Emerging evidence indicates that associations between landscape contexts and population change may vary with prevailing weather conditions. Agricultural landscapes are often more homogeneous, receive greater agrochemical inputs, and support altered or reduced insect prey communities, influencing buffering capacity during challenging conditions. Because weather can constrain avian thermoregulation, alter foraging patterns, and insect availability, associations between agricultural intensity and aerial insectivore fitness may vary in magnitude and direction under different weather conditions. We therefore evaluate whether the relationship between row-crop cover and fitness varies with weather across breeding attempts. We paired a 13-yr dataset of Hirundo rustica (Barn Swallow) clutch size, hatching ratio, and fledging success, gathered across a gradient of agricultural intensity in southwestern Ontario, Canada, with historical weather conditions collected during periods relevant to each parameter of reproductive success. We used this dataset to test the hypothesis that the relationship between agricultural intensity and the outcome of a breeding attempt may be modulated by the experienced weather conditions. Our analyses treated first and second breeding attempts separately. For first attempts, temperature modified the association between agricultural intensity and both hatching and fledging success, and precipitation modified the association for fledging success only. Interactions were not supported for clutch size in either attempt or for hatching in second attempts. Together, these results indicate that the effects of agricultural intensity on reproduction likely depends on prevailing weather conditions, and the stage of the breeding season. Considering projected increases in heat extremes and heavy rainfall, this context dependence suggests that climate change may alter, and in some cases intensify, the demographic consequences of intensive agriculture. Declines of aerial insectivores are often attributed to agricultural intensification and climate change, yet their combined effects on breeding performance are rarely evaluated. We monitored Hirundo rustica (Barn Swallow) reproduction for 13 yr (2006-2018) at 32 barn-based colonies in southern Ontario, Canada. We evaluated the extent to which row crop cover (a proxy for agricultural intensity) was associated with clutch size, hatching ratio, and fledging success, and whether these relationships varied with temperature and precipitation measured during biologically relevant windows. Clutch size showed little evidence of weather-by-row crop interactions, although clutch size tended to decline under warmer and wetter conditions. In contrast, hatching ratio and especially fledging success provided strong evidence that weather conditions modified the relationship between row crop cover and breeding outcomes. This implies that climate variability can amplify or dampen land-use effects, and that models incorporating weather may improve predictions of reproductive performance.

To determine whether we can reduce the impacts of forest loss on biodiversity by altering forest pattern, we need to estimate the effects of forest pattern independent of forest amount. We evaluated the independent and interactive effects of forest amount, fragmentation, and connectivity (wooded corridors) on diversity of forest-associated plants, small mammals, and birds. We selected 70 forest sites in eastern Ontario, Canada with low correlations between these landscape predictors. We found positive effects of forest amount, neutral or positive effects of forest fragmentation, and an interaction effect between connectivity and forest amount. In landscapes with low forest amount, biodiversity increased with connectivity, while at high forest amount, biodiversity decreased with connectivity. Thus, forest patches should be protected regardless of size, and conservation actions aimed at improving connectivity by adding wooded corridors should be prioritized in areas where forest is scarce, for example agricultural and urban areas.

Bat populations are declining globally. Maintaining high-quality habitat for bats can help mitigate extinction risk. Natural and semi-natural linear vegetation features have been shown to provide shelter and foraging habitat for bats in temperate agroecosystems in Europe, yet their value for bats in North America has received little attention. Using automated ultrasonic recorders, we assessed bat species richness and activity across agricultural drainage ditches that varied in mean vegetation height, variability in vegetation height, and mean width in agroecosystems in eastern Ontario, Canada. Landscapes surrounding recording sites also varied in forest amount and mean field size, and recording sites were located at different distances from the nearest forest patch. We found that in general, bat activity at the community level and at the individual species level was positively associated with mean vegetation height and mean vegetation width; however, species appeared to vary in their response to variation in vegetation height. We also found a general positive relationship within and across species for bat activity with forest amount at the landscape scale. Overall, our results suggest maintaining or increasing vegetation height along drainage ditches and field margins as well as maintaining or increasing forest amount at the landscape scale will best support bats in temperate North American agroecosystems.

