Phosphorus In Water Column Research Articles

Historical Phosphorus Mass and Concentrations in Utah Lake: A Case Study with Implications for Nutrient Load Management in a Sorption-Dominated Shallow Lake

Utah Lake is unusual due to its large surface area, shallow depth, phosphorus-rich sediments, and well-mixed, unstratified waters. This creates conditions where water column phosphorous concentrations tend toward equilibrium, with lake sediments containing high concentrations of geologic phosphorus. To help understand the potential impact of phosphorous load reductions, we computed a time history of phosphorus mass in the lake using state and federal records of lake volume, dissolved phosphorus concentrations, and outflow. We show that historically, Utah Lake phosphorus concentrations have remained stable over time, in the range of 0.02 to 0.04 mg/L, despite large changes in lake volume and internal phosphorus mass. We performed sorption calculations using data from the literature, demonstrating that it would take unrealistically large load changes to alter water column phosphorus concentrations under sorption processes. The sorption model produces results consistent with historical data that show relatively constant phosphorous concentrations despite large lake volume changes. We show, through several lines of evidence, that water column phosphorus concentrations are insensitive to external loads. Phosphorous load reduction is unlikely to have a significant effect on phosphorus concentrations in Utah Lake and, by extension, in other sorption-dominated shallow lakes with phosphorus-rich sediment.

Linking Sediment and Water Column Phosphorus Dynamics to Oxygen, Temperature, and Aeration in Shallow Eutrophic Lakes

AbstractWater quality improvements in shallow eutrophic lakes are commonly delayed due to loading from legacy phosphorus (P)‐enriched sediments, even with reduced external nutrient loads. It is critical to understand the drivers of internal P loading to suppress or remove this source of P and meet water quality goals. We contrast the drivers of internal P loading in two shallow eutrophic systems (Lake Carmi and Missisquoi Bay). Legacy P dynamics in the unmanipulated systems were compared to Lake Carmi during aeration. In‐situ high frequency water column monitoring along with water and sediment sampling was used to study P dynamics in response to changing lake conditions and aeration. Despite both systems exhibiting P mobility controlled by iron redox cycling, we observed distinct differences in the spatial extent and drivers of internal P loading. Legacy P loading was controlled by seasonal drivers in Lake Carmi, but by spatially variable and highly transient wind driven forcing of hydrodynamics in Missisquoi Bay. Aeration altered the mixing regime of Lake Carmi and shifted loading dynamics to frequent wind‐driven pulses of legacy P to surface waters akin to those of Missisquoi Bay. Mean hypolimnetic dissolved oxygen increased with aeration, but greater oxygen demand rates and periods of anoxia under transient stratification still resulted in internal P loading. Surface P concentrations were higher in summer months with aeration compared to previous years. This research illustrates the dynamic nature of legacy P behavior within and between shallow eutrophic lakes and the challenges in addressing this common water resources threat.

Changes in the spatial distribution of total phosphorus in sediment and water column of a shallow subtropical lake

Julian PJ II, Schafer T, Cohen MJ, Jones P, Osborne TZ. 2023. Changes in the spatial distribution of total phosphorus in sediment and water column of a shallow subtropical lake. Lake Reserv Manage. 39:213–230. In shallow lakes, interactions between bed sediment and the water column strongly influence the availability and transport of nutrients. Lake Okeechobee, in South Florida, is a eutrophic, shallow, polymictic lake that exhibits frequent mixing of the water column and resuspension of unconsolidated sediments. The objective of this study was to evaluate the temporal and spatial patterns of water and sediment characteristics and evaluate the linkage of sediment and water column total phosphorus (TP) concentrations. Spatiotemporal generalized additive models identified key periods during which both surface water total suspended solids (TSS) and TP increased, corresponding to hurricane and tropical storm activity. Our models identified regions with persistently greater water column concentrations than the lake average, indicating potential hot spots in resuspension processes and/or internal loading. Sediment bulk density (BD) and TP were inversely correlated, indicating that light, less dense sediments have greater TP concentrations, potentially contributing to the redistribution of sediments and P. An integrated evaluation of surface water and sediment characteristics using a generalized additive model revealed that sediment TP strongly influences water column TP concentrations, potentially explaining the spatial distribution of surface water TP concentrations. The area of the lake with sediment TP concentration ≤500 mg/kg is increasing while the low bulk density sediments area is decreasing, marking a potential improvement in sediment conditions. If this trend persists, it indicates that the lake is increasingly storing P in sediments that can resist entrainment, with significant implications for assessing the trajectory of lake restoration.

