Phytoplankton pigments constitute many more compounds than chlorophyll a that can be applied to study phytoplankton diversity, populations, and primary production. In this study, field measurements were applied to develop ocean color satellite algorithms of phytoplankton pigments from in-water radiometry measurements. The match-up comparisons showed that the satellite-derived pigments from our algorithms agree reasonably well (e.g. 30-55% of uncertainty for SeaWiFS and 37-50% for MODIS-Aqua) to field data, with better agreement (e.g. 30-38% of uncertainty for SeaWiFS and 39-44% for MODIS-Aqua) for pigments abundant in diatoms. The seasonal and spatial variations of satellite-derived phytoplankton biomarker pigments, such as fucoxanthin, which is abundant in diatoms, peridinin, which is found only in peridinincontaining dinoflagellates, and zeaxanthin, which is primarily from cyanobacteria in coastal waters, revealed that higher densities of diatoms are more likely to occur on the inner shelf and during winter-spring and obscure other abundant phytoplankton groups. However, relatively higher densities of other phytoplankton, such as dinoflagellates and cyanobacteria, are likely to occur on the mid-to outer-continental shelf and during summer. Seasonal variation of riverine discharge may play an important role in stimulating algal blooms, in particular diatoms, while higher abundances of cyanobacteria coincide with warmer water temperatures and lower nutrient concentrations.

[1] At present, satellite remote sensing of coastal water quality and constituent concentration is subject to large errors as compared to the capability of satellite sensors in oceanic waters. In this study, field measurements collected on a series of cruises within United States southern Middle Atlantic Bight (SMAB) were applied to improve retrievals of satellite ocean color products from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectrometer (MODIS-Aqua) in order to examine the factors that regulate the bio-optical properties within the continental shelf waters of the SMAB. The first objective was to develop improvements in satellite retrievals of absorption coefficients of phytoplankton (a ph ), colored dissolved organic matter (CDOM) (a g ), nonpigmented particles (a d ), nonpigmented particles plus CDOM (a dg ), and chlorophyll a concentration ([Chl_a]). Several algorithms were compared to derive constituent absorption coefficients from remote sensing reflectance (R rs ) ratios. The validation match-ups showed that the mean absolute percent differences were typically <35%, although higher errors were found for a d retrievals. Seasonal and spatial variability of satellite-derived absorption coefficients and [Chl_a] was apparent and consistent with field data. CDOM is a major contributor to the bio-optical properties of the SMAB, accounting for 35-70% of total light absorption by particles plus CDOM at 443 nm, as compared to 30-45% for phytoplankton and 0-20% for nonpigmented particles. The overestimation of [Chl_a] from the operational satellite algorithms may be attributed to the strong CDOM absorption in this region. River discharge is important in controlling the bio-optical environment but cannot explain all of the regional and seasonal variability of biogeochemical constituents in the SMAB.

Streaks of elevated concentrations of surface chlorophyll a (Chl_a) of various spacing were found to be associated with internal waves in their transmission zone and dissipation zone in the summertime in the deep open northern South China Sea. At an anchored station in the dissipation zone north of the Dongsha Atoll with a water depth of ca. 600 m, undulations of the mixed layer depth with an amplitude of ca. 30 m and a periodicity of ca. 12 h were observed, and they were accompanied by similar undulation in the isotherm and isopleth of the nutrients. These observations are consistent with the enhancement of vertical mixing by internal waves and the resulting transfer of cold, nutrient-rich subsurface water to the surface mixed layer to fuel biological productivity. In the transmission zone and dissipation zone, respectively, the summertime (May-October) average sea surface temperature was 0.5 and 0.8°C lower and Chl_a was 19 and 43 % higher than those in a nearby subregion that was minimally affected by internal waves. The mean net primary productivity was elevated by 15 and 37 %. These results indicate that the enhancement of biological activity by internal waves is not confined to the shallow waters on the shelf. The effect can be detected in all phases of the internal waves although it may be especially prominent in the dissipation zone where mixing between subsurface and surface waters is more effective.

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