# Distribution Characteristics of Suspended Macroalgae in the Southern Yellow Sea Before the Green Tide Outbreak

## TL;DR

This study examines the distribution of suspended macroalgae in the Southern Yellow Sea before green tides, finding a link between their abundance and nutrient levels.

## Contribution

The study identifies key species and spatial patterns of suspended macroalgae, offering insights into green tide outbreak mechanisms.

## Key findings

- Suspended macroalgae were predominantly found in nearshore surface waters of the Southern Yellow Sea.
- Nutrient levels showed a positive correlation with macroalgae abundance.
- Ulva prolifera and other species were identified as dominant contributors to green tides.

## Abstract

Green tides have become a widespread ecological concern around the world, with the Southern Yellow Sea (SYS) in China experiencing large-scale events dominated by Ulva prolifera. A total of 31 and 38 sampling stations were established in the SYS in 2023 and 2024, respectively. At these stations, environmental parameters, the abundance of suspended macroalgae, and species composition were analyzed. The study revealed that suspended macroalgae were predominantly distributed in the surface waters of nearshore areas, showing a positive correlation with nutrient levels. The main species identified were U. prolifera O.F.Müller, Ulva linza Linnaeus, Ulva flexuosa Wulfen, and Blidingia sp. Investigating the distribution characteristics of suspended macroalgae filaments during green tide outbreaks can substantially enhance our understanding of the underlying mechanisms driving these events.

For nearly two decades, the Yellow Sea has experienced recurrent green tides, which are now considered the largest of their kind globally; the mechanism behind these outbreaks remains highly complex and not fully understood. This study investigates the pre-outbreak distribution, abundance, and species composition of suspended macroalgae in the Southern Yellow Sea (SYS) during 2023–2024, along with environmental parameters. The results indicate that suspended macroalgae were predominantly distributed in the nearshore waters, particularly along the shallow beaches of northern Jiangsu. Furthermore, their abundance in the surface water layer significantly exceeded that in the bottom water. A total of 1353 and 493 algal filament samples were collected in 2023 and 2024, respectively. Dominant species included Ulva prolifera, Ulva linza, Ulva flexuosa, and Blidingia sp. Nutrient levels positively correlated with filament abundance. As a primary means of rapid proliferation for U. prolifera, suspended macroalgae contribute significantly to the initial expansion of green tides. Furthermore, their abundance holds promise as a biological indicator for forecasting the scale and extent of impending blooms, thereby providing a critical foundation for elucidating the underlying outbreak mechanisms.