The fate of salp blooms: decomposition and sinking of salp carcasses

Gelatinous zooplankton (GZ) biomass is an important, yet often overlooked, vector of the particulate organic matter downward export and a nutritional prey source for the mesopelagic and benthic communities. To better quantify the potential impact of their blooms on biogeochemistry and food webs, we performed decomposition and sinking experiments under two different temperature regimes, 6 and 12 °C using 260 Salpa aspera, sampled in the Northeast Pacific (48.39°–50.40°N, 126.40°–145.00°W) during May 2021. Salps sank 1312 and 1424 m day⁻¹ on average in 6 and 12 °C, respectively. The fast sinking is common among other salp species. Salp decay was exponential and occurred ~ 1.5 times faster under warmer conditions. Comparison of the published GZ decay rates supported their strong temperature dependence (Q₁₀ = 3.46) and revealed that S. aspera decayed slower than most GZ taxa. Carcass sinking rates were higher than previously reported for this species and slowed after a prolonged decay. Biochemical (proteins, carbohydrates, lipids) and elemental (C: carbon, N: nitrogen) compositions were determined for salps at various decomposition stages. The high water content (~ 97%) and low organic content (27.8 ± 7.1% dry weight) were typical of other thaliaceans. The high C:N ratio (6.61 ± 1.14) of S. aspera, compared to many thaliaceans, suggested that their carcasses are valuable sources of carbon beyond the euphotic zone.