ABSTRACT(1) The succession of primary producer communities in coastal lagoons is analysed in the light of the regime shift theory. Pristine coastal lagoons are considered to be dominated by extensive meadows of seagrass species, which are assumed to take advantage of nutrient supply from sediments. An increasing nutrient input is thought to favour phytoplankton and/or epiphytic micro-, macroalgae as well as opportunistic ephemeral macroalgae that coexist with seagrasses. In the latest stages of this succession, the imbalance of phosphorus to nitrogen ratio can favour macroalgal, cyanobacteria and/or picoplankton blooms, often causing dystrophy.(2) The primary causes of shifts and succession in the macrophyte community are nutrient loadings, mainly nitrogen, as well as changes in coastal hydrology or interactions between them. To some extent, in very shallow choked lagoons, benthic vegetation is mainly controlled by loading rates, while in open deep estuaries hydromorphological factors predominate.(3) External stressors/perturbations cause an amplification in benthic biogeochemical processes, e.g. wide variations in primary productivity and dark respiration, with large oscillations in oxygen and sulphide concentrations. Altered biogeochemical processes can determine positive feedbacks inducing a shift from pristine to altered macrophyte communities, which in turn amplify the perturbation until the shift becomes irreversible.(4) Macrophyte typology, organic matter composition and sedimentary geochemistry are primary factors in controlling feedbacks and shifts. For example, the sedimentary buffering capacity of iron controls sulphide and phosphates, while nitrogen cycling is mainly controlled by primary producers -microbial process interactions.(5) The alternative states which occur through the transition from pristine to modified primary producer communities can also be viewed as a sequence of stable states with different degrees of embedded information and with different ecological functions.

The influences of suspended mussel and infaunal clam cultivation on benthic metabolism and nutrient cycling were compared in Goro lagoon, Italy. Both aquaculture types stimulated benthic metabolism, with sediment oxygen demand (SOD), CO 2 and ammonium effluxes of up to 14, 16 and 1.2 mmol m -2 h -1. However, whilst mussel farming preferentially stimulated anaerobic metabolism and sediment reduction, clam farming did not. The mussel ropes were also large oxygen sinks and ammonium sources, with oxygen consumption and ammonium production rates of 1.4 to 1.5 and 0.18 to 0.43 mmol kg -1 h -1. Consequently, the overall impacts of mussel farming on oxygen and nutrient dynamics were much greater than those of clam farming. There were also differences in nitrate-reduction processes and the nitrate sources that fuelled them. In winter, at high water column nitrate concentrations, highest nitrate reduction rates (~320 µmol m -2 h -1 ) occurred at the mussel farm. Nitrate reduction was driven predominantly by water column nitrate and ~30% of nitrate reduced was recycled to ammonium via dissimilatory nitrate reduction to ammonium (DNRA). At the control and clam farm sites, nitrate reduction rates were lower (~180 µmol m -2 h -1 ), nitrification supplied ~30% of nitrate and denitrification was dominant. In summer under low nitrate conditions, nitrate reduction was highest (~130 µmol m -2 h -1 ) at the mussel farm site, but this activity was completely dependent upon water column nitrate and 95% of nitrate was reduced via DNRA. In contrast, at the clam farm station, DNRA was unimportant and nitrification was the major nitrate source for denitrification. Consequently, whilst nitrate reduction processes eliminated fixed N from the clam farm sediments via coupled nitrificationdenitrification, the dominance of DNRA at the mussel farm site resulted in a net N input to the sediment compartment. These large differences in the impacts of clam and mussel farming can be explained by the fact that infaunal clams stimulate transfer of both organic matter and oxygen to the sediment, whereas suspended mussels enhance only organic matter inputs.

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Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from \5 to 1,133 lM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35-1,888 lmol N m -2 h -1 ) were up to two orders of magnitude higher than rates measured in isolated wetlands (2-231 lmol N m -2 h -1 ), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9-1,888 lmol N m -2 h -1 ) than in winter (2-365 lmol N m -2 h -1 ). Denitrification supported by water column nitrate (D W ) accounted for 60-100% of total denitrification (Dtot); denitrification coupled to nitrification (D N ) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.