Turbid State Research Articles

Flipping Lakes: Explaining concepts of catchment‐scale water management through a serious game

AbstractOngoing anthropogenic and climatic pressures on inland waters have made water quality management a challenge of the 21st century. A holistic catchment‐scale approach to water management which includes stakeholder participation will be a key in maintaining lake health. A first step toward community engagement is to bolster environmental literacy on lake management, ecology, and eutrophication concepts of stakeholders now and in future generations. However, communicating with nonwater professionals about effects of pollution on water quality and catchment‐scale interactions across space and time can be difficult. Here, we present “Flipping Lakes,” a games‐based method for lake professionals to communicate and educate about catchment‐level water quality management to diverse audiences. In Flipping Lakes, the players take on the role of water managers in a catchment and are tasked to prevent a lake from “flipping” from a clear to a turbid state. During the game, the catchment slowly becomes polluted by a range of sources of which the effects are exacerbated by societal or climatic scenarios. Players need to implement measures while taking into consideration the intrinsic properties of the catchment in order to keep lakes clean. The game was tested with a diverse range of user groups and was well‐received. With its entertaining and accessible content, Flipping Lakes can lower communication barriers and increase understanding of difficult water quality concepts. The game is highly customizable, making it applicable to a variety of settings to support education and engagement of stakeholders and the broader community in order to address local water challenges around the globe.

Using hidden Markov models to inform conservation and management strategies in ecosystems exhibiting alternative stable states

Abstract Successful conservation of ecosystems exhibiting alternative stable states requires tools to accurately classify states and quantify state transition risk. Methods that utilize early warning signals are promising approaches for helping managers anticipate impending state transitions, but they require high‐resolution temporal or spatial data for individual sites, and they do not directly implicate causes or quantify their impacts on transition risk. There is need for a modelling approach that can assess state transition risk with lower resolution temporal data, identify drivers of state transitions and assess the impact of changes in these drivers on the persistence of ecosystem states. We developed a novel, integrated modelling framework that (a) classifies states in a way that reflects the qualitative dynamics of catastrophic regime shifts, (b) estimates state transition probabilities and (c) uses annual survey data to identify top predictors of state transitions and quantify their effects on transition risk. We applied our model to short time series from 123 shallow lakes that exhibit clear‐ and turbid‐water alternative stable states. We found that clear lakes were more likely to transition to the turbid state as total phosphorus levels increased or occurrence of submerged vegetation decreased. Additionally, increases in planktivorous or benthivorous fish biomass elevated transition risk. Too few turbid‐to‐clear transitions were observed to identify predictors of transitions in this direction. Synthesis and applications. Our study will inform conservation and management strategies for ecosystems exhibiting alternative stable states by providing a new tool to accurately classify states, compare state transition risk among sites based on resilience and system perturbations, and identify key variables to target to prevent undesirable transitions. Although we focus on shallow lakes as a case study for our modelling approach, we emphasize that our framework is applicable to any ecosystem known to exhibit alternative stable states.

Winter Oxygen Regimes in Clear and Turbid Shallow Lakes

AbstractDissolved oxygen controls important processes in lakes, from chemical reactions to organism community structure and metabolism. In shallow lakes, small volumes allow for large fluctuations in dissolved oxygen concentrations, and the oxygen regime can greatly affect ecosystem‐scale processes. We used high frequency dissolved oxygen measurements to examine differences in oxygen regimes between two alternative stable states that occur in shallow lakes. We compared annual oxygen regimes in four macrophyte‐dominated, clear state lakes to four phytoplankton‐dominated, turbid state lakes by quantifying oxygen concentrations, anoxia frequency, and measures of whole‐lake metabolism. Oxygen regimes were not significantly different between lake states throughout the year except for during the winter under‐ice period. During winter, clear lakes had less oxygen, higher frequency of anoxic periods, and higher oxygen depletion rates. Winter oxygen depletion rates correlated positively with peak summer macrophyte biomass. Due to lower levels of oxygen, clear shallow lakes may experience anoxia more often and for longer duration during the winter, increasing the likelihood of fish winterkills. These observations have important implications for shallow lake management, which typically focuses efforts on maintaining the clearwater state.

