Effects of fish farming on seagrass (Posidonia oceanica) in a Mediterranean bay: seagrass decline after organic loading cessation

Abstract

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.

Individual and population indicators of Zostera japonica respond quickly to experimental addition of sediment-nutrient and organic matter

“…Nutrients absorption of seagrass roots may impact on uptake of seagrass leaves to nitrogen Harlin, 1982, 1984). Also epiphyte overgrowth due to additive sediment-nutrient and organic matter loading may hinder N uptake by seagrass leaves, therefore decrease N uptake of seagrass leaves Balata et al, 2010;Cancemi et al, 2003;Delgado et al, 1999), although we did not observe epiphytes in this study.…”

“…In recent years, large-scale losses of seagrasses and risk of extinction for individual seagrass species were reported (Orth et al, 2006;Short et al, 2011). Eutrophication is identified as the main causes for the decline of seagrass meadows worldwide (Delgado et al, 1999;Govers et al, 2014;Orth et al, 2006;Waycott et al, 2009).…”

Tracing sewage and natural freshwater input in a Northwest Mediterranean bay: Evidence obtained from isotopic ratios in marine organisms

“…Epiphytic communities have also been used to assess the impacts caused by fish farms (Vizzini and Mazzola, 2004;Delgado et al, 1999) or urban disturbances (Piazzi et al, 2004). Epiphytes can reveal the complexity of the nitrogen budget in sea grasses because they are exclusively associated with the water column.…”

Is the vegetative development of the invasive chlorophycean, Caulerpa taxifolia, favored in sediments with a high content of organic matter?

“…However, the organic matter content of marine sediments can be high (Bourcier 1989, Delgado et al 1999, Ruiz et al 2001, leading to high pools of dissolved organic carbon (DOC) in sediment pore water (Burdige andHomstead 1994, Kö ster andMeyer-Reil 2001). External glucose and acetate at mM concentration allowed Ulva lactuca L. to maintain an intact photosynthetic apparatus and survive during long periods of darkness or low-light in laboratory conditions (Markager and Sand-Jensen 1990).…”