No abstract

Phytoplankton biomass in Monterey Bay, California is typically dominated by diatoms, but it shifted to dinoflagellates twice in the past 18 years (2004–2007, 2017–2018), which was associated with increased harmful algal blooms. Located within the central California Current System (CCS), Monterey Bay is strongly influenced by cycles of upwelling‐favorable winds and their relaxation or reversal. Both dinoflagellate‐dominated periods were linked to a negative North Pacific Gyre Oscillation (NPGO) and increased river discharge, but each had a different relationship with upwelling. To examine the connection between large‐scale and local forcings underlying floristic shifts in the phytoplankton assemblage, an Imaging FlowCytobot (IFCB) was deployed within the Monterey Bay upwelling shadow for a full year. A machine learning classifier differentiating IFCB images of the phytoplankton assemblage was developed. Despite anomalously strong upwelling in 2018, dinoflagellates comprised 57% of the annual phytoplankton‐specific biomass. During upwelling, dinoflagellates appear to have accumulated at convergent fronts, while during relaxation these frontal populations were transported to the nearshore where they seeded local blooms. Frequent upwelling‐relaxation cycles and local wind anomalies generated an unusually retentive circulation pattern in the upwelling shadow, producing a warm and stratified bloom incubator. Thus, local features and forcings (upwelling shadow, winds, river discharge) modified the effects of regional‐ and basin‐scale oceanographic variability (regional upwelling, NPGO), altering local phytoplankton patterns. As North Pacific decadal variability and CCS upwelling intensity increase under climate warming, dinoflagellates may become more common in some CCS regions, due to the enhancement or mitigation of large‐scale trends by local forcings.

No abstract

New resting cyst production is crucial for the survival of many microbial eukaryotes including phytoplankton that cause harmful algal blooms. Production in situ has previously been estimated through sediment trap deployments, but here was instead assessed through estimation of the total number of planktonic cells and new resting cysts produced by a localized, inshore bloom of Alexandrium catenella, a dinoflagellate that is a globally important cause of paralytic shellfish poisoning. Our approach utilizes high frequency, automated water monitoring, weekly observation of new cyst production, and pre‐ and post‐bloom spatial surveys of total resting cyst abundance. Through this approach, new cyst recruitment within the study area was shown to account for at least 10.9% ± 2.6% (SE) of the bloom's decline, ∼ 5× greater than reported from comparable, sediment trap based studies. The observed distribution and timing of new cyst recruitment indicate that: (1) planozygotes, the immediate precursor to cysts in the life cycle, migrate nearer to the water surface than other planktonic stages and (2) encystment occurs after planozygote settlement on bottom sediments. Near surface localization by planozygotes explains the ephemerality of red surface water discoloration by A. catenella blooms, and also enhances the dispersal of new cysts. Following settlement, bioturbation and perhaps active swimming promote sediment infiltration by planozygotes, reducing the extent of cyst redistribution between blooms. The concerted nature of bloom sexual induction, especially in the context of an observed upper limit to A. catenella bloom intensities and heightened susceptibility of planozygotes to the parasite Amoebophrya, is also discussed.

Many phytoplankton species, including many harmful algal bloom (HAB) species, survive long periods between blooms through formation of benthic resting stages. Because they are crucial to the persistence of these species and the initiation of new blooms, the physiology of benthic stages must be considered to accurately predict responses to climate warming and associated environmental changes. The benthic stages of dinoflagellates, called resting cysts, germinate in response to the combination of favorable temperature, oxygen-availability, and release from dormancy. The latter is a mechanism that prevents germination even when oxygen and temperature conditions are favorable. Here, evidence of temperature-mediated control of dormancy duration from the dinoflagellates Alexandrium catenella and Pyrodinium bahamense-two HAB species that cause paralytic shellfish poisoning (PSP)-is reviewed and presented alongside new evidence of complementary, temperature-based control of cyst quiescence (the state in which cysts germinate on exposure to favorable conditions). Interaction of the two temperature-based mechanisms with climate is explored through a simple model parameterized using results from recent experiments with A. catenella. Simulations demonstrate the importance of seasonal temperature cycles for the synchronization of cysts' release from dormancy and are consistent with biogeography-based inferences that A. catenella is more tolerant of warming in habitats that experience a larger range of seasonal temperature variation (i.e., have higher temperature seasonality). Temperature seasonality is much greater in shallow, long-residence time habitats than in deep, open-water ones. As warming shifts species' ranges, cyst beds may persist longer in more seasonally variable, shallow inshore habitats than in deep offshore ones, promoting HABs that are *