Preferential recycling of nutrients—the ocean's way to increase new production and to pass nutrient limitation?

Abstract

Uptake of atmospheric CO2 by the oceans and the export of carbon into deeper waters via the biological CO2 pump is driven by the production of particulate organic matter (POM). The elemental ratios of carbon, nitrogen, and phosphorus within POM follow Redfield ratios, suggesting that the concentrations of dissolved inorganic carbon (DIC) and nutrients decrease during new production corresponding to these ratios. Subsequently, new production and the efficiency of the biological CO2 pump are usually estimated using Redfield ratios. However, our observations in the Baltic Sea and obser‐vations elsewhere show significantly greater decreases in DIC during the productive season than that predicted from the decline in nutrients with reference to Redfield ratios. Using new DIC, nutrient, and oxygen data from the Baltic Sea, we discuss this discrepancy and provide evidence that preferential recycling of nutrients fuels the productive community with nutrients. Limiting nutrients are preferentially recycled and become available for new production. These findings derived from hydrochemical bulk data confirm the picture of the microbial loop but question the common description of new production and nutrient limitation. Finally, we argue for a carbon‐based efficiency estimate of the biological CO2 pump probably exceeding significantly nutrient‐based estimates.