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A key phenological event in the annual cycle of many pelagic ecosystems is the onset of the spring algal bloom (OAB). Descriptions of the factors controlling the OAB in temperate to polar lakes have been limited to isolated studies of single systems and conceptual models. Here we present a validated modelling approach that, for the first time, enables a quantitative prediction of the OAB and a systematic assessment of the processes controlling its timing on a continental scale. We used a weather-driven, 1-dimensional lake model to simulate the seasonal dynamics of the underwater light climate in 16 lake types characterized by the factorial combination of 4 lake depths with 4 levels of water transparency. We did so at 1,962 locations across western Europe and over 31 years (1979-2009). Assuming that phytoplankton production is light-limited in winter, we identified four patterns of OAB control across lake types and climate zones. OAB timing is controlled by (i) the timing of ice-off in ice-covered clear or shallow lakes, (ii) the onset of thermal stratification in sufficiently deep and turbid lakes, and (iii) the seasonal increase in incident radiation in all other lakes, except for (iv) ice-free, shallow and clear lakes in the south, where phytoplankton is not light-limited. The model predicts that OAB timing should respond to two pervasive environmental changes, global warming and browning, in opposite ways. OAB timing should be highly sensitive to warming in lakes where it is controlled by either ice-off or the onset of stratification, but resilient to warming in lakes where it is controlled by incident radiation. Conversely, OAB timing should be most sensitive to browning where it is controlled by incident radiation, but resilient to browning where it is controlled by ice-off or the onset of stratification. Available lake data are consistent with our findings. published

© The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adap- tation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statu- tory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons. org/licenses/by/4.0/. ake surfaces are warming worldwide, raising concerns about lake organism responses to thermal habitat changes. Species may cope with temperature increases by shifting their seasonality or their depth to track suitable thermal habitats, but these responses may be constrained by ecological interactions, life histories or limiting resources. Here we use 32 million temperature measurements from 139 lakes to quantify thermal habitat change (percentage of non-overlap) and assess how this change is exacerbated by potential habitat constraints. Long-term temperature change resulted in an average 6.2% non-overlap between thermal habitats in baseline (1978–1995) and recent (1996–2013) time periods, with non-overlap increasing to 19.4% on aver- age when habitats were restricted by season and depth. Tropical lakes exhibited substantially higher thermal non-overlap com- pared with lakes at other latitudes. Lakes with high thermal habitat change coincided with those having numerous endemic species, suggesting that conservation actions should consider thermal habitat change to preserve lake biodiversity. Faculty yes

In situ experiments on sediment resuspension were carried out along a depth transect in shallow polytrophic Lake Langer See, northeast Germany (area = 1.27 km2, Zmax = 3.8 m), in winter under ice and in summer 2006, using a hydrodynamically calibrated erosion chamber (diameter 20 cm). Shear velocity U* was incrementally increased 11 times for 10 min each (0–2.57 cm s−1) to initiate resuspension. Entrainment rates (E) of suspended particulate matter (ESPM) and total P (ETP) were determined by a mass balance. Sandy nearshore sediments at 1–2 m depth showed low ESPM (0.01–6.64 g m−2 h−1) and ETP (0.2–2.96 mg m−2 h−1). There was no difference in critical shear velocity (U*crit) of incipient resuspension between winter and summer. Muddy offshore sediments at 2–3.8 m depth showed higher ESPM (0.09–106.1 g m−2 h−1) and ETP (0.06–532.3 mg m−2 h−1). Under ice, U*crit was 0.9–1.1 cm s−1 higher than in summer, indicating that the wave-unaffected sediment had ample time for consolidation and biostabilization. Wind-wave modeling, using measured ESPM and ETP, revealed that resuspension is primarily (90%) restricted to the nearshore sediments (17% lake area). Our scenarios show that annual entrainment of SPM and TP increased linearly with the duration of ice-free period. Since ice coverage increased shear resistance, climate-driven absence or shortening of ice-cover would reduce the period for settling of particles and their integration into sediments, lowering their biostabilization, thus facilitating resuspension and phytoplankton recruitment in spring after ice thaw.

