Christopher A. Edwards

Integrated optics N*N multiplexer on silicon

The authors report the measured performance of an integrated N*N multiplexer fabricated using SiO/sub 2//Si waveguides. The insertion loss for N=7 was typically lower than 2.5 dB, and crosstalk was less than -25 dB. The multiplexer response is approximately periodic. In each period the multiplexer accepts, from each input waveguide, a total of N channels, and it efficiently transmits each channel to a particular output port. Similarly, each output port receives N channels, one from each input port. Thus, the total number of channels that can be transmitted simultaneously is N/sup 2/, which requires N optical frequencies.<<ETX>>

Ideal microlenses for laser to fiber coupling

The design and fabrication of ideal microlenses for semiconductor laser to fiber coupling are reported. Properly coated for reflections, lenses of the new design can theoretically collect 100% of the radiated energy of a modal-symmetric laser source. The crucial feature is its hyperbolic shape. Microlenses fabricated directly on the end of the fiber by laser micromachining have demonstrated up to 90% coupling efficiency. This performance represents a major advance in microlens technology when compared to currently fabricated hemispherical microlenses which are at best 55% efficient. A theoretical comparison of the two lens shapes illuminates the advantages of the hyperbolic profile. The ability to couple all of the light from a semiconductor laser into a fiber has far-reaching implications for all optical communication systems. >

Individual-based models of copepod populations in coastal upwelling regions : implications of physiologically and environmentally influenced diel vertical migration on demographic success and nearshore retention

We link a two-dimension coastal upwelling circulation hydrodynamic-ecosystem (NPZ) model with an individual-based model (IBM) for an intermediate sized (ca. 2.5 mm) copepod capable of diel vertical migration (DVM) at larger sizes. The NPZ model is that of Franks, Wroblewski and Flierl (1986), with the zooplankton state variable parameterized for macrozooplankton. IBM simulations are done with different scenarios for behavioral responses; the interaction of the organisms with the circulation is evaluated by examining growth/development, reproduction, survival and distribution. Since ocean productivity in coastal upwelling systems is greatest nearshore, zooplankton production is favored by nearshore retention. Model results, using an idealized, intermittently wind-forced, upwelling circulation, indicate that non-migrating copepods are flushed from the nearshore system in offshore zonal surface flow; highest population abundances occur offshore, in a region of relatively low food resources. Conversely, migrating copepods interact with the stratified zonal flow within the upwelling system and are retained nearshore when the amplitude of the DVM is sufficient to place the individuals in near-bottom onshore flow during the day. Environmental features, like deep-extending food resources, and physiological controls, like satiation or body size, that permit copepods to remain deeper, or spend more time away from the surface, favor nearshore retention. Diel vertical migration is one mechanism, which may permit animals to exploit favorable habitats located nearshore in upwelling systems.

Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N2 fixation in the tropical Atlantic Ocean.

The abundances of six N₂-fixing cyanobacterial phylotypes were profiled at 22 stations across the tropical Atlantic Ocean during June 2006, and used to model the contribution of the diazotrophs to N₂ fixation. Diazotroph abundances were measured by targeting the nifH gene of Trichodesmium, unicellular groups A, B, C (UCYN-A, UCYN-B and UCYN-C), and diatom-cyanobiont symbioses Hemiaulus-Richelia, Rhizosolenia-Richelia and Chaetoceros-Calothrix. West to east gradients in temperature, salinity and nutrients [NO₃⁻ + NO₂⁻, PO₄³⁻, Si(OH)₄] showed the influence of the Amazon River plume and its effect on the distributions of the diazotrophs. Trichodesmium accounted for more than 93% of all nifH genes detected, dominated the warmer waters of the western Atlantic, and was the only diazotroph detected at the equatorial upwelling station. UCYN-A was the next most abundant (> 5% of all nifH genes) and dominated the cooler waters of the eastern Atlantic near the Cape Verde Islands. UCYN-C was found at a single depth (200 m) of high salinity and low temperature and nutrients, whereas UCYN-B cells were widespread but in very low abundance (6.1 × 10¹ ± 4.6 × 10² gene copies l⁻¹). The diatom-cyanobionts were observed primarily in the western Atlantic within or near the high Si(OH)₄ input of the Amazon River plume. Overall, highest diazotroph abundances were observed at the surface and declined with depth, except for some subsurface peaks in Trichodesmium, UCYN-B and UCYN-A. Modelled contributions of Trichodesmium, UCYN-B and UCYN-A to total N₂ fixation suggested that Trichodesmium had the largest input, except for the potential of UCYN-A at the Cape Verde Islands.

Impacts of the 2015–2016 El Niño on the California Current System: Early assessment and comparison to past events

The 2015–2016 El Nino is by some measures one of the strongest on record, comparable to the 1982–1983 and 1997–1998 events that triggered widespread ecosystem change in the northeast Pacific. Here we describe impacts of the 2015–2016 El Nino on the California Current System (CCS) and place them in historical context using a regional ocean model and underwater glider observations. Impacts on the physical state of the CCS are weaker than expected based on tropical sea surface temperature anomalies; temperature and density fields reflect persistence of multiyear anomalies more than El Nino. While we anticipate El Nino-related impacts on spring/summer 2016 productivity to be similarly weak, their combination with preexisting anomalous conditions likely means continued low phytoplankton biomass. This study highlights the need for regional metrics of El Ninos effects and demonstrates the potential to assess these effects before the upwelling season, when altered ecosystem functioning is most apparent.

