Depth attenuation of organic matter export associated with jelly falls

Abstract

We explore the attenuation in the export ratio of jelly-POM (particulate organic matter) with depth as a function of the decay rate, temperature, and sedimentation rate. Using data from the Vertical Transport In the Global Ocean project, we compare ratios computed with the Martin-curve, with a particle-based parameterization, and with sediment-trap data. Owing to the temperature dependence of the decay rate (Q10 5 4.28), the jelly-POM export ratio below 500 m is 20–45% larger in subpolar and temperate areas than in the tropics. Vertical migration of gelatinous zooplankton leads to a variable starting depth of a jelly fall (death depth), which governs the start of remineralization, and the fate of the biomass. Owing to the absence of observations, we employ a sinking speed matrix ranging from 100 m d21 to 1500 m d21 to represent slowand fast-sinking carcasses. The assumption of a constant decay rate k independent of temperature in other particle-based models may not be appropriate. These results provide information for including jelly-POM in global biogeochemical model formulations. Particulate and gelatinous material export—Vertical fluxes of biogenic and particulate organic matter (POM) govern chemical gradients and, thus, drive the ocean’s biological carbon pump. Sinking POM varies in size and composition, originating in every trophic level as exudates, detritus, fecal material, aggregates, biogenic carbonates, or the carcasses of the organisms themselves (Turner 2002). The remineralization profile of POM generally depends on the sinking speed (McDonnell and Buesseler 2010) and the decay rate (Martin et al. 1987). The contribution of gelatinous zooplankton to POM export has been assessed for detrital particles and fecal pellets (Turner 2002). The fate of the biomass of gelatinous organisms (jelly-POM), mainly from Cnidaria and Thaliacea, however, has been rarely assessed and, thus, not included in biogeochemical models. Yet, there is substantial evidence of sedimentation events in the last decades, so-called ‘jelly falls,’ that can deposit large amounts of biomass on the seafloor (Lebrato and Jones 2009). Remineralization of gelatinous material releases dissolved organic matter (jelly-DOM), creating a ‘jelly carbon shunt’ (Condon et al. 2011). Inorganic nutrients are also released (Pitt et al. 2009) and oxygen is consumed (West et al. 2009). Thus, remineralization of jelly falls has broad biogeochemical implications, similar to other sinking particles, although the different biochemical composition (C : N : P) and the absence of mineral ballast imply different stoichiometric relationships. Jelly-POM representation in biogeochemical models— The parameterization of sinking organic matter in regional and large-scale biogeochemical models has evolved from the early use of static remineralization profiles, and the depth dependence of the flux of sinking particles. Recent work on sinking particles, covering a size range of about 1– 1000 mm, is not truly applicable for the size spectrum of particles found in jelly falls (millimeters to meters). The sedimentation of POM has been modeled using constant rates or an acceleration with depth, which can be related to the Martin et al. (1987) formulation. Observations of sinking material are limited to particles whose size fractions are small enough (, 1 mm to , 50 mm) to be collected by sediment traps. However, larger particles (such as those associated with jelly falls) have rarely been investigated (salps: , 100–800 m d21, siphonophores: , 200 m d21 [Apstein 1910]). Information on carcasses from other organisms could give an indication of jelly-POM sinking rates (e.g., cladocerans: , 140 m d21, amphipods: , 900 m d21, chaetognaths: , 400 m d21, copepods: 30– 700 m d21 [Kuenen 1950]). The sedimentation rate of jellyPOM is governed by a combination of size, diameter, biovolume, and geometry (Walsby and Xypolyta 1977), material density (Yamamoto et al. 2008), and drag coefficients (McDonnell and Buesseler 2010). For large gelatinous carcasses (e.g., 100–200 kg Nemopilema nomurai carcasses), the weight per se dominates over any other forcing. Owing to their size, gelatinous carcasses sink individually, and do not coagulate as smaller particles do. Coagulation may occur during decomposition, however, when the material accumulates in high densities, forming mats at the seabed (Billett et al. 2006). Although scyphozoans and thaliaceans sometimes occur on the seabed in enormous numbers, other gelatinous taxa, such as ctenophores and hydrozoans, have never been observed. This implies that the transfer efficiency of gelatinous biomass varies among taxa. The attenuation concept (Buesseler and Boyd 2009) motivated the current study. Here, we examine the remineralization of gelatinous biomass to assess the fraction arriving at a given reference * Corresponding author: mlebrato@ifm-geomar.de Limnol. Oceanogr., 56(5), 2011, 1917–1928 E 2011, by the American Society of Limnology and Oceanography, Inc. doi:10.4319/lo.2011.56.5.1917

Topics

    2 Figures and Tables

    Download Full PDF Version (Non-Commercial Use)