Application of nitrogen and phosphorus criteria for streams in agricultural landscapes

Efforts to control eutrophication of water resources in agriculturally dominated ecosystems have focused on managing on-farm activities to reduce nutrient loss; however, another management measure for improving water quality is adoption of environmental performance criteria (or 'outcome-based standards'). Here, we review approaches for setting environmental quality criteria for nutrients, summarize approaches developed in Canada for setting 'ideal' and 'achievable' nutrient criteria for streams in agricultural watersheds, and consider how such criteria could be applied. As part of a 'National Agri-Environmental Standards Initiative', the Government of Canada committed to the development of non-regulatory environmental performance standards that establish total P (TP) and total N (TN) concentrations to protect ecological condition of agricultural streams. Application of four approaches for defining ideal standards using only chemistry data resulted in values for TP and TN spanning a relatively narrow range of concentrations within a given ecoregion. Cross-calibration of these chemically derived standards with information on biological condition resulted in recommendations for TP and TN that would likely protect aquatic life from adverse effects of eutrophication. Non-point source water quality modelling was then conducted in a specific watershed to estimate achievable standards, i.e. chemical conditions that could be attained using currently available and recommended management practices. Our research showed that, taken together, short-term achievable standards and ultimate ideal standards could be used to set policy targets that should, if realized, lower N and P concentrations in Canadian agricultural streams and improve biotic condition.     

Temporal changes in the hydrology and nutrient concentrations of a large tropical river: Anthropogenic influence in the Lower Grijalva River,Mexico

Dam construction and nutrient loading are among the greatest threats to freshwater ecosystems, altering ecological processes and the provisioning of ecosystem services. Temporal change in hydrology and ambient nitrogen and phosphorus concentrations was studied on the Grijalva, a large tropical river in southern Mexico, where four hydroelectric dams operate and where land conversion has impacted the freshwater environment. Temporal changes in discharge and in river chemistry were examined by analysing long‐term discharge and nutrient data using the software Indicators of Hydrologic Alteration and Mann–Kendall tests. Furthermore, additional water chemistry samples were collected to examine seasonal nutrient dynamics in the lower Grijalva. Long‐term discharge data indicated dam construction has severely altered temporal patterns in discharge and other hydrological characteristics. The lower Grijalva has also experienced increase in nitrate concentrations through time, which may be attributed to the expansion of agricultural and urban areas in the watershed. In contrast, total phosphorus appeared to decline at the sites influenced by dam construction. Lower nutrient concentrations were recorded upstream from the city of Villahermosa, suggesting that inputs from urban areas may have contributed to nutrient loading. Additionally, higher nitrate and total phosphorus concentrations were detected in tributaries draining intensive agricultural and suburban areas. Collectively, the results from the study suggest that dam construction and land conversion in large, tropical watersheds can produce chemical and hydrological changes, which may negatively impact important ecosystem services—such as fisheries and the provisioning of sources of drinking water—and may compromise the integrity of coastal zones.     

Spatial and temporal patterns in macronutrient concentrations and stoichiometry of tributaries draining the lower Great Lakes-St. Lawrence basin

Nutrient management in the Great Lakes-St. Lawrence basin has focused on the reduction of tributary phosphorus inputs to control lake eutrophication. However, the regional implications of nutrient enrichment on stream eutrophication and management remain understudied. We compared nutrient concentrations and stoichiometric ratios of carbon, nitrogen, and phosphorus to evaluate spatial and temporal patterns in the potential for nutrient limitation in streams of the lower Great Lakes-St. Lawrence basin in Ontario, Canada. Monitoring data from 127 streams was used to describe macronutrient concentrations and stoichiometry over a 10-year period (2007–2016). Nutrient enrichment was widespread as 65% and 68% of studied streams had nitrogen and phosphorus concentrations above regional guidelines, respectively. Macronutrient stoichiometry indicated that 35% of streams were depleted for phosphorus and 65% were co-depleted for nitrogen and phosphorus relative to the Redfield ratio. However, algal production in most streams was likely nutrient saturated such that only 2% of streams showed the potential for phosphorus limitation and 21% for nitrogen and phosphorus co-limitation. Temporal assessment of individual nutrients that were depleted indicated minimal variation within most streams. In contrast, macronutrient stoichiometry was associated with spatial patterns in catchment land-cover whereby a shift from nitrogen and phosphorus co-depletion to phosphorus depletion occurred with increasing agriculture and decreasing natural and wetland cover. Our results suggest that phosphorus reductions alone may be insufficient to manage stream eutrophication in the Great Lakes-St. Lawrence basin and that dual nutrient reductions needed to improve stream conditions may be achievable through land-cover management.     

