Phytoplanktonic nutrient utilisation and nutrient signature in the Southern Ocean

The separation in Southern Ocean provinces of silicate excess at nitrate exhaustion and of nitrate excess at silicate exhaustion was already introduced by Kamykowski and Zentara (Kamykowski, D., Zentara, S.J., 1985. Nitrate and silicic acid in the world ocean: patterns and processes. Mar. Ecol. Prog. Ser. 26, 47–59; and Kamykowski, D., Zentara, S.J., 1989. Circumpolar plant nutrient covariation in the Southern Ocean: patterns and processes. Mar. Ecol. Prog. Ser. 58, 101–111) and our investigations of the silicate to nitrate uptake ratios confirm the earlier distinction. Oligotrophic antarctic waters mainly exhibit proportionally higher silicate removal what induces a potential for nitrate excess. The nitrogen uptake regime of such areas is characterised by low absolute as well as specific nitrate uptake rates throughout. Maximal values did not exceed 0.15 µM d-1 and 0.005 h-1, respectively. Corresponding f-ratios ranged from 0.39 to 0.86. This scenario contrasts strikingly to the more fertile ice edge areas. They showed a drastic but short vernal increase in nitrate uptake. Absolute uptake rates reached a maximum value of 2.18 µM d-1 whereas the maximal specific uptake rate was 0.063 h-1. In addition to an optimal physical environment for bloom development, accumulation of ammonium stimulated nitrate uptake in a direct or indirect way. Since ammonium build-up in surface waters traces enhanced remineralisation, release of other essential compounds during degradation of organic matter might have been the main trigger. This peak nitrate utilisation during early spring led to the observed potential for silicate excess. With increasing seasonal maturity the nitrate uptake became inhibited by the presence of enhanced ammonium availability (up to 8% of the inorganic nitrogen pool), however, and after a short period of intensive nitrate consumption the uptake rates drop to very low levels, which are comparable to the ones observed in the area of nitrate excess at silicate exhaustion

The taxonomic identity of the cosmopolitan prymnesiophyte Phaeocystis: a morphological and ecophysiological approach

Phaeocystis species diversity has been reviewed by comparing the morphological and physiological characteristics of Phaeocystis cells and colonies of different geographical origin. These analyses gave evidence for four Phaeocystis species: P. globosa, P. scrobiculata, P. pouchetii and one undefined antarctic species, distinguishing themselves by colony and single cell morphology and temperature tolerance. Typical colonial shape constitutes the most apparent morphological characteristics distinguishing P. pouchetii from P. globosa. Differences between colonies referable to pouchetii and globosa can be confirmed on the basis of variation in temperature and light requirements, as well as morphological descriptions of palmelloid stages, e.g. colony shape and size, organisation of the cells inside the colonies. The most striking features of the motile single cell are the thread-like appendages, which are much longer than the cell itself, the organic scales covering the cells, varying in shape and size, the haptonema and the flagella. On this basis, previous Phaeocystis records were analysed and the geographical distribution of the genus reported. There was no evidence for strain specific elemental composition or photosynthesis or growth performance of cells and colonies. This indicates that more elaborate molecular and biochemical analyses are required to identify different species. Possible opportunities available through modern chemical and molecular biological advances are describedSCOPUS: ar.jinfo:eu-repo/semantics/publishe

Nitrogen uptake regime and phytoplankton community structure in the Atlantic and Indian sectors of the Southern Ocean

Phytoplankton nitrogen uptake is studied in relation to the biomass and structure of phytoplankton community in the Atlantic and Indian sectors of the Southern Ocean. Two scenarios of seasonal evolution of uptake regime and phytoplankton community structure are described. The first scenario includes the Marginal Ice Zone areas of the Weddell Sea and adjacent areas where a predominantly nitrate based, diatom dominated assemblage, thriving in a stable water column at the beginning of the season was transformed into a mainly ammonium based, flagellate dominated assemblage, towards the end of the season. The change in phytoplankton community structure was caused by selective grazing by large grazers and reduced stability of the water column and the shift in uptake regime was due to increased ammonium availability and changes in community structure. In the second scenario, in the Coastal and Continental Shelf Zone (CCSZ) and Open Oceanic Zone (OOZ) of the Indian sector, a shift in uptake regime occurred without a big change in phytoplankton community structure. These areas were sampled late in the growth season and were characterized by prolonged water column stability, less grazing pressure on large diatoms and high ammonium availability. Diatoms dominated the assemblage and about 80% phytoplankton biomass was on the > 10 μm size fraction. Unlike the first scenario, diatoms were largely based on ammonium. Thus, in areas of persistent water column stability and less selective grazing pressure, a shift in uptake regime can occur without change in community structure. The dominance of diatoms under regenerated production provides a physiological evidence for the excess net removal of silicate over nitrate occurring in certain provinces of the Southern Ocean.info:eu-repo/semantics/publishe