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The seasonal growth pattern of the seagrass Posidonia oceanica (L.) Delile was examined in 5 meadows in NE Spain to assess the relative importance of large-scale versus local factors in controbng the seasonal patterns observed. Large-scale seasonal forcing, resulting from changes in light and temperature associated with the solar cycle, was assessed from the coherence of seasonal growth palterns among the meadows and accounted for 46 and 43% of variability in shoot size and growth, respectively. The local component of seasonality, which results from local variation in environmental variables (e.g. nutrients, dissolved inorganic carbon, redox potential) was assessed as the differences in the annual time course of shoot size and growth among the meadows, and accounted for 9 and 1 2 % of the variability, respectively. These results support the contention that seagrass seasonality is primarily controlled by the solar cycle, and secondarily by seasonal changes in the environment which are at leas1 in part caused by the temporal variability of seagrass growth. This indirect link between light and temperature and local conditions needs to be taken into account to interpret correlations between such variables and seagrass growth.

The rates of seagrass defoliation exerted by the herbivorous fish Sarpa salpa and by the sea urchin Paracentrotus lividus were evaluated through both direct (tethering experiment) and indirect (bite marks) methods. Sampling was conducted once per season in 10 shallow meadows of Posidonia oceanica (L.) Delile from the continental NW Mediterranean coast covering a spatial scale of > 300 km. Results indicated that a large proportion (ca. 57%) of the annual leaf production is lost to herbivory, yet with considerable spatial variation. Patterns of seagrass defoliation showed high temporal variability, with a peak in summer with values that exceeded about 2.5 times those of leaf production and a minimum during the winter period. On average, defoliation exerted by S. salpa accounted for 40% of leaf production (ca. 70% of total annual losses to herbivory), while P. lividus was also responsible for a substantial 17% removal of leaf production. High discrepancies encountered when comparing direct and indirect measurements suggest that the latter are inappropriate to achieve accurate estimates of herbivory pressure. This study evidences that P. oceanica leaf losses to herbivores are not marginal, but a widespread process that occurs at much higher rates than previously estimated through indirect methods (ca. 2%), resetting the paradigm of the negligible importance of herbivory in temperate systems.

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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.

We present the seasonal carbon (C) balance of the Mediterranean seagrass Posidonia oceanica (L.) Delile calculated from seasonal rates of C gain (photosynthesis), C loss (respiration) and growth. We compare our balance with the evolution of seasonal C reserves in order to determine the parameters (shoot:root biomass, reserve allocation, photosynthetic parameters, etc.) that influence the seasonal cycle of the plant. Additionally, we examine whether the annual C balance can be used as a valid tool for testing the vulnerability of seagrasses to light reduction. The seasonal whole-plant C balance showed alternate negative (from September to June) and positive (July and August) values. This trend was the result of the interplay among several seasonal factors such as irradiance, water turbidity, photosynthetic parameters, respiratory rates, shoot growth, within-shoot age distribution, and principally, the low photosynthetic:non-photosynthetic biomass ratio. The lack of significant correlation between seasonal growth and metabolic balance (C gain -C demand) did not permit the prediction of plant growth. Conversely, the seasonal pattern of carbon storage was consistent with the periods of positive and negative C balance. Consequently, reserve mobilization allows overwintering and re-growth under conditions of negative C balance. Using different calculations the annual C balance was found to be negative during 1993; this is in accordance with the carbohydrate interannual depletion and the shoot density decline. Since Posidonia oceanica is regressing in the Mediterranean, our carbon budget may notably contribute to future carbon models that can be essential tools for defining the minimum light requirements for survival. More insight into the functioning of some of the parameters that definitively influence this carbon budget (e.g.: the rhizome/root oxygen consumption and the O 2 to C conversion) is needed to fully understand the vulnerability of seagrasses to light reduction.