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Morphological evolution from a unicellular to multicellular state provides greater opportunities for organisms to attain larger and more complex living forms. As the most common freshwater cyanobacterial genus, Microcystis is a unicellular microorganism, with high phenotypic plasticity, which forms colonies and blooms in lakes and reservoirs worldwide. We conducted a systematic review of field studies from the 1990s to 2017 where Microcystis was dominant. Microcystis was detected as the dominant genus in waterbodies from temperate to subtropical and tropical zones. Unicellular Microcystis spp. can be induced to form colonies by adjusting biotic and abiotic factors in laboratory. Colony formation by cell division has been induced by zooplankton filtrate, high Pb2+ concentration, the presence of another cyanobacterium (Cylindrospermopsis raciborskii), heterotrophic bacteria, and by low temperature and light intensity. Colony formation by cell adhesion can be induced by zooplankton grazing, high Ca2+ concentration, and microcystins. We hypothesise that single cells of all Microcystis morphospecies initially form colonies with a similar morphology to those found in the early spring. These colonies gradually change their morphology to that of M. ichthyoblabe, M. wesenbergii and M. aeruginosa with changing environmental conditions. Colony formation provides Microcystis with many ecological advantages, including adaption to varying light, sustained growth under poor nutrient supply, protection from chemical stressors and protection from grazing. These benefits represent passive tactics responding to environmental stress. Microcystis colonies form at the cost of decreased specific growth rates compared with a unicellular habit. Large colony size allows Microcystis to attain rapid floating velocities (maximum recorded for a single colony, ∼ 10.08 m h−1) that enable them to develop and maintain a large biomass near the surface of eutrophic lakes, where they may shade and inhi

This essay investigates the limiting capacity of the planet to support humans, making various assumptions about current practices and the intensities of per caput resource consumption. Supposing people to be exclusively vegetarian, consuming cereals produced by present methods, at the highest reported yields, and also eschewing the cultivation of non-edible crops, the Earth is argued to be capable of sustaining a population up to 55 billion. Consuming mixed diets including meat and beverages while continuing to raise non-food crops reduces the capacity by 7- to 10-fold, closer to the actual population at the present time. When the availability and distribution of exploitable water supplies are considered, it is difficult to argue for a sustainable population much exceeding 10 billion, without considerable changes in the equity of supply. All such extrapolations are subject to unknown consequences of rapid and chaotic climate change. The possibility that the rate of human population growth may be stabilising for other reasons, with numbers perhaps peaking at 10 to 11 billion, may yet allow increasingly widespread and severe water shortages to be avoided. This coincidence offers the opportunity to improve human sustainability through new social structures and new, cleaner, more resource-efficient technologies. They need to be directed towards solving inequities in resource use—not only of food and energy, but especially also of water. Though ultimately speculative and polemical, the essay is a genuine attempt to promote the case for recognising our real problems and the need to evolve strategies for survival.

We explore here the sensitivity and predictive power of quantified morphological and physiological properties of phytoplankton. We have adopted groups of species, distinguished by the clear morphological- and physiological-traits (Kruk et al., 2010; Litchman et al., 2010), and simulated their growth in various environments, using the model PROTECH (Phytoplankton RespOnses To Environmental CHange). This well-tested model uses equations fitted to optimal growth performances of various (“real”) phytoplankton species, described in terms of their functional attributes. Thus, we are able to reconstruct successional series in a range of shallow and deep lakes, situated at set latitudes (temperate or “Mediterranean”). All sequences are initiated at equal biomass levels but they soon become dominated by Groups which responded positively to the changing environmental conditions. We show the selection of small algae under conditions of good light and resource abundance and their subsequent collapse, especially as a consequence of zooplankton consumption. We show the advantages accruing to larger organisms that escape grazing but may also be motile and better defend assembled biomass against loss. Lengthening of cells or filaments of many cells increases organismic size but preserves the high surface-to-volume ratios that aid optimal interception of underwater light in turbid, mixed layers. Nitrogen-fixing capability confers to appropriately adapted species the opportunity to dominate when diminishing supplies of nitrogen limit the growth of other algae. Models founded on trait-separated phytoplankton demonstrate the value of these properties to the description and interpretation of phytoplankton succession.