Water‐Quality Assessment of the Lower Grand River Basin,Missouri and Iowa,USA, in Support of Integrated Conservation Practices

The effectiveness of agricultural conservation programmes to adequately reduce nutrient exports to receiving streams and to help limit downstream hypoxia issues remains a concern. Quantifying programme success can be difficult given that short‐term basin changes may be masked by long‐term water‐quality shifts. We evaluated nutrient export at stream sites in the 44 months that followed a period of increased, integrated conservation implementation within the Lower Grand River Basin. These short‐term responses were then compared with export that occurred in the main stem and adjacent rivers in northern Missouri over a 22‐year period to better contextualize any recent changes. Results indicate that short‐term (October 2010 through May 2014) total nitrogen (TN) concentrations in the Grand River were 20% less than the long‐term average, and total phosphorus (TP) concentrations were 23% less. Nutrient reductions in the short term were primarily the result of the less‐than‐average precipitation and, consequently, streamflow that was 36% below normal. Therefore, nutrient concentrations measured in tributary streams were likely less than normal during the implementation period. Northern Missouri streamflow‐normalized TN concentrations remained relatively flat or declined over the period 1991 through 2013 likely because available sources of nitrogen, determined as the sum of commercial fertilizers, available animal manures and atmospheric inputs, were typically less than crop requirement for much of that time frame. Conversely, flow‐normalized stream TP concentrations increased over the past 22 years in northern Missouri streams, likely in response to many years of phosphorus inputs in excess of crop requirements. Stream nutrient changes were most pronounced during periods that coincided with the major tillage, planting and growth phases of row crops and increased streamflow. Nutrient reduction strategies targeted at the period February through June would likely have the greatest impact on reducing nutrient export from the basin. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Analysis of the Impacts of the Zebra Mussel, Dreissena polymorpha, on Nutrients,Water Clarity,and the Chlorophyll-Phosphorus Relationship in Lower Green Bay

Zebra mussels, Dreissena polymorpha, invaded Green Bay, Lake Michigan in the early 1990s. In 1986, prior to zebra mussel invasion, the Green Bay Metropolitan Sewerage District initiated a long-term water quality monitoring program involving 12 stations in three distinct zones along a trophic gradient in lower Green Bay. We analyzed this data set pre and post invasion using various regression models to determine the impacts of the zebra mussel on water clarity, nutrient concentrations, and the relationship between chlorophyll and phosphorus in this system. Following zebra mussel invasion, Secchi depths did not change in all three zones. Chlorophyll a concentrations decreased post zebra mussels in all zones. These differences were attributed to the filter feeding abilities of zebra mussels. Lower Green Bay exhibits a strong trophic gradient and zebra mussel impacts on the chlorophyll-phosphorus relationship differed between the three zones. We saw no changes in the chlorophyll-phosphorus relationship in zone 1, zone 2 appeared to be a transition zone with slight changes in the chlorophyll-phosphorus relationship, and in zone 3 there was evidence of an altered chlorophyll-phosphorus relationship post zebra mussels. These results indicate that the impact of zebra mussels on water quality parameters and on chlorophyll-phosphorus dynamics may differ depending on initial trophic status and on zebra mussel densities.     

