Total Phosphorus Thresholds Research Articles

Setting nutrient boundaries to protect aquatic communities: The importance of comparing observed and predicted classifications using measures derived from a confusion matrix

Defining nutrient thresholds that protect and support the ecological integrity of aquatic ecosystems is a fundamental step in maintaining their natural biodiversity and preserving their resilience. With increasing catchment pressures and climate change, it is more important than ever to develop clear methods to establish thresholds for status classification and management of waters. This must often be achieved using complex data and should be robust to interference from additional pressures as well as ameliorating or confounding conditions. We use both artificial and real data to examine challenges in setting nutrient thresholds in unbalanced and skewed data. We found significant advantages to using binary logistic regression over other techniques. However, one of the key challenges is objectively selecting a probability from which to derive the nutrient threshold. For this purpose, the examination of the proportions of matching and mismatching status classifications of nutrients and a biological quality element using a confusion matrix is a key step that should be more widely adopted in threshold selection. We examined a large array of statistical measures of classification accuracy and their performance over combinations of skewness and imbalance in the data. The most appropriate threshold probability is a compromise between maximising overall classification accuracy and reducing mismatches expressed as commission (false positives) without excessive omission (false negatives). An application to a lake type indicated total phosphorus thresholds that would be around 50 μg l−1 lower than the threshold achieved by an ‘unguided’ approach, indicating that this approach is a very significant development meriting attention from national authorities responsible for water management.

Developing Ecological Thresholds for Nitrogen and Phosphorus in the Haihe River Basin in China.

Many aquatic ecosystems are eutrophicated due to human inputs of nitrogen and phosphorus. Therefore, it is now considered important to establish nutrient criteria to prevent eutrophication. In this study, the water quality and biological data for 26 stations in the lower reaches of the Haihe River Basin were collected. The total nitrogen (TN) and total phosphorus (TP) ecological thresholds were derived from the threshold indicator taxa analysis (TITAN) and frequency distribution approach. The results showed that the TN threshold was 1.8 mg/L based on the TITAN and the TP threshold was 0.039 mg/L based on the frequency distribution approach. The TITAN also identified 29 indicator species of TN and 28 indicator species of TP. Based on these criteria, we found 73% sites with TN concentrations higher than the 1.8 mg/L. As for TP, 58% sites exceeded 0.039 mg/L. This study showed that most of the downstream areas of the Haihe River are subject to relatively serious disturbances. This finding could provide implications for identifying the water quality traits of and generating protection strategies for the Haihe River in Tianjin.

Eutrophication evolution trajectory influenced by human activities and climate in the shallow Lake Gehu, China

Lake eutrophication has become a global environmental pollution problem. Analysis of the eutrophication recovery trajectory of lakes can help to accurately implement regulations and reduce the cost of management. The degradation and recovery trajectories of Lake Gehu were firstly analyzed based on 34-years monitoring data of water quality. A dynamic correlation analysis of the relationship between chlorophyll-a (Chla) and the total phosphorus (TP) (total nitrogen [TN]) at an annual scale was conducted using the filtering trajectory method (FTM). The result showed that the recovery trajectory of Lake Gehu was nonlinear, and showed a short lag time in Chla decrease. The regime shift from a clear-water state to a turbid-water state occurred in Lake Gehu when the TP threshold was 120 μg/L. Combined the filtering trajectory diagram with a principal component analysis, the results revealed that the adverse climate change did not completely offset the positive effect of nutrient load reduction, which required further attention and had significance for eutrophication control. According to the dynamic changes in the algae production efficiency of total phosphorus (ETP), the result showed that the sensitivity of lake eutrophication increased and the TP carrying concentration reduced by 40%-60%. The water quality goal for controlling algae was determined as TP = 40 μg/L (Chla = 10 μg/L), which was 20% stricter than the national standard (TP = 50 μg/L). In conclusion, aquatic vegetation restoration and biological manipulation can be effective alternatives for controlling eutrophication in addition to nutrient load reduction. The eutrophication control target and adaptive strategies were determined according to the recovery trajectory of Lake Gehu in this study. The ETP has reference significance for the management of shallow lakes.