Modelling phosphorus loading to the largest shallow lake in northern China in different shared socioeconomic pathways

Excessive nutrients inputs cause anthropogenic lake eutrophication worldwide. It is thus crucial to provide information on future trends of nutrient loading and its implications for the lake’s trophic state. Here we quantified total phosphorus (TP) loading to the Lake Baiyangdian (BYD), the largest shallow lake in northern China, and assessed their future trend and the trophic state of the lake. The TP loading to the Lake BYD by rivers in 2015 was 276 ton, which was in a relatively low level in lakes from China. Based on the five shared socioeconomic pathways (SSPs), the TP loading to the Lake BYD in 2050 will increase only at the fragmentation scenario (SSP3) by an increment of 34.4%, in which the directly discharged animal manure become the main source. The lowest TP loading in 2050 is observed at the sustainability scenario (SSP1), in which sewage system will be the dominant contributor. At the sub-basin scale, compositions of river exported TP in rural areas will be more influenced by future socioeconomic developments than those in urban areas. The twenty years nutrient dynamics from clear or turbid state in the Lake BYD were simulated by the PCLake model to calculate the critical P loading (CPL). The results indicated that the CPL in the Lake BYD was 2.06 mg P m−2 d−1 for eutrophication and 0.84 mg P m−2 d−1 for oligotrophication. The only scenario in which Lake BYD will become eutrophic is SSP3, which should be avoided in the future development. To restore the lake water to the oligotrophic state, more efforts are necessary to reduce direct discharge of animal manure and increase the TP removal rates, especially in rural areas.

Principal Determinants of Aquatic Macrophyte Communities in Least-Impacted Small Shallow Lakes in France

Small shallow lakes (SSL) support exceptionally high and original biodiversity, providing numerous ecosystem services. Their small size makes them especially sensitive to anthropic activities, which cause a shift to dysfunctional turbid states and induce loss of services and biodiversity. In this study we investigated the relationships between environmental factors and macrophyte communities. Macrophytes play a crucial role in maintaining functional clear states. Better understanding the factors determining the composition and richness of aquatic plant communities in least-impacted conditions may be useful to protect these shallow lakes. We inventoried macrophyte communities and collected chemical, climatic, and morphological data from 89 least-impacted SSL widely distributed in France. SSL were sampled across four climatic ecoregions, various geologies, and elevations. Hierarchical cluster analysis showed a clear separation of four macrophyte assemblages strongly associated with mineralization. Determinant factors identified by distance-based redundancy analysis (db-RDA) analysis were, in order of importance, geology, distance from source (DIS, a proxy for connectivity with river hydrosystems), surface area, climate, and hydroperiod (water permanency). Surprisingly, at a country-wide scale, climate and hydroperiod filter macrophyte composition weakly. Geology and DIS are the major determinants of community composition, whereas surface area determines floristic richness. DIS was identified as a determinant in freshwater lentic ecosystems for the first time.

Eutrophication-induced regime shifts reduced sediment burial ability for polycyclic aromatic hydrocarbons: Evidence from Lake Taihu in China

Regime shifts from a vegetated state with clear water to a turbid state with high contents of phytoplankton and suspended particles have been found in numerous waters worldwide. The fate and risks of hydrophobic organic contaminants (HOCs) in such waters may be altered, and the effects on burial ability of HOCs remain unknown. Influences of regime shifts on sediment burial ability for 16 polycyclic aromatic hydrocarbons (PAHs) (defined as burial/emission ratio) were investigated based on the evidence from the third largest freshwater lake (Lake Taihu) in China. The results of δ13C, δ15N, atomic ratio of Corg/N, and the content of total organic carbon testing and historical data suggested that the regime shifted abruptly from macrophytes to phytoplankton dominance in Lake Taihu in the late 1980s. The annual burial ability for the PAHs decreased gradually over time by 63.2%–98.9% in the period from 1980 to 2016. Meanwhile, the decrease rates of PAH burial ability varied from −1.65% y−1 to −2.98% y−1, depending on the hydrophobicity of the compound. The PAH burial ability varied with the dominant primary producers associated with the trophic level index of the water column. Regime shifts had a stronger influence on the burial ability of PAHs with higher hydrophobicity. This study helps to understand the fate and potential risks of HOCs in waters due to eutrophication-induced regime shifts.

