The effects of nutrient additions to phytoplankton size structures based on chlorophyll in the Equatorial Pacific Ocean

Abstract

Phytoplankton, the primary producers of ocean, have produced over half of the oxygen in the atmosphere and use dissolved carbon dioxide to accumulate biomass which is critical to the biological pump. In High-Nutrient, Low-Chlorophyll zones such as the equatorial Pacific, smaller phytoplankton (<10 μm) are more abundant than larger phytoplankton (>10 μm). The composition of size structure of phytoplankton communities changes with the abundance of nutrients, due to larger phytoplankton having greater nutrient needs. The addition of nutrients can give insight into how the biological pump may be affected by changing conditions in the ocean, as larger size classes of phytoplankton have faster sinking rates. In incubation experiments conducted on the TN413 cruise, size-fractionated chlorophyll concentrations were compared between treatments of different nutrients as well as nutrient-rich water from below the mixed layer to simulate upwelling in the region. I hypothesize that an increase in nutrient availability will result in greater abundance of larger phytoplankton due to nutrient co-limitation. The addition of deep water had the largest affect on chlorophyll concentration measured across all size classes in the incubation experiments while the addition of nitrate, silicate, and iron with deep water had little observable effect on chlorophyll concentrations. Results of added nutrients to incubations were contrasting, with phytoplankton larger than 10 μm increasing in abundance in incubations from the equator and smaller phytoplankton less than 10 μm dominating at 5° North, while at 5° South almost no growth was observed. Co-limiting nutrients are strongly implied, although future work at this location without deep water additions to incubations is necessary to determine what nutrients are limiting in this area to better understand how the biological pump may be affected by nutrient availability.