Occurrence forms and environmental characteristics of phosphorus in water column and sediment of urban waterbodies replenished by reclaimed water

To illustrate the evolution process, environmental feature and phytoremediation effect of phosphorus (P) in waterbodies with long-term replenishment by reclaimed water (RW), an urban waterbody using RW as the sole supply was selected as a case study. The concentration and distribution of soluble reactive phosphate (SRP), dissolved organic P (DOP) and particulate P (PP) in water column, as well as organic P (OP), inorganic P (IP), exchangeable P (Ex-P), BD-P (redox-sensitive P), NaOH-P (P bound to Fe and Al oxyhydroxides) and HCl-P (P bound to Ca) in sediment were investigated. Results showed that the seasonal average concentrations of total phosphorus (TPw) in water column ranged from 0.048 to 0.130 mg∙L−1, with the highest in summer and the lowest in winter. P in water column was predominantly present in dissolved state, with the similar proportions of SRP and DOP. SRP decreased apparently in midstream, where the phytoremediation was applied extensively. PP content obviously increased in non-phytoremediation area in downstream, due to visitor activity and sediment resuspension. Total phosphorus (TPs) in sediments was between 352.9 and 1331.3 mg∙kg−1, with average 365.7 mg∙kg−1 of IP and 382.8 mg∙kg−1 of OP. Among IP, HCl-P had the highest proportion, followed by BD-P, NaOH-P and Ex-P. OP was significantly higher in phytoremediation areas than that in non-phytoremediation areas. Coverage of aquatic plants was positively correlated with TP, OP, BAP, while it was negatively correlated with BD-P. Hydrophyte stabilized and conserved active P in sediment and prevented release of active P. Moreover, hydrophyte increased NaOH-P and OP contents in sediment through regulating the abundance of phosphorus-solubilizing bacteria (PSB), such as Lentzea and Rhizobium. Four sources were identified through two multivariate statistical models. RW and runoff were the dominant sources of P accounting for 52.09 %, which mainly contributed to P in sediment especially IP.

Modeling Lake Recovery Lag Times Following Influent Phosphorus Loading Reduction.

Internal feedback of nutrients may impede timely improvement in lake water quality. We describe a parsimonious, mechanistic framework for modeling lag times to recovery of phosphorus-enriched lakes, given decreases in external loading. The approach assumes first-order kinetics in a two-compartment system taking account of phosphorus storage in and loading from benthic sediments. Bayesian parameter modeling, published sediment phosphorus release rates, and a prior dynamic calibration for one lake are used to derive estimates of key parameters. Applications are developed for an example lake, as are maps displaying estimated times to attainment of a phosphorus criterion in lakes across a midwestern state, and lag time estimates for fractional water column concentration decrease averaged over HUC-8s. Mean lag times to 50 and 75% declines in water column phosphorus concentration were estimated as 13.1 and 39.0 years respectively, across more than 70,000 lentic water bodies in the continental United States.

A palaeoecological study investigating the impacts of multiple tephra depositions on a lacustrine ecosystem in Northeast China, using diatoms as environmental indicators

Tephra layers are common in lake sediments and although they have often been used as chronological controls, few studies have investigated the impacts of past tephra depositions on lake ecosystems (Tephropalaeoecology). For the first time we systematically assess how different types of tephra layers vary in their ecological impact on the same lacustrine system. We use a diatom-based tephropalaeoecological approach to infer the impacts of five tephra deposits on Lake Sihailongwan, a well-studied volcanic lake in Northeast China, over the past 30,000 years. The five tephra layers (including two micro-tephras) have varying thicknesses and were deposited in time periods with different climatic conditions. Changes in diatom communities and chrysophyte-cyst concentrations between pre- and post-tephra samples were used to infer changes in lake conditions and highlight the importance of lake background conditions in mediating the impact of tephra. While the two micro-tephra layers did not cause observable changes, the three thicker tephras induced pronounced changes in lake conditions and thus diatom communities. The two thick tephras deposited in more eutrophic and warmer lake conditions caused larger responses from diatoms. We argue that water-column phosphorus decreased due to reduced sediment–water-phosphorus loading as thick tephra layers formed an impermeable layer at the lake bottom. This is supported by a decrease in total diatom concentration and a decline in high phosphorus-requiring taxa such as Discostella stelligeroides and Stephanodiscus minutulus as well as modern limnological observations which showed that groundwater influxes from the lake bottom are the main source of nutrients to the lake. By contrast, the thick tephra deposited in more oligotrophic and colder lake conditions caused less conspicuous changes. When the lake was already low in phosphorus, diatoms did not respond to a further decline in phosphorus but rather responded to the minor increase in silica from the dissolution of tephra particles in the water column. This was inferred from the slight increases in overall diatom concentration and opportunistic taxa such as Pantocsekiella comensis f. minima. Diatom analysis of the post-tephra sediments above the three thick tephras showed that the aquatic ecosystem did not completely recover, indicating the long-lasting effects of these thick tephras and shifts to new lake-ecosystem equilibria.