Nitrogen and phosphorus control thresholds of cyanobacterial blooms in Lake Taihu

藻类生长与营养盐浓度存在藻类几何级数增长的营养盐浓度变化的下限阈值和藻类生长不受氮磷浓度增加影响的上限阈值,但由于蓝藻水华的形成受多种因素的综合影响,不同湖泊、不同区域及不同时段的氮磷浓度对蓝藻水华的影响差别较大,使得蓝藻生长的氮磷控制阈值难以确定.针对控制蓝藻水华暴发的氮磷阈值的研究虽然有所开展,但多集中在实验室研究阶段或对经验值的判断,虽然也有基于野外实测数据的研究,但也限制于某一特定区域,而基于野外长序列实测数据并且覆盖整个湖泊的氮磷阈值研究则是空白.太湖作为具有较高营养背景的富营养化浅水湖泊,蓝藻水华的发生受氮磷影响较大.对太湖总磷(TP)、总氮(TN)和叶绿素a(Chl.a)浓度的时空变化分析发现,太湖西北湖区的TP、TN与Chl.a浓度明显较高,并且TP、TN与Chl.a均呈显著性正相关.为探究太湖蓝藻水华暴发的TP和TN控制阈值,以轻富营养化等级下的Chl.a分级标准(10,26］作为表征水华暴发的条件,采用郑丙辉等的频率分布法,确定了太湖蓝藻水华暴发的TP和TN控制阈值分别为0.05~0.06和1.71~1.72 mg/L;通过空间验证,太湖藻型区TP和TN浓度远高于同级营养水平下全湖区TP和TN控制阈值,表明藻型区高氮磷水平为蓝藻水华发生提供充足营养盐条件,即使氮磷全湖平均浓度控制在蓝藻水华暴发的氮磷阈值水平之下,但在气象水文等因素适宜条件下,藻型区水华发生风险仍然较高;并且在高氮磷背景下,即便在水华发生风险低的季节,水华发生风险仍然较大.近十几年来,虽然太湖经历了大规模的高强度治理,但由于环太湖流域的湖西区入湖负荷占比大,导致太湖藻型区氮磷浓度仍处于高位运行状态,为蓝藻水华的暴发提供了充足的营养盐基础,因此,湖西区的控源减排仍然是太湖富营养化及蓝藻水华防控的重点.;Algae growth and nutrient concentration has a lower threshold for algae affected by nutrient concentration with geometric increase and an upper threshold for algae that is not affected by the increase of nitrogen and phosphorus concentration. However, the formation of cyanobacterial blooms is affected by many factors, the effects of nitrogen and phosphorus on cyanobacterial blooms vary greatly in different lakes, different areas, and different periods, making it difficult to determine the nitrogen and phosphorus control thresholds for cyanobacteria growth. Although studies on nitrogen and phosphorus thresholds for algal growth and control of cyanobacterial blooms have been carried out, they are mostly focused on laboratory studies or empirical judgments. Although there are also studies based on field-measured data, they are also limited to a specific area, while studies on nitrogen and phosphorus thresholds based on long series of field measurements and covering the entire lake are lacking. As a eutrophic shallow lake with a high nutrient background, the occurrence of cyanobacterial blooms in Lake Taihu is strongly influenced by nitrogen and phosphorus. Analyzing the temporal and spatial changes of total phosphorus (TP), total nitrogen (TN) and chlorophyll-a concentration (Chl.a) in Lake Taihu, it is found that TP, TN and Chl.a in the northwestern of the lake were significantly higher, and TP, TN and Chl.a are all positively correlated. To investigate the control thresholds of TP and TN for cyanobacterial bloom outbreaks in Lake Taihu, the Chl.a grading standard (10, 26] under the light eutrophication level was used as the condition to characterize the bloom outbreak, referring to the frequency distribution method put forward by Zheng Binghui, the TP and TN control thresholds were determined to be 0.05-0.06 mg/L and 1.71-1.72 mg/L, respectively. Through spatial verification, the concentration of TP and TN in the phytoplankton-dominated area of Lake Taihu is much higher than the control thresholds of TP and TN in the whole lake area under the same nutritional level. This indicated that the high nitrogen and phosphorus levels in the phytoplankton-dominated area provide sufficient nutrient basis for the occurrence of cyanobacterial blooms. Even if the average concentration of nitrogen and phosphorus in the whole lake is controlled below the threshold level, the risk of algal blooms in the phytoplankton-dominated area is still relatively high under suitable conditions of meteorology and hydrology. In addition, under the background of high nitrogen and phosphorus in the phytoplankton-dominated area, even in the low-risk seasons for cyanobacterial blooms that were once considered, the possibility of blooms is still high. In the past ten years, although Lake Taihu has undergone large-scale and high-intensity treatment, the nitrogen and phosphorus concentrations in the phytoplankton-dominated area of Lake Taihu are still at a high level because of the large proportion of the inflow pollution load from Huxi District around Lake Taihu Basin, which provides sufficient nutrients for the cyanobacterial blooms. In consequence, source control and emission reduction in Huxi District is still the key measurement for the prevention and control of eutrophication and cyanobacterial blooms in Lake Taihu.