Does differential phosphorus processing by plankton influence the ecological state of shallow lakes?

Shallow lakes have a tendency to settle into turbid or clear-water states, the latter having lower concentrations of total phosphorus (TP). However, how P-cycling is affected by and perhaps contributes to maintaining the different states is not well understood, in part because quantifying the processes involved by traditional methods is difficult. To elucidate these processes, we conducted experiments using 32P-PO4 as a tracer on samples collected from the unrestored, unvegetated sections of Huizhou West Lake where turbid water prevails as well as the restored, clear-water, macrophyte-rich waters of the lake. We measured PO4 uptake rates, 32P-PO4 accumulation by various plankton size-fractions (picoplankton (0.2–2 μm), nanoplankton (2–20 μm) and microplankton (>20 μm)) as well as release rates of 32P-PO4 by labelled plankton. Our results revealed slow PO4 uptake in the turbid state due to low PO4 concentration, slow recycling of the high particulate P, and high levels of particulate 32P which may allow for continuous high growth and biomass of phytoplankton. In contrast, in the clear water state, the uptake of PO4 was rapid due to a higher PO4 concentration, the recycling rates of particulate 32P were high and the levels of particulate 32P were low, potentially constraining the phytoplankton growth. A greater proportion of particulate 32P was in the microplankton fraction in clear waters, suggesting that grazing by microplankton may play an important role in the rapid P recycling in clear-waters. Our results provide some evidence for a reinforcement of the turbid conditions (low recycling rate) when the lake is in a turbid state and vice versa when in the clear water state. The results add new knowledge to the understanding of P cycling in shallow lakes and illustrate the utility of using P-kinetics in contrasting states in plankton communities.

Sedimentary Diatom Records Reveal the Succession of Ecosystem in Lake Xihu, Dali over the Past 50 Years

In recent decades, intense human activities have caused a decline in many lake ecosystems in Yunnan Province, rendering the transformation of the lake from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state. Improved understanding of the ecological changes in lake ecosystem has significant implications for management. In this study, a small lake in Dali Prefecture of Yunnan Province, i.e., Lake Xihu, was selected. Combined with diatom records and physicochemical proxies from the lake sediments, this paper focuses on the long-term ecological changes in Lake Xihu, Dali since the mid-1960s. The results show that the Lake Xihu, Dali has undergone a significant shift in stable states over the past 50 years. Prior to 2000, the benthic-epiphytic species (i.e., Cocconeis placentula, Staurosira construens, Gomphonema angustum, and Achnanthidium minutissimum) dominated in diatom assemblages, indicating oligotrophic conditions; since 2000, benthic-epiphytic species (i.e., Encyonopsis microcephala and Navicula cryptocephala) and planktonic species (i.e., Cyclotella atomus, Cyclotella meneghiniana, Stephanodiscus hantzschii, and Aulacoseira granulata) dominated successively, indicating mesotrophic to eutrophic conditions. Principal component analysis based on the diatom assemblages in temporal scale showed the response of diatoms succession to nutrients. Redundancy analysis also confirmed that nutrient enrichment was the main driving force for the succession of diatom assemblages in Lake Xihu, Dali. In the past 50 years, climate change and human activities (i.e., agricultural reclamation, fertilization, animal husbandry, and fishery) have enhanced the accumulation of nutrients in the lake. The continuous loading of nutrients promoted the propagation of planktonic algae, and also the productivity of the lake, rendering the transformation to a turbid phytoplankton-dominated state.

