An eco-physiological interpretation of phytoplankton primary production and bulk community properties within the photic layer of marine water columns – Part I: Primary findings

Using models and field observations, we evaluated dominant eco-physiological factors governing phytoplankton bulk community properties in the photic layer. We find that open ocean phytoplankton populations are either single-factor light limited or co-limited by light and nutrients. Single-factor limitation occurs when light levels are insufficient for phytoplankton to completely utilize available nutrients (e.g., below the nutricline in oligotrophic regions or seasonally at higher latitudes). Phytoplankton are light-nutrient co-limited in oligotrophic surface mixed layers and the mid-column between the base of the mixed layer and nutricline. In these waters, light jointly limits phytoplankton division rates by modifying nutrient recycling, but has little impact on standing stocks. In the co-limited condition, phytoplankton chlorophyll-to-carbon ratios ( Chl : C phyto ) exhibit a dampened dependence on growth irradiance ( I g ) compared to single-factor light-limitation because cellular signals to decrease chlorophyll synthesis with increasing I g (i.e., ‘photoacclimation’) are countered by signals to increase chlorophyll synthesis with enhanced light-driven nutrient recycling. At the base of the photic layer, chlorophyll synthesis is actively (i.e., reductant consuming) down-regulated, mechanistically explaining the common appearance of deep Chl : C phyto maxima. A Depth-Explicit Eco-physiological Production (DEEP) model is described capturing these and other new insights on natural phytoplankton populations. The DEEP model provides robust estimates of net primary production (NPP) when compared to field long-term and survey data, with improved performance statistics compared to previous algorithms. Sources of uncertainty and a path to DEEP model implementation with satellite data are discussed, along with potential satellite retrievals of nutricline depth, mixed layer nutrient concentration, and phytoplankton size distribution.