Michael Ondrusek

Spatial and temporal variability of phytoplankton pigment distributions in the central equatorial Pacific Ocean

Abstract During the 1992 U.S. JGOFS Equatorial Pacific (EqPac) study, we participated in survey (12°N-12°S, along 135°–140°W) and time-series (0°, 140°W) cruises to identify the factors that control spatial and temporal variations in phytoplankton pigment biomass, taxonomic composition and size structure. To achieve this goal, we determined pigment marker distributions in conjunction with collaborative measurements performed by EqPac investigators. Distributions of phytoplankton pigments measured during early 1992 (El Nino conditions) were different from those measured during late 1992 (“normal” conditions). Most notably, the accessory pigment distributions revealed that the 1991–1992 El Nino event produced a significant reduction in eukaryotic phytoplankton biomass, especially that contributed by prymnesiophytes, pelagophytes and diatoms. This variability was most likely caused by a combination of physical (Kelvin waves, tropical instability waves and advection), chemical (iron limitation) and biological (growth-grazing imbalances) processes. The results of this pigment study underscore the need for sampling biological properties on the appropriate time and space scales, and the necessity of physical measurements for interpreting their distributions.

EVIDENCE A PHOTOPROTECTIVE FOR SECONDARY CAROTENOIDS OF SNOW ALGAE1

Snow algae occupy a unique habitat in high altitude and polar environments. These algae are often subject to extremes in nutrient availability, acidity, solar irradiance, desiccation, and ambient temperature. This report documents the accumulation of secondary carotenoids by snow algae in response to the availability of nitrogenous nutrients. Unusually large accumulations of astaxanthin esters in extra‐chloroplastic lipid globules produce the characteristics red pigmentation typical of some snow algae (e.g. Chlamydomonas nivalis (Bauer) Wille). Consequently these compounds greatly reduce the amount of light available for absorption by the light‐harvesting pigment‐protein complexes, thus potentially limiting photoinhibition and photodamage caused by intense solar radiation. The esterification of astaxnthin with fatty acids represents a possible mechanism by which this chromophore can be concentrated within cytoplasmic globules to maximize its photoprotective efficiency.

Iron deficiency and phytoplankton growth in the equatorial Pacific

Several experiments were conducted in the equatorial Pacific at 140°W during the Joint Global Ocean Flux Study, equatorial Pacific, 1992 Time-series I (TS-I, 23 March–9 April), Time-series II (TS-II, 2–20 October) and FeLINE II cruises (10 March–14 April), to investigate the effects of added Fe on phytoplankton communities. Seven series of deckboard iron-enrichment experiments were performed, with levels of added Fe ranging from 0.13 to 1000 nM. Time-course measurements included nutrients, chlorophyll a and HPLC pigments. Results of these experiments showed that subnanomolar (sub-nM) additions of Fe increased net community specific growth rates, with resultant chlorophyll a increases and nutrient decreases. Community growth rates followed Michaelis-Menten type kinetics resulting in maximum rates of 0.99 doublings per day and a half-saturation constant of 0.12 nM iron. The dominant group responding to iron enrichment was diatoms.

Influence of the Orinoco River outflow on distributions of algal pigments in the Caribbean Sea

Seasonal measurements of photosynthetic pigments were performed in the Caribbean Sea to investigate the potential effects of Orinoco River discharge on the vertical distributions of the major phytoplankton groups. Samples were collected during periods of low (April 1988) and high (September 1988) riverine input. The chlorophyll a concentration of the subsurface chlorophyll maximum (SCM) sampled at inshore and offshore stations displays only minor seasonal variations. However, the depth and pigment composition of the SCM during periods of low and high outflow are markedly different. A “two-layer” phytoplankton distribution pattern is observed for all stations occupied during the spring and those stations not influenced by Orinoco River discharge in the fall: an upper “light-adapted” population dominated by prokaryotic phytoplankton (cyanobacteria and possibly prochlorophyte-like phytoplankton) and a deeper “shade-adapted” population comprised of chromophytes and “green algae.” The depth of the SCM at these stations averaged 77 ± 12 m. By contrast, stations influenced by Orinoco River discharge during the fall where characterized by near-surface diatom-dominated communities and shallower SCM depths (39 ± 16 m). Nutrient loading, elevation of absorption and attenuation coefficients, and transport of coastal “seed” phytoplankton populations are possible mechanisms by which the Orinoco River modifies the composition and distribution of phytoplankton in the Caribbean Sea.

