| Abstrakt: |
In July 1985, diurnal patterns of photosynthesis and pigmentation were characterized for whole water (>0.4 μm) and size-fractioned (>5 μm and 0.4 to 5 μm) communities from three light depths sampled across a coastal thermal front in the Southern California Bight. Samples were collected predawn and held for 20 h in deck incubators. Variations in chlorophyll a and accessory pigment-to-chlorophyll a ratios showed no obvious diurnal trends. Timing of peak photosynthetic potential ( P) and its coincidence with variations in light-limited rates of photosynthesis (alpha), as well as diurnal amplitudes in P and alpha, often differed between size fractions sampled within the same community. The same was true for identical size fractions collected from different depths and stations transecting the front. Primary productivity was 20-fold greater on the cold water side, where >5 μm diatoms dominated the mixed layer and accounted for 80% of daytime productivity. Diatoms collected from the top and bottom of the upper mixed layer displayed nearly identical diurnal patterns in P and alpha, with midday peaks exceeding predawn values by four-fold and two-fold respectively. Above the pycnocline, the 0.4 to 5 μm fraction had lower assimilation rates than the >5 μm fraction and smaller diurnal amplitudes in P and/or alpha, with daytime patterns often characterized by two peaks interspersed by a short period of photoinhibition. Within the front, the 0.4 to 5 μm fraction accounted for two-thirds of plant biomass and >90% of primary production. Pigment analyses by high-performance liquid chromatography revealed enrichment in 19′-hexanoyloxyfucoxanthin, indicative of enhanced numbers of prymnesiophtes. Photosynthetic activity in confined surface communities was susceptible to daytime photoinhibition, but subsurface communities exhibited midday P peaks that were three-to seven-fold predawn values. In the warm-water mass, both algal size fractions contributed equally to photosynthesis and chlorophyll a in surface waters, with the 0.4 to 5 μm fraction becoming dominant at the base of the euphotic zone. At all depths, peak P of the 0.4 to 5 μm fraction occurred before noon, while P of the >5 μm fraction was clearly evident in the afternoon. Elevated chlorophyll b-, 19′hexanoyloxyfucoxanthin- and zeaxanthin-to-chlorophyll a ratios indicated a mixture of algal groups, including chlorophytes, cyanobacteria and prymnesiophytes. [ABSTRACT FROM AUTHOR] |
Full text from SpringerLink