The optical, chemical and biological characteristics of a cyclonic eddy were investigated in the Arabian Sea during November 1995. This eddy was 3,000 km2 in area and located 350 km offshore of the coast of Oman. The mixed layer of this feature extended to a depth of 17 m, below which oxygen concentrations were depleted to 10 percent of surface values. Chlorophyll a concentrations within the mixed layer averaged 1.7 mg m-3. Microscopic observations and flow cytometric measurements revealed that the algal community was dominated by the coccoid cyanobacterium Synechococcus. Detailed pigment analyses documented additional phytoplankton biomass contributions by Prochlorococcus, diatoms, dinoflagellates, pyrmnesiophytes, cryptophytes, chlorophytes and pelagophytes. This algal community caused a marked attenuation of the blue to bluegreen wavelengths of light, resulting in a preferential transmittance of green light with increasing depth. Measurements of photosynthetic performance and the spectral absorption coefficient document that the phytoplankton community was photo- and chromatically-adapted to the light environment within the eddy. The results of this field work support a previous laboratory study which found that the nonphotosynthetic carotenoid zeaxanthin produces significant decreases in the maximum quantum yield of photosynthesis of the marine cyanobacterium Synechococcus.
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.
Although nutrient-limited growth of phytoplankton in the laboratory produces well-defined responses in the absorption coefficient, pigmentation, and quantum yield of photosynthesis, natural populations show only weak or inconclusive effects. Further analysis of the data from Biowatt-II, a seasonal study in the NW Atlantic in 1987, suggests two things. First, the phytoplankton absorption coefficient changes as a function of chlorophyll (alpha) , although photoprotectant accessory pigments are important in summer. Second, with the exception of one cruise, the data suggest that an estimated maximum quantum yield varies by less than 20%. Resolution of the issue of a nutrient effect may come with detailed measurements in a tropical region, in an ocean province without seasonally varying irradiance but having a strong seasonal signal in nutrient concentrations.
Estimates of the in vivo specific absorption coefficients (m2 mg'; 400-750 nm, 2 nm intervals) for the major algal pigment groups (chlorophylls, carotenoids and phycobilins) are presented. "Unpackaged" absorption coefficients were initially obtained by measuring the absorption properties of pure pigment standards spectrophotometrically and "shifting" their absorption maxima to match in vivo positions. Two approaches for estimating the phytoplankton absorption coefficient (spectral reconstruction and spectral decomposition) are compared by linear regression analysis, incorporating concurrent measurements of particulate absorption and pigmentation performed in the Sargasso Sea. Results suggest that "pigment package" effects are minimal for natural assemblages of open-oceanic phytoplankton and that accessory pigments do not always co-vary with chlorophyll a over depth and time.