UC Santa Barbara

Approximately 50 percent of marine primary production passes through the dissolved organic matter (DOM) pool (Myklestad 2000; Nagata 2000). The major consumers of DOM are marine bacterioplankton, which can direct DOM down three pathways: respiration to inorganic constituents, incorporation into biomass, or modification or lack of consumption leading to persistence as DOM. The aspects of DOM source and composition that determine its bioavailability, and its interactions with the clades comprising the bacterial community, are broadly understood, but many of the finer details remain to be studied. This dissertation investigates bottom-up controls on bacterial community composition (BCC) and activity through a time-series study in the Santa Barbara Channel (SBC), CA, and examines the effects of phytoplankton DOM on bacterial metabolism, in field and laboratory experiments.

Chapter II describes the results of a four-year time-series study of BCC and metabolic activity in the SBC. BCC showed the greatest variability over depth; at the surface, the seasonal cycle was significantly more influential on BCC than spatial variability within cruises. Community types showed repeating patterns following the annual upwelling and phytoplankton bloom season; however, new community types appeared in the second half of the time-series, possibly in association with the Pacific warm anomaly observed in 2014. Individual operational taxonomic units (OTUs) were strongly correlated with environmental parameters, reflecting a successional pattern in OTUs following the spring upwelling. Seasonality was moderately repeatable in both the community as a whole and in individual OTUs, with long-term changes in BCC possibly linked to broader climatological phenomena.

In Chapter III, interactions between dissolved organic carbon (DOC) and bacterioplankton were examined during a diatom and Phaeocystis bloom in the SBC over 5 days following an upwelling event, encompassing phytoplankton physiological states from a healthy bloom through the onset of silicon (Si) stress. DOC bioavailability, bacterial growth, and BCC responses were assessed with dilution batch-culture bioassays. In these experiments, as the bloom state progressed: bacterioplankton DOC usage increased; bacterial growth efficiencies increased; and measureable DOC that accumulated during the bloom remained unutilized in the bioassays. Thus, DOC released by the plankton community during a bloom simultaneously contributes to several DOM pools of variable longevity.

In Chapter IV, the ecological role of carbon-rich DOM exuded by nutrient-stressed phytoplankton was assessed using DOM from four coastal diatoms following depletion of nitrogen (N), Si, or both N+Si. In bioassay experiments, short-term responses were affected both by diatom source species and by the nutrient stress under which the DOM was produced. Si-stress DOM was generally the most bioavailable over several days and led to higher bacterial growth efficiencies. However, the amount of diatom-derived DOC that persisted over months differed among source diatom species, with no evidence of a nutrient stress effect. The identity of the nutrient that terminates a phytoplankton bloom can therefore impact heterotrophic activity in the short term, while source phytoplankton species is more likely to influence DOC persistence in surface waters and its potential export.