Abstract
Oceanographic studies in the last three decades have provided insights on the correlation between cadmium (Cd) and phosphate (PO43-) in seawater. That the concentrations of these two are closely related in open ocean waters was first observed in mid-1970s (Boyle, 1976; Martin et al., 1976). The seeming consistency of this relationship has been exploited to provide a proxy for historic PO43- based on measurements of Cd in fossil material. However, at present, there is no single generally acceptable explanation for the Cd-PO43- relationship particularly in certain areas of the surface ocean and the use of Cd as a paleo-nutrient proxy in the surface ocean is therefore controversial. Experimental work described in this thesis investigates the Cd–PO43- relationship in two study systems with distinct physicochemical and biological properties (Subantarctic and Subtropical waters, east of New Zealand). This study involves field measurement of dissolved and particulate concentrations of some bioactive trace metals (Cd, Fe, Zn, Co), nutrients (PO43, NO3-, Si(OH)4) and biological indicators (chlorophyll a, particulate organic carbon and particulate organic nitrogen) as well as a laboratory culture study of Cd uptake in the diatom Nitzschia sp. (Clone AI).
A pragmatic investigation of the factors responsible for seasonal variation in the Cd and PO43- relationship along with other relevant oceanographic parameters off the southeast coast of New Zealand has been conducted. Strong seasonality was seen in the biological parameters and also in the trace element concentrations. The correlation of the trends in Cd concentrations and the Cd/PO43- ratios with indicators of biological activity (chlorophyll a, POC/PON and micronutrients) in this study indicate that the seasonal variation in Cd/PO43- ratios observed in these waters is due to biological activities. The drawdown in dissolved Cd concentration corresponded well with the drawdown in micronutrients and particulate Cd concentration was higher during algae growth as Cd partitioned from dissolved into the particulate phase at this time. From the timing of the changes in these parameters it is postulated that the mechanism of seasonal drawdown in dissolved Cd (and evolution of the Cd/PO43-) in Subantarctic Surface Water (SA) is a seasonal supply of Si(OH)4 into a Zn-depleted water in which certain species of phytoplankton are able to used Cd as a substitute for Zn. This results in microplankton (diatoms) dominating the phytoplankton assemblage in the late spring and coinciding with an intense drawdown in DCd (15 fold) and Si(OH)4. Underlying factors (during the algal growth season) such as Si(OH)4 (and Fe) supply, low dissolved Zn concentration and changes in the composition of phytoplankton community are factors modulating the Cd/PO43- ratio in the SA east of New Zealand.
Cd partitioned from dissolved to particulate phase due to biological uptake during a natural spring bloom event in an anticyclonic eddy off the east coast of North Island, New Zealand. Cd/PO43- ratios also decreased in the surface water as the bloom progressed. Mixed layer depth, biomass production and phytoplankton distribution were the contributing factors to the modulation of Cd/PO43- ratios observed in this bloom event. Results also show Cd uptake is not limited to diatoms as there was significant increase in particulate Cd during the nano-flagellates dominated period (phase 2). This is comparable to the observation in the Otago transect (Chapter 3) where Cd partitioned into different size fractions.
Higher Cd uptake and partitioning were observed in the diatom cultures (Nitzchia sp) with lower dissolved Zn concentrations in agreement with previous studies. However, Fe and Zn addition did not significantly increase the growth rate and biomass production in this organism.
Seasonal dependency of Cd/PO43- ratios on Cd uptake as shown in this thesis indicates that the Cd proxy is probably not reliable to reconstruct PO43- concentration in the past Southern Ocean.