General background

The atoll lagoon floors are largely covered with loose sediments (fine sand), which sustain a great deal of animal and plant life. This chapter deals with the unicellular photosynthetic organisms living at the surface and buried a few centimeters below the surface of the sediments.  These organisms are mainly algae and cyanobacteria.

Since these organisms are very difficult to observe with a microscope, their biomass was assessed by determining the proportions of the various molecules characterising the plant kingdom, the pigments and the living organisms, the ATP levels.

Assessing the production rates of these organisms is of particular interest as a means of understanding how the coastal ecosystems located at shallow depths function. These production rates can be measured by determining the increase in the dissolved oxygen occurring in closed tanks.

Methods

The present studies were carried out using the following methods:

Photosynthetic pigments were analysed using fluorimetric and spectrophotometric methods.

The ATP content of the sediments was measured as described by Bancroft et al. (1976).

The percentage energy reaching the incubated material was determined using a LICOR quantum-metre equipped with a spherical cell.

Tikehau

On Tikehau, which was the main island studied, the following data and findings were obtained.

In terms of the biomass :

• The ATP (adenosine triphosphate) levels were low, averaging only 4.1 mg m^-2 at the top of the sediments. The vertical pattern of ATP distribution is uneven: the peaks observed may be attributable to the existence of heterotrophic micro-organisms, since no correlations have been found to exist with the pigment content recorded in these sedimentary layers. Phytobenthic carbon accounts for 76% of the total living carbon present in the most superficial layer.

• The pigments : The variability of the photosynthetic pigment concentrations does not seem at first sight to depend on the depth of the stations prospected (0.60 m to 40 m) in the case of any of the pigments studied. On the other hand, although the excess light energy may be one of the factors limiting the photo-oxidative process of photosynthesis, the microphytobenthos at the bottom of the lagoons do not seem to be inhibited by the light, which reaches a maximum at these latitudes. On the whole, the mean values recorded on the first 5 millimetres of sediment in all the atoll lagoons  prospected amounted to only about 10 mg of chlorophyll per m². This value is 4 times lower than those obtained by Plante-Cuny on the Nosy-Bé sediments in Madagascar (1978).

Based on comparisons with the mean phytoplankton biomass values obtained on Tikehau, active chlorophyll, which was homogeneously distributed along the water column, was found to be produced at a rate of  about 0.2 mg m^-3 (Charpy 1985). The values of the benthic and plaktonic microalgal biomasses are therefore probably both  quite similar, after integrating below depths of 50 m. The phytobenthic carbon production rates are much higher than the phytoplanktonic carbon production rates, which were estimated by Charpy (1985) to amount to 14%.

In terms of the production rates :

The production rates were studied from 1983 to 1986 on all the loose white lagoon floors (at depths of 0m to 40m), which were by far the main type of lagoon floor encountered during this period. The rate of colonisation, the pattern of distribution and the density of the photo-autotrophic biofilms have all increased since 1990, and the total primary microbenthic production has therefore also increased at the bottom of the lagoons. The results presented here were obtained on white lagoon floors only, and therefore do not include the cyanobacterial communities, which also actively colonise the  sediments.

Studies on the light energy have shown that the percentages measured at the surface decrease with the depth according to the following exponential relation: % incident light energy = e (40.45-0.066Z) Z = depth in metres

In an initial study, the phytobenthic production rate was monitored kinetically by measuring the oxygen levels in diving bells every hour. The kinetic data obtained in this way showed that: 

The following results emerged from the data obtained on samples incubated  for short periods. These data were much more numerous than the kinetic data:

1) the daily oxygen production rate was found to be correlated with the depth : PJ (g O2 m-2 d-1) = - 25.5 x Depth + 1005 2) the depth at which PJ(O2) drops to zero is 39 m; practically all the Tikehau lagoon floors are therefore involved in primary production. In view of the areas of the bathymetric steps, the [20-25] m depth range can be said to contribute most to the production process. (see diagram)

The gross production rate (GP) can be calculated using the equation developed by Mc Closey et al. (1978) : 

where : NP = net production rate  (g O₂ m^-2 d^-1)

  R = respiration (g O₂ m^-2 d^-1

  PQ = the ratio between the number of C atoms and the number of O₂ molecules resulting from the process of photosynthesis

  RQ = the ratio between the number of C atoms and the number of O₂ molecules used for respiratory purposes. 

Since PQ and RQ were taken to be equal to 1, the mean daily phytobenthic GP on the loose white  Tikehau floors works out at:

Comparisons between the benthic, planktonic and total primary production rates in the lagoon have shown that  the phytobenthic production rate is greater than that occurring in the water column at depths of 0-10 m, whereas the two rates are similar at depths of  [10-15]m; below this threshold, the phytoplanktonic production process predominates. The two types of production process are therefore complementary: the total production was found to be fairly constant at all the depths investigated (see diagram).

Takapoto

Experiments were carried out at stations located all over the lagoon.

The biomass (6 mg Chl m^-2 on the average) was found to be slightly lower here than on Tikehau. The highest values were those obtained at the stations where the pearl oysters were the most abundant. However, the proximity of pearl rearing stations were not found to affect the micro-algal production rates; the benthic micro-plants therefore do not seem a priori to provide the pearl oysters with nutrients. 

The gross production rate, which was found to average 0.14 g C m^-2 d^-1 (which is only half the mean value obtained on Tikehau), was found to depend almost entirely on the depth, according to the following relation:

At the mean depth of the lagoon (25 m), the net oxygen production rate is negative
(-25 mg O₂ m^-2 d^-1). 

This page was based on the article :

Charpy-Roubaud  C.J., (1988). Production primaire des fonds meubles du lagon de Tikehau (Atoll des Tuamotu, Polynésie Française). Oceanol. Acta, 11: 241-248.

References 

Charpy L. (1985) Distribution and composition of particulate organic matter in the lagoon of Tikehau (Tuamotu archipelago, French  Polynesia). Proceed of the fifth intern coral reef Symp. 3 : 353-357.

Charpy-Roubaud C.J. & Charpy L. (1994). Productions primaires. Rap. Final du PGRN, EVAAM, Tahiti,  pp 29

MacClosket I.R., Wethey D.S., Porter J.W. (1978) Measurement and interpretation of photosynthesis and respiration in Coral reefs research methods, edited by D.R. Stoddart and J.E. Johannes, Unesco : 379-396

Plante-Cuny M.-R. (1973). Recherches sur la production primaire en milieu tropical. 1 : Variation de la production primaire et des teneurs en pigments photosynthetiques sur quelques fonds sableux. Valeurs des résultats obtenus par la méthode du 14C. Cah. ORSTOM, Sér. Océanogr., 11(3): 317-348.

Plante-Cuny M.-R. (1984). Le microphytobenthos et son rôle à l'échelon primaire dans le milieu marin. Oceanis 10: 417-427.