University of Bath

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.

Project outline

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 inhibit 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. The morphological properties of phytoplankton species have major effects on the functioning of aquatic ecosystems. It is of the utmost importance to faithfully characterize the effects of turbulence, a major factor that affects these properties.

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 Center (Sweden) from April to May 2018. The experiments used natural phytoplankton species from the Bothnian Sea in 5m deep mesocosms that permitted the manipulation of light conditions, nutrient concentration 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 species in the tanks was observed, in addition to turbulence, primary productivity, and other water quality parameters. 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.

Science

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 regime.

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.

Impact

As well as being responsible for 99% of all marine food chains, phytoplankton also produces 50 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 seas, 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.