Assessing the Application of Cloud–Shadow Atmospheric Correction Algorithm on HICO

Several ocean color earth observation satellite sensors are presently collecting daily imagery, including the Hyperspectral Imager for the Coastal Ocean (HICO). HICO has been operating aboard the International Space Station since its installation on September 24, 2009. It provides high spatial resolution hyperspectral imagery optimized for the coastal ocean. Atmospheric correction, however, still remains a challenge for this sensor, particularly in optically complex coastal waters. In this paper, we assess the application of the cloud-shadow atmospheric correction approach on HICO data and validate the results with the in situ data. We also use multiple sets of cloud, shadow, and sunlit pixels to correct a single image multiple times and intercompare the results to assess variability in the retrieved reflectance spectra. Retrieved chlorophyll values from this intercomparison are similar and also agree well with the in situ chlorophyll measurements.

Radiance transmittance measured at the ocean surface

The radiance transmittance (Tr) is the ratio of the water-leaving radiance (Lw(0+)) to the sub-surface upwelling radiance (Lu(0-)), which is an important optical parameter for ocean optics and ocean color remote sensing. Historically, a constant value (~0.54) based on theoretical presumptions has been adopted for Tr and is widely used. This optical parameter, however, has never been measured in the aquatic environments. With a robust setup to measure both Lu(0-) and Lw(0+) simultaneously in the field, this study presents Tr in the zenith direction between 350 and 700 nm measured in a wide range of oceanic waters. It is found that the measured Tr values are generally consistent with the long-standing theoretical value of 0.54, with mean relative difference less than 10%. In particular, the agreement within the spectral domain of 400-600 nm is found to be the best (with the averaged difference less than 5%). The largest difference is observed for wavelengths longer than 600 nm with the average difference less than 15%, which is related to the generally very small values in both Lu(0-) and Lw(0+) and rough environmental conditions. These results provide a validation of the setup for simultaneous measurements of upwelling radiance and water-leaving radiance and confidence in the theoretical Tr value used in ocean optics studies at least for oceanic waters.

Diurnal changes in ocean color in coastal waters

Coastal processes can change on hourly time scales in response to tides, winds and biological activity, which can influence the color of surface waters. These temporal and spatial ocean color changes require satellite validation for applications using bio-optical products to delineate diurnal processes. The diurnal color change and capability for satellite ocean color response were determined with in situ and satellite observations. Hourly variations in satellite ocean color are dependent on several properties which include: a) sensor characterization b) advection of water masses and c) diurnal response of biological and optical water properties. The in situ diurnal changes in ocean color in a dynamic turbid coastal region in the northern Gulf of Mexico were characterized using above water spectral radiometry from an AErosol RObotic NETwork (AERONET -WavCIS CSI-06) site that provides up to 8-10 observations per day (in 15-30 minute increments). These in situ diurnal changes were used to validate and quantify natural bio-optical fluctuations in satellite ocean color measurements. Satellite capability to detect changes in ocean color was characterized by using overlapping afternoon orbits of the VIIRS–NPP ocean color sensor within 100 minutes. Results show the capability of multiple satellite observations to monitor hourly color changes in dynamic coastal regions that are impacted by tides, re-suspension, and river plume dispersion. Hourly changes in satellite ocean color were validated with in situ observation on multiple occurrences during different times of the afternoon. Also, the spatial variability of VIIRS diurnal changes shows the occurrence and displacement of phytoplankton blooms and decay during the afternoon period. Results suggest that determining the temporal and spatial changes in a color / phytoplankton bloom from the morning to afternoon time period will require additional satellite coverage periods in the coastal zone.

