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In recent years, harmful algal blooms (HABs) have increased in their severity and extent in many parts of the world and pose serious threats to local aquaculture, fisheries, and public health. In many cases, the mechanisms triggering and regulating HAB events remain poorly understood. Using underwater microscopy and Residual Neural Network (ResNet‐18) to taxonomically classify imaged organisms, we developed a daily abundance record of four potentially harmful algae (Akashiwo sanguinea, Chattonella spp., Dinophysis spp., and Lingulodinium polyedra) and major grazer groups (ciliates, copepod nauplii, and copepods) from August 2017 to November 2020 at Scripps Institution of Oceanography pier, a coastal location in the Southern California Bight. Random Forest algorithms were used to identify the optimal combination of environmental and ecological variables that produced the most accurate abundance predictions for each taxon. We developed models with high prediction accuracy for A. sanguinea (R2=0.79±0.06), Chattonella spp. (R2=0.63±0.06), and L. polyedra (R2=0.72±0.08), whereas models for Dinophysis spp. showed lower prediction accuracy (R2=0.24±0.07). Offshore nutricline depth and indices describing climate variability, including El Niño Southern Oscillation, Pacific Decadal Oscillation, and North Pacific Gyre Oscillation, that influence regional‐scale ocean circulation patterns and environmental conditions, were key predictor variables for these HAB taxa. These metrics of regional‐scale processes were generally better predictors of HAB taxa abundances at this coastal location than the in situ environmental measurements. Ciliate abundance was an important predictor of Chattonella and Dinophysis spp., but not of A. sanguinea and L. polyedra. Our findings indicate that combining regional and local environmental factors with microzooplankton populations dynamics can improve real‐time HAB abundance forecasts. [ABSTRACT FROM AUTHOR]

Studies over the last two decades have shown that submicron particulates (SMPs) can be transferred from the seawater into sea spray aerosol (SSA), potentially impacting SSA cloud seeding ability. This work reports the first concurrent bulk and sea surface microlayer (SSML) SMP (0.4–1.0 μm) measurements, made during two mesocosm phytoplankton blooms in a region devoid of active wave breaking and bubble formation, providing insight into how biological and physicochemical processes influence seawater SMP distributions. Modal analyses of the SMP size distributions revealed contributions from multiple, biologically related particulate populations that were controlled by the microbial loop. With negligible bubble scavenging occurring, SSML enrichment of SMPs remained low throughout both experiments, suggesting scavenging is vital for SMP enrichment in the SSML. Our findings are discussed in the context of SMP transfer into SSA and its potential importance for SSA cloud seeding ability. Plain Language Summary: Research has shown that particulates can be transferred from the ocean into sea spray aerosol (SSA) when bubbles burst at the ocean surface. This transfer is important because incorporation of seawater particulates into SSA can impact its ability to seed water and ice clouds. During the Sea Spray Chemistry and Particulate Evolution (SeaSCAPE) study, submicron particulates (SMPs, 0.4–1.0 μm) were measured daily in the sea surface microlayer (SSML), the topmost 1–1,000 μm of the ocean surface, and the underlying bulk seawater over the course of two phytoplankton growth experiments. The objective of this study was to assess the influence of biological activity on SMP concentrations and distributions in the seawater. Our results indicate that biological growth led to increased SMP production, and the size distribution of SMPs produced was dependent on the phase of the bloom growth. Additionally, almost no SSML enrichment of SMPs occurred without active wave breaking and bubble formation, highlighting the significance of biological activity and bubble scavenging/bursting for particulate transfer into SSA. These findings are discussed in the context of potential SMP impacts on SSA cloud forming ability. Key Points: The concentration of 0.4–1.0 μm seawater particulates is higher during a phytoplankton bloomThe biologically induced increase in seawater submicron particulates is size dependent and influenced by the phase of the bloomParticulate enrichment in the sea surface microlayer is low in the absence of wave breaking and bubble scavenging [ABSTRACT FROM AUTHOR]

With oceans covering 71% of the Earth's surface, sea spray aerosol (SSA) particles play an important role in the global radiative budget by acting as cloud condensation nuclei and ice nucleating particles (INPs). By acting as INPs, SSA particles affect the structure and properties of mixed‐phase clouds by inducing freezing at warmer temperatures than the homogeneous freezing temperature. Climate models that incorporate marine INPs use the emission of submicron SSA in INP parameterizations because these particles contain a higher fraction of organic mass. Here we show supermicron SSA particles, produced using a natural breaking wave analogue, are the major source of INPs throughout the lifecycle of a phytoplankton bloom. Additionally, supermicron SSA particles are shown to be more efficient INPs than submicron SSA particles, because they carry a greater number of ice active components. Thus, supermicron SSA needs to be incorporated in INP parameterizations for future climate models. Plain Language Summary: Clouds over high latitude oceans are a major source of uncertainty in global climate models. Sea spray aerosol (SSA) particles are the dominant source of cloud seeds over remote marine locations. Ice nucleating particles (INPs) initiate cloud freezing. Previous climate models approximated INPs released from oceans using the number of submicron SSA. However, field studies show supermicron particles are effective INPs over marine locations. This study was designed to understand the role of particle size for marine INPs in an isolated ocean/atmosphere laboratory setting. Two phytoplankton blooms were induced in seawater and a breaking wave proxy generated SSA. The ice nucleating activities of SSA particles in two size‐segregated populations were measured. Throughout both blooms, supermicron SSA particles were the most ice active population. The efficiency of supermicron particles was still apparent after normalizing to particle surface area. Following normalization to aerosol volume, submicron and total particle populations were comparable, suggesting ice active components in SSA scale with particle volume in the observed temperature range (−26°C to −8°C). These results suggest that climate models use size‐dependent INP activities for SSA. Proper representation of marine INPs will increase model accuracy of ice content in clouds and radiative forcing over high latitude oceans. Key Points: Supermicron sea spray aerosols have higher ice nucleating particle (INP) concentrations at warm temperatures than submicron sea spray aerosolsThe higher concentrations of INPs in supermicron sea spray aerosols occur independent of phytoplankton bloom stateThe ice nucleating ability of a sea spray aerosol (SSA) is roughly proportional to the dry particle volume [ABSTRACT FROM AUTHOR]