Is Diel Vertical Migration Important to Oceanic Export Carbon Flux?

BackgroundThe global carbon cycle is of interest due to the large and increasing amount of anthropogenic CO2 that has been released into the atmosphere. Much of this CO2 has been taken up by the ocean at a net rate of ~2 Pg y-1. The air-sea flux of CO2 results from, and is proportional to, the partial pressure differences between the atmosphere and the ocean surface. About 70% of the CO2 concentration gradient in the top 1000 m of the ocean is maintained by biological processes (Volk and Hoffert, 1985), referred to as the “biological pump”. The biological pump is broadly divided into passive (sinking) and active transport terms, which together amount to 11 PgC y-1 or 91 mgC m-2 d-1 (Falkowski et al., 2003). Nearly all calculations of the carbon export flux are based only upon the passive transport term (Falkowski et al., 2003).

The potential importance of active transport is illustrated by the Diel Vertical Migration (DVM) behavior of the sonar DSL (Figure 1). DVM is thought to be an adaptation to high visual predation risk in the epipelagic layer, which also contains the highest food density (Angel, 1985). The DSL is chiefly composed of mesopelagic fish with swim bladders. The global biomass of mesopelagic fish is estimated at ~1 Pg, or 1 billion tons. The fish eat 1-5% of their dry body weight each day (0.5-2.1 PgC y-1). These large numbers are known to be underestimates. Biomass measured with trawls is biased low due to net avoidance and escapement by the fish. Trawling has been shown to capture only 4-20% of the true biomass (Gjosaeter, 1984; Koslow et al., 1997; May and Blaber, 1989).

Carbon Flux ModelingMeasurement of carbon transport by migrating fish requires the use of appropriate physiological and ecological rates. The rates of interest for carbon transport are respiration, defecation, and mortality. Excretion has not been included in the model because its carbon content is small in comparison to the other fluxes. Of special interest is the potential interaction between physiology, DVM behavior, and the temperature changes associated with DVM. We have selected published rates and parameters to combine with DVM into a model of the carbon transport due to a single fish. The model is constructed as arrays with depth rows and time columns (Figure 6). Location of the fish is used as a mask. This model was then applied to the weight distribution of captured fish. Figures 7 and 8 show the carbon export by trawl and by cycle respectively. Figure 9 displays the relative size of the fish active flux to the passive flux measured with sediment traps (Stukel, unpublished data) at Cycles 2 and 4. Figure 9 also shows the relative sizes of the components of the biological pump if the expected capture efficiency of 14% is applied (Koslow et al., 1997).

Conclusions

The export carbon flux mediated by dielly migrating mesopelagic fish is of comparable magnitude to published values for zooplankton.

The export carbon flux mediated by these fish is likely to comprise a large fraction of the biological pump.

Quantitative sonar data in combination with the trawl sampling are necessary to reduce bias in the biomass measurements.

Pete Davison, David M. Checkley, Jr., Tony KoslowScripps Institution of Oceanography, pdavison@ucsd.edu

AbstractThe active transport of carbon out of the surface ocean by migrating fish that form the sonar Deep Scattering Layer (DSL) is poorly known, but potentially large. Biomass measurements and CTD profiles from the CCE-P0704 cruise were combined with physiological and mortality rates from the literature using a computer model. The model results indicate an overall fish mediated transport of 0.7-3.2 mgC m-2 d-1 in the California Current. The fish carbon flux is similar to estimates of the migratory zooplankton flux from other ecosystems (2-3 mgC m-2 d-1, summarized in Al-Mutairi and Landry, 2001), and is 6-7% of the passive carbon flux measured concurrently with sediment traps (Stukel, unpublished data). The measured fish biomass is biased low due to net avoidance. We hope to measure this bias on future CCE-LTER cruises by combining quantitative sonar data with the mesopelagic trawl sampling.

Materials and MethodsMesopelagic fish were sampled thirteen times at three stations using a high-speed 5 m2 Oozeki midwater trawl (Figure 2, Oozeki et al., 2004). The trawls were oblique from 500 m depth with a target velocity of 4 knots. Four of the night trawls were only to 150 m due to wire-time constraints. Trawl locations are shown in Figure 3. Fish captured from the trial tow (“Cycle 0”) were preserved in 90% ethyl alcohol. Fish from Cycles 2 and 4 were preserved in 10% buffered formalin and then transferred to 50% isopropyl alcohol on shore. The preservation process resulted in a 15% loss of wet weight. Each fish was identified, weighed, and measured for standard length. The weight distribution of fish from migratory taxa was used as an input parameter to a fish carbon flux model. The CTD profiles reaching 500 m were averaged and used as the temperature input parameter to the carbon flux model.

CCE-P0704 ResultsCTD profiles from the cruise are shown in Figure 4. Migratory fish biomass is shown in Figure 5.

AcknowledgementsThis work was supported by NSF Grant NSF 0321167. CCE-LTER advisors graciously provided wire time during the 2007 process cruise. The SIO Vertebrate Collection and Pelagic Invertebrate Collection provided chemicals, lab space, gear, and advice. CCE-LTER volunteers were essential for the task of processing the catch. Lastly, we would like to thank the crew of the R/V Thomas G. Thompson who in addition to helping deploy and recover the trawl, repaired it after it was damaged by the ship’s propeller.

ReferencesAl-Mutairi, H., Landry, M.R., 2001. Active export of carbon and nitrogen at Station ALOHA by diel migrant zooplankton. Deep-Sea Research Part II-Topical Studies in Oceanography 48 (8-9), 2083-2103.Angel, M.V., 1985. Vertical Migrations in the Oceanic Realm: Possible Causes and Probable Effects. In: Rankin, M.A. (Ed.), Migration. Mechanisms and Adaptive Significance, pp. 45-70.Falkowski, P.G., Laws, E.A., Barber, R.T., Murray, J.W., 2003. Phytoplankton and Their Role in Primary, New, and Export Production. In: Fasham, M.J.R. (Ed.), Ocean Biogeochemistry. Springer-Verlag, Berlin, pp. 99-121.Gjosaeter, J., 1984. Mesopelagic Fish, a Large Potential Resource in the Arabian Sea. Deep-Sea Research Part A-Oceanographic Research Papers 31 (6-8), 1019-1035.Koslow, J.A., Kloser, R.J., Williams, A., 1997. Pelagic biomass and community structure over the mid-continental slope off southeastern Australia based upon acoustic and midwater trawl sampling. Marine Ecology-Progress Series 146 (1-3), 21-35.May, J.L., Blaber, S.J.M., 1989. Benthic and Pelagic Fish Biomass of the Upper Continental-Slope Off Eastern Tasmania. Marine Biology 101 (1), 11-25.Oozeki, Y., Hu, F.X., Kubota, H., Sugisaki, H., Kimura, R., 2004. Newly designed quantitative frame trawl for sampling larval and juvenile pelagic fish. Fisheries Science 70 (2), 223-232.Stukel, M., 2007. Unpublished sediment trap data from CCE-P0704 cruise.Volk, T., Hoffert, M.I., 1985. Ocean carbon pumps: Analysis of relative strengths and efficiencies in ocean-driven atmospheric CO 2 changes. In: Sundquist, E., Broecker, W.S. (Eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present.

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