The role of mixing in an Antarctic ocean ecosystem: observations and model computations of vertical distributions of related parameters

Observations during the Swedish–South African Antarctic marine expedition, SWEDARP 97/98, from December 1997 to February 1998 along longitude 6°E in the Southern Ocean demonstrate geographical differences in vertical distributions of chlorophyll-a and salts in three special study areas between latitude 61°S and 40°30′S. The aim of this work is to describe the functioning of the Antarctic ecosystem with emphasis on the inter-link between physical, chemical and biological processes in surface waters (0–200 m). Model calculations of fluxes of salts provide possible explanations of measured concentrations of vertical variations regarding salinity, chlorophyll-a and nitrate. The area of investigation is described by vertical mean gradients of salinity, temperature and nitrate for the upper layer (0–400 m) in every special study area together with chlorophyll-a for 0–100 m. The model includes physical processes in terms of vertical mixing and sinking velocity together with biological processes in terms of light-dependent phytoplankton growth, zooplankton grazing and regeneration. The vertical mixing is parameterised as an eddy diffusivity (Kz), assessed from salinity measurements. Both a depth constant Kz and a depth variable Kz are used. Model computations for a high Kz (2000–15 000 m2 day−1) in the upper layer, rapidly decreasing below 20–50 m and then a low and constant Kz (375–600 m2 day−1) down to 200 m depth supply the best reproduction of measured profiles. A subsurface maximum of chlorophyll-a can be formed only at very low Kz (50 m2 day−1). The irradiance quanta absorption coefficient is used together with the self-absorption of light of 0.06 times the mean chlorophyll-a in the water column. The specific algal gross primary production is calculated according to Steele (Limnol. Oceanogr. 7 (1962) 137). The specific loss rates are varied between 0.15 and 0.30 day−1 together with a regeneration factor of 0.3. The carbon:chlorophyll ratio ranges from 75 to 150, with the highest ratios in the Antarctic Polar Front area at around 51°S, where the integrated chlorophyll-a in the upper 200 m is highest (137 mg m−2). The sinking velocity of chlorophyll-a is judged to be 1 m day−1. sult of this work implies that the vertical mixing is a major limiting factor for the total amount of chlorophyll-a in the surface layer in this part of the Southern Ocean ecosystem on a shorter time scale. Changes of the vertical mixing, for example as an effect of climate change, will induce effects on the functioning of the ecosystem. For future oceanographic work in this area a lot of effort should be put into research with emphasis on losses of chlorophyll and an increased knowledge about vertical mixing.