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Rhizaria is a group of protists contributing significantly to plankton in the oligotrophic ocean. With our high resolution underwater optical system, the Focused Shadowgraph Imaging system, we collected rhizarian images from surface to 5000 m in the Sargasso Sea in the vicinity of the Bermuda Atlantic Time Series stations. Images were processed using a novel image analysis algorithm including illumination correction and a newly developed edge detection approach. We found high abundances of Rhizaria such as Acantharia, Collodaria Foraminifera, Phaeodaria and Spumellaria throughout the water column to 4532m depth. While Rhizaria were most abundant in the epipelagic layer, they were present in the mesopelagic zone at approximately 1/10th of the abundance levels at the surface. Many Rhizaria in the deep sea displayed intact pseudopodial extensions indicating that they are active flux feeders at depth. Our image collection further serves as a training set for the automated detection of rhizarian plankton using deep-learning algorithms.

A Focused Shadowgraph Imaging System (FoSI) camera with a new image analysis algorithm has been developed to study particles from the surface to the deep sea. FOSI is uniquely able to capture low-density and small particles that are underestimated using conventional optical systems and uses an image algorithm incorporating an improved edge detector to reconstruct particle contours. Using data from deployments in the Sargasso Sea, we found that particle numbers throughout the water column better correlate with beam attenuation than total particle volume. This results from the fact that larger particles are rare events which contribute to total particle volume but are undetected by the transmissometer. Chlorophyll a was poorly correlated with both particle number and volume. Assuming that optical density is correlated with the physical density of particles, the observed patterns in the relative abundance of fainter particles may provide new insights into the average sinking velocities of particles throughout the water column.

Optical surveys of aquatic particles and their particle size spectra have become important tools in studies of light propagation in water, classification of water masses, and the dynamics of trophic interactions affecting particle aggregation and flux. Here, we demonstrate that typical settings used in image analysis vastly underestimate particle numbers due to the particle – gel continuum. Applying a wide range of threshold values to change the sensitivity of our detection system, we show that macrogels cannot be separated from more dense particles, and that a true particle number per volume cannot be ascertained; only relative numbers in relation to a defined threshold value can be reported. A quandary thus presents itself between choosing a detection threshold low enough to accurately record orders of magnitude more particles on one hand or selecting a higher threshold to yield better image quality of plankton on the other. By observing the dynamics of coagulation and dissolution steps unique to cation-bridged gels abundant in aquatic systems, we find naturally occurring gels, and microscopic particles attached to them, to cause the ill-defined particle numbers. In contrast, the slopes in particle number spectra remained largely unaffected by varying sensitivity settings of the image analysis. The inclusion of fainter particles that are not typically captured by imaging systems provides a window into the true microscale spatial heterogeneity at scales relevant to small plankton organisms and processes that are dependent on particle density such as surface-associated chemical reactions as well as particle coagulation and aggregation dynamics.