Sediment and water-column phosphorus chemistry in streams at baseflow across varying catchment geologies

ABSTRACT Streams can attenuate inputs of phosphorus (P) and therefore dampen the likelihood of ecosystem eutrophication. This P attenuation derives from many processes and remains poorly understood, particularly in reference to the geochemical mechanisms involved. We studied P attenuation in the form of (1) potential for mineral (co-)precipitation from the water column and (2) P sorption with benthic sediments. We hypothesized that both mechanisms vary with catchment geology and that P sorption depends on sediment reactive iron (Fe). We sampled 31 streams at baseflow, covering a gradient of P inputs (via land use), hydrological characteristics, and catchment geologies. Geochemical equilibria in the water column were measured and benthic sediments were analyzed for sorption properties and P and Fe fractions. Neither P-containing minerals nor calcite-phosphate co-precipitation had the potential to form. However, in-stream dissolved reactive P (DRP) correlated with labile sediment P (water soluble and easily reduced Fe-P) for streams where hyporheic exchange between the water column and sediment porewaters was likely sufficient. Because this labile P was associated with poorly crystalline Fe oxides, which determined P sorption capacity, we observed that more sorptive sediments were positively related to DRP concentrations. Our results suggest that DRP attenuation in these streams at baseflow depends in part on the combination of biogeochemical Fe and P cycles and the hydrological exchange with the hyporheic zone. Such combinations will likely vary spatiotemporally within a catchment and must be considered alongside inputs of P and sediment if the P concentrations at baseflow, and eutrophication risk, are to be well managed.

Dynamics of nitrogen and phosphorus profile and its driving forces in a subtropical deep reservoir.

Nitrogen and phosphorus stratification is crucial for ecosystem dynamics in deep lakes and reservoirs. It is critical for water quality management to understand the characteristics of nitrogen and phosphorus stratification and its driving forces. In this study, high-frequency total nitrogen (TN) and total phosphorus (TP) from January 2017 to October 2019 were estimated using the datasets of high-frequency buoy parameters, including water temperature, pH, chlorophyll-a, oxidation-reduction potential, dissolved oxygen, and fluorescent dissolved organic matter. The results revealed that both nitrogen and phosphorus in water column were periodically stratified. Specifically, the stratification of nitrogen and phosphorus occurred from April to December or January of the following year. Moreover, indices of TN stratification (IC-TN) and TP stratification (IC-TP) were - 0.29 ~ 0.05 and - 0.78 ~ 0.28, respectively. Significant (P < 0.01) positive correlations were observed between RWCS (an index of thermal stability) and IC-TN (or IC-TP), indicating thermal stratification may be the main driving force of nutrient stratification at inter-month scales. Further analysis indicated that the thermal stratification may affect nitrogen and phosphorus stratification though (1) influencing algal growth and (2) affecting the release of internal sources and the material exchange between water columns. Furthermore, precipitation is also suggested as an important factor affecting the stability of nitrogen and phosphorus vertical profile in the flood season. These findings may provide important information for optimizing water quality management efforts in Qiandaohu and other subtropical deep reservoirs. In addition, the knowledge of the effect of temperature and precipitation on nutrient stratification are essential to understand future ecosystem dynamics of deep reservoirs.

