Franks
Peter J. S.
Franks
Peter J. S.
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ArticleVertical migration of dinoflagellates : model analysis of strategies, growth, and vertical distribution patterns(Inter-Research, 2007-08-23) Ji, Rubao ; Franks, Peter J. S.Dinoflagellates demonstrate a variety of vertical migration patterns that presumably give them a competitive advantage when nutrients are depleted in the surface layer of stratified waters. In this study, a simple quota-based model was used to examine the relationships between the vertical migration pattern and internal nutritional status, and to assess how external environmental conditions, such as mixing layer depth (MLD) and internal waves, can influence these relationships. Dinoflagellates may form subsurface aggregations or conduct vertical migration (diel or non-diel) in response to their internal nutrient quota, but within a limited physiological parameter space. The model was implemented in a 1D (vertical) domain using an individual-based modeling approach, tracking the change in nutrient quota and the trajectory of many individual cells in a water column. The model shows that dinoflagellate cells might change from one vertical migration pattern to another when the external environmental conditions change. Using the average net growth rate as an index of fitness, 2 migration strategies, photo-/geotaxis vs. quota-based migration, were assessed with regard to MLD and internal wave regime. It was found that dinoflagellates might choose different migration strategies under different mixing/stratification regimes. In addition, under the same environmental conditions, different species might display unique vertical migration patterns due to inherent physiological differences. This study reveals the sensitivity of dinoflagellate vertical migration to biological and physical factors and offers possible explanations for the various vertical distributions and migration patterns observed in the field.
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ArticleInfluence of diurnal heating on stratification and residual circulation of Georges Bank(American Geophysical Union, 2003-11-22) Chen, Changsheng ; Beardsley, Robert C. ; Franks, Peter J. S. ; Van Keuren, J.The influence of the diurnal heat flux on summer stratification and residual circulation over Georges Bank was examined using a three-dimensional primitive equation numerical circulation model. For a given spatially uniform and time-varying heat flux the model results show that the surface water is heated much faster on the southern flank than on the northern flank and much faster in the stratified region than in the mixed region. Heating significantly strengthens the tidal mixing front and intensifies the frontward convergence near the surface. As seasonal stratification develops, the location of the tidal mixing front gradually shifts on bank on the southern flank, while remaining almost unchanged on the northern flank. Response of the tidal currents to the diurnal variation in the heat flux varies across Georges Bank. It changes periodically with tidal cycles on the southern flank but is locked to the phase of the eastward tidal current on the northern flank. This phase-lock feature directly contributes to the intensification of the along-bank residual current jet on the northern flank. Diagnostic analysis suggests that this intensification is mainly caused by the heat-enhanced, cross-bank momentum flux. Model-computed variations of near-surface temperature and residual currents are in good agreement with satellite-derived sea surface temperature data and drifter measurements.
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ArticleEpisodic vertical nutrient fluxes and nearshore phytoplankton blooms in Southern California(Association for the Sciences of Limnology and Oceanography, 2012-11) Omand, Melissa M. ; Feddersen, Falk ; Guza, R. T. ; Franks, Peter J. S.Three distinct phytoplankton blooms lasting 4–9 d were observed in approximately 15-m water depth near Huntington Beach, California, between June and October of 2006. Each bloom was preceded by a vertical NO3 flux event 6–10 d earlier. NO3 concentrations were estimated using a temperature proxy that was verified by comparison with the limited NO3 observations. The lower–water-column vertical NO3 flux from vertical advection was inferred from observed vertical isotherm displacement. Turbulent vertical eddy diffusivity was parameterized based on the observed background (< 0.3 cycles h−1) stratification and vertical shear in the horizontal currents. The first vertical nitrate flux event in June contained both advective and turbulent fluxes, whereas the later two events were primarily turbulent, driven by shear in the lower part of the water column. The correlation between the NO3 flux and the observed chlorophyll a (Chl a) was maximum (r2 = 0.40) with an 8-d lag. A simple nitrate–phytoplankton model using a linear uptake function and driven with the NO3 flux captured the timing, magnitude, and duration of the three Chl a blooms (skill = 0.61) using optimal net growth rate parameters that were within the expected range. Vertical and horizontal advection of Chl a past the measurement site were too small to explain the observed Chl a increases during the blooms. The vertical NO3 flux was a primary control on the growth events, and estimation of both the advective (upwelled) and turbulent fluxes is necessary to best predict these episodic blooms.