Increasing Carbon-to-Phosphorus Ratio (C:P) from Seston as a Prime Indicator for the Initiation of Lake Reoligotrophication.

Decline in total phosphorus (TP) during lake reoligotrophication does not apparently immediately influence carbon assimilation or deep-water oxygen levels. Traditional monitoring and interpretation do not typically consider the amount of organic carbon exported from the productive zone into the hypolimnion as a measure of net ecosystem production. This research investigated the carbon-to-phosphorus ratios of suspended particles in the epilimnion, (C:P)epi, as indicators of changing productivity. We report sestonic C:P ratios, phytoplankton biomass, and hypolimnetic oxygen depletion rates in Lake Hallwil, a lake whose recovery from eutrophic conditions has been documented in 35 years of historic water-monitoring data. This study also interpreted long-term (C:P)epi ratios from reoligotrophication occurring in four other lakes. Lake Hallwil exhibited three distinct phases. (i) The (C:P)epi ratio remained low when TP concentrations did not limit production. (ii) (C:P)epi increased steadily when phytoplankton began optimizing the declining P and biomass remained stable. (iii) Below a critical TP threshold of ∼15 to ∼20 mg P m-3, (C:P)epi remained high and the biomass eventually declined. This analysis showed that the (C:P)epi ratio indicates the reduction of productivity prior to classic indicators such as deep-water oxygen depletion.

Climate and Nutrient-Driven Regime Shifts of Cyanobacterial Communities in Low-Latitude Plateau Lakes.

Cyanobacterial blooms that form in response to climate warming and nutrient enrichment in freshwater lakes have become a global environmental challenge. Historical legacy effects and the mechanisms underlying cyanobacterial community succession are not well understood, especially for plateau lakes that are important global freshwater resources. This study investigated the temporal dynamics of cyanobacterial communities over centuries in response to nutrient enrichment and climate warming in low-latitude plateau lakes using high-throughput DNA sequencing of sedimentary DNA combined with traditional paleolimnological analyses. Our results confirmed that nutrients and climate warming drive shifts in cyanobacterial communities over time. Cyanobacterial community turnover was pronounced with regime shifts toward new ecological states, occurring after exceeding a tipping point of aquatic total phosphorus (TP). The inferred species interactions, niche differentiation, and identity of keystone taxa significantly changed after crossing the aquatic TP ecological threshold, as demonstrated by network analysis of cyanobacterial taxa. Further, the contribution of aquatic TP to cyanobacterial community dynamics was greater than that of air temperature when lakes were in an oligotrophic state. In contrast, as the aquatic TP threshold was exceeded, the contribution to community dynamics by air temperature increased and potentially surpassed that of aquatic TP. Overall, these results provide new evidence for how past nutrient levels in lacustrine ecosystems influence contemporary cyanobacterial community responses to global warming in low-latitude plateau lakes.

Ecosystem Shift from Submerged to Floating Plants Simplifying the Food Web in a Tropical Shallow Lake

Shifts between the alternative stable states have been a popular topic of ecological studies for over 50 years. However, identifying the mechanisms that drive these regime shifts remains a challenging task in the field of applied ecology and ecosystem management. Herein, we applied a Bayesian latent variable regression (BLR) to the dataset obtained for a shallow lake experiencing a shift from the dominance of submerged plants to the dominance of floating plants to classify the observed lake states, determine total phosphorus (TP) thresholds, and establish a steady-state relationship between the phosphorus content and the size of an aphotic zone. In addition, a structural equation model was utilized to elucidate the mechanisms driving this shift and their effects on the trophic web of the ecosystem. The BLR model exhibited high state classification rates (median/mean accuracy > 97.5%) and accurately determined the steady-state relationships between the TP thresholds and the aphotic zone size. We also found that the top-down control via piscivorous fish was stronger in the state dominated by submerged macrophytes, while the bottom-up control via TP was stronger in the state dominated by floating macrophytes. Additionally, the dominance of floating macrophytes was related to the low degree of biodiversity and simplification of the trophic web. Our results show that top predators can be used to control nutrient levels and ensure the dominance of submerged macrophytes. It also provides a new perspective on the theory of alternative states and potential tools for the management of ecosystems experiencing regime shifts.

