During the past decades, on-line monitoring of freshwater lakes has developed rapidly. To use high frequency time-series in lake management, novel models are needed that are simple and provide ...insight into the complexity of phytoplankton dynamics. Chlorophyll a (Chl), a proxy for phytoplankton biomass and environmental drivers were monitored on-line in large, shallow Lake Balaton during the vegetation periods between 2001 and 2018. Growth and non-growth (G and non-G) states of algae were deduced from daily change in Chl. Random forests (RF) were used to find stochastic response rules of phytoplankton to growth-supporting environmental habitat templates. The stochastic G/non-G state was translated into long-term daily biomass dynamics by a deterministic biomass model to assess uncertainty and to distinguish between inevitable and unpredictable blooms. A biomass peak was qualified as inevitable or unpredictable if the lower 95% confidence limit of simulations exceeded or remained at the baseline Chl level, respectively. Compared to a stochastic null model based on monthly Markovian transition probabilities, RF-based models captured wax and wane of biomass realistically. Timing of peaks could be better simulated than their magnitude, likely because habitat templates were primarily determined by light whereas peak sizes might depend on unmeasured processes, such as phosphorus availability. In general, algal growth was favored by wind-induced sediment resuspension that decreased light availability but simultaneously enhanced the P supply. Seasonal temperature and an integral of departures from the “normal” seasonal temperature over 2 to 3 generations were important drivers of phytoplankton growth, whereas short-term (diel and day to day) changes in water temperature appeared to be irrelevant. Four types of years could be distinguished during the study period with respect to algal growth conditions. The present modeling approach can reasonably be used even in highly variable aquatic environments when 3 to 4 years of daily data are available.
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•To use on-line time-series in lake management, new phytoplankton models are needed.•Chlorophyll and its drivers were recorded for 18 years in a large, shallow lake.•Random forests were used to predict stochastic response rules of algae.•Low light and elevated P supply due to sediment resuspension favored growth.•Application of the model is recommended with 3–4 years of high frequency data.
After 25 years of apparently successful eutrophication management, a record‐breaking mixed bloom of Ceratium furcoides and Aphanizomenon flos‐aquae developed unexpectedly in 2019 in the large, ...polymictic Lake Balaton. The peak concentration of chlorophyll exceeded 300 mg/m3, 1.5 times higher than the pre‐management maxima. The external load was insufficient to provide the phosphorus (P) required to support this bloom, and its taxonomic composition was radically different from blooms in the 1980s. We hypothesised: (1) that unusually long periods of intermittent stratification led to anoxic P release from the sediments, providing the required P; and (2) that the surprising bloom composition indicated a potential regime shift.
To test hypothesis 1, we analysed decade‐long high‐frequency data on drivers of lake metabolism recorded at a shallow near‐shore observatory. To explore likely dissolved oxygen (DO) conditions in deep water in 2019, we performed a sensitivity analysis of a one‐dimensional DO model driven by the General Ocean Turbulence Model, the latter being calibrated against high‐frequency hydrometeorological data measured in the open water. Hypothesis 2 was examined by a sequential t‐test analysis applied to multidecadal (1976–2020) data on phytoplankton biomass and composition, nutrient loads and summer mean air temperature.
We found that DO depletion (<1 g/m3) was the immediate precursor of the large 2019 bloom. The synergistic impact of a climatic regime shift and multiple coincident management actions provoked a shift in phytoplankton composition in 2013 and forced the ecosystem into the state of internal eutrophication.
In the present ecosystem state, similarly large blooms could develop in any summer depending on the sequence of weather events. Within the present, warmer climate regime identified here, the effectiveness of external nutrient control in eutrophication management may be limited by changes in the thermal structure. Therefore, it is vital to supplement external nutrient control with internal nutrient control strategies, such as water level regulation, to manage eutrophication and prevent future blooms.
Large, continuous polymictic lakes are susceptible to small changes in water level, because the likelihood of stratification (and thus the likeliness of a eutrophication event) increases steeply and non‐linearly with increasing depth. With a climate that continues to warm, and the associated changes expected in internal lake processes, it is possible that we will see regime shifts (such as that identified in Lake Balaton) across more lakes globally. Managing these conditions will require case‐specific modifications of lake management plans.