Role of submerged macrophytes in sediment phosphorus stabilization in shallow lakes from the Brazilian semiarid region

ABSTRACT Allochthonous or autochthonous disturbances may change waterbodies from a clear to a turbid state, increasing the availability of nutrients and promoting blooms of harmful cyanobacteria. Submerged macrophytes may play an important role in alleviating the adverse effects of phosphorus resuspension and release from bottom sediments. We assessed the effects of allochthonous disturbances (rainfall and wind) on phosphorus release by constructing models using data from 2 shallow lakes with contrasting steady states. Using partial least squares path modeling, we examined submerged macrophytes, phytoplankton, and possible drivers of their abundance. In the clear-water lake, the concentrations of total phosphorus (TP) had the greatest direct influence on the macrophyte percentage of volume infested and phytoplankton biomass, followed by concentrations of TP in the sediment. In the turbid lake, the TP concentration in the water was the most important factor affecting phytoplankton biomass directly, followed by disturbances on TP concentration in the sediment and euphotic zone. Our models support the hypothesis that submerged macrophyte coverage acts as a stabilizing force to prevent the proliferation of cyanobacteria and stabilize the clear-water phase in semiarid lakes. However, potential mutual inhibition mechanisms between the submerged macrophyte Nitella cernua and the cyanobacterium Microcystis aeruginosa need to be better addressed for restoration purposes in lakes with shallow clear waters.

The potential persistence of abundant submerged macrophyte and phytoplankton in a shallow system at very high nutrients loading: results from a mesocosm study.

Shallow lakes are typically either in a clear water state with high abundance of submerged macrophytes or in a turbid state dominated by phytoplankton. A 10-week mesocosm experiment was set up to test the hypothesis that in a very shallow aquatic ecosystem impacted by very high nutrient loading, both submerged macrophytes and phytoplankton may be abundant. Nutrient levels, phytoplankton biomass (measured as chlorophyll a, Chl a), total suspended solids (TSS), inorganic suspended solids (ISS), and submerged macrophyte biomass were measured in three different treatments: DH (deep (1 m) water with very high nutrient loading), SH (very shallow (0.5 m) water with very high nutrient loading), and SL (very shallow (0.5 m) water with low nutrient loading). Of the three treatments, the DH mesocosms exhibited the highest nutrient, the TSS and ISS levels, the highest phytoplankton biomass, and the lowest submerged macrophyte biomass, while the lowest nutrient, the TSS and ISS levels, the lowest phytoplankton biomass, and the highest submerged macrophyte biomass were observed in the SL treatment. All measured parameters exhibited intermediate levels in the SH treatment. Our study demonstrates that in very shallow aquatic ecosystems with very high nutrient loading, submerged macrophytes and phytoplankton may persist in an intermediate but potentially unstable state with medium water clarity. These findings may contribute to the understanding of the mechanisms involved in regime shifts in shallow lakes.

Regime shifts in shallow lakes observed by remote sensing and the implications for management

Abstract Shallow lakes are one of the most complex aquatic systems and are known to shift between a macrophyte-dominated clear-water state and a phytoplankton-dominated turbid state. We developed a decision-tree classification based on the normalized difference vegetation index (NDVI) to classify the major vegetation cover types in shallow lakes and documented the vegetation in Gehu Lake using Landsat time series images from 1984 to 2018. The area of aquatic vegetation in Gehu Lake (a large shallow lake in China) showed a significant decreasing trend (R2 = 0.839, p

Decreasing underwater ultraviolet radiation exposure strongly driven by increasing ultraviolet attenuation in lakes in eastern and southwest China

Underwater light attenuation plays an important role in modulating aquatic ecosystems and is considered a sentinel of climate change and human activity. However, knowledge of the long-term exposure of underwater ultraviolet radiation (UVR) in aquatic ecosystem is still very limited. We carried out extensive UVR measurements in different seasons in five lakes at different altitudes, collected long-term Secchi disk depth (SDD) data, developed the models between UVR diffuse attenuation coefficient (Kd) and SDD, and further assessed the long-term underwater UVR exposure. Observation results from five lakes including 259 samples showed large spatial variabilities of Kd(313) (UVB) from 0.83 to 5.91 m−1 and Kd(340) (UVA) from 0.51 to 4.67 m−1. Chromophoric dissolved organic matter (CDOM) absorption coefficients were significantly correlated with Kd(313) and Kd(340). Thus, the effects of climate change and human activity on CDOM abundance, source and composition may significantly alter UVR attenuation in aquatic environments. The long-term underwater UVR exposure, which was estimated from significant positive correlations between 1/SDD and Kd(313) and Kd(340), and incident UVR, significantly decreased in Lake Fuxianhu, Lake Erhai, and Lake Qiandaohu. The regime shift from clear water state to turbid state in Lake Erhai around 2001–2003 dramatically decreased underwater UVR exposure. In conclusion, increasing UVR attenuation played a more important role in determining underwater UVR exposure than decreasing incident UVR with the relative contributions of 89.9% and 87.7% in Lake Fuxianhu, 98.0% and 97.7% in Lake Erhai, 94.4% and 92.5% in Lake Qiandaohu for UVB and UVA exposure, respectively. This is the first study to elucidate the long-term trend of underwater UVR exposure considering both increasing UVR attenuation and decreasing incident UVR.