Monarch populations have declined precipitously over the past decades, largely due to the loss of their breeding host plant, milkweed. One mitigation strategy is to plant milkweed along rights-of-ways. However, many rights-of-ways undergo routine mowing, which can result in egg, caterpillar, and chrysalis mortality. To minimize this risk, it is critical to time mowing activity to avoid the peak breeding activity of monarchs. In this study we used community science data to define breeding patterns and timing of monarch breeding throughout the United States and Canada. We identified four breeding patterns: (1) year-round, (2) spring-only, (3) summer, and (4) disjunct breeding. Year-round and disjunct breeding were concentrated around the Gulf of Mexico, including Florida, and the southern United States, respectively. As expected, we found that monarch breeding was later with increased latitude but with some longitudinal variation; for summer breeding regions, breeding occurred earlier in the western portion of the study area relative to the east, but the end of breeding was later in the east relative to the west, resulting in breeding seasons of similar duration. Additionally, in the east, breeding occurred later along the Appalachian Mountains. We suggest adapting our findings into mowing practices that benefit monarchs while considering the life histories of milkweed and the broader insect community. Implications for insect conservation. Mowing and other maintenance activities in habitat where milkweed is present can be detrimental to monarch breeding. Our analysis provides guidance to minimize monarch mortality and loss of milkweed during peak breeding periods.

The eastern North American monarch butterfly, known for its spectacular annual migration between Mexico, USA, and Canada, is currently the focus of intense conservation attention to minimize extinction risk and reach a conservation target of approximately 132 million individuals (equivalent to occupying 6 ha of overwintering habitat in Mexico).1 Given that migratory eastern North American monarchs breed over successive breeding generations across a vast area,1,2,3,4,5,6 reaching this conservation objective must account for the inherent uncertainties of undertaking conservation actions across space and time.7,8,9,10,11 We integrated a density-dependent full-annual-cycle matrix population model with stochastic dynamic programming to identify the optimal sequence of conservation actions (restoring habitat in three different breeding regions or protecting habitat on the wintering grounds), spanning three countries, to reach each of three objectives: maximize population viability, meet the population target, and maximize population size. Using an annual budget of $30 million over 5 years, we find that a coordinated approach that would primarily focus on habitat restoration in the central USA would best help achieve each of the three recovery objectives. Importantly, monarchs had a higher chance of reaching each conservation objective following a strategy that was coordinated across all three nations and commenced immediately rather than be delayed. Our results provide quantitative evidence for the necessity of coordinated international efforts to conserve migratory species and a tool for decision-makers in Canada, USA, and Mexico to recover this iconic and highly threatened butterfly.

Domestic cats (Felis catus) play a dual role in society as both companion animals and predators. When provided with unsupervised outdoor access, cats can negatively impact native wildlife and create public health and animal welfare challenges. The effective implementation of management strategies, such as buffer zones or curfews, requires an understanding of home range size, the factors that influence their movement, and the types of habitats they use. Here, we used a community/citizen scientist approach to collect movement and habitat use data using GPS collars on owned outdoor cats in the Kitchener-Waterloo-Cambridge-Guelph region, southwestern Ontario, Canada. Mean (± SD) 100% minimum convex polygon home range size was 8 ± 8 ha (range: 0.34–38 ha) and was positively associated with road density but not with intrinsic factors such as boldness, sex, or age. With regards to habitat selection, cats used greenspaces, roads, and agricultural land less often than predicted but strongly selected for impervious surfaces (urban areas other than greenspaces or roads). Our results suggest that wildlife near buildings and residential areas are likely at the greatest risk of cat predation and that a buffer size of 840 m would be needed to restrict cats from entering areas of conservation concern.

