M. B. Cunha-Santino

Oxygen demand during mineralization of aquatic macrophytes from an oxbow lake

This study presents a kinetic model of oxygen consumption during aerobic decomposition of detritus from seven species of aquatic macrophytes: Cabomba furcata, Cyperus giganteus, Egeria najas, Eichhornia azurea, Salvinia auriculata, Oxycaryum cubense and Utricularia breviscapa. The aquatic macrophytes were collected from Oleo Lagoon situated in the Mogi-Guaçu river floodplain (SP, Brazil). Mineralization experiments were performed using the closed bottles method. Incubations made with lake water and macrophytes detritus (500 mL and 200 mg.L(-1) (DM), respectively) were maintained during 45 to 80 days at 20 degrees C under aerobic conditions and darkness. Carbon content of leachates from aquatic macrophytes detritus and dissolved oxygen concentrations were analyzed. From the results we concluded that: i) the decomposition constants differ among macrophytes; these differences being dependent primarily on molecular and elemental composition of detritus and ii) in the short term, most of the oxygen demand seems to depend upon the demineralization of the dissolved carbon fraction.

Anaerobic decomposition of a native and an exotic submersed macrophyte in two tropical reservoirs

Some aquatic plants have fast metabolism and growth, even at sub-optimal conditions, and become dominant in lentic environments such as large reservoirs, altering the nutrient cycle and impairing their environmental quality. There is great need in the knowledge impact processes of invasive species in aquatic environments, among the major, those related to the decomposition. This study evaluated the anaerobic decomposition of invasive submerged macrophytes Egeria densa Planch, native, and Hydrilla verticillata (L.f.) Royle, exotic in Porto Primavera and Jupiá reservoirs, Paraná basin. We evaluated the decay of organic matter, humification degree of the leached material, electrical conductivity and pH of the decomposition process. Mathematical models were utilised to describe the decomposition patterns over time. Both species showed the same heterogeneous pattern of decay of organic matter and carbon mineralisation. The models of carbon mineralisation, compared with the experimentally obtained data presented were adequate. Both species show no significant differences in the decomposition processes. Incubations of both species presented rapid t ½ for POC mineralisation and low DOC mineralisation.

Oxygen uptake from aquatic macrophyte decomposition from Piraju Reservoir (Piraju, SP, Brazil)

The kinetics of oxygen consumption related to mineralisation of 18 taxa of aquatic macrophytes (Cyperus sp, Azolla caroliniana, Echinodorus macrophyllus, Eichhornia azurea, Eichhornia crassipes, Eleocharis sp1, Eleocharis sp2, Hetereanthera multiflora, Hydrocotyle raniculoides, Ludwigia sp, Myriophyllum aquaticum, Nymphaea elegans, Oxycaryum cubense, Ricciocarpus natans, Rynchospora corymbosa, Salvinia auriculata, Typha domingensis and Utricularia foliosa) from the reservoir of Piraju Hydroelectric Power Plant (São Paulo state, Brazil) were described. For each species, two incubations were prepared with ca. 300.0 mg of plant (DW) and 1.0 L of reservoir water sample. The incubations were maintained in the dark and at 20 ºC. Periodically the dissolved oxygen (DO) concentrations were measured; the accumulated DO values were fitted to 1st order kinetic model and the results showed that: i) high oxygen consumption was observed for Ludwigia sp (533 mg g-1 DW), while the lowest was registered for Eleocharis sp1 (205 mg g-1 DW) mineralisation; ii) the higher deoxygenation rate constants were verified in the mineralisation of A. caroliniana (0.052 day-1), H. raniculoides (0.050 day-1) and U. foliosa (0.049 day-1). The oxygen consumption rate constants of Ludwigia sp and Eleocharis sp2 mineralisation (0.027 day-1) were the lowest. The half-time of oxygen consumption varied from 9 to 26 days. In the short term, the detritus of E. macrophyllus, H. raniculoides, Ludwigia sp, N. elegans and U. foliosa were the critical resources to the reservoir oxygen demand; while in the long term, A. caroliniana, H. multiflora and T. domingensis were the resources that can potentially contribute to the benthic oxygen demand of this reservoir.

Aerobic and anaerobic decomposition of Pistia stratiotes leachates from a tropical eutrophic reservoir (Barra Bonita, SP, Brazil)

The present study discussed the kinetic aspects of leachate decomposition from an aquatic macrophyte, Pistia stratiotes L (water lettuce). This species was collected from Barra Bonita Reservoir located in the State of São Paulo (Brazil). Decomposition chambers were prepared with high molecular weight (HMW), low molecular weight (LMW) and integral (INT = HMW + LMW) dissolved organic matter (DOM) diluted with reservoir water. The samples were incubated at 20 degrees C, in darkness and under aerobic or anaerobic conditions. For 79 days, the concentrations of dissolved oxygen (DO) and organic carbon (OC) were measured. For calculating the deoxygenation coefficients (k d) and maximum oxygen consumption (COmax) the concentration of DO was integrated and fitted to a first-order kinetics model, which also applied to the depletion of OC concentrations. The COmax of INT incubations were 4% higher than the sum of HMW and LMW fractions. The deoxygenation coefficients, k d, had the same order of magnitude for all treatments. In relation to carbon decay, regardless of the availability of oxygen, the INT DOM also showed higher mineralisation. These results suggest that the leachate mineralisations are short-term processes; when the fractionation of the leachates occurs, the LMW had organic compounds with more accessibility for heterotrophic metabolism. On the other hand, when compared to INT DOM, the HMW and LMW were less consumed suggesting an interaction of the reactivity of the leachate. Our data suggest that in the Barra Bonita Reservoir the mineralisation of P. stratiotes leachates occurs through two competitive pathways (i.e. mineralisation of the labile compounds and formation of recalcitrant organic resources and their mineralisation) in which the oxygen availability and the molecular mass of DOM can interfere in the rates of reactions.

