Marine primary producers in a darker future – a meta-analysis of light effects on pelagic and benthic autotrophs

The availability of underwater light, as the primary energy source for all aquatic photoautotrophs, is (and will further be) altered by changing precipitation, water turbidity, mixing depth, and terrestrial input of chromophoric dissolved organic matter (CDOM). While experimental manipulations of CDOM input and turbidity are frequent, they often involve multiple interdependent changes (light, nutrients, C-supply). To create a baseline for the expected effects of light reduction alone, we performed a weighted meta-analysis on 240 published experiments (from 108 studies yielding 2,500 effect sizes) that directly reduced light availability and measured marine autotroph responses. Across all organisms, habitats, and response variables, reduced light led to an average 23% reduction in biomass-related performance, whereas the effect sizes on physiological performance did not significantly differ from zero. Especially pigment content increased with reduced light, which indicated strong physiological plasticity in response to diminished light. This acclimation potential was also indicated by light reduction effects minimized if experiments lasted longer. Nevertheless, performance (especially biomass accrual) was reduced the more the less light intensity remained available. Light reduction effects were also more negative at higher temperatures if ambient light conditions were poor. Macrophytes or benthic systems were more negatively affected by light reduction than microalgae or plankton systems, especially in physiological responses where microalgae and plankton showed slightly positive responses. Otherwise, effect magnitudes remained surprisingly consistent across habitats and aspects of experimental design. Therefore, the strong observed log-linear relationship between remaining light and autotrophic performance can be used as a baseline to predict marine primary production in future light climate.