coverage occurred at TN >2 mg N L^?1 and TP >0.13–0.2 mg P L^?1. These results concur with a survey of Danish shallow lakes, showing that high macrophyte coverage occurred only when summer mean TN was below 2 mg N L^?1, irrespective of the concentration of TP, which ranged between 0.03 and 1.2 mg P L^?1. 4. Zooplankton biomass and the zooplankton : phytoplankton biomass ratio, and probably also the grazing pressure on phytoplankton, remained overall low in all treatments, reflecting the high fish abundance chosen for the experiment. We saw no response to nutrition addition in total zooplankton biomass, indicating that the loss of plants and a shift to the turbid state did not result from changes in zooplankton grazing. Shading by phytoplankton and periphyton was probably the key factor. 5. Nitrogen may play a far more important role than previously appreciated in the loss of submerged macrophytes at increased nutrient loading and for the delay in the re‐establishment of the nutrient loading reduction. We cannot yet specify, however, a threshold value for N that would cause a shift to a turbid state as it may vary with fish density and climatic conditions. However, the focus should be widened to use control of both N and P in the restoration of eutrophic shallow lakes.     

TN : TP ratio and planktivorous fish do not affect nutrient-chlorophyll relationships in shallow lakes

1. In previous work, phytoplankton regulation in freshwater lakes has been associated with many factors. Among these, the ratio of total nitrogen to total phosphorus (TN : TP) has been widely proposed as an index to identify whether phytoplankton are N‐ or P‐limited. From another point of view, it has been suggested that planktivorous fish can be used to control phytoplankton. 2. Large‐scale investigations of phytoplankton biomass [measured as chlorophyll a, (chl‐a)] were carried out in 45 mid‐lower Yangtze shallow lakes to test hypotheses concerning nutrient limitation (assessed with TN : TP ratios) and phytoplankton control by planktivorous fish. 3. Regression analyses indicated that TP was the primary regulating factor and TN the second regulating factor for both annual and summer phytoplankton chl‐a. In separate nutrient–chl‐a regression analyses for lakes of different TN : TP ratios, TP was also superior to TN in predicting chl‐a at all particular TN : TP ranges and over the entire TN : TP spectrum. Further analyses found that chl‐a : TP was not influenced by TN : TP, while chl‐a : TN was positively and highly correlated to TP : TN. 4. Based on these results, and others in the literature, we argue that the TN : TP ratio is inappropriate as an index to identify limiting nutrients. It is almost impossible to specify a ‘cut‐off’ TN : TP ratio to identify a limiting nutrient for a multi‐species community because optimal N : P ratios vary greatly among phytoplankton species. 5. Lakes with yields of planktivorous fish (silver and bighead carp, the species native to China) >100 kg ha^?1 had significantly higher chl‐a and lower Secchi depth than those with yields <100 kg ha^?1. TP–chl‐a and TP–Secchi depth relationships are not significantly different between lakes with yields >100 kg ha^?1 or <100 kg ha^?1. These results indicate that the fish failed to decrease chl‐a yield or enhance Z_SD. Therefore, silver carp and bighead carp are not recommended as a biotic agent for phytoplankton control in lake management if the goal is to control the entire phytoplankton and to enhance water quality.     

太湖夏季浮游细菌群落多样性的空间格局

为了研究太湖夏季浮游细菌群落多样性与水体营养盐的关系,在太湖全湖范围内开展了一次大规模浮游细菌采样调查,分析了太湖不同湖区浮游细菌丰度和多样性组成。研究发现,浮游细菌丰度在不同湖区中存在明显的空间差异,从北部和西部湖区沿湖流向东南方向至湖心和南部沿岸再到东部湖区呈下降趋势,这与太湖水体营养水平从高到低变化趋势一致。浮游细菌丰度与营养盐浓度回归分析结果显示,总磷（TP）与细菌丰度存在较好的正相关（R2=0.6392,n=29,P0.01）,而总氮（TN）与细菌丰度无显著相关（R2=0.0663,n=29,P0.05）。因此,磷是太湖夏季浮游细菌生长的限制因子。不同湖区营养盐与浮游细菌群落多样性也具有显著的正相关,随着营养水平的升高,浮游细菌多样性增加。此外,细菌群落的组成在不同湖区间亦具有明显的空间异质性,与不同湖区营养水平空间变化一致。研究结果将有助于人们更好地理解淡水湖泊中微生物循环和生态系统功能。