Phytoplankton response to fluctuating light intensity and spectrum II - How fluctuations in the multiple dimensions of LIGHt shape phytoplankTON (LIGHT-ON)

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A fluctuating environment constrains the ability of a population or community to adapt. Thereby highly predictable fluctuations will trigger different responses of organisms and communities than more stochastic ones. These responses are non-linear and therefore do not average across time, which hampers predicting future ecosystem functionality based on mean environments. A steadily increasing number of studies demonstrate the importance of thermal variability around the average, but less studies exist for other environmental variables besides temperature. Here, we ask the fundamental question how phytoplankton organisms cope with light variation often between stressful and advantageous conditions, especially if anthropogenic forcing shifts the variance beyond the level of fluctuations that would naturally occur in these habitats. Light fluctuations range from predictable seasonal and diurnal components to stochastic changes within seconds through cloud cover and scattering, comprising the intensity and spectral composition of light, as absorption and scattering are wavelength-specific. Thus, light needs to be addressed as a multi-dimensional and hypervariable resource, as moments of variability (amplitude, frequency and stochasticity) apply to multiple wavelengths. Highly variable light conditions in aquatic systems directly impact phytoplankton photosynthesis, respiration, growth rates, and biochemical composition. However, most of the empirical studies on light used regular fluctuations as drivers of community dynamics and the majority focused on a single dimension of the resource (mostly total irradiance). The innovative approach here is to study the interactive effect of light intensity and light spectrum, to understand the combined effects of multiple fluctuating resources on phytoplankton by manipulating the frequency, amplitude and stochasticity of fluctuations around different means. We start with analyzing how fluctuations in light intensity and spectrum occur in nature and conduct experiments with monoclonal cultures to understand species-specific responses. In the next step we test whether interspecific interactions alter the responses using controlled lab settings and manipulations for multispecies assemblages. Experiments will be associated with simulations to shed further light into underlying mechanisms and predict scenarios that cannot be sufficiently tested experimentally. Finally, we will test our hypotheses on in-situ phytoplankton communities in controlled indoor mesocosms and in field experiments. This project helps understanding and predicting species and community responses to changes in light intensity and spectral composition fluctuations (e.g. changed amplitude, frequency and stochasticity, resp.).