The evolution of mating systems reflects a balance of the often-conflicting interests of males and females. Polygyny, a mating system in which males have multiple mates, presents a fitness benefit to males, but the consequences for females are less clear. Females with polygynous social mates may suffer reduced fitness, especially secondary females who typically receive less male support. We used 32 yr of detailed reproductive data on a population of Savannah sparrows (Passerculus sandwichensis) breeding on Kent Island, NB, Canada, to investigate the effects of females' social mating status on 6 indices of female fitness: survival, clutch size, fledging success, number of fledglings produced per nest and annually, and recruitment of offspring. Secondary females produced fewer fledglings per nest and annually than did monogamous or primary females, and their young were less likely to recruit into the breeding population. Yearling secondary females also had lower survival rates than older secondary females. Combined with higher rates of partial brood loss among secondary females, our results suggest that secondary females are unable to provide enough care to consistently fledge all nestlings in their broods, likely due to reduced male provisioning. Given that the sex ratio of breeders in the population is female-biased, we suggest that polygyny persists despite its fitness costs because some females must mate polygynously to "make the best of a bad situation." Our study demonstrates the value of detailed, long-term population monitoring data for understanding mating systems and using multiple indices of fitness to analyze the costs of polygyny.

First detected in Atlantic Canada in December 2021, highly pathogenic avian influenza virus (HPAIV) subtype H5N1 clade 2.3.4.4b, A/Goose/Guangdong/1/96 lineage, has caused massive mortality in wild birds and domestic poultry in North America. Swallows (Hirundinidae), abundant in North American agricultural ecosystems, have been proposed as possible (bridge) species for HPAIV transmission between wild and domestic birds. We aimed to seek evidence of the potential role of swallows in bridging AIV infection between wild bird reservoirs and poultry flocks in eastern Canada. During a wide-scale outbreak of HPAIV in wild birds and poultry farms across eastern Canada, 200 samples were collected from swallow breeding sites in the Canadian provinces of New Brunswick, Nova Scotia, Ontario, and Quebec, June-August 2022. Samples came from Barn Swallow (Hirundo rustica; n=142), Tree Swallow (Tachycineta bicolor; n=56), and Cliff Swallow (Petrochelidon pyrrhonota; n=2) nests. All samples tested negative for AIV, suggesting that HPAIV and low pathogenic AIV (LPAIV) strains were probably not circulating widely in swallows during the 2022 breeding season in eastern Canada; thus swallows may present a low risk of transmitting AIV. Within a management context, these findings suggest that removing nests of Barn Swallows, a species at risk in Canada, from the exterior of biosecure domestic poultry facilities may not significantly reduce risks of HPAI transmission to poultry.

Identifying strategies that offer co-benefits for biodiversity protection, forest restoration and human well-being are important for successful conservation outcomes. In this study, we identified opportunities where forest restoration and rehabilitation programs in Colombia also align with priority areas for the conservation of Neotropical migratory birds. We used citizen science eBird-based abundance estimates to define regions with the highest richness of Neotropical migratory birds of conservation concern at montane elevations in Colombia and aligned these high richness areas with domestic initiatives for forest protection (Forest Areas), restoration (Restoration Areas) and rehabilitation (Rehabilitation Areas). We quantified the location and amounts of these three areas as well as the type of land protection and designation within them, specifically, National Protected Areas, Indigenous Reserves, Afro-descendent territories, and regions affected by poverty and violence that are prioritized for rural development by the Colombian government in Post-conflict Territorially Focused Development Programs (PDET). Almost half of Forest Areas overlapped with PDETs where goals for economic development present a risk of forest loss if not done sustainably. There was a 20% overlap between Forest Areas and Afro-descendant territories and indigenous reserves; most of this overlap was outside of established protected areas thus presenting an opportunity for community forest conservation that benefits migratory birds. We found an alignment of less than 6% between migrant bird focal areas and the priority Restoration and Rehabilitation Areas identified by the Colombian National Restoration Plan indicating less opportunity for these programs to simultaneously benefit Neotropical migrant species. Our approach highlights that timely and efficient conservation of declining migrants depends on identifying the regions and strategies that incorporate local communities as part of the solution to forest loss and degradation in Colombia.