Identification of Regime Shifts and Their Potential Drivers in the Shallow Eutrophic Lake Yilong, Southwest China

Regime shifts in shallow lakes can lead to great changes in ecosystem structures and functions, making ecosystem management more complicated. Lake Yilong, located in Yunnan Province, is one of the most eutrophic lakes in China. Although there is a high possibility that this lake has undergone regime shift one or more times, the presence of regime shifts and their drivers remain unknown. Here, we employed the sequential t-test analysis of regime shifts to detect the regime shifts based on the long-term (1989–2018) dataset of the lake. We further determined their potential drivers, and explored the nutrient thresholds of regime shifts and hysteresis. The results showed that during the testing period, three regime shifts occurred in 1996 (restorative type), 2009 (catastrophic type) and 2014 (restorative type). The potential key drivers for the first two regime shifts (1996 and 2009) were both related to aquaculture. The abolition of cage fish culture may have led to the restorative regime shift in 1996, and the stocking of crabs and excessive premature releasing of fry possibly caused the catastrophic regime shift in 2009. However, the third regime shift, which occurred in 2014, was possibly related to the drought and succedent hydration. These results indicate that adjustments of aquaculture strategy and hydrological conditions are critical for the lake ecosystem’s recovery. Moreover, the total phosphorus thresholds were identified to be lower than 0.046 mg/L (restorative type) and higher than 0.105 mg/L (catastrophic type), respectively. In addition, an obvious hysteresis was observed after 2014, suggesting that nutrient reduction is important for this lake’s management in the future.

Quantifying the dependence of cyanobacterial growth to nutrient for the eutrophication management of temperate-subtropical shallow lakes

The increasing global occurrence of cyanobacterial blooms, attributed primarily to human-induced nutrient enrichment, significantly degrades freshwater ecosystems and poses serious risks to human health. The current study examined environmental variables and cyanobacterial biovolume (BCyano) of 28 shallow lakes in the eastern China plains during the spring and summer of 2018. We used a 95% quantile regression model to explore season-specific response of BCyano to total nitrogen (TN), or total phosphorus (TP), and robust linear relationships were observed between log(BCyano+0.001) and log(TN), or log(TP) in both spring and summer periods. Based on these regressions, regional-scale and season-specific TN and TP thresholds are proposed for these lakes to ensure the safety for recreational waters and drinking water source. However, actual BCyano for a given concentration of TN (or TP) for many observations were considerably lower than the results of the 95% regression model predict, indicating that other factors significantly modulated nutrient limitation of BCyano. Generalized additive model and quantile regression model were used together to explore potentially significant modulating factors, of which lake retention time, macrophytes cover and N: P ratio were identified as most important. Thus, it is necessary to develop type-specific nutrient thresholds with the consideration of these significant modulating factors. Furthermore, nutrient-BCyano relationships of our studied lakes with lake retention time>100 days and no macrophyte were further explored and nutrient thresholds of this lake type were proposed. Nutrient thresholds proposed in this study may play an essential role in achieving a cost-effective eutrophication management for shallow lakes both in the eastern China plains and elsewhere with similar climatic background. On a broader scale, the approaches and findings of this study may provide valuable reference to formulate reasonable nutrient reduction targets for other ecoregions with different climatic conditions.

Diatom–water quality thresholds in South Australian streams indicate a need for more stringent water quality guidelines

Water quality guidelines are an important tool that guide stream protection. In South Australia, as in other Australian states, guidelines have been set using statistical properties of physical and chemical stressors in reference streams. Given that water quality guidelines are designed to protect ecosystems, biologically based guidelines are more desirable. In this context, we investigated diatom–water quality relationships in South Australian streams. Our analysis focused on electrical conductivity (EC) and total phosphorus (TP), which explained significant variance in diatom assemblages. Threshold indicator taxa analysis was conducted to determine thresholds of diatom community change along EC and TP gradients. There were significant declines in the relative abundance of sensitive species at an EC of ~280μScm–1 and a TP concentration of 30μgL–1. The TP threshold is considerably lower than the trigger value in South Australia’s guidelines (100μgTPL–1). The change in species composition in relation to EC is considerably lower than the upper limit of the water quality guidelines (which range from 100 to 5000μScm–1). Hence, particularly in the case of TP, but also for EC, the current water quality guideline trigger values are too high in South Australia, and indeed in other temperate Australian states.

