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Abstract Mixed‐layer dynamics exert a first order control on nutrient and light availability for phytoplankton. In this study, we examine the influence of mixed‐layer dynamics on net community production (NCP) in the Southern Ocean on intra‐seasonal, seasonal, interannual, and decadal timescales, using biogeochemical Argo floats and satellite‐derived NCP estimates during the period from 1997 to 2020. On intraseasonal timescales, the shoaling of the mixed layer is more likely to enhance NCP in austral spring and winter, suggesting an alleviation of light limitation. As expected, NCP generally increases with light availability on seasonal timescales. On interannual timescales, NCP is correlated with mixed layer depth (MLD) and mixed‐layer‐averaged photosynthetically active radiation (PAR) in austral spring and winter, especially in regions with deeper mixed layers. Though recent studies have argued that winter MLD controls the subsequent growing season's iron and light availability, the limited number of Argo float observations contemporaneous with our satellite observations do not show a significant correlation between NCP and the previous‐winter's MLD on interannual timescales. Over the 1997–2020 period, we observe regional trends in NCP (e.g., increasing around S. America), but no trend for the entire Southern Ocean. Overall, our results show that the dependence of NCP on MLD is a complex function of timescales. 

An inherent issue in high-throughput rRNA gene tag sequencing microbiome surveys is that they provide compositional data in relative abundances. This often leads to spurious correlations, making the interpretation of relationships to biogeochemical rates challenging. To overcome this issue, we quantitatively estimated the abundance of microorganisms by spiking in known amounts of internal DNA standards. Using a 3-year sample set of diverse microbial communities from the Western Antarctica Peninsula, we demonstrated that the internal standard method yielded community profiles and taxon cooccurrence patterns substantially different from those derived using relative abundances. We found that the method provided results consistent with the traditional CHEMTAX analysis of pigments and total bacterial counts by flow cytometry. Using the internal standard method, we also showed that chloroplast 16S rRNA gene data in microbial surveys can be used to estimate abundances of certain eukaryotic phototrophs such as cryptophytes and diatoms. In Phaeocystis, scatter in the 16S/18S rRNA gene ratio may be explained by physiological adaptation to environmental conditions. We conclude that the internal standard method, when applied to rRNA gene microbial community profiling, is quantitative and that its application will substantially improve our understanding of microbial ecosystems.