WATER QUALITY DURING TWO HIGH‐FLOW YEARS ON THE LOWER MISSOURI RIVER: THE EFFECTS OF RESERVOIR AND TRIBUTARY CONTRIBUTIONS

Complex socioeconomic and ecological issues, ranging from impaired streams to Gulf of Mexico hypoxia, have made nutrient management an increasingly important issue across the USA. High flows during 2010 and 2011 provided a unique opportunity to investigate trends in discharge, total nitrogen, nitrate/nitrite, total phosphorus, ortho‐phosphorus, suspended sediment and total suspended solids during two distinct high‐flow years on the Missouri River. We compared collections taken during 2010 and 2011 at 12 lower Missouri River locations (river kilometers 1212 to 71) and 22 Missouri River tributary locations. During 2011, average concentrations for all sampled parameters were significantly lower, despite significantly higher total discharge, than 2010 concentrations. Differences in water chemistry between years are likely attributed to the primary source of water. Tributary inflow created high flows during 2010, whereas record releases from Gavins Point Dam created high flows during 2011. Analysis of flow estimated the contribution of these releases at each site and revealed strong positive relationships between the percentage of estimated tributary flow at each site and the concentrations of total nitrogen, total phosphorus and total suspended solids. These monitoring efforts underline the contrasting impacts that tributary streams and reservoir releases have on nutrient export of the Missouri River during high‐flow events and reveal a larger trend of increased nutrient concentrations as the proportion of Missouri River tributary flow increased. Published 2013. This article is a U.S. Government work and is in the public domain in the USA. River Research and Applications published by John Wiley & Sons, Ltd.     

Nutrient reference concentrations and trophic state boundaries in subtropical reservoirs

Nutrient criteria as reference concentrations and trophic state boundaries are necessary for water management worldwide because anthropogenic eutrophication is a threat to the water uses. We compiled data on total phosphorus (TP), nitrogen (TN) and chlorophyll a (Chl a) from 17 subtropical reservoirs monitored from 2005-2009 in the S?o Paulo State (Brazil) to calculate reference concentrations through the trisection method (United States Environmental Protection Agency). By dividing our dataset into thirds we presented trophic state boundaries and frequency curves for the nutrient levels in water bodies with different enrichment conditions. TP and TN baseline concentrations (0.010 mg/L and 0.350 mg/L, respectively) were bracketed by ranges for temperate reservoirs available in the literature. We propose trophic state boundaries (upper limits for the oligotrophic category: 0.010 mg TP/L, 0.460 mg TN/L and 1.7 μg Chl a/L; for the mesotrophic: 0.030 mg TP/L, 0.820 mg TN/L and 9.0 μg Chl a/L). Through an example with a different dataset (from the Itupararanga Reservoir, Brazil), we encouraged the use of frequency curves to compare data from individual monitoring efforts with the expected concentrations in oligotrophic, mesotrophic and eutrophic regional systems. Such analysis might help designing recovery programs to reach targeted concentrations and mitigate the undesirable eutrophication symptoms in subtropical freshwaters.     

The effects of drying and re‐flooding on nutrient availability in ephemeral deflation basin lakes in western New South Wales,Australia

We examined the response of nutrient concentrations to the drying and re‐flooding of ephemeral deflation basin lakes (EDBL) in western New South Wales, Australia. As lakes dried total nitrogen (TN) and total phosphorus (TP) concentrations increased. TN concentrations increased more quickly and TP concentrations increased more slowly than could be attributed to evaporation alone. This suggested that additional nitrogen was being sequestered from the atmosphere or sediments and that some phosphorus loss to the sediments was occurring. Concentrations of nitrogen oxides (NOx) and filterable reactive phosphorus (FRP), however, declined as lakes dried, suggesting a tighter coupling of nutrient release and uptake mechanisms. Inorganic nutrient concentrations rose sharply in response to re‐flooding in all lakes. Evidence is provided to suggest that post‐flood nutrient pulses are the net result of both riverine inputs and sediment releases and that the relative significance of either may be influenced by regulation. Copyright © 2002 John Wiley & Sons, Ltd.     

