James M. Sullivan

Use of optical scattering to discriminate particle types in coastal waters

The particulate scattering characteristics of coastal waters were examined at nine locations around the United States, including near-shore sites in the Gulf of Mexico and the Atlantic and Pacific oceans. The scattering measurements were used in conjunction with inversion models to estimate particle size distributions and bulk refractive indices of the suspended particles. The relationships between various scattering properties and chlorophyll concentration were also investigated and compared with previous relationships described for case I waters. Although the general patterns of scattering and particle characteristics in coastal waters were fairly consistent, fine-scale variability within the water column was substantial. Combining optical measurements with inversion techniques provided a more informative view of the environment and a better understanding of the nature of particle populations in the coastal ocean.

Hyperspectral temperature and salt dependencies of absorption by water and heavy water in the 400-750 nm spectral range

The temperature and salt dependencies of absorption by liquid water (H2O) and heavy water (D2O) were determined using a hyperspectral absorption and attenuation meter (WET Labs, AC-S). Sodium chloride (NaCl) was used as a proxy for seawater salts. There was no significant temperature (PsiT) or salt (PsiS) dependency of absorption at wavelengths <550 nm. At wavelengths >550 nm, PsiT exhibited peaks at approximately 604, 662, and 740 nm. A small negative trough in PsiS occurred at approximately 590 nm, followed by a small positive peak approximately 620 nm, a larger negative trough at approximately 720 nm, and a strong positive peak at approximately 755 nm. The salt dependency of absorption by heavy water, Psis(H), exhibited a negative power-law shape with very low Psis(H), at wavelengths >550 nm. Our experiments with NaCl, clean open ocean seawater, and artificial seawater support the hypothesis that salts modify the absorption spectra of seawater by modifying the molecular matrix and vibrations of pure water.

Microscale Quantification of the Absorption by Dissolved and Particulate Material in Coastal Waters with an ac-9

Abstract Measuring coastal and oceanic absorption coefficients of dissolved and particulate matter in the visible domain usually requires a methodology for amplifying the natural signal because conventional spectrophotometers lack the necessary sensitivity. The WET Labs ac-9 is a recently developed in situ absorption and attenuation meter with a precision better than ±0.001 m−1 in the raw signal, which is sufficient to make these measurements in pristine samples. Whereas the superior sensitivity of the ac-9 has been well documented, the accuracy of in situ measurements for bio-optical applications has not been rigorously evaluated. Obtaining accurate results with an ac-9 requires careful attention to calibration procedures because baselines drift as a result of the changing optical properties of several ac-9 components. To correct in situ measurements for instrument drift, a pressurized flow procedure was developed for calibrating an ac-9 with optically clean water. In situ, micro- (cm) to fine- (m) scale...

Angular shape of the oceanic particulate volume scattering function in the backward direction.

Analysis of several million particulate volume scattering functions (VSFs) from different field sites around the worlds oceans and coastlines revealed that the shape of the VSF in the backward direction was remarkably consistent (5% or less variability at angles between 90 degrees and 170 degrees ). In agreement with theoretical models and past field measurements, the variability of the VSF shape (the VSF normalized to the backscattering coefficient) was found to be lowest between 110 degrees and 120 degrees . This study concludes that under most oceanic conditions, estimates of the particulate backscattering coefficient, using single angle scattering measurements near 110 degrees to 120 degrees and suitable conversion factors, are justified and should have a maximum uncertainty of less than a few percent once instrument noise is accounted for.

EFFECTS OF SMALL‐SCALE TURBULENCE ON NET GROWTH RATE AND SIZE OF TEN SPECIES OF MARINE DINOFLAGELLATES1

Field observations and results from previous laboratory studies on the effects of turbulence on dinoflagellates have led to a paradigm in phytoplankton ecology that dinoflagellate growth is negatively affected by turbulence. To test the paradigm, 10 species of autotrophic dinoflagellates were exposed to quantified three‐dimensional turbulence generated by vertically oscillating cylindrical rods in 20‐L rectangular culture tanks. Turbulence was quantified in the tanks (as the turbulent energy dissipation rate, ε) using an acoustic Doppler velocimeter. Dinoflagellates were exposed to two turbulence treatments: high turbulence (ε∼ 10−4 m2·s−3), low turbulence (ε∼ 10−8 m2·s−3), and an unstirred control. In accord with the paradigm, Ceratium fusus (Ehrenberg) Dujardin had lower net growth rates in high turbulence, whereas Pyrocystis noctiluca Murray ex Haeckel and Ceratium tripos (O. F. Müller) Nitzsch did not increase their numbers in high turbulence. However, Alexandrium tamarense (Lebour) Balech, Pyrocystis fusiformis Wyville‐Thomson ex Murray, Alexandrium catenella (Whedon and Kofoid) Balech, and a Gyrodinium sp. Kofoid and Swezy were apparently unaffected by turbulence and had the same net growth rates across all turbulence treatments. Contradicting the paradigm, Lingulodinium polyedrum (Stein) Dodge (= Gonyaulax polyedra), Gymnodinium catenatum Graham, and Alexandrium fundyense Balech had increased net growth rates in high turbulence treatments. Cross‐sectional area (CSA) varied little across turbulence treatments for 8 of 10 dinoflagellate species tested, CSA in C. fusus increased when net growth rate decreased in high turbulence, and, conversely, CSA decreased in L. polyedrum when net growth rate increased in high turbulence.

