The Stability Of Size-structured Aquatic Ecosystems

Size-spectrum models are a recent class of models describing the dynamics of a whole community based on a description of individual organisms.The models can either cover the size range from multicellular plankton to the largest fish in an aquatic ecosystem,or depict the fish ontogeny during its life history.We propose to extend the size spectrum model with spatial components,and explore the responses of community structures as well as ecosystem functions in the context of global changes.The work is structured as follows.Chapter 1:We firstly introduce the background of aquatic ecosystems,including the formation mechanism of patterns in marine ecosystems,the impact of human forces on aquatic ecosystems,and the predator-prey relationships in shallow lake ecosystems.Then we describe the relevant research methods,including Lotka-Volterra model and sizestructured model.Finally we state the main contents of this paper.Chapter 2:For the aim of exploring spatial drivers of instability in marine ecosystems,we augment the size-spectrum model to the ecosystem level,which synthetically takes fishes'growth,maintenance,reproduction and mortality into account,and further extend it with diffusion and fitness taxis in the local and non-local systems,respectively.Making use of the Fourier analyses and stability analyses,we derive the equilibrium and discuss what effects these spatial movements have on the stability of marine ecosystem.Results show that diffusion always plays a stabilizing role whereas fitness taxis destabilizes the system.There is a strong asymmetry between fitness-induced destabilizing and diffusion-induced stabilizing effects with the latter dominating over the former.These findings reveal that fitness taxis acts as a possible mechanism behind pattern formations in ecosystems with high diversity of organism sizes,which can drive the emergence of spatial heterogeneity even in a spatially homogeneous environment.Chapter 3:In the present chapter,we focus on shallow lake ecosystems,developing a process-based model describing light and nutrient dynamics as well as multiple trophic interactions including algae,grazers and size-structured fish.Specifically,we try to elucidate how human-driven environmental changes affect community structure,stability and ecosystem functioning when fish ontogeny is taken into account.Our study supports the previous finding on trophic controls,and further provides important understanding of how bottom-up forcing mediates species coexistence.Fish ontogeny destabilizes ecosystem across nutrient gradients,and the shallow lake experiences a reduction in total biomass.Nutrient enrichment and irradiance subsidy can induce community structure changes including increased maximal fish length and declined average fish length.Results further disclose that the primary production follows a dome-shaped pattern along nutrient gradient,and the benthic algae makes a considerable contribution to whole-lake primary production.Our results highlight the significant role of the size-structured trophic interactions in regulating shallow lake dynamics,and emphasize the need to include both spatial asymmetry and fish ontogeny for comprehensively understanding the responses of shallow lakes to anthropogenic forces in a context of global change.Chapter 4:We focus on the impacts of human activities on aquatic ecosystems.We introduce a boldness trait for consumer individuals,and further apply it to a physiologically structured population model(three trophic levels including grazers,perch and roach),well fed with empirical data,to simulate how previously suggested alterations of fish boldness traits due to anthropogenic stressors affect ecosystem structure.Our results suggest that anthropogenic stressors,such as chemicals,microplastics,acoustic emissions and fisheries,may cause ecosystem stability and structure effects,such as skewed size distributions,reduced fish biomass and reduced reproduction success,by altering the foraging behavior of fish.The counterintuitive effects may lead to possible extinction of predators when the foraging behavior of the prey is hampered.We conclude that anthropogenic forcing of fish behavior may be a hidden mechanism behind ecosystem structure changes in aquatic ecosystems.Chapter 5:Finally,we make a summary of our results,and put forward some problems needed to be studied in the future.