Effect of food and copepod trophic level on zooplankton community structure in a subtropical stratified reservoir

于2014年10-12月，采集厦门溪东水库浮游动物、浮游植物、浮游细菌与悬浮物样品，分析浮游动物群落与食物质量和食物浓度的关系，探讨桡足类营养级变化对浮游动物的影响.结果表明，空间上后生浮游动物群落结构在水库不同水层间的差异不显著；时间上桡足类在水华期和非水华期的差异不显著，枝角类和轮虫在水华期和非水华期的差异显著.桡足类群落组成受食物质量的影响较大，枝角类群落组成受食物浓度的影响较显著，轮虫群落组成受食物质量和食物浓度的共同影响.进一步分析表明，食物质量和食物浓度与水温显著相关，枝角类生物量受食物浓度的影响，桡足类生物量与轮虫生物量呈显著相关.为了探究桡足类与食物质量的关系，我们以优势种舌状叶镖水蚤（Phyllodiaptomus tunguidus）为对象分析化学计量特征，发现舌状叶镖水蚤体内的磷含量与水柱总磷平均浓度和水温呈显著正相关，其营养级变化范围为1.28~2.84，且营养级随着水温下降而显著增加.非水华期的食物磷含量增加可能是舌状叶镖水蚤丰度升高的原因之一，降温和种群繁殖的共同影响导致非水华期舌状叶镖水蚤体内磷含量逐渐下降，营养级增加.研究结果可为揭示浮游动物在富营养化内陆水体中的群落演变过程机制提供基础数据.;To analyze the relationship between zooplankton community structure and food quality or food concentration, and to explore the effect of copepod trophic level change on zooplankton community, the suspended matter, free-living bacteria, phytoplankton and zooplankton samples were collected in Xidong Reservoir in Xiamen from October to December of 2014. The results showed that the community structure of zooplankton did not exhibit significant difference along the water column. The community structure of copepod between the cyanobacterial bloom stage and the non-bloom stages was similar, whereas the cladocera and rotifera during cyanobacterial bloom and non-bloom stages were significantly different. Further, our results showed that the effect of food quality on copepod was significant, the effect of food concentration on cladocera was significant, while the effect of food quality and food concentration on rotifera was significant. Food quality and food concentration were significantly correlated with water temperature, copepod biomass was significantly related with rotifera biomass, cladocera biomass was significantly correlated with food concentration. In order to explore the relationship between copepod and food quality, the dominant species (Phyllodiaptomus tunguidus) was selected to analyze the stoichiometric characteristics. The results showed that the phosphorus content of P. tunguidus body was significantly and positively correlated with the average concentration of total phosphorus in water column and water temperature. The trophic level of P. tunguidus varied from 1.28 to 2.84 during the investigation period, and the trophic level increased significantly with the decrease of water temperature. Our results showed that the high quality of food in non-bloom stage was one of the reasons for the high abundance of P. tunguidus. In non-bloom stage, the joint effects of water temperature decrease and population reproduction led to decrease in the phosphorus content of P. tunguidus, and increase in the trophic level. These results provide the fundamental data for revealing the dynamic processes and mechanism of zooplankton community in eutrophic inland waters.

Mitigation of CyanoHABs Using Phoslock® to Reduce Water Column Phosphorus and Nutrient Release from Sediment.

Cyanobacterial blooms can be stimulated by excessive phosphorus (P) input, especially when diazotrophs are the dominant species. A series of mesocosm experiments were conducted in a lake dominated by a cyanobacteria bloom to study the effects of Phoslock®, a phosphorus adsorbent. The results showed that the addition of Phoslock® lowered the soluble reactive phosphate (SRP) concentrations in water due to efficient adsorption and mitigated the blooms. Once settled on the sediments, Phoslock® serves as a barrier to reduce P diffusion from sediments into the overlying waters. In short-term (1 day) incubation experiments, Phoslock® diminished or reversed SRP effluxes from bottom sediments. At the same time, the upward movement of the oxic–anoxic interface through the sediment column slightly enhanced NH4+ release and depressed N2 release, suggesting the inhibition of nitrification and denitrification. In a long-term (28 days) experiment, Phoslock® hindered the P release, reduced the cyanobacterial abundance, and alleviated the bloom-driven enhancements in the pH and oxygen. These results suggest that, through suppression of internal nutrient effluxes, Phoslock® can be used as an effective control technology to reduce cyanobacteria blooms common to many freshwater systems.

Animal aggregations promote emergent aquatic plant production at the aquatic-terrestrial interface.

