UC San Diego

Coastal upwelling regions, such as the California Current Ecosystem, are among the most biologically productive regions in the ocean. Eukaryotic phytoplankton, particularly diatoms, underlie this high productivity, forming the base of the food web and driving biogeochemical cycling in the region. As a result, understanding how anthropogenic change, which is superimposed on the high variability of coastal upwelling regions, may influence phytoplankton abundances and physiology is critical towards forecasting the future state of the ocean. Concurrently, advancements in molecular-based approaches offer the ability to examine these organisms both within their environment and with an unprecedented level of detail.

In Chapter 1, we review, from a molecular perspective, the diverse mechanisms that marine eukaryotic phytoplankton have evolved related to the acquisition and use of the micronutrient iron. Iron frequently limits phytoplankton growth in the large areas of the ocean including the California Current, and its bioavailability for phytoplankton is expected to be reduced by ocean acidification, which is driven by rising atmospheric carbon dioxide concentrations. Therefore, in Chapter 2, we examine the effects of ocean acidification on natural phytoplankton communities in the California Current Ecosystem. We show that eukaryotic phytoplankton respond to increased acidification with a variety of iron uptake and conservation mechanisms that are indicative of increased iron stress, further supporting that acidification will negatively impact phytoplankton communities.

In Chapter 3, we describe how pigment-based approaches to measure phytoplankton abundances and community composition align with quantitative DNA- or RNA-based measurements. As phytoplankton pigments may be modeled from hyperspectral remote sensing data, these results further support that phytoplankton community composition, including harmful bloom-forming taxa, can be assessed from satellite-based ocean color observations.

In Chapter 4, these quantitative molecular approaches are leveraged to examine patterns in diatom abundances, diversity, and gene expression the California Current Ecosystem. In contrast to other phytoplankton groups, seasonal upwelling conditions are strongly associated with increases in diatom abundances and diversity as well as differences in gene expression. Furthermore, dominant bloom-forming diatoms in the nearshore environment are shown to be dispersal limited over relatively small spatial scales, as they sink out of the euphotic zone and contribute to carbon export rather than becoming established offshore.

Collectively, these results improve our understanding of phytoplankton in upwelling regions and support our ability to predict their future responses. In some areas, upwelling may intensify, resulting in an abundance of diverse diatoms that contribute to carbon export. In other areas, increased stratification may result in a diatom community with both low abundances and low diversity. Meanwhile, increased acidification may exacerbate iron limitation leading to declines in productivity. Continued environmental genomic investigations of phytoplankton in upwelling regions that further span phases of climate modes or experiment with other projected future conditions may reveal additional patterns and responses, increasing our understanding of phytoplankton from individual genes to entire communities.