Seasonal dynamics of macroalgal dominance and its ecological impacts on coral reefs in Mangadu, Palk Bay, India

Imagine the ocean floor as a bustling underwater city. In a healthy neighborhood, the buildings are made of living coral, colorful and intricate, providing homes for fish and protecting the shore. But sometimes, this city faces a different kind of tenant: seaweed. While seaweed is a natural part of the ocean, acting like a giant, floating sponge that cleans the water and provides food, it can sometimes grow too fast and take over. This is called "macroalgal dominance." Think of it like a garden where the weeds grow so thick they choke out the flowers, blocking sunlight and crowding out the plants that were supposed to be there. Scientists study this because when seaweed takes over, the coral city can get sick, turn white (bleach), and eventually die, leaving the underwater neighborhood empty and gray. The big question is: why does the seaweed win sometimes, and what happens to the coral when it does?

This paper dives into that exact drama in a place called Mangadu, along the southeast coast of India in Palk Bay. The researchers, a team from Annamalai University and Madurai Kamaraj University, acted like underwater detectives, tracking how different types of seaweed—green, brown, and red—moved in and took over the coral reefs over the course of a year. They wanted to see if the seasons (like summer, monsoon, and winter) changed who was in charge, and how the seaweed's growth affected the health of the coral. They found that the seaweed didn't just sit there; it actively fought the coral for space, shading it out and creating low-oxygen pockets that stressed the coral. The study suggests that human activities, like boats dropping anchors and farming certain types of seaweed nearby, are making the problem worse, turning a balanced ecosystem into a seaweed-dominated one.

The Underwater Battle for Space

The story begins in Mangadu, a coastal spot in India known for its rich marine life. Here, the researchers set up a "belt transect," which is basically a long measuring tape laid out on the ocean floor, and placed square frames (quadrats) along it to count what was living there. They did this four times a year, covering the summer, pre-monsoon, monsoon, and post-monsoon seasons, from June 2024 to May 2025.

What they found was a clear seasonal shift in the "weed war." The ocean floor wasn't static; it changed with the weather.

• During the Monsoon: The green algae (Chlorophyta) took the lead, making up about 66% of the seaweed cover. It was like a green tide washing over the reef.
• During Summer and Pre-Monsoon: The brown algae (Phaeophyta) stepped up to the plate, dominating with 61% and 52% cover respectively.
• Red algae (Rhodophyta): These were the underdogs, showing up only in small numbers (never more than 6%) and mostly hiding in quiet, low-energy spots.

Two specific species were the main troublemakers. Caulerpa sedoides (a green alga) was the most consistent winner, showing up in huge numbers all year round, peaking at 39% cover during the monsoon. Sargassum wightii (a tough, brown alga) was the summer champion, thriving when the water was warm and energetic, reaching 36.5% cover.

The Weight of the Invasion

It wasn't just about how much space the seaweed covered; it was also about how heavy it was. The researchers weighed the seaweed to see how much "biomass" was piling up. They found that the post-monsoon season was the peak time for growth.

• The wet weight of the seaweed hit a high of 7.82 kg per square meter in the post-monsoon.
• The dry weight (the actual solid stuff after removing water) was 1.98 kg per square meter during the same time.

This suggests that after the heavy rains of the monsoon, the water became rich in nutrients, acting like a super-charged fertilizer that made the seaweed explode in size.

The Coral's Struggle

The most concerning part of the story is what happened to the coral. As the seaweed grew, the coral suffered.

• In the summer, when seaweed cover was highest at 62%, the live coral cover dropped to about 34%.
• The seaweed acted like a heavy blanket, blocking sunlight that the coral needs to survive.
• It also created "hypoxic microenvironments," which are tiny pockets of water with very little oxygen, especially at night. This is like suffocating the coral.
• Some seaweeds, like Sargassum and Halimeda, physically touched the coral, causing the coral tissue to recede and die.

Interestingly, the data showed that coral bleaching (when coral turns white due to stress) was actually highest during the monsoon and pre-monsoon seasons (around 19.69% and 16.83%), suggesting that the stressors causing the bleaching might be different from just the physical crowding of the seaweed, perhaps related to water temperature or chemical changes.

Why is this happening?

The paper points a finger at a mix of nature and human activity.

1. Nutrient Overload: The water chemistry changed with the seasons. Before the monsoon, the water had high levels of nutrients like nitrogen and phosphorus. This is like pouring fertilizer on a lawn; it makes the weeds (seaweed) grow faster than the flowers (coral).
2. Human Disturbance: The researchers noted that activities like boats dropping anchors and moving through shallow water stir up the sand. This damages the coral and creates bare spots where seaweed can easily take root.
3. Farming: The area is known for farming a specific red seaweed called Kappaphycus alvarezii. While this helps local fishermen, the paper suggests that this farming, along with overfishing of the fish that usually eat seaweed, has upset the natural balance. The fish that keep the weeds in check are gone, and the farmed seaweed sometimes escapes or alters the environment, letting wild seaweeds take over.

The Verdict

The study concludes that the Mangadu reef is shifting from a coral-dominated city to a seaweed-dominated one. The data shows a clear link: when the water gets nutrient-rich and human disturbances increase, the seaweed wins. The authors suggest that without intervention, this creates a "negative feedback loop." As the coral dies, it leaves behind empty space, which the seaweed immediately fills, making it even harder for new coral to grow back.

The paper doesn't claim to have solved the problem, but it sounds an alarm. It suggests that to save the reef, we need to manage how seaweed is farmed, stop boats from damaging the reef, and protect the fish that eat the seaweed. Without these steps, the underwater city of Mangadu risks becoming a quiet, green wasteland instead of a vibrant coral home.