A cross-regional examination of patterns and environmental drivers of Pseudo-nitzschia harmful algal blooms along the California coast

Image-derived indicators of phytoplankton community responses to Pseudo-nitzschia blooms

Phytoplankton populations in the natural environment interact with each other. Despite rising global concern with Pseudo-nitzschia blooms, which can produce the potent neurotoxin domoic acid, we still do not fully understand how other phytoplankton genera respond to the presence of Pseudo-nitzschia. Here, we used a 4-year high-resolution imaging dataset for 9 commonly found phytoplankton genera in Narragansett Bay, alongside environmental data, to identify potential interactions between phytoplankton genera and their response to elevated Pseudo-nitzschia abundance. Our results indicate that Pseudo-nitzschia tends to bloom either concurrently with or right after other phytoplankton genera. Such bloom periods coincide with higher water temperatures and lower salinity. Pseudo-nitzschia image abundance tends to increase the most from March-May and peaks during May-Jun, whereas the image-derived biovolume and width of Pseudo-nitzschia chains increase the most during Jan-Feb. For most phytoplankton genera, their relationship with Pseudo-nitzschia abundance is noticeably different from their relationship with Pseudo-nitzschia image features. Despite the complexity in the phytoplankton community, our analysis suggests several ecological indicators that may be used to determine the risk of harmful algal blooms.

Modeling the dynamics of harmful algal bloom events in two bays from the northern Chilean upwelling system

The bays of Tongoy and Guanaqueros are located in the Humboldt Current system, where Argopecten purpuratus has been the subject of intense aquaculture development. These bays lie in one of the most productive marine ecosystems on Earth and are dominated by permanent coastal upwelling at Lengua de Vaca Point and Choros Point, one of the three upwelling centers on the Chilean coast. Significantly, this productive system experiences a high recurrence of harmful algal bloom (HAB) events. This paper examines 9-year (2010-2018) samples of three toxic microalgal species collected in different monitoring programs and research projects. During this period, nine HAB events were detected in Guanaqueros Bay and 14 in Tongoy Bay. Among these, three HAB events were produced simultaneously in both bays by Pseudo-nitzschia australis, and two events produced simultaneously were detected in one bay by Alexandrium spp. and the other by Dinophysis acuminata. Before El Niño 2015-16, there were more HAB events of longer duration by the three species. Since El Niño, the number and duration of events were reduced and only produced by P. australis. HAB events were simulated with the FVCOM model and a virtual particle tracker model to evaluate the dynamics of bays and their relationship with HAB events. The results showed retention in bays during the relaxation conditions of upwelling and low connectivity between bays, which explains why almost no simultaneous events were recorded.

Effect of ocean outfall discharge volume and dissolved inorganic nitrogen load on urban eutrophication outcomes in the Southern California Bight

Climate change is increasing drought severity worldwide. Ocean discharges of municipal wastewater are a target for potable water recycling. Potable water recycling would reduce wastewater volume; however, the effect on mass nitrogen loading is dependent on treatment. In cases where nitrogen mass loading is not altered or altered minimally, this practice has the potential to influence spatial patterns in coastal eutrophication. We apply a physical-biogeochemical numerical ocean model to understand the influence of nitrogen management and potable wastewater recycling on net primary productivity (NPP), pH, and oxygen. We model several theoretical management scenarios by combining dissolved inorganic nitrogen (DIN) reductions from 50 to 85% and recycling from 0 to 90%, applied to 19 generalized wastewater outfalls in the Southern California Bight. Under no recycling, NPP, acidification, and oxygen loss decline with DIN reductions, which simulated habitat volume expansion for pelagic calcifiers and aerobic taxa. Recycling scenarios under intermediate DIN reduction show patchier areas of pH and oxygen loss with steeper vertical declines relative to a "no recycling" scenario. These patches are diminished under 85% DIN reduction across all recycling levels, suggesting nitrogen management lowers eutrophication risk even with concentrated discharges. These findings represent a novel application of ocean numerical models to investigate the regional effects of idealized outfall management on eutrophication. Additional work is needed to investigate more realistic outfall-specific water recycling and nutrient management scenarios and to contextualize the benefit of these management actions, given accelerating acidification and hypoxia from climate change.

