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.