上海市淀山湖水域春夏季水质特征及环境容量分析

根据2011-2015年间春夏季对上海市淀山湖水域中pH、水温、溶解氧、总氨氮、亚硝酸盐氮、硝酸盐氮、总氮及总磷等环境因子的调查结果,分析水体中氮磷变化特征及营养盐限制状态,采用综合营养状态指数法对水体富营养化现状进行评价,利用数学模型估算淀山湖主要特征污染物的环境容量。结果表明:调查期间TN、TP春季与夏季存在显著差异,除2014年TP季节规律不明显外,春季TN、TP含量显著高于同年夏季;淀山湖三态氮含量排序,NO_3~-—N含量最高,TAN其次,NO-2—N最低,淀山湖水域三态氮基本达到热力学平衡;氮磷Pearson相关性分析表明湖库整体呈氧化性环境,氮磷补给具有异源性。根据水中营养物限制性分类标准分析得出,大多数情况下淀山湖水域氮磷比例合理,少数情况下营养盐限制状态以磷限制为主,氮磷限制状态交替存在的情况,可能会对浮游植物生长和群落演替产生影响。综合营养状态指数法计算结果表明调查期间淀山湖均处于富营养化状态,其中2014年之前基本处于中度富营养化状态,2014年以后处于轻度富营养化状态。环境容量分析表明除2014年夏季CODMn外,调查期间淀山湖水体CODMn、TN及TP的污染负荷现状均大于同期保护目标要求的水环境容量,为达到水环境保护的目标,需要进行不同程度的削减。     

Snowmelt and its role in the hydrologic and nutrient budgets of prairie streams

Small watersheds in the Canadian Prairies are characterized by seasonally disconnected hydrologic networks whereby stream channels are hydrologically connected during snowmelt but have disconnected reaches throughout the remainder of the year. Snowmelt is the most significant hydrological event in the Canadian Prairies, yet few studies have investigated the role of snowmelt in the nutrient budget of prairie streams. We quantified hydrologic and nutrient dynamics during snowmelt for ten agricultural subwatersheds distributed along a gradient of human activity in the Red River Valley, Canada, to evaluate the timing of nitrogen (N) and phosphorus (P) export. Elevated concentrations of total P (TP) and total N (TN) were observed during the snowmelt peak, with maximum concentrations reaching 3.23 mg TP L(-1) and 18.50 mg TN L(-1). Dissolved P and N dominated the total nutrient pool throughout snowmelt, likely due to reduced erosion and sediment transport resulting from the combination of the flat topography, frozen soil and stream banks, and gradual snow cover melt. Significant correlations were observed between snowmelt N load (r=0.91; p<0.05) and both agricultural land cover and fertilizer usage, with a weaker correlation between snowmelt P load (r=0.81; p<0.05) and agricultural area. Our results showed that snowmelt plays a key role in nutrient export to prairie aquatic ecosystems and this may have serious impacts on downstream ecosystems. Land use management practices need to consider the snowmelt period to control nutrient loads to Lake Winnipeg and other waterbodies in the Great Plains.     

阳宗海叶绿素a、磷、氮动态特征及富营养化趋势

分析2002-2012年阳宗海TN、TP、Chl-a、氮磷比的动态变化特征及相互关系,并用综合营养状态指数法评价阳宗海的富营养化状态。结果表明：阳宗海富营养化呈上升趋势,2007年从之前的贫营养级上升为中营养级;Chl-a、TN、TP浓度在2007年后均呈快速上升趋势。 Chl-a浓度与TN、TP浓度呈正相关,且氮磷比越接近阈值16∶1,Chl-a浓度上升就越快。指出若营养盐输入得不到有效控制,预测阳宗海将在2017年前后达到富营养级水平,水质将进一步下降。     

Periphytic algal biomass as a bioindicator of phosphorus concentrations in agricultural headwater streams of southern Ontario

