Iron is a key micronutrient for marine biogeochemistry, limiting growth and nitrogen fixation in over a third of the ocean. However, the impacts of iron-binding ligands and iron source processes on dissolved iron distributions is not well known. The goal of this dissertation is to better understand the cycling of iron-binding ligands and the controls on their distributions and how that impacts dissolved iron. This dissertation also seeks to understand how individual iron sources influence iron distributions and biogeochemistry. To accomplish this, a new prognostic ligand tracer and iron-ligand speciation chemistry are incorporated into the Community Earth System Model (CESM). The CESM is now able to simulate realistic distributions of iron-binding ligands. The results show that with relatively few ligand sources and sinks the model was able to match observations of ligands, and that inclusion of a dynamic ligand tracer improves simulation of dissolved iron. To better understand the influence iron sources have on dissolved iron concentrations and biogeochemistry, sensitivity experiments for each source are conducted with the CESM. The results show that atmospheric dust and sedimentary iron inputs have the largest impact on dissolved iron concentrations and biogeochemistry. Hydrothermal vent inputs are important for deep ocean iron, and their inclusion in global biogeochemical ocean models would allow for more realistic iron simulation.

This dissertation work also reevaluates the parameters governing the temperature influence on community phytoplankton growth rates. A dataset of in situ community phytoplankton growth rates was compiled and parameter values for the Q10 and Arrhenius models were optimized for use in global biogeochemical and ecosystem models. The results show and optimized Q10 value of 1.47 and an activation energy of 0.277 eV. Both the Q10 and Arrhenius models do equally well for estimating the temperature influence on community phytoplankton growth rates against the dataset. Evaluation of global biogeochemical and ecosystem models against our dataset will allow for further constraints on phytoplankton ecology and associated biogeochemitry.

Humans, as all members of the animal kingdom, have always interacted with the natural environment. However, as populations grew exponentially due to agricultural and technological advances, their impact on ecosystems has become disproportionate. Anthropogenic influence on coastal marine environments has detrimental effects on ecosystem balance and function. Negative impacts of human activity can be referred to collectively as pollution. Sources of pollution include, but are not limited to, chemical waste, human effluent, plastic, and heavy metals. In this text, I explore the impacts of effluent and microplastic on the marine environment, within the context of annual and diel biogeochemical cycles. I investigated bottom-up control on phytoplankton growth over two seasonal cycles in the Southern California Bight off the coast of Orange County. I measured nutrient concentrations, in addition to a suite of biogeochemical variables, to assess the dynamics of primary production. I observed moderate Spring nutrient pulses and corresponding high chlorophyll concentrations. Higher nutrient concentrations were attributed to weak coastal upwelling and a strong Pineapple Express event that contributed a significant pulse of nutrients from shore. Through this I demonstrate that isolated heavy rain events carry the capacity to cause punctuated algal blooms in low-nutrient environments. During Summer, I found strong nutrient drawdown, rendering the upper layer nutrient-deplete. The water column was highly stratified, indicated by both high temperature and salinity signatures. During Fall and Winter, the water column was less thermally stratified, and we observed lower salinity in the upper layer. I found that wastewater effluent can impact already anthropogenically altered coastal environments, by providing a constant influx of nutrients. Throughout the year, there is a constant influx of wastewater input from the Orange County Sanitation District outflow pipe. Though the pipe has multiple ports, to allow for more efficient diffusion into the seawater, it is difficult to distinguish where the influence of the wastewater effluent ends. A reference sampling site was chosen for comparison with the effluent samples; however, it is unclear whether the reference site is truly outside the influence of the wastewater outflow. I explored the intricacies of the interactions between microplastic and marine invertebrates and note that microplastic will differentially impact species depending on habitat, feeding strategy, and size. Marine species who inhabit the water column will be most impacted by neutrally and positively buoyant plastic particles, as they are more likely to come into contact. In addition, species who passively filter the water column will ingest more plastic than species who have the capacity to selectively feed and reject microplastic. However, once microplastic enters the marine environment, it is rapidly colonized by microbes. These microbes may make plastic more appealing, fooling marine invertebrates into thinking they are food particles. Though the particles themselves may rapidly exit an organism, the chemicals embedded within microplastic could adsorb into the digestive tract during passage. In addition, the effect of plastic particles on fecal pellet sinking rates may have significant impacts on carbon export by decreasing sinking rates. In addition, I show that a marine zooplankton, Artemia salina, does indeed ingest microplastic. As a passive feeder, A. salina will ingest more microplastic as concentration in the environment increases. However, ingestion rate will saturate after 90 minutes of exposure to high concentrations of microplastic. Further, I develop and refine methodologies for studying microplastic ingestion for chitinous zooplankton in situ. The only successful digesting agent was nitric acid (HNO3) at 50% dilution and 80ºC. Dissolution on plastic or GF/F filters was not possible, because the acid destroyed the vessel. In addition, neutralization of the solution post-digestion was not possible due to the formation of a hydrogel after the addition of NaOH and K2HPO4. Finally, I examine the diel dynamics of biogeochemical variables and mercury in a tidally influenced estuary in Southern California. I found that only a few samples contained detectable mono methylmercury concentrations—all at low tide, indicating the estuary had a high flushing rate. Therefore, the diel dynamics of MMHg could not be analyzed. However, I found that dissolved oxygen and chlorophyll concentrations were highly dependent upon the dominant source of water at the time, with higher DO at high tide and higher chlorophyll at low tide.