The roles mobile animals and abiotic processes play as vectors for resource transfers between ecosystems ("subsidies") are well studied, but the idea that resources from animals with limited mobility may be transported across boundaries through intermediate taxa remains unexplored. Aquatic plants ("macrophytes") are globally distributed and may mediate transfers of aquatic-derived nutrients from aggregations of aquatic animals to terrestrial ecosystems when consumed by terrestrial herbivores. We used mesocosms (94×44cm) to test whether aquatic animal-generated biogeochemical hotspots increase growth and nutrient content in macrophytes using the macrophyte Justicia americana and freshwater mussels. Justicia americana biomass production increased and belowground biomass allocation changed with increasing mussel density. At high mussel density, water-column phosphorus increased and carbon:phosphorus ratios in J. americana tissues decreased. We deployed motion-sensing cameras to explore herbivory on J. americana growing along the margins of the Kiamichi River, Oklahoma, and documented feeding by large mammals (Odocoileus virginianus, Sus scrofa, and Bos taurus). Thus, biogeochemical hotspots generated by aquatic animal aggregations can promote macrophyte production that subsequently is transferred to terrestrial animals. More broadly, this suggests that reductions in aquatic animal biomass may have bottom-up impacts that indirectly affect terrestrial ecosystems via plant-animal interactions bridging ecosystem boundaries.

Water quality simulation for rehabilitation of a eutrophic lake in Selangor, Malaysia

More than 50 % of lakes worldwide, including 60 % in Malaysia, are contaminated by eutrophication due to excessive nutrient inputs. These nutrients include phosphorus and nitrogen derived from agriculture, industry and domestic wastes. Eutrophication is an undesirable lake condition characterized by turbid water and dominated by abundant growth of algae, resulting in suppressed macrophytes and in reduced ecosystem services. Healthy lakes provide ecosystem services that have been valued to be worth billions to the Malaysian Gross Domestic Production annually. Rehabilitation of eutrophic lakes located in urban areas is therefore urgently needed. A recreational lake located in Selangor Malaysia has shown persistent signs of eutrophication with chlorophyll-a level fluctuating between 10 and 25 µg/L. Left untreated, the lake water quality will deteriorate further, potentially inducing a regime shift to hypereutrophic condition, caused by intense interaction between water column phosphorus and sediment phosphorus. Based upon water and sediment quality data compiled over the last eight years, this paper presents analysis and model simulations for implementation of a sustainable program for controlling eutrophication using low cost technology.

Effects of eutrophication on sedimentary organic carbon cycling in five temperate lakes

Abstract. Even though human-induced eutrophication has severely impacted temperate lake ecosystems over the last centuries, the effects on total organic carbon (TOC) burial and mineralization are not well understood. We study these effects based on sedimentary records from the last 180 years in five Swiss lakes that differ in trophic state. We compare changes in TOC content and modeled TOC accumulation rates through time to historical data on algae blooms, water column anoxia, wastewater treatment, artificial lake ventilation, and water column phosphorus (P) concentrations. We furthermore investigate the effects of eutrophication on rates of microbial TOC mineralization and vertical distributions of microbial respiration reactions in sediments. Our results indicate that the history of eutrophication is well recorded in the sedimentary record. Overall, eutrophic lakes have higher TOC burial and accumulation rates, and subsurface peaks in TOC coincide with past periods of elevated P concentrations in lake water. Sediments of eutrophic lakes, moreover, have higher rates of total respiration and higher contributions of methanogenesis to total respiration. However, we found strong overlaps in the distributions of respiration reactions involving different electron acceptors in all lakes regardless of lake trophic state. Moreover, even though water column P concentrations have been reduced by ∼ 50 %–90 % since the period of peak eutrophication in the 1970s, TOC burial and accumulation rates have only decreased significantly, by ∼ 20 % and 25 %, in two of the five lakes. Hereby there is no clear relationship between the magnitude of the P concentration decrease and the change in TOC burial and accumulation rate. Instead, data from one eutrophic lake suggest that artificial ventilation, which has been used to prevent water column anoxia in this lake for 35 years, may help sustain high rates of TOC burial and accumulation in sediments despite water column P concentrations being strongly reduced. Our study provides novel insights into the influence of human activities in lakes and lake watersheds on lake sediments as carbon sinks and habitats for diverse microbial respiration processes.