Algal blooms in Lake Erie have worsened in recent decades and are driven by diffuse export of phosphorus (P) from a large stream network that drains predominately agricultural land. Given the diffuse nature of nonpoint source pollution, best management practices (BMPs) must target areas where P levels are high. This requires long-term watershed-wide monitoring programs that do not currently exist in many jurisdictions. Instead of conventional nutrient analyses that can be costly and time-consuming, we propose the use of periphyton biomass as a bioindicator of trophic status in low-order streams, where agricultural runoff first enters watercourses. We carried out 2-week in-stream bioassays to measure periphytic algal biomass (CHL_peri) in 19 low-order streams in southern Ontario across an agricultural gradient (8 % to 89 %). CHL_peri was significantly related to total P (TP) concentration (r² = 0.46; p = 0.0015) but was not significantly related to soluble reactive P (SRP). A relationship between TP and turbidity (r² = 0.52; p = 0.0007) is consistent with previous observations of increasing SRP uptake in streams draining agriculturally-dominated landscapes. Stream temperature (°C) was correlated with the proportion of agricultural land (R = 0.55; p = 0.019) and may reflect the warming effects of the sun in unshaded agricultural streams. This method involving substrate rods (Peristix) is cost-effective, requires very little training, and yielded data that were significantly related to TP concentrations in agricultural streams. We recommend that environmental agencies and landowners use this bioassay to identify areas for implementing BMPs to reduce P export from the Lake Erie watershed.     

Winter application of manure on an agricultural watershed and its impact on downstream nutrient fluxes

Whole Farm Planning was instituted and monitored over a 5-year period within the Graywood Gully sub-watershed of Conesus Lake, NY (USA). An array of agricultural Best Management Practices (BMPs) (strip cropping, fertilizer reduction, tiling, manure disposal practices, etc.) were simultaneously introduced to determine the impact of a concentrated management effort on nutrient and soil loss from one watershed within the Conesus Lake catchment. During the study period, significant decreases in winter concentrations of dissolved and particulate fractions, including total phosphorus (TP), soluble reactive phosphorus (SRP), total Kjeldahl nitrogen (TKN), and nitrate (NO₃) but not total suspended solids (TSS), were observed. These decreases may or may not be attributed to cessation of manuring practices. Three years into the study, an opportunity existed to test the responsiveness of the watershed to the curtailment of a single BMP — winter manure application to fields. We field-tested the hypothesis that a change in winter manure applications would impact dissolved and particulate fractions in stream water draining this watershed. We found that the water quality of Graywood Gully is very responsive to winter manure application on environmentally sensitive portions of the sub-watershed. With the short-term resumption of manure application, TP, SRP, TKN, and NO₃ concentrations rose dramatically in stream water; elevated phosphorus concentrations persisted over a 5-week period. Total suspended solids, however, were not elevated after short-term manure application. Factors that affected these results were slope of the land, application of manure over snow and during a snowfall, warm air and soil temperatures, and possibly tile drainage of snowmelt water. Managers of agricultural systems must recognize that phosphorus losses from the watershed during the nongrowing season may detrimentally affect nuisance population of algae in lakes during the summer.     

Comparison of different trophic state indices applied to tropical reservoirs

Reservoirs represent impoundments of river flows for multiple human purposes, including hydropower generation and industrial, recreational and agricultural activities. In considering the current state of dammed rivers and future projects, there is a major concern in tropical areas regarding the water quality of these aquatic ecosystems, most being subjected to eutrophication process. To this end, the Carlson (1977), Toledo (1983), Lamparelli (2004) and Cunha (2013) trophic state indices were applied and compared in six interconnected reservoirs from the Paraíba do Sul Ecoregion in Brazil that differ in their morphometry, nutrient loadings, water retention times and altitudes. Limnological variables were measured in an extensive monitoring programme carried out from 2011 to 2014 during both the rainy and dry seasons. Application of the different numerical models resulted in different trophic state classifications. Toledo's trophic state index was considered the best for assessing the trophic state of the study reservoirs since it under‐evaluates water transparency, which can be related more to inorganic turbidity rather than to phytoplankton biomass in tropical water systems. The Carlson index, developed for temperate zone lakes, resulted in higher trophic classifications, thereby not being considered a reliable model for the study reservoirs. Both the Lamparelli and Cunha indices generally classified the study reservoirs as being in an oligotrophic state, thereby underestimating their trophic status. All the trophic state indices varied seasonally, with water transparency being the only variable significantly affected by rainfall variations. Based on the results of the present study, it was concluded that an arbitrary choice of existing trophic state indices to evaluate the water quality in tropical environments might lead to erroneous conclusions regarding the actual trophic conditions of tropical waterbodies. Further, the weak correlation observed between total phosphorus and chlorophyll‐a concentrations indicates total phosphorus might not be a key factor limiting algal growth in tropical water systems, thereby meriting more research focusing on tropical reservoirs, as well as development of new trophic state indices, including consideration of other potentially limiting factors.     

