Competition for Light and Nutrients in Layered Communities of Aquatic Plants

Dominance of free-floating plants poses a threat to biodiversity in many freshwater ecosystems. Here we propose a theoretical framework to understand this dominance, by modeling the competition for light and nutrients in a layered community of floating and submerged plants. The model shows that at high supply of light and nutrients, floating plants always dominate due to their primacy for light, even when submerged plants have lower minimal resource requirements. The model also shows that floating-plant dominance cannot be an alternative stable state in light-limited environments but only in nutrient-limited environments, depending on the plants’ resource consumption traits. Compared to unlayered communities, the asymmetry in competition for light—coincident with symmetry in competition for nutrients—leads to fundamentally different results: competition outcomes can no longer be predicted from species traits such as minimal resource requirements ( rule) and resource consumption. Also, the same two species can, depending on the environment, coexist or be alternative stable states. When applied to two common plant species in temperate regions, both the model and field data suggest that floating-plant dominance is unlikely to be an alternative stable state

The resilience and resistance of an ecosystem to a collapse of diversity

Diversity is expected to increase the resilience of ecosystems. Nevertheless, highly diverse ecosystems have collapsed, as did Lake Victoria's ecosystem of cichlids or Caribbean coral reefs. We try to gain insight to this paradox, by analyzing a simple model of a diverse community where each competing species inflicts a small mortality pressure on an introduced predator. High diversity strengthens this feedback and prevents invasion of the introduced predator. After a gradual loss of native species, the introduced predator can escape control and the system collapses into a contrasting, invaded, low-diversity state. Importantly, we find that a diverse system that has high complementarity gains in resilience, whereas a diverse system with high functional redundancy gains in resistance. Loss of resilience can display early-warning signals of a collapse, but loss of resistance not. Our results emphasize the need for multiple approaches to studying the functioning of ecosystems, as managing an ecosystem requires understanding not only the threats it is vulnerable to but also pressures it appears resistant t

Exploring how cyanobacterial traits affect nutrient loading thresholds in shallow lakes: A modelling approach

Globally, many shallow lakes have shifted from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state due to eutrophication. Such shifts are often accompanied by toxic cyanobacterial blooms, with specialized traits including buoyancy regulation and nitrogen fixation. Previous work has focused on how these traits contribute to cyanobacterial competitiveness. Yet, little is known on how these traits affect the value of nutrient loading thresholds of shallow lakes. These thresholds are defined as the nutrient loading at which lakes shift water quality state. Here, we used a modelling approach to estimate the effects of traits on nutrient loading thresholds. We incorporated cyanobacterial traits in the process-based ecosystem model PCLake+, known for its ability to determine nutrient loading thresholds. Four scenarios were simulated, including cyanobacteria without traits, with buoyancy regulation, with nitrogen fixation, and with both traits. Nutrient loading thresholds were obtained under N-limited, P-limited, and colimited conditions. Results show that cyanobacterial traits can impede lake restoration actions aimed at removing cyanobacterial blooms via nutrient loading reduction. However, these traits hardly affect the nutrient loading thresholds for clear lakes experiencing eutrophication. Our results provide references for nutrient loading thresholds and draw attention to cyanobacterial traits during the remediation of eutrophic water bodies

Alternative stable states in large shallow lakes?

Many lakes worldwide are experiencing great change due to eutrophication. Consequently, species composition changes, toxic algal blooms proliferate, and drinking water supplies dwindle. The transition to the deteriorated state can be catastrophic with an abrupt change from macrophyte to phytoplankton domination. This has been shown repeatedly in small lakes. Whether such alternative stable states also exist in large shallow lakes is less clear, however. Here we discuss the characteristics that give rise to alternative stable states in large shallow lakes either in the lake as whole or restricted to specific regions of the lake. We include the effect of lake size, spatial heterogeneity and internal connectivity on a lake's response along the eutrophication axis. As a case study, we outline the eutrophication history of Lake Taihu (China) and illustrate how lake size, spatial heterogeneity and internal connectivity can explain the observed spatial presence of different states. We discuss whether these states can be alternatively stable by comparing the data with model output (PCLake). These findings are generalised for other large, shallow lakes. We conclude that locations with prevailing size effects generally lack macrophytes; and, therefore, alternative stable states are unlikely to occur there. However, most large shallow lakes have macrophytes whose presence remains unexplained when only size effect is taken into account. By including spatial heterogeneity in the analysis, the presence of macrophytes and alternative stable states in large shallow lakes is better understood. Finally, internal connectivity is important because a high internal connectivity reduces the stability of alternative states