Establishment of season-specific nutrient thresholds and analyses of the effects of nutrient management in eutrophic lakes through statistical machine learning

Eutrophication and subsequent harmful cyanobacteria blooms are global water quality problems, and identifying the key drivers of water eutrophication and estimating nutrient thresholds for it in waterbodies have long been challenges for water quality managers. Data-intensive machine learning models have been shown to be better able to reveal the nonlinear relationships between variables in the study of complex biotic community dynamics than traditional mechanistic models. In this study, we applied random forest models to long-term datasets from nutrient monitoring and meteorological observations to characterize the relationships between algal growth and different environmental drivers in three eutrophic lakes in China. We further attempted to estimate the season-specific nutrient thresholds in these lakes, and assess the potential decreases in chlorophyll a concentrations that could be achieved through nutrient management. In general, chlorophyll a concentrations predicted by the random forest models were consistent with the values observed in the lakes, and successfully displayed the same seasonal variations. The estimated total nitrogen (TN) and total phosphorus (TP) nutrient thresholds were quite variable among months, and were higher in summer than in winter. To maintain chlorophyll a concentrations below 20 μg/L, the estimated TN thresholds in Lakes Taihu, Dianchi, and Chaohu in August were 2145 ± 683, 2372 ± 918 and 1527 ± 71 μg/L (mean ± standard deviation), respectively, and the corresponding TP thresholds were 82 ± 24, 149 ± 22, and 120 ± 22 μg/L. The modelling results indicated that it was more important to control the TP concentrations in these lakes than the TN concentrations to control algal growth in summer. In summary, the strong seasonal variation in the estimated nutrient thresholds suggests that a ‘one-size-fits-all’ nutrient control target could overprotect these water bodies. Seasonal variation in nutrient concentrations and environmental drivers should thus be considered when establishing nutrient criteria and setting nutrient control targets.

Comparison of water quality thresholds for macroinvertebrates in two Mediterranean catchments quantified by the inferential techniques TITAN and HEA

Catchment managers face complex decisions to balance between competing targets such as water quantity and quality, land use and biodiversity. To balance these targets satisfactorily, decisions need to be grounded in reliable information. The present study aimed to identify potential water quality drivers that may impact the sustainability of the River Torrens and the Onkaparinga River catchments in South Australia by means of the Threshold Indicator Taxa Analysis (TITAN) and the Hybrid Evolutionary Algorithm (HEA). Fourteen years of data comprising abundance for 518 macroinvertebrate taxa and water quality variables were examined to determine possible stimulating and inhibiting effects of water quality conditions on macroinvertebrates at catchment and stream site level. When applied to the Torrens (n = 262) and Onkaparinga (n = 227), TITAN identified positive (z+) and negative (z−) responding species for both catchments and detected community change points in response to total nitrogen (Torrens: z+ = 305 µg L−1, z− = 277 µg L−1; Onkaparinga: z+ = 420 µg L−1, z− = 397 µg L−1), total phosphorus (Torrens: z+ and z− = 22 µg L−1; Onkaparinga: z+ = 73 µg L−1, z− = 71 µg L−1) and electrical conductivity (Torrens: z+ = 931 µS cm−1, z− = 407 µS cm−1; Onkaparinga: z+ = 951 µS cm−1, z− = 428 µS cm−1). These results suggested similar change points along electrical conductivity (EC) and total nitrogen (TN) gradients in both catchments, but three-times higher change points for total phosphorus (TP) in the Onkaparinga River catchment than in the Torrens catchment.Since TITAN identified the species Austrochiltonia australis as responding positively to TP, TN and EC increases, and Paramerina spp. as responding negatively, these taxa were selected for modelling by HEA. In contrast to TITAN, HEA used averaged data from 37 stream sites in each of the two catchments. Resulting models for A. australis revealed much higher change points for TN, TP and EC in the Torrens catchment compared to the TITAN results. Models for Paramerina spp. suggested twice as high thresholds for TN and EC in Onkaparinga as compared to the Torrens. Whilst TN and TP thresholds specified by HEA fall within the range identified by TITAN, thresholds for EC exceeded significantly those identified by TITAN.Overall, the study has shown that the nature of the data used in the two threshold approaches is likely to have resulted in differences in the derived thresholds. Importantly, despite the differences observed in thresholds based either on a catchment or the modelling technique, overall results indicate that South Australian water quality guidelines for freshwater systems are too high.

