Détermination par bioessai de la biodisponibilité des ressources azote et phosphore, dans les eaux du Golfe du Morbihan Bioassay determination of the bioavailability of nitrogen and phosphorus resources in Morbihan Gulf waters

Résumé

Des dosages physico-chimiques et des numérations du phytoplancton sont réalisés sur des prélèvements d'eau du Golfe de Morbihan (France), conjointement à des bioessais en milieu non renouvelé :

1. Avec le peuplement autochtone, par enrichissement en azote et en phosphore de l'eau filtrée sur 200 µm.

2. Avec deux algues tests épuisées en azote et en phosphore, sur l'eau filtrée stérilement. Les quotas cellulaires des algues tests en fin de croissance sont comparés aux quotas cellulaires minimaux et maximaux préalablement établis.

Les bioessais avec enrichissement révèlent une limitation de la croissance des algues par l'azote. Les bioessais avec Fragilaria elliptica présentent une relation linéaire croissante simple entre la concentration en azote assimilée par l'algue test et la densité cellulaire du peuplement phytoplanctonique naturel. L'azote total en solution représente la fraction utilisée par ces algues : azote biodisponible. Par contre, la fraction « phosphore soluble » ne représente qu'une partie du phosphore biodisponible, et se trouve consommée de manière très diverses par les algues testées. Cependant, l'analyse des quotas cellulaires en fin de croissance des algues tests montre que le phosphore est l'élément impliqué dans la limitation de leur croissance.

Les bioessais exposent des résultats contradictoires de limitation de la croissance. Les algues autochtones utilisent des ressources en phosphore supérieures à celles qui sont mises à disposition des algues tests, par stockage interne ou par consommation du phosphore minéral adsorbé sur les particules en suspension.

Mots-clés:

• Phytoplancton,
• estuaire,
• bioessais,
• azote,
• phosphore,
• biodisponibilité

Abstract

In estuarine water, the knowledge of the resources bioavailability is indispensable for understanding phytoplankton dynamics. For assessing nitrogen and phosphorus bioavailability, algal bioassays are the most indicated. Nutrient limitations of an alga exhibit a threshold rather than an additive or a multiplicative response (ELRIFI et al., 1985). In contrast, numerous nutrient limitations are described for an algal population (TILMAN,1982).

Samples are taken monthly at 1 meter’s depth from two French river estuaries, Noyalo River and Vannes River, in the Morbihan Gulf from April to July. Main physico-chemical measures (temperature, salinity, nitrate, nitrite, ammonia, total nitrogen, reactive phosphorus and total phosphorus) were taken. Total soluble nitrogen and total soluble phosphorus concentrations were determined after filtration en 0,2 µm, for comparison with bioassays. In addition two kinds of batch bioassays with 250 ml erlenmeyers into controlled environment (17 °C and 180~200 µEinstein), were employed :

- Estuarine waters filtered on 200 µm, to remove most of the zooplankton, were enriched with sodium nitrate and/or dipotassium hydrogen phosphate. Algal Growth response, was recorded by fluorescence « in vivo », against a blank.

- With the total growth on sterilely filtered (0,2 µm) estuarine waters of two test algae, starved simultaneously in phosphorus and nitrogen in artificial sea water, it was possible to determine the bioavailability of these two elements in the soluble traction. The cellular density of the two test algae : Fragilaria elliptica Schumann and Phaeodactylum tricornutum Bohlin, was related to optical density measures at 750 nm. Nitrogen and phosphorus intracellular concentrations, in the whole culture at the end of the growth, gave their bio-availability for the test algae provided. Nitrogen and phosphorus cell quotas, intracellular concentration per cell, were then compared with their higher and lower cell quotas. Higher and lower cell quotas were measured at the end of the test algae growth in an artificial medium without phosphorus or nitrogen resources. They were similar to those described by DAUTA (1982).

In 1989, an exceptional drought reduced the river flow. Consequently, nutrient flux decreased and phytoplankton density, mainly diatoms, was reduced to between 100 and 800 cells/ml. Sea water dominated in the Morbihan Gulf with salinities higher Chan 30 %o.

First, waters filtered on 200 µm and added to which was nitrate and/or phosphate revealed a nitrogen limitation of natural phytoplankton.

Secondly, phosphorus limitation were found in bioassays on sterilely filtered waters, in all the test algae starved for phosphorus and nitrogen. At the end of the growth in bioassays, phosphorus cell quotas of the test algae were generaly similar to the lower ones. The growth of the tests algae was limited by that resource.

These contrary results are related to the intracellular storage by the natural phytoplankton and/or the major role of particulate fractions in phosphorus bioavailability.

Phosphorus and nitrogen concentrations assimilated by the test alga : Fragilaria elliptica, were inversely related. From May to July 1989 the results showed a simple linear relation between soluble nitrogen source (filtered on 0.2 µm) which was bioavailable for the test alga and natural phytoplankton density (R² = 0.98). The two measurements of April did not agree with the latter relation, rather because of environmental factors such as temperature (11 °C against 20~21 °C for the next ones), than because of the resources. Moreover, the station B in July exhibited a natural phytoplankton density significantly lower than the station A and independent of the nitrogen bioavailability for the two test algae. The bioavailable phosphorus for Phaeodactylum tricornutum, was also significantly lower at station B. That resource can explain the difference in density monitored in July.

The total soluble nitrogen source, inorganic and organic, and just a fraction of the total soluble phosphorus source, were assimilated. The soluble phosphorus assimilated by test algae, after sterile filtration, is not the total bioavailable phosphorus in the water of the Morbihan Gulf and seems to be inversely related to the natural phytoplancton density. A part from mineral phosphorus adsorbed on suspended matter is also bioavailable (BERLAND et al., 1973). However, for the phosphorus resource, bioassays have to be performed on particulate fractions.

Keywords:

• Phytoplankton,
• estuary,
• bioassays,
• nitrogen,
• phosphorus,
• bioavailabitity