Summer phytoplankton assemblages and their environmental correlates in the Southern California Bight

Weekly observations of chemical and physical variables, and of phytoplankton abundances, were made over a 21-week period at three stations in the Southern California Bight. Principal component analysis was employed to resolve four phytoplankton assemblages among the 25 taxa with the highest variances for their log transformed abundances. Two of these components were described by the abundance of taxa characteristic of upwelling assemblages—both dominated by diatoms. A third assemblage was dominated by the red tide dinoflagellate, Gonyaulax polyedra, along with some small diatoms and a coccolithophorid.Canonical correlation analysis of the four phytoplankton components against 13 environmental variables revealed distinct sets of temperature-salinity-nutrient conditions associated with periods of abundance of each assemblage. Two were interpretable, on physical grounds, as upwelling situations, and these were associated with periods of abundance of the phytoplankton assemblages that were identified, on floristic grounds, as upwelling assemblages. The red tide assemblage was associated with nonupwelling conditions.The pattern of correlations of 89 other taxa, besides the 25 employed in the principal component analysis, with the four principal components and the four environmental canonical variates were also consistent with this interpretation. Comparison with phytoplankton assemblages described in other studies reveals substantial consistency in broad outline, but many differences in detail, especially with respect to the presence and absence of species.The species assemblages defined by the principal components analysis exhibited episodes of abundance of a duration of 2–3 weeks at a given location. Current meter records, from nearby stations, but from another year, suggest that a persistence time of 2–3 weeks at a stationary site corresponds to a spatial patch scale of 20–40 km. These same current meter records show approximately 50% coherence in low-frequency currents at a separation of 25 km, indicating a possible common scale for spatial coherence of currents and spatial extent of phytoplankton blooms in this system.Implications of the analysis are discussed in terms of hypotheses concerning the structure and dynamics of plankton communities

Modelling RF sources using 2-D PIC codes

In recent years, many types of RF sources have been successfully modelled using 2-D PIC codes. Both cross field devices (magnetrons, cross field amplifiers, etc.) and pencil beam devices (klystrons, gyrotrons, TWT'S, lasertrons, etc.) have been simulated. All these devices involve the interaction of an electron beam with an RF circuit. For many applications, the RF structure may be approximated by an equivalent circuit, which appears in the simulation as a boundary condition on the electric field ( port approximation''). The drive term for the circuit is calculated from the energy transfer between beam and field in the drift space. For some applications it may be necessary to model the actual geometry of the structure, although this is more expensive. One problem not entirely solved is how to accurately model in 2-D the coupling to an external waveguide. Frequently this is approximated by a radial transmission line, but this sometimes yields incorrect results. We also discuss issues in modelling the cathode and injecting the beam into the PIC simulation

Optimization of a lasertron double output cavity

Double output cavities have been used experimentally to increase the efficiency of high-power klystrons. We have used particle-in-cell simulations with the 2 + 1/2 dimensional code MASK to optimize the design of double output cavities for the lasertron under development at SLAC. We discuss design considerations for double output cavities (e.g., optimum choice of voltages and phases, efficiency, wall interception, breakdown). We describe how one calculates the cavity impedance matrix from the gap voltages and phases. Some results of the effect of varying voltage, perveance, and pulse are reported

Particulate organic matter in surface waters off Southern California and its relationship to phytoplankton

Particulate carbon, nitrogen, chlorophyll a, adenosine triphosphate, particle count and particle volume were measured in the euphotic zone during six quarterly cruises (between September, 1974, and March, 1976) in the Southern California Bight. The distribution and quantitative relationships among these parameters were examined in an attempt to estimate the relative contributions of plankton to the total particulate matter...

Nitrate use by plankton in the eastern subtropical North Atlantic, March-April 1989

Nitrate concentration was measured in seawater samples from the euphotic zone at the beginning and end of 12-h, daytime, in situ incubations. The changes in concentration are considered to be measurements of new production. During periods of 2-3 weeks in March-April 1989, important time scales for NO3- input to the euphotic zone (i.e. residence times) and new production were approximately 26 d at 18-degrees-N, 31-degrees-W and approximately 10 d near 33-degrees-N, 21-degrees-W. The average rate of NO3- use in the two areas was 2.63 and 0.62 mmol N m-2 (12 h)-1, or, in carbon equivalents 209 and 49 mg C m-2 d-1, respectively. These values bracket the large-scale estimate by Jenkins of new production in the nearby beta triangle of 150 mg C m-2 d-1