The influence of land use and potamodromous fish on ecosystem function in Lake Superior tributaries

Allochthonous nutrients and carbon are recognized as dominant controls on biogeochemistry of low-order streams. In some systems, potamodromous fish may provide a complementary source of material as they deliver lake-derived materials to spawning streams. This study examines nutrient and carbon inputs from terrestrial ecosystems and migratory fishes to streams in undeveloped watersheds in northern Michigan, USA. We compared watershed and riparian area, slope, and landcover to nutrient concentrations at 26 sites, as well as whole-stream metabolism at 5 sites. Despite low levels of agricultural land use (0–3%), agriculture had the largest influence on stream chemistry as indicated by higher dissolved organic carbon (DOC), ammonium, silica, and chloride concentrations at the watershed level, and increased DOC and chloride at the riparian level. Ecosystem respiration and net primary production increased with watershed and riparian area, and the proportion of managed forest. To quantify inputs from fish, we monitored the spawning migrations of white (Catostomus commersonii) and longnose (C. catostomus) suckers at one site, and measured nutrients and stream metabolism above and below an impassable dam. Nutrient concentrations were uniformly low and did not increase during the fish migration; however, temporal shifts in stream metabolism during sucker migration suggest that fish influenced respiration, presumably by providing high-quality carbon and bioavailable nutrients. We conclude that both watershed land use and fish migrations provide important sources of allochthonous material to these oligotrophic streams. Recognizing the bi-directional nature of allochthonous inputs is important for understanding controls on ecosystem functioning in low-order streams.     

Nutrient Retention Associated with Phenological Features in Sparganium Erectum Stands in a Lowland Stream

In a previous study, we found that Sparganium erectum, an emergent macrophyte, accumulates sediment inside the stand because of its phenological features. Because this species usually grows in a nutrient‐rich environment, we hypothesize that a high trophic level is maintained in the accumulated sediment. To test this hypothesis, we intensively studied flow velocity, nutrient budget and nutrient flux in and outside of S. erectum stands along the Moto‐Arakawa River, Saitama, Japan and found that the growth stages of S. erectum substantially affected the flow conditions inside the stand. The growth stages of this plant also controlled the depositional rate of sediment and organic particles. After the collapse of each cohort, a substantial amount of organic matter accumulated on the stand bed. Because the accumulated organic matter mineralized very fast, the concentration of total carbon (TC), total nitrogen (TN) and total phosphorus (TP) increased substantially after the collapse of the shoots. While the concentrations of TC, TN and TP within the stand's sediment varied seasonally, the concentrations of these elements were always higher inside than outside of the stand. More than five times the amount of carbon and twice the amount of nitrogen and phosphorus were physically retained in the stand for 1 year compared with the amounts assimilated by the plants. The ratio of carbon to nitrogen in the stand's sediment also remained constant throughout the growing period. Conversely, the nitrogen to phosphorus ratio of the accumulated sediment was much lower than that of other plant tissues. The main flow contains suspended organic solids that constantly supply the stand and partially occupy the channel where they eventually settle. Copyright © 2014 John Wiley & Sons, Ltd.     

Long-term and seasonal nitrate trends illustrate potential prevention of large cyanobacterial biomass by sediment oxidation in Hamilton Harbour,Lake Ontario

Several studies have shown that large, experimental additions of nitrate (NO₃) to eutrophic systems can mitigate large populations of nuisance cyanobacteria and that high NO₃ concentrations can oxidize anoxic sediments. These studies are consistent with observations from numerous aquatic systems across a broad trophic range showing development of reduced surficial sediments precedes the formation of large cyanobacteria populations. We use 50+ years of data to explore whether high NO₃ concentrations may have been instrumental both in the absence of large populations of cyanobacteria in eutrophic Hamilton Harbour, Lake Ontario in the 1970s when total phosphorus (TP) and total nitrogen (TN) concentrations were high, and in delaying large populations until August and September in recent decades despite much lower TP and TN. Our results indicate that large cyanobacteria population events do not occur at the central station in July-September when epilimnetic NO₃ > 2.2 mg N L^?1. The results further suggest that remedial improvements to wastewater treatment plant oxidation capacity may have been inadvertently responsible for high NO₃ concentrations > 2.2 mg N L^?1 and thus for mitigating large cyanobacteria populations. This also implies that large cyanobacteria populations may form earlier in the summer if NO₃ concentrations are lowered.     

