Phytoplankton biomass and photosynthetic competency in the summertime Mertz Glacier Region of East Antarctica

Vertical profiles of water temperature, salinity, beam transmission, density, pressure, wind speed, wind direction, phytoplankton biomass (chlorophyll a (Chl a) plus phaeophytin a), and photosynthetic competency (by fast repetition-rate fluorometry) are presented for the Mertz Glacier region, East Antarctica, for a 3-week period during the austral summer 2000–2001. Injection of low-salinity water from the melting of the ice pack formed a shallow (ca. 25 m) mixed layer offshore. Two distinct deep mixing features were observed associated to varying degrees with high chlorophyll levels and high photosynthetic competency. Along the Adelie Land coast, a deep mixing layer (1400 m) with Chl a concentrations of 386 mg m−2 and elevated Fv/Fm (>0.5) was observed. At the eastern end of the study area, along a seaward extension of fast-ice, a bloom of Phaeocystis antarctica bloom formed within a shallow (24 m) mixed layer, with surface Chl a concentrations of ca. 8 mg m−3 and elevated Fv/Fm (0.5). This feature also had high surface salinity (>34) and was contiguous with a deep-mixing feature (236 m). FRRF parameters Fv/Fm and σPSII were strongly suppressed in surface waters (<20 m). Results of photoinhibition experiments showed that the photo-suppression of Fv/Fm and σPSII in surface waters was reversible over a time period of minutes to hours and suggested that the surface suppression was due to non-photochemical quenching by photo-protective pigments and photo-damage to Photosytem II reaction centers and pigment beds. FRRF data at a depth of 45–55 m were independent of irradiance, and provided a reasonable index of the nutrient status of the phytoplankton. The spatial coherence of plant biomass, photo-competency, high salinity and deep-mixing suggested that blooms of Phaeocystis and diatoms form in this region after physical disturbances result in mixing of nutrient-rich subsurface waters into the euphotic zone. The chlorophyll fluorescence and beam transmissometer profiles at the coastal diatom bloom indicated that much of this biomass was transported to shelf sediments by sinking or a downward advection and/or mixing mechanism. Possible physical mechanisms to account for this process are discussed, such as downward advection from sea-ice and brine formation, katabatic wind-induced mixing, and upwelling/downwelling, but insufficient data preclude the identification of the definitive causative mechanism. It is hypothesized that the coastal region in Adelie Land is a site of intense and frequent diatom blooms that transport large amounts of organic matter to depth. The presence of large accumulations of silicious ooze below the Mertz Polynya in the Adelie Depression suggests that this ‘biological pumping’ mechanism has been functioning for most of the Holocene.