Phosphorus in estuarine and oligotrophic ocean waters : analytical and biogeochemical studies

Abstract

Analytical difficulties have precluded scientists from studying in a comprehensive manner in natural waters the main forms of P, i.e. dissolved inorganic (DIP) and organic (DOP), and particulate total (PTP) phosphorus, and their cycles. A long capillary cell (LCC) system for measuring DIP in natural waters at nanomolar concentrations in the range 1-500 nM was developed and proved easy to construct with relatively cheap and versatile components, compact and portable, and able to work on ship. Amongst 5 techniques for measuring DOP, that using Mg(NO₃)₂ as oxidant gave the highest DOP values, whilst the ultra-violet (UV) irradiation oxidation technique gave the lowest. The importance of dissolved poly-phosphates (PPs) was revealed in this work. The Mg(NO₃)₂ oxidation technique was modified for use in determining PTP. Surfactants widely used in automate systems proved to interfere in the colour formation of the phosphomolybdenum blue complex. 

 A study of DIP, DOP and PTP in four English estuaries was carried out. DIP varied in both concentration and behaviour. Concentration ranged from < 0.05 μM (Beaulieu and Humber) to up to 55 μM (Itchen). Anthropogenic discharges were the dominant source of P in the Itchen and Thames estuaries. Non-conservative behaviour of DIP in the Beaulieu, and possibly also in the Humber, was detected with concentrations generally below 0.5 μM. In these estuaries the dominant fraction of P was PTP, which ranged from about 1 to 6 μM (average 3 μM), followed in importance by DOP. However in the Itchen and Thames DIP was the dominant fraction. The ratio of DOP/DTP varied greatly from < 1 to 80%, with high values at the salinity end members. Previous studies of riverine PTP fluxes to the oceans appear low if the measurements reported are representative of world rivers. 

 The different forms of P were also comprehensively studied in the Sargasso Sea. The first nanomolar vertical profiles of DIP, and the processes affecting the well defined vertical structure observed are presented. It was found that 1) DIP is being utilized in biological processes at least down to concentrations of 1 nM. 2) DOP was constantly the dominant fraction (75%) of the total P and proved to be an important reservoir of P for micro-organisms. DOP concentrations measured by the Mg(NO_3)_2 method were generally double those determined using the UV oxidation. The differences between these values were ascribed to the presence of dissolved PPs. 3) DIP was shown to be the limiting nutrient rather than nitrate. 4) Turnover rates within the euphotic layer for DIP were 0.3 hours^-1, and up to 0.5 days ^-1 for DOP, whilst the overall residence time for P in mixed layer is about 2 years. 5) The application of a Fickian model to compute the diffusion of DIP to the euphotic layer from the nutricline was not adequate to balance export fluxes of particulate P from this layer. 6) The use of the LCC technique for following the net removal rates of DIP in situ et vitro conditions was successfully demonstrated. Data from two profiles (0-4200m) show that PPs and DOP are present as well as the expected DIP in deep waters.

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