High frequency time series of dissolved oxygen (DO), delayed chlorophyll fluorescence (Chl) and relevant background variables were recorded on nearly 900 d during 7 yr in large, shallow, ...meso-eutrophic Lake Balaton (Hungary). Novel models were developed for coupled simulation of diel dynamics of DO and Chl using sequential learning and uncertainty assessment in a Bayesian framework. Despite the generally good model fit for both variables, the uncertainty of the metabolic estimates was high, due primarily to the identification problem of individual metabolic processes. Deviations between observed and simulated DO concentrations suggested that neglect of transient stratification might be responsible for the bulk of the systematic model errors. Net ecosystem production (NEP) was uncertain. Unless air-water oxygen exchange can be estimated from direct measurements, the free-water DO method cannot reliably estimate NEP. Gross primary production (GPP) could satisfactorily be hindcasted assuming non-linear multiplicative dependence on Chl, water temperature and light. Hindcast of community respiration (CR) was less successful, possibly due to the impact of local benthic respiration. Results suggested a major shift in lake metabolism at about 16°C. Below and above this temperature, 70% and 90% of net primary production could be utilized by heterotrophs within a day, respectively. Indirect evidence suggested that biomass-specific net primary production was determined by phosphorus. The large difference between reproductive rates and net growth rates estimated from GPP and Chl and from daily change in Chl, respectively indicated that loss rates of phytoplankton were as important determinants of algal dynamics as reproductive rates.
Inverse modeling is a common practice to decompose observed processes into constituents that are unobservable or difficult to measure. To achieve this goal, a mechanistic model is calibrated to fit ...the observations and thereby the model produces a coherent set of constituent estimates. A disadvantage of this procedure is that any disagreement between the model assumptions and reality potentially introduces bias and other statistical artifacts into the constituents and their relations. Lake metabolism is recently most often followed by high‐frequency measurements of dissolved oxygen, and inverse modeling with simple conceptual models is used to couple oxygen dynamics to ecosystem‐wide aggregated metabolic rates, such as net ecosystem production (NEP). These models rely on estimates of gas exchange and community respiration. Using a model of a simple ecosystem and field data, we demonstrate that typical relations between modeled metabolic rates frequently do not follow patterns expected from synthetic ecosystems and that estimation errors strongly influence calculations by producing strong, spurious correlations. Correlation artifacts can be expected during inverse modeling, whenever observed time series are decomposed into poorly known or unmeasured processes that can compensate for the effect of each other.
Ecosystem respiration Solomon, Christopher T.; Bruesewitz, Denise A.; Richardson, David C. ...
Limnology and oceanography,
20/May , Letnik:
58, Številka:
3
Journal Article
Recenzirano
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We assembled data from a global network of automated lake observatories to test hypotheses regarding the drivers of ecosystem metabolism. We estimated daily rates of respiration and gross primary ...production (GPP) for up to a full year in each lake, via maximum likelihood fits of a free-water metabolism model to continuous high-frequency measurements of dissolved oxygen concentrations. Uncertainties were determined by a bootstrap analysis, allowing lake-days with poorly constrained rate estimates to be down-weighted in subsequent analyses. GPP and respiration varied considerably among lakes and at seasonal and daily timescales. Mean annual GPP and respiration ranged from 0.1 to 5.0 mg O2 L−1 d−1 and were positively related to total phosphorus but not dissolved organic carbon concentration. Within lakes, significant day-to-day differences in respiration were common despite large uncertainties in estimated rates on some lake-days. Daily variation in GPP explained 5% to 85% of the daily variation in respiration after temperature correction. Respiration was tightly coupled to GPP at a daily scale in oligotrophic and dystrophic lakes, and more weakly coupled in mesotrophic and eutrophic lakes. Background respiration ranged from 0.017 to 2.1 mg O₂ L−1 d−1 and was positively related to indicators of recalcitrant allochthonous and autochthonous organic matter loads, but was not clearly related to an indicator of the quality of allochthonous organic matter inputs.
Environmental management decisions should be made based on solid scientific evidence that relies on monitoring and modeling. In practice, changing economic, societal, and political boundary ...conditions often interfere with management during large, long, and complex projects. The result may be a sub-optimal development path that may finally diverge from the original intentions and be economically or technically ineffective. Nevertheless, unforeseen benefits may be created in the end. The Kis-Balaton wetland system is a typical illustration of such a case. Despite tremendous investments and huge efforts put in monitoring and modeling, the sequence of decisions during implementation can hardly be considered optimal. We use a catchment model and a basic water quality model to coherently review the impacts of management decisions during the 30-year history. Due to the complexity of the system, science mostly excelled in finding explanations for observed changes after the event instead of predicting the impacts of management measures a priori. In parallel, the political setting and sectoral authorities experienced rearrangements during system implementation. Despite being expensive as a water quality management investment originally targeting nutrient removal, the Kis-Balaton wetland system created a huge ecological asset, and thereby became worth the price.
