John E. O'Reilly

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.

Role of satellites in estimating primary productivity on the northwest Atlantic continental shelf

Abstract Variance in primary productivity over the northwest Atlantic continental shelf has been studied to determine its dependence on variables amenable to satellite remote sensing. A large data set consisting of 14C uptake rates, chlorophyll concentrations, incident and underwater light (PAR) and sea surface temperatures was analysed for empirical and theoretical relationships. These data were obtained at 1047 stations surveyed by the National Marine Fisheries Service between 1977 and 1982 as a part of the MARMAP program. Empirically, we found a well-defined seasonal pattern in the ratio of integral productivity to surface layer chlorophyll; its annual maximum occurred in August, lagging the maximum insolation by about 30 days, and coinciding with maximum surface water temperature. The annual minimum occurred during December and January. Surface chlorophyll was highly correlated with mean euphotic chlorophyll concentration (r2 = 0.93) and moderately correlated with mean euphotic productivity (r2 = 0.62). Theoretical models of the dependence of integral productivity on chlorophyll and light were considered. It was found that parameters in such models can be highly variable, in particular those that characterize light absorption characteristics of the phytoplankton. We conclude that the goal of estimating primary productivity from satellite measurements requires improved techniques for measuring and/or modeling the PAR absorbed by photosynthetic organisms.

Accelerated Warming and Emergent Trends in Fisheries Biomass Yields of the World's Large Marine Ecosystems

Abstract Information on the effects of global climate change on trends in global fisheries biomass yields has been limited in spatial and temporal scale. Results are presented of a global study of the impact of sea surface temperature (SST) changes over the last 25 years on the fisheries yields of 63 large marine ecosystems (LMEs) that annually produce 80% of the worlds marine fisheries catches. Warming trends were observed in 61 LMEs around the globe. In 18 of the LMEs, rates of SST warming were two to four times faster during the past 25 years than the globally averaged rates of SST warming reported by the Intergovernmental Panel on Climate Change in 2007. Effects of warming on fisheries biomass yields were greatest in the fast-warming northern Northeast Atlantic LMEs, where increasing trends in fisheries biomass yields were related to zooplankton biomass increases. In contrast, fisheries biomass yields of LMEs in the fast-warming, more southerly reaches of the Northeast Atlantic were declining in response to decreases in zooplankton abundance. The LMEs around the margins of the Indian Ocean, where SSTs were among the worlds slowest warming, revealed a consistent pattern of fisheries biomass increases during the past 25 years, driven principally by human need for food security from fisheries resources. As a precautionary approach toward more sustainable fisheries utilization, management measures to limit the total allowable catch through a cap-and-sustain approach are suggested for the developing nations recently fishing heavily on resources of the Agulhas Current, Somali Current, Arabian Sea, and Bay of Bengal LMEs.

NOAA-NASA Coastal Zone Color Scanner Reanalysis Effort

Satellite observations of global ocean chlorophyll span more than two decades. However, incompatibilities between processing algorithms prevent us from quantifying natural variability. We applied a comprehensive reanalysis to the Coastal Zone Color Scanner (CZCS) archive, called the National Oceanic and Atmospheric Administration and National Aeronautics and Space Administration (NOAA-NASA) CZCS reanalysis (NCR) effort. NCR consisted of (1) algorithm improvement (AI), where CZCS processing algorithms were improved with modernized atmospheric correction and bio-optical algorithms and (2) blending where in situ data were incorporated into the CZCS AI to minimize residual errors. Global spatial and seasonal patterns of NCR chlorophyll indicated remarkable correspondence with modern sensors, suggesting compatibility. The NCR permits quantitative analyses of interannual and interdecadal trends in global ocean chlorophyll.

Satellite-derived estimates of primary production on the northwest Atlantic continental shelf

Abstract Synoptic estimation of rates of primary production from remote observation of ocean color is essential for determination of the oceans role in global carbon cycles. Models having a physiological and optical basis have been developed to predict water-column primary production, normalized to water-column biomass, as a linear function of sea-surface irradiance. Such a model is applied using 41 successive images of sea-surface chlorophyll concentration derived from the Coastal Zone Color Scanner (CZCS) and covering the northeast coast of the U.S. and Canada in the spring of 1979. The CZCS level III data set for the northwest Atlantic continental shelf includes estimates of surface chlorophyll concentration and percent cloud cover on a 10 km grid. Linear regression techniques are used to calibrate the CZCS chlorophyll estimates with ship measurements and to convert percent cloud cover into sea-surface irradiance; such satellite-derived irradiance estimates are significant predictors of irradiances recorded at the earths surface. The satellite-derived primary production values have been composited as monthly images. The enhanced production of the spring bloom is evident in areas corresponding to major fisheries such as Georges Bank and offshore Yarmouth, Nova Scotia, while the lower production in the Gulf Stream and an associated warm-core ring can be seen farther to the south. Bottom topography seems to have a major influence on rates of primary production, particularly in waters shallower than 100 m. The relative error estimated for these calculations of primary production is approximately 75%.

Determination of optimum aerosol optical thickness ratios for atmospheric correction of coastal zone color scanner data in the Georges Bank-Gulf of Maine Region

Abstract The influence of varying aerosol optical thickness ratios (AOTR) on the precision and accuracy of satellite-derived CZCS pigment was evaluated using 92 full-resolution CZCS scenes collected along the U.S. northeast coast (NEC) from 1979 to 1986. CZCS pigment was estimated using the standard CZCS two channel bio-optical algorithm and seven sets of AOTR values including individual ratios computed interactively for each NEC image using the “clear water” radiance technique (INDIVID); means of the individual NEC ratios (NECAVE, N = 92); means of individual ratios computed from Middle Atlantic Bight (MAB) and NEC scenes (NECMAB, N = 276); means of individual ratios computed from South Atlantic Bight (SAB), MAB and NEC scenes (ECOAST, N = 1091); “default” values for global marine aerosols (GLOBAL); and time-varying ratios derived from weak annual signals present in the individual ratios used to compute NECMAB and ECOAST means. Precision was estimated using geometric mean CZCS pigment values computed from the seven sets of images for three 70 × 70 km sub-domains located in the western Gulf of Maine, on southern Georges Bank and over the continental slope. The smallest root mean square difference (RMSD) for geometric mean CZCS pigment ( 0.1 log10 units) was obtained between images processed using GLOBAL and INDIVID ratios. Accuracy was estimated using geometric mean CZCS pigment values computed from the seven sets of images and a sea truth data set. After removal of suspect CZCS pigment values, agreement between CZCS pigment and sea truth data was highest using ECOAST mean ratios and a spatial window diameter (temporal window) of 7 km (±0.5 days) for N = 546, displaying RMSD (log10 units), linear regression slope and correlation coefficient of 0.19, 1.00 and 0.88, respectively. ECOAST “mean” AOTR values resulted in a superior atmospheric correction for computation of CZCS pigment values from the Georges Bank-Gulf of Maine region when compared with values computed using “default” global or individual “interactively determined” ratios. ECOAST mean AOTR values should provide a consistent regional atmospheric correction for all CZCS scenes from the Georges Bank-Gulf of Maine region, including scenes which lack low pigment water and therefore cannot make use of the “clear-water” radiance technique.