Dissolved dimethylsulfide and its relationship with dimethylsulphfoniopropionate production, phytoplankton pigments, and grazing during the SOLAS-SAGE iron enrichment experiment in subantarctic waters southeast of New Zealand

Full description

Carbon dioxide levels continue to rise in the atmosphere with predictions of a 2-3 oC increase in temperature by the end of the century. One possible strategy to prevent a climate catastrophe is to use iron fertilisation of the ocean to stimulate phytoplankton blooms to capture carbon dioxide. Studies have shown that natural iron fertilization during past glacial periods has repeatedly drawn up to 60 billion tons of carbon into the ocean depths. However, most small-scale iron fertilization experiments have not been very successful in capturing carbon. There are many reasons for this but a major one is that it is not easy to stimulate growth of large-cell diatoms, which seem to be needed to fix large amounts of CO2. Another reason is that increasing iron can also increase microzooplankton grazing, so that as soon as the CO2 is fixed by phytoplankton it is released during the grazing of the phytoplankton. Such grazing activities also affect the production and consumption of dimethylsulfide (DMS), a trace sulfur gas produced by phytoplankton and implicated in formation of low levels clouds that lower sea surface temperatures. This communication describes processes that affected dissolved DMS during the SAGE (SOLAS air-sea gas experiment).

Gas chromatography was used to determine DMS and DMSP in seawater during the 3 week voyage in subantarctic waters east of NZ.

Dissolved dimethylsulfide (DMS), total and dissolved dimethylsulfoniopropionate (DMSPt, DMSPd) measurements were made from March-April 2004 during an iron enrichment experiment in subantarctic HNLC waters east of Dunedin, New Zealand. During the first two iron enrichments chl a and DMS production were constrained but during the third enrichment (Lagrangian days 9-11) large pulses of dissolved DMS occurred in the fertilised IN patch ranging from 0.81-5.7 nM (mean 2.38 nM, n = 115), and were significantly higher than measurements made in the unfertilised OUT patch (range 0.09-2.94 nM, mean 1 nM, n = 59). During the 3rd and 4th iron infusions total chl a concentrations doubled from 0.52-1.02 µg/L. Type 8 haptophytes (plus pelagophytes) > prasinophytes > chlorophytes >dinoflagellates > cyanobacteria > cryptophytes; with hapto8s and prasinophytes accounting for 50%, and 20%, respectively, of the total chl a. Prasinophytes and cyanobacteria increased slightly, whilst haptophytes decreased over the experiment. Diatoms were < 1% of chl a. DMSP and chl a were used to model the response of DMSP and chl a to iron addition. Modelling results suggest that DMSPt production seemed to increase between Lagrangian days 6-12, due to an increase in DMSPd production from increased grazing activities. DMSPt concentrations were more significantly (p<0.01) correlated with phytoplankton pigments fucoxanthin, 19´-hexanolyoxyfucoxanthin, peridinin, and β, β-carotene. Large pulses of dissolved DMS during the third iron enrichment coincided with increased microzooplankton grazing on phytoplankton, causing low DMSPp and high DMSPd concentrations which were converted to DMS by bacteria that constitute the microbial loop.

Gas chromatography was used to determine DMS and DMSP in seawater during the 3 week voyage in subantarctic waters east of NZ.

Dissolved dimethylsulfide (DMS), total and dissolved dimethylsulfoniopropionate (DMSPt, DMSPd) measurements were made from March-April 2004 during an iron enrichment experiment in subantarctic HNLC waters east of Dunedin, New Zealand. During the first two iron enrichments chl a and DMS production were constrained but during the third enrichment (Lagrangian days 9-11) large pulses of dissolved DMS occurred in the fertilised IN patch ranging from 0.81-5.7 nM (mean 2.38 nM, n = 115), and were significantly higher than measurements made in the unfertilised OUT patch (range 0.09-2.94 nM, mean 1 nM, n = 59). During the 3rd and 4th iron infusions total chl a concentrations doubled from 0.52-1.02 µg/L. Type 8 haptophytes (plus pelagophytes) > prasinophytes > chlorophytes >dinoflagellates > cyanobacteria > cryptophytes; with hapto8s and prasinophytes accounting for 50%, and 20%, respectively, of the total chl a. Prasinophytes and cyanobacteria increased slightly, whilst haptophytes decreased over the experiment. Diatoms were < 1% of chl a. DMSP and chl a were used to model the response of DMSP and chl a to iron addition. Modelling results suggest that DMSPt production seemed to increase between Lagrangian days 6-12, due to an increase in DMSPd production from increased grazing activities. DMSPt concentrations were more significantly (p<0.01) correlated with phytoplankton pigments fucoxanthin, 19´-hexanolyoxyfucoxanthin, peridinin, and β, β-carotene. Large pulses of dissolved DMS during the third iron enrichment coincided with increased microzooplankton grazing on phytoplankton, causing low DMSPp and high DMSPd concentrations which were converted to DMS by bacteria that constitute the microbial loop.