Decadal trends and ecological shifts in backwater lakes of a large floodplain river: Upper Mississippi River

Shallow lakes are typically found in one of two stable states, a macrophyte-dominated clear water state or a turbid state due to excessive phytoplankton and suspended sediment. Whether shallow backwater lakes in large river floodplains exhibit similar alternate stable states is less understood. This study considers mechanisms, interactions and feedbacks associated with a shift in environmental conditions and biotic community structure in backwater lakes of a hydrologically dynamic floodplain river system. We use long-term data from backwater lakes to show an increase in submersed aquatic vegetation, improved water quality, and resulting shifts in the community structure of aquatic vegetation and fish following a 4 year period of summer low water discharge on the Upper Mississippi River. Backwater lakes in our study span a gradient of environmental conditions. Backwater lakes located in the upper reach of our study area were chronically turbid and support only sparse aquatic macrophytes, whereas those downriver exhibited clearer water and abundant vegetation. An increase in submersed aquatic vegetation in the lower backwater lakes resulted in a fish community shift to more vegetation-associated species. A lesser response in submersed aquatic vegetation abundance and fish community shift was observed in the upper, more turbid backwater lakes. The combination of vegetative cover and turbidity were key environmental variables associated with fish community structure in lower backwater lakes. Turbidity was the key environmental variable associated with submersed aquatic vegetation in both upper and lower backwaters. Providing further insight into the physical, chemical and biological interactions associated with ecological shifts will help guide management and restoration decisions towards more resilient, macrophyte-rich floodplain backwaters.

Effects of warming, climate extremes and phosphorus enrichment on the growth, sexual reproduction and propagule carbon and nitrogen stoichiometry of Potamogeton crispus L

Contemporary evidence suggests that submerged macrophytes are experiencing a global decline due to the multiple compounding anthropogenic stressors impacting shallow lake ecosystems. Eutrophication and climate change are two main widespread, often co-occurring stressors, yet evidence concerning their interactive effects on aquatic plants remains partial and fragmented. Predicting the response of submerged aquatic vegetation to the combined effects of nutrient pollution and compound climate warming (mean + variability) is therefore crucial for the conservation and management of these valuable and vulnerable ecosystems. Here, we present the results of an outdoor mesocosm experiment examining the combined effects of nutrient enrichment (phosphorus addition) and warming (a 4 °C increase in mean temperature above present ambient conditions applied as either a constant increase or a variable increase ranging between 0 and 8 °C to mimic the effect of extreme events but keeping an equivalent total amount of warming) on Potamogeton crispus L. Warming accelerated the growth and senescence of P. crispus suggesting a more important role in maintaining the clear water state in winter-early spring but concomitant to possible earlier turbid states in summer. Warming also consistently advanced the flowering phenology but had no significant effect on flowering duration. There were no significant differences in the life cycle between the two warming treatments, while phosphorus addition also had little effect. However, under phosphorus enrichment, P. crispus increased sexual reproduction investment producing higher seed setting rate per infructescence. In contrast, warming, especially variable warming, may decrease sexual reproduction investment by reducing the number of infructescences. Seed and turion stoichiometry were altered by the combination of warming and phosphorus addition, but the changes were complex and difficult to interpret.

