Seston is suspended particulate organic matter, comprising a mixture of autotrophic, heterotrophic and detrital material. Despite variable proportions of these components, marine seston often exhibit relatively small deviations from the Redfield ratio (C:N:P = 106:16:1). Two time-series from the Norwegian shelf in Skagerrak are used to identify drivers of the seasonal variation in seston elemental ratios. An ordination identified water mass characteristics and bloom dynamics as the most important drivers for determining C:N, while changes in nutrient concentrations and biomass were most important for the C:P and N:P relationships. A generalized linear model was used to differentiate between the live autotrophic and non-autotrophic sestonic fractions, and for both stations the non-autotrophic fractions dominated with respective annual means of 24 and 45 % live autotrophs. The autotrophic percentage calculated from the statistical model was compared with estimated phytoplankton carbon, and the two independent estimates of autotrophic percentage were comparable with similar seasonal cycles. The estimated C:nutrient ratios of live autotrophs were in general lower than Redfield, while the non-autotrophic C:nutrient ratios were higher than the live autotrophic ratios and above or close to the Redfield ratio. This is due to preferential remineralization of nutrients (especially phosphorus), while carbon gradually builds up in the detritus pool. Despite the seasonal variability in seston composition and the generally low contribution of autotrophic biomass, the variation observed in the total seston ratios was low compared to the variation found in dissolved and particulate pools. This study shows that sestonic Redfield ratios cannot automatically be interpreted as phytoplankton with "balanced growth", but could instead reflect varying contributions of sestonic compartments that sum up to an elemental ratio close to Redfield
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