Community metabolism and buffering capacity of nitrogen in a ruppia cirrhosa meadow

Fluxes of oxygen, inorganic nitrogen (DIN) and denitrification i(sotope pairing)were measured fromJanuary 1997 to February 1998 via intact cores incubation in a shallow brackish area within the eutrophic Valli di Comacchio (northern Adriatic coast, Italy). Ratesweremeasured in the light and in the dark in sediments colonized by the rooted macrophyte Ruppia cirrhosa and in adjacent sediments with benthicmicroalgae. Ruppia biomass (25–414 g DWm−2) exhibited a seasonal evolutionwhilst that ofmicrophytobenthos (12–66mg chlam−2)wasmoreerratic.Net (NP) and gross (GP) primary productivity was 1.15 and 6.89 mol Cm−2y−1 for bare and 25.4 and 51.7 mol Cm−2y−1 for Ruppia vegetated sediments. Nitrogen pools in Ruppia standing stock varied from 43.6 to 631.4 (annual average 201.2) mmol N m−2; the macrophyte N content was correlated with DIN concentration in the water column. Estimated N pool in microphytobenthos was one order of magnitude lower (from 2.4 to 14.5 mmol N m−2, annual average 7.2). Theoretical DINassimilation calculated fromNPwas 127.8 and 1112.6mmolNm−2y−1 whilst that calculated fromGP was 765 and 2282 mmol N m−2y−1 for microphytobenthos and Ruppia respectively. Measured annual fluxes of DIN were 974.6 and −577 mmol N m−2y−1 in bare and Ruppia vegetated sediments meaning that the two sites were a source and sink for DIN and that from 25 to 50% of Ruppia annual DIN requirements came from the water column. During the period of this study total denitrification was lower in the macrophyte colonized (92.3 mmol N m−2y−1) compared to bare sediments (163.3 mmol N m−2y−1) as a probable consequence of higher competition between denitrifiers and phanerogams. At both sites the ratio between denitrification of water column nitrate (DW) and denitrification coupled to nitrification (DN) was N1.6 due to little oxygen penetration in reducing sediments (b1.2mm) and scarce nitrification activity.DW(0–35 μmolNm−2h−1)was significantly correlatedwithwater column NO3 − (2–16 μM). Theoretical DINassimilation to denitrification ratio varied from12.0 to 24.8 for Ruppia vegetated and from 0.8 to 4.7 for unvegetated sediments. At Valle Smarlacca, Ruppia may influence nitrogen cycling by incorporating large DIN pools in biomass which is scattered in surrounding areas and fuels intense bacterial activity. With increasing anthropogenic nutrient input and insignificant organic matter export in the open sea the already severe eutrophic conditions are enhanced and may accelerate the decline of the macrophytemeadow.