Distinct shifts in bacterioplankton community composition and functional gene structure between macrophyte‐ and phytoplankton‐dominated regimes in a large shallow lake

AbstractShallow lakes often exhibit a macrophyte‐dominated (MD) clear state or a phytoplankton‐dominated (PD) turbid state, each of which has profound impacts on biological communities, including bacterioplankton community composition (BCC). However, the response of microbial functional gene structure (FGS) to regime shift has not been thoroughly elucidated to date. In this study, we used a functional gene array (GeoChip 5.0) in combination with high‐throughput sequencing of the bacterial 16S rRNA gene to investigate the FGS and BCC between the two regimes in Lake Taihu, a large shallow freshwater lake in China. A simultaneous change in the BCC and FGS was observed between the two different regimes. We observed higher numbers of bacterial operational taxonomic units and functional genes detected by the GeoChip 5.0 in the MD samples than in the PD samples. By performing network analyses, we observed stronger microbe‐microbe interactions but weaker functional gene interactions involved in nutrient metabolism in the MD community than in the PD community. We speculate that at higher concentrations of phosphorus, the domination of Microcystis bloom may weaken the interactions among microbial species by releasing more labile dissolved organic carbon, but enhance the selection and interactions of functional genes involved in carbon cycling in the PD community. In contrast, the lower levels of phosphorus may enhance microbe‐microbe interactions but weaken the interactions between the primary nutrients and particular genes. Overall, this study demonstrated contrasting patterns in the BCC and FGS between the MD and PD communities in a large shallow freshwater lake.

Modeling Decreased Resilience of Shallow Lake Ecosystems toward Eutrophication due to Microplastic Ingestion across the Food Web.

The discovery of microplastic (MP) being present in freshwaters has stimulated research on the impacts of MP on freshwater organisms. To date, research has focused on primary effects, leaving questions with respect to secondary effects at the level of freshwater food webs unanswered. Here, we use a theoretical modeling approach to investigate the hypothesis that MP imposes negative impacts on the level of freshwater shallow lake food webs. We find that increasing MP levels have the potential to affect the critical phosphorus loading (CPL), which is defined as the threshold for regime shifts between clear and turbid states of the water column. The possible occurrence of catastrophic cascades due to MP pollution is predominantly driven by the negative effects of MP on zooplankton. We explore the possible states of the food web by scenario analysis and show that the secondary effects of MP at current concentrations are likely to be negligible. However, at the current rate of MP production, a 20-40% reduction in the CPL would occur by the end of this century, suggesting a loss of resilience in shallow lakes that would be subject to abrupt changes in the food web under lower nutrient loading.

Relationship between bream (Abramis brama) activity and water turbidity in a shallow lake under different season conditions

The foraging activity of large-bodied benthivorous fish has been suggested to be of key importance for maintaining shallow lakes in a turbid state. Since especially the spring ecosystem dynamics determines the successive state of shallow lakes, such impact could depend on seasonality in benthivore activity patterns. However, we do not yet know to what extent the activity of large-bodied benthivorous fish affects lake turbidity across the year. In order to investigate seasonal dynamics in bream activity and its impact on water turbidity under natural conditions, bream daily activity was studied in a small (39 ha) shallow Danish lake using passive biotelemetry technology, i.e. a modified Passive Integrated Transponder (PIT)-tag antenna system. We tracked the activity of 448 benthivorous bream over a period of four years (2012 to 2016) and during the same period wind conditions, water turbidity and temperature was measured. Results showed a clear relationship between bream activity and water turbidity at water temperature below 15°C indicating that winter season activity of benthivorous bream may play an important role for maintaining lake ecosystems in a turbid state. Also wind speed and wind direction affected water turbidity, suggesting that wind induced resuspension can be important even in small shallow lakes. This is to our knowledge the first full-scale study under natural conditions to describe how bream activity influence lake turbidity on a day-to-day basis. Our findings also add a seasonal component to previous findings by showing that benthivorous feeding bream have the potential to increase water turbidity also in the winter season and thereby, ultimately, impact ecosystem functioning within shallow lakes.

