The waters in the Tuamotu atoll lagoons originate from the central South Pacific Ocean, which is one of the most improductive waters in the whole world. The dissolved mineral nitrogen concentrations they contain are so low that they are only just detectable (0.02 µatg/l): they are way below the levels required to provide most phytoplankton species with the optimum nutrient assimilation conditions (Table 1).

Table 1 : Dissolved nutrient salt concentrations in the surface ocean waters around the  Tuamotu Archipelago. Means, standard errors and number of samples. DIN : NO₂^-+ NO₃ + NH₄⁺. The optimum concentrations  for most phytoplankton species are DIN> 1µM, PO₄> 0.2 µM, and for diatom species, they are SiO₂> 1.5 µM.

From the point of view of the phytoplankton's nutrient requirements, the lack of nitrogen in the central South Pacific waters is much more severe than the shortage of phosphorus and silica, the other nutrient elements on which the growth of the phytoplankton depends.  Phytoplankton with no nutrient deficits have a balanced internal composition with a  N/P ratio of 16 at/at and a  N/Si ratio of 1 at/at, whereas the N/P and N/Si  ratios between the nutrient salts dissolved in the surface waters of the Pacific Ocean are as low as < 0.2 at/at and < 0.04 at/at, respectively. The fact that the central Pacific waters lack nitrogen was confirmed by the results of the bio-assays given in the accompanying Figure.

Figure: mineral elements were added to surface water samples collected at 5 marine stations on the Tuamotu archipelago. The growth rate of the phytoplankton  was found to be proportional to the height of the water columns. Since adding iron (+ Fe), silica (+ Si) and phosphorus (+ P) did not result in any increase in the growth rates of the phytoplankton in comparison with the control sample (un-supplemented sea-water), it can be concluded that these elements are not growth-limiting factors; whereas adding nitrogen alone (+ N) enhanced the growth of the phytoplankton. Nitrogen is therefore the element on which the growth depends most strongly. However, the growth of the phytoplankton was soon inhibited by the lack of another nutrient element, phosphorus, which had to be added to nitrogen (+ N + P) for the growth to continue. Phosphorus is therefore the second most important growth-limiting element. The naturally occurring concentrations  of all the other nutrient elements suffice to sustain the increase in the phytoplankton biomass induced by adding N et P. These substances are therefore not growth-limiting factors (adapted from Dufour and Berland, 1999). 

In comparison with the open Pacific waters on which they draw, the lagoon waters have a higher nitrogen content, a lower phosphorus content and similar levels of silica. The concentrations measured in 12  of the Tuamotu atoll  lagoons were below  the saturation levels in terms of the assimilation capacities of most of the phytoplankton species, i.e., 1µM in the case of N, 0.2 µM in the case of P and 1.5 µM in the case of Si (Table 2).  The existence of a nutritional shortage was confirmed by the smallness of the phytoplankton cells and by the low light energy yields recorded. The growth of bacteria, which constitute a larger biomass than the phytoplankton in most of the atoll lagoons (click here for further information), is also limited by the shortage of nutrient salts. Since bacteria assimilate nitrogen and phophorus more efficiently than phytoplankton do, they compete with the latter for access to these sparse resources. The dissolved nutrient salt ratios N/P, N/Si and P/Si (Table 2) show that the growth limiting factors, in decreasing order of importance, are nitrogen, phosphorus and silica.

Table 2 : Dissolved nutrient salt concentrations in the Tuamotu Archipelago atoll lagoons. Medians of the mean concentrations in 12 lagoons, and the mean concentrations in the most abundantly supplied and the least abundantly supplied lagoon. DIN : NO₂^-+ NO₃ + NH₄⁺  

Nutrient enrichment tests of the kind described in the above Figure were carried out on the waters from 11 lagoons. All the nutrient elements liable to limit the growth of the phytoplankton were tested, namely nitrogen, phosphorus, silica, iron, molybdenum, magnesium and vitamin B1, B12 and H. The conclusions reached are shown in Table 3. The findings obtained are in line with those deduced from the in situ concentrations and with the data obtained on the bacterioplankton.

Table 3: The nutrient deficits were studied using 3 different methods: by measuring the concentrations in situ and by determining the responses of phytoplankton and bacterioplankton to supplementary nutrients (bio-assays). N>P>Si means that nitrogen is more growth-limiting than phosphorus, which in turn is more growth-limiting  than silica. G: glucose; -: no data obtained.

The lack of nitrogen was usually found to limit the growth of the phytoplankton more than the lack of phosphorus and silica. The greatest phosphorus deficits are to be found on the smallest atolls and those which are completely cut off from the open sea (click here for further information). The fact that silica is not a decisive factor as far as the growth of the phytoplankton is concerned is in line with the small numbers of diatoms detected (click here for further information), since the algae in this class have high silica requirements. The finding that iron is not growth-imiting is not in agreement with data previously published in the literature on the central South Pacific and on some coral environments. This finding is supported, however, by the fact that cyanobacteria were the predominant type of organisms found to inhabit the Tuamotu atoll lagoons (click here for further information): the iron using capacities of these algae are much greater than those of  other classes of algae, thanks to the siderophores with which they are endowed.

Dufour P. and Berland B. (1999). Nutrient control of phytoplanktonic biomass in atoll lagoons and Pacific ocean waters: studies with factorial enrichment bioassays. J. Exp. Mar. Biol. Ecol., 234(2):147-166.

Dufour P., L. Charpy, S. Bonnet and N. Garcia (1999). Phytoplankton nutrient control in the oligotrophic South Pacific sub tropical (Tuamotu archipelago). Marine Ecology Progress Series, 179 : 285-290.