New constraints on assemblage‐driven variation in the relationship amongst diatom‐bound, biomass, and nitrate nitrogen isotope values

Tracking variations in the surface ocean supply and demand of nitrate, a key marine nutrient, can help constrain the contribution of biological production in driving past climate shifts. The nitrogen isotopic composition (as delta N-15) of organic matter in marine sediments is a proxy for surface ocean nitrate supply and demand over time, but it may be subject to alteration during sinking and burial. The isotopic composition of nitrogen occluded in the opal shells, or frustules, of diatoms (delta N-15(DB)) is protected and is, therefore, a potentially more robust tracer of nitrate use in the past. Here, we show that delta N-15(DB) in Southern Ocean growout cultures of natural communities does not depend on species composition. We found that the epsilon(DB) (= biomass delta N-15-delta N-15(DB)) of the community growouts was -4.8 +/- 0.8 parts per thousand, more than 10 parts per thousand different from previous monospecific growouts, but statistically indistinguishable from previous Southern Ocean and North Pacific surface ocean observations. The two community growouts, seeded with populations from similar to 66 degrees to similar to 61 degrees S, had distinct community compositions, but indistinguishable epsilon(DB), suggesting that species composition does not primarily set delta N-15(DB) values, at least in Antarctic and Polar Frontal Zones of the Southern Ocean. Our results demonstrate that under nitrate-replete conditions, delta N-15(DB) values of frustules sinking from the surface ocean robustly track surface ocean nitrate delta N-15 values, and therefore nitrate supply and demand.