The effect of a shift from macrophyte to phytoplankton dominance on phosphorus forms and burial in the sediments of a shallow hard-water lake

In shallow lakes, increasing phosphorus (P) loading has often been accompanied by a shift from a clear-water, macrophyte-dominated state to a turbid state featuring phytoplankton dominance. The effect of a regime shift on P burial and P fractions in lake sediments, however, is poorly understood. We used sediment cores from a eutrophic hard-water lake (Lake Gollinsee, Germany) that had undergone a regime shift (in approximately 1917) to investigate the effect on the accumulation rate of P and on changes in P forms. The cores were dated using Hg contents and radioisotopes (210Pb, 137Cs, and 241Am). A combination of total organic carbon to total nitrogen ratios (TOC:TN), δ13TOC values, X-ray fluorescence calcium (Ca) counts, and sediment colour clearly distinguished sediment layers that were deposited during periods of macrophyte or phytoplankton dominance. The accumulation rate of total P (TP) in the sediments was 1.8 times higher after the regime shift and was associated with changes in the distribution of P fractions. The proportions of loosely-(NH4Cl-extracted TP) and Ca-(HCl-extracted TP) bound P decreased significantly, whilst the proportions of biogenic P (NaOH-extracted NRP) and aluminium-bound P (NaOH-extracted SRP) increased significantly. A higher dry mass deposition rate, reduced burial of stable Ca-P complexes, and increased contents and proportions of the mobile iron-bound (BD-extracted TP) and biogenic P fractions in the near-surface sediment layers are assumed to have enhanced the internal cycling of P and hence to have helped to maintain a state of phytoplankton dominance.

Experimental test of abiotic and biotic factors driving restoration success of Vallisneria americana in the Lower Bay of Green Bay

Abstract The eutrophication of aquatic habitats is a primary driver of ecosystem degradation, often culminating in a switch from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. While numerous studies have documented the ecological implications of this switch, subsequent reductions in nutrient and sediment loading do not consistently result in predictable reversals to macrophyte dominance. Re-introduction of rooted aquatic macrophytes at appropriate scales and species combinations may disrupt these negative feedbacks, although our current limited understanding of these complex mechanisms hinders the development of effective, targeted restoration strategies. We evaluated the potential for restoration of Vallisneria americana (wild celery) in the Lower Bay of Green Bay by altering restoration size and co-planting with the emergent species Schoenoplectus acutus (hardstem bulrush). Wild celery survival among all sites exceeded 90% in 2015 and 110% in 2016. However, in contrast to our expectations the effect of restoration size and interspecific facilitation on survival was either marginal or non-significant. Instead, various environmental effects focused largely on the interaction of water depth, substrate characteristics, and the abundance of unrestored floating macrophytes (i.e. Lemna, Ceratophyllum, and Utricularia spp.) drove variability in restoration success, suggesting that future work focus on identifying restoration methods that can withstand a highly dynamic environment. Our results provide insight into the factors continuing to limit the re-establishment of aquatic macrophytes in degraded systems, suggesting a more limited role of water quality and greater role of interspecific competition and propagule limitation or seedling establishment than previously recognized.

Using Bayesian change point model to enhance understanding of the shifting nutrients-phytoplankton relationship

Possibility of shifting nutrients-phytoplankton relationship in lakes requires methods with the ability to testify abrupt relationship change to facilitate efficient management. Bayesian change point model (BCPM) can handle multiple shifts in coefficients and/or in residual errors and therefore suits this requirement. We employed BCPMs to enhance understanding of the shifting nutrients-Chlorophyll a (Chla) relationship in Yilong Lake, which has undergone a regime shift from clear state to turbid state. We developed four candidate models to simulate nutrients-Chla relationship. Model selection results showed the relationship has changed and only one change point exists. Further research based on the selected model showed that (1) the change point was around the 96th observation, (2) nutrients increase did not drive the relationship change, (3) total phosphorus (TP) plays a more important role than total nitrogen on Chla increase, and (4) nutrients reduction is a better strategy than ecosystem recovery to effectively reduce Chla concentration. Therefore, TP reduction should have the priority in Yilong Lake. BCPM is convenient for model selection, posterior distribution acquisition, and relationship change quantification. It could provide critical information for causality deduction. These characters make it useful and extendable to explore shifting relationships in ecological field.