Morphometry and retention time as forcing functions to establishment and maintenance of aquatic macrophytes in a tropical reservoir

Macrophytes may constitute an important resource for several chemical, physical and biological processes within aquatic ecosystems. This study considers that in tropical reservoirs with low retention time and with low values of shoreline development (DL), the expansion and persistence of aquatic macrophytes are mainly reported to local conditions (e.g., hydrodynamic and wind exposure) rather than trophic status and depth of the euphotic zone. In this context, this study aimed at describing and comparing the incidence of aquatic macrophytes in a throughflowing, non-dendritic tropical reservoir. During February 2006 to November 2007, eight limnological surveys were performed quarterly within the Ourinhos Reservoir, and in the mouth areas of its tributaries. At the six sampling stations 30 variables were measured. The number of sites with plants varied between 21 and 38 and at the end of the 1st year the total richness was found. The sampling survey outcome the recognition of 18 species of aquatic macrophytes; Cyperaceae (2 genera and 1 species), Pontederiaceae (3 species) and Onarograceae (3 genera) were the families with higher diversity. Seven species (Typha domingensis Pers., Myriophyllum aquaticum (Vell.) Verdec, Salvinia auriculata Aubl., Eichhornia azurea (Sw.) Kunth, Eleocharis sp1, Eichhornia crassipes (Mart.) Solms, Oxycaryum cubense (Poepp. & Kunth) Lye) always were present and were more frequent in the sites. The occurrence of emergent species predominated (45.9%), followed by submersed rooted (24.5%), free floating (19.5%), floating rooted (9.7%) and free submersed (0.3%). Although limnological variables and the distribution of macrophytes have discriminated the same sampling points, the stepwise multiple linear regressions did not pointed out strong correspondences (or coherence) among the most constant and distributed macrophyte species and the selected limnological variables, as well the trophic statuses. Seeing the low relationship among limnological variables and macrophytes distribution, in the case of Ourinhos Reservoir, the results pointed out that the water turbulence, low DL and wind exposure are the main driving forces that determine its aquatic plant distribution, life forms and species composition.

Differential aerobic decomposition between a native and exotic macrophytes of tropical reservoirs.

Decomposition is essential for carbon and nutrients cycling for all ecosystems (Hoorens et al., 2003). In lentic aquatic environments (e.g., lakes and reservoirs), aquatic macrophytes are the main autochthonous detritus source, and their mineralization is fundamental for maintaining biogeochemical cycles, associated with energy flux and detritus food-webs. (Bianchini Junior et al., 2014). In disturbed systems, some of these aquatic plants are potentially invasive due its high growth rates and elevated productivity (Pieterse and Murphy, 1990). The excessive growth of invasive submerged macrophytes, and consequently accumulation of biomass, provides increased rates of decomposition, leading to reductions in dissolved oxygen concentrations, changes of redox potential, increases of detritus accumulation rates and consequential changes in biogeochemical cycles (Reddy and Delaune, 2008). Considering the need for understand nutrient and carbon cycling processes involving invasive macrophytes, we compared the aerobic decomposition of two important same niche submerged macrophytes, the Brazilian native Egeria densa Planch. and the exotic Hydrilla verticillata (L.f.) Royle. We collected and processed samples of H. verticillata, E. densa and water in the Jupiá and Porto Primavera reservoirs, both located in Paraná River Basin, Brazil (Chiba de Castro et al., 2013) and aerobic incubations and carbon fraction estimations were carried out in the laboratory (Cunha-Santino and Bianchini Junior, 2002) during 80 days. The oxygen consumption (OC) kinetics from mineralization of aquatic plant detritus was fitted using a non-linear method (Levenberg-Marquardt iterative algorithm; Bianchini Junior et al., 2014). To verify differences among aerobic decomposition, a co-variance analysis test (ANCOVA) was applied (PAST, v.2.01). The kinetics parameterization obtained from the fitting of experimental data presented high determination coefficients; H. verticillata presented significant (ANCOVA; F = 36.42, p<0.001) higher oxygen consumption than E. densa (Figure 1). The significant differences between the OC from H. verticillata and E. densa mineralization can be explained by the lignin content. H. verticillata presents 50% higher refractory fraction content than E. densa due to lignin content (Chiba de Castro et al., 2013). The most recalcitrant quality of lignin, when compared with the other fibers (i.e. cellulose and hemicellulose), requires a great proportion of energy allocated to mineralization (Kourtev et al., 2002) and consequently high OC. The accentuated OC in the first days occurred due mineralization of labile compounds, displaying a high velocity and high oxygen demand. The subsequent decrease in OC is related with predominance of refractory compounds, with lower mineralization rates. Refractory fraction can be