Aggregates and their distributions determined from LOPC observations made using an autonomous profiling float
- a Department of Oceanography, Texas A&M University, College Station, TX 77843, USA
- b Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA
- Received 1 September 2012
- Revised 7 December 2012
- Accepted 30 December 2012
- Available online 9 January 2013
Abstract
The vertical flux of particles in the ocean drives the movement of organic carbon to the deep ocean. We have been studying the distribution and flux of these particles using the SOLOPC, a profiling Lagrangian (SOLO) float with a Laser Optical Particle Counter (LOPC). We have been able to distinguish between aggregate-like and zooplankton-like particles with diameters 
but needed a way to separate the smaller particles into aggregates and zooplankton. Observations included a lognormal-shaped fraction in the normalized volume distribution similar to that observed in results for simulations of particles in the euphotic zone. By fitting a lognormal distribution to the volume spectrum of particles with diameters 
, we have been successful at making a separation of marine snow material from other, presumably living, particles. The particle volumes derived using the separations are positively correlated with fluorescence, particulate organic carbon, and the volume of larger particles classified as aggregate-like, which supports the conclusion that these particles are truly aggregates, in some cases derived from phytoplankton. The residual volumes (total less the above fit) are highly correlated with the volumes of large, zooplankton-like particles. Downward velocities of the aggregate fraction calculated from time series of particle profiles are consistent with previous estimates of particle settling rates 
. We now have a tool to estimate aggregate distributions, properties, and vertical fluxes in the euphotic zone, including when and where they change.
Highlights
► Fit a lognormal model to the particle spectrum to isolate aggregates. ► The modeled fraction shares many properties associated with aggregates. ► The residual fraction is correlated with zooplankton-like particles. ► Have a tool to estimate aggregate distributions, properties, and vertical fluxes. ► Estimated aggregate fluxes are consistent with flux feeding by zooplankton.
Keywords
- Particle distributions;
- Marine snow;
- Particle counter;
- Vertical flux;
- Zooplankton
1. Introduction
Particles and their motions relative to water (falling, rising, swimming) create distinctive biological and chemical zonation in the ocean. Such particles range from living organisms to aggregates containing mixtures of dead and living material. One such particle type observed by divers and cameras is known as marine snow; it can be composed of a mixture of plankton, detritus, and inorganic materials. Most aggregates originate from primary production in the euphotic zone and may form by coagulation or fecal pellet production, and can be fragmented by turbulence, remineralized by attached microbes, repackaged and redistributed by zooplankton feeding, or sink from the euphotic zone to the underlying mesopelagic. The resulting movement of particles out of the euphotic zone is a key part of the biological carbon pump that moves carbon out of the atmosphere and into the deep ocean.
The importance of particles in mediating oceanic biological and chemical processes has stimulated the development of techniques to detect and characterize particles automatically. Such techniques tend to have been developed separately by the aggregate and the plankton communities, although their instruments often detect both types of particles (e.g., Ashjian et al., 2001 (VPR); Checkley et al., 2008 (LOPC); Picheral et al., 2010 (UVP)). One such instrument, the Laser Optical Particle Counter (LOPC), was developed to measure zooplankton concentrations but also measures concentrations of aggregates ( Herman et al., 2004 and Jackson and Checkley, 2011). The LOPC has been attached to an autonomous Lagrangian profiling float to make the SOLOPC (Checkley et al., 2008). The SOLOPC allows sampling of particle distributions that are highly variable in time and space at scales relevant to them.
The LOPC measures the attenuation of particles passing through a light sheet sensed by multiple photodiode elements (Herman et al., 2004). When a small particle occludes all or part of 1 or 2 photodiode elements (single element particle; SEP) the LOPC estimates its equivalent spherical diameter (desd; see Table 1 for all notation) and then adds a count to the appropriate size bin. In this case, desd is the diameter of an opaque sphere that attenuates the same amount of light. For larger particles that span at least 3 photodiode elements (multiple element particles; MEPs), there is an additional measurement of diameter, the occluded diameter (dod), the number of 1 mm light beams a particle occludes either partially or completely while passing through the light sheet. Thus, the LOPC provides a single measurement, based on opacity and size, for an SEP and data on both opacity and size for each MEP. Such data have been used to classify the larger particles into either marine snow or zooplankton (Jackson and Checkley, 2011). The information about the two particle groups can be combined to describe the particle distribution across the entire size range of desd.