Cyanobacteria Dominance in Lakes and Evaluation of Its Predictors: a Study of Southern Appalachians Ecoregion, USA

Owing to their ecological and economic implications, the abundance of cyanobacteria has been an important subject in aquatic ecology. There has been a debate about which nutrients are a better predictor of cyanobacteria abundance, total nitrogen (TN), total phosphorus (TP) or nitrogen to phosphorus ratio (N:P). A classic study by Downing and coworkers in 2001 concluded that total N and total P are better predictors than TN:TP ratio. We picked lakes from a different region of the US, the Southern Appalachians (SAP), to test the same hypotheses using the National Lakes Assessment (NLA, 2007) database. This dataset consists of 116 selected sampling sites throughout the ecoregion. In this study, total cyanobacteria abundance was related to three predictors (N, P and N:P) using linear regression, non-parametric statistics and boosted regression trees (BRTs). Total N and total P were more strongly correlated to cyanobacteria abundance than N:P. Partial dependence plots from BRT analysis confirmed results of correlations showing higher relative influence values of TN and TP on cyanobacteria abundance. An important observation from both analyses was that cyanobacteria abundance increased rapidly with increasing TP above 5 μg TP/L. This is a similar response relationship between abundance of cyanobacteria and TP as reported by Downing et al. (2001). These TP thresholds for cyanobacteria dominance are important for TP management targets to control blooms in lakes.

Total phosphorus thresholds for regime shifts are nearly equal in subtropical and temperate shallow lakes with moderate depths and areas

Summary Published research suggests that the total phosphorus (TP) thresholds for the regime shifts between a clear‐water state dominated by submersed macrophytes and a turbid‐water state dominated by phytoplankton in shallow lakes vary with forms of lake basins and climates. However, such hypotheses remain untested by direct field evidence. We therefore tested the hypotheses with empirical data from subtropical lakes on the Yangtze floodplain and also from other lakes in temperate to tropical zones. TP thresholds were found to vary little at moderate depths, but to decrease notably when depth exceeds a level of probably 3–4 m, and increase sharply when depth is below a level of around 1–2 m. TP thresholds were found to be nearly equal in shallow lakes (1–2 m <mean depth <3–4 m; c. 0.1 km2 <area <at least 350 km2) from temperate to subtropical (probably to tropical) zones, being 80–120 mg m−3 for the forward shift from a clear‐water state to a turbid‐water state and 40–60 mg m−3 for the backward shift. The threshold of turbidity for the forward shift was found to be higher than that for the backward shift, amending the previous hypothesis of the equality of turbidity thresholds for both shifts. Our findings suggest that according to the subequality of TP thresholds, similar target concentrations for in‐lake TP can be set in most shallow lakes world‐wide to mitigate eutrophication.

A Synoptic Survey of Nitrogen and Phosphorus in Tributary Streams and Great Rivers of the Upper Mississippi, Missouri, and Ohio River Basins

We combined stream chemistry and hydrology data from surveys of 436 tributary stream sites and 447 great river sites in the Upper Mississippi, Missouri and Ohio River basins to provide a regional snapshot of baseflow total nitrogen (TN) and total phosphorus (TP) concentrations, and to investigate the relationships between land use and stream chemistry. Catchments in the Upper Mississippi River basin had more land in agricultural uses (51%) than the Missouri or Ohio River basin catchments (25% and 29%, respectively). The difference in agriculture is reflected in the TN concentrations in tributary streams and the great rivers: 5,431 and 2,112 μg L−1 for the Upper Mississippi, 1,751 and 978 μg L−1 for the Missouri, and 1,074 and 1,152 μg L−1 for the Ohio River basins. This agricultural effect was not as evident for tributary stream or great river TP concentrations: 165 and 181 μg L−1 in the Upper Mississippi, 177 and 171 μg L−1 in the Missouri, and 67 and 53 μg L−1 in the Ohio River basins. We set reference thresholds based on the 75th percentile TN and TP concentrations at our least disturbed sites. The TN threshold was exceeded for 50–63% of the tributary stream and 16–55% of great river lengths, with the greatest proportion in the Upper Mississippi River basin. The TP threshold was exceeded in 32–48% of tributary stream and 12–41% of great river lengths. Tributary stream N/P ranged from 67:1 (Ohio) to 210:1 (Upper Mississippi); river N/P ranged from 20:1 (Missouri) to 60:1 (Ohio). N/P indicated that potential N-limitation occurred in 10–21% of total tributary stream length and in 0–46% of great river length; potential P-limitation ranged from 60–83% of cumulative tributary stream length and from 21–98% of cumulative great river length. Total N flux (concentration × discharge) was highest in the Upper Mississippi River basin; TP flux was lowest in the Ohio River basin. River TN yields and TP yields for both tributary streams and great rivers, was not significantly different between the sub-basins. Our study empirically links catchment land use and stream chemistry, and demonstrates using monitoring data for estimating nutrient yields at a large regional scale.

