Winter diatom blooms and their effect on lake ecosystems (Acronym: DIABLO - DIAtom BLOoms)

• Applicant: Dr. Tom Shatwell
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 436793656

Global change is rapidly altering winter conditions in lakes. However it is difficult to predict the consequences because limnologists have historically focused on the “vegetation period” from spring through autumn and little is known about winter ecology. Until recently winter was generally considered to be ecologically dormant because there is insufficient light for phytoplankton to grow until stratification begins in spring. Contrary to this view, there are many lakes in which phytoplankton – specifically large-celled diatoms – can form dense blooms in late winter before stratification. This phenomenon is little known yet not uncommon in temperate lakes. Moreover, there is evidence that these blooms can strongly alter lake ecosystems in the subsequent seasons by sequestering nutrients and decreasing phytoplankton biomass. We urgently need to better understand these blooms because ice cover is decreasing and winter mixing is changing in temperate zones and we expect these blooms to become more frequent as a result. In this project we want to characterize the causes and consequences of late-winter diatom blooms. Our main hypothesis is that large-celled diatom species can develop high biomasses in late winter if 1) the mixing period is of sufficient duration and 2) lake depth and water clarity permit a certain amount of average light. In turn, late-winter diatom blooms strip nutrients from surface water through sedimentation, which alters lake biogeochemistry and ecology in spring and summer. Our approach combines 1) an analysis of over 100 collective years of high quality data from four of Germany’s best-monitored lakes and reservoirs, all of which have the required depth and water clarity range and support large winter biomasses of diatoms, 2) field measurements of key processes during winter and spring in one of our study lakes, and 3) coupled 1-D hydrodynamic-ecological modelling of three of the study lakes. We plan to expose the relationships between the winter conditions and the seasonal phytoplankton and nutrient dynamics, and determine the presence of feedbacks that promote late-winter diatoms and can lead to alternative stable states. We will quantify vertical fluxes of carbon and nutrients, winter phytoplankton inocula, and internal nutrient storage. Finally the modelling effort will integrate the knowledge gained into mathematical process descriptions. The expected outcome is a comprehensive systems understanding in the form of a conceptual framework and predictive models of the occurrence of late-winter diatom blooms and their effect ecology in the rest of the year. This will help to close the seasonal loop and better understand the factors that control water quality in lakes.

• DFG Programme: Research Grants