Coupling of several predator-prey oscillations can generate intriguing patterns of synchronization and chaos. Theory predicts that prey species will fluctuate in phase if predator-prey cycles are coupled through generalist predators, whereas they will fluctuate in anti-phase if predator-prey cycles are coupled through competition between prey species. Here, we investigate predator-prey oscillations in a long-term experiment with a marine plankton community. Wavelet analysis of the species fluctuations reveals two predator-prey cycles that fluctuate largely in anti-phase. The phase angles point at strong competition between the phytoplankton species, but relatively little prey overlap among the zooplankton species. This food web architecture is consistent with the size structure of the plankton community, and generates highly dynamic food webs. Continued alternations in species dominance enable coexistence of the prey species through a non-equilibrium 'killing-the-winner' mechanism, as the system shifts back and forth between the two predator-prey cycles in a chaotic fashion.

Dense surface blooms of toxic cyanobacteria in eutrophic lakes may lead to mass mortalities of fish and birds, and provide a serious health threat for cattle, pets, and humans. It has been argued that global warming may increase the incidence of harmful algal blooms. Here, we report on a lake experiment where intermittent artificial mixing failed to control blooms of the harmful cyanobacterium Microcystis during the summer of 2003, one of the hottest summers ever recorded in Europe. To understand this failure, we develop a coupled biological-physical model investigating how competition for light between buoyant cyanobacteria, diatoms and green algae in eutrophic lakes is affected by the meteorological conditions of this extreme summer heatwave. The model consists of a phytoplankton competition model coupled to a one-dimensional hydrodynamic model, driven by meteorological data. The model predicts that high temperatures favour cyanobacteria directly, through increased growth rates. Moreover, high temperatures also increase the stability of the water column, thereby reducing vertical turbulent mixing, which shifts the competitive balance in favour of buoyant cyanobacteria. Through these direct and indirect temperature effects, in combination with reduced wind speed and reduced cloudiness, summer heatwaves boost the development of harmful cyanobacterial blooms. These findings warn that climate change is likely to yield an increased threat of harmful cyanobacteria in eutrophic freshwater ecosystems.

Microcystis is one of the most notorious phytoplankton species, because it can form harmful toxic blooms that cause problems in freshwaters all over the world. Since Microcystis colonies are buoyant, high concentrations of toxic Microcystis accumulate at the water surface in scums during calm, warm conditions. The aim of this study was to investigate the physiology and population dynamics of benthic and pelagic Microcystis, and to use this knowledge to predict which lake management strategies could be effective in the battle against Microcystis blooms in Lake Volkerak-Zoommeer, The Netherlands. Changes in benthic and pelagic populations of Microcystis were monitored in the water and sediments of the lake. Sedimentation and recruitment rates were measured using traps, and the physiology of benthic colonies was investigated. In addition, the response of Microcystis to light, temperature, salinity and the aggregation of Microcystis to clay was determined in the laboratory. Large amounts of Microcystis sink from the water column in summer due to attachment of sediment particles. The attachment of clay particles to colonies depends on the concentration and type of clay and on the composition of the mucus that surrounds Microcystis. Colonies that have sunk overwinter in the sediments. A small amount of colonies overwinter in the water column. During autumn and winter, benthic colonies from shallow sediments are gradually transported to the deeper parts of the lake. The concentration of benthic Microcystis therefore increases with lake depth. Although all benthic colonies remain vital during winter, colonies in shallow euphotic sediments have a higher vitality then colonies in deep anoxic sediments. The overwintering benthic population inoculates the water column in spring. Since the density of benthic colonies does not change in spring, colonies do not actively recruit by using their buoyancy, but are probably passively resuspended by wind-induced mixing or bioturbation. Since colonies from shallow sediments have the highest vitality and are more frequently resuspended then deeper parts, most recruitment originates from the shallow sediments. Model simulations indicate that benthic recruitment contributes for 50-75% to the size of the summer bloom. To suppress harmful blooms of Microcystis in Lake Volkerak-Zoommeer, two management strategies have been suggested: flushing the lake with freshwater or reintroducing saline water. A mechanistic model of the population dynamics of Microcystis predicts that flushing with freshwater will suppress Microcystis blooms when the current flushing rate is sufficiently increased. Furthermore, the inlet of saline water will suppress Microcystis blooms for salinities exceeding 14 g/l. Both management strategies are technically feasible.

