Nutrient anomalies in Fragilariopsis kerguelensis blooms, iron deficiency and the nitrate/phosphate ratio (A. C. Redfield) of the Antarctic Ocean

Abstract

During seasonal development of blooms in the Polar Frontal region, concentrations of nitrate and phosphate decreased in surface waters. In blooms of Fragilariopsis kerguelensis at the southern rim (49–50°S) of the Polar Frontal region the dissolved ratio NO 3 PO 4 increased from the winter value of ∼14 to 15.8 (18 October 1992) to as high as 25 (23 November 1992). Ambient dissolved Fe in these blooms was subnanomolar compared to ∼1.1–1.9 nM in the overall Polar Frontal region. Blooms more northerly in the Polar Frontal region were dominated by other diatoms and higher dissolved Fe (> 1 nM), and showed only very modest NO 3 PO 4 anomalies. From nutrient inventories the biogenic pools (PON and DON) and export of settling biogenic debris would have N P ratios as low as 4.4–6.1 compared to ∼14 in deep Antarctic waters. Such shifts are consistent with decreasing availability of Fe for nitrate reduction, but also may be due to intrinsically low N P in Fragilariopsis kerguelensis cells. Moreover, a low ratio DON DOP in dissolved organic matter and enhanced recycling of N versus P cannot be excluded either. Triplicate mesocosm (20 l) experiments were performed with a diatom-dominated community in ambient seawater (initial Fe = ∼0.9 nM) collected at the Polar Front during early spring. Three other triplicates were enriched with 2 nM Fe to total Fe ∼ 2.9 nM. During the incubations, the Fe-enriched experiments showed assimilation at near-perfect Redfield N P ratios of ∼15 and a virtually near-zero intercept. The untreated incubations showed significantly lower uptake ratios at ∼13 and non-zero intercepts, suggesting leftover nitrate after all phosphate was utilised. At initial Fe = ∼0.9 nM, the Fe-containing algal enzymes for reduction of nitrate appeared to be impaired, hence nitrate assimilation was less efficient. The observed N P fractionation in Fragilariopsis kerguelensis blooms at the Polar Frontal region, in combination with the local formation of AAIW flowing northward, might help maintain the lower N P ratio at ∼14 in Antarctic waters, as compared to a ∼15 as an average value for the other oceans. The functionality of Fe in C-fixation, nitrate assimilation as well as N 2 fixation may partly explain the large variability of the NO 3 PO 4 ratio in this and other ocean basins (Fanning, 1992; Journal of Geophysical Research, 97, 5693–5712), as well as recently reported variations in the extended C/N/P ratio.