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1.
氮、磷浓度是制约湖泊营养状态和生产力水平的重要环境因子,而氮磷化学计量比是湖泊生态系统的主要指标,因此,判识氮磷比变化趋势及其驱动力对湖泊生态恢复具有重要意义.研究基于19882018年连续观测数据,分析了滇池氮磷浓度和氮磷摩尔比(简称氮磷比)的时空分布演变特征;采用多元线性回归模型分别对滇池草海和外海氮磷比驱动效应进行定量解析,筛选出影响湖体氮磷比变化的潜在驱动因子.结果表明:①19882018年滇池氮磷比呈现显著的线性上升趋势,其中草海和外海氮磷比分别上升1.3和0.7 a^-1.②草海和外海分别在2008年和2004年发生了氮磷比上升突变,突变前上升归因于总氮浓度快速增加,突变后则是由于总磷浓度下降较快.③滇池的氮磷浓度变化主要是受流域氮磷输入负荷、跨流域调水、流域氮磷削减、风速和水位的综合影响,但受控因子在不同区域可能存在差异.④气温是滇池氮磷比变化的主要驱动因子,流域人为氮磷输入差异是滇池氮磷比变化的次要驱动因子.  相似文献   

2.
To show the relative availability of nitrogen, phosphorus and silica for diatoms as well as non-siliceous algae in phytoplankton communities triangular diagrams are most appropriate. Triangular diagrams for TN-TP-SRSi show the nutrient ratios TN:TP, SRSi:TN and SRSi:TP in proportion to each other at the same time.Comparison of waters, grouped according to their limnological characteristics, indicate that these groups have also different seasonal dynamics of TN, TP and SRSi in proportion to each other. From these groups, which include meso-eutrophic and hypertrophic, shallow and deep, polymictic and dimictic lakes,only the group of "flushed, hypertrophic polymictic lakes" has a high seasonal dynamic of the ratios of all three macronutrients. Moreover the relative availability of TN:TP-ratios changes from > 16:1 in spring to <16:1 in summer. Significant changes in phytoplankton structure are associated with this high dynamic of the nutrient ratios. Considerable changes in the share of cyanobacteria to diatom biomass during summer are synchronised with a powerful modification of the relative availability of TN-TP-SRSi as illustrated by the triangular diagrams.For practical purposes trophic situations are most commonly evaluated from concentrations of a single nutrient or a nutrient ratio, such as N:P. However, a complex assessment would often be more appropriate. In this context triangular diagrams have the benefit of synoptically presenting relative nutrient availability for phytoplankton communities as a whole.  相似文献   

3.
This study aims at investigating the composition and biomass of the phytoplankton community in 15 urban shallow eutrophic lakes as well as the effects of main environmental factors, including nutrient concentrations and the ratio of nitrogen to phosphorus, temperature, COD, BOD, water depth, etc. on the phytoplankton community structure. Lake water samples were taken and analyzed on a bimonthly basis during the period from March 2004 to March 2006. The redundancy analysis (RDA) and regression analysis (RA) were performed to identify the effects of nutrients on the phytoplankton community and biomass in these typical urban lakes. The results indicate that most of these urban lakes were hypertrophic due to high concentrations of total phosphorus (TP) and total nitrogen (TN), with mean levels of 490 and 5380 mg m−3, respectively. The phytoplankton community was dominated by Microcystis aeruginosa and Euglena caudate in summer and Cryptomonas ovata and Cyclotella meneghiniana in winter. The mean biomass of the phytoplankton reached 456.87 mg L−1 in summer months and the annual level was 189.24 mg L−1. Temperature and TP content were found to be the principal limiting factors for phytoplankton growth on an annual basis. On the other hand, the results of RDA and RA demonstrate that the dominant phytoplankton species were not nutrient-limited during summer months. Low TN:TP ratios (<10) were detected accompanied with fewer occurrences of N-fixing cyanobacteria and other filamentous algae in most lakes in summer, which implies that low N:P ratio does not always shifts the dominance of phytoplankton community to the N-fixing cyanobacteria. Moreover, TP always had higher correlation with chlorophyll a (Chl-a) than TN, even when the TN:TP ratios of most samples were lower than 10. Therefore, it is concluded that the TN:TP ratio is not always a suitable index to determine whether nitrogen or phosphorus limits the phytoplankton biomass in urban shallow eutrophic lakes.  相似文献   

