AUTHOR=Bochdansky Alexander B. , Huang Huanqing , Conte Maureen H. TITLE=The aquatic particle number quandary JOURNAL=Frontiers in Marine Science VOLUME=Volume 9 - 2022 YEAR=2022 URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.994515 DOI=10.3389/fmars.2022.994515 ISSN=2296-7745 ABSTRACT=Optical surveys of aquatic particles and their particle size spectra have become important tools in 11 studies of light propagation in water, classification of water masses, and the dynamics of trophic 12 interactions affecting particle aggregation and flux. Here, we demonstrate that typical settings 13 used in image analysis vastly underestimate particle numbers due to the particle – gel continuum. 14 Applying a wide range of threshold values to change the sensitivity of our detection system, we 15 show that macrogels cannot be separated from more dense particles, and that a true particle 16 number per volume cannot be ascertained; only relative numbers in relation to a defined 17 threshold value can be reported. A quandary thus presents itself between choosing a detection 18 threshold low enough to accurately record orders of magnitude more particles on one hand or 19 selecting a higher threshold to yield better image quality of plankton on the other. By observing 20 the dynamics of coagulation and dissolution steps unique to cation-bridged gels abundant in 21 aquatic systems, we find naturally occurring gels, and microscopic particles attached to them, to 22 cause the ill-defined particle numbers. In contrast, the slopes in particle number spectra remained 23 largely unaffected by varying sensitivity settings of the image analysis. The inclusion of fainter 24 particles that are not typically captured by imaging systems provides a window into the true 25 microscale spatial heterogeneity at scales relevant to small plankton organisms and processes 26 that are dependent on particle density such as surface-associated chemical reactions as well as 27 particle coagulation and aggregation dynamics.