| Abstract |
Fish farming impact on carbon and nutrient (nitrogen and phosphorus) dynamics of the endemic Mediterranean seagrass Posidonia oceanica (L.) Delile was assessed in Sounion Attica (Aegean Sea, Greece) in order to detect changes in the magnitude and fate of production and nutrient incorporation with organic loading of the meadow. Two stations were selected along the direction of main currents; one (called ‘cages’) as close to the fish cages as possible (about 20 m away from the edge of the cages) at 16 m depth and one (called ‘control’) located about 1 km away from the cages at 14.5 m depth. The site was visited on bi-monthly basis for 14 months (i.e. 8 sampling events, from April 2006 to June 2007).
Concentration of dissolved inorganic nitrogen and phosphorus in the water column were not significantly different between cages and control stations. Ammonium and phosphate concentration in the pore water was 2 and 3 times, respectively, higher under the cages than at the control station and was positively correlated to nitrogen and phosphorus, respectively, input and pool in the sediment. Carbon, nitrogen and phosphorus sedimentation was 4, 3 and 4, respectively, times higher at the cages station, resulting in accumulation of these elements under the cages, where concentration of carbon, nitrogen and phosphorus was 14, 9 and 1.4 times, respectively, higher than those at the control station. On annual basis, the impacted meadow took up (-7724 mmol C m-2yr-1) dissolved inorganic carbon (DIC) whereas the pristine one released (1842 mmol C m-2 yr-1) DIC. Organic loading induced DOC release by 158%, DON by 1639%, NH4 by 122% and NO3 by 26%, in respect to the pristine community, and shifted the meadow from a typical sink (-0.3 mmol DOP m-2 yr-1, -7 mmol DIP m-2 yr-1) to a source (3 mmol DOP m-2 yr-1, 20 mmol DIP m-2 yr-1) for dissolved organic (DOP) and inorganic (DIP) phosphorus under the fish cages. Release of phosphorus from the impacted meadow and increased phosphorus input and pool in the sediment, in combination with the fact that phosphorus limited seagrass growth, resulted in accumulation of phosphorus in seagrass tissue under the cages. Biomass decreased by 64%, production by 66%, carbon, nitrogen and phosphorus incorporation by 66%, 56% and 58%, respectively, shedding by 81% and loss of carbon, nitrogen and phosphorus through shedding by 82%, 74% and 72%, respectively, under the cages. Steeper decay rates, less decomposed mass, lower litter export and less burial were recorded under the cages rather than the control station. Higher export and loss of elements through this path, representing grazing and mechanical breakage of leaves, were measured at the cages than those at the control station. Net community production and respiration decreased by 60% and 34%, respectively, under the cages. On annual basis, P. oceanica meadow at the control station produced 13.98 g C m-2 yr-1, 1.91 g N m-2 yr-1 and 0.05 g P m-2 yr-1 in excess of consumption and loss and was therefore a net sink for these elements. On the contrary, the impacted meadow was a net source of carbon and nutrients, by releasing 12.69 g C m-2 yr-1, 0.31 g N m-2 yr-1 and 0.04 g P m-2yr-1, annually, implying divergence from metabolic balance of seagrass meadow with organic loading and possible dependence on allochthonous nutrient inputs.
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Δυναμική του άνθρακα και των θρεπτικών σε υγιείς και υποβαθμισμένους λειμώνες Posidonia oceanica
