F. Brian Griffiths

Limitation of algal growth by iron deficiency in the Australian Subantarctic region

In March 1998 we measured iron in the upper water column and conducted iron- and nutrient-enrichment bottle-incubation experiments in the open-ocean Subantarctic region southwest of Tasmania, Australia. In the Subtropical Convergence Zone (∼42°S, 142°E), silicic acid concentrations were low (< 1.5 µM) in the upper water column, whereas pronounced vertical gradients in dissolved iron concentration (0.12-0.84 nM) were observed, presumably reflecting the interleaving of Subtropical and Subantarctic waters, and mineral aerosol input. Results of a bottle-incubation experiment performed at this location indicate that phytoplankton growth rates were limited by iron deficiency within the iron-poor layer of the euphotic zone. In the Subantarctic water mass (∼46.8°S, 142°E), low concentrations of dissolved iron (0.05-0.11 nM) and silicic acid (< 1 µM) were measured throughout the upper water column, and our experimental results indicate that algal growth was limited by iron deficiency. These observations suggest that availability of dissolved iron is a primary factor limiting phytoplankton growth over much of the Subantarctic Southern Ocean in the late summer and autumn.

A persistent subsurface chlorophyll maximum in the Interpolar Frontal Zone south of Australia: Seasonal progression and implications for phytoplankton-light-nutrient interactions

A subsurface chlorophyll maximum (SCM), composed principally of large diatoms, has been consistently observed in summer and autumn between 53° and 58°S along 140°E on Australian World Ocean Circulation Experiment and Joint Global Ocean Flux Study cruises from 1994 to 1998. This region lies in a zone of weak northward flow, between two branches of the Polar Front (hence the Interpolar Frontal Zone (IPFZ)). In the IPFZ, mixed layer nitrate concentrations are high (>24 μM) year-round, while mixed layer silicic acid is intermediate (about 15 μM) in winter but is depleted to 2 μM or less in late summer. Dissolved Fe concentrations, only available for summer, are low (<0.2 nM). Mixed layer chlorophyll concentrations are generally <0.3 μg L−1 and surface waters thus fit the high-nitrate low-chlorophyll (HNLC) definition. In spring and early summer, the SCM is relatively shallow (about 60 m), intense (up to 1.5 μg chl a, L−1), and contributes 30–50% of column production. By March the SCM is deep (100 m or greater), less intense (about 0.5 μg chl a L−1), and contributes at most 20% of column production. The existence of a SCM in a HNLC region is surprising, and we consider a number of possible explanations. The SCM may be partly explained by changes in C:chl a ratios, but phytoplankton species composition in the SCM also differs from that in the mixed layer. Sinking of diatoms, mediated by Fe and/or silicic acid availability, appears to play an important role in the formation and maintenance of the SCM.

Influence of different water masses and biological activity on dimethylsulphide and dimethylsulphoniopropionate in the subantarctic zone of the Southern Ocean during ACE 1

Measurements of salinity, temperature, phytoplankton biomass and speciation, dissolved nitrate, dimethylsulfide (DMS) in seawater and air, and dimethylsulfoniopropionate (DMSP), were made in the subantarctic zone of the Southern Ocean from 40°-54°S, and 140°-153°E during the southern hemisphere marine First Aerosol Characterization Experiment (ACE 1). DMSP concentrations were highest in subtropical convergence zone (STCZ) waters, intermediate in subantarctic waters, and lowest in polar waters. DMSP appeared to decrease at frontal regions between these major water masses. In subantarctic waters, high levels of DMSP were generally associated with an increase in dinoflagellate biomass and low microzooplankton grazing rates. Lower DMSP concentrations occurred in polar waters when the diatom biomass and grazing rates were high. DMS levels measured on Southern Surveyor ranged from not detectable (nd) to 5.6 nM (mean 1.7 nM), with below average levels in subantarctic waters (mean 1.25 nM), and above average levels (mean = 1.93 nM) in polar waters. Pulses of DMS occurred as Southern Surveyor traveled south into polar waters, with a large pulse (mean = 2.3 nM) highlighted as the vessel traveled back into subantarctic waters (46°-47°S, 148°-151°E) in early December. By using the dissolved DMSP (DMSPd) to DMS ratio as an index of the bacterial conversion of DMSPd to DMS some evidence was found that, in polar waters, increased microzooplankton (MZP) grazing in diatom dominated waters, may lead to above average concentrations of DMS. This does not appear to be the case when the biomass was dominated by dinoflagellates in subantarctic waters.

Light and dark uptake and loss of 14C: methodological problems with productivity measurements in oceanic waters

Measurements of the uptake and loss of 4C in the light and in the dark in the Tasman and Coral Seas have revealed methodological problems with the estimation of productivity in these waters. Rates of productivity estimated without replication, time series incubations and dark controls frequently overestimated the true rates of autotrophic production. The data showed unexpectedly high rates of both uptake and loss in the dark in oligotrophic waters. In oligotrophic oceanic waters, dark incorporation of 14C sometimes equalled the uptake of 14C in the light bottle. Rapid uptake of isotope in the dark controls appeared to be the result of rapid bacterial growth and metabolism. This problem was exacerbated by agitation of the sample before or during the incubation. Tropical samples were particularly susceptible to problems arising from the agitation of the samples. Latitudinal gradients of dark uptake and loss were revealed in these incubations. The loss of label during 8–12 hours in the dark (after 12 hr in the light) was as high as 50% in subtropical waters. The loss was frequently unmeasurable (< 10%) in temperate waters. The time course of 14C uptake indicated active grazing in the bottles and suggested that most of the nighttime losses of label were due to grazing by microheterotrophs. Respiratory losses appeared to be small. Calculated values of the assimilation number (or photosynthetic capacity) which did not correct for dark 14C uptake were too high to be biochemically realistic. The errors were due to the heterotrophic uptake of label and the lack of dark controls. Rapid release of 14C in the dark after incubation in the light meant that the estimate of ‘productivity’ was dependant on the trophic state of the sample and on the period of incubation.

Response of Phytoplankton Photophysiology to Varying Environmental Conditions in the Sub-Antarctic and Polar Frontal Zone

Climate-driven changes are expected to alter the hydrography of the Sub-Antarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia, in which distinct regional environments are believed to be responsible for the differences in phytoplankton biomass in these regions. Here, we report how the dynamic influences of light, iron and temperature, which are responsible for the photophysiological differences between phytoplankton in the SAZ and PFZ, contribute to the biomass differences in these regions. High effective photochemical efficiency of photosystem II (/ 0.4), maximum photosynthesis rate (), light-saturation intensity (), maximum rate of photosynthetic electron transport (1/), and low photoprotective pigment concentrations observed in the SAZ correspond to high chlorophyll and iron concentrations. In contrast, phytoplankton in the PFZ exhibits low / ( 0.2) and high concentrations of photoprotective pigments under low light environment. Strong negative relationships between iron, temperature, and photoprotective pigments demonstrate that cells were producing more photoprotective pigments under low temperature and iron conditions, and are responsible for the low biomass and low productivity measured in the PFZ. As warming and enhanced iron input is expected in this region, this could probably increase phytoplankton photosynthesis in this region. However, complex interactions between the biogeochemical processes (e.g. stratification caused by warming could prevent mixing of nutrients), which control phytoplankton biomass and productivity, remain uncertain.