4.
Regression results based on data from 46 northern temperate lakes show that total phosphorus (TP) is the best predictor for phytoplankton (as chl-a) at lower trophic levels, TP < 200 mg · m–3. A regression including both TP and TN as regressors is the best predictor for lakes with TP > 200 mg · m–3. However, the good correlation is probably due to a high correlation between lake average chl-a (all years observed) and lake average TP and TN. Within single hypereutrophic lakes, TN alone is the best predictor. It was not possible to identify a medium trophic domain where TN and TP in combination was the best predictor for chl-a. The ratio TN:TP in the water decreases from about 40 to about 5 with increasing trophic level. Optimum TN:TP ratio for algal species with high abundance during late summer and autumn reflects this decreasing ratio, but within a lesser range, i.e., 20 to 5. In contrast, TN:TP ratios for species abundant during the early vernal period showed no, or an inverse, relation to the TN:TP ratio of the water.  相似文献   

5.
李静  陈光杰  黄林培  孔令阳  索旗  王旭  朱云  张涛  王露 《湖泊科学》2023,35(6):2170-2184
区域增温和大气氮沉降作用已成为高山湖泊面临的重要环境胁迫,已有高山湖泊生物群落响应的长期模式研究主要集中于藻类而缺乏更高营养级生物(如浮游动物)的系统调查。本研究选择滇西北地区深水型的高山湖泊沃迪错开展沉积物调查,通过多指标分析(总氮、总磷、叶绿素a、氮稳定同位素等)并结合区域气候重建记录,识别近两百年来该湖泊及流域环境的变化历史,进一步利用枝角类群落指标(物种组成、生物量等)定量评价了湖泊生物群落的响应模式与驱动因子。结果表明,湖泊营养水平(如总氮浓度)和初级生产力(叶绿素a浓度等)在过去近两百年总体呈上升趋势。相关分析显示,大气氮沉降和流域外源输入是影响总氮上升的主要因素,同时区域增温和营养盐富集促进了湖泊初级生产力的不断上升。自1960s以来区域升温明显,湖泊营养水平和叶绿素a浓度呈现加速上升的趋势。钻孔中枝角类群落以浮游属种(Daphnia longispina等)为优势种,在1900AD以前D.longispina相对丰度较为稳定(40.83%±8.02%),之后出现下降趋势且在1948—1965年间明显下降,之后再次明显上升并成为主要优势种。排序分析显示,气温、叶绿素a和总...  相似文献   

6.
Temporal discontinuity of nutrient limitation in plankton communities   总被引：1，自引：0，他引：1  
Ideas on how various measures of nutrient limitation relate to plankton biomass and species composition are re-examined. While long-term and multi-lake studies typically focus on determining overall biomass, seasonal studies are more focused toward understanding species composition. We use physiological assays to assess short-term nutrient deficiency of nitrogen and phosphorus in two moderately fertile lakes. While biomass in the lakes was considered to ultimately be limited by total phosphorus, nutrient assays were variable in time. Nutrient ratios (TN:TP, PN:PP, PC:PP and PC:PN) did not predict short-term deficiencies, notably that nitrogen deficiency occurred in these phosphorus-limited lakes. In one of our study lakes, there was a relaxation of phosphorus deficiency despite phosphate concentrations occurring below traditional detection limits. Following this period, there was an autumn bloom of Aphanizomenon flos-aquae. This relationship corresponds with other studies that have found A. flos-aquae to be a poor competitor for phosphorus. In contrast, phosphorus deficiency remained high prior to the autumn diatom bloom in our other study lake. Deficiency measures remain an excellent means of assessing physiological status of plankton communities and provide greater insight into species compositional changes, especially when other potential indicators like dissolved nutrient concentrations are inconclusive. Regardless of the nutrient limitation indicator used for a given study, it is critical to consider the appropriate scale of the measure.  相似文献   

