The neglected role of phytoplankton adaptation in marine ecosystem models — Implications for climate projections and future perspectives

Abstract

Marine ecosystems perform functions that are vital to global climate and human life, including the long-term storage of atmospheric carbon and food production. Despite their small size, phytoplankton are key players in marine ecosystem functioning, not only as primary producers, but also as a crucial component of biogeochemical cycles and climate-relevant feedback loops. Global warming, however, leads to a rapid reorganization of phytoplankton communities, with severe consequences for ecosystem functioning. To reliably assess future changes in ecosystem functioning, we need robust approaches.

While sedimentary records and marine monitoring data provide information on the past to present state of an ecosystem, albeit on different temporal scales, laboratory and mesocosm experiments allow to study marine organisms and communities under future levels of warming. Still, experiments can neither replicate the complexity of real ecosystems nor provide realistic rates of environmental change. Ecosystem models can fill this gap and are therefore a very well-suited tool for estimating future species- to ecosystem-level changes. However, while ecosystem models are increasingly being used to assess future changes in ecosystem functions such as net primary production and carbon export, their projections are inconsistent and differ not only in magnitude but also in direction. Since model projections represent a valuable resource for political decision-making, improving their informative value is crucial.

This thesis aims to help improve model projections of future ecosystem change by addressing two major uncertainties in current ecosystem models. First, most ecosystem models ignore the high adaptive potential of phytoplankton, which can be relevant on perennial time scales. We show that phytoplankton adaptation can significantly change simulated phytoplankton and ecosystem dynamics under both present-day and future climate conditions. Second, most ecosystem models are developed for ecosystems in their current state, so that they may no longer be applicable when ecosystem structure has changed in the future. We present a conceptual framework that makes use of data from sediment archives to develop evolutionary ecosystem models that are equally applicable to the past, the present, and, by implication, the future. This work not only emphasizes the need to revise current ecosystem models, especially those relevant for political decisions, but also provides approaches for solutions.