Measurements of photophysiological parameters and primary production in the Central North Pacific Ocean

The North Pacific Central Gyre is one of the largest homogenous bodies of water on Earth. Phytoplankton distributions appear to remain relatively constant for thousands of kilometers throughout the year. However, recent studies conducted at Station ALOHA as part of the Hawaii Ocean Time-series (HOT) program reveal significant seasonal and interannual variability in phytoplankton biomass and production rates. Despite the high resolution sampling performed at the HOT site, spatial and temporal variations in phytoplankton pigment biomass are difficult to resolve. This will require remote sensing platforms such as moorings and satellites. In situ measurements of the photo- physiological parameters necessary to bio-optically model primary production rates are an essential element for the interpretation of data that will result from the HOT and MOBY moorings and the SeaWiFS and OCTS satellite sensors. We participated in a transect cruise in the North Pacific Ocean from Station ALOHA to the CLIMAX site to document the spatial variability of photo-physiological parameters and to determine if the conditions at the HOT site are representative of the central gyre and, in particular, are comparable to the CLIMAX site. We measured the light limited rate of photosynthesis ((alpha) ), the irradiance at which photosynthesis becomes light saturated, the maximum rate of photosynthesis (Pmax), the phytoplankton spectral absorption coefficient, and the maximum quantum yield of photosynthesis ((Phi) max). The photosynthetical parameters were similar at the HOT and CLIMAX locations, however a diatom bloom at intermediate stations resulted in a doubling of Pmax, (alpha) , and (Phi) max. If these variations in photosynthetic parameter estimates are not accounted for when modeling production rates for the diatom- dominated stations, then carbon uptake estimates would be underestimated by 2-fold. This study demonstrates the need for temporally dynamic algorithms that account for variations in phytoplankton composition and physiology.

Inter-satellite comparison and evaluation of Navy SNPP VIIRS and MODIS-Aqua ocean color properties

Navy operational ocean color products of inherent optical properties and radiances are evaluated for the Suomi–NPP VIIRS and MODIS-Aqua sensors. Statistical comparisons with shipboard measurements were determined in a wide variety of coastal, shelf and offshore locations in the Northern Gulf of Mexico during two cruises in 2013. Product consistency between MODIS-Aqua, nearing its end-of-life expectancy, and Suomi-NPP VIIRS is being evaluated for the Navy to retrieve accurate ocean color properties operationally from VIIRS in a variety of water types. Currently, the existence, accuracy and consistency of multiple ocean color sensors (VIIRS, MODIS-Aqua) provides multiple looks per day for monitoring the temporal and spatial variability of coastal waters. Consistent processing methods and algorithms are used in the Navy’s Automated Processing System (APS) for both sensors for this evaluation. The inherent optical properties from both sensors are derived using a coupled ocean-atmosphere NIR correction extending well into the bays and estuaries where high sediment and CDOM absorption dominate the optical signature. Coastal optical properties are more complex and vary from chlorophyll-dominated waters offshore. The in-water optical properties were derived using vicariously calibrated remote sensing reflectances and the Quasi Analytical Algorithm (QAA) to derive the Inherent Optical Properties (IOP’s). The Naval Research Laboratory (NRL) and the JPSS program have been actively engaged in calibration/validation activities for Visible Infrared Imager Radiometer Suite (VIIRS) ocean color products.

Remote sensing of normalized diffuse attenuation coefficient of downwelling irradiance

The diffuse attenuation of downwelling irradiance, Kd (m−1), is an important property related to light penetration and availability in aquatic ecosystems. The standard Kd(490) product (the diffuse attenuation coefficient at 490 nm) of the global oceans from satellite remote sensing has been produced with an empirical algorithm, which limits its reliability and applicability in coastal regions. More importantly, as an apparent optical property (AOP), Kd is a function of the angular distribution of the light field (e.g., solar zenith angle). The empirically derived product thus contains ambiguities when compared with in situ measurements as there is no specification regarding the corresponding solar zenith angle associated with this Kd(490) product. To overcome these shortcomings, we refined the Kd product with a product termed as the normalized diffuse attenuation coefficient (nKd, m−1), which is equivalent to the Kd in the absence of the atmosphere and with the sun at zenith. Models were developed to get nKd from both in situ measurements and ocean color remote sensing. Evaluations using field measurements indicated that the semi-analytically derived nKd product will not only remove the ambiguities when comparing Kd values of different light fields, but will also improve the quality of such a product, therefore maximizing the value offered by satellite ocean color remote sensing. This article is protected by copyright. All rights reserved.