Abstract
Phytoplankton regulate internal pigment concentrations in response to light and nutrient availability. Chlorophyll a to phytoplankton carbon ratios (chl:Cphyto) are commonly reported as a function of growth irradiance (Eg) for evaluating the photoacclimation response of phytoplankton. In contrast to most culture experiments, natural phytoplankton communities expe- rience fluctuating environmental conditions, making it difficult to compare field and lab observa- tions. Observing and understanding photoacclimation in nature is important for deciphering changes in chl:Cphyto resulting from environmental forcings and for accurately estimating net pri- mary production (NPP) in models which rely on a parameterized description of photoacclimation. Here we employ direct analytical measurements of Cphyto and parallel high-resolution biomass estimates from particulate backscattering (bbp) and flow cytometry to investigate chl:Cphyto in nat- ural phytoplankton communities. Chl:Cphyto observed over a wide range of Eg in the field was con- sistent with photoacclimation responses inferred from satellite observations. Field-based photoac- climation observations for a mixed natural community contrast with laboratory results for single species grown in continuous light and nutrient-replete conditions. Applying a carbon-based NPP model to our field data for a northsouth transect in the Atlantic Ocean results in estimates that closely match 14 C depth-integrated NPP for the same cruise and with historical records for the dis- tinct biogeographic regions of the Atlantic Ocean. Our results are consistent with previous satel- lite and model observations of cells growing in natural or fluctuating light and showcase how direct measurements of Cphyto can be applied to explore phytoplankton photophysiology, growth rates, and production at high spatial resolution in situ.
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