Recent winter warming over Central Europe associated with a positive phase of the North Atlantic Oscillation (NAO) strongly influenced the thermal and water column stability properties of deep Lake Constance (zmean = 101 m).Volumet- rically weighted average water temperatures have increased since the 1960s by an average of 0.017 degree C yr-1, and its interannual variability was strongly related to the variability in winter air temperature and the NAO winter index (NAOW). The influence of NAOw on water temperature was more persistent than its influence on air temperature. The seasonal persistence of the NAOw signal increased with water depth. Deep-water temperatures were related to the NAOw from one spring mixing period to the next. This caused a time lag of 1 yr in the response of deep-water winter temperatures to the NAOw. Reduced winter cooling during high-NAOw years resulted in the persistence of small temperature gradients that possibly resisted complete mixing. This, in turn, resulted in less upward mixing of nutrients (total phosphorus and total silica), which accumulated in the hypolimnion during the previous stratification period. A second effect of incomplete mixing was the lack of the replenishment of deepwater oxygen during high-NAOw years. Hence, besides its strong impact on the thermal regime, climate variability influenced both the causes (nutrient supply for phytoplankton growth) and symptoms (the degree of hypolimnetic oxygen deficiency) of trophic changes in Lake Constance.

A coupled model is presented for simulating physical and biological dynamics in fresh water lakes. The physical model rests upon the assumption that the turbulent kinetic energy in a water column of the lake is fully contained in a mixed layer of variable depth. Below this layer the mechanical energy content is assumed to vanish. Additionally, the horizontal currents are ignored. This one-dimensional two-layered model describes the internal conversion of the mechanical and thermal energy input from the atmosphere into an evolution of the mixed layer depth by entrainment and detrainment mechanisms. It is supposed to form the physical domain in which the simulation of the biological processes takes place. The biological model describes mathematically the typical properties of phyto- and zooplankton, their interactions and their response to the physical environment. This description then allows the study of the behaviour of Lagrangian clusters of virtual plankton that are subjected to such environments. The essence of the model is the dynamical simulation of an arbitrary number of nutrient limited phytoplankton species and one species of zooplankton. The members of the food web above and below affect the model only statically. The model is able to reproduce the typical progression of a predator-prey interaction between phyto- and zooplankton as well as the exploitative competition for nutrients between two phytoplankton species under grazing pressure of Daphnia. It suggests that the influence of the biological system on the physical system results in a weak increase of the surface temperature for coupled simulations, but a considerably higher seasonal thermocline in spring and a lower one in autumn.

To prevent flooding of the Dutch delta, former estuaries have been impounded by the building of dams and sluices. Some of these water bodies, however, experience major ecological problems. One of the problem areas is the former Volkerak estuary that was turned into a freshwater lake in 1987. From the early 1990s onward, toxic Microcystis blooms dominate the phytoplankton of the lake every summer. Two management strategies have been suggested to suppress these harmful algal blooms: flushing the lake with fresh water or reintroducing saline water into the lake. This study aims at an advance assessment of these strategies through the development of a mechanistic model of the population dynamics of Microcystis. To calibrate the model, we monitored the benthic and pelagic Microcystis populations in the lake during two years. Field samples of Microcystis were incubated in the laboratory to estimate growth and mortality rates as functions of light, temperature, and salinity. Recruitment and sedimentation rates were measured in the lake, using traps, to quantify benthic–pelagic coupling of the Microcystis populations. The model predicts that flushing with fresh water will suppress Microcystis blooms when the current flushing rate is sufficiently increased. Furthermore, the inlet of saline water will suppress Microcystis blooms for salinities exceeding 14 g/L. Both management options are technically feasible. Our study illustrates that quantitative ecological knowledge can be a helpful tool guiding large-scale water management. [KEYWORDS: benthic–pelagic coupling ; cyanobacteria ; harmful algal blooms ; population dynamics ; recruitment ; residence time ; salinity ; sedimentation ; water management]