Nutrient emission dropped significantly during the last two decades in the Danube Basin. To assess the effect of reduced nutrient loads on the trophic status of running waters, this regional study ...analyzed the relationships between nutrients (P and N) and suspended chlorophyll (Chl) using long-term monitoring data in Hungary. Including the upstream catchments of trans-boundary rivers, the study covered an approximate area of 400,000 km
2
, equivalent to the half of the entire Danube catchment. Decadal median Chl was unrelated to P and N concentrations in the whole data set and weakly related to total P (TP) at natural-moderately polluted (N-MP) sites, which were distinguished from highly polluted (HP) sites by using cutoff values for chloride, chemical oxygen demand and TP. At both the N-MP sites and most of the HP sites, Chl increased with channel length. This indicated that water residence time was a more important determinant of Chl than nutrients. Nutrient concentrations showed a significant downward trend in time at half of our sites. With a nearly equal frequency, a parallel trend might or might not occur in Chl. The apparent efficiency of nutrient management was expressed as the quotient of the slopes of linear trends in Chl and nutrients. At sites within 150 km from source, this efficiency was marginal. In larger rivers, efficiency improved steeply. The highest efficiency was observed in the downstream reach of the Danube (upstream length >1,300 km) where P availability might frequently limit algal growth. The results suggest that eutrophication management in rivers should be based on Chl response functions, rather than universal nutrient criteria. Four Chl response classes were identified based on the observed longitudinal P and Chl gradients.
Phytoplankton growth in three rivers Istvánovics, Vera; Honti, Márk
Limnology and oceanography,
July 2011, Letnik:
56, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We analyzed long-term (5–8 yr) hourly time series of chlorophyll (Chl) converted from fluorescence measurements in relation to discharge and light in three medium- to large-size rivers, where ...planktonic diatoms dominated during the growing season. Overall, there was an inverse relationship between discharge and Chl. At fine temporal scales, flow pulses were typically accompanied by an increase in diatom Chl. In contrast, chlorophytes were usually diluted. The increase in diatoms was likely due to resuspension of meroplanktonic species from the bottom. The benthic retention hypothesis proposes that rapidly sedimenting diatoms take advantage of a prolonged benthic residence provided that the enhanced retention is sufficient to compensate for slower light-supported growth at the bottom relative to water. This hypothesis was tested with simple growth models. Although the rivers were highly turbid and did not support net growth when flow exceeded a rather low threshold, benthic retention might have favored low-light–adapted algae during more than half of the period when net growth was possible. Among the physical factors, the rate of resuspension might be the critical factor that determines the ultimate success of a meroplanktonic life cycle strategy. The three rivers of this study rarely supported persistent planktonic populations. We propose that self-sustaining populations of riverine algae are primarily based on meroplanktonic diatom species, whereas truly planktonic populations, mostly chlorophytes, depend on periodic inoculations from out-of-channel sources.
Recent technological developments have increased the number of variables being monitored in lakes and reservoirs using automatic high frequency monitoring (AHFM). However, design of AHFM systems and ...posterior data handling and interpretation are currently being developed on a site-by-site and issue-by-issue basis with minimal standardization of protocols or knowledge sharing. As a result, many deployments become short-lived or underutilized, and many new scientific developments that are potentially useful for water management and environmental legislation remain underexplored. This Critical Review bridges scientific uses of AHFM with their applications by providing an overview of the current AHFM capabilities, together with examples of successful applications. We review the use of AHFM for maximizing the provision of ecosystem services supplied by lakes and reservoirs (consumptive and non consumptive uses, food production, and recreation), and for reporting lake status in the EU Water Framework Directive. We also highlight critical issues to enhance the application of AHFM, and suggest the establishment of appropriate networks to facilitate knowledge sharing and technological transfer between potential users. Finally, we give advice on how modern sensor technology can successfully be applied on a larger scale to the management of lakes and reservoirs and maximize the ecosystem services they provide.
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