Cryptic peaks: invisible vertical structure in fluorescent particles revealed using a planar laser imaging fluorometer

Abstract

Small‐scale vertical structure in the distribution of phytoplankton could be fundamentally important for zooplankton foraging, trophic coupling, and carbon cycling in planktonic ecosystems. Here, we identify regions of structure in phytoplankton communities that would be undetected by fluorometers by comparing the vertical distribution of chlorophyll fluorescence to the concentration of fluorescent particles over submeter scales. Images acquired with a free‐falling planar laser fluorescence imaging system were used to calculate vertical profiles of the concentrations and spatial distributions of fluorescent particles (e.g., eukaryotic phytoplankton, aggregates) and bulk fluorescence. We frequently observed peaks in the concentration of fluorescent particles with no coincident peak in bulk fluorescence: we define these features as cryptic peaks. These cryptic peaks can occur because the integrated fluorescence of the particles that are resolved by the imaging system is a small fraction of the total fluorescence; thus, a dramatic local change in the abundance of fluorescent particles can occur without significantly changing the bulk fluorescence. We also observed bulk fluorescence‐only peaks: peaks in bulk fluorescence with no coincident peak in fluorescent particle concentration. These features suggest that peaks in bulk fluorescence or chlorophyll a do not necessarily indicate increases in the concentration of the fluorescent particles resolved by our system, again emphasizing the difference between these two measures of phytoplankton structure. Comparing the relative abundances of two size classes of the fluorescent particles in the images revealed that the size composition of the fluorescent particles also varied over small scales. Phytoplankton less than ~ 500 µm in length numerically dominated the composition of most (65%) of the cryptic peaks we observed. By comparing vertical profiles of fluorescent particle concentration from two drops separated by less than an hour, we hypothesize that the peaks formed through vertical shearing of existing patches. Cryptic peaks contained almost 20% of the total number of fluorescent particles counted in all drops during our study and thus could represent disproportionately intense regions for important ecological processes relative to the rest of the water column.