The role of phytoplankton morphology on dominance under different turbulent regimes (PhytoMix)
This project studies phytoplankton communities and primary production, the modelling of which is crucial for fisheries management as well as climatic forcing.
The PhytoMix project involves a multi-disciplinary approach to achieve the aim of determining how morphology influences the response of a natural phytoplankton community to turbulent mixing. This will be accomplished via a series of mesocosm experiments which used convection-induced turbulence to create various turbulent regimes ranging from quiescent, stratified systems through to well-mixed systems. Over the course of the experiment, nutrient levels, growth rates, primary production were monitored and samples were taken for species identification and enumeration. The objective is to determine whether there has been a species response to the different turbulent regimes and if so, to what degree the species’ morphology played a role in this.
For ease, current ecosystem models parameterise the typical phytoplankton as simple shapes such as spheres, ellipsoids and cylindrical rods. However, it is known that phytoplanktonic cells display remarkable variation in size and shape (aka morphology) across the 30,000 to one million species thought to inhabit the world’s waterways. Furthermore, subtle phytoplankton-turbulence feedback mechanisms can have significant impacts on determining which species becomes dominant given a specific turbulent environment. While there has been some research into the influence of morphology on sinking rates, little work has been undertaken to ascertain how cell morphology affects a species’ ability to prosper in a given turbulent environment.
With support from the EU AQUACOSM program and the UK Royal Society, a 10-day experiment was performed on the large mesocosm facilities at Umeå Marine Science Centre (UMSC), Sweden from April to May 2018. The experiment used a natural phytoplankton community from the Gulf of Bothnia, placing them in 5m-deep mesocosms that permitted the manipulation of temperature, light conditions and turbulence regime. The 12 tanks were divided into four treatments (strongly stratified, weakly stratified, weakly mixed, and strongly mixed), and the evolution of the phytoplankton community in the tanks was observed. In addition, the turbulence environment was characterised in each tank while measurements of primary productivity and other water quality parameters were taken concurrently. The advantage of using mesocosms over natural sites is that the turbulence can be controlled and precisely quantified. Response of the phytoplankton to different turbulence regimes is being assessed with subsequent morphometric analyses of key species using flow cytometry and FlowCAM imaging.
The innovative aspect of this project is the combination of experimental and mathematical approaches at the physical, ecological and environmental expertise in one team. We will generate models of turbulent phytoplankton growth as deliverables that can be used in more complex biogeochemical or ecological models of marine environments. The data is still being processed, but preliminary analysis indicates different phytoplankton responses to different turbulence regimes. In addition, this is the first time that convection-induced turbulence has been used in the study of phytoplankton interactions. To this end, the turbulent regimes have been quantified and compared to those that exist in nature. It is postulated that convection-induced turbulence of this nature offers a number of benefits when compared to other methods of turbulence generation namely working on more realistic length scales while also avoid moving apparatus in the fluid medium that can prohibit sensors and damage phytoplankton cells.
As well as being responsible for 99% of all marine food chains, phytoplankton also produces up to 80% of our atmospheric oxygen. Being able to model phytoplankton primary production is crucial for fisheries management as well as climatic forcing. Furthermore, as climate change continues to alter the nature of our ocean, the abundance of various phytoplankton groups is shifting both temporally and spatially. In order to more accurately predict phytoplankton dynamics, physical parameters need to be considered yet too often, the role of turbulence in ecosystem modelling is omitted or at best vastly simplified. It is hoped that this research will add to the growing body of biological-physical interactions in allowing us to more accurately predict which species will become prosper / retreat given a set of physical forcings. It also hopes to introduce a new method of turbulence generation into the phytoplankton-turbulence community.
While the project is led via the University of Bath (Russell Arnott and Dr Lee Bryant, an international team of researchers is facilitating its progress. Dr Danielle Wain (Director of the 7 Lakes Alliance, Maine, USA and lecturer at Colby College, USA) is consulting on the physical analyses with a focus on the turbulent dynamics. Dr Mehdi Cherif (Ecologist at the Department of Ecology and Environmental Science, Umeå University, Sweden) is assisting with the statistical analyses as well as the biological considerations. Finally, Anastasia Tsotkou from the Hellenic Centre for Marine Research assisted with the laboratory-based biological analyses during the main experiment.