Jelly biomass sinking speed reveals a fast carbon export mechanism

Sinking of gelatinous zooplankton biomass is an important component of the biological pump removing carbon from the upper ocean. The export efficiency, e.g., how much biomass reaches the ocean interior sequestering carbon, is poorly known because of the absence of reliable sinking speed data. We measured sinking rates of gelatinous particulate organic matter (jelly-POM) from different species of scyphozoans, ctenophores, thaliaceans, and pteropods, both in the field and in the laboratory in vertical columns filled with seawater using high-quality video. Using these data, we determined taxon-specific jelly-POM export efficiencies using equations that integrate biomass decay rate, seawater temperature, and sinking speed. Two depth scenarios in several environments were considered, with jelly-POM sinking from 200 and 600 m in temperate, tropical, and polar regions. Jelly-POM sank on average between 850 and 1500 m d21 (salps: 800–1200 m d21; ctenophores: 1200– 1500 m d21; scyphozoans: 1000–1100 m d21; pyrosomes: 1300 m d21). High latitudes represent a fast-sinking and low-remineralization corridor, regardless of species. In tropical and temperate regions, significant decomposition takes place above 1500 m unless jelly-POM sinks below the permanent thermocline. Sinking jelly-POM sequesters carbon to the deep ocean faster than anticipated, and should be incorporated into biogeochemical and modeling studies to provide more realistic quantification of export via the biological carbon pump worldwide. Sinking marine particles comprise a size continuum ranging from individual cells and exudates to aggregates of degraded biogenic detritus and fecal pellets to carcasses of zooplankton, including large gelatinous zooplankton (gelatinous particulate organic matter [jelly-POM]). The sedimentation of particles to the ocean’s interior is central to the biological pump concept, redistributing chemical elements in the water column and sequestering carbon to depth (Lee and Fisher 1993; Buesseler et al. 2007). Sinking jelly-POM (also referred to as jelly falls) originates from post-bloom biomass following the seasonal collapse of gelatinous zooplankton populations (Lebrato et al. 2012), the main known contributors of which are jellyfish (Scyphozoa and Hydrozoa; Billett et al. 2006) and pelagic tunicates (Salpa and Pyrosomida; Wiebe et al. 1979; Lebrato and Jones 2009). Because gelatinous biomass is distributed from the surface to mesopelagic and deeper waters worldwide, jelly-POM is an important source of labile carbon (Condon et al. 2012). These gelatinous carcasses are occasionally exported and deposited at the seafloor, sequestering carbon and providing food for benthic communities. During jelly-POM sedimentation, large amounts of nutrients are released, providing labile resources for bacterioplankton and microzooplankton (Titelman et al. 2006; Pitt et al. 2009; Tinta et al. 2010). The existence of jelly falls on the seafloor suggests a high export efficiency of at least some gelatinous taxa from the upper ocean. Alternatively, the apparent sporadic existence of jelly-POM on the seabed might suggest a poor transfer efficiency (of these or other taxa) and thus rapid remineralization of sinking jelly-POM that contributes to the dissolved carbon pool (Lebrato et al. 2011, 2012). Yet, to date, no sinking rate estimations have been made in order to investigate transfer efficiency of sinking jellyPOM. Our understanding of particulate fluxes from field observations is limited to particles in the size range collected by sediment traps (Fowler and Knauer 1986; Asper 1987) or visible with cameras (from , 1 to , 50 mm; Jackson et al. 1997). In the laboratory, phytoplanktonand zooplankton-derived materials (marine snow and fecal pellets) from 0.5 to 1000 mm have been commonly studied (Shanks and Trent 1980; Walsby and Holland 2006; Ploug et al. 2008). However, the study of sinking rates of gelatinous zooplankton-derived POM (particles of millimeters to meters) has received little attention beyond their fecal pellets or mucous feeding webs (Silver et al. 1998; Turner 2002). The absence of fundamental data on gelatinous zooplankton carcasses prevents the implementation of a jelly-POM formulation in biogeochemical models of the biological carbon pump. Particle sinking rates increase with individual size and length (Shanks and Trent 1980) but sedimentation with depth is modeled in different ways based on theory and observations. These include particle sinking rates that increase with depth * Corresponding author: [email protected] 1 Present address: Department of Microbiology, Oregon State University, Corvallis, Oregon Limnol. Oceanogr., 58(3), 2013, 1113–1122 E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2013.58.3.1113