# Effects of Marine Ranching on Phytoplankton Community: A Case Study in the Bailong Pearl Bay National Marine Ranching Demonstration Zone, China

## TL;DR

This study examines how marine ranching affects phytoplankton in China's Bailong Pearl Bay, finding it boosts phytoplankton in summer and may reduce harmful algal blooms in autumn and winter.

## Contribution

The study provides new insights into how marine ranching influences phytoplankton dynamics and may help control harmful algal blooms.

## Key findings

- Marine ranching areas had higher phytoplankton density than surrounding waters in summer.
- During autumn and winter, the ranching area had lower phytoplankton levels compared to surrounding areas experiencing blooms.
- Phytoplankton abundance was positively correlated with nitrate and negatively with phosphate.

## Abstract

Scientists are still learning how marine ranching affects the tiny algae (phytoplankton) that form the base of the marine food web. This study investigated the Bailong Pearl Bay National Marine Ranching Demonstration Zone in China to find answers. Researchers discovered 101 different phytoplankton species in the area, including 19 types known to cause harmful algal blooms, or “red tides”. We found that during summer, the marine ranching area had more phytoplankton than the surrounding waters, showing that it helps these organisms thrive. However, during autumn and winter, when certain algae species began to bloom excessively, the ranching area actually had lower algae levels than other locations. This suggests that marine ranching not only boosts ocean productivity but may also naturally control harmful algal blooms. These findings are valuable for coastal communities and environmental managers because they demonstrate that well-designed marine ranching can help maintain healthy ocean ecosystems while potentially reducing the damaging effects of algal blooms on fisheries and tourism.

Marine ranching is an important strategy for restoring marine habitats and replenishing aquatic populations. However, the effects of marine ranching on phytoplankton dynamics remain unclear. In this context, this study takes the Bailong Pearl Bay National Marine Ranching Demonstration Zone as an example to evaluate the influence of marine ranching on the spatial and temporal variation in phytoplankton abundance and community structure. A total of 101 phytoplankton species, spanning 44 genera and 26 families, were documented in the Bailong Pearl Bay National Marine Ranching Demonstration Zone, with 19 of these species identified as harmful algal blooms (HABs) or potential HABs. In spring, phytoplankton abundance remained relatively uniform across sampling stations, with community structure characterized by varying combinations of co-dominant species. In summer, phytoplankton density within the demonstration zone was higher than in adjacent regions. In contrast, lower phytoplankton abundance was observed within the demonstration zone during autumn and winter, periods marked by phytoplankton blooms in surrounding areas (autumn: Chaetoceros lorenzianus, Rhizosolenia alata, and Skeletonema costatum; winter: Nitzschia pungens). Correlation analysis indicated that phytoplankton abundance was positively correlated with nitrate and negatively correlated with phosphate, suggesting nutrient availability as a key driver of phytoplankton dynamics. These findings provide baseline information on how phytoplankton communities vary spatially and seasonally in relation to a marine ranching zone and offer insights to support the management and rehabilitation of marine ecosystems.