7.
The changes of NH3-N, NO3-N, NO2-N and TN/TP were studied during growth and non-growth season in 33 subtropical shallow lakes in the middle and lower reaches of the Yangtze River. There were significant positive correlations among all nutrient concentrations, and the correlations were better in growth season than in non-growth season. When TP>0.1 mgL-1, NH3-N increased sharply in non-growth season with increasing TP, and NO3-N increased in growth season but decreased in non-growth season with TP. These might be attributed to lower dissolved oxygen and low temperature in non-growth season of the hypereutrophic lakes, since nitrification is more sensitive to dissolved oxygen and temperature than antinitrification. When 0.1 mgL-1>TP>0.035 mgL-1, TN and all kinds of inorganic nitrogen were lower in growth season than in non-growth season, and phytoplank-ton might be the vital regulating factor. When TP<0.035 mgL-1, inorganic nitrogen concentrations were relatively low and NH3-N, NO2-N had significant correlations with phytoplankton, indicating that NH3-N and NO2-N might be limiting factors to phytoplankton. In addition, TN/TP went down with decline in TP concentration, and TN and inorganic nitrogen concentrations were obviously lower in growth season than in non-growth season, suggesting that decreasing nitrogen (especially NH3-N and NO3-N) was an important reason for the decreasing TN/TP in growth season. The ranges of TN/TP were closely related to trophic level in both growth and non-growth seasons, and it is apparent that in the eutrophic and hypertrophic state the TN/TP ratio was obviously lower in growth season than in non-growth season. The changes of the TN/TP ratio were closely correlated with trophic levels, and both declines of TN in the water column and TP release from the sediment were important factors for the decline of the TN/TP ratio in growth season.  相似文献   

8.
In 2001, the Hong Kong government implemented the Harbor Area Treatment Scheme (HATS) under which 70% of the sewage that had been formerly discharged into Victoria Harbor is now collected and sent to Stonecutters Island Sewage Works where it receives chemically enhanced primary treatment (CEPT), and is then discharged into waters west of the Harbor. The relocation of the sewage discharge will possibly change the nutrient dynamics and phytoplankton biomass in this area. Therefore, there is a need to examine the factors that regulate phytoplankton growth in Hong Kong waters in order to understand future impacts. Based on a historic nutrient data set (1986-2001), a comparison of ambient nutrient ratios with the Redfield ratio (N:P:Si=16:1:16) showed clear spatial variations in the factors that regulate phytoplankton biomass along a west (estuary) to east (coastal/oceanic) transect through Hong Kong waters. Algal biomass was constrained by a combination of low light conditions, a rapid change in salinity, and strong turbulent mixing in western waters throughout the year. Potential stoichiometric Si limitation (up to 94% of the cases in winter) occurred in Victoria Harbor due to the contribution of sewage effluent with high N and P enrichment all year, except for summer when the frequency of stoichiometric Si limitation (48%) was the same as P, owing to the influence of the high Si in the Pearl River discharge. In the eastern waters, potential N limitation and N and P co-limitation occurred in autumn and winter respectively, because of the dominance of coastal/oceanic water with low nutrients and low N:P ratios. In contrast, potential Si limitation occurred in spring and a switch to potential N, P and Si limitation occurred in eastern waters in summer. In southern waters, there was a shift from P limitation (80%) in summer due to the influence of the N-rich Pearl River discharge, to N limitation (68%) in autumn, and to N and P co-limitation in winter due to the dominance of N-poor oceanic water from the oligotrophic South China Sea. Our results show clear temporal and spatial variations in the nutrient stoichiometry which indicates potential regulation of phytoplankton biomass in HK waters due to the combination of the seasonal exchange of the Pearl River discharge and oceanic water, sewage effluent inputs, and strong hydrodynamic mixing from SW monsoon winds in summer and the NE monsoon winds in winter.  相似文献   