Migratory populations of monarch butterflies have declined substantially in North America. In Canada, where the monarch is listed as a species at risk, protecting the butterfly’s critical breeding habitat has been identified as a conservation priority. However, protection of the breeding habitats in this country was not readily possible due to the scarcity of monarch breeding data. Here, we describe Mission Monarch, a community science program launched in 2016 to address this knowledge gap. We compare the Mission Monarch dataset with the datasets of eButterfly, iNaturalist and the Monarch Larva Monitoring Project, three other longer-established community science programs, to highlight the unique knowledge gap that Mission Monarch helps fill at national to continental scales. Even though Mission Monarch is a relatively young program, it has already provided more records of monarch caterpillars and milkweed than any other Canadian dataset from the aforementioned programs near the species’ northern range. It has become the main monarch breeding monitoring program in Canada. We discuss the relevance of community science in filling knowledge gaps and addressing conservation issues. Implications for insect conservation Community science programs can be specifically designed to fill knowledge gaps for insect conservation. They can be unique, yet complementary to other established programs.

Early definitions of conservation focused largely on the end goals of protection or restoration of nature, and the various disciplinary domains that contribute to these ends. Conservation science and practice has evolved beyond being focused on just issues of scarcity and biodiversity decline. To better recognize the inherent links between human behaviour and conservation, “success” in conservation is now being defined in terms that include human rights and needs. We also know that who engages in conservation, and how, dictates the likelihood that conservation science will be embraced and applied to yield conservation gains. Here we present ideas for reconceptualizing conservation. We emphasize the HOW in an attempt to reorient and repurpose the term in ways that better reflect what contemporary conservation is or might aspire to be. To do so, we developed an acrostic using the letters in the term “CONSERVATION” with each serving as an adjective where C = co-produced, O = open, N = nimble, S = solutions-oriented, E = empowering, R = relational, V = values-based, A = actionable, T = transdisciplinary, I = inclusive, O = optimistic, and N = nurturing. For each adjective, we briefly describe our reasoning for its selection and describe how it contributes to our vision of conservation. By reconceptualizing conservation we have the potential to center how we do conservation in ways that are more likely to result in outcomes that benefit biodiversity while also being just, equitable, inclusive, and respectful of diverse rights holders, knowledge holders, and other actors. We hope that this acrostic will be widely adopted in training to help the next generation of conservation researchers and practitioners keep in mind what it will take to make their contributions effective and salient.

UV radiation (UVR) has significant physiological effects on organisms living at or near the Earth’s surface, yet the full suite of genes required for fitness of a photosynthetic organism in a UVR-rich environment remains unknown. This study reports a genome-wide fitness assessment of the genes that affect UVR tolerance under environmentally relevant UVR dosages in the model cyanobacterium Synechococcus elongatus PCC 7942. Our results highlight the importance of specific genes that encode proteins involved in DNA repair, glutathione synthesis, and the assembly and maintenance of photosystem II, as well as genes that encode hypothetical proteins and others without an obvious connection to canonical methods of UVR tolerance. Disruption of a gene that encodes a leucyl aminopeptidase (LAP) conferred the greatest UVR-specific decrease in fitness. Enzymatic assays demonstrated a strong pH-dependent affinity of the LAP for the dipeptide cysteinyl-glycine, suggesting an involvement in glutathione catabolism as a function of night-time cytosolic pH level. A low differential expression of the LAP gene under acute UVR exposure suggests that its relative importance would be overlooked in transcript-dependent screens. Subsequent experiments revealed a similar UVR-sensitivity phenotype in LAP knockouts of other organisms, indicating conservation of the functional role of LAPs in UVR tolerance.

