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Studie van waterdieren.

Hydrobiologisch onderzoek RIVON

Hydrobiologie RIN

1. Eutrophication constitutes a serious threat to many European lakes and many approaches have been used during the past 20–30 years to improve lake water quality. Results from the various lake restoration initiatives are diverse and the long-term effects are not well described. 2. In this study we evaluated data from more than 70 restoration projects conducted mainly in shallow, eutrophic lakes in Denmark and the Netherlands. Special focus was given to the removal of zooplanktivorous and benthivorous fish, by far the most common internal lake measure. 3. In more than half of the biomanipulation projects, Secchi depth increased and chlorophyll a decreased to less than 50% within the first few years. In some of the shallow lakes, total phosphorus and total nitrogen levels decreased considerably, indicating an increased retention or loss by denitrification. The strongest effects seemed to be obtained 4–6 years after the start of fish removal. 4. The long-term effect of restoration initiatives can only be described for a few lakes, but data from biomanipulated lakes indicate a return to a turbid state within 10 years or less in most cases. One of reasons for the lack of long-term effects may be internal phosphorus loading from a mobile pool accumulated in the sediment. 5. Synthesis and applications. Lake restoration, and in particular fish removal in shallow eutrophic lakes, has been widely used in Denmark and the Netherlands, where it has had marked effects on lake water quality in many lakes. Long-term effects ( > 8–10 years) are less obvious and a return to turbid conditions is often seen unless fish removal is repeated. Insufficient external loading reduction, internal phosphorus loading and absence of stable submerged macrophyte communities to stabilize the clear-water state are the most probable causes for this relapse to earlier conditionsEutrophication constitutes a serious threat to many European lakes and many approaches have been used during the past 20-30 years to improve lake water quality. Results from the various lake restoration initiatives are diverse and the long-term effects are not well described. In this study we evaluated data from more than 70 restoration projects conducted mainly in shallow, eutrophic lakes in Denmark and the Netherlands. Special focus was given to the removal of zooplanktivorous and benthivorous fish, by far the most common internal lake measure. In more than half of the biomanipulation projects, Secchi depth increased and chlorophyll a decreased to less than 50% within the first few years. In some of the shallow lakes, total phosphorus and total nitrogen levels decreased considerably, indicating an increased retention or loss by denitrification. The strongest effects seemed to be obtained 4-6 years after the start of fish removal. The long-term effect of restoration initiatives can only be described for a few lakes, but data from biomanipulated lakes indicate a return to a turbid state within 10 years or less in most cases. One of reasons for the lack of long-term effects may be internal phosphorus loading from a mobile pool accumulated in the sediment. Synthesis and applications. Lake restoration, and in particular fish removal in shallow eutrophic lakes, has been widely used in Denmark and the Netherlands, where it has had marked effects on lake water quality in many lakes. Long-term effects (> 8-10 years) are less obvious and a return to turbid conditions is often seen unless fish removal is repeated. Insufficient external loading reduction, internal phosphorus loading and absence of stable submerged macrophyte communities to stabilize the clear-water state are the most probable causes for this relapse to earlier conditions.

Cette étude a pour but la caractérisation hydrobiologique d’une lagune méditerranéenne (lagune de Bizerte, nord de la Tunisie) sur la base de dix campagnes de mesures réalisées à une fréquence mensuelle durant l’année 2004, de janvier à décembre. Deux parmi ces dix campagnes ont fait l’objet d’un maillage fin avec 118 stations en mai et 40 stations en septembre. Le reste n’a couvert que 10 stations seulement. Pendant ces campagnes, des paramètres physicochimiques ont été mesurés (Température, Salinité, Oxygène dissous). Des mesures de paramètres chimiques et biologiques ont été effectuées au laboratoire (Ammonium, Nitrates, Phosphates, Azote total, Phosphore total, Chlorophylle a et Phéopigment).Une analyse multivariée des données collectées a permis de distinguer deux principaux modes de fonctionnement de la lagune. Le premier mode, expliquant 26,3 % de la variance totale, montre un contraste nord-sud et reflète l’effet relatif de la Méditerranée et du bassin versant de la lagune, alors que le second mode, expliquant 18,2 % de la variance totale, montre un contraste est-ouest qui traduit l’effet du lac Ichkeul. Les paramètres qui contribuent significativement au premier mode sont la température, la salinité, la transparence absolue et relative. Ceux qui contribuent au second mode sont la transparence absolue et relative ainsi que la chlorophylle a et le phéopigment. Cependant, les paramètres dont les poids moyens sont non significatifs ont montré des oscillations spatiales importantes en fonction des saisons qui sont en liaison avec les rejets terrestres., The purpose of this study was the hydrobiological characterization of a Mediterranean lagoon (Bizerte Lagoon, North Tunisia) based on ten sampling campaigns carried out during the year 2004, from January to December, on a roughly monthly frequency. Two of these ten campaigns were subject to a high spatial coverage with 118 stations in May and 40 stations in September; the eight remaining campaigns covered only ten stations. During these campaigns, physicochemical parameters were measured (temperature, salinity, dissolved oxygen) and laboratory analyses were performed for chemical and biological parameters (ammonium, nitrate, phosphate, total nitrogen, total phosphorus, chlorophyll-a and pheopigments). A multivariate analysis distinguished two principal functioning hydrobiologic modes. The first mode, explaining 26.3% of the total variance, shows a North-South contrast reflecting the relative effect of the Mediterranean Sea and the catchment area. The second mode, explaining 18.2% of the total variance, shows an East-West contrast that reflects the effect of Lake Ichkeul. The parameters that contribute significantly to the first mode are temperature, salinity, absolute and relative transparency, whereas those that contribute to the second mode are absolute and relative transparency as well as chlorophyll-a and pheopigments. However, parameters for which the average weights were not significant showed important spatial oscillations according to seasons, in relation with terrestrial discharges.

Une étude spatio-temporelle a été menée de mars 1990 à mars 1991 sur deux stations ostréicoles (nord et sud), de la baie de Bourgneuf. Cette étude se divise en deux parties distinctes : - L'une, hydrobiologique a permis, par des prélèvements hebdomadaires à basse mer plus trois heures, de caractériser les masses d'eau qui baignent chaque point d'étude, - L'autre, a eu pour but d'observer, par un suivi mensuel, l'évolution de la croissance, de l'engraissement et de la mortalité d'un échantillon d'huîtres (Crassostrea gigas) élevé sur chacun des secteurs; L'examen des résultats obtenus amène aux constatations suivantes : - Le secteur nord présente un milieu hétérogène d'une richesse nutritive importante mais est accompagnée d'une charge sestonique minérale élevée. Cette charge particulaire est liée aux conditions météorologiques et notamment dépendante de la direction et de la vitesse du vent. Le bilan de la production biologique des huîtres en élevage est médiocre. - Le secteur sud est un milieu pauvre en nourriture disponible, mais de caractéristiques hydrobiologiques homogènes au cours de l'année. Les quantités de sestons sont faibles et la remise en suspension négligeable du fait de la position abritée des vents dominants de cette station. La production des huîtres en élevage est bonne. Enfin, une relation huîtres/milieu a démontré que la croissance des huîtres est mieux corrélée à la qualité trophique par rapport à la quantité de nourriture. La part minérale de la matière en suspension, est considérée comme un facteur de dilution de la composante organique et pourrait être préjudiciable à la physiologie de Crassostrea gigas.