9.
The changes of NH3-N, NO3-N, NO2-N and TN/TP were studied during growth and non-growth season in 33 subtropical shallow lakes in the middle and lower reaches of the Yangtze River. There were significant positive correlations among all nutrient concentrations, and the correlations were better in growth season than in non-growth season. When TP>0.1 mgL?1, NH3-N increased sharply in non-growth season with increasing TP, and NO3-N increased in growth season but decreased in non-growth season with TP. These might be attributed to lower dissolved oxygen and low temperature in non-growth season of the hypereutrophic lakes, since nitrification is more sensitive to dissolved oxygen and temperature than antinitrification. When 0.1 mgL?1>TP>0.035 mgL?1, TN and all kinds of inorganic nitrogen were lower in growth season than in non-growth season, and phytoplankton might be the vital regulating factor. When TP<0.035 mgL?1, inorganic nitrogen concentrations were relatively low and NH3-N, NO2-N had significant correlations with phytoplankton, indicating that NH3-N and NO2-N might be limiting factors to phytoplankton. In addition, TN/TP went down with decline in TP concentration, and TN and inorganic nitrogen concentrations were obviously lower in growth season than in non-growth season, suggesting that decreasing nitrogen (especially NH3-N and NO3-N) was an important reason for the decreasing TN/TP in growth season. The ranges of TN/TP were closely related to trophic level in both growth and non-growth seasons, and it is apparent that in the eutrophic and hypertrophic state the TN/TP ratio was obviously lower in growth season than in non-growth season. The changes of the TN/TP ratio were closely correlated with trophic levels, and both declines of TN in the water column and TP release from the sediment were important factors for the decline of the TN/TP ratio in growth season.  相似文献   

10.
《Marine pollution bulletin》2009,58(6-12):335-348
In 2001, the Hong Kong government implemented the Harbor Area Treatment Scheme (HATS) under which 70% of the sewage that had been formerly discharged into Victoria Harbor is now collected and sent to Stonecutters Island Sewage Works where it receives chemically enhanced primary treatment (CEPT), and is then discharged into waters west of the Harbor. The relocation of the sewage discharge will possibly change the nutrient dynamics and phytoplankton biomass in this area. Therefore, there is a need to examine the factors that regulate phytoplankton growth in Hong Kong waters in order to understand future impacts. Based on a historic nutrient data set (1986–2001), a comparison of ambient nutrient ratios with the Redfield ratio (N:P:Si = 16:1:16) showed clear spatial variations in the factors that regulate phytoplankton biomass along a west (estuary) to east (coastal/oceanic) transect through Hong Kong waters. Algal biomass was constrained by a combination of low light conditions, a rapid change in salinity, and strong turbulent mixing in western waters throughout the year. Potential stoichiometric Si limitation (up to 94% of the cases in winter) occurred in Victoria Harbor due to the contribution of sewage effluent with high N and P enrichment all year, except for summer when the frequency of stoichiometric Si limitation (48%) was the same as P, owing to the influence of the high Si in the Pearl River discharge. In the eastern waters, potential N limitation and N and P co-limitation occurred in autumn and winter respectively, because of the dominance of coastal/oceanic water with low nutrients and low N:P ratios. In contrast, potential Si limitation occurred in spring and a switch to potential N, P and Si limitation occurred in eastern waters in summer. In southern waters, there was a shift from P limitation (80%) in summer due to the influence of the N-rich Pearl River discharge, to N limitation (68%) in autumn, and to N and P co-limitation in winter due to the dominance of N-poor oceanic water from the oligotrophic South China Sea. Our results show clear temporal and spatial variations in the nutrient stoichiometry which indicates potential regulation of phytoplankton biomass in HK waters due to the combination of the seasonal exchange of the Pearl River discharge and oceanic water, sewage effluent inputs, and strong hydrodynamic mixing from SW monsoon winds in summer and the NE monsoon winds in winter.  相似文献   

