B. Greg Mitchell

Ocean color chlorophyll algorithms for SeaWiFS

A large data set containing coincident in situ chlorophyll and remote sensing reflectance measurements was used to evaluate the accuracy, precision, and suitability of a wide variety of ocean color chlorophyll algorithms for use by SeaWiFS (Sea-viewing Wide Field-of-view Sensor). The radiance-chlorophyll data were assembled from various sources during the SeaWiFS Bio-optical Algorithm Mini-Workshop (SeaBAM) and is composed of 919 stations encompassing chlorophyll concentrations between 0.019 and 32.79 μg L−1. Most of the observations are from Case I nonpolar waters, and ∼20 observations are from more turbid coastal waters. A variety of statistical and graphical criteria were used to evaluate the performances of 2 semianalytic and 15 empirical chlorophyll/pigment algorithms subjected to the SeaBAM data. The empirical algorithms generally performed better than the semianalytic. Cubic polynomial formulations were generally superior to other kinds of equations. Empirical algorithms with increasing complexity (number of coefficients and wavebands), were calibrated to the SeaBAM data, and evaluated to illustrate the relative merits of different formulations. The ocean chlorophyll 2 algorithm (OC2), a modified cubic polynomial (MCP) function which uses Rrs490/Rrs555, well simulates the sigmoidal pattern evident between log-transformed radiance ratios and chlorophyll, and has been chosen as the at-launch SeaWiFS operational chlorophyll a algorithm. Improved performance was obtained using the ocean chlorophyll 4 algorithm (OC4), a four-band (443, 490, 510, 555 nm), maximum band ratio formulation. This maximum band ratio (MBR) is a new approach in empirical ocean color algorithms and has the potential advantage of maintaining the highest possible satellite sensor signal: noise ratio over a 3-orders-of-magnitude range in chlorophyll concentration.

Massive phytoplankton blooms under Arctic Sea ice

In midsummer, diatoms have taken advantage of thinning ice cover to feed in nutrient-rich waters. Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

Observations of modeling of the Antartic phytoplankton crop in relation to mixing depth

The multi-disciplinary program RACER (Research on Antarctic Coastal Ecosystem Rates) conducted eight surveys of a 69-station grid in a 100 × 250 km area in the southwestern Bransfield Strait from December 1986 to March 1987. Mean phytoplankton crop size in the upper 50 m during December, January, February and March was 291, 176, 58 and 50 mg Chl α m−2, respectively, and was inversely proportional to the increasing mean depth of the upper mixed layer (UML) (15, 17, 26 and 30 m, respectively). Massive mid-summer phytoplankton blooms (> 10 mg Chl α + phaeo m−3) were persistent nearshore where we observed shallow UMLs ( 20 m) with small density gradients (Δσt 0.05–0.20 from 0 to 75 m). Proximity to stabilizing meltwater and protection from intense Antarctic storm activity appear to be essential for the development of persistent massive blooms. A model of Antarctic phytoplankton growth based on mixing depth and pigment-specific light attenuation and in situ photosynthesis-irradiance relationships indicates that the depth of the UML (ZUML) can be used to predict the upper limit of the phytoplankton crop size. Observed phytoplankton biomass for diverse Southern Ocean ecosystems is discussed in relation to the mean light level of the UML, growth and loss rates of Antarctic phytoplankton, and the depth and duration of stratification required before a bloom ensues. Assuming nutrients do not limit the crop size, a best-fit to observations indicates specific loss rates must be approximately 0.3–0.35 day−1 and massive blooms occur only if ZUML <25 m. The grazing component of this predicted loss rate is higher than previously estimated. We conclude that grazing rates are greater than previously reported, or vertical flux rates of nutrients limit massive blooms.

