Sea sparkle (type of marine plankton) blooms. Source: Sander van der Wel from Netherlands via Wikimedia Commons.
Sea sparkle blooms are often admired for their striking blue glow. Yet beneath this spectacular natural phenomenon lies a larger story about how changes in rivers are reshaping India's coastal waters. New research suggests that the increasing frequency of blooms triggered by Noctiluca scintillans (NS), a marine plankton commonly known as sea sparkle, is linked to declining levels of dissolved silicates in the ocean.
These blooms are becoming more common in Indian waters, destabilising marine ecosystems, altering food webs, reducing dissolved oxygen and encouraging bacterial proliferation. The findings point to an unexpected connection between rivers and oceans: rivers naturally transport sediments rich in dissolved silica that sustain diatoms, the microscopic algae forming the base of healthy marine food webs. As this flow is disrupted, the ecological balance of coastal waters begins to shift. What is driving this change?
These are caused due to sudden, explosive growth of plankton (tiny drifting organisms) found in the marine waters.
These can include :
Phytoplankton, plant-like, photosynthetic organisms such as diatoms, cyanobacteria and dinoflagellates, like sea sparkles
Zooplankton, animal-like organisms that feed on phytoplankton
Blooms can discolour water into green, red, brown, or blue patches, sometimes even visible from space.
Conditions that trigger blooms include:
Nutrients: Excess nitrogen and phosphorus from agriculture, sewage, or aquaculture act like fertilisers, aiding the growth of plankton.
Sunlight: Abundant light in the upper water column can fuel photosynthesis.
Temperature: Warm waters accelerate plankton reproduction, so blooms often occur in spring/summer.
Water stratification: Stable layers keep phytoplankton concentrated near the surface.
Natural processes: Upwelling brings nutrient-rich deep waters to the surface, sparking blooms.
Among the many plankton species found in the ocean, Noctiluca scintillans is perhaps the most visually recognisable. Commonly known as "Sea Sparkle", it is a free-living marine plankton that grows up to 0.2 millimetres in diameter and produces a brilliant blue glow when disturbed by waves, swimming fish or passing boats.
Unlike most phytoplankton, sea sparkle does not produce its own food through photosynthesis. Instead, it feeds on diatoms, microalgae, fish eggs and other microscopic organisms using specialised feeding tentacles.
Its characteristic glow is created by a chemical reaction inside tiny cellular structures known as scintillons. During large bloom events, millions of these organisms accumulate at the water's surface, forming dense gelatinous layers that appear pink or red during daylight, giving rise to the phenomenon commonly known as a red tide. Although visually striking, these blooms can disrupt marine ecosystems when they become widespread, altering food webs, reducing oxygen levels and affecting water quality.
A sea sparkle.
A recent study titled 'Towards understanding the paradox associated with the rise in algal blooms of Noctiluca sp. (Dinoflagellate) around the Indian continental waters' by Rajdeep Roy et al. published in Global Environmental Change Advances looks at the frequency of outbreaks of sea sparkle blooms between 1900 and 2025 in the Arabian Sea (AS) and the Bay of Bengal (BoB) and explores factors affecting their proliferation
The study reveals that:
Sea sparkle blooms are increasing in frequency in Indian waters
Sea sparkle blooms are showing an increasing trend over the last 25 years, with blooms in the Bay of Bengal occurring in the months of August and September (monsoon) and in February (post-monsoon) in the Arabian Sea. Nearly 40 episodes of sea sparkle blooms have been documented between 2000 and 2025 compared to years before.
Lack of dissolved silicates in the water is linked to increase in sea sparkle blooms
It was thought that anthropogenic pollution and low oxygen levels are the main causes of sea sparkle blooms. However, recent evidence links lack of silicates (Si) that are important components of sediments in the water to sea sparkle blooms. Silicates form the vast majority of river sediments, making up over 70-90% of the solid material carried by and deposited in rivers. Rivers ultimately flow to the sea and carry parts of these sediments that have silicates into the oceans. However, this study finds that Indian rivers are unable to deliver sediments and silicates to the oceans, triggering sea sparkle blooms.
Dams are blocking rivers from carrying silicates and sediments to the ocean.
Many dams have been built on Indian rivers. Dams drastically reduce the sediment load of rivers. By creating reservoirs, they slow down water flow, causing 70% to 90% of suspended sediment (sand, gravel, and silt) to settle at the bottom. This traps nutrients, starves downstream ecosystems, and accelerates coastal erosion, besides reducing the amount of silica transferred to the oceans, triggering planktonic blooms.
The study finds a positive association between the number of dams built over two decades and the reported number of sea sparkle blooms occurring over the same time.
For example, as high as 60–80% of sediment load reaching the Arabian Sea has decreased on the Indian west coast due to the Sardar Sarovar Dam traps on the Narmada basin. Construction of an irrigation barrage in the Indus River has led to an 80% reduction in sediment load in the northern Arabian Sea. Studies suggest that major hot spots of dissolved Si release are located in South-east Asia, wherein the Ganges and its neighbouring small rivers contribute large amounts of silica to the Bay of Bengal.
However, historical Si data from some major rivers in India clearly indicates a reduction in the silica values compared to 4 decades earlier. This further supports the results of this study, which suggests a strong reduction in silica input, particularly in the Bay of Bengal due to the damming effect. A reduction of 17% in silica input is projected due to the damming effect in the Bay of Bengal in the years to come.
Sea sparkle blooms may appear visually spectacular, but when they occur frequently or over large areas, they can alter the ecological balance of coastal waters. The study highlights that the rapid proliferation of Noctiluca scintillans affects marine ecosystems in several interconnected ways, influencing oxygen availability, aquatic habitats and food webs.
One of the most immediate consequences is the depletion of dissolved oxygen. As dense blooms die and decompose, microorganisms consume large amounts of oxygen, reducing its availability in the surrounding water. This process can create low oxygen conditions, often referred to as dead zones, where fish and other aquatic organisms struggle to survive.
Large blooms also reduce the amount of sunlight reaching underwater ecosystems. With less light penetrating the water column, the growth of seagrasses and other aquatic plants declines, affecting habitats that support fish, crustaceans and numerous marine species.
Some bloom-forming organisms, including sea sparkle and certain cyanobacteria, are also associated with Harmful Algal Blooms (HABs). These blooms can release compounds that affect marine organisms and further disrupt coastal ecosystems. Together, these changes alter the structure of marine food webs, with consequences that extend beyond individual species to the functioning of entire coastal ecosystems.
The ecological impacts of sea sparkle blooms are closely linked to the wellbeing of people who depend on the sea. Reduced oxygen levels can lead to fish mortality and declining fish populations, affecting both marine biodiversity and the availability of fish for commercial and small-scale fisheries. For coastal communities whose livelihoods depend on fishing, repeated bloom events can translate into reduced catches and economic uncertainty.
The study also notes that harmful bloom-forming organisms may accumulate toxins in shellfish and fish. When contaminated seafood is consumed, it can lead to illnesses such as paralytic shellfish poisoning, posing risks to public health.
Beyond fisheries, recurring harmful algal blooms can affect tourism and aquaculture. Discoloured coastal waters, declining fish stocks and concerns about seafood safety can have wider economic consequences for communities whose livelihoods depend on healthy marine ecosystems. These findings illustrate that sea sparkle blooms are not simply an ecological phenomenon. They are increasingly becoming an issue of food security, livelihoods and coastal resilience.
The study also points to a less visible consequence of expanding sea sparkle blooms: their influence on the ocean's role in storing carbon.
Unlike diatoms, which contribute significantly to carbon fixation by transporting carbon into deeper ocean layers, Noctiluca scintillans behaves differently. Sea sparkle possesses large storage sacs filled with ammonium ions that help it remain buoyant near the water's surface. During bloom events, these organisms accumulate in dense surface layers rather than sinking through the water column.
This reduces the vertical flux of carbon, limiting the transfer of carbon from surface waters to deeper parts of the ocean where it can be stored over longer periods.
Under natural conditions, when dissolved silica is abundant, diatoms dominate surface waters. As some of the ocean's most important primary producers, they generate oxygen, support marine food webs and play a central role in removing carbon dioxide from the atmosphere through carbon fixation.
However, when silica availability declines, sea sparkle increasingly replaces diatoms. According to the study, this shift not only affects water quality and ecological health but may also weaken the ocean's capacity to sequester carbon.
The findings therefore suggest that changes in river sediment transport have implications that extend beyond coastal ecology. By altering the balance between diatoms and sea sparkle, they may also influence broader climate-regulating processes carried out by healthy marine ecosystems.
One of the study's central recommendations is to improve the movement of sediments and dissolved silica through river systems.
Since dams trap much of the sediment that would naturally reach estuaries and coastal waters, modifying dam operations to allow greater sediment transport downstream could help restore ecological processes. The study also points to the importance of incorporating sediment passage structures into the design of new dams so that rivers can continue delivering sediments to downstream ecosystems.
Restoring floodplains and wetlands is another important strategy. These ecosystems naturally regulate the movement of water, nutrients and sediments through river basins while supporting biodiversity and improving water quality. Together, these measures could help maintain the flow of dissolved silica that supports healthy coastal ecosystems.
The researchers also highlight the importance of improving monitoring systems to better understand changing coastal conditions. Technologies that track nutrient concentrations, including silica and nitrogen, could help identify conditions that favour bloom formation before large outbreaks occur.
The study recommends expanding early warning systems that combine satellite observations with in situ monitoring to detect bloom development. Timely information could help fisheries, aquaculture operators and coastal managers respond more effectively to changing marine conditions.
Protecting and restoring wetlands and mangroves can also contribute to healthier coastal waters. These ecosystems act as natural buffers, helping regulate nutrient flows before they reach the sea and reducing conditions that favour harmful blooms.
The findings suggest that managing sea sparkle blooms cannot be separated from broader river basin planning. The study calls for:
Integrated basin management, where hydropower generation, irrigation and ecological requirements are considered together during the planning and operation of dams. Such an approach recognises that decisions affecting rivers can have consequences that extend well beyond freshwater systems.
Exercising greater caution when planning new dams in river basins where sediment delivery is essential for maintaining coastal ecosystems.
Community participation forms another important part of this approach. Fishers and coastal communities, who are often the first to observe changes in marine conditions, can contribute valuable local knowledge to monitoring efforts and early response systems.
Sea sparkle blooms may appear to be an ocean phenomenon, but this study shows that their origins often lie much farther upstream. For decades, discussions around these blooms have focused largely on pollution, warming seas and declining oxygen levels. While these remain important drivers, the research highlights another dimension that has received far less attention: the role of rivers in sustaining marine ecosystems through the continuous transport of sediments and dissolved silica.
By interrupting these natural flows, dams are influencing ecological processes that extend from river basins to estuaries and coastal waters. The consequences are reflected not only in the growing frequency of sea sparkle blooms but also in changing food webs, declining oxygen levels, reduced carbon sequestration and pressures on fisheries and coastal livelihoods.
The study ultimately underscores the close relationship between freshwater and marine ecosystems. Rivers do not end where they meet the sea. They continue to shape the ecological health of coastal waters through the sediments, nutrients and minerals they carry. As India continues to balance water storage, irrigation and hydropower with environmental sustainability, recognising these river-to-ocean connections will be essential for protecting both freshwater resources and the coastal ecosystems that depend on them.