11.
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12.
浅水湖泊中的初级生产者主要由分布在底栖生境中的底栖植物和生活在敞水生境中的浮游植物组成.底栖植物主要包括维管束沉水植物和底栖藻类等,浮游植物则主要为浮游藻类.贫营养浅水湖泊湖水营养盐浓度低,透明度高,底栖植物因能直接从沉积物中获取营养盐,往往是浅水湖泊的优势初级生产者.随着外源营养盐负荷的增加,湖水中的营养盐浓度不断升高,浮游植物受到的营养盐限制作用减小,加上其在光照方面的竞争优势,逐步发展成为湖泊的优势初级生产者,湖泊逐步从底栖植物为优势的清水态转变为浮游植物为主的浑水态,即稳态转换.在稳态转换过程中,浅水湖泊生态系统结构与功能发生了一系列变化,本文综述了浅水湖泊沉积物性质和生物(浮游植物、底栖植物、底栖动物和鱼类等)群落结构的变化,分析了这些变化对底栖植物、浮游植物之间竞争优势和底栖敞水生境间磷交换的影响,探讨了富营养化驱动的底栖敞水生境耦合过程变化和稳态转换机理.了解浅水湖泊底栖敞水生境耦合过程与稳态转换机理对富营养化浅水湖泊修复有重要意义.富营养化浅水湖泊修复实际就是重建其清水态,在制定修复目标时应该关注评价清水态的指标,如透明度、浮游植物生物量、底栖植物的覆盖度或优势度等.在开展湖泊修复技术研发与工程应用时,应该重点关注对底栖敞水生境耦合有重要影响的关键技术,如沉积物磷释放和底栖生物食性鱼类控制以及底栖植物(尤其是沉水植物)恢复等有关技术.  相似文献   

13.
滇池沉水植物生长过程对间隙水氮、磷时空变化的影响   总被引：4，自引：0，他引：4  
2015年6-10月通过原位采集滇池沉水植物分布区和无植物对照区柱状沉积物间隙水,分析其溶解性总氮(DTN)和溶解性总磷(DTP)、溶解性无机氮(DIN)和溶解性无机磷(DIP)及溶解性有机氮(DON)和溶解性有机磷(DOP)浓度的时空变化,探讨沉水植物分布对间隙水氮、磷浓度、形态贡献及氮磷比的影响.结果表明:滇池沉水植物生长过程显著影响间隙水氮、磷浓度.与无植物对照区相比,沉水植物生长过程对间隙水氮浓度的削减主要发生在6、8月,而对间隙水磷浓度的削减主要发生在7月,反映了沉水植物对氮、磷两种元素的生物地球化学循环作用机制不同;间隙水氮形态贡献受季节性影响较大,6-7月以DON贡献为主,沉水植物分布区和无植物对照区分别达到61%和84%;而8-10月以DIN贡献为主,沉水植物分布区和无植物对照区分别为76%和75%;沉水植物分布区磷形态贡献随季节波动变化,沉水植物分布区以DOP贡献为主(63%),无植物对照区以DIP贡献为主(62%);沉水植物生长对沉积物间隙水各形态氮磷比影响显著.沉水植物生长显著增加间隙水DTN/DTP比,尤其是DIN/DIP比,相反降低DON/DOP比.沉水植物对间隙水氮、磷吸收及转化过程改变了沉积物氮、磷释放机制,从而影响上覆水氮、磷组成及氮磷比,很可能会影响到浮游植物生长及藻类水华过程,这对于湖泊水质管理具有重要意义.  相似文献   