SummaryDiatoms, dinoflagellates, and coccolithophorids are the dominant groups of marine eukaryotic phytoplankton collectively responsible for the majority of primary production in the ocean1. These phytoplankton contain additional intracellular membranes around their chloroplasts derived from ancestral engulfment of red microalgae by unicellular heterotrophic eukaryotes that led to secondary endosymbiosis2. This symbiogenesis hypothesis for the origin of modern secondary endosymbiotic phytoplankton is supported by a wealth of palaeontologic, morphologic, and genomic evidence3–6. However, the selectable evolutionary advantage of these membranes and the physiological significance for extant phytoplankton are unknown. We report that the proton-pumping enzyme V-type H+-ATPase (VHA), ubiquitously used in eukaryotic intercellular digestion, is localized around the chloroplasts of centric diatoms and that VHA-activity significantly enhances photosynthesis over a wide range of oceanic irradiances. Similar results in pennate diatoms, dinoflagellates, and coccolithophorids, but not green or red microalgae, imply a mechanism resulting from the co-option of phagocytic VHA activity into a carbon concentrating mechanism that is common to secondary endosymbiotic phytoplankton. Furthermore, analogous VHA-dependent mechanisms in extant photosymbiotic marine invertebrates7–9 provide functional evidence for an adaptive advantage throughout the transition from endosymbiosis to symbiogenesis. Our results suggest that VHA-dependent enhancement of photosynthesis contributes at least 7% of primary production in the ocean, providing an example of a symbiosis-derived evolutionary innovation with global environmental implications.

Diatoms, dinoflagellates, and coccolithophores are dominant groups of marine eukaryotic phytoplankton that are collectively responsible for the majority of primary production in the ocean.1 These phytoplankton contain additional intracellular membranes around their chloroplasts, which are derived from ancestral engulfment of red microalgae by unicellular heterotrophic eukaryotes that led to secondary and tertiary endosymbiosis.2 However, the selectable evolutionary advantage of these membranes and the physiological significance for extant phytoplankton remain poorly understood. Since intracellular digestive vacuoles are ubiquitously acidified by V-type H+-ATPase (VHA),3 proton pumps were proposed to acidify the microenvironment around secondary chloroplasts to promote the dehydration of dissolved inorganic carbon (DIC) into CO2, thus enhancing photosynthesis.4,5 We report that VHA is localized around the chloroplasts of centric diatoms and that VHA significantly contributes to their photosynthesis across a wide range of oceanic irradiances. Similar results in a pennate diatom, dinoflagellate, and coccolithophore, but not green or red microalgae, imply the co-option of phagocytic VHA activity into a carbon-concentrating mechanism (CCM) is common to secondary endosymbiotic phytoplankton. Furthermore, analogous mechanisms in extant photosymbiotic marine invertebrates6,7,8 provide functional evidence for an adaptive advantage throughout the transition from endosymbiosis to symbiogenesis. Based on the contribution of diatoms to ocean biogeochemical cycles, VHA-mediated enhancement of photosynthesis contributes at least 3.5 Gtons of fixed carbon per year (or 7% of primary production in the ocean), providing an example of a symbiosis-derived evolutionary innovation with global environmental implications.

