Ideal temperature and sunlight, along with nutrient influx from rivers, fuel the growth of brown algae in coastal waters
Published : 20 Jan 2025, 03:30 AM
A phytoplankton bloom is sweeping across the vast coastline of Cox’s Bazar, a phenomenon unique compared with other beaches worldwide.
The bloom, a natural occurrence, is short-lived, and its layer settles on the water's surface.
Research has begun in Bangladesh to determine how beneficial or harmful it is to the marine ecosystem and resources.
Phytoplankton are microscopic aquatic plants or algae. A sudden increase in their population is called a bloom, which alters the water’s colour. It is also known as "algal bloom”.
Experts say the red algal blooms seen on different beaches worldwide are harmful, referred to as Harmful Algal Blooms, or HAB.
However, the brown algae bloom in the Bay of Bengal is primarily caused by diatoms.
They explain that the river water flowing into the sea contains an abundance of nutrients essential for the growth of diatoms.
In addition, the bloom occurs due to the ideal salinity, sunlight, and favourable temperature in the sea.
While it remains uncertain whether the algal bloom in Cox’s Bazar is harmful or beneficial, initial observations indicate potential benefits.
Experts believe that further research is needed to understand the extent of the positive and negative impacts of the bloom on the marine ecosystem, as well as its trends and quantities.
A researcher from India highlighted that similar algal blooms have been seen at the Kolpakkam beach along the southeast coast of the Bay of Bengal in Tamil Nadu.
The researcher emphasised the need for further research to determine whether harvesting these diatom algae could yield economic benefits.
WHAT CAUSES ALGAL BLOOMS?
Microscopic aquatic algae rapidly multiply when they receive adequate nutrients and sunlight, a phenomenon known as a bloom.
The key nutrients for this are nitrates, phosphates, and silicates.
The area around Cox’s Bazar beach in the Bay of Bengal is particularly rich in these nutrients for several reasons.
Sayeed Mahmood Belal Haider, former secretary of the fisheries and livestock ministry and oceanographer, told bdnews24.com: “The rivers flowing into the Bay of Bengal carry more sediment compared to those in other countries.
“During the monsoon, heavy rainfall further increases the flow of water into the sea. This brings a significant amount of nitrates, phosphates, and silica—ideal for the proliferation of diatoms.”
He added, “The TSP [Triple Super Phosphate] and urea fertilisers used in agriculture, as well as household and industrial waste, flow into the sea through rivers. These also serve as a major source of nitrates and phosphates.”
Md Tarikul Islam, a senior scientific officer at the Bangladesh Oceanographic Research Institute, or BORI, has been monitoring the algal bloom in the Cox’s Bazar region since 2021.
To better understand the rate of the bloom and its impact, he initiated a research project in December under BORI titled "Assessment of Eutrophication at the Eastern Coast of Bangladesh”
At different times, Tarikul has observed algal blooms along the beaches of Saint Martin, Teknaf, Shamlapur, Patuartek, Naf River estuary, Sonar Para, Himchari, Nuniarchara, and Sonadia.
According to him, the nutrients from the river water accumulate in the deeper layers of the sea.
During the monsoon (June to September), the southwest monsoon winds push these nutrient-rich deep waters to the surface.
In addition, frequent cyclones in the Bay of Bengal cause nutrient-rich bottom water to rise to the surface.
As temperatures begin to rise after the monsoon, regular algal blooms occur during the full moon tides in the sea.
Moreover, during the northeast monsoon winds (October to February), this same phenomenon occurs.
However, due to cooler air, the surface temperature of the sea remains lower, so the blooms are less intense than during the monsoon.
Oceanographer Sayeed explained that on many beaches around the world, like those along the Arabian Sea, "red tide" or harmful algal blooms occur primarily due to the proliferation of dinoflagellates, which are toxic and harmful to marine life and humans.
He added that when samples of the algal bloom at Cox’s Bazar beach were collected and analysed at BORI, it was found that 99 percent of the algae belonged to the diatom group, which is silicate-rich and non-toxic. Their colour is brown or dark brown.
BENEFICIAL OR HARMFUL?
Algal blooms generally last for a few days or weeks, and they typically end when the phytoplankton or algae naturally decompose.
While algal blooms are generally beneficial for the ocean's food chain, some can be harmful.
When asked whether the algal blooms occurring on Cox's Bazar beach are good or bad, Tarikul Islam explained that he has collected samples from different locations along the beach over time.
He found plenty of shrimp larvae in these areas, as shrimp and small fish feed on these minuscule algae.
He said, “Moreover, these algae serve as food for marine zooplankton, which are then consumed by larger fish and marine animals. Therefore, the blooms positively impact marine biodiversity.
“However, there are negative consequences to these blooms as well.”
The senior scientific officer at BORI added, "In the Bay of Bengal, most of the algae are diatoms. However, there are other species too.
“If the blooms persist for a long time, oxygen levels in the water can drop while carbon dioxide levels rise. Ammonia can even be produced.”
He continued, “A decrease in oxygen makes survival difficult for marine creatures.
“In addition, these toxic algae can contaminate fish, which can then enter the human food chain, potentially causing long-term poisoning.”
Excessive blooms also reduce water clarity, blocking sunlight from reaching deeper into the water.
This impedes the growth of marine plants and corals, which could have a negative impact on the entire marine ecosystem, according to Tarikul.
His observation over the past four years has been that algal blooms along the beach rarely last long.
Most of the time, they dissipate within hours or a day or two.
NEED FOR RESEARCH
Scientists say that while algal blooms in the Bay of Bengal are a natural phenomenon, understanding their impact is crucial.
If the consequences are negative, it could harm the marine ecosystem and resources, which would, in turn, affect the local economy. This is why research is essential.
Oceanographer Sayeed said, "Research is necessary to determine whether these blooms are linked to global warming and climate change.
"Research will help determine the frequency and scale of these blooms. If they are excessive, efforts must be made to control human-induced nutrient sources, such as agricultural, industrial, and household waste that flow into rivers and canals."
He believes that it is also important to investigate whether the excess algae generated by the bloom can be harvested for use in food products or commercial purposes.
"There are examples of resources harvested from different seas being converted for commercial use. This potential should also be explored.”
The senior BORI official said he has started research on the nature of algal blooms in the Bay of Bengal, using his experience from the past few years.
After this research, a better understanding will be gained regarding the frequency of algal blooms in the Bay of Bengal, their duration, and the proportion of beneficial versus harmful algae present.