Phytoplankton diversity and nitrogen use in an upwelling system
The ocean’s most productive regions are associated with eastern boundary currents where the upwelling of cold, nutrient-rich waters from depth stimulates phytoplankton blooms. The phytoplankton community in these regions tends to be dominated by diatoms, although the exact mechanism for diatom success has never been explicitly demonstrated. By simulating upwelling events in large mesocosms and monitoring nutrient uptake, biomass concentrations, and phytoplankton community composition over the course of the subsequent bloom, we investigated the response of diatoms (along with the entire phytoplankton community) to upwelling conditions (figure 1). We found that the biomass was initially dominated by the smallest phytoplankton, but the largest phytoplankton (which light microscopy counts and pigment analyses suggest were mainly diatoms) increased most rapidly to dominate the biomass, and rate of nitrogen assimilation and carbon fixation after less than four days. While all phytoplankton size fractions achieved similar maximum specific nitrate uptake rates (i.e., absolute uptake rates normalized to biomass; VNO3), this occurred most rapidly and was maintained longest by the largest phytoplankton. This strategy appears to be the mechanism by which diatoms exploit upwelling conditions; their initial rapid acceleration and subsequent maintenance of a high VNO3 allows diatoms to dominate algal biomass in upwelling environments as it gives them access to a disproportionate fraction of the available nutrients. Pigment measurements and light microscopy independently documented changing phytoplankton abundance, with diatoms demonstrating a characteristic pattern of succession: initially, the diatom community was dominated by a few ubiquitous oceanic and coastal species that are known to succeed under low nutrient conditions, and diversity was low. As nitrate became available, diversity increased due to an increase in the abundance of upwelling diatom species that were able to efficiently exploit the new environmental conditions. By the time all available nitrate had been consumed, the diatom assemblage was completely dominated by upwelling species, and diversity was once again low.
The mesocosm experiments suggest that the ability of large diatoms to accelerate and maintain elevated nitrate uptake rates explains their dominance over other phytoplankton groups during upwelling. Moreover, the observed delay in biomass accumulation following nitrate supply after the initiation of upwelling events has been hypothesized to result from changes in the diatom community structure or from physiological acclimation. To investigate these hypotheses, we calibrated the “Tracers Of Phytoplankton and Allometric Zooplankton” (TOPAZ) ecosystem model with a coastal upwelling species assemblage (figure 2), and used it as a framework to evaluate the results of the mesocosm experiments. We found that to accurately reproduce the observed patterns and timescales of size-partitioned new production in a non-steady state (i.e., upwelling) environment, variations in functional group-specific traits must be taken into account, through adjustments of group-dependent maximum production rates (PCmax, s-1; figure 3). The model required neither nutrient acclimation nor diatom diversity to simulate the patterns observed in the mesocosms, provided that lower-than-theoretical-maximum production rates were implemented. We conclude that this physiological feature, PCmax, is critical for representing the early, relatively higher specific nitrate uptake rate of large diatoms, and explains the differential success of small and large phytoplankton communities in response to nitrate supply during upwelling.
Flynn, R.F., Burger, J.M., Pillay, K., Fawcett, S.E. Wintertime rates of net primary production and nitrate and ammonium uptake in the southern Benguela upwelling system. African Journal of Marine Science 40, 253-266 (2018). Flynn AJMS 2018
Van Oostende, N., Dunne, J.P., Fawcett, S.E., Ward, B.B. Phytoplankton succession and acclimation explains nitrate uptake following an upwelling event. Journal of Marine Systems 148: 14-25 (2015). Van Oostende JMarSys 2015
Fawcett, S.E., Ward, B.B. Phytoplankton succession and nitrogen utilization during the development of an upwelling bloom. Marine Ecology Progress Series 428: 13-31 (2011). Fawcett Ward MEPS 2011