Historically, elemental stoichiometry (Redfield ratios) have been used as a tool in biogeochemical models. More and more we are realizing that long-term changes in Redfield ratios can be used as indicators of shifts in ecosystem function, either because of shifts in phytoplankton species assemblages, or because of shifts in micronutrient availability. Work on alterations in N:P stoichiometry at the Hawaii Ocean Time-series and the Bermuda Atlantic Time Series has shown that environmental shifts favoring nitrogen fixers can alter marine ecosystems and potentially C flux. Similarly, work on alterations in Si:N stoichiometry in HNLC regions has shown that low Fe availability can induce silicon limitation and further regulate diatom production. Emerging work using microassays to elucidate single-cell nutrient status in mixed phytoplankton populations may lead to a mechanistic understanding of these phenomena. This session will highlight recent work on mechanisms behind shifts in Refield nutrient stoichiometry and the implications behind such shifts for ecosystem function.