Mathematical modeling of colony formation in algal blooms: phenotypic plasticity in cyanobacteria

Time scale of resilience loss: Implications for managing critical transitions in water quality

Regime shifts involving critical transitions are a type of rapid ecological change that are difficult to predict, but may be preceded by decreases in resilience. Time series statistics like lag-1 autocorrelation may be useful for anticipating resilience declines; however, more study is needed to determine whether the dynamics of autocorrelation depend on the resolution of the time series being analyzed, i.e., whether they are time-scale dependent. Here, we examined timeseries simulated from a lake eutrophication model and gathered from field measurements. The field study involved collecting high frequency chlorophyll fluorescence data from an unmanipulated reference lake and a second lake undergoing experimental fertilization to induce a critical transition in the form of an algal bloom. As part of the experiment, the fertilization was halted in response to detected early warnings of the algal bloom identified by increased autocorrelation. We tested these datasets for time-scale dependence in the dynamics of lag-1 autocorrelation and found that in both the simulation and field experiment, the dynamics of autocorrelation were similar across time scales. In the simulated time series, autocorrelation increased exponentially approaching algal bloom development, and in the field experiment, the difference in autocorrelation between the manipulated and reference lakes increased sharply. These results suggest that, as an early warning indicator, autocorrelation may be robust to the time scale of the analysis. Given that a time scale can be shortened by increasing sampling frequency, or lengthened by aggregating data during analysis, these results have important implications for management as they demonstrate the potential for detecting early warning signals over a wide range of monitoring frequencies and without requiring analysts to make situation-specific decisions regarding aggregation. Such an outcome provides promise that data collection procedures, especially by automated sensors, may be used to monitor and manage ecosystem resilience without the need for strict attention to time scale.

Spatiotemporal dynamics of cell abundance, colony size and intracellular toxin concentrations of pelagic and benthic Microcystis in Lake Caohai, China

Lake Caohai has experienced extensive Microcystis blooms in recent years, and to improve its water quality, the local government carried out a series of water control measures. To better understand the dynamics of both pelagic and benthic Microcystis and their characteristics in Lake Caohai, we conducted a 1-year investigation from December 2015 to December 2016 to gain a seasonal outlook on the distribution and dynamics of cell abundance, colony size and intracellular microcystins (MCs) of Microcystis. The results indicated that the Microcystis bloom occupied primarily the northeastern region and then moved gradually from lakeshore to lake center. The perennial southwesterly winds and the water inflow from northeast to southwest in Lake Caohai determined the spatiotemporal distribution of pelagic Microcystis. Benthic Microcystis was mainly distributed in the northeastern region in summer, occupied the lake center in autumn and then occupied the southeastern region in winter, determined by the sedimentation of pelagic Microcystis and the death of benthic Microcystis. Small colonies (20-60 μm) overwintered more easily in both water column and sediment. The concentrations of intracellular toxin of benthic Microcystis were observed to be significantly higher than those of pelagic Microcystis. This might be because Microcystis synthesized large amount of MCs to acclimate to an unfavorable benthic environment. This knowledge on the dynamics of Microcystis expands our understanding of mechanisms underpinning the formation of Microcystis blooms.

The importance of small colonies in sustaining Microcystis population exposed to mixing conditions: an exploration through colony size, genotypic composition and toxic potential

Microcystis is a toxic colony-forming cyanobacterium, which can bloom in a wide range of freshwater ecosystems. Despite the ecological advantage of the colonial form, few studies have paid attention to the size of Microcystis colonies in the field. With the aim of evaluating the impact of a fluctuating physical environment on the colony size, the genotypic composition and the toxic potential of a Microcystis population, we investigated five different colony size classes of a Microcystis bloom in the Grangent reservoir (France). By sequencing the internal transcribed spacer of the ribosomal operon, we evidenced changes in the genetic structure among size classes in response to environmental change. While similar genotypes were seen in every size class in stable conditions, new dominant genotypes appeared in the smallest colonies (< 160 μm) concomitantly with mixing conditions, strongly suggesting the importance of these colonies in response to disturbances. Moreover, these small colonies played a major role in microcystin production during this bloom, since very high microcystin contents (> 1 pg.cell.(-1)) were found in their cells. These findings indicate that the colony size distribution of a Microcystis population in response to disturbance could be an adaptive strategy that may explain its ecological success in freshwater ecosystems.

Comparing the qualitatively different effects rapidly evolving and rapidly induced defences have on predator-prey interactions

Interspecific interactions depend not only on the population densities of the interacting species, but on their phenotypes as well. Phenotypic variation can be plastic or heritable and both mechanisms can drive phenotypic change at rates comparable to or faster than those of ecological dynamics (e.g. changes in population abundances or spatial distributions). In this study, we compare the effects rapidly induced and rapidly evolving defences have on community dynamics by considering the fast phenotypic change limit using fast-slow systems theory. Our approach allows us to study phenotypically plastic and evolving systems with one overarching theory, thus capturing the effects rapidly induced defences have on ecological dynamics and how those effects differ from the effects of evolving defences. Our results show that rapidly induced defences tend to stabilize community dynamics and that some behaviours observed in rapidly evolving systems cannot be produced by phenotypic plasticity.