Spatial and temporal distribution of phytoplankton and primary production in the western Bransfield Strait region

Abstract

Studies on phytoplankton were one component of the multi-disciplinary RACER program which had 69 stations within a 100 × 250-km rectangle in the southwestern Bransfield Strait and contiguous waters. Data were acquired during eight cruises between December 1986 and March 1987. All deep stations north of the continental shelf break were low in phytoplankton biomass (<41 mg Chl a m −2) and in rates of primary production (mean of 0.34 g C m −2 day −1) as compared to stations in continental shelf waters. Phytoplankton biomass exceeded 700 mg Chl a m −2 at some stations in shelf waters, with rates of primary production in December exceeding 3.0 g C m −2 day −1. The mean rate of primary production at the shelf stations for the 4-month period was 1.05 g C m −2 day −1. Greatest phytoplankton biomass was found at stations in Gerlache Strait and in nearby Bransfield Strait. Dramatic seasonality in phytoplankton crop size was observed, as massive blooms during December to January declined abruptly to low levels in February to March. As the decrease in phytoplankton biomass was much more abrupt than the corresponding decrease in incident solar irradiation, light does not appear to be the major factor involved in decline of the bloom. Except for stations in Gerlache Strait, nutrient levels remained sufficiently high (> 10 μM inorganic nitrogen) that nutrient depletion is not likely to have caused the rapid decline of the phytoplankton bloom. Grazing, sinking and advection all appear to be important mechanisms of massive bloom decline. Phytoplankton populations appeared to be low-light adapted, as they showed low P max values (1.1 mg C mg Chl a −1 h −1), low saturating ligh values ( I k ∼ 18 μEins m −2 s −1), high initial slope [ α = 0.06 (mg C mg Chl a −1h −1/ μEin m −2 s −1)] and a compensation point for net light-activated fixation of CO 2 of ≈ 1.0 μEin m −2 s −1.