High Water Phase Research Articles

Estrutura da comunidade das larvas de Chironomidae (Insecta: Diptera), em diferentes substratos artificiais e fases hídricas, no trecho superior do rio Paraná, Estado do Paraná, Brasil

This study investigated the influence of hydrological phases on species composition, density and dominance of Chironomidae larvae in different types of artificial substrates. Samplings were undertaken fortnightly, between August 2004 and December 2005. Cluster analysis evidenced differences in the species composition and density of Chironomidae morphospecies among hydrological phases; according to the Kownacki dominance index, Cricotopus sp. was dominant or subdominant in all substrates and hydrological phases. During the low water phase, the greatest similarity was observed among the substrates with the same shape. In this period, the highest values of species richness and density of larvae were registered in X-shaped substrates. During the high water phase, the rise in the level of the Paraná River influenced on the reduction of mean larvae density, and only in wooden X-shaped and tube-shaped PVC substrates, whose materials best favored larvae fixing, high values of density were verified.

Heavy oil upgrading in the presence of high density water: Basic study

Heavy oil (Canada oil sand bitumen) upgrading in high density water (100 and 200 kg/m 3) at 723 K was performed by a batch reactor. Yields of asphaltene, maltene, and coke were evaluated. With increasing water density, the rate of coke formation was promoted. To get some hints of coke formation mechanism, the formed coke was observed by scanning electron microscope (SEM). The most part of the coke formed st neat pyrolysis (pyrolysis in the absence of high density water) was coalescent structure of some small coke particles, while that at pyrolysis in the presence of water (200 kg/m 3 of water density) was porous structure that indicated occurrence of phase inversion of coke precursors. Based on the results, the reaction mechanism of the heavy oil upgrading was considered: lighter oil was extracted in high density water and the concentration of light hydrocarbon decreased in a heavier oil phase, while the concentration of heavier oil in the oil phase increased. Thus, the lighter oil decomposed further in high density water phase and the heavier oil in the oil phase combined together to form coke due to its higher concentration.

The Solute Diffusion Coefficient in Variably Compacted, Unsaturated Volcanic Ash Soils

The solute diffusion coefficient in soil (Ds) and its dependency on the soil water content (θ), soil type, and compaction govern the transport and fate of dissolved chemicals in the soil vadose zone. Only a few studies have quantified solute diffusivity (Ds/D0, where Ds and D0 are the solute diffusion coefficients in soil and pure water, respectively) for variably compacted soils with different textures. We measured the Ds for KCl on five different soils from Japan: two volcanic ash soils (Andisols) at different bulk densities, two sandy soils, and a loamy soil. The Ds was measured across a wide range of θ using the half‐cell method. The Ds/D0 values for Andisols with bimodal pore size distribution were comparatively lower than for the other soils. Opposite to the behavior for sandy soils, the Ds/D0 for Andisols at a given θ decreased markedly with increasing bulk density under wet conditions but increased with increasing bulk density under dry conditions. Data for all soil types including sandy soils with unimodal pore size distribution implied a two‐region behavior when plotted as log(Ds/D0) vs. θ. We suggest that the similar behavior across soil types can be explained by regions of low and high water phase connectivity for relatively structureless soils and by high intraaggregate and low interaggregate water phase tortuosity for aggregated soils. Among a number of tested predictive models for Ds/D0, the Penman–Millington–Quirk model, which requires knowledge of only θ and total porosity, performed best across soil types.

