Geology topics

Daniel Killam

Publications and source records attributed to Daniel Killam.

2 recordsLinked to original sources

USGS2026

Revisiting chlorophyll a thresholds for San Francisco Bay: Insights from observations of phytoplankton molecular abundance

Harmful Algal Blooms (HABs) are a hazard for coastal environments worldwide; identifying screening thresholds of chlorophyll- a (chl- a) associated with increased risk of HABs is a management priority. Molecular surveillance of coastal phytoplankton and bivalve biotoxins could be used to link chl- a with HAB risk, but requires an understanding of whether the HAB risks increase uniformly as chl-a rises, or whether some taxa are disproportionately favored, and if these relationships vary by season. In this study, we present a novel use of molecular abundance data to investigate the scientific bases for estuarine chl- a thresholds protective against HABs. In San Francisco Bay (SFB), California, the relationship between molecular relative abundance (as measured by 18S metabarcoding) of nine different HAB taxa, absolute quantitative polymerase chain reaction (qPCR) abundance, and mussel toxin concentrations of a subset of the taxa were investigated for thresholds as a function of increasing chl- a . Our results show most HAB taxa did not increase in absolute or relative abundance during SFB’s spring bloom interval, when chl- a levels were highest (>10 µg/L) but the assemblage was dominated by non-harmful diatoms. However, several flagellated, mixotrophic taxa did increase above their molecular baseline in fall, and the combined probability of any HAB occurring above baseline was elevated when chl- a reached ∼4.6 µg/L in the fall. This work demonstrates the promise of molecular approaches in disentangling the seasonally complex interplay between stressors and phytoplankton/HAB community responses and has the potential to provide clearer, more cost-effective monitoring and mitigation strategies for managers.

California↗

USGS2025

Assessing environmental drivers and protist community dynamics that shaped the historic August 2022 Heterosigma akashiwo bloom in San Francisco Bay, California

San Francisco Bay, California, typically has chlorophyll a (chl- a ) concentrations below 10 µg L −1 , despite nutrient loadings exceeding those in many estuaries with recurring harmful algal blooms (HABs). However, in August 2022 there was a Heterosigma akashiwo (raphidophyte) bloom with chl- a concentrations exceeding 450 µg L −1 , resulting in widespread hypoxia and fish die-off. We used protist community (18S ribosomal ribonucleic acid gene metabarcoding) and environmental data collected 2–3 times per month from 2015–2023 to assess differences between 2022 and other years, and capture dynamics pre-, during, and post-bloom. The 2022 protist community clustered separately from other years in non-metric multidimensional scaling and hierarchical cluster analyses. A diatom bloom in June generated the highest median June chl- a concentration (15 µg L −1 ) recorded between 1977–2023 and drew down nutrients (nitrogen, silicate), leading to the establishment of a heterotrophic community in July with low dinoflagellate abundance (0.9% of protists versus 5.9% average in other years). The absence of dinoflagellates, potentially including H. akashiwo predators , plus suitable habitat (13–14 daylight hours, temperatures 17–21˚C, nitrate concentrations 17–24 µmol L −1 , neap tides) likely provided opportunity for H. akashiwo to proliferate from 0.03% to 48% of protists between July 20 and August 25. As the bloom collapsed, the genus Gyrodinium (dinoflagellate predator of H. akashiwo ) increased from 3.0% to 52% of protists between August 25 and August 31, likely exerting grazing pressure. Results show that antecedent protist community dynamics play important roles in shaping HABs, and interactions between biotic and abiotic factors govern the development of blooms.

California↗