Optical remote sensing in support of eutrophication monitoring in the southern North Sea

Spring mean and maximum chlorophyll a (chla) concentrations are main factors to determine the eutrophication status of the Belgian waters as agreed within OSPAR in 2002. Other important assessment parameters to measure the degree of nutrient enrichment - the amounts of inorganic phosphate and nitrogen in winter - appeared to be above thresholds for most measurements performed in the period 1974-2002. As the standard in situ monitoring programme does not give a clear picture of the temporal and spatial distribution of chl a, it is logical to complement these measurements with optical remote sensing. However, chlorophyll concentrations derived from sensors such as SeaWiFS are unreliable in the Case 2 waters of this region because of high particulate and dissolved yellow substance absorption. Another important limitation of ocean colour sensors is the amount of useful images due to cloud cover. The combination of data from different ocean colour sensors in order to enable a better temporal coverage might be hampered by the different chlorophyll retrieval algorithms used. This study compares different global chl a algorithms (MODIS, SeaWiFS, MERIS) as well as a turbid water algorithm for the Southern North Sea. This is done by running the different algorithms on in situ reflectance spectra collected at 107 stations in the period 2001-2002 over the Southern North Sea and comparing them with in situ chl a concentrations, as well as by running the algorithms on a MERIS image of the 29th of July 2002. Based on this validation the accuracy of these products and their suitability for eutrophication monitoring in the Southern North Sea are assessed

Variation à différentes échelles spatiales de l'herbier à <i>Posidonia oceanica</i> (L.) Delile ; effets sur les paramètres physico-chimiques du sédiment

The biometric parameters (shoot density, length and width of leaves, leaf and epiphyte biomasses, leaf surface) of the P. oceanica meadow present a significant spatial variability at the mesoscale level (100m2). All the parameters studied, excepting the number of leaves, decrease according to the depth. The horizontal spatial variations (centrifuge and centripetal distributions) reach a maximum in the shallower areas. The analysis of the microstructures (m2) of the meadow has not allowed a relationship to be established between the spatial variations of the biometric parameters and the environmental factors in the sediment (pH, water and organic matter content, red-ox potential, nutrient content). The heterogeneity of the meadow in the Revellata Bay quite probably results from the internal shoot and rhyzome growth dynamics. On the other hand, it appears that the presence of the prairie modifies the physical-chemical characteristics of the sediment. In fact, the sandy patches present different physical-chemical characteristics: lower organic matter, different granulometric and much higher interstitial water nutrient levels, as well as a higher pH and oxydo-reduction potential

Variations à différentes échelles spatiales de l'herbier à Posidonia oceanica (L.) Delile ; effets sur les paramètres physico-chimiques du sédiment

The biometric parameters (shoot density, length and width of leaves, leaf and epiphyte biomasses, leaf surface) of the P. oceanica meadow present a significant spatial variability at the mesoscale level (100m(2)). All the parameters studied, excepting the number of leaves, decrease according to the depth. The horizontal spatial variations (centrifuge and centripetal distributions) reach a maximum in the shallower areas. The analysis of the microstructures (m(2)) of the meadow has not allowed a relationship to be established between the spatial variations of the biometric parameters and the environmental factors in the sediment (pH, water and organic matter content, red-ox potential, nutrient content). The heterogeneity of the meadow in the Revellata Bay quite probably results from the internal shoot and rhyzome growth dynamics. On the other hand, it appears that the presence of the prairie modifies the physical-chemical characteristics of the sediment. In fact, the sandy patches present different physical-chemical characteristics: lower organic matter, different granulometric and much higher interstitial water nutrient levels, as well as a higher pH and oxydo-reduction potential.Les paramètres biométriques (densité en pousses, longueur et largeur des feuilles, biomasses épiphyte et foliaire, surface foliaire) de l'herbier à Posidonia oceanica présentent une variabilité spatiale significative à mésoéchelle (100 m2). Tous les paramètres étudiés, sauf le nombre de feuilles, diminuent en fonction de la profondeur. Les variations horizontales (distributions centrifuge et centripète) sont maximales dans les zones moins profondes. L'analyse de la microstructures (m2) de l'herbier n'a pas permis de mettre en relation les variations spatiales des paramètres biométriques avec les facteurs environnementaux du sédiment (pH, teneurs en eau et en matière organique, potentiel oxydo-réduction, teneur en nutriments). L'hétérogénéité de l'herbier de la baie de La Revellata résulte certainement de la dynamique interne de croissance des pousses et des rhizomes. En revanche, la présence de l'herbier modifie les conditions physico-chimiques du sédiment. En effet, les taches de sable présentent des conditions physico-chimiques différentes : teneurs en matière organique plus faible, granulométrie différente, teneurs en nutriments de l'eau interstitielle beaucoup plus élevées, pH et potentiel d'oxydo-réduction plus élevés