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.
ABSTRACT1. Transitional waters are ecotones between terrestrial, freshwater and marine ecosystems, being characterized by high spatial heterogeneity and temporal variability.2. The EU Water Framework Directive (WFD) posed to the scientific community the challenge to classify these ecosystems into a small number of types, while retaining a functional classification of ecosystem types.3. A niche theory approach is proposed to identify the limiting forcing factors organizing biological quality elements, i.e. the limiting niche dimensions.4. The analysis of a macro-invertebrate dataset from published papers on 36 Italian lagoons suggested a two-level typological classification of Mediterranean lagoons.5. Basic ecological theories, such as niche and island biogeography theories, have fundamental implications for the process of developing a typological classification for all aquatic ecosystems, as required by the WFD.
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 .
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.