Effect of nutrient amendments on the summer phytoplankton dynamics on the Bering Sea shelf under experimental conditions

Abstract

Experiments on nutrient and iron amendments were performed with phytoplankton on the eastern shelf of the Bering Sea in June 2000 and August 2001. The nutrient amendments (NO3, NH4, SiO4, NO3 + SiO4, NH4 + SiO4, and Fe + NO3) increasing their initial concentrations by ∼20 μM were put into test bottles 10 l in volume each. With iron addition (Fe or Fe + NO3), its concentration increased by 5 nM. The experiments performed showed that the main nutrient that limited the phytoplankton development was nitrogen. Regardless of the composition of the dominant algae in the background community, the amendments caused massive development of diatoms. The intense growth was characteristic for diatoms of both the spring and spring-summer assemblages. At high abundances of Phaeocystis pouchetii or of the coccolithophore Emiliania huxleyi in the natural water, nitrogen-containing amendments caused an intense growth of these species, along with the massive development of diatoms. In the case of the diatom prevalence in the initial sample, the intensities of the utilization of NO3 and NH4 in combination with SiO4 in the course of the experiment were 1.7 and 3 times as high as their intensities with no silicon amendments. Likewise, NO3 + SiO4 and NH4 + SiO4 mixed amendments caused an increase in the silicon assimilation by a factor of 4–5 as compared to pure silicon amendments. During one of the experimental series in which both diatoms and Phaeocystis pouchetii actively developed, virtually complete nitrogen utilization (90–99.8%) in 4–5 days was observed for both the NO3 and NH4. The addition of silicon and iron only caused no significant growth of the phytoplankton abundance. It was assumed that the destruction of the seasonal thermocline and the supply of nutrients into the surface layer as a result of strong wind forcing might cause a phytoplankton bloom in the summer time and result in the much pronounced qualitative and quantitative spatial heterogeneity of the phytoplankton characteristic of the eastern shelf of the Bering Sea.