Ascending marine particles: Significance of transparent exopolymer particles (TEP) in the upper ocean

The high abundance of transparent exopolymer particles (TEP) in marine and freshwater greatly affects particle dynamics. TEP act as glue for colliding particles and form the matrix in aggregates, thereby altering aggregation dynamics. We studied the sinking behavior of freshly produced, particle-free TEP and of aggregates composed of TEP and latex spheres in a laboratory using water collected from Santa Barbara Channel, California. Particle-free TEP ascend and accumulate in the surface layer of a settling column at an average velocity of 1.6 3 1024 cm s21. The estimated density of TEP ranges from 0.70 to 0.84 g cm23. TEP also transported latex spheres of 45.6 and 1.82 mm in diameter and a density of 1.05 g cm23 to the surface layer. We describe a simple model illustrating the role of TEP for the vertical transport of solid particles. The densities and relative proportions of TEP, solid particles, and interstitial water within an aggregate determine its sinking or ascending velocity. High ratios of TEP to solid particles retard the sinking of aggregates, prolonging their residence time in the surface ocean. Our results demonstrate that TEP can provide a vehicle for the upward flux of biological and chemical components in the marine environment, including bacteria, phytoplankton, carbon, and reactive trace elements. Transparent exopolymer particles (TEP) are important for many aspects of particle dynamics in aquatic systems. This stems from their central role in coagulation (Passow et al. 1994; Jackson 1995; Logan et al. 1995) and sedimentation of particles (Passow et al. 2001). TEP are formed abiotically from dissolved precursors released by phytoplankton and bacteria (Passow 2000) and consist predominantly of surface-active polysaccharides (Mopper et al. 1995; Zhou et al. 1998). TEP are extremely ‘‘sticky’’ (Dam and Drapeau 1995; Logan et al. 1995; Engel 2000) and readily form aggregates of all sizes with ambient solid particles such as bacteria, phytoplankton, molts, mineral clays, or detritus (Alldredge et al. 1993). TEP exist as microaggregates of tenths of micrometers (Simon et al. 1990) and as an integral part of snow-sized aggregates of .500 mm (Alldredge et al. 1993). Marine particulate matter settles to great depths as snowsized aggregates (Knauer 1991). Sinking velocities of diatoms embedded in snow-sized aggregates are generally very fast (50–200 m d21; Asper 1987; Alldredge and Gotschalk 1988) compared with those of individually sinking cells (,1–10 m d21; Culver and Smith 1989), allowing aggregated particles to reach the deep ocean more effectively. Factors determining sinking velocities of aggregates include size, dry weight, porosity, and excess density of aggregates (Alldredge and Gotschalk 1988), as well as density gradients in 1 Corresponding author ([email protected]). Acknowledgments We thank Chris Gotschalk for help with the sampling, Bill Li and Karen Saunders for their help with the analyses of latex spheres, and Alice Alldredge, Dave Scott, and Christopher Cogan for comments and discussions. This work was partly supported by the Panel on Energy Research Development. the water (Alldredge and Crocker 1995; MacIntyre et al. 1995) and turbulence (Shanks 2002). The density of aggregates is a function of the presence of gas enclosures (Riebesell 1992), the density of the enclosed solid matter (Asper 1987; Azetsu-Scott and Johnson 1992), and the TEP to cell ratio (Engel and Schartau 1999). The present study evaluates in detail the effect of TEP on the density and sinking velocity of aggregates. The sinking behavior of particle-free TEP and of aggregates composed of latex spheres and TEP was investigated to better understand the transport of solid particles and substances sorbed to TEP in aquatic systems. We present experiments showing that particle-free TEP are positively buoyant, and we provide the first estimates of TEP density. The volume fraction of TEP required for aggregates to be neutrally buoyant was estimated for different solid particle types. The effect of TEP on the flux of solid particles, as well as on the biogeochemical cycles of elements associated with TEP, is also discussed. Materials and methods Seawater was collected in the Santa Barbara Channel (348209N, 1198509W) from the chlorophyll a (Chl a) maximum (0–20 m) for 10 different experiments in March, April, and May 1997 (Table 1). Chl a concentrations at the sampling stations ranged from 0.41 to 3.81 mg L21. All experiments were conducted with freshly collected water. Samples were prefiltered through 0.2-mm polycarbonate (Nuclepore) filters to remove all particles, including preexisting TEP. Then, clean (particle-free) TEP was generated from dissolved precursors by rotating 1.2 liters of the prefiltered seawater in a Couette flocculator (at a shear rate of 742 Azetsu-Scott and Passow Ta bl e 1. C ha ra ct er is ti cs of se aw at er co ll ec te d in th e S an ta B ar ba ra C ha nn el , C al if or ni a, in M ar ch , A pr il , an d M ay 19 97 .