Patrick Schultz

High-latitude phytoplankton blooms support productive fisheries and play an important role in oceanic uptake of atmospheric carbon dioxide. In the subarctic North Atlantic Ocean, blooms are a recurrent feature each year, while in the eastern subarctic Pacific only small changes in chlorophyll (Chl) are seen over the annual cycle. Here we show that when evaluated using phytoplankton carbon biomass (Cphyto) rather than Chl, an annual bloom in the North Pacific is evident and can even rival blooms… Show more

High-latitude phytoplankton blooms support productive fisheries and play an important role in oceanic uptake of atmospheric carbon dioxide. In the subarctic North Atlantic Ocean, blooms are a recurrent feature each year, while in the eastern subarctic Pacific only small changes in chlorophyll (Chl) are seen over the annual cycle. Here we show that when evaluated using phytoplankton carbon biomass (Cphyto) rather than Chl, an annual bloom in the North Pacific is evident and can even rival blooms observed in the North Atlantic. The annual increase in subarctic Pacific phytoplankton biomass is not readily observed in the Chl record because it is paralleled by light- and nutrient-driven decreases in cellular pigment levels (Cphyto:Chl). Specifically, photoacclimation and iron stress effects on Cphyto:Chl oppose the biomass increase, leading to only modest changes in bulk Chl. The magnitude of the photoacclimation effect is quantified using descriptors of the near-surface light environment and a photophysiological model. Iron stress effects are diagnosed from satellite chlorophyll fluorescence data. Lastly, we show that biomass accumulation in the Pacific is slower than that in the Atlantic but is closely tied to similar levels of seasonal nutrient uptake in both basins. Annual cycles of satellite-derived Chl and Cphyto are reproduced by in situ autonomous profiling floats. These results contradict the long-standing paradigm that environmental conditions prevent phytoplankton accumulation in the subarctic Northeast Pacific and suggest a greater seasonal decoupling between phytoplankton growth and losses than traditionally implied. Further, our results highlight the role of physiological processes in shaping bulk properties, such as Chl, and their interpretation in studies of ocean ecosystem dynamics and climate change. Show less

Nutrient inputs to lakes, reservoirs, estuaries, and continental shelf waters are often dominated by nutrient release from sediments. Rates of nutrient release from the decomposition of sedimentary organic matter are determined by bacterial demands for food and energy. Here, we present a new model of early diagenesis that is based on soil decomposition models and that explicitly includes bacterial biomass as a state variable. The model predicts that bacterial abundance in the sediments and… Show more

Nutrient inputs to lakes, reservoirs, estuaries, and continental shelf waters are often dominated by nutrient release from sediments. Rates of nutrient release from the decomposition of sedimentary organic matter are determined by bacterial demands for food and energy. Here, we present a new model of early diagenesis that is based on soil decomposition models and that explicitly includes bacterial biomass as a state variable. The model predicts that bacterial abundance in the sediments and nutrient release rates increase significantly with higher substrate inputs and higher quality of organic substrate. High growth efficiencies, on the other hand, reduce nutrient release rates and lead to more nutrient immobilization into bacterial biomass. Efficiency driven increases in bacterial abundances must be counteracted by higher mortality rates, if bacterial pool sizes are to remain within the range of values reported in the literature. Second, we simulate eutrophication and lake recovery over a 20-year period and the effect of a short-term peak in substrate input from an algal bloom. The model predicts that nutrient immobilization in the bacterial pool during eutrophication reduces nutrient release, while the decline of bacterial biomass in response to reduced substrate loadings prolongs lake recovery. The model further suggests that bacterial dynamics dampen the response of nutrient release to short-term peak inputs of organic matter. When bacterial biomass is allowed to vary over time, nutrient release rates are suppressed at their peak but remain elevated for a longer time. Models that do not consider the role of bacterial growth dynamics for organic matter decomposition are not able to simulate the potentially important effects of nutrient immobilization and variable decomposition rates on nutrient release. Show less