Coastal eddies, frontal zones and microscale oceanographic features are now easily observable from satellite measurements of SST and Chl a. Enhancing the utility of these space-borne measurements for biological productivity, biogeochemical cycling and fisheries investigations will require novel bio-optical methods capable of providing information on the community structure, biomass and photo-physiology of phytoplankton associated on spatial scales that match these features. This study showcases high-resolution in-situ measurements of sea water hydrography (SeaBird CTD®), CDOM (WetLabs ALF®), phytoplankton functional types (PFTs, FlowCAM®), biomass (bbe Moldaenke AlgaeOnlineAnalyzer® and WetLabs ALF®) and phytoplankton photosynthetic competency (mini-FIRe) across microscale features encountered during a recent (Nov. 2014) cruise in support of NOAA's VIIRS ocean color satellite calibration and validation activities. When mapped against binned daily, Level 2 satellite images of Chl a, Kd490 and SST over the cruise period, these high-resolution in-situ data showed great correspondence with the satellite data, but more importantly allowed for identification of PFTs and water types associated with microscale features. Large assemblages of phytoplankton communities comprising of diatoms and diatom-diazotroph associations (DDAs), were found in mesohaline frontal zones. Despite their high biomass, these populations were characterized by low photosynthetic competency, indicative of a bloom at the end of its active growth possibly due to nitrogen depletion in the water. Other prominent PFTs such as Trichodesmium spp., Synechococcus spp. and cryptophytes, were also associated with specific water masses offering the promise and potential that ocean remote sensing reflectance bands when examined in the context of water types also measurable from space, could greatly enhance the utility of satellite measurements for biological oceanographic, carbon cycling and fisheries studies.
Particulate absorption (aP()) including phytoplankton (aPHY()) and non-algal particles (NAP) (aNAP()) were measured in southeastern Bering Sea during a cruise in July 2008. This study analyzes the aP() properties through in-situ and quasi analytical algorithm (QAA) derived ocean color satellite Medium Resolution Imaging spectrometer (MERIS) and Moderate resolution Imaging Spectroradiometer (MODIS) observations. We found that the aP() and aPHY() correlated well with chlorophyll-a and were lower as a function of chlorophyll-a as compared to low latitudes. The specific phytoplankton absorption (a*PHY()) showed more variability in the blue as compared to the red part of the spectrum indicating pigment packaging and/or change in pigment composition. The remote sensing reflectance (Rrs()) showed significant variability in spectral shape and magnitude which was consistent with the variable total absorption minus pure water absorption (aT-W()) spectra observed in the study area. Simple satellite retrieved Rrs() ratios were related to in-situ aPHY() and aDG() by applying an inverse power fit; Rrs(490)/Rrs(510) gave the best results for aPHY(443) and aDG(443) (R2 - 0.80 and 0.75) respectively. The match-ups of in-situ and MERIS retrieved aPHY() and NAP plus colored dissolved organic matter (aDG()) using QAA after log-transformation showed reasonable agreement with R2 of 0.71 and 0.61 and RMSE of 0.316 and 0.391 at 443 nm, respectively. Although the QAA derived aPHY() and aDG() from MERIS overestimated and underestimated, respectively the in-situ measurements at all wavelengths, the match-up analysis was encouraging.
Measurements of particulate absorption, namely absorption by phytoplankton and non-algal particles (NAP) are
important components in bio-optical models; only a few studies have been reported for the southeastern Bering Sea. This
study analyzes variability in spectral particulate absorption (aP(λ)) including phytoplankton (aPHY(λ)) and NAP
absorption (aNAP(λ)) from in-situ data in conjunction with ocean color satellite data (MODIS - Moderate Resolution
Imaging Spectroradiometer) along four transects in the southeastern Bering Sea shelf during a cruise in July 2008.
Results obtained indicate that surface aPHY(λ) at 443 nm is higher in middle shelf near the Pribilof Islands with aNAP(λ)
decreasing from north to south across the shelf. Greater than 90% of variability in aP(λ) could be explained by aPHY(λ)
indicating biogenic matter dominates changes in particulate absorption. Good correlations were found between aP(λ),
aPHY(λ) at 443 nm and chlorophyll-a (R2 = 0.65 and 0.80, respectively). aPHY(λ) spectra were highly variable, with larger
variability in blue than red part of the spectrum, indicating change in pigment composition or package effect. MODIS
satellite derived aPHY(λ) using quasi-analytical algorithms (QAA) revealed patterns similar to in-situ absorption data for a
major part of the study area. Inconsistencies seen between in-situ absorption and QAA retrieved satellite absorption
could probably be attributed to temporal differences between in-situ data collection and satellite overpass.
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