# Photosynthesis and Spatial Distribution of Surface Phytoplankton in the Yangtze Estuary and Adjacent Waters During Spring

## TL;DR

This study explores how phytoplankton in the Yangtze River Estuary respond to environmental factors like temperature and salinity during spring.

## Contribution

The study provides new insights into phytoplankton photosynthetic activity and distribution in the Yangtze River Estuary using in situ measurements.

## Key findings

- Diatoms dominated all regions, while chlorophytes and cryptophytes thrived in low-salinity waters.
- Warm, high-salinity waters showed stronger photosynthetic activity compared to colder coastal areas.
- Low spring temperatures limited phytoplankton bloom development despite nutrient availability.

## Abstract

Phytoplankton are key players in marine ecosystems and the carbon cycle, yet their in situ physiological responses to environmental gradients remain underexplored. This study investigated the surface phytoplankton communities and photosynthetic activity in spring across the Yangtze River Estuary and the adjacent East China Sea. By combining chlorophyll fluorescence measurements and pigment analysis, we found distinct spatial differences driven by temperature and salinity. Diatoms dominated throughout, while chlorophytes and cryptophytes thrived in low-salinity plume waters, and dinoflagellates and chrysophytes were more abundant offshore. Phytoplankton in warmer, high-salinity waters exhibited stronger photosynthetic activity, while colder coastal waters showed reduced efficiency despite nutrient enrichment. These findings highlight how environmental gradients shape phytoplankton physiology and distribution, and suggest that low spring temperatures may limit bloom development even when nutrients are abundant. This work improves our understanding of estuarine algal dynamics and supports early warning efforts for harmful algal blooms in rapidly changing coastal ecosystems.

Chlorophyll fluorescence provides direct insights into the physiological status of algae, contributing to the understanding of the marine carbon cycle. However, in situ measurements of phytoplankton photosynthetic physiology remain relatively scarce in the Yangtze River Estuary (YRE), an ecosystem under intense anthropogenic pressure. Consequently, quantifying composition and physiological status variations of phytoplankton in this region is critical for understanding their ecological functions and responses. Spring cruise revealed that the warm and high-salinity oceanic region exhibited a greater abundance of dinoflagellates and chrysophytes, while the lower-salinity plume region was characterized by higher abundances of chlorophytes and cryptophytes. Diatoms dominated across all regions. The highest chlorophyll a concentration (6.3 μg/L) was observed in the oceanic region. Chlorophyll fluorescence indicated that the warm offshore community was more active, suggesting favorable phytoplankton growth. As temperature decreased and seawater mixed, the maximum relative electron transport rate (rETRmax) and the minimal saturated light intensity (Ik) decreased, yet the overall community remained healthy. Despite the plume delivering abundant nutrients, phytoplankton activity was relatively low due to the cold spring water temperature. This study will provide a foundation for understanding phytoplankton dynamics under anthropogenic influences in the YRE and the adjacent East China Sea, supporting algal bloom monitoring and early warning efforts.