This paper is written in honour of the influential career of Professor T.J. Smayda. It celebrates the brief, but productive, period in which we collaborated on the formulation of the principles and mechanisms of community assembly in plankton. The topics considered then provide the structure of the present paper – the origin of communities (nascence), the common and differential requirements of phytoplankton species that might affect their selection, the general and specialist adaptive traits that differentiate their performances and their relative successes, and the sensitivity of performances to sub-ideal or hostile environmental conditions. In all these instances, it is the reactivity of the net dynamic responses of the species present to imposed environmental variations that shapes the structure of the assemblage and the composition of the community. These are the high-level properties that emerge from the behaviour of low-level components. The patterns that emerge at the community level – selection by performance or competition, dominance and temporal succession – are qualitatively predictable and, at the assemblage level, are amenable to quantitative modelling. In turn, it is theoretically possible to make deductions about the likelihood of biological events that involve species occurring in abundances sufficient to affect water quality and/or constitute risks to human health (harmful algal blooms).

1. The paper explores the predictability of ecological conditions in aquatic systems and of their responses to catchment management schemes, in which the hydrological and material loads can be accurately and precisely quantified. Linkages between catchment hydrology and impacts on water quality are considered. 2. New approaches to modelling the complex biological behaviour of recipient systems are briefly reviewed. The case is made for simulating emergent outcomes based on the adaptive strategies and dynamic behaviours of species attempting to meet their material demands within the supportive capacities furnished by local environments. This ‘bottom-up’ approach is justified by the fact that ecosystems are the sum of their smallest components; their behaviour is the aggregate of the activities of individuals. 3. A framework model template founded upon the emergent properties of high-performing organisms is developed within the context of exergy fluxes and their deployment and against axes representing the resource-carrying capacity versus the energy (photosynthetic and chemical) available to process it. 4. Some worked examples of simulated emergence in lakes are given, using the phytoplankton-community model, PROTECH, to demonstrate the acuity of predictions of the responses to quite subtle variations in biologically active solutes and suspensoids to simulated communities of the pelagic.

Twenty years after model equations describing the in situ growth rates of phytoplankton were first devised and eight since their successful incorporation into a computer simulation was first published, we set out to affirm the general validity and utility of PROTECH (Phytoplankton RespOnses To Environmental Change). Elaborated originally for commercial purposes, PROTECH has been shown to be capable of simulating simultaneous seasonal fluctuations in the standing crops of several contrasting species of alga, making it attractive for testing the impacts of various simulated management of the growth conditions. These have been sufficiently convincing to persuade us to use PROTECH as a research tool; over a number of years, it has been used to simulate such ‘traditional’ problems of ecology as succession, competitive exclusion and species diversity, in the context of intermediate disturbance. In this paper, we review critically the workings of the model, especially how complex but consistent outcomes emerge in compliance with simple trait-based rules of community assembly. We affirm that temperature-specific growth rates of algae are strongly influenced by algal morphology, that slender species are tolerant of low average light exposure and that periodicity is related to species-specific characteristics of motility and buoyant behaviour. The results of some applications of PROTECH are presented, simulating responses of the phytoplankton community to adjustments in nutrient loading, light penetration and hydrological flushing rates; an explicit investigation of the sensitivity of population responses of Cyanobacteria to eutrophication is also reported, in the context of varying availabilities of combined inorganic nitrogen. Considering future developments of PROTECH, we affirm the virtues of its central growth equations; we anticipate that future applications will mostly depend upon improved representation of the physical environments it is sought to simulate and that th

1. A logical way of distinguishing functional groups of phytoplankton is to cluster species according to their functional traits, such as growth rate and nutrient assimilation constants. However, data for such an approach are lacking for the vast majority of the species. 2. In this study, we show that a classification based on simple morphological traits may capture much of the variability in functional properties among the phytoplankton. We used information on more than 700 freshwater species, from more than 200 lakes situated in climate zones ranging from subpolar to tropical. 3. Morphological characteristics correlated well with functional properties, such as growth rate and sinking rate, and also with the population size and biomass attained in the field. This suggests that morphology is a good predictor of the functional characteristics of species. 4. Cluster analysis was used to define seven species groups based on morphology. Although some of the clusters are taxonomically homogeneous, others include species of several separate divisions. Functional traits (not used for the classification) differed significantly among the clusters, suggesting that the clusters may indeed represent meaningful functional groups. 5. Advantages of our morphological approach to classification include its objectivity, its independence from taxonomic affiliations, and the relative ease of its application to the majority of species for which physiological traits are unknown and are not readily determined.