Nitrogen fixation does not axiomatically lead to phosphorus limitation in aquatic ecosystems

A long-standing debate in ecology deals with the role of nitrogen and phosphorus in management and restoration of aquatic ecosystems. It has been argued that nutrient reduction strategies to combat blooms of phytoplankton or floating plants should solely focus on phosphorus (P). The underlying argument is that reducing nitrogen (N) inputs is ineffective because N 2 -fixing species will compensate for N deficits, thus perpetuating P limitation of primary production. A mechanistic understanding of this principle is, however, incomplete. Here, we use resource competition theory, a complex dynamic ecosystem model and a 32-year field data set on eutrophic, floating-plant dominated ecosystems to show that the growth of non-N 2 -fixing species can become N limited under high P and low N inputs, even in the presence of N 2 fixing species. N 2 -fixers typically require higher P concentrations than non-N 2 -fixers to persist. Hence, the N 2 fixers cannot deplete the P concentration enough for the non-N 2 -fixing community to become P limited because they would be outcompeted. These findings provide a testable mechanistic basis for the need to consider the reduction of both N and P inputs to most effectively restore nutrient over-enriched aquatic ecosystems

Quantum Computing with Atomic Josephson Junction Arrays

We present a quantum computing scheme with atomic Josephson junction arrays. The system consists of a small number of atoms with three internal states and trapped in a far-off resonant optical lattice. Raman lasers provide the "Josephson" tunneling, and the collision interaction between atoms represent the "capacitive" couplings between the modes. The qubit states are collective states of the atoms with opposite persistent currents. This system is closely analogous to the superconducting flux qubit. Single qubit quantum logic gates are performed by modulating the Raman couplings, while two-qubit gates result from a tunnel coupling between neighboring wells. Readout is achieved by tuning the Raman coupling adiabatically between the Josephson regime to the Rabi regime, followed by a detection of atoms in internal electronic states. Decoherence mechanisms are studied in detail promising a high ratio between the decoherence time and the gate operation time.Comment: 7 figure

Matching scope, purpose and uses of planetary boundaries science

Background: The Planetary Boundaries concept (PBc) has emerged as a key global sustainability concept in international sustainable development arenas. Initially presented as an agenda for global sustainability research, it now shows potential for sustainability governance. We use the fact that it is widely cited in scientific literature (>3500 citations) and an extensively studied concept to analyse how it has been used and developed since its first publication. Design: From the literature that cites the PBc, we select those articles that have the terms 'planetary boundaries' or 'safe operating space' in either title, abstract or keywords. We assume that this literature substantively engages with and develops the PBc. Results: We find that 6% of the citing literature engages with the concept. Within this fraction of the literature we distinguish commentaries—that discuss the context and challenges to implementing the PBc, articles that develop the core biogeophysical concept and articles that apply the concept by translating to sub-global scales and by adding a human component to it. Applied literature adds to the concept by explicitly including society through perspectives of impacts, needs, aspirations and behaviours. Discussion: Literature applying the concept does not yet include the more complex, diverse, cultural and behavioural facet of humanity that is implied in commentary literature. We suggest there is need for a positive framing of sustainability goals—as a Safe Operating Space rather than boundaries. Key scientific challenges include distinguishing generalised from context-specific knowledge, clarifying which processes are generalizable and which are scalable, and explicitly applying complex systems' knowledge in the application and development of the PBc. We envisage that opportunities to address these challenges will arise when more human social dimensions are integrated, as we learn to feed the global sustainability vision with a plurality of bottom-up realisations of sustainability