Presenter: Polina Orlov (University of British Columbia)

Description:
Gelatinous zooplankton (GZ) biomass is an important, yet often overlooked, vector of particulate organic matter (POM) export in the ocean and a nutritional prey source for the mesopelagic and benthic environments. To better quantify the potential impact of their blooms on surrounding biogeochemistry and food webs, we performed decomposition and sinking experiments on 244 Salpa aspera, sampled from the Northeast Pacific in May 2021, under two different temperature regimes, 6 and 12 °C. Salps were suspended in large seawater tanks with continuous water filtration systems and kept in the dark for a maximum of 30 days. Decay was exponential and occurred ~1.5 times faster in the warmer conditions. Comparison of GZ across literature studies supported the strong influence of temperature on their decomposition (Q₁₀ = 3.46) and revealed S. aspera to decay at a lower rate compared to most other GZ taxa. Carcass sinking rates were higher than previously reported for this species but slowed down with prolonged decay. Biochemical (proteins, carbohydrates, lipids) and elemental (C: carbon, N: nitrogen) composition was determined for samples at various stages of degradation. Energetic content, estimated from total organics using published conversions, was typical of GZ but notably lower than common crustacean zooplankton. High water fraction (97%) and organic content of 27.8 ± 7.1 percent of dry weight (% DW) was also typical of other thaliaceans, while body C was relatively high (15.6 % DW). Carbohydrates accounted for the major fraction of total organics (45.3%), followed by lipids (42.1%) and protein (13.3%). However, unresolved uncertainty remains with total protein detection in the salp tunic. The high C:N ratio (7.14 ± 0.71) of S. aspera suggests their carcasses to be valuable sources of vertical C flux to the deep. Changes in biochemical and elemental composition of decaying carcasses are described with potential implications for nutrient availability for deep ocean communities.