Laser micromachining of efficient fiber microlenses

We present results of the use of a CO(2) waveguide laser for the reproducible fabrication of efficient microlenses on single-mode optical fibers. Short intense laser pulses are used both to melt microlens tips to specific radii and to micromachine microlenses by ablative removal of small (~1-mum(2)) areas of glass from the fiber surface. CO(2) laser heating of fiber tips results in more consistent lens curvature than the electric arc commonly used in microlens fabrication. The microlenses formed using a laser microlathe, in which spinning fibers are shaped by simultaneous cutting and heating in the pulsed CO(2) laser beam, show excellent laser-fiber coupling with losses in the 1.5-3-dB range, an improvement of more than 2 dB over coupling with standard microlenses made by the etch and melt technique.

GROWTH AND CARBON CONTENT OF THREE DIFFERENT-SIZED DIAZOTROPHIC CYANOBACTERIA OBSERVED IN THE SUBTROPICAL NORTH PACIFIC1

To develop tools for modeling diazotrophic growth in the open ocean, we determined the maximum growth rate and carbon content for three diazotrophic cyanobacteria commonly observed at Station ALOHA (A Long‐term Oligotrophic Habitat Assessment) in the subtropical North Pacific: filamentous nonheterocyst‐forming Trichodesmium and unicellular Groups A and B. Growth‐irradiance responses of Trichodesmium erythraeum Ehrenb. strain IMS101 and Crocosphaera watsonii J. Waterbury strain WH8501 were measured in the laboratory. No significant differences were detected between their fitted parameters (±CI) for maximum growth rate (0.51 ± 0.09 vs. 0.49 ± 0.17 d−1), half‐light saturation (73 ± 29 vs. 66 ± 37 μmol quanta · m−2 · s−1), and photoinhibition (0 and 0.00043 ± 0.00087 [μmol quanta · m−2 · s−1]−1). Maximum growth rates and carbon contents of Trichodesmium and Crocosphaera cultures conformed to published allometric relationships, demonstrating that these relationships apply to oceanic diazotrophic microorganisms. This agreement promoted the use of allometric models to approximate unknown parameters of maximum growth rate (0.77 d−1) and carbon content (480 fg C · μm−3) for the uncultivated, unicellular Group A cyanobacteria. The size of Group A was characterized from samples from the North Pacific Ocean using fluorescence‐activated cell sorting and real‐time quantitative PCR techniques. Knowledge of growth and carbon content properties of these organisms facilitates the incorporation of different types of cyanobacteria in modeling efforts aimed at assessing the relative importance of filamentous and unicellular diazotrophs to carbon and nitrogen cycling in the open ocean.

A 5 million year comparison of Mg/Ca and alkenone paleothermometers

Geochemical sea surface temperature (SST) proxies such as the magnesium to calcium ratio (Mg/Ca) in foraminifera and the alkenone unsaturation index (UK′37) are becoming widely used in pre-Pleistocene climate records. This study quantitatively compares previously published Mg/Ca and UK′37 data from Ocean Drilling Program (ODP) site 847 in the eastern equatorial Pacific to assess the utility of these proxies to reconstruct tropical SST over the last 5 Ma. Foraminiferal Mg/Ca–SST calibrations that include a dissolution correction are most appropriate at this location because they provide SST estimates for the youngest sample that are close to modern mean annual SST. The long-term trends in the two records are remarkably similar and confirm a ∼3.5°C cooling trend from the early Pliocene warm period to the late Pleistocene noted in previous work. Absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate SST from Mg/Ca are taken into account. Comparing the two SST records at ODP site 847 to other records in the region shows that the eastern equatorial Pacific was 2–4°C warmer during the early Pliocene compared to today.

The stability of an NPZ model subject to realistic levels of vertical mixing

The linear stability of a vertically-distributed, Nutrient-Phytoplankton-Zooplankton (NPZ) ocean ecosystem model is analyzed to understand how vertical mixing influences biological dynamics. In the absence of vertical diffusion, the model generally exhibits both stable fixed point and limit cycle behavior, depending on the depth and choice of parameters. Diffusion couples the dynamics of nearby levels and can induce stable profiles as well as oscillatory dynamical trajectories that become vertically phase-locked for large mixing levels. Calculations of the Lyapunov exponent reveal that vertical diffusion can drive this model into a chaotic state, though this occurs only for levels of diffusion well below those found in nature. The dynamics of the model, assuming macrozooplankton are the dominant grazers in the ecosystem, are compared to those in which microzooplankton dominate, with a faster grazing rate and poor assimilation efficiency. While the coupled physical-macrozooplanton system has a stable profile, the coupled microzooplankton profile remains unstable, even at large mixing levels. Fluctuations occur on time scales varying between a few days and a few months, depending on the parameters and magnitude of diffusion.

Spatially resolved upwelling in the California Current System and its connections to climate variability

A historical analysis of California Current System (CCS) circulation, performed using the Regional Ocean Modeling System with four-dimensional variational data assimilation, was used to study upwelling variability during the 1988-2010 period. We examined upwelling directly from the vertical velocity field, which elucidates important temporal and spatial variability not captured by traditional coastal upwelling indices. Through much of the CCS, upwelling within 50 km of the coast has increased, as reported elsewhere. However, from 50 to 200 km offshore, upwelling trends are negative and interannual variability is 180 ◦ out of phase with the nearshore signal. This cross-shore pattern shows up as the primary mode of variability in central and northern CCS vertical velocity anomalies, accounting for ∼40% of the total variance. Corresponding time series of the dominant modes in the central and northern CCS are strongly correlated with large-scale climate indices, suggesting that climate fluctuations may alternately favor different biological communities.