Measuring optical backscattering in water

Knowledge of light scattering can provide important information on underwater radiative transfer and the nature and dynamics of suspended particulate matter within a water mass.

Bioluminescent organisms and bioluminescence measurements in the North Atlantic Ocean near latitude 59.5°N, longitude 21°W

We investigated mixed-layer bioluminescence from early April to late September (in April 1989, May 1991, July 1983 and 1990, August 1991, September 1988 and 1989) at stations near the Marine-Light - Mixed Layers (MLML) bio-optical moorings site. Volume-specific bioluminescence potential (BPOT, photons per unit volume) from epipelagic organisms was estimated directly with a pump-through bioluminescence photometer (BP) in 1983, 1988, and 1991. For all cruises, BPOT was also estimated by summing for a volume of seawater, the measurements of each species′ total stimulable bioluminescence multiplied by each species′ numerical abundance in the volume. The abundance data were taken from bottle casts, net tows, and BP effluent nets. After the onset of the spring bloom, from May through September, mixed layer BPOT was fairly constant, ∼1–4×1014 photons m−3. On one early April cruise (1989) before the spring bloom, BPOT was two orders of magnitude lower. Heterotrophic dinoflagellates in the genus Protoperidinium generally produced most (90% or more) of the mixed layer BPOT in the spring, summer, and fall. On one cruise in September (1988), the autotrophic dinoflagellate Ceratium fusus produced the bulk of the mixed layer BPOT (more than about 4×1014 photons m−3). Other autotrophic dinoflagellates in the genus Gonyaulax and mesozooplankton produced a minor part of BPOT at most stations. The relative contribution of all autotrophic dinoflagellates to BPOT increased from a few percent during the May–June–July period to ∼10% during the August–September period. In situ mechanically stimulable bioluminescence was reduced when underwater scalar irradiance (wavelengths 400–700 nm) was greater than 0.1 μmol photons m−2 s−1.

Measurements and simulations of polarization states of underwater light in clear oceanic waters.

Polarization states of the underwater light field were measured by a hyperspectral and multiangular polarimeter and a video polarimeter under various atmospheric, surface, and water conditions, as well as solar and viewing geometries, in clear oceanic waters near Port Aransas, Texas. Some of the first comprehensive comparisons were made between the measured polarized light, including the degree and angle of linear polarization and linear Stokes parameters (Q and U), and those from Monte Carlo simulations that used concurrently measured water inherent optical properties and particle volume scattering functions as input. For selected wavelengths in the visible spectrum, measured and model-simulated polarization characteristics were found to be consistent in most cases. Measured degree and angle of linear polarization are found to be largely determined by an in-water single-scattering model. Model simulations suggest that the degree of linear polarization (DoLP) at horizontal viewing directions is highly dependent on the viewing azimuth angle for a low solar elevation. This implies that animals can use the DoLP signal for orientation.

In Situ Phytoplankton Analysis: There’s Plenty of Room at the Bottom

In Situ Phytoplankton Analysis: There’s Plenty of Room at the Bottom Jeffrey S. Erickson, Nastaran Hashemi, James M. Sullivan, Alan D. Weidemann, and Frances S. Ligler* Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Code 6900, Washington, D.C. 20375-5438, United States WET Laboratories, Inc., Department of Research, 70 Dean Knauss Drive, Narragansett, Rhode Island 02882, United States Hydro-Optics, Sensors, and Systems Section, Naval Research Laboratory, Code 7333, Stennis Space Center, Mississippi 39529-5004, United States

Photoenhancement of bioluminescence capacity in natural and laboratory populations of the autotrophic dinoflagellate Ceratium fusus (Ehrenb.) Dujardin

Cultures of Ceratium fusus were grown at seven light intensities (5 to 400 μmol photons m−2 s−1 as photosynthetically active radiation (PAR)) in the laboratory. Measurements of bioluminescence capacity (BCAP), division rate, and cell size were made at each intensity and compared to similar data from natural populations. In cultures, BCAP increased by a factor of 5–10 with increasing light. Division rates ranged from 0 d−1 (5 μmol photons m−2 s−1) to about 0.16 d−1 (100 μmol photons m−2 s−1). In these cultures, BCAP was highly correlated with both the log of light (r2 = 0.89) and division rate (r2 = 0.69), suggesting that BCAP may be of use as an in-situ indicator of division rate. In both cultures and natural populations, BCAP was poorly correlated with cell size (r2 ≈ 0.3 and r2 ≈ 0.1 respectively). For populations from the Gulf of Maine ( ∼42°N, 69° W) and the Marine light-Mixed Layers (MLML) site (∼59°N, 21° W), division rates in the mixed-layer could only be determined with statistical certainty at the Gulf of Maine station (0.04 to 0.17 d−1). At the MLML site, BCAP in C. fusus was greater in the mixed-layer in May and below the mixed-layer in August. The August result is opposite to what culture photoenhancement studies would predict if light was the factor controlling BCAP.