Domoic acid in Canadian Pacific waters, from 2016 to 2021, and relationships with physical and chemical conditions

Domoic acid, a phycotoxin produced by species of the marine diatom Pseudo-nitzschia, can cause deleterious impacts to marine food webs and human health. Domoic acid and Pseudo-nitzschia spp. were surveyed from 2016 to 2021 in the Pacific waters of Canada to assess their occurrences, concentrations, and relationships with physical and chemical conditions. Domoic acid was common, occurring in measurable concentrations in 73 % of the 454 samples. It occurred in all regions (west coast of Vancouver Island, Salish Sea, Queen Charlotte Sound / Hecate Strait, deep oceanic NE Pacific), in all years and all seasons. Median concentrations were highest along the west coast of Vancouver Island, and lowest in the oceanic waters of the NE Pacific. Winter had the lowest concentrations; no significant differences occurred between spring, summer, and autumn. High domoic acid concentrations equal to or above 100 ng/L were not common, occurring in about 5 % of samples, but in all seasons and all years except 2019. All six Pseudo-nitzschia taxa identified had similar median concentrations, but different frequencies of occurrence. P. cf. australis appeared to be the major contributor to high concentrations of domoic acid. Physico-chemical conditions were described by ten variables: temperature, salinity, density difference between 30 m and the surface (a proxy for vertical stability), chlorophyll a, nitrate, phosphate, silicate, and the ratios nitrate:phosphate, nitrate:silicate, and silicate:phosphate. Statistical analyses, using general linear models, of their relationships with the absence/presence of Pseudo-nitzschia spp. found silicate (negative) to be the most influential variable common in both the west coast of Vancouver Island and Salish Sea regions. Temperature and chlorophyll a were the most influential variables which determined the log10 abundance of Pseudo-nitzschia spp. in both regions. Analyses of the absence/presence of particulate domoic acid per Pseudo-nitzschia cell (excluding P. americana) found chlorophyll a to be the most influential variable common in both regions, whereas no common influential variable determined the log10 concentration of particulate domoic acid per Pseudo-nitzschia cell (excluding P. americana). These results were generally similar to those of other studies from this area, although this study extends these findings to all seasons and all regions of Canada's Pacific waters. The results provide important background information against which major outbreaks and unusual events can be compared. A domoic acid surveillance program during synoptic oceanographic surveys can help to understand where and when it reaches high concentrations at sea and the potential impacts to the marine ecosystem.

Simulated upwelling and marine heatwave events promote similar growth rates but differential domoic acid toxicity in Pseudo-nitzschia australis

Along the west coast of the United States, highly toxic Pseudo-nitzschia blooms have been associated with two contrasting regional phenomena: seasonal upwelling and marine heatwaves. While upwelling delivers cool water rich in pCO2 and an abundance of macronutrients to the upper water column, marine heatwaves instead lead to warmer surface waters, low pCO2, and reduced nutrient availability. Understanding Pseudo-nitzschia dynamics under these two conditions is important for bloom forecasting and coastal management, yet the mechanisms driving toxic bloom formation during contrasting upwelling vs. heatwave conditions remain poorly understood. To gain a better understanding of what drives Pseudo-nitzschia australis growth and toxicity during these events, multiple-driver scenario or 'cluster' experiments were conducted using temperature, pCO2, and nutrient levels reflecting conditions during upwelling (13 °C, 900 ppm pCO2, replete nutrients) and two intensities of marine heatwaves (19 °C or 20.5 °C, 250 ppm pCO2, reduced macronutrients). While P. australis grew equally well under both heatwave and upwelling conditions, similar to what has been observed in the natural environment, cells were only toxic in the upwelling treatment. We also conducted single-driver experiments to gain a mechanistic understanding of which drivers most impact P. australis growth and toxicity. These experiments indicated that nitrogen concentration and N:P ratio were likely the drivers that most influenced domoic acid production, while the impacts of temperature or pCO2 concentration were less pronounced. Together, these experiments may help to provide both mechanistic and holistic perspectives on toxic P. australis blooms in the dynamic and changing coastal ocean, where cells interact simultaneously with multiple altered environmental variables.