The Contribution of Cyanobacteria Bloom Decline to Phosphorus in Water Column of Dianchi Lake, China

The Contribution of Cyanobacteria Bloom Decline to Phosphorus in Water Column of Dianchi Lake, China

Distribution, sedimentation, and bioavailability of particulate phosphorus in the mainstream of the Three Gorges Reservoir

The transportation and sedimentation of particulate phosphorus (PP) in a huge reservoir such as the Three Gorges Reservoir (TGR) are closely related to the phosphorus distribution characteristics and nutritional status of the water body. In this study, the PP distribution, sedimentation, and bioavailability in the mainstream section of the TGR were investigated through a field survey, indoor simulated settlement experiment, and historical data analysis. The results indicated that PP was the major component of the total phosphorus (TP) and that the Three Gorges Dam (TGD) trapped nearly 76.25% of suspended sediment (SS) and 75.35% of PP in the TGR, even during the flood season. A decline in flow velocity promoted the deposition of PP; additionally, PP concentrations gradually dropped from 0.35 mg/L in Chongqing to 0.02 mg/L in Zigui. The static PP sedimentation process adequately fitted a pseudo-second-order kinetic equation with a maximum correlation coefficient of 0.97. Moreover, more than half of the PP sedimentation process was achieved in less than 60 min for samples collected from the upper river reaches within simulated sedimentation process. The median particle size of SS and absolute value of the water column's zeta potential were negatively and positively related to the t12 values of PP sedimentation, respectively. Compared with the concentration and particle size of SS obtained in the pre-TGR period, the values in the mainstream section of the TGR were lower. However, the TP and Fe/Al–P contents in SS increased several times. Due to the combined effects of flow velocity reduction and SS trapping, the water transparency and bioavailability of water column phosphorus were enhanced. Thus, the risk of water bloom outburst significantly increased when the impounded water level of 175 m in the TGR became the normal state.

Nutrient Utilization Strategies of Algae and Bacteria after the Termination of Nutrient Amendment with Different Phosphorus Dosage: A Mesocosm Case

ABSTRACTThe impacts of nutrient amendment termination on the growth strategies of algae and bacteria were conducted in experimentally designed mesocosm in which two different phosphorus (P) dosages were treated. The algal community composition did not change greatly in Group A (low phosphorus) and Group B (high phosphorus). In Group A, the secretion of bacterial alkaline phosphatase (AP) after nutrient termination stimulated bacterial phosphorus acquisition, which caused the decrease in algal phosphorus levels, in terms of the increase of bacterial abundance and bacterial production, as well as the decrease in chlorophyll a and particulate organic carbon. The algal collapse resulted in dissolved organic carbon secretion, further fuelling bacterial growth. In Group B, excess phosphorus input urged algae to store phosphorus as poly-phosphate. When phosphorus input ceased, in order to maintain their used high phosphorus demand, algae strengthened to gain phosphorus through the hydrolysis of dissolved organic phosphorus in water column and ploy-phosphate inside the cells by AP, evidenced by high algal alkaline phosphatase activity, algal growth continuation, and bacterial growth decline. These facts indicated that phosphorus content should reduce to a lower level than expected, so that algal bloom can be effectively controlled in eutrophic water bodies.

Spatial distributions of external and internal phosphorus loads in Lake Erie and their impacts on phytoplankton and water quality

Re-eutrophication in Lake Erie has led to new programs to reduce external phosphorus loads, and it is important to understand the interrelated dynamics of external and internal phosphorus loads. In addition to developing phosphorus load response curves for algal biomass in the western basin and hypoxia in the central basin, we used a two-dimensional (vertical-longitudinal) hydrodynamic and ecological model to show that both external and internal phosphorus loads were distributed homogeneously in the water column in Lake Erie's western basin. In the stratified central and eastern basins phosphorus released by organic matter decay and crustacean zooplankton excretion was concentrated in the upper water column, contributing 100–119% of the phytoplankton phosphorus demand, while phosphorus released by dreissenids and from anoxic sediments was distributed primarily in the hypolimnion during the growing season. Simulated reductions in external phosphorus loads decreased individual phytoplankton groups most at times when they were normally most abundant, e.g., Microcystis decreased the most during September. Phosphorus was limiting over the simulation periods, but water temperature and light conditions also played critical roles in phytoplankton succession. While water column phosphorus responded quickly to external phosphorus reduction, pulses of phosphorus (riverine input or sediment resuspension) occurring immediately before the Microcystis bloom period could allow it to bloom despite long-term external phosphorus load reduction. Studies are warranted to assess the contribution of seasonal dynamics in phosphorus loading (including sediment resuspension) to Microcystis bloom development.