Chlorophyll α specific absorption and fluorescence excitation spectra for light-limited phytoplankton

Methods are described for the measurement of spectral absorption coefficients, fluorescence excitation, and fluorescence yields for pigmented particles retained on filters. The corrections required for absorption coefficients include determining increased optical pathlength while corrections for fluorescence include determining system spectral variability, mean light level and reabsorption. The empirical technique is consistent with and validated by theoretical relationships for light transmission and fluorescence of absorbing particulate material embedded in a medium with intense scattering.These methods were applied to a study of photoadaptation in several phytoplankton species and revealed variations in the blue for chlorophyll α specific absorption [αph*(λ)] and fluorescence excitation [F*(λ)] of greater than 3− and 10-fold, respectively. Variations in the spectral shapes and the magnitude of αph*(λ) and F*(λ) with photoadaptation are determined largely by the effect of pigment absorption in discrete particles, sometimes referred to as the sieve or package effect. A model is presented expressing F*(λ) in terms of α*(λ) which predicts large variability in F*(λ) due to cell size and cellular pigmentation and which may help reconcile the previously reported, but unexplained variations in F*(λ). Spectral variations in the fluorescence yield appear to be caused by variations in the fraction of light absorbed by photosystem II which fluoresces as compared to photosystem I or photoprotective pigments which do not fluoresce. The techniques presented provide a rapid, reproducible, and simple approach for routine analysis, particularly for field applications where particle densities are too low for direct analysis of absorption spectra.

Spatial and temporal distribution of phytoplankton and primary production in the western Bransfield Strait region

Abstract Studies on phytoplankton were one component of the multi-disciplinary RACER program which had 69 stations within a 100 × 250-km rectangle in the southwestern Bransfield Strait and contiguous waters. Data were acquired during eight cruises between December 1986 and March 1987. All deep stations north of the continental shelf break were low in phytoplankton biomass ( 10 μM inorganic nitrogen) that nutrient depletion is not likely to have caused the rapid decline of the phytoplankton bloom. Grazing, sinking and advection all appear to be important mechanisms of massive bloom decline. Phytoplankton populations appeared to be low-light adapted, as they showed low Pmax values (1.1 mg C mg Chl a−1 h−1), low saturating ligh values (Ik ∼ 18 μEins m−2 s−1), high initial slope [α = 0.06 (mg C mg Chl a−1h−1/μEin m−2 s−1)] and a compensation point for net light-activated fixation of CO2 of ≈ 1.0 μEin m−2 s−1.

Bio-optical properties of Antarctic Peninsula waters: differentiation from temperate ocean models

Abstract An extensive biological and optical data set was collected during a 4 month cruise as part of the Research on Antarctic Coastal Ecosystem Rates (RACER) program conducted in coastal waters of the Antarctic Peninsula and adjacent open ocean waters of Drake Passage. Chlorophyll plus phaeopigment (Chl + Phaeo) concentration in the upper mixed layer ranged 2 orders of magnitude from 0.5 to 50 mg Chl + Phaeo m −3 during the study. The large variations in pigment corresponds to variations in the beam attenuation coefficient at 660 nm ( c t ranging from 0.5 to >2.5 m −1 and in the diffuse attenuation coefficient ( k d ) for 441 nm ranging from 0.04 to >1.0 m −1 . Chl + Phaeo specific particulate beam attenuation and spectral absorption coefficients suggest that detrital contributions are relatively low and that pigment package effects are relatively important compared to low latitude observations. The combination of these effects causes low pigment specific absorption and scattering. This regional differentiation in particulate optical properties has a significant effect on models of the relationship between Chl + Phaeo and spectral values of k d and upwelled radiance ( L u ). Implications of these effects for modeling light propagation through the water column and for remote sensing of phytoplankton pigments are discussed.

Pilot-scale data provide enhanced estimates of the life cycle energy and emissions profile of algae biofuels produced via hydrothermal liquefaction.

Life cycle assessment (LCA) has been used widely to estimate the environmental implications of deploying algae-to-energy systems even though no full-scale facilities have yet to be built. Here, data from a pilot-scale facility using hydrothermal liquefaction (HTL) is used to estimate the life cycle profiles at full scale. Three scenarios (lab-, pilot-, and full-scale) were defined to understand how development in the industry could impact its life cycle burdens. HTL-derived algae fuels were found to have lower greenhouse gas (GHG) emissions than petroleum fuels. Algae-derived gasoline had significantly lower GHG emissions than corn ethanol. Most algae-based fuels have an energy return on investment between 1 and 3, which is lower than petroleum biofuels. Sensitivity analyses reveal several areas in which improvements by algae bioenergy companies (e.g., biocrude yields, nutrient recycle) and by supporting industries (e.g., CO2 supply chains) could reduce the burdens of the industry.

