Different Roles of Top-Down and Bottom-Up Processes in the Distribution of Size-Fractionated Phytoplankton in Gwangyang Bay

Yoonja Kang,Yeongji Oh

doi:10.3390/w13121682

Abstract

The interactive roles of zooplankton grazing (top-down) and nutrient (bottom-up) processes on phytoplankton distribution in a temperate estuary were investigated via dilution and nutrient addition experiments. The responses of size-fractionated phytoplankton and major phytoplankton groups, as determined by flow cytometry, were examined in association with zooplankton grazing and nutrient availability. The summer bloom was attributed to nanoplankton, and microplankton was largely responsible for the winter bloom, whereas the picoplankton biomass was relatively consistent throughout the sampling periods, except for the fall. The nutrient addition experiments illustrated that nanoplankton responded more quickly to phosphate than the other groups in the summer, whereas microplankton had a faster response to most nutrients in the winter. The dilution experiments ascribed that the grazing mortality rates of eukaryotes were low compared to those of the other groups, whereas autotrophic cyanobacteria were more palatable to zooplankton than cryptophytes and eukaryotes. Our experimental results indicate that efficient escape from zooplankton grazing and fast response to nutrient availability synergistically caused the microplankton to bloom in the winter, whereas the bottom-up process (i.e., the phosphate effect) largely governed the nanoplankton bloom in the summer.

Highlights

IntroductionPhytoplankton is responsible for more than half of primary production worldwide [1]
DIN levels were significantly high in the summer at 68.8 μM and sharply declined in the other seasons, with the lowest value detected in the winter (1.0 μM; Figure 2D)
We examined the interactive role of top-down and bottom-up processes in phytoplankton dynamics in Gwanyang Bay via dilution and nutrient amendment experiments

Summary

Introduction

Phytoplankton is responsible for more than half of primary production worldwide [1]. The key role of phytoplankton in biogeochemical cycles has received much attention [2,3]. The physical and chemical variables regulating phytoplankton dynamics are well disclosed [4,5], while nutrients are critical in partitioning niches among co-existing phytoplankton in the ocean [6,7]. Beyond these controlling factors, zooplankton grazing plays an essential role in shaping phytoplankton communities [8,9]. The interactive role of zooplankton grazing and nutrient supply often causes the formation of ecosystem disruptive blooms, during which harmful algae are able to avoid grazing and efficiently utilize various types of nutrient resources (e.g., the utilization of dissolved organic nitrogen when dissolved inorganic nutrients are limited) [10,11]

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Discussion

Conclusion