Regulation of sedimentary processes by benthic communities
Vallisneria spiralis L.
Freshwater ecosystems in
human-impacted watersheds are subjected to increasing rates of nutrient and organic
matter supply. The decline of submerged macrophytes in eutrophic water bodies and the switch to phytoplankton or floating plant dominated states have been addressed to
a combination of increased turbidity and interstitial chemical conditions
unfavorable to roots. Some tolerant plant species have developed adaptations
that allow not only their survival along pronounced gradients of sedimentary
organic content, but also their fast response to short-term variations of pore
water chemistry, as those occurring seasonally in freshwater temperate ecosystems.
Vallisneria spiralis L. (Hydrocharitaceae family), a perennial stoloniferous species, is tolerant to eutrophication and colonizes both lentic and lotic environments. It performs photosynthesis in low light conditions, grows in nutrient-rich waters and on a wide range of substrates, from gravel bottoms to organic-rich muddy sediments. We performed several experiments with the aim of evaluate the rhizosphere-microbial communities interactions in freshwater ecosystems under excess nitrogen and organic matter availability. Different methodological approaches have been adopted (i.e. hydroponic incubations of plants, characterization of pore water, sediment microbial activity assays, incubations of intact cores or microcosms, measurements of growth rates) to investigate the following aspects:
I) direct (uptake) and indirect (oxygen release) effects of V. spiralis presence on pore water features and benthic fluxes of gases and nutrients;
II) V. spiralis plasticity to colonize substrates with increasing organic content and changes of its influence on redox-dependent processes along the gradient;
III) relation between assimilative (mediated by vegetation) and dissimilative nitrogen processes (mediated by bacteria) under excess nitrogen.
I) direct (uptake) and indirect (oxygen release) effects of V. spiralis presence on pore water features and benthic fluxes of gases and nutrients;
II) V. spiralis plasticity to colonize substrates with increasing organic content and changes of its influence on redox-dependent processes along the gradient;
III) relation between assimilative (mediated by vegetation) and dissimilative nitrogen processes (mediated by bacteria) under excess nitrogen.
Multiple evidences support the hypothesis
that V. spiralis varies seasonally
the oxygen quota transported to the below-ground tissues to counteract the
changing interstitial chemical conditions. Radial oxygen loss increases in the transition winter-summer and
peaks in
early autumn, when the lowest sediment redox occurs, due to a combination of
exhaustion of energy yielding electron acceptor pools and input of labile
organic matter from senescent meadows. Even if radial oxygen loss represents
only a small fraction in the plant oxygen economy, it can significantly affect
the sediment biogeochemistry of eutrophic sites. The oxygen injected in the pore
water by V. spiralis meadows constitutes
a relevant amount of the daily benthic oxygen consumption, with relevant
implications for the quality of both sediment and water column compartments in
term of oxidation status.
V. spiralis acts as an engineer species controlling actively interstitial features (NH4+, NOx-, PO43-, Fe2+ and CH4) over a wide range of trophic conditions and along its whole vegetative cycle. This plant promotes less reducing conditions in the rhizosphere and the consequent maintenance of an active nitrifying community. In sediments with a moderate organic enrichment, radial oxygen loss promotes denitrification coupled to nitrification, thus enhancing the ecosystem capacity to control nitrogen contamination. Furthermore, the high nitrogen availability in both pore water and water column weakens the competition between macrophytes and nitrifying and denitrifying bacteria, favoring nitrogen removal through a combination of plant uptake and dissimilative microbial processes. However, at extremely elevated organic enrichment, vegetated sediment lose their role as nitrogen traps due to inhibition of nitrification and plant stress induced by very reduced conditions.
This macrophyte plays a crucial role in driving water-sediment exchanges of gases and nutrients, partially buffering the negative effects of organic enrichment. Moreover, it modifies benthic dynamics with positive feedbacks for water bodies restoration (i.e. regeneration of ferric iron buffer and phosphorus retention in sediment, stimulation of coupled nitrification-denitrification, attenuation of internal organic load), which makes this plant an interesting option in programs for improving sediment conditions and favoring ecosystem recovery.
