Deep Sea Research Part II: Topical Studies in Oceanography
Assessing Sub-Antarctic Zone primary productivity from fast repetition rate fluorometry
Introduction
The Southern Ocean plays an important role in distributing global nutrients (Sarmiento et al., 2004) and controlling global climate (Sarmiento et al., 1998). The waters of the Southern Ocean are predicted to significantly change in response to climate change (Sarmiento et al., 1998). Climate-driven atmospheric and oceanic changes, which modify the nutrient supply and light regime, are also expected to affect phytoplankton composition dynamics and growth rates in the Southern Ocean. However, the response of phytoplankton to these changes in this region remains unclear (Boyd, 2002).
On site primary productivity (PP) measurements in the Southern Ocean have mostly been carried out using conventional radiocarbon methods (El-Sayed, 2005). The 14C method requires abundant bottle incubations, which are time consuming and labour intensive, and spatially and temporally limited. Remote sensing techniques, which have overcome some of these constraints, have been frequently applied to investigate the spatial and temporal primary production in the Southern Ocean (Sullivan et al., 1993, Arrigo et al., 1998, Arrigo et al., 2008, Moore and Abbott, 2000) but the limited number of in situ measurements available for large areas of the Southern Ocean has hindered the sea truth validation and interpretation of estimates from satellite imagery.
The fast repetition rate fluorometry (FRRF) technique, which was only introduced in the last decade, has been applied widely to study the photosynthesis of phytoplankton in the ocean (Suggett et al., 2005). Based on variable chlorophyll a (chl a) fluorescence, the FRRF provides in situ non-destructive, instantaneous measurements of photosynthetic parameters and allows both the derivation of gross primary production at fixed depth and continuous observations with a temporal resolution of seconds (Falkowski and Kolber, 1995). Attempts to determine the reliability of the FRRF in estimating PP have been carried out in several different regions. These studies, in either highly productive regions (Suggett et al., 2001, Moore et al., 2003, Raateoja et al., 2004, Smyth et al., 2004, Estevez-Blanco et al., 2006, Melrose et al., 2006) or oligotrophic waters (Corno et al., 2006) found significant correlations between FRRF measurements and traditional radiocarbon-derived estimates. However, the relationship between the two methods is not consistent. The inconsistencies are thought to be linked to differences in how the two techniques estimate PP. The 14C method measures the uptake and assimilation of dissolved inorganic carbon whilst the FRRF measurement is equivalent to gross oxygen evolution based on the electron transport rate in photosystem II (PSII) (Suggett et al., 2001). As a bio-optical instrument, FRRF measures instantaneous photosynthesis in the water column that reflects real-time irradiance, whilst 14C-uptake rates are a product of incubations integrated over a certain period of time that are subjected to average irradiance. Several key assumptions (e.g. photosynthetic unit size, maximum quantum yield of electron transport and photosynthetic quotient) are required in the FRRF-based PP model and these have been found to vary between different water bodies (Falkowski and Kolber, 1995).
Previous applications of FRRF in the Southern Ocean were mainly focused on the physiological conditions of phytoplankton in response to nutrients (Olson et al., 2000, Strutton et al., 2000, Boyd and Abraham, 2001, Gervais et al., 2002, Vaillancourt et al., 2003, Holeton et al., 2005). So far, there have been no published studies on the use of the FRRF to derive phytoplankton productivity in the Southern Ocean, despite its flexibility. In this study, we present PP data collected from the Sub-Antarctic Zone (SAZ), which lies between the Subtropical Front (STF) and the Sub-Antarctic Front (SAF) (Fig. 1), south of Tasmania using both the FRRF and 14C techniques. This region, which is reported to be a large sink for atmospheric CO2 (Metzl et al., 1999, McNeil et al., 2001), has a high degree of spatial and temporal variability in water properties. The mixed layer south of SAZ, for instance, is lower in temperature and salinity but higher in nutrients than that to the north (Rintoul and Trull, 2001). By comparing the FRRF technique with the 14C technique, we assess the reliability of FRRF-derived PP for this region.
Section snippets
Sampling
FRRF and 14C measurements were obtained from the SAZ south of Australia (43–55°S, 140–154°E) during the Sub-Antarctic Zone Sensitivity to Environmental Change (SAZ-Sense) survey. The voyage was conducted aboard RSV Aurora Australis from 17 January to 20 February 2007 and consisted of three process (P) stations and 24 transit (T) stations. For the purpose of this study, a total of 11 stations were investigated, at which there were 14 conductivity–temperature–depth (CTD) casts and FRRF
Hydrological conditions
This study covered an area that consisted of several different water masses with variable physical and biological characteristics. Sea surface temperature (SST) at Process station 1 (P1, SAZ-West) ranged from 12.8 to 13.1 °C with a salinity of approximately 34.7. The mixed layer (ML) depths in this area ranged from 25 to 46 m. SST and salinity at Process station 3 (P3, SAZ-East) were approximately 13.5 °C and 35.0, respectively. Two MLs was present at P3, a shallow ML from the surface to 16 m and
Variability of fluorescence measurements
The fluorescence parameters exhibit a close relationship with the light levels and nutrients. The reduction in both the photochemical efficiency (F′v/F′m and F′q/F′m) at the surface during midday implies the effects of non-photochemical quenching (qN) at high irradiances (Fig. 2C). At high light, qN acts as a self-protection mechanism that dissipates excess light energy in the form of heat, rather than in the form of fluorescence to avoid photodamage (Falkowski and Raven, 2007). The divergence
Conclusion
This study provides both validation and constraints for determining gross primary productivity (GPP) by FRRF in the Southern Ocean. The FRRF can provide in situ measurements of photosynthesis parameters at a much higher frequency than conventional radiocarbon techniques and can be applied to moorings as well as to remote surveillance. These characteristics make the FRRF a convenient tool in oceanographic research, especially in the vast and remote Southern Ocean. However, although strong
Acknowledgments
We would like to thank the captain and crew of RSV Aurora Australis, Australian Antarctic Division's gear officers, the CTD team and fellow expeditioners for their support during the cruise. We are indebted to Mark Rosenberg for the oceanography data, and Neil Johnson and Alicia Navidad for the nutrients data. We would also like to thank Philip Boyd (NIWA) and Klaus Meiners (ACE CRC) and four anonymous reviewers for their comments on the manuscript. This work was supported financially by the
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