Interactions between phytoplankton and ultraviolet radiation (UVR: 280 – 400 nm) are undergoing changes dictated by variability in ocean temperature, the depth of mixed layers, nutrient availability, and the thickness of the ozone layer. There are a variety of mechanisms for phytoplankton to cope with UVR stress, one of the most prevalent being the presence of mycosporine-like amino acids (MAAs). Despite the importance of these molecules to phytoplankton fitness under UVR stress, knowledge of the diversity and distribution of these molecules in the world’s oceans is relatively limited. Here, the composition and distribution of MAAs in phytoplankton were examined in a transect across the Southern Ocean, crossing multiple fronts, from eastern New Zealand to the West Antarctic Peninsula in March and April of 2018. The highest concentration of MAAs (> 0.2 μg/L) was found between 50 and 60°S, as well as along a longitudinal gradient between 137.47 and 144.78°W. A strong correlation was found between a model of the preceding month’s UVR dosage experienced in the mixed layer and the ratio of MAAs to chlorophyll-a across the transect, indicating a relationship between the integrated history of light exposure and phytoplankton physiology. Haptophytes accounted for the majority of biomass north of the polar front (PF) and were strongly correlated with a diversity of MAAs. South of the PF a transition to a community dominated by diatoms was observed, with community composition changes strongly correlated to porphyra-334 concentrations. The data presented here provide a baseline for MAA abundance and association with specific phytoplankton taxa across the Southern Ocean amid a changing climate.

Phenotypic plasticity allows organisms to adjust the timing of life-history events in response to environmental and demographic conditions. Shifts by individuals in the timing of breeding with respect to variation in age and temperature are well documented in nature, and these changes are known to scale to affect population dynamics. However, relatively little is known about how organisms alter phenology in response to other demographic and environmental factors. We investigated how pre-breeding temperature, breeding population density, age, and rainfall in the first month of life influenced the timing and plasticity of lay date in a population of Savannah Sparrows (Passerculus sandwichensis) monitored over 33 yr (1987-2019). Females that experienced warmer pre-breeding temperatures tended to lay eggs earlier, as did older females, but breeding population density had no effect on lay date. Natal precipitation interacted with age to influence lay date plasticity, with females that experienced high precipitation levels as nestlings advancing lay dates more strongly over the course of their lives. We also found evidence for varied pace of life; females that experienced high natal precipitation had shorter lifespans and reduced fecundity, but more nesting attempts over their lifetimes. Rainfall during the nestling period increased through time, while population density and fecundity declined, suggesting that increased precipitation on the breeding grounds may be detrimental to breeding females and ultimately the viability of the population as a whole. Our results suggest that females adjust their laying date in response to pre-breeding temperature, and as they age, while presenting new evidence that environmental conditions during the natal period can affect phenological plasticity and generate downstream, population-level effects.

Migratory insects use a variety of innate mechanisms to determine their orientation and maintain correct bearing. For long-distance migrants, like the monarch butterfly (Danaus plexippus), these journeys could be affected by exposure to environmental contaminants. Neonicotinoids are synthetic insecticides that work by affecting the nervous system of insects resulting in impairment of their mobility, cognitive performance and other physiological and behavioural functions. To examine how neonicotinoids might affect the ability of monarch butterflies to maintain a proper directional orientation on their ∼4,000 km migration, we grew swamp milkweed (Asclepias incarnata) in soil that was either untreated (0 ng/g: control) or mixed with low (15 ng/g of soil), or high (25 ng/g of soil) levels of the neonicotinoid clothianidin. Monarch caterpillars were raised on control or clothianidin-treated milkweed and, after pupation, either tested for orientation in a static flight simulator or radio-tracked in the wild during the fall migration period. Despite clothianidin being detectable in milkweed tissue consumed by caterpillars, there was no evidence that clothianidin influenced the orientation, vector strength (i.e., concentration of direction data around the mean), or the rate of travel of adult butterflies and nor was there evidence that morphological traits (i.e., mass and forewing length), testing time, wind speed, or temperature impacted directionality. While samples sizes for both flight simulator and radio-tracking tests were limited, our preliminary results suggest that clothianidin exposure during early caterpillar development does not affect the directed flight of adult migratory monarch butterflies or influence their orientation at the beginning of migration.

