Water Quality Improvement Shifts the Dominant Phytoplankton Group From Cryptophytes to Diatoms in a Coastal Ecosystem

Yoonja Kang,Chang-Ho Moon,Chang-Keun Kang,Hyun-Jung Kim,Yang Ho Yoon

doi:10.3389/fmars.2021.710891

Abstract

We investigated long-term variations in the dominant phytoplankton groups with improvements in water quality over 11 years in the Yeongil Bay on the southeastern coast of Korea. River discharge declined during the study period but TN from river discharge remained stable, indicating the input of enriched nutrients to the bay was fairly consistent. NH4+ levels decreased with a decrease in TN from the POSCO industrial complex. While the study region was characterized by the P-limited and deficient environment, cryptophytes dominated with the intensified P-limitations. The relative abundance of cryptophytes declined from 70% in 2010 to 10% in 2016, but that of diatoms increased from 70% in 2009 to 90% in 2016. Correlation analysis showed a positive correlation of cryptophytes with NH4+ and a negative correlation with photic depth. Generalized additive models also exhibited an increase in diatom dominance and a decrease in cryptophyte dominance with an increase in water quality, indicating that a decrease in NH4+ and increase in light favored the diatom growth but suppressed the cryptophyte growth. Thus, water quality improvements shift the dominant group in the coastal ecological niche from cryptophytes to diatoms.

Highlights

Coastal ecosystems in bays and estuaries experience strong land-ocean interactions (Spatharis et al, 2007; Wang et al, 2020)
Cryptophytes are a cosmopolitan phytoplankton group found under a wide range of environmental conditions (Clay et al, 1999); previous studies on cryptophyte blooms have been confined to either the mixotrophic strategy or light conditions (Barone and Naselli-Flores, 2003; Adolf et al, 2008; Mendes et al, 2013; Johnson et al, 2018)
Long-term phytoplankton dynamics in Yeongil Bay in response to environmental variations were investigated to assess the dominant groups, which are altered by water quality improvements

Summary

Introduction

Coastal ecosystems in bays and estuaries experience strong land-ocean interactions (Spatharis et al, 2007; Wang et al, 2020). Phytoplankton biomass (i.e., chlorophyll a) is often utilized as an indicator of water quality conditions because of its strong interaction with environmental conditions (Boyer et al, 2009; Sathicq et al, 2017), and phytoplankton photopigments identifying the functional phytoplankton groups can be applied to assess coastal eutrophication (Paerl et al, 2003). The ecological niche of phytoplankton is based on the nutrient strategies and functional differences among phytoplankton groups (Gobler et al, 2011; Alexander et al, 2015b). It is necessary to understand how the dominant group within a phytoplankton niche changes with variations in environmental conditions because it reveals the underlying mechanisms of phytoplankton bloom dynamics associated with group-specific or speciesspecific ecological and physiological features (Litchman et al, 2012; Brun et al, 2015). Cryptophytes are a cosmopolitan phytoplankton group found under a wide range of environmental conditions (Clay et al, 1999); previous studies on cryptophyte blooms have been confined to either the mixotrophic strategy or light conditions (Barone and Naselli-Flores, 2003; Adolf et al, 2008; Mendes et al, 2013; Johnson et al, 2018)

Methods

Results

Conclusion