Elsevier

Oceanologica Acta

Volume 22, Issue 1, January–February 1999, Pages 109-117
Oceanologica Acta

Effects of fish farming on seagrass (Posidonia oceanica) in a Mediterranean bay: seagrass decline after organic loading cessationEffets de l'élevage de poissons sur les herbiers de Posidonia oceanica dans une baie méditerranéenne

https://doi.org/10.1016/S0399-1784(99)80037-1Get rights and content

Abstract

The effects of fish farming on a seagrass (Posidonia oceanica) meadow at Fornells Bay, Minorca (Balearic Islands) were studied. Changes in plant and meadow features (e.g. shoot morphology, shoot density, biomass, rhizome growth, nutrient and soluble sugars concentrations…) in three stations along a transect from a disturbed (organic pollution due to fish cultures) to an undisturbed site were assessed. The fish culture had ceased in 1991; however, seagrass decline, already reported in a previous study for the period 1988–1990, was still taking place at the time of sampling (July 1994). Differences between stations were very clear; the station closest to the fish cages showed reduced shoot density, shoot size, underground biomass, sucrose concentration and photosynthetic capacities. In contrast, shoots from the most polluted station showed higher P-concentration in tissues and higher epiphyte biomass than the other two. Since water conditions had recovered completely by the time of the sampling mission, it is proposed that the persistence of the seagrass decline was due to the excess organic matter remaining in the sediment.

Résumé

Les effets de l'élevage de poissons en cages flottantes sur un herbier de Posidonia oceanica ont étéétudiés dans la baie de Fornells (Minorque, îles Baléares). Des modifications dans les caractéristiques de la plante et de l'herbier (morphologie et densité des faisceaux, biomasse, croissance des rhizomes, teneurs en nutriments et en sucres solubles) sont observées en trois stations d'une radiale allant d'une zone polluée (par la matière organique due aux élevages de poissons) à une zone non polluée. Bien que l'élevage ait cessé en 1991, la dégradation notée en 1988–1990 s'est poursuivie jusqu'au moment de l'échantillonage décrit ici (juillet 1994). Les différences entre les stations sont très nettes : la proximité des cages est associée aux plus faibles valeurs de densité et taille des faisceaux, biomasse, teneur en saccharose et taux photosynthétique, tandis que dans cette même zone polluée, la concentration du phosphore dans les tissus et la biomasse des épiphytes sont maximales. La colonne d'eau ayant retrouvé son état initial après la disparition des élevages, la persistance de la dégradation pourrait être liée à des modifications du sédiment dont la teneur en matière organique est restée très élevée.

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