Phosphatase Activity Along Soil C and P Gradients in a Reed-Dominated Wetland of North China

Previous studies in oligotrophic wetlands showed that phosphatase activity (PHA) was positively related to soil total phosphorus (TP) content, and that PHA can serve as a predictor of wetland degradation. However, phosphatase dynamics are poorly known for eutrophic wetlands, where anthropogenic nutrient inputs elevate soil P content. We reported variations in PHA along a nutrient content gradient in soils of the eutrophic Baiyangdian wetland in North China. Soil PHA mainly responded to the soil organic carbon (C) content, and microbial biomass carbon. A positive correlation existed between PHA and C but not between PHA and TP in the wetland soils. The Baiyangdian wetland has high soil P content, averaging 598 mg kg(-1). The high TP content caused the lower mass ratios between sediment C and TP (C:TP) that are lower than those reported in many wetlands. Results suggest that there may be a TP threshold above which PHA has no relationship with TP. A laboratory incubation study showed that PHA increased with added C, suggesting that the soil C content was a limiting factor for P mineralization in the Baiyangdian wetland. PHA, together with its limiting factors, could reflect the state of eutrophication of the wetland.

Cyanobacteria dominance: Quantifying the effects of climate change

An increase in cyanobacteria bloom formation within lakes has been forecasted as a result of global warming. We investigated the particular physical and chemical thresholds for cyanobacteria performance in a lake model system, the polymictic eutrophic Müggelsee, which has been affected by significant warming trends and substantial reductions in external nutrient load. To identify key physical and nutrient thresholds favoring cyanobacterial performance, we applied classification tree analysis to water temperature, Schmidt stability, oxygen, pH, nutrients (including phosphorus, nitrogen, and their relative ratios), and zooplankton abundance during periods of summer thermal stratification. Although total phosphorus (TP) concentration was the principal force driving cyanobacteria contribution to total algal mass, climate‐induced changes in the thermal regime, rather than direct temperature effects, positively influenced cyanobacteria dominance. Stratification periods exceeding 3 weeks and exhibiting a Schmidt stability of >44 g cm cm−2 favored cyanobacteria proliferation within a critical TP concentration range (70‐215 µg L−1). The dominating genera Aphanizomenon, Anabaena, and Microcystis achieved the highest biomass in cases in which total nitrogen concentrations exceeded 1.29 mg L−1, stratified conditions exceeded a duration of 3 weeks, and TP concentrations exceeded 215 mg L−1, respectively. Given the observed broad range of TP thresholds within which climate warming enhances the probability of cyanobacteria dominance, the incidence of cyanobacteria blooms will certainly increase in many lakes under future climate scenarios.

