UC San Diego

Phytoplankton are the building blocks of marine pelagic food webs, fixing carbon through photosynthesis and providing food to higher trophic levels. Thus, how phytoplankton are distributed spatially can significantly affect important ecological processes such as carbon cycling and trophic dynamics. In this dissertation, I present analyses of vertical phytoplankton distributions on scales of a meter and less using data from a novel free -falling planar laser imaging fluorometer, the FIDO-[Phi]. I frequently observed peaks in fluorescent particle concentration with no coincident peaks in bulk fluorescence. These cryptic peaks may be regions of local importance for zooplankton foraging and aggregate formation, and are undetectable using methods that only measure bulk fluorescence. Small-scale variability was also observed in particle size spectra and the distribution of total particles throughout the water column. However, changes in these variables did not explain much of the observed structure in fluorescent particle concentration. This result suggests that small- scale phytoplankton distributions are likely controlled by complex interactions between different factors, and cannot be predicted accurately from simple correlations. To resolve these interactions, I developed a model that describes the control of gradients in phytoplankton abundance by biological and physical dynamics. The model predicted that the maximum strength of phytoplankton gradients is determined by a balance between turbulent mixing and the rate of phytoplankton layer formation. Comparing these results with phytoplankton gradient data and simultaneous physical measurements allowed estimation of the rate of layer formation and the minimum possible phytoplankton layer thickness. This understanding of the mechanisms underlying phytoplankton layer formation will allow better prediction of small-scale phytoplankton distributions. Analyzing images from the FIDO-[Phi] also allowed the investigation of the spatial distributions of phytoplankton on centimeter scales. The data revealed that individual large phytoplankton or fluorescent aggregates can cause microscale peaks in chlorophyll a fluorescence. The investigation of small-scale vertical phytoplankton distributions presented in this dissertation provides knowledge of local phytoplankton variability - the backdrop upon which fundamental ecological processes take place