Seasonal and nonseasonal variability of satellite‐derived chlorophyll and colored dissolved organic matter concentration in the California Current

Time series of surface chlorophyll a concentration (Chl) and colored dissolved organic matter (CDOM) derived from the Ocean Color and Temperature Sensor and Sea-Viewing Wide Field-of-View Sensor were evaluated for the California Current area using regional algorithms. Satellite data composited for 8-day periods provide the ability to describe large-scale changes in surface parameters. These changes are difficult to detect based on in situ observations alone that suffer from undersampling the large temporal and spatial variability, especially in Chl. We detected no significant bias in satellite Chl estimates compared with ship-based measurements. The variability in CDOM concentration was significantly smaller than that in Chl, both spatially and temporally. While being subject to large interannual and short-term variations, offshore waters (100–1000 km from the shore) have an annual cycle of Chl and CDOM with a maximum in winter-spring (December-March) and a minimum in late summer. For inshore waters the maximum is more likely in spring (April-May). We detect significant increase in both Chl and CDOM off central and southern California during the La Nina year of 1999. The trend of increasing Chl and CDOM from October 1996 to June 2000 is statistically significant in many areas.

A chlorophyll‐dependent semianalytical reflectance model derived from field measurements of absorption and backscattering coefficients within the Southern Ocean

A semianalytical model was developed for the prediction of spectral remote sensing reflectance (Rrs) as a function of fluorometric chlorophyll a concentration (Chl) for two regions within the Southern Ocean: the Ross Sea and the Antarctic Polar Front Zone (APFZ). The model is based upon Chl-dependent parameterizations of the spectral absorption, a(λ), and backscattering, bb(λ), coefficients of seawater which were derived from field measurements. The relationships between a(λ) and Chl were similar in both regions, but for comparable Chl the particulate backscattering was on average 4 times greater in the APFZ. Measurements of particle size distributions suggest that particle assemblages in the APFZ were characterized by a greater predominance of smaller particles, consistent with the observed regional differences in backscattering properties. The model is used to examine the separate influences of absorption and backscattering on the blue to green ratio of reflectance, Rrs(490)/Rrs(555). Variability in the spectral absorption ratio, resulting principally from changes in the relative contribution of water to total absorption in each band, contributes >75% to changes in the Rrs(490)/Rrs(555) ratio as a function of Chl. However, variability in the spectral backscattering ratio appears to be the primary cause for the observed differentiation in the Rrs versus Chl relationships between the two regions.

Light absorption by phytoplankton, photosynthetic pigments and detritus in the California Current System

Abstract Pigment-specific absorption by total particulates, detritus and phytoplankton was measured throughout the euphotic zone at > 275 stations on three cruises off California in late 1991 and early 1992. A new spectral fluorescence method for assessing photosynthetically active absorption in natural samples was developed and applied. Spatial variability in specific absorption coefficients at the mesoscale was found to be as high as previously observed between mid- and high latitudes, while differences between cruises were very low. In surface waters, the highest values of specific absorption were found in warm, low-pigment surface waters offshore and in the Southern California Bight. Vertical sections reveal that low values occur near the surface only where the pycnocline and nitracline slope toward the sea surface. The highest values of phytoplankton specific absorption occurred at shallow optical depths for stations with deep nitraclines, whereas the lowest values always occurred close to or below the depth of the nitracline. Specific absorption generally increased with increasing temperature, but there were large differences in the relationships between cruises. In the context of previous laboratory observations, these results imply that nutrient availability plays a greater role than direct temperature effects in controlling natural variance in phytoplankton specific absorption. Specific absorption of photosynthetically active phytoplankton pigments was found to be less variable than that of total phytoplankton and showed no systematic trends with temperature, optical depth, or distance from the nitracline. This result leads to a new version of a bio-optical model for primary production which is based only on the photosynthetically active component rather than total phytoplankton absorption.