MULTIPLE REGIME SHIFTS IN A SUBTROPICAL PEATLAND: COMMUNITY‐SPECIFIC THRESHOLDS TO EUTROPHICATION

Ecosystems have a natural resilience to perturbations, where resilience is the magnitude of a disturbance that an ecosystem can resist before changes in structure, function, and services result in a regime shift. The Everglades region of Florida, USA, has been detrimentally impacted by phosphorus (P) enrichment and a regime shift from Cladium (sawgrass) to Typha (cattail) marsh has been described. We examine another facet of the low‐nutrient Everglades stability regime, open‐water sloughs, to determine if eutrophication leads to similar regime shifts.We analyzed surface water P and soil P as controlling variables that, once a critical threshold is surpassed, alter ecosystem state variables. Nonlinear relationships between P and vegetation were observed along a northern Everglades eutrophication gradient. In addition to the Cladium–Typha regime shift, a second independent regime shift, slough–Typha, was identified. Synoptic surveys of 49 sloughs within the boundary between the slough and Typha regime revealed that surface water total phosphorus (TP) and the benthic algal floc layer (BAFL) were the controlling variables, with critical thresholds of 11 μg/L and 412 mg/kg, respectively. The slough regime below these thresholds was characterized by calcareous periphyton (BAFL TP = 298 mg P/kg; BAFL calcium = 149 g Ca/kg). Above the TP thresholds, vegetation composition shifted to open‐marsh species with significantly higher BAFL TP (700 mg P/kg) and total organic carbon (TOC) (350 g C/kg). A second BAFL TP threshold occurred at 712 mg P/kg, above which Nymphaea dominated and BAFL TP (1034 mg P/kg) and TOC (417 g C/kg) significantly increased. Nymphaea sloughs transitioned to the Typha regime. The boundary reflects the loss of ecosystem resilience due to eutrophication. Both low‐nutrient stability regimes (slough and Cladium) lie precariously close to the P critical threshold but differ in how eutrophication is absorbed and resisted. The slough regime transitions rapidly through a series of ecosystem state changes linked to positive feedback loops that affect P dynamics, whereas the Cladium regime does not. An adaptive management strategy has been implemented to address the surface water TP threshold; however, to ensure successful restoration of the Everglades, the BAFL and soil TP thresholds also need to be considered.

Estimating Ecological Thresholds for Phosphorus in the Everglades

The Florida Everglades, a wetland of international importance, has been undergoing a significant shift in its native flora and fauna due to excessive total phosphorus (TP) loadings (an average of 147 t per annum from 1995to 2004) and an elevated mean TP concentration (69 microg L(-1) of TP in 2004) from agricultural runoff and Lake Okeechobee outflow despite the use of 16000 ha of stormwater treatment areas. Here, we present a Bayesian changepoint analysis of long-term experimental research and show that exceeding a surface water geometric mean TP threshold concentration of 15 microg L(-1) causes an ecological imbalance in algal, macrophyte, and macroinvertebrate assemblages as well as slough community structure. A phosphorus threshold for all trophic levels may be more realistic and protective when presented as a threshold zone (12-15 microg L(-1)) because estimates of uncertainty must be utilized to accurately define TP thresholds, which change with seasons and water depths. Most interior areas of the Everglades are currently at or below this threshold zone, but the exterior areas near inflow structures (except for the Everglades National Park) are presently receiving double or triple the proposed threshold. Our Bayesian approach, used hereto address ecological imbalance along nutrient gradients, is applicable to determining thresholds and stable states in other aquatic ecosystems.

Frequent regime shifts in trophic states in shallow lakes on the Boreal Plain: Alternative "unstable" states?

Shallow lakes are known to exhibit alternative states in their biotic structure. Lakes dominated by submersed aquatic vegetation (SAV) are ‚‚clear,’’ while lakes dominated by algae (high concentrations of chlorophyll a [Chl a]) are ‚‚turbid.’’ The roles of total phosphorus (TP) and SAV in defining these alternative states were examined for up to 6 yr in 24 naturally eutrophic shallow lakes (12.4‐670.4 µg TP L−1) with variable SAV cover (0–100%) on the western Boreal Plain in Alberta, Canada. Clear lakes had <18 mg Chl a L−1. Sixty‐seven percent of lakes were clear in any given year, but individual lakes did not remain clear over consecutive years. While 29% did not switch, 71% of lakes were unstable, with 57% switching states once and 14% switching more than once. To increase the temporal and spatial scale of analysis, we used Landsat Thematic Mapper satellite imagery (1984‐ 2003) to classify clarity for up to 20 yr in 82 naturally eutrophic shallow lakes. Approximately 80% of lakes were unstable, with 7% switching once and more than 73% switching 2–9 times. Only 20% of lakes were stable and clear. Switches in lake clarity were related to TP but were also dependent on the abundance of SAV. For lakes with high SAV, the TP threshold for the transition from clear to turbid was 275 µg TP L−1, while for lakes with low SAV, the TP threshold was 50 µg TP L−1. Given the harsh winter conditions, including lakes with ice depths of up to 0.7 m and anoxia leading to winterkill of aquatic communities, these systems are strongly abiotically regulated and lack mechanisms that maintain a lake in a stable state.