Elsevier

Progress in Oceanography

Volume 189, November–December 2020, 102445
Progress in Oceanography

Size does not matter after all: No evidence for a size-sinking relationship for marine snow

https://doi.org/10.1016/j.pocean.2020.102445Get rights and content

Highlights

  • In situ measurements of individual untouched and undamaged particles.

  • Simultaneous measurements of aggregate size, abundance, type, and sinking velocity.

  • In situ aggregate sinking velocity is not controlled by aggregate size.

  • Caution required when using only particle sizes to calculate export flux.

Abstract

The biological carbon pump is a critical component for uptake of carbon by the oceans. Most of this is mediated by gravitational sinking of particles and it is generally assumed that there is a positive relationship between sinking velocities and sizes of particles. Due to the difficulties inherent in measuring the sinking velocity of untouched and undamaged particles, the majority of studies have been based on artificial solid particles or laboratory generated marine snow formed from homogenous material, e.g. phytoplankton cultures. Here, we present results from a newly developed optical method that measures sizes and sinking velocities of undisturbed in situ aggregates in the mesopelagic zone. The measurements were done at depths between 90 and 530 m throughout day and night. In total we measured 55 image sequences of aggregate sizes and sinking velocities, resulting in measurements of 1060 individual aggregates. Only 10 sequences showed significant correlations between aggregate size and sinking velocity. Furthermore, only 8 of the 10 significant correlations between size and settling were positive. Despite similar optical appearance, similar sized aggregates had different sinking velocities, suggesting that aggregates formed in situ are heterogeneous with different compositions, structure and densities. This complicates estimates of sinking velocity from aggregate size only and one has to be careful when estimating downward particle flux based on in situ size-distribution and abundance of aggregates and theoretical relationships between size and sinking velocity. Hence, this study forces the conclusion that estimates of downward particle flux which use particle size distributions must also include information about aggregate composition, compactness and density from in situ optics or direct particle and aggregate sampling.

Introduction

Understanding the nature of downward flux of organic matter remains a key goal of oceanography. The sinking velocity of a marine snow particle relative to its surrounding water will change over time partly due to externalities in the water through which it is sinking (grazing pressure, particle concentration, water density (temperature and salinity) and turbulence) and partly as a result of internal autochthonous modification (aggregation, disintegration, porosity, stickiness, chemistry, hydrated density, surface roughness, internal diffusivity and biotic assemblage). The influence of the externalities will be dependent on its size in most instances and a clear understanding of size and sinking is therefore essential.

Large sinking aggregates, such as zooplankton faecal pellets and marine snow, transport organic matter to depth in the ocean. These particles affect nutrient distribution in the water column, feed life in the dark ocean, control carbon dioxide removal from the atmosphere, and determine deposition rates of surface material in the sediments several kilometres deep (Trull et al., 2008). More than 90% of the organic matter sinking below the euphotic zone is respired before it reaches a depth of a thousand meters (Robinson et al., 2010) and only organic matter sinking to depths greater than a thousand meters can be considered to be stored in the ocean for more than 100 years (Lampitt et al., 2008a). Most flux attenuation occurs in the upper few hundred meters of the water column where zooplankton grazing and microbial degradation is high (Iversen et al., 2010, Jackson and Checkley, 2011). It therefore seems reasonable that fast-sinking particles will make a greater contribution to the deep ocean flux than slow-sinking particles, since the latter will be rapidly recycled at shallow depths. The broader implication of this is that reduced attenuation in flux with depth will lead to a reduction in atmospheric carbon dioxide concentration (Kwon et al., 2009).

