The effects of fish farming on a seagrass (Posidonia oceanica) meadow at Fomells 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. 0 Elsevier, Paris / Ifremer / Cnrs / Ird aquaculture I seagrass I Posidonia oceanica I eutrophication I sediment characteristics R&urn6-Effets de Nlevage de poissons sur les herbiers de Posidonia oceanica dans une baie m6diterrant5enne. Les effets de l'elevage de poissons en cages flottantes sur un herbier de Posidonia oceanica ont et6 CtudiCs dans la baie de Fornells (Minorque, iles Baleares). Des modifications dans les caracteristiques de la plante et de l'herbier (morphologie et densitt des faisceaux, biomasse, croissance des rhizomes, teneurs en nutriments et en sucres solubles) sont observees en trois stations d'une radiale allant d'une zone poll&e (par la mat&e organique due aux elevages de poissons) a une zone non pollube. Bien que l'elevage ait cesse en 1991, la degradation notee en 1988-1990 s'est poursuivie jusqu'au moment de l'tchantillonage d&it ici (millet 1994). Les differences entre les stations sont tres nettes : la proximite des cages est associte aux plus faibles valeurs de densite et taille des faisceaux, biomasse, teneur en saccharose et taux photosynthetique, tandis que dans cette m&me zone polluee, la concentration du phosphore darts les tissus et la biomasse des epiphytes sont maximales. La colonne d'eau ayant retrouve son &at initial apt& la disparition des Blevages, la persistance de la degradation pourrait $tre lice ?I des modifications du sediment dont la teneur en mat&e organique est restee tres ClevCe.
LONG-TERM GOALS An overall goal of the CoBOP program is to produce a working radiative-transfer model for selected sub-littoral environments. From a microbiological context, it is important to investigate the entire community of microorganisms associated with the benthic environments of focus, i.e., sediments and seagrass. Light must pass through a "microbial gateway", both before it reaches the sediment or seagrass and prior to its return to the water column. To understand time-and-space variations in optical parameters, we must understand the microbial milieu in which they exist. OBJECTIVES Determine the interannual variation in the biomass and composition of sedimentary microorganisms at field sites nearby Lee Stocking Island, Bahamas, and in Monterey Bay, California. Similarly, determine the biomass, composition, and temporal variation of microorganisms epiphytic on seagrass blades at Lee Stocking Island (turtle grass, Thalassia testudinum) and in Monterey Bay (eel grass, Zostera marina). Assess how the microbial community affects the flux of photons to and from the sediments and seagrass blades and how temporal changes in the microbiological community influence temporal changes in benthic optical characteristics. APPROACH Our approach is to combine extensive field sampling of sediments and seagrasses with biochemical determination (membrane lipids-Dobbs) and microscopic examination (light and scanning electron microscopy-Drake) of their microbiological constituents. These fundamental observations then serve as the basis for comparison with the sediments' and seagrasses' optical properties.
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