Nutrient shortages and other factors limiting the development of phytoplankton

Philippe Dufour , IRD

Figure 1 : The ocean waters (bottom right) being driven over the coral ring of an atoll by the sea swell mingle with the lagoon waters.

The morphological diversity of the atolls  is reflected in the availability of the nutrient salts on which the growth of the phytoplankton depends.

The ocean waters which enter the lagoons are ultra-oligotrophic. The concentrations of dissolved mineral nitrogen (DIN = NO₂^-+ NO₃^- + NH₄⁺), phosphate and silica have been found to be 0.02, 0.21 and 1 µM, respectively, which means that they lack nitrogen, and the development of diatom species is limited by the shortage of silica (Click here for further information). Similar concentrations might therefore be expected to occur in those lagoons which are not cut off from the sea. In fact, this is far from being the case in any of the lagoons. In all the lagoons, the nutrient salt concentrations differ from those of the surrounding sea, due to the nutrient regeneration and production processes at work within the lagoons themselves, as well as to the inputs originating from the "motus", bird colonies and ground water. The inter-lagoon differences in the nutrient concentrations also depend on the residence times of the oceanic waters in the lagoons. On the scale of a single lagoon, assuming the sea swell and wind conditions to be identical in all the lagoons, the residence times depend mainly on the degree of aperture of the atolls with respect to the surrounding sea and on the size of the lagoon (its depth and area).

The concentrations of nitrogen, phosphorus and silica are growth-limiting factors as far as the phytoplankton species in most of the lagoons are concerned (Table 1).

The dissolved mineral nitrogen concentrations decrease with the size of the lagoons (i.e., with their area and their mean depth, Figure 2). The phosphate concentrations do not depend on the size of the lagoons, but the most self-contained lagoons seem to have the greatest phosphate shortages (Figure 2). A gradual process therefore seems to occur between the lack of nitrogen in the largest and most widely open lagoons and the lack of phosphorus in those which are the smallest and the most closely confined. This process can be expressed by the following regression: DIN / P-PO₄ = -29.7 aperture - 0.35 mean depth + 17.2, r² = 0.56, n = 10.

The shift from a limiting shortage of nitrogen to a limiting shortage of phosphorus can be observed in the smallest lagoons and those communicating the least with the open sea. It results from the decrease in the nitrogen  concentrations, which depend on the size of the lagoons, and the increase in the phosphorus concentrations, which depend on the degree of aperture of the atolls with respect to the surrounding sea.

Figure 2 

The silica concentrations are inversely correlated to the size of the lagoons (i.e., to their area and their mean depth)

Figure 3

Low silica concentrations do not limit the growth of the main types of picophytoplankton inhabiting the  Tuamotu atoll lagoons. In some lagoons, however, low levels of silica restrict the development of diatom species and explain why this class of algae are to be found so rarely in the Tuamotu atolls. The silica available is partly diffused from the ground water present on the "motus", where it exists in particularly high concentrations. Now the relative size of the "motus" decreases with that of the lagoons. This might explain why the silica concentrations also decrease with the size of the lagoons. (Figure 3).

The fact that the nutrient concentrations are growth-limiting factors was confirmed  in studies using two other methods (Table 2). The lack of nitrogen usually has stronger inhibitory effects than the lack of phosphorus and silica. The other vital nutrient elements, metals and vitamins are present in excess, contrary to these 3 elements. As can be seen from Table 2, the shortage of phosphorus is a growth-limiting factor on the smallest completely enclosed lagoon, Reka Reka.

The shift from a shortage of nitrogen to a shortage of phosphorus results from a combination of 4 processes, the effects of which depend on the capacity, the depth and the degree of enclosure of the  lagoons :

The following mechanism has been put forward to account for these findings :

The waters in the shallowest lagoons are enriched with  nitrogen to a significant extent as the result of the molecular nitrogen fixation activities of the benthic cyanobacteria and the diffusion of nitrogen originating from the sediments. This supplementary nitrogen enhances the growth of the  phytoplankton and simultaneously gives rise to greater phosphorus requirements. Given the high N/P ratio present in the sediments, the phosphorus requirements have to be supplied by the water column. In the smallest lagoons only infrequently receiving fresh sea water inputs, such as Reka Reka, the P concentrations decrease to such low levels that they limit the development of the phytoplankton. on other small atolls with medium-sized or wide apertures, the lagoon waters are completely replaced by fresh sea water every few hours or days.The nutrient fluxes originating from the bottom of the lagoons are greatly diluted by the vast sea water inputs, which also lack nitrogen. In the largest and deepest lagoons, the N fluxes of benthic origin are also diluted in the dense water column and do not suffice to meet the requirements of the phytoplankton. The occurrence of a shift from a shortage of nitrogen to a shortage of phosphorus therefore depends on the magnitude of the benthic processes relative to the pelagic ones involved. It reflects an increasing level of confinement, which depends in turn on the degree of aperture of the coral ring, the depth of the lagoons and their area - in other words, on the morphological status of the atolls.

Dufour P., Andréfouët S., Charpy L. and Garcia N.  (2001) Atoll morphometry controls lagoon nutrient regime Limnology and Oceanography Vol. 46, No. 2 : 456-461