The effects of changing temperatures on diatom-chytrid dynamics and carbon fluxes to lake sediments

Diatoms are globally important primary producers that form the base of marine and freshwater food

webs. They possess relatively large, heavy cells and so tend to be favoured during spring and

autumn, when water columns in temperate lakes are unstratified, and water mixing prevents losses

due to sinking. As a consequence, spring diatom blooms are a regular and well-recognised feature of

temperate lakes and seas, with cell densities increasing by several orders of magnitude in a few

weeks. These blooms are terminated by the onset of stratification, grazing, or infection by parasites,

with cells sinking to the sediment or being recycled at the lake surface. The termination and sinking

of these diatom blooms is an important contributor to aquatic carbon export. In recent years, the

importance of freshwater phytoplankton (including diatoms) for global carbon budgets has

increasingly been recognised. Lakes remain net emitters of carbon because of high gas fluxes, but

carbon fluxes to lake sediments exceed fluxes to ocean sediments despite their vastly smaller area.

The magnitude of these fluxes means that as warming alters thermal regimes and causes an earlier

onset of stratification, it may change the magnitude of diatom carbon export to the sediment at a

globally meaningful scale.

Asterionella formosa is a large, common diatom species that dominates spring blooms in many

temperate lakes worldwide. These blooms are often terminated by a fungal parasite, the chytrid

Zygorhizidium. As a consequence, this host-parasite interaction shifts phytoplankton community

composition and likely affects the amount of carbon exported to lake sediments in spring. Increasing

temperatures are likely to alter this interaction, with uncertain consequences for ecosystems. In a

typical cold winter, Zygorhizidium is present as inactive resting spores. This allows Asterionella a

window of unimpeded growth during the winter-spring transition, leading to the formation of a

spring diatom bloom. Once the chytrid resting spores hatch, they rapidly infect Asterionella cells and

increase in abundance till they kill the bloom. In contrast, warmer winters allow Zygorhizidium to

maintain infections over winter, preventing Asterionella from increasing in abundance and forming a

bloom. This shifts the community composition, often towards smaller species that sink more slowly,

which alters both lake food webs and carbon fluxes. We aim to test the hypothesis that winter

temperatures above 3 °C prevent the formation of spring Asterionella blooms by enabling chytrid

growth. We will use a combination of lab experiments and models to understand how temperature

affects Asterionella-Zygorhizidium dynamics. Furthermore, we aim to provide an initial estimate of

how warming-driven changes in these biotic interactions may alter future aquatic carbon fluxes. We

will use models and field studies to link Asterionella-Zygorhizidium dynamics to carbon fluxes in two

lakes, Lake Geneva in Switzerland and Lake Maarsseveen in the Netherlands. Our work attempts to

understand how a changing abiotic environment (temperature) affects a complex ecological

interaction (parasitism) to ultimately alter a globally important ecosystem parameter (carbon flux). It

will develop a fundamental theoretical and empirical understanding of the temperature-dependence

of host-parasite interactions, while addressing the consequences of environmental change for

ecosystem processes