Challenges and opportunities of Earth observation for the prediction of water quality in inland waters

Water quality in lakes and river systems has deteriorated worldwide due to intensification in land use and associated nutrient loading or changes in natural flow regimes. The most obvious impacts are increase in the frequency of harmful algal blooms caused by potentially toxic cyanobacteria and fish kills due to hypoxia. Other problems are not immediately visible or have indirect impacts like contamination by metals and pathogens, or vector borne diseases depending on wetting and increased temperature. To reduce health and economic risks posed by such water quality issues, there is an increased need for early warning systems. While Earth observation of inland aquatic systems can give an account of historic conditions and current state, integrating hydrodynamic and hydrologic modelling tools with predictive capabilities allow for timely intervention and optimised management options. On a local scale Earth observation can be used to drive hydrodynamic simulations for short term prediction of harmful algal blooms in specific water bodies allowing for early warning and providing operating strategies for risk minimisation for, e.g., water treatment plants or reservoirs (case studies shown here). Combined with local hyperspectral sensors it is even possible to discriminate cyanobacteria species based on their pigments and thus infer potential toxicity. A generalisation of these methods on a regional or continental scale not only yields an early warning account for a larger region, e.g. state wide, but can yield a risk estimation based on weather forecast. In combination with hydrologic modelling tools EO is applied in ecological impact studies of flood inundation, e.g., the generation of hypoxic conditions in lowland rivers, or the spread of a carp virus for pest eradication in a large basin. Although there is a large spectrum of water quality issues where EO can lead to better insight, spatial and temporal resolution of satellite sensors limits their application. Other techniques of remote sensing are necessary to fill these gaps

Boom‐bust dynamics in biological invasions: towards an improved application of the concept

Boom‐bust dynamics – the rise of a population to outbreak levels, followed by a dramatic decline – have been associated with biological invasions and offered as a reason not to manage troublesome invaders. However, boom‐bust dynamics rarely have been critically defined, analyzed, or interpreted. Here, we define boom‐bust dynamics and provide specific suggestions for improving the application of the boom‐bust concept. Boom‐bust dynamics can arise from many causes, some closely associated with invasions, but others occurring across a wide range of ecological settings, especially when environmental conditions are changing rapidly. As a result, it is difficult to infer cause or predict future trajectories merely by observing the dynamic. We use tests with simulated data to show that a common metric for detecting and describing boom‐bust dynamics, decline from an observed peak to a subsequent trough, tends to severely overestimate the frequency and severity of busts, and should be used cautiously if at all. We review and test other metrics that are better suited to describe boom‐bust dynamics. Understanding the frequency and importance of boom‐bust dynamics requires empirical studies of large, representative, long‐term data sets that use clear definitions of boom‐bust, appropriate analytical methods, and careful interpretations

Integration of near-surface and satellite observations for algal bloom detection

Retrieval of water quality information from satellite imagery can provide resource managers with an improved understanding into the spatial variability of the water body. In light of the increasing availability of ‘analysis ready data’ (ARD) satellite imagery in open datacubes*, either on cloud-based services or on high performance computing environments, development of operational monitoring systems is becoming feasible. Near-surface sensors can assist in more rapid and widespread algal bloom monitoring at a much higher temporal resolution. Remote sensing imagery, whilst cost effective, may not be optimal in terms of spatial or spectral resolution and can be greatly enhanced with the integration of near-surface observations. We describe pathways to use field-based near-surface sensors to calibrate and validate satellite remote sensing. These methods allow early detection of algal blooms and assist in the early warning for management intervention. We have designed and deployed several low-cost, near-surface sensors at several inland water sites around eastern Australia. The data is transferred using mobile networks where it is processed into spectral information. From this data and coincident field bio-optical measurements, we have developed algorithms for quantitative estimation of blue-green algal-specific pigments (phycocyanin) and chlorophyll concentrations. We have tested these algorithms for detection using a number of existing satellite sensors and report on results here. These methods have applied next-generation monitoring technology and when combined with hydrologic modelling will provide aquatic observations and forecasts. These will lead to improved management preparedness to respond to environmental challenges, e.g., a harmful algal blooms

Simulation of surface energy fluxes and stratification of a small boreal lake by a set of one-dimensional models

Peer reviewe

Amplified surface temperature response of cold, deep lakes to inter-annual air temperature variability

Summer lake surface water temperatures (LSWTs) have previously been shown to respond more rapidly to climatic warming compared to local summer surface air temperatures (SATs). In a global- scale analysis, we explore the factors underpinning the observation of an amplified response of summer LSWT to SAT variability using 20 years of satellite-derived temperatures from 144 lakes. We demonstrate that the degree of amplification in inter-annual summer LSWT is variable, and is greater for cold lakes (e.g. high latitude and high altitude), which are characterised by a short warming season, and deep lakes, that exhibit long correlation timescales of temperature anomalies due to increased thermal inertia. Such lakes are more likely to display responses in excess of local inter-annual summer SAT variability. Climatic modification of LSWT has numerous consequences for water quality and lake ecosystems, so quantifying this amplified response at a global scale is important

