UC San Diego

The southern CCS (sCCS), part of the California Current Ecosystem (CCE), is home to the California Cooperative Oceanic Fisheries Investigation (CalCOFI) survey program and the California Current Ecosystem Long Term Ecological Research (CCE-LTER) program. CalCOFI, begun in 1949, was designed as a survey program, sampling the same stations each quarter; whereas, the CCE-LTER program was designed to study interactions between various physical perturbations and ecosystem processes using Lagrangian cycles. The goal of this thesis was to examine microbial dynamics within these two programs and present a model for carbon fluxes into and out of the bacterial compartment. Chapter 1 ‘binned’ findings from CCE-LTER cruises into the relevant CalCOFI climatology and hydrography allowing the mechanistic studies into microbial dynamics enabled by the CCE-LTER program to be interpreted in the context of long-term observations. For example, the underlying mechanism of the enhanced microbial loop hypothesis, originally based on CalCOFI data, was supported using bacterial production data from CCE-LTER cycles in 2017. Additionally, the picoplankton community responded to broad climate variability with increased abundances and shifts in spatial distribution following a switch to the positive phase of the Pacific Decadal Oscillation at the end of 2013. Using the binned oceanic regions designated in Chapter 1, Chapter 2 showed that including the bacterial component in the food web strengthened previous conclusions that the nearshore is net autotrophic, providing excess carbon available to be laterally exported to the offshore. Examination of the westward propagating 2017 Morro Bay filament supported the hypothesis that lateral transport of organic material could support offshore, net heterotrophic communities. Bacterial production was strongly correlated to organic carbon pools, as observed in Chapter 3. Bottom-up control of bacterial production across the CCE was comparable to global trends, but did not scale with net primary production, perhaps due to enhanced grazing nearshore. Furthermore, the observed temperature control of bacterial production was a proxy for the role of upwelling and substrate production, not metabolic rate. Strong bottom-up controls and weak temperature controls observed during the 2014 warm anomaly suggest future warming and stratification within the sCCS could be rapidly experienced by the bacterial community.