This project aimed to characterise the organic speciation of iron in various oceanic waters associated with the New Zealand marine environment, and to examine the possible production of natural organic Fe-binding ligands by the Southern Ocean phytoplankton Phaeocystis antarctica. The location of New Zealand at the edge of the Southern Ocean provides a natural laboratory for studying ocean processes in a variety of oceanic conditions.
A time series of a surface transect across the Otago Continental Shelf was undertaken between April 2002 and January 2005 to measure the organic complexation of iron, hydrographic parameters and macronutrient concentrations. The study area contains three distinct water masses: 1) neritic water; 2) the Southland Current, derived from the Subtropical Surface Water (STW); 3) Subantarctic Surface Water (SASW). Hydrological measurements outlined the positions of the three water masses as being fixed within predictable boundaries. Variations in nutrient concentrations in the study area indicated that SASW is the predominant source of nitrate and phosphate to the shelf. Dissolved iron concentrations dropped sharply seaward from several nanomolar to sub-nanomolar levels. The dissolved iron was fully complexed with strong organic ligands in all three water masses, and the ligand concentrations also showed a slightly seaward decreasing trend. Trends in dissolved iron and the iron-binding ligand concentrations related to season were only obvious in neritic waters. Concentration maxima occurred during late spring and summer months, and concentration minima occurred in the middle of each year (winter months). Dissolved iron concentration was low (~0.1 nM) in SASW year round. Data from the present study are in support of that the SASW as a whole is classified as a high nitrate low chlorophyll (HNLC) water body and has an iron-limited phytoplankton population.
East of the New Zealand landmass, the Subtropical Convergence (STC) is topographically locked to the Chatham Rise. This is a dynamic region of enhanced primary production (Bradford-Grieve et al., 1997), which separates macronutrient-depleted STW from macronutrient-replete SASW. Dissolved iron concentrations were low (~0.1-0.2 nM) in SASW, while elevated dissolved iron concentrations were observed at the north flank of the Chatham Rise. The iron data imply that the regional currents may be an important vehicle for transporting the elevated iron across the front. Total dissolved iron-binding ligand concentrations were consistently higher (~0.5 nM) in the STW and STC waters than in SASW. The discrepancy in the ligand concentrations between STW and SASW may reflect a different contribution to the ligand pool from the local planktonic community.
The organic complexation of iron in the oligotrophic subtropical water columns in the Tasman Sea was also studied and comparison of waters to the north and south of the Tasman front were reported. The iron speciation data imply the potential biological origin for the iron-binding ligands, and the difference in ligand concentrations across the Tasman Front may represent slight differences in algal biomass.
A limited investigation to examine the production of natural organic iron-binding ligands by the Southern Ocean phytoplankton Phaeocystis antarctica was undertaken in laboratory culture experiments. Release of nanomolar levels of a strong iron-binding ligand was detected by cathodic stripping voltammetry (CSV) even under relatively high iron concentrations (> 1nM). The estimated iron binding strength for the ligands was similar to those observed in the open ocean. Moreover, the kinetic data suggested the presence of another weaker ligand class, which had a higher Fe concentration (>2 nM) and was not detectable by the CSV ligand titration technique.
Our field observations and culture experiment results suggest that the Fe-binding ligands are biologically produced. It appears algae engineer their environment to make Fe more available/accessible for longer by producing these ligands. Therefore, the organic speciation of Fe plays a very important role in the sustained nutrition of ocean primary productivity and must be accounted for in geochemical modelling.
| Identifer | oai:union.ndltd.org:ADTP/217438 |
| Date | January 2006 |
| Creators | Tian, Feng, n/a |
| Publisher | University of Otago. Department of Chemistry |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Feng Tian |
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