V. spiralis acts as an engineer species controlling actively interstitial features (NH4+, NOx-, PO43-, Fe2+ and CH4) over a wide range of trophic conditions and along its whole vegetative cycle. This plant promotes less reducing conditions in the rhizosphere and the consequent maintenance of an active nitrifying community. In sediments with a moderate organic enrichment, radial oxygen loss promotes denitrification coupled to nitrification, thus enhancing the ecosystem capacity to control nitrogen contamination. Furthermore, the high nitrogen availability in both pore water and water column weakens the competition between macrophytes and nitrifying and denitrifying bacteria, favoring nitrogen removal through a combination of plant uptake and dissimilative microbial processes. However, at extremely elevated organic enrichment, vegetated sediment lose their role as nitrogen traps due to inhibition of nitrification and plant stress induced by very reduced conditions.
This macrophyte plays a crucial role in driving water-sediment exchanges of gases and nutrients, partially buffering the negative effects of organic enrichment. Moreover, it modifies benthic dynamics with positive feedbacks for water bodies restoration (i.e. regeneration of ferric iron buffer and phosphorus retention in sediment, stimulation of coupled nitrification-denitrification, attenuation of internal organic load), which makes this plant an interesting option in programs for improving sediment conditions and favoring ecosystem recovery.
References
Pinardi, M., Bartoli, M., Longhi, D., Marzocchi, U., Laini, A., Ribaudo, C., Viaroli, P. (2009) Benthic metabolism and denitrification in a river reach: A comparison between vegetated and bare sediments. Journal of Limnology, 68 (1), pp. 133-145.
Racchetti E, Bartoli M, Ribaudo C, Longhi D, Brito EQL, Naldi M, Iacumin P and Viaroli, P. (2010) Short term changes in pore water chemistry in river sediments during the early colonization by Vallisneria spiralis. Hydrobiologia 652, 127–137.
Ribaudo, C., Bartoli, M., Racchetti, E., Longhi, D., Viaroli, P. (2011) Seasonal fluxes of O2, DIC and CH4 in sediments with Vallisneria spiralis: Indications for radial oxygen loss. Aquatic Botany, 94 (3), pp. 134-142.
Soana E., Bartoli M. (2013) Seasonal variation of radial oxygen loss in Vallisneria spiralis L.: an adaptation to sediment redox? Aquat. Bot. 104, 228–232.
Soana E, Naldi M and Bartoli M. (2012) Effects of increasing organic matter loads on pore water features of vegetated (Vallisneria spiralis L.) and plant-free sediments. Ecol. Eng. 47, 141–145.
Soana E., Bartoli M. (2014) Seasonal regulation of nitrification in a rooted macrophyte (Vallisneria spiralis L.) meadow under eutrophic conditions. Aquatic Ecology, DOI 10.1007/s10452-013-9462-z
Pinardi, M., Bartoli, M., Longhi, D., Marzocchi, U., Laini, A., Ribaudo, C., Viaroli, P. (2009) Benthic metabolism and denitrification in a river reach: A comparison between vegetated and bare sediments. Journal of Limnology, 68 (1), pp. 133-145.
Racchetti E, Bartoli M, Ribaudo C, Longhi D, Brito EQL, Naldi M, Iacumin P and Viaroli, P. (2010) Short term changes in pore water chemistry in river sediments during the early colonization by Vallisneria spiralis. Hydrobiologia 652, 127–137.
Ribaudo, C., Bartoli, M., Racchetti, E., Longhi, D., Viaroli, P. (2011) Seasonal fluxes of O2, DIC and CH4 in sediments with Vallisneria spiralis: Indications for radial oxygen loss. Aquatic Botany, 94 (3), pp. 134-142.
Soana E., Bartoli M. (2013) Seasonal variation of radial oxygen loss in Vallisneria spiralis L.: an adaptation to sediment redox? Aquat. Bot. 104, 228–232.
Soana E, Naldi M and Bartoli M. (2012) Effects of increasing organic matter loads on pore water features of vegetated (Vallisneria spiralis L.) and plant-free sediments. Ecol. Eng. 47, 141–145.
Soana E., Bartoli M. (2014) Seasonal regulation of nitrification in a rooted macrophyte (Vallisneria spiralis L.) meadow under eutrophic conditions. Aquatic Ecology, DOI 10.1007/s10452-013-9462-z