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.

Organic enrichment may deeply affect benthic nitrogen (N) cycling in macrophyte meadows, either promoting N loss or its recycling. This depends upon the plasticity of plants and of the associated microbial communities, as those surrounding the rhizosphere. Rates of denitrification, dissolved inorganic N fluxes and N uptake were measured in sediments vegetated by the submerged macrophyte Vallisneria spiralis L. under increasing organic matter loads. The aim was to investigate how the combined N assimilation and denitrification, which subtract N via temporary retention and permanent removal, respectively, do vary along the gradient. Results showed that V. spiralis meadows act as regulators of benthic N cycling even in organic enriched sediments, with negative feedbacks for eutrophication. A moderate organic load stimulates N uptake and denitrification coupled to nitrification in the rhizosphere. This is due to a combination of weakened competition between macrophytes and N cycling bacteria and enhanced radial oxygen loss by roots. An elevated organic enrichment affects N uptake due to hostile conditions in pore water and plant stress and impairs N mineralisation and its removal via denitrification coupled to nitrification. However, the loss of plant performance is almost completely compensated by increased denitrification ofwater column nitrate, resulting in a shift between the relative relevance of temporary and permanent N removal processes

The responses of macroalgae to ocean acidification could be altered by availability of mac-ronutrients, such as ammonium (NH 4 +). This study determined how the opportunistic macro-alga, Ulva australis responded to simultaneous changes in decreasing pH and NH 4 + enrichment. This was investigated in a week-long growth experiment across a range of predicted future pHs with ambient and enriched NH 4 + treatments followed by measurements of relative growth rates (RGR), NH 4 + uptake rates and pools, total chlorophyll, and tissue carbon and nitrogen content. Rapid light curves (RLCs) were used to measure the maximum relative electron transport rate (rETR max) and maximum quantum yield of photosystem II (PSII) photochemistry (F v /F m). Photosynthetic capacity was derived from the RLCs and included the efficiency of light harvesting (α), slope of photoinhibition (β), and the light saturation point (E k). The results showed that NH 4 + enrichment did not modify the effects of pH on RGRs, NH 4 + uptake rates and pools, total chlorophyll, rETR max , α, β, F v /F m , tissue C and N, and the C:N ratio. However, E k was differentially affected by pH under different NH 4 + treatments. E k increased with decreasing pH in the ambient NH 4 + treatment, but not in the enriched NH 4 + treatment. NH 4 + enrichment increased RGRs, NH 4 + pools, total chlorophyll, rETR max , α, β, F v /F m , and tissue N, and decreased NH 4 + uptake rates and the C:N ratio. Decreased pH increased total chlorophyll content, rETR max , F v /F m , and tissue N content, and decreased the C:N ratio. Therefore, the results indicate that U. australis growth is increased with NH 4 + enrichment and not with decreasing pH. While decreasing pH influenced the carbon and nitrogen metabolisms of U. australis, it did not result in changes in growth.

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The impact of macrophyte communities on benthic fluxes has been analyzed in three shallow coastal environments : Etang du Provost (Mediterranean coast of France), characterized by the large floating macro-alga Ulva rigida ; Certes fishponds (Bassin d'Arcachon), covered by Ruppia cirrhosa ; and the inner intertidal mud-flat in the Arcachon Bay (French Atlantic coast), which has extensive Zostera noltii meadows . In these bodies of water, primary production is dependent primarily on the dominant seagrasses and macroalgae that are also responsible for the large quantity of organic matter deposited on the sediment surface . In 1993 and 1994, fluxes of oxygen, sulphide and nutrients were measured in early and late summer, which were selected in order to represent the production and decomposition phases of the dominant macrophytes . Experimental work was undertaken to measure : (1) standing crop of dominant macroalgae and rooted phanerogams and the elemental and macromolecular composition of plant biomass ; (2) benthic fluxes of oxygen, sulphide, nitrogen and phosphorus using incubation of multiple dark and light benthic chambers ; (3) water-sediment profiles of free-sulphide in sediment cores with rooted phanerogams (Ruppia) as well as with floating seaweeds (Ulva) .At the selected sampling sites, in addition to external (tides) and/or internal (sediment reactivity) factors, we observed differences in benthic fluxes which were clearly related to growth patterns and structure of the macrophyte communities . The Z noltii meadows were stable and characterized by slow growth and almost constant biomass . In the more sheltered sampling station in the Certes fishponds, R . cirrhosa beds showed a summer decrease due to extensive epiphyte growth . During the decomposition phase, significant fluxes of free-sulphide occurred inside the dark benthic chambers, probably due to the metabolism of the epiphytic layer . In the Etang du Provost, U. rigida achieved high biomass levels, even though the macroalgal beds exhibited a patchy distribution due to wind action and the hydrodynamics of the lagoon . In the decomposition phase, which was coincident with the annual dystrophic crisis the rapid decomposition of Ulva led to high fluxes of free sulphide .The shift from the production to decomposition phase resulted in substantial changes in nutrient recycling only in the macro-algal-dominated system . During the growth period dissolved inorganic nitrogen and phosphorus were kept at low levels due to macrophyte uptake . In contrast during the decomposition phase when the macroalgal biomass was mineralised, nitrogen and phosphorus were rapidly recycled . The same processes did not occur in the Certes fishponds probably because of the greater internal buffering capacity linked either to plant morphology/physiology or to the properties of the sediment .