The Bank Swallow (Riparia riparia), a threatened species in Canada, breeds primarily in banks at lakeshores and rivers and in artificial (typically inland) aggregate mining pits. Inland pits may be ecological traps for this species, but relative dietary trade-offs between these two nesting habitats have not been investigated. The availability of aquatic emergent insects at lakeshores may have associated nutritional benefits for growing nestlings due to increased omega-3 fatty acids (FAs) in prey. We compared the diets of juvenile swallows from lakeshore and inland pit sites using assays of stable isotope values (δ13C, δ15N, δ2H) of feathers, faecal DNA metabarcoding and blood plasma FAs. Colony proximity to Lake Erie influenced the use of aquatic versus terrestrial insects by Bank Swallow adults and juveniles. Feather δ2H was particularly useful as a tracer of aquatic emergent versus terrestrial prey, and inland juveniles had feathers enriched in 2H, reflective of diets composed of fewer aquatic emergent insects. DNA metabarcoding of juvenile and adult faecal material indicated that lakeshore birds consumed more aquatic-emergent chironomids than inland birds. Lakeshore juveniles had elevated plasma omega-3 eicosapentaenoic acid levels compared with inland pit-breeding birds. We discuss the need to consider 'nutritional landscapes' and the importance of this concept in conservation of declining species and populations.

Benthic surveys are a key component of monitoring and conservation efforts for coral reefs worldwide. While traditional image-based surveys rely on manual annotation of photographs to characterise benthic composition, automatic image annotation based on computer vision is becoming increasingly common. However, accurate classification of some benthic groups from reflectance images presents a challenge to local ecologists and computers alike. Most coral reef organisms produce one or a combination of fluorescent pigments, such as Green Fluorescent Protein (GFP)-like proteins found in corals, chlorophyll-a found in all photosynthetic organisms, and phycobiliproteins found in red macroalgae, crustose coralline algae (CCA) and cyanobacteria. Building on the potential of these pigments as a target for automatic image annotation, we developed a novel imaging method based on off-the-shelf components to improve classification of coral and other biotic substrates using a multi-excitation fluorescence (MEF) imaging system. We used RGB cameras to image the fluorescence emission of coral and algal pigments stimulated by narrow-waveband blue and green light, and then combined the information into three-channel pseudocolour images. Using a set of a priori rules defined by the relative pixel intensity produced in different channels, the method achieved successful classification of organisms into three categories based on the dominant fluorescent pigment expressed, facilitating discrimination of traditionally problematic groups. This work provides a conceptual foundation for future technological developments that will improve the cost, accuracy and speed of coral reef surveys.

As harmful algae blooms are increasing in frequency and magnitude, one goal of a new generation of higher spectral resolution satellite missions is to improve the potential of satellite optical data to monitor these events. A satellite-based algorithm proposed over two decades ago was used for the first time to monitor the extent and temporal evolution of a massive bloom of the dinoflagellate Lingulodinium polyedra off Southern California during April and May 2020.The algorithm uses ultraviolet (UV) data that have only recently become available from the single ocean color sensor on the Japanese GCOM-C satellite. Dinoflagellates contain high concentrations of mycosporine-like amino acids and release colored dissolved organic matter, both of which absorb strongly in the UV part of the spectrum. Ratios <1 of remote sensing reflectance of the UV band at 380 nm to that of the blue band at 443 nm were used as an indicator of the dinoflagellate bloom. The satellite data indicated that an observed, long, and narrow nearshore band of elevated chlorophyll-a (Chl-a) concentrations, extending from northern Baja to Santa Monica Bay, was dominated by L. polyedra. In other high Chl-a regions, the ratios were >1, consistent with historical observations showing a sharp transition from dinoflagellate- to diatom-dominated waters in these areas. UV bands are thus potentially useful in the remote sensing of phytoplankton blooms but are currently available only from a single ocean color sensor. As several new satellites such as the NASA Plankton, Aerosol, Cloud, and marine Ecosystem mission will include UV bands, new algorithms using these bands are needed to enable better monitoring of blooms, especially potentially harmful algal blooms, across large spatiotemporal scales.