Geomorphic history of low-perched, transgressive dune complexes along the southeastern shore of Lake Michigan

Abstract A general geomorphic history of low-perched coastal dunes along southeastern Lake Michigan is developed by combining new chronological data from P.J. Hoffmaster and Warren Dunes State Parks (SP) with published data from Van Buren SP, Silver Lake SP and dunes near Holland, Michigan. Fragmentary evidence of dunes older than 6 ka has been almost obliterated by active dune growth since the mid-Holocene Nipissing transgression of ancestral Lake Michigan. Aeolian activity continued during the drop from peak water levels ∼4.7 ka resulting in broad fields of low dunes. Aeolian activity halted during a period of low lake levels but was renewed with the development of large parabolic dune during the Algoma high-water phase of Lake Michigan at ∼3.2 ka. This was followed by reduced aeolian activity and development of the Holland Paleosol. Subsequent dune remobilization predates European settlement. High lake levels and land use practices cannot completely account for the pattern of aeolian activity which may be affected by changes in storm winds linked to changes in the paths of extratropical cyclones. Dune field morphology depends on the whether the shore is receding, prograding or stable. Simple lake-plain complexes form along receding shorelines where lakefront erosion exposes sediment to aeolian transport, leading to the preservation of a single set of large parabolic dunes migrating eastward with the shoreline. Compound lake-plain complexes form along stable or prograding shorelines. Here progressively younger dune ridges develop and blowouts migrate inland forming overlapping and nested parabolic dunes.

The role of vegetated areas on fish assemblage of the Paraná River floodplain: effects of different hydrological conditions

In this paper, we analyze the changes in composition and abundance of fish assemblages in seven vegetated floodplain wetlands with different connectivity across different hydrologic conditions: after a prolonged connection of the floodplain with the main channel, during receding water, and after a prolonged isolation. We also investigated the size and abundance of large-sized migratory species found in these wetlands and the food resources exploited by the dominant fish. Fishes were captured by diurnal seining (8.0 m x 1.50 m, 5 mm mesh) along macrophyte banks. Despite the high total number of species registered (100), sample species richness varied between 7 and 31, depending on the sampling site and the sampling date. Cluster analysis indicated low similarity between sites during both the isolation and the prolonged connection. Species turnover decreased from high water (β = 40.33) to low water (β = 33.83), with the minimum value of beta diversity index obtained during the isolation of the floodplain wetlands (β = 26.83). Our results indicated that different dominant populations of fish occur in different hydrological conditions, even though high water and isolation phases occur in the same season of different years. The ordination (NMDS) indicated the importance of hydrologic conditions in structuring fish assemblages in the studied floodplain. Small-sized characids, typically associated with macrophytes, dominated the fish assemblages, whereas the younger stages of large sized migratory species were found in low abundance. The maximum standard length of the fish captured was 28 cm and for large migratory fish, standard length varied between 1.6 and 25.0 cm. The dominant fish used several food resources, but littoral macrophytes-associated organisms had a high frequency of occurrence in the three hydrologic conditions. The high species richness of fish in the small, vegetated lakes was related to the high spatial heterogeneity during different hydrological conditions. Disturbances in the hydrological pulses could reduce the biodiversity by modifying the connectivity of the floodplain with the river channel. Conservation of these vegetated wetlands requires maintenance of actual width range of connectivity that provide diverse habitat along the time.

Stability and spatio-temporal structure in fish assemblages of two floodplain lagoons of the lower Orinoco River

Fish assemblage structure and variability were analyzed in two floodplain lagoons (Las Arhuacas and Los Cardonales) along the lower Orinoco over a hydrological cycle. Every three months during continuous three-day sampling, experimental gill nets (5 to 12.5 cm of mesh opening) and 1 mm-mesh seine nets were utilized according to the types of habitats presents. A total of 133 fish species were found in Las Arhuacas and 95 species in Cardonales. Fifty five and 17 species were exclusive to Las Arhuacas and Los Cardonales respectively, and 77 were common to both lagoons. In Las Arhuacas, the most speciesrich orders were Characiformes, Siluriformes, Perciformes and Gymnotiformes and in Los Cardonales, the most species-rich orders were Characiformes, Siluriformes, Clupeiformes and Perciformes. The richness, abundance and biomass were significantly higher (p < 0.001) in Arhuacas than in Cardonales. In general, the fishