Metabolic rates in the Agua Hedionda Lagoon during the 2020 Southern California red tide event

A standing time series from autonomous sensors (pH, dissolved oxygen, salinity, temperature) in the Agua Hedionda Lagoon, Carlsbad, CA, captured the effects of a massive red tide occurring along the Southern and Baja California coast during the spring of 2020. Biogeochemical data (pH and dissolved oxygen) were examined using an open-source weighted regression model designed to filter out the influence of tides and estimate net ecosystem metabolism. Contemporaneous pH and dissolved oxygen observations allowed simultaneous, independent evaluations of production, respiration, and net ecosystem metabolism. Under normal conditions, the Agua Hedionda Lagoon tends toward net heterotrophy, averaging 10 mmol C m–2 d–1. During a 2-month period, centered around the peak of the event, trophic status in the lagoon shifted multiple times between net heterotrophic and net autotrophic, with a pronounced period of anoxia. Fueled by the intense local bloom, at its peak, respiration reached rates of 140 mmol C m–2d–1. We found that the co-location of pH and oxygen sensors affords independent assessment of metabolic rates, which often agree, as expected under baseline (oxic) conditions, but diverge during an extreme event. This observation allowed us to identify non-Redfieldian behavior and speculate on the source of anoxic reactions. Similar to many coastal environments, the Agua Hedionda Lagoon serves a multitude of functions (including a natural habitat for hundreds of marine and avian species, and several commercial and recreational activities), which makes characterizing the dominant mechanisms controlling the ecosystem state (such as metabolic rate) of great interest to scientists, stakeholders, decision-makers, and regulators alike.