Salp Blooms: Shaping Marine Food Webs and Carbon Flow

Massive swarms of gelatinous organisms called salps periodically appear in the ocean, dramatically altering marine ecosystems. These blooms, spanning hundreds of kilometers, significantly impact food webs and carbon cycling. Scientists are increasingly interested in their role in regulating oceanic processes, particularly as climate change affects bloom frequency and intensity.

Understanding the factors driving salp blooms and their ecological consequences is crucial for predicting shifts in marine environments.

Salp Morphology And Reproductive Cycle

Salps are barrel-shaped tunicates with a transparent, gelatinous body that allows them to drift with ocean currents. Their structure is optimized for efficient locomotion and filter feeding, with a muscular outer tunic that contracts rhythmically to propel water through their bodies. This movement not only enables swimming but also drives a continuous flow of water through a feeding apparatus lined with a fine mucus net. As water passes through, microscopic phytoplankton and other suspended particles become trapped, providing a steady food source. Their simple body structure, lacking a centralized brain or rigid skeleton, supports rapid growth and high reproductive output, key factors in their ability to form massive blooms.

Salps follow a dual-phase reproductive cycle, alternating between solitary asexual individuals (oozoids) and colonial sexual chains (blastozooids). An oozoid reproduces asexually by budding off a chain of genetically identical blastozooids, which can number in the hundreds. These mature into hermaphrodites, first functioning as females by producing eggs fertilized by sperm from older individuals in the chain. This sequential hermaphroditism maintains genetic diversity while enabling rapid population expansion. The fertilized eggs develop into new oozoids, completing the cycle and allowing populations to surge under favorable conditions.

Physical Conditions Influencing Bloom Formation

Salp blooms form in response to oceanographic and climatic factors that create favorable conditions for rapid proliferation. Temperature plays a major role, as salps thrive in temperate and subtropical waters, with many species reproducing more rapidly in warmer environments. Seasonal and long-term climate shifts can trigger population surges by enhancing metabolic efficiency and reproductive success. Studies have linked higher bloom frequencies in the Southern Ocean to anomalously warm sea surface temperatures, highlighting a direct connection between thermal conditions and salp dynamics.

Water column stratification also influences bloom development by concentrating food resources and stabilizing habitats. In highly stratified waters, where temperature and salinity differences create distinct layers, salps exploit concentrated phytoplankton populations while avoiding competition with other grazers. Reduced wind mixing reinforces stratification, limiting nutrient upwelling and favoring smaller, fast-growing phytoplankton species that align with salps’ feeding preferences. Events like El Niño–Southern Oscillation (ENSO) cycles often lead to pronounced salp outbreaks as these organisms capitalize on altered ecosystem structures.

Ocean currents and circulation patterns further shape bloom intensity. Mesoscale eddies create retention zones where salps accumulate in high densities, forming large swarms. These features not only concentrate food sources but also provide a stable environment for reproduction. Boundary currents like the Agulhas or Gulf Stream extend bloom persistence by dispersing individuals across vast distances, allowing populations to expand beyond their initial spawning grounds. Even minor shifts in ocean circulation can significantly affect salp distribution.

Feeding Dynamics And Impact On Phytoplankton

Salps are highly efficient filter feeders, using continuous water-pumping to capture microscopic phytoplankton. Their mucus net traps particles as water flows through their bodies, allowing them to extract nutrients with remarkable efficiency. Some species filter up to 60 liters per hour, enabling them to rapidly deplete local phytoplankton populations, particularly during bloom conditions. Their ability to consume a broad range of particle sizes, from picoplankton to larger diatoms, gives them a competitive edge over more selective grazers like copepods.

Their grazing pressure can significantly alter phytoplankton community composition. In regions where salp populations surge, smaller, fast-growing species often dominate over slower-reproducing taxa. This shift favors nanoplankton and cyanobacteria that thrive in nutrient-depleted conditions, altering primary production. Additionally, by outcompeting other grazers, salps disrupt traditional trophic pathways, reducing phytoplankton availability for organisms that depend on them. These changes cascade through the ecosystem, affecting everything from nutrient cycling to higher trophic levels.

Role In Marine Carbon Cycling

Salps play a major role in the biological carbon pump, which transfers carbon from the surface ocean to the deep sea. Their rapid filtration enables them to ingest large amounts of phytoplankton, which absorb atmospheric carbon dioxide during photosynthesis. Once consumed, this organic carbon is either metabolized for energy or compacted into dense, fast-sinking fecal pellets. These pellets, which can sink more than 1000 meters per day, efficiently transport carbon away from the surface, reducing its potential to re-enter the atmosphere.

Beyond fecal output, salps contribute to carbon cycling through their life cycle. When large blooms collapse, the gelatinous bodies of deceased individuals sink rapidly, providing an additional pulse of organic material to the deep sea. This biomass serves as an energy source for deep-sea organisms while also locking away carbon for extended periods. In regions where salps dominate, their carbon export efficiency can rival or exceed that of krill and copepods, significantly influencing regional carbon budgets.

Salps In Food Web Interactions

Salps influence marine food webs not only as grazers but also as prey. Despite their gelatinous composition, which offers less caloric value than lipid-rich zooplankton like krill, they are consumed by various predators, including fish, seabirds, and sea turtles. Some deep-sea fish, such as myctophids, adapt their feeding behavior to take advantage of dense salp swarms. While opportunistic predators benefit from these temporary abundances, it remains unclear if salps can fully replace more energy-dense prey in sustaining long-term population health.

Beyond direct consumption, salps impact food webs through energy transfer and nutrient cycling. Their rapid phytoplankton ingestion can reduce primary producer availability for other grazers, such as copepods and krill, altering competitive dynamics. These shifts can affect species that rely on traditional grazers, including commercially important fish populations. Additionally, the sinking of salp fecal pellets and decomposing bodies supplies organic matter to the deep sea, supporting detritivores and microbial communities. In regions where salps proliferate, their contribution to deep-sea energy pathways rivals that of other major zooplankton groups, highlighting their complex role in structuring marine ecosystems.