Sinkers or floaters? Contribution from salp pellets to the export flux during a large bloom event in the Southern Ocean

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Abstract

Salp fecal pellets are rich in organic matter and have been shown to sink at very high velocities. In recent years, salp abundances have been increasing in the Southern Ocean where they seem to be replacing krill as the dominant grazers on phytoplankton. As salps can form large swarms with high pellet production rates, it has been suggested that they will become increasingly important for the vertical export of particulate organic matter in the Southern Ocean. However, detailed studies combining both investigations of pellet production rates, turnover, and export are still needed in order to determine whether salp pellets are important for export (‘sinkers’) or recycling (‘floaters’) of organic matter. Our results suggest that pellets are produced at high rates in the upper few hundred meters of the water column. Although we observed high sinking velocities and low microbial degradation rates of the produced salp pellets, only about one third of the produced pellets were captured in sediment traps placed at 100 m and about ~13% of the produced pellets were exported to sediment traps placed at 300 m. The high retention of these fast-settling pellets seems to be caused by break-up and loosening of the pellets, possibly by zooplankton and salps themselves. We measured 3-fold lower size-specific sinking velocities in loosened and fragmented compared to freshly produced intact pellets-. This enhanced the residence times (>1 day) of both small and large pellets in the upper water column. We postulate that the fragile nature of salp pellets make them more important for recycling of organic matter in the upper mesopelagic layer rather than as a conduit for export of particulate organic matter to the seafloor.

Introduction

Salp fecal pellets have long been considered to be important contributors to the vertical export of particulate organic carbon (e.g. Berner, 1967; Ramaswamy et al., 2005; Urrere and Knauer, 1981). Recent discoveries of declining krill populations and a possible rise in salp populations (Atkinson et al., 2004, Loeb et al., 1997) may have affected the ecosystem structure and function in the Southern Ocean (Smetacek and Nicol, 2005). For example, more phytoplankton might be packed into large, rapidly sinking salp fecal pellets in comparison to krill fecal pellets, and it is assumed that this enhances the export flux and the efficiency of the biological pump (Loeb et al., 1997, Pakhomov et al., 2002). At the same time, the shift from krill to salps might channel less primary production to large pelagic krill feeders such as penguins and marine mammals (Schofield et al., 2010).

Salpa thompsoni is the dominant salp species in the Southern Ocean (Casareto and Nemoto, 1986, Foxton, 1966, Pakhomov et al., 1994) and is capable of feeding at high rate on particles ranging in size from ~1 µm to several mm (Bone et al., 2003, Harbison and McAlister, 1979, Kremer and Madin, 1992). The large, fast-settling pellets produced by salps (Bruland and Silver, 1981) are rich in organic nitrogen and carbon (Andersen, 1998) but do not seem to be degraded at high rates by microbes (Caron et al., 1989). This indicates that salp pellets are mainly ‘sinkers’ that export organic matter from the surface to the deep ocean and seafloor at high rates. However, the majority of the Southern Ocean studies assessing the impact of salp pellets on the export flux have been “potential” estimates, derived from salp biomass and on-board pellet production incubations (Pakhomov et al., 2002, Phillips et al., 2009) or from sediment trap studies and seafloor observations (Bathmann, 1988, Matsueda et al., 1986, Smith Jr. et al., 2014). In isolation, these two methods provide information on either potential pellet production rates but not the amount of export, or pellet export rates without information about the total amount of pellets produced. Only studies combining both these measurements can provide the information necessary to determine whether salp pellets mainly recycle organic matter in the mixed layer (‘floaters’) or export organic matter to the deep ocean (‘sinkers’).

Here we followed the abundance and distribution of S. thompsoni using net trawls over 18 days during a large bloom event in the Southern Ocean. Abundance measurements were combined with on-board incubations for pellet production rates, measurements of size-specific sinking velocities and microbial degradation of the salp pellets, as well as determinations of the pellet organic carbon and chlorophyll a (Chl a) contents. Vertical export of both POC and salp pellets were assessed using free-drifting sediment traps, with both conventional traps and traps equipped with a viscous gel that preserves fragile particles (Thiele et al., 2015). By combining these measurements with standing stocks of particulate organic carbon (POC) and Chl a measured in the upper 100 m of the water column we aimed to understand the role of salp pellets in both POC vertical export (’sinkers’) and recycling (‘floaters’) within the epi- and upper mesopelagic layer of water near the Antarctic Polar Front.

Section snippets

Methods

This study was conducted from 29 January to 17 February 2012 in the Atlantic sector of the Southern Ocean during the RV Polarstern voyage ANT-XXVIII/3, in the framework of the ‘Eddy-Pump’ project (Wolf-Gladrow, 2013). We determined salp fecal pellet production and export during a large bloom covering an area of about 8000 km2 between the Antarctic Polar Front and the Southern Antarctic Circumpolar Current Front (Strass et al., This issue).

POC and salp pellet fluxes were determined at 100 and 300

Study area

Our study area spatially coincided with a large bloom event situated within the Antarctic Circumpolar Current. The bloom occupied an area of 8000 km2 and was centred at 51.2°S and 12.7°W, as estimated from Chl a satellite images (Ocean Colour Climate Change Initiative) (Fig. 1). The mixed layer depth of the region during the study was 82 ± 13 m (Hoppe et al., 2015).The oceanographic conditions of the study area are presented in detail in Strass et al. (This issue). The average satellite-derived

Salp abundance and fecal pellet production

During a phytoplankton bloom in the Southern Ocean, we observed salps at low to medium abundances (63 to 217 ind 1000 m3) in the upper 250 m of the water column. Despite their relatively low abundance, compared to previous observations (Foxton, 1966, Pakhomov et al., 2006, Pakhomov et al., 2002, Phillips et al., 2009), salps were successfully reproducing in the study area (see Pakhomov and Hunt, This issue).

The fecal pellet production rates in this study (~1.5 µg Chl a l−1 and from 0.1 to 0.5 FP

Acknowledgements

We thank Ann-Christine Zinkann for assistance at sea, Christiane Lorenzen for assistance during POC and Chl. a measurements, and Gerhard Fischer for comments and discussions during the writing of the manuscript. We thank the crew and scientific party of the R/V Polarstern ANT-XXVIII/3 cruise for 10 weeks of exploration. We thank two anonymous reviewers for valuable thoughts that have improved the manuscript. This study was supported by the Helmholtz Association (to MHI), the Alfred Wegener

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