UC San Diego

The deep chlorophyll maximum (DCM) is a ubiquitous but poorly understood feature of the ocean ecosystem. Chapter 2 explores possible explanations for the existence of the DCM with simple, one-dimensional, steady-state mathematical models. Sinking of plankton or detritus is shown to be critical in the formation of a deep maximum in phytoplankton biomass. A model with species-dependent chlorophyll-to-biomass ratios and growth rate characteristics demonstrates the formation of a DCM which does not represent a maximum in biomass. During the Spice cruise of 1997, SeaSoar measurements were made with 4-m horizontal resolution on two isopycnals and two isobars along a 1000-km transect in the northeastern Pacific. Chapter 3 is an investigation, based on these data, of the relationship between the distribution of chlorophyll and that of temperature. Fluctuations in the two tracers tend to align with each other, either in phase or out of phase, at scales above 10 km. Enhancement of gradients by stirring is shown to be a likely explanation for this behavior, and models suggest that turbulent diffusion or rapid phytoplankton growth could be responsible for destroying alignment at small scales. Chapter 4 presents results from recent glider measurements along CalCOFI Line 93. Three glider missions were carried out along the 700- km transect, and chlorophyll and salinity measurements are compared with historical CalCOFI data. Sharp, density- compensated salinity fronts are found to be common, often with corresponding features in the chlorophyll field. The deep chlorophyll maximum tends to track vertical fluctuations in the depth of nearby isopycnals, but on average the depth of the DCM does not move as far as that of the isopycnals