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ThesisDinoflagellate blooms and physical systems in the Gulf of Maine(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1990-05) Franks, Peter J. S.Numerous studies have shown dinoflagellate blooms to be closely related to density discontinuities and fronts in the ocean. The spatial and temporal patterns of the dinoflagellate population depend on the predominant mode of physical forcing, and its scales of variability. The present study combined field sampling of hydrographic and biological variables to examine the relationship of dinoflagellate population distributions to physical factors along the southwestern cost of the Gulf of Maine. A bloom of Ceratium longipes occurred along this coast during the month of June, 1987. A simple model which coupled along-isopycnal diffusion with the logistic growth equation suggested that the cells had a growth rate of about 0.1 d-1 , and had reached a steady horizontal across-shelf distribution within about 10 d. Fur~her variations in population density appeared to be related to fluctuations of light with periods of -10 d. To our knowledge, this was the first use of this simple diffusion model as a diagnostic tool for quantifying parameters describing the growth and movement of a specific phytoplankton population. Blooms of the toxic dinoflagellate, Alexandrium tamarense have been nearly annual features along the coasts of southern Maine, New Hampshire and Massachusetts since 1972; however the mechanisms controlling the distribution of cells and concomitant shellfish toxicity are relatively poorly understood. Analysis of field data gathered from April to September, 1987-1989, showed that in two years when toxicity was detected in the southern part of this region, A. tamarense cells were apparently transported into the study area between Portsmouth and Cape Ann, Massachusetts, in a coastally trapped buoyant plume. This plume appears to have been formed off Maine by the outflow from the Androscoggin and Kennebec Rivers. Flow rates of these rivers, hydrographic sections, and satellite images suggest that the plume had a duration of about a month, and extended alongshore for several hundred kilometers. The distribution of cells followed the position of the plume as it was influenced by wind and topography. Thus when winds were downwelling-favourable, cells were moved alongshore to the south, and were held to the coast; when winds were upwelling-favourable, the plume sometimes separated from the coast, advecting the cells offshore. The alongshore advection of toxic cells within a coastally trapped buoyant plume can explain the temporal and spatial patterns of shellfish toxicity along the coast. The general observation of a north-to-south temporal trend of toxicity is consistent with the southward advection of the plume. In 1987 when no plume was present, Alexandrium tamarense cells were scarce, and no toxicity was recorded at the southern stations. A hypothesis was formulated explaining the development and spread of toxic dinoflagellate blooms in this region. This plume-advection hypothesis included: source A. tamarense populations in the north, possibly associated with the Androscoggin and Kennebec estuaries; a relationship between toxicity patterns and river flow volume and timing of flow peaks; and a relationship between wind stresses and the distribution of low salinity water and cells. Predictions of the plume-advection hypothesis were tested with historical records of shellfish toxicity, wind speed and direction, and river flow. The predictions tested included the north-south progression of toxic outbreaks, the occurrence of a peak in river flow prior to the PSP events, the relationship of transit time of PSP toxicity along the coast with river flow volume, and the influence of surface wind stress on the timing and location of shellfish toxicity. All the predictions tested were supported by the historical records. In addition it was found that the plume-advection hypothesis explains many details of the timing and spread of shellfish toxicity, including the sporadic nature of toxic outbreaks south of Massachusetts Bay, and the apparently rare occurrence of toxicity well offshore on Nantucket Shoals and Georges Bank.
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ArticleDinoflagellate vertical migration fuels an intense red tide(National Academy of Sciences, 2023-08-28) Zheng, Bofu ; Lucas, Andrew J. ; Franks, Peter J. S. ; Schlosser, Tamara L. ; Anderson, Clarissa R. ; Send, Uwe ; Davis, Kristen ; Barton, Andrew D. ; Sosik, Heidi M.Harmful algal blooms (HABs) are increasing globally, causing economic, human health, and ecosystem harm. In spite of the frequent occurrence of HABs, the mechanisms responsible for their exceptionally high biomass remain imperfectly understood. A 50-y-old hypothesis posits that some dense blooms derive from dinoflagellate motility: organisms swim upward during the day to photosynthesize and downward at night to access deep nutrients. This allows dinoflagellates to outgrow their nonmotile competitors. We tested this hypothesis with in situ data from an autonomous, ocean-wave-powered vertical profiling system. We showed that the dinoflagellate Lingulodinium polyedra’s vertical migration led to depletion of deep nitrate during a 2020 red tide HAB event. Downward migration began at dusk, with the maximum migration depth determined by local nitrate concentrations. Losses of nitrate at depth were balanced by proportional increases in phytoplankton chlorophyll concentrations and suspended particle load, conclusively linking vertical migration to the access and assimilation of deep nitrate in the ocean environment. Vertical migration during the red tide created anomalous biogeochemical conditions compared to 70 y of climatological data, demonstrating the capacity of these events to temporarily reshape the coastal ocean’s ecosystem and biogeochemistry. Advances in the understanding of the physiological, behavioral, and metabolic dynamics of HAB-forming organisms from cutting-edge observational techniques will improve our ability to forecast HABs and mitigate their consequences in the future.