14.
秦镕聪  肖艳  郭劲松  李哲  方芳  刘静  杨梅 《湖泊科学》2018,30(5):1284-1294
本文通过选取水华鱼腥藻(Anabaena flos-aquae)、铜绿微囊藻(Microcystis aeruginosa)、空球藻(Eudorina sp.)和湖北小环藻(Cyclotella hubeiana)为三峡库区流域中典型的优势藻,在澎溪河进行原位培养试验,跟踪监测了三峡水库不同运行时期4种典型优势藻细胞氮(Cell-N)、磷(Cell-P)浓度以及环境水体中的总氮(TN)、总磷(TP)浓度,并且结合团队之前的研究成果,根据临界(供应)N/P摩尔比、细胞N/P摩尔比和Guildford的TN/TP摩尔比评价标准对库区水体的营养限制状态进行了评价,为进一步揭示三峡水库运行下支流水域富营养化过程与生理特征的限制性评价提供研究基础.原位试验结果表明:3个评价标准下水体的营养(氮或磷)限制状态基本相同,低水位时期水体无机磷营养盐比较贫乏,在试验初期没有显著的营养盐限制而在末期受到了磷的相对限制;高水位试验初期和末期均未呈现显著的营养盐限制;泄水期营养盐变动频繁,试验初期表现为氮限制而末期表现为磷限制.虽然各藻种在不同时期会表现出不同的营养盐限制性,但总的来看,临界N/P比和细胞N/P比的阈值范围具有一定指示作用,能够从藻类细胞生理的角度对水体的营养状况作出初步评价.  相似文献   

15.
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16.
SD模型在洱海流域营养物削减策略研究中的应用   总被引：1，自引：1，他引：0  
针对洱海富营养化问题,本文在深入分析洱海流域营养物输运转移特征的基础上,应用VENSIM-DSS构建了洱海流域的社会、经济、技术、环境SD耦合模型.模型由7个子系统组成,确定了一套适用于洱海流域的耦合模型特征参数.对入湖TN、TP进行追根溯源,模拟结果表明洱海流域入湖TN主要来源于种植业子系统、畜牧业子系统、生活污水子系统和干湿沉降子系统,以上4大子系统占入湖TN的88%;入湖TP主要来源于种植业子系统、畜牧业子系统、生活污水子系统和水土流失子系统,以上4大子系统占入湖TP的89%.以此为基础通过设置4种不同的营养物削减情景,模拟未来10年入湖TN、TP的变化,并通过构建的政策评价子系统和DILLON模型定量评价不同削减方案的可行性,提出最优洱海流域营养物削减方案.  相似文献   

17.
以太湖重度蓝藻水华发生的西北湖区为研究对象,从河口至湖心区设置5个采样点,于2012年10月至2013年10月逐月采集表层水体样品,测定了水温、溶解氧和浮游细菌丰度,并分析了浮游植物群落结构的组成、溶解性无机氮(DIN)和有机氮(DON)浓度以及氮磷比.研究结果表明,太湖西北湖区浮游植物主要由蓝藻、硅藻、绿藻和隐藻组成.可能由于风、浪等混合作用使太湖西北湖区不同采样点之间蓝藻细胞密度没有显著差异.蓝藻生物量在浮游植物中所占比例最高为34%±15%,春季部分点位隐藻生物量高于50%,表明隐藻与蓝藻的相互竞争趋势显著.CCA排序图结果表明,DIN、DON浓度以及总氮∶总磷比(TN∶TP比)是影响西北湖区浮游植物优势属分布的重要环境因子.5个采样点铵态氮(NH_4~+-N)与DIN浓度具有显著差异,与DON浓度没有显著差异.夏季蓝藻水华暴发期间,可能由于蓝藻的吸收利用引起NH_4~+-N和硝态氮(NO_3~--N)浓度迅速降低.此外,由于NH_4~+-N浓度还可能受到沉积物NH_4~+-N释放的影响,因此,蓝藻细胞密度与NO_3~--N的相关系数和显著水平均高于NH_4~+-N.夏季TN∶TP比和DIN∶TP比降至最低,表明该湖区浮游植物,尤其是蓝藻的生长可能受到氮限制.蓝藻细胞密度与DON浓度呈显著负相关,表明在氮限制条件下,DON可能是蓝藻氮素利用的重要补充.  相似文献   