Vertical motion of the thermocline, nitracline and chlorophyll maximum layers in relation to currents on the Southern California Shelf

A continuous four-day time series of nitrate concentration, temperature, chlorophyll fluorescence, and currents, sampled at fixed depths, revealed that distributions of temperature and nitrate could be accounted for by vertical motions in the water column associated with the semidiurnal internal tide and internal waves. A probable mixing event was observed: the transport of nitrate into the surface-layer associated with shear instabilities generated by internal waves. On temporal scales of less than a few hours, the variation of chlorophyll fluorescence could also be explained by vertical advection. However, on longer scales, swimming behavior of the phytoplankton assemblage (dominated by Ceratium spp.), along with vertical motions in the water column, appears to account for the vertical distribution of chlorophyll. These results indicate that the nitracline maintains a stable relationship with the density structure of the water column on a scale of days, whereas the subsurface chlorophyll maximum can change significantly over several hours

Absence of "Ghost Images" Excludes Large Values of the Cosmological Constant

We used the 1.4 GHz NRAO NVSS survey to search for ghost images of radio sources, expected in cosmologies with a positive cosmological constant and positive space curvature. No statistically significant evidence for ghost images was found, placing constraints on the values of L, the space curvature or the duration of the radio-luminous phase of extragalactic radio sources.Comment: 11 pages 2 figure

Impacts of Atmospheric Nitrogen Deposition and Coastal Nitrogen Fluxes on Oxygen Concentrations in Chesapeake Bay

Although rivers are the primary source of dissolved inorganic nitrogen (DIN) inputs to the Chesapeake Bay, direct atmospheric DIN deposition and coastal DIN concentrations on the continental shelf can also significantly influence hypoxia; however, the relative impact of these additional sources of DIN on Chesapeake Bay hypoxia has not previously been quantified. In this study, the estuarine‐carbon‐biogeochemistry model embedded in the Regional‐Ocean‐Modeling‐System (ChesROMS‐ECB) is used to examine the relative impact of these three DIN sources. Model simulations highlight that DIN from the atmosphere has roughly the same impact on hypoxia as the same gram‐for‐gram change in riverine DIN loading, although their spatial and temporal distributions are distinct. DIN concentrations on the continental shelf have a similar overall impact on hypoxia as DIN from the atmosphere (~0.2 mg L−1); however, atmospheric DIN impacts dissolved oxygen (DO) primarily via the decomposition of autochthonous organic matter, whereas coastal DIN concentrations primarily impact DO via the decomposition of allochthonous organic matter entering the Bay mouth from the shelf. The impacts of atmospheric DIN deposition and coastal DIN concentrations on hypoxia are greatest in summer and occur farther downstream (southern mesohaline) in wet years than in dry years (northern mesohaline). Integrated analyses of the relative contributions of all three DIN sources on summer bottom DO indicate that impacts of atmospheric deposition are largest in the eastern mesohaline shoals, riverine DIN has dominant impacts in the largest tributaries and the oligohaline Bay, while coastal DIN concentrations are most influential in the polyhaline region

Can rates of ocean primary production and biological carbon export be related through their probability distributions?

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Global Biogeochemical Cycles 32 (2018): 954-970, doi:10.1029/2017GB005797.We describe the basis of a theory for interpreting measurements of two key biogeochemical fluxes—primary production by phytoplankton (p, μg C · L−1 · day−1) and biological carbon export from the surface ocean by sinking particles (f, mg C · m−2 · day−1)—in terms of their probability distributions. Given that p and f are mechanistically linked but variable and effectively measured on different scales, we hypothesize that a quantitative relationship emerges between collections of the two measurements. Motivated by the many subprocesses driving production and export, we take as a null model that large‐scale distributions of p and f are lognormal. We then show that compilations of p and f measurements are consistent with this hypothesis. The compilation of p measurements is extensive enough to subregion by biome, basin, depth, or season; these subsets are also well described by lognormals, whose log‐moments sort predictably. Informed by the lognormality of both p and f we infer a statistical scaling relationship between the two quantities and derive a linear relationship between the log‐moments of their distributions. We find agreement between two independent estimates of the slope and intercept of this line and show that the distribution of f measurements is consistent with predictions made from the moments of the p distribution. These results illustrate the utility of a distributional approach to biogeochemical fluxes. We close by describing potential uses and challenges for the further development of such an approach.National Science Foundation Grant Number: OCE-1315201; Simons Foundation Grant Numbers: 329108, 553242; National Aeronautics and Space Administration Grant Numbers: NNX16AR47G, NNX16AR49