Sediment–water interaction in phosphorus cycling as affected by trophic states in a Chinese shallow lake (Lake Donghu)

Lake sediment substantially accumulates nutrients, while little is known regarding the conditions under which it tends to be a sink or source of phosphorus. It is postulated that the above functions were largely dependent on trophic state. To test this hypothesis, composition, abundance, and size-spectrum of phytoplankton were studied in a Chinese shallow lake (Lake Donghu), together with concentrations and sorption behaviors of phosphorus in water column and sediment. Relationships among these variables were also examined by structural equation model. In the basins with the lowest phosphorus concentration, sediment was a phosphorus source as judged by equilibrium phosphorus concentration, directly affecting the abundance of phytoplankton with smaller size. Contrastingly, in the basins with the highest phosphorus concentration, sediment tended to uptake phosphorus, companied by the lowest ratio of nitrogen to phosphorus, diversity, and evenness of phytoplankton with the smaller size one dominating. Oscillatoria sp. was directly affected by ammonia and phosphorus. Moreover, with a balanced exchange of phosphorus between sediment and water column, phytoplankton increased markedly with the highest diversity and evenness in the basins having intermediate phosphorus concentration. Overall, trophic state modulated sediment functions to supply nutrient and was in turn greatly affected by the sediment in shallow lakes.

TMDL reevaluation: reconciling internal phosphorus load reductions in a eutrophic lake

Total maximum daily loads (TMDLs) are assigned to impaired waterbodies and provide a prescription for recovery. It is implicit that both the existing conditions and targets identified in a TMDL are based on credible and accurate information, ensuring that management actions directed to recovery are appropriate and effective. We evaluated the TMDL for phosphorus in Bear Lake, Michigan, to assess if the prescribed reduction was appropriately based on existing annual internal loads. We developed 5 different annual internal load scenarios based on phosphorus release rates from laboratory-based sediment incubations and diel dissolved oxygen measurements, ranging from very conservative to very liberal estimates of phosphorus release. The most realistic scenarios indicate that the previously established TMDL for internal phosphorus loading is 3–7 times greater than what actually occurs in Bear Lake. Based on our assessment, Bear Lake is likely already meeting the TMDL internal loading target, without the implementation of any management action. Thus, management efforts aimed at reducing the water column phosphorus concentration to reach the TMDL target should instead be directed at controlling the external phosphorus load.

Recognizing cross‐ecosystem responses to changing temperatures: soil warming impacts pelagic food webs

The energy and materials that move across ecosystem boundaries influence food web structure and key ecosystem functions. Despite the acknowledged importance of such ecological subsidies, surprisingly little information is available regarding the role of environmental temperature in influencing subsidy quality and the response of the recipient ecosystem. We evaluated the impacts of temperature‐mediated changes in leaves from deciduous trees, an important subsidy from terrestrial to freshwater ecosystems, on both the producer‐based and detritivore‐based components of a pelagic pond food web in a field mesocosm experiment. We hypothesized that variation in leaf chemistry driven by increased soil temperature would alter both the quality of leaf subsidies and the pond response. We collected red maple Acer rubrum leaves from heated and ambient temperature plots from the long‐term soil warming experiment at the Harvard Experimental Forest and added them to 167‐l field mesocosms containing established plankton communities, creating ‘no leaf’, ‘ambient leaf’ and ‘heated leaf’ treatments during autumn 2012. We then monitored physical, chemical, and biological responses to treatments until the mesocosms froze six weeks later. Experimental soil warming altered the chemical composition of deciduous leaves, the physical and chemical environment of the aquatic ecosystems to which leaves were added, and the pelagic pond food webs as measured by community composition. Compared to leaves from ambient‐temperature soils, leaves from warmed soils initially resulted in lower water column phosphorus and dissolved organic carbon, reducing bacterial densities. However, the diminished carbon and phosphorus resulting from soil warming also increased light availability that ultimately stimulated cladoceran zooplankton relative to ambient‐temperature leaves. Our results suggest that changes in temperature can alter ecological subsidies in unanticipated ways, and suggest that accurately predicting the potential consequences of climate change will require conducting research across ecosystem boundaries.