18.
Mean dissolved inorganic nitrogen concentrations ([DIN]) in deep, seasonally stratified lakes with comparable DIN inputs can differ by up to a factor of 3 depending on hydraulic and morphometric properties and/or different trophic states of the lakes. In such lakes, net N sedimentation rates were estimated with two independent methods (sediment core analysis and input-output mass balances). They were higher in eutrophic lakes (Mean: 5.1; SD: ± 1.6 g m–2 yr–1; n = 13) than in oligotrophic lakes (1.6 ± 1.0 g m–2 yr–1; n = 3), but independent of [DIN]. Gaseous N loss rates to the atmosphere, as calculated from combined N- and P-mass balances from selected lakes, ranged from 0.9 to 37.4 g m–2 yr–1 (n = 10) and were positively correlated with [DIN]. Reduction of NO 3 - to N2 is assumed to be the main cause for gaseous N losses. A simple one-box mass balance model for [DIN], based on DIN input and rates and kinetics of N removal processes (net sedimentation and gaseous N loss) is proposed, and validated with a data base on [DIN] and DIN input in 19 deep, seasonally stratified lakes of central Europe. The model illustrated that the amount of water loading per unit surface area of a lake (called water discharge height q) is the critical parameter determining mean lake [DIN] relative to mean input [DIN]. Lakes with a q > 50 m yr–1 have average [DIN] similar to the [DIN] of the inflows regardless of their trophic states, because input and outflow exceed lake-internal N removal processes. A high primary production favors DIN removal in lakes with q < 50 m yr–1. It is concluded that measures to decrease primary production, e.g. by means of P removal programs, lead to an increase of [DIN] in lakes.  相似文献   

19.
We conducted a statistical analysis to discern the relative strengths of the loading of various forms of nitrogen, phosphorus, dissolved silicate and their molar ratios on the variance in the size of the summertime low oxygen zone found off the Mississippi River, northern Gulf of Mexico. A stable statistical model that included Year and riverine nitrate+nitrite loading for the 2 months prior to the measurement of hypoxic zone size described 82% of its variation in size from 1978 to 2004. The usefulness of the term Year is consistent with the documented increase in carbon stored in sediments after the 1970s, which is when the hypoxic zone is predicted to have become a regular feature on the shelf and to have expanded westward. The increased carbon storage is anticipated to cause a sedimentary respiratory demand influencing the size of the zone, and whose temporal influence is cumulative and transcends the annual variations in nitrogen loading. The variable Year is negatively correlated with the TN:TP ratio in a way that suggests N, not P, has become more important as a factor limiting phytoplankton growth in the last 20 years. Nitrogen, in particular nitrate+nitrite, and not phosphorus, dissolved silicate, or their molar ratios, appears to be the major driving factor influencing the size of the hypoxic zone on this shelf. This conclusion is consistent with cross-system analyses that conclude that the TN:TP ratio in the Mississippi River, currently fluctuating around 20:1, is indicative of nitrogen, not phosphorus, limitation of phytoplankton growth. Nutrient management that places stronger emphasis on reducing nitrogen loading as compared to phosphorus loading, is justified.  相似文献   

20.
To test if phytobenthic algae provide additional important information to macrophytes and phytoplankton for lake monitoring, we sampled two large lakes in Norway. In each lake, we analyzed water chemistry and phytoplankton above the deepest site, recorded macrophytes and non-diatom phytobenthic algae at 20 sites around the shoreline and estimated site-specific nutrient input from land cover. Since no ready-to-use phytobenthos index exists for lakes in Norway, we tested the PIT index developed for rivers, commonly perceived signs of disturbance such as high algal cover, and taxon richness as well as similarity patterns. Both lakes were nutrient poor, but had potential local nutrient inputs (villages, agriculture). In neither of the lakes did phytobenthos indicate a worse overall ecological status than macrophytes and phytoplankton. Our data therefore, did not suggest that it would be useful to add phytobenthos into surveillance monitoring of lakes in Norway. There was a loose correlation between macrophyte and phytobenthic site-specific taxon richness and similarities. This means that macrophytes and phytobenthos do indeed give partly redundant information. High algal cover was found at sites with both high and low phosphorus input. Using algal cover as indicator of site-specific nutrient input is therefore overly simplistic. Urban and cultivated areas were associated with a more eutrophic PIT. This indicates that the PIT, despite being developed for lotic waters, may be used to detect site specific nutrient input in lakes.  相似文献   

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