HIGH-RESOLUTION STUDY OF THE PLATELET ICE ECOSYSTEM IN MCMURDO SOUND, ANTARCTICA - BIOMASS, NUTRIENT, AND PRODUCTION PROFILES WITHIN A DENSE MICROALGAL BLOOM

Vertical distributions of inorganic nutrients (nitrate, nitrite, ammonium, phosphate, and silicic acid) and microalgal and bacterial biomass were monitored within both the lower congelation ice and the platelet ice layer in McMurdo Sound, Antarctica from 31 October to 3 December 1989. Profiles of dissolved organic matter (DOM) (amino acids and simple sugars) and heterotrophic protists were also obtained in early December. Microalgal standing crop increased from 9.4 to 37.4 g C m(-2) during our 34 d study (peak accumulation rate of >1 g C m(-2) d(-1)), and maximum pigment concentrations exceeded 6500 mg chl a m(-3) near the congelation ice/platelet ice interface. Estimates of seawater exchange within the platelet ice ranged from 0.06 to 0.61 m(2) d(-1) when Si(OH)(4) was used as a tracer. The biochemical composition of the autotrophic community and the presence of high nutrient concentrations within the platelet layer indicated that the growth of platelet ice algae was limited by Light, rather than nutrients, throughout the season. NO3 and Si(OH)(4) concentrations in the platelet ice were generally high (30 to 32 and 65 to 82 mmol m(-3), respectively), but became depleted in the uppermost layers as chl a peaked late in November. NH4 was far more abundant in the platelet ice (5 to 10 mmol m(-3)) than in the underlying water column (0.5 mmol m(-3)) throughout the field season. Microbial regenera- tion was primarily responsible for the high background concentrations of NH4 in the platelet ice, although physical processes also may have contributed. A large NH4 pulse (>150 mmol m(-3)) was observed in the platelet layer in early November, possibly attributable to an isolated macrozooplankton grazing and excretion event. PO4 concentrations (2.0 mmol m(-3) in the water column) were minimal (<1.7 mmol m(-3)) in the upper platelet ice on November 11, but began to increase thereafter due to PO4 regeneration. By early December, concentrations reached their maximum of 3.73 mmol m(-3). Rates of PO4 regeneration were highest (peak of 35 mmol m(-3) d(-1)) in the upper layers of the platelet ice where the bulk of the microbial biomass was located. The platelet ice was unusual because, although concentrations of regenerated nutrients were often high, the elemental composition of particulates remained near the Redfield ratios and detrital abundance was low. The coincidence of NO2 and NO3 peaks at a depth of 0.65 to 0.75 m suggests that bacterial nitrification may also occur in the platelet ice.