Atmospheric stilling leads to prolonged thermal stratification in a large shallow polymictic lake

To quantify the effects of recent and potential future decreases in surface wind speeds on lake thermal stratification, we apply the one-dimensional process-based model MyLake to a large, shallow, polymictic lake, Võrtsjärv. The model is validated for a 3-year period and run separately for 28 years using long-term daily atmospheric forcing data from a nearby meteorological station. Model simulations show exceptionally good agreement with observed surface and bottom water temperatures during the 3-year period. Similarly, simulated surface water temperatures for 28 years show remarkably good agreement with long-term in situ water temperatures. Sensitivity analysis demonstrates that decreasing wind speeds has resulted in substantial changes in stratification dynamics since 1982, while increasing air temperatures during the same period had a negligible effect. Atmospheric stilling is a phenomenon observed globally, and in addition to recent increases in surface air temperature, needs to be considered when evaluating the influence of climate change on lake ecosystems

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.Peer reviewe

Exploring, exploiting and evolving diversity of aquatic ecosystem models: A community perspective

Here, we present a community perspective on how to explore, exploit and evolve the diversity in aquatic ecosystem models. These models play an important role in understanding the functioning of aquatic ecosystems, filling in observation gaps and developing effective strategies for water quality management. In this spirit, numerous models have been developed since the 1970s. We set off to explore model diversity by making an inventory among 42 aquatic ecosystem modellers, by categorizing the resulting set of models and by analysing them for diversity. We then focus on how to exploit model diversity by comparing and combining different aspects of existing models. Finally, we discuss how model diversity came about in the past and could evolve in the future. Throughout our study, we use analogies from biodiversity research to analyse and interpret model diversity. We recommend to make models publicly available through open-source policies, to standardize documentation and technical implementation of models, and to compare models through ensemble modelling and interdisciplinary approaches. We end with our perspective on how the field of aquatic ecosystem modelling might develop in the next 5–10 years. To strive for clarity and to improve readability for non-modellers, we include a glossary

Physical and chemical impacts of a major storm on a temperate lake: a taste of things to come?

Extreme weather can have a substantial influence on lakes and is expected to become more frequent with climate change. We explored the influence of one particular extreme event, Storm Ophelia, on the physical and chemical environment of England's largest lake, Windermere. We found that the substantial influence of Ophelia on meteorological conditions at Windermere, in particular wind speed, resulted in a 25-fold increase (relative to the study-period average) in the wind energy flux at the lake-air interface. Following Ophelia, there was a short-lived mixing event in which the Schmidt stability decreased by over 100 Jm-2 and the thermocline deepened by over 10 m during a 12-hour period. As a result of changes to the strength of stratification, Ophelia also changed the internal seiche regime of Windermere with the dominant seiche period increasing from ~17 h pre-storm to ~21 h post-storm. Following Ophelia, there was an upwelling of cold and low-oxygenated waters at the southern-end of the lake. This had a substantial influence on the main outflow of Windermere, the River Leven, where dissolved oxygen concentrations decreased by ~48 %, from 9.3 mg L-1 to 4.8 mg L-1, while at the mid-lake monitoring station in Windermere, it decreased by only ~3%. This study illustrates that the response of a lake to extreme weather can cause important effects downstream, the influence of which may not be evident at the lake surface. To understand the impact of future extreme events fully, the whole lake and downstream-river system need to be studied together

Climate change and freshwater zooplankton: what does it boil down to?

Recently, major advances in the climate–zooplankton interface have been made some of which appeared to receive much attention in a broader audience of ecologists as well. In contrast to the marine realm, however, we still lack a more holistic summary of recent knowledge in freshwater. We discuss climate change-related variation in physical and biological attributes of lakes and running waters, high-order ecological functions, and subsequent alteration in zooplankton abundance, phenology, distribution, body size, community structure, life history parameters, and behavior by focusing on community level responses. The adequacy of large-scale climatic indices in ecology has received considerable support and provided a framework for the interpretation of community and species level responses in freshwater zooplankton. Modeling perspectives deserve particular consideration, since this promising stream of ecology is of particular applicability in climate change research owing to the inherently predictive nature of this field. In the future, ecologists should expand their research on species beyond daphnids, should address questions as to how different intrinsic and extrinsic drivers interact, should move beyond correlative approaches toward more mechanistic explanations, and last but not least, should facilitate transfer of biological data both across space and time