We know very little about particle settling behaviour in the ocean. This is because it is extremely difficult to measure the sinking velocity of individual untouched and undamaged particles in the water column where horizontal flows are typically several orders of magnitude higher than the sinking velocities of particles. Several in situ estimates of particle sinking velocities have been made using a variety of approaches such as settling chambers that excluded ambient water movements (Diercks and Asper, 1997, Nowald et al., 2009, Pilskaln et al., 1998), by measuring the arrival time of peak fluxes at traps deployed at different depths (Fischer and Karakas, 2009), from the arrival times of material on the deep-sea floor (Billett et al., 1983) or by relating concentrations of particles obtained with in situ cameras with fluxes of particles collected in gel-traps (McDonnell and Buesseler, 2010). Only very few direct measurements of undisturbed aggregates while still sinking in an unrestricted water column have been done and those measurements have all been obtained using SCUBA diving and hence confined to the surface ocean (Alldredge and Gotschalk, 1988, Alldredge and Gotschalk, 1989).

Settling velocities of similar types of aggregates generally increase with aggregate size (Alldredge and Gotschalk, 1988, Alldredge and Gotschalk, 1989, Iversen and Ploug, 2010). However, aggregate size alone is not a good indicator for sinking velocity (Kiørboe et al., 1998), which mainly depends on the aggregate composition, shape and porosity (Iversen and Ploug, 2010). Still, many processes can alter the sinking velocity of aggregates while they sink through the water column, such as zooplankton feeding (Iversen et al., 2010, Stemmann et al., 2004) transforming marine snow into faecal pellets, fragmentation by zooplankton (Dilling and Alldredge, 2000, Iversen and Poulsen, 2007, Lampitt et al., 1990) and disaggregation due to physical shear, resulting in smaller and possibly slower settling aggregates.

In an attempt to obtain in situ settling velocities from undisturbed aggregates in an unrestricted water column at depths down to 530 m, we modified a neutrally buoyant platform to carry an optical system. Our study was over the Porcupine Abyssal Plain in the Northeast Atlantic (49°N 16.5°W) (Lampitt et al., 2001) at the PAP Sustained Observatory (PAP-SO) during the research cruise with James Cook (JC087) from the 31st May to the 18th June 2013. We choose this site because it has little horizontal input of particles (Weaver et al., 2000), less eddy activity than most other marine regions (Chelton et al., 2007), and generally has weak currents (Lampitt et al., 2001) with limited lateral advection (Hartman et al., 2010, Williams et al., 2006).

Section snippets

PELAGRA – neutrally buoyant sediment traps

We equipped two PELAGRA sediment traps (neutrally buoyant platforms) with camera systems to image settling aggregates in the free water column. The PELAGRAs are neutrally buoyant sediment traps based on an APEX float (Webb Research Corporation, USA), which has been equipped with four funnel traps (Lampitt et al., 2008b, Saw et al., 2004). The APEX floats provide active buoyancy control that enables the PELAGRAs to maintain a level of constant pressure or density (according to the protocol

Results

The 55 image sequences provided direct observations of size-specific sinking velocities for 1060 individual aggregates, n.b. only 12 sequences provided absolute sinking velocities, i.e. 158 individual aggregates. The 1060 particles varied in ESD from 0.04 to 2.98 mm (Mean 0.69 ± 0.40 mm). The aspect ratios of the aggregates averaged 1.40 ± 1.21, suggesting that the majority of the aggregates were ellipsoidal. Generally, the aggregates were similar in appearance and apart from a few faecal

Discussion

We first consider the 12 sequences which were obtained during periods when the PELAGRAs were in equilibrium with the surrounding water mass. These sequences provided absolute sinking velocities of the aggregates (Fig. 2). We observed some variation in size-specific sinking velocities between the 12 image sequences. The majority of the aggregates had sinking velocities in the range from 20 m d−1 to 300 m d−1 which is typical for in situ measurements of aggregates smaller than 1.5 mm (Alldredge

Declaration of Competing Interest

None.

Acknowledgments

We thank Kevin Saw for outstanding engineering support during PELAGRA design, preparations, deployments and recoveries. We thank Christian Konrad for valuable assistance and help with image analyses in C++ and the laboratory calibrations. We thank two reviewers for valuable input to the manuscript. This study was supported by the Helmholtz Association and the Alfred Wegener Institute for Polar and Marine Research (to MHI), the DFG-Research Center/Cluster of Excellence “The Ocean Floor - Earth's

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