# Ecological Effects of Sargassum fusiforme Cultivation on Coastal Phytoplankton Community Structure and Water Quality: A Study Based on Microscopic Analysis

## TL;DR

This study shows that cultivating Sargassum fusiforme improves coastal water quality and supports a more diverse phytoplankton community.

## Contribution

The study provides empirical evidence of the ecological benefits of Sargassum fusiforme farming on water quality and phytoplankton diversity.

## Key findings

- Sargassum fusiforme cultivation increased dissolved oxygen and pH while reducing excess nutrients like nitrogen and phosphorus.
- Phytoplankton diversity increased due to reduced dominance of Skeletonema costatum during cultivation periods.
- Water quality declined during non-cultivation periods, indicating the role of seaweed farming in maintaining coastal health.

## Abstract

Seaweed farming is a green and sustainable way to produce food and to protect the marine environment. In this study, we examined how large-scale cultivation of Sargassum fusiforme affects seawater quality and tiny plant-like organisms in the ocean called phytoplankton. These organisms are important because they form the base of marine food chains. We collected water samples during both farming and non-farming seasons. The results showed that S. fusiforme farming helped raise oxygen levels and made nutrient concentrations, such as nitrogen and phosphorus, less excessive. Farming also helped reduce the overgrowth of certain dominant species and supported a more diverse and balanced phytoplankton community. We used microscopes to observe and to count the phytoplankton. Our findings show that seaweed farming can improve ocean health and can offer useful guidance for sustainable aquaculture and coastal ecosystem protection.

This study used microscopy-based quantitative enumeration to investigate the effects of large-scale Sargassum fusiforme cultivation on coastal water quality and phytoplankton communities. Data from April (cultivation period) and June (non-cultivation period) in 2018 and 2019 showed that cultivation increased pH and dissolved oxygen (DO). It also reduced nitrate–nitrogen (NO3–N), nitrite–nitrogen (NO2–N), phosphate–phosphorus (PO4–P), total phosphorus (TP), and silicate–silicon (SiO3–Si) concentrations. These changes indicate improved coastal water quality from S. fusiforme cultivation. Nutrient levels rose again during the non-cultivation period. This suggests that water purification decreased without cultivation. Cultivation also lowered the dominance of Skeletonema costatum. This led to a more diverse and stable phytoplankton community. Microscopic observation is valuable for quantifying larger phytoplankton species, and plays an important role in ecological monitoring. These findings provide insights for sustainable aquaculture and ecological restoration.