Salp blooms are large-scale oceanic phenomena, often appearing suddenly as vast, gelatinous swarms across the surface of the sea. These organisms, which are frequently mistaken for jellyfish due to their transparent, barrel-shaped bodies, are actually invertebrates called salps. Unlike jellyfish, salps belong to the subphylum Tunicata, making them more closely related to vertebrates than to cnidarians. The massive scaling of these populations has profound implications for the ocean’s food web and the global carbon cycle.
Salps and Their Unique Biology
Salps are classified as pelagic tunicates, marine invertebrates that possess a notochord during their larval stage, placing them within the phylum Chordata. Their anatomy is built for efficient filter-feeding, using a muscular, barrel-like body to pump water through a sophisticated internal mucous net. This mechanism allows them to capture microscopic prey, primarily phytoplankton, from the water column. The speed of a salp bloom is attributed to an unusual life cycle that alternates between two distinct forms.
The life cycle begins with the solitary phase, known as the oozoid, which reproduces asexually by budding off a chain of clones. This chain forms the aggregate phase, or blastozoid, where individuals remain attached, sometimes forming colonies over 50 feet long. Each individual in the aggregate chain is a sequential hermaphrodite, starting as a female and maturing into a male. This rapid alternation of asexual cloning and sexual reproduction allows salps to grow to maturity in as little as 48 hours, increasing their body length by up to 10% per hour.
Environmental Triggers for Blooms
Salp populations exist in a “bloom or bust” cycle, held in check until a specific external signal initiates the explosion of numbers. The primary environmental trigger is the sudden, massive availability of their food source: phytoplankton. Events like coastal upwelling, where deep, nutrient-rich water is pulled to the surface, or seasonal shifts can cause phytoplankton populations to surge. When these plants become abundant, the salps’ rapid asexual cloning mechanism is leveraged, leading to exponential population growth.
The speed of this biological response allows salps to quickly capitalize on the dense food source before slower-reproducing zooplankton can compete. A massive, but often short-lived, bloom can cover thousands of square miles of ocean surface. This rapid grazing can effectively strip the surface water of phytoplankton, leading to a population crash when the food runs out.
This rapid grazing significantly reduces the primary production of phytoplankton in the area, impacting the entire food web. The bloom ends when the food resource is depleted, and the enormous biomass of salps begins to die off and sink.
The Critical Role in Ocean Carbon Cycling
The most profound impact of a salp bloom on the global ocean is its disproportionate role in the biological carbon pump, the process that transfers carbon from the surface to the deep ocean. Salps consume vast quantities of carbon-rich phytoplankton from the surface layer and then package this material into large, dense fecal pellets. This packaging process is a highly efficient mechanism for carbon export.
These salp fecal pellets are significantly larger and heavier than the detritus produced by most other zooplankton, such as copepods or krill. Due to their density, the pellets sink rapidly through the water column, minimizing the time they spend in the surface layers where they could be recycled. Sinking rates for these pellets can allow them to travel 100 meters in a matter of hours, accelerating the delivery of carbon to the deep sea.
A major salp bloom can increase the rate of carbon absorption and export by an average of five-fold in the affected area, shifting an ecosystem from one with low carbon export efficiency to one that is highly efficient. Once the carbon is sequestered in the deep ocean, it is removed from interaction with the atmosphere for long periods.
The sheer volume of dead salp bodies following the collapse of a bloom also sinks. This provides a second, substantial pulse of carbon flux to the seafloor, contributing significantly to the deep-sea ecosystem.