Response to comment on 'Impacts of aeolian dust deposition on phytoplankton dynamics in Queensland coastal waters'

Abstract

In our earlier paper (Shaw et al. 2008; hereafter SGM), we related an increase in phytoplankton biomass in the coastal waters of Queensland, Australia, to the large October 2002 dust storm. Mackie (2010) has questioned these findings, stating that (i) the timing of the phytoplankton response precludes the dust storm as a causative agent for the chlorophyll (CHL) increase, and that (ii) it is not clear that there was actually any change in CHL in response to the dust storm. Mackie has highlighted that for the three regions where a CHL response was observed, the increase in CHL concentration began during the 8-day period ending on 23 October in two regions and on the 8-day period ending on 15 October in one of the regions. We agree with Mackie that these increases are not related exclusively to the large dust storm on 23 October. As stated in SGM, we believe this initial increase to be in response to smaller dust events before the major dust event. In our paper, we provided aerosol optical depth (AOD) data that supported this hypothesis. Mackie has questioned whether AOD is a reliable indicator of particulate load during the dust storm. As noted in SGM, the magnitude of the AOD value does not necessarily reflect the concentration of dust. However, AOD has been widely used in similar studies (e.g. Lenes et al. 2001; Gabric et al. 2002; Boyd et al. 2004) and we believe that, when used in conjunction with dust transport models, it is an appropriate method to indicate the presence of dust, although it does have limitations in determining dust loads. In further support of our hypothesis that there were smaller dust storms in the drought conditions before the CHL response, Australian Bureau of Meteorology (BoM) records show that therewere dust storms inwesternQueensland on 8 of the 22 days leading up to the dust storm on 23 October (BoM 2009). This activity peaked on 19 October, with an intense dust storm in the Birdsville area (reducing visibility to 300m), associated with a small low-pressure system. The red dusts from this localised event in the Simpson Desert–Channel Country would have been much more highly enriched in iron (Bullard et al. 2007) than those from the event on 23 October, which sourced dusts from a much larger area of diverse soils. Independent evidence of these events is available from the Earth Probe Total Ozone Mapping Spectrometer (EP/TOMS) aerosol data. Daily aerosol index (AI) data, calculated with the version 8 algorithm, were obtained for October 2002 from the National Aeronautics and Space Administration (NASA, http://macuv.gsfc.nasa.gov, verified 18 July 2009). TOMSAImeasurements have been used to determine the sources and distributions of aerosols and have been shown to be an effective measure of atmospheric dust loading (Israelevich et al. 2002 and references therein). EP/TOMS images of AI are shown in Fig. 1, where positive index values indicate the presence of UV-absorbing aerosols. High AI values can be seen in western Queensland on 6–8 October (Fig. 1a–c), in agreement with the BoM record that there were dust storms at this time. From these images, the region of high AI value shifts east, where moderate AI values can be seen off the Queensland coast on 8 and 9 October (Fig. 1c–d). Similarly, very high AI values were observed over central Australia during 17–20 October (Fig. 1e–h), corresponding with dust-storm records in Queensland on 17 and 19October (BoM 2009). On 20 October, high AI values were observed off the coast of Queensland and New SouthWales (Fig. 1h). Therefore, we contend that these dust storms, which occurred before themajor event on 23October, resulted in deposition of dust into Queensland coastal waters and resulted in the initial increase in phytoplankton biomass before 23 October. The second argument put forward byMackie is that the CHL concentration did not increase further following the major dust event in two of the three regions in which a response was observed and therefore it was concluded that the dust storm had no effect on phytoplankton biomass. This conclusion relates only to the intensity of the bloom and fails to acknowledge any effect that themajor dust event had on prolonging the duration of the phytoplankton response. Aperiodic phytoplankton blooms are often short-lived events where high biomass is rapidly attained in response to the addition of new nutrients (Behrenfeld et al. 1996; Cloern 1996). Nutrients are rapidly taken up by large populations of phytoplankton and are quickly depleted, causing the dissipation of the bloom. The duration of the phytoplankton CSIRO PUBLISHING