Seasonal Variation of Nutrient Limitation in Western Lake Erie

Monthly nutrient enrichment experiments were performed from April through October, 1983, to identify growth limiting nutrients of natural phytoplankton assemblages collected from a station in the Pigeon Bay waters off the north shore of Lake Erie's western basin. Data from these experiments suggest that silica was the major limiting nutrient in April, phosphorus was the major limiting nutrient in May-September, and trace metals were limiting in October. These results show that nutrient enrichment experiments should be performed often enough to account for seasonal changes in the physicochemical environment as well as seasonal succession in phytoplankton. The conclusion that phosphorus was the major limiting nutrient in summer is different from the conclusions of the late 1960s and early 1970s that nitrogen was the major limiting nutrient in the summer in the western basin and suggests a possible shift from nitrogen to phosphorus limitation. Pigeon Bay nutrient trend data show that summer nitrate:soluble reactive phosphorus ratios have increased from approximately 4 in the late 1960s to over 40 in the late 1970s and early 1980s, corroborating that a shift has probably occurred. This shift in nutrient limitation is probably due to reduced bioavailable phosphorus loadings and increased nitrate loadings to the western basin. The southern portion of the western basin is physicochemically different from the northern portion and may not have responded in the same manner.     

南方丘陵山区湿地氮磷时空变化及影响因素分析

对陈墩库区湿地从2015年9月到2016年8月进行水质监测,通过方差分析和相关性分析归纳总结湿地氮磷浓度变化规律,探究不同植物与湿地氮磷的相互作用。结果表明:陈墩库区TN、TP浓度在不同湿地季节变化有所差异,各湿地TN浓度在春、夏、冬差异显著;TP在冬季有显著差异,其余季节均无明显差异,陈墩库区TN、TP浓度季节变化与进水氮磷浓度和湿地面积、植物类型及生长状况密切相关;不同植物与湿地TN、TP呈现不同的相关性,这与水生植物对氮磷的吸收能力不同有关;轮叶黑藻、再力花、芦苇对氮磷的去除效果较好。     

Trophic state evolution in a subtropical reservoir over 34 years in response to different management procedures

Despite their importance for water management, long-term studies on trophic state are relatively scarce in subtropical reservoirs. We analyzed total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl a) concentrations in the Billings Reservoir (Brazil) over time: Phase 1 (1977-1992, Tietê River water was pumped to Billings to increase energy generation, 100 m(3) s(-1)); Phase 2 (1992-2007, Tietê water was conveyed to Billings only in special cases for flood avoidance, 8 m(3) s(-1)); and Phase 3 (2007-2010, besides flood control, Billings received Tietê water treated by an in situ flotation system, 13 m(3) s(-1)). We compared our results with data from 12 reservoirs to evaluate current (2005-2009) enrichment conditions. Phosphorus and nitrogen concentrations decreased (p < 0.05, MANOVA) from Phase 1 to 2 and were stable thereafter. TN/TP ratios increment (1977-2010) indicated shift from N- to P-limitation in the reservoir, affecting the phytoplankton. Nutrient levels in Billings are currently between the expected concentrations in mesotrophic and eutrophic reservoirs (0.03 mg L(-1) < TP < 0.42 mg L(-1), 0.8 mg L(-1) < TN < 7.6 mg L(-1)) and Chl a concentrations exceeded 34 μg L(-1), median for the eutrophic waterbodies from the dataset. Although water quality in Billings has improved, nutrient inputs from Tietê River pumping episodes, diffuse and internal sources are still favoring biomass accrual and compromising water uses.