Macrophyte communities and their impact on benthic fluxes of oxygen, sulphide and nutrients in shallow eutrophic environments

Abstract

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 .

Differences in the growth cycle ofRuppia cirrhosa(Petagna) Grande in a Mediterranean shallow system

“…Moreover, algal growth may limit the photosynthetic oxygen production by the Ruppia plants and consequently the oxygen transport to the roots, with a high accumulation of sulphide concentrations, resulting in the death of the roots and rhizomes (Azzoni et al 2001). Furthermore, algal decomposition led to significant fluxes of free sulphide (Viaroli et al 1996). In addition, different physiological requirements of algae and phanerogams led to mutual exclusion (Giusti & Marsili-Libelli 2005).…”

Nitrogen fluxes, denitrification and the role of microphytobenthos in microtidal shallow-water sediments:an annual study

“…2). Applying the 2 trophic indices Trophic Oxygen State Index (TOSI; Viaroli et al 1996) and the Benthic Trophic State Index (BTSI; Rizzo et al 1996), based on net oxygen production and community respiration, shows that both sites can be categorised as net autotrophic to highly autotrophic (BTSI 2-3) (Fig. 4).…”

Benthic fluxes of oxygen and nutrients in sublittoral fine sands in a north-western Mediterranean coastal area

“…Summer 2010). R and NPP rates were used to apply the Trophic State Oxygen Index (TOSI, Viaroli et al, 1996) and to assess benthic metabolism. The sampling station was photoautotrophic throughout the year as seen in Figure 4: highly photoautotrophic during Spring 2010, Spring 2011 and Autumn 2011 campaigns and net photoautotrophic in the remaining campaigns.…”

“…In these autotrophic conditions, phototrophs produce an excess of organic matter and play the role of nutrient sink (Gazeau et al, 2005). Viaroli et al, 1996) based on benthic respiration and benthic net primary production at sampling station in each campaign. TH: totally heterotrophic, NH: net heterotrophic, NA: net photoautotrophic, TA: highly photoautotrophic.…”

“…Viaroli et al, 1996) based on benthic respiration and benthic net primary production at sampling station in each campaign. TH: totally heterotrophic, NH: net heterotrophic, NA: net photoautotrophic, TA: highly photoautotrophic.…”