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 .

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In this paper we investigate ecological flow networks via graph theory in search of the real sequential chains through which energy passes from producers to consumers in complex food webs. We obtain such fundamental pathways by identifying strongly connected components (SCCs), subsystems that groups species that take part in cycling, and performing topological sorting on the acyclic graphs that are obtained. Topological sorting identifies preferential directions for energy to flow from sources to sinks, while recycling remains confined within each SCC. Resolving food web networks for SCC highlights the possibility that compartments can be found in ecosystems, but this does not seem a general rule. The four aquatic food webs described in detail show a rather clear subdivision between benthic and pelagic subcommunities, a result that is discussed in the light of other studies. Should further research confirm these results, new insight into the way ecosystems use energy will be provided, with implications on cycling, reciprocal dependency of variables and indirect effects.

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Growth of the seaweed Ulva rigida C . Agardh was investigated in relation to biomass densities, internal nutrient pools and external nutrient supply . Research was carried out from 23 March to 5 July 1994 in the Sacca di Goro (Po Delta, Northern Italy), whose south-eastern part was covered by extensive mats of Ulva rigida . Two types of field experiments were conducted by incubating Ulva thalli inside large cages . In the first experiment, beginning on 23 March, 100 g of wet thalli were placed into the cages, allowed to grow for two weeks, then collected and replaced. This procedure was repeated 8 times over the study period. In the second experiment, Ulva thalli were left inside the cages and collected at selected time intervals (14, 27, 41, 64 and 76 days) in order to simulate the effects of increased density on growth and nutrient storage .We recorded specific growth rates (NGR) ranging from 0 .025 to 0 .081 d -' for a period up to two months in the repeated short-term experiments performed at relatively low initial algal densities (300-500 g AFDW m -3 ) . These NGR resulted significantly related to dissolved inorganic nitrogen (DIN) in the water column . Tissue concentrations of total Kjeldahl nitrogen (TN) were almost constant, while extractable nitrate decreased in a similar manner to DIN in the water column . Total phosphorus showed considerable variation, probably linked to pulsed freshwater inflow.In the long-term incubation experiment, NGR of Ulva was inversely related to density. Internal concentrations of both total P and TN reached maximum values after one month ; thereafter P concentration remained almost constant, while TN decreased below 2% w/w (by dry weight) . The TN decrease was also accompanied by an abrupt decrease in nitrate tissue concentration . The biomass incubated over the two month period suffered a progressive N limitation as shown by a decreasing N :P ratio (49 .4 to 14 .6) . The reciprocal control of Ulva against biogeochemical environment and viceversa is a key factor in explaining both resource competition and successional stages in primary producer communities dominated by Ulva . However, when the biomass exceeds a critical threshold level, approximately 1 kg AFDW m --', the macroalgal community switches from active production to rapid decomposition, probably as a result of selfshading, biomass density and development of anaerobic conditions within the macroalgal beds . IntroductionThe wide spread distribution of large opportunistic seaweeds has been recognized to depend on their growth In recent years there has been an increased production strategy, viz . the species-specific growth rate and its of large seaweeds in shallow coastal waters . These seasonal timing (Rosenberg & Ramus, 1981 ; Luning, macroalgae are mostly ephemeral Chlorophyceans, 1993) . Moreover, macroalgal growth is modulated by able to survive in fluctuating environments because factors such as light, nutrient availability, and temof their ability to rapidly take-up and store nutrients .perature, although in ...