Uncoupling of Seasonal Variations Between Phytoplankton Chlorophyll a and Production in the East China Sea

Abstract

Phytoplankton chlorophyll a (Chla), primary production (PP), and nutrient uptake rates are fundamental and important parameters in marine biogeochemical studies. However, the relationships between them are associated with much uncertainty that reflects gaps in basic understanding of ecosystem dynamic. In this study, we simultaneously measured PP and nitrate assimilation (NA) along with estimated concentrations of nutrients, particulate organic carbon concentrations, phytoplankton Chla, pigments, and carbon biomass in the dynamic shelf ecosystems of the East China Sea (ECS) on four cruises during different seasons. There were large spatial gradients of these parameters, and seasonal patterns of phytoplankton Chla and PP varied between water masses. The results of this study and previous work indicated that seasonal variations of Chla and PP were similar in coastal waters but opposite in off‐shelf waters. During summer in off‐shelf waters, there were patches of high PP and NA associated with low Chla concentrations; this pattern was reversed in the winter. We suggest that there is a global tendency for PP and Chla patterns to be out of phase. This asynchrony is reflected mainly in the seasonal variations of C:Chla ratios driven by the combined influences of light, temperature, and nutrients. Stronger grazing pressure by microzooplankton in summer likely contributed to the high production but low biomass in the ECS. The significant correlation between PP and NA suggested that variations of PP in the ECS were driven largely by nitrate uptake, which may be inhibited by high ammonium concentrations in the Changjiang River plume.

New Insights Into the Non‐Conservative Behaviors of Nutrients Triggering Phytoplankton Blooms in the Changjiang (Yangtze) River Estuary

“…These primary production estimations all fall into the range reported by Liu et al. (2019) in ECS (i.e., 87–5,327, 167–3,710, and 92–1,005, mg C/(m 2 day) for May–June, August, and November–December, respectively).…”

“…The parameter of chlorophyll‐ a concentration has long been regarded as an index to evaluate phytoplankton biomass, primary productivity, and trophic conditions in estuarine, coastal, and other marine waters (e.g., Boyer et al., 2009). Interactions and relationships among parameters such as nutrient concentrations and their uptake rates, chlorophyll‐ a concentrations, and mechanisms for bloom events have long been of interest in marine biogeochemical studies (Bode et al., 2005; Dugdale et al., 2007; Heil et al., 2014; Liu et al., 2019; Medina‐Gómez et al., 2019; Nausch et al., 2012; Rees et al., 1999; Vaz et al., 2019). However, whether high nutrient uptake rates result in the high chlorophyll‐ a content or whether the high chlorophyll‐ a level results in high nutrient uptake remains a contentious topic (e.g., Liu et al., 2019).…”

Physiological characteristics of phytoplankton in response to different light environments in the Philippine Sea, Northwestern Pacific Ocean

“…Additionally, Colijn et al (1990) reported a positive linear correlation (r 2 = 0.81) between MVChl a and primary production within the euphotic layer in the German Bight. Liu X et al (2019) observed that surface MVChl a concentrations had a significantly positive correlation (Spearman, r = 0.41, p<0.01) with integrated primary production in the East China Sea. These previous results imply that photosynthetic pigments may be involved in the production activity of phytoplankton via light energy absorption.…”

Spatial Variations of Phytoplankton Biomass Controlled by River Plume Dynamics Over the Lower Changjiang Estuary and Adjacent Shelf Based on High-Resolution Observations

“…The contribution of biological consumption to the nutrient reduction was estimated by the ratio of Chl to C (Chl/C) and the Redfield stoichiometry. The Chl concentration in this region was 2.0-4.0 µg L −1 , yielding a phytoplankton carbon biomass of 6.0-12.0 µM based on a Chl/C ratio of 30 for the Changjiang plume water in summer (Liu et al, 2019). The estimated consumption of DIN and DIP from the Redfield stoichiometry (106:16:1 for C:N:P) was, respectively, 0.91-1.81 and 0.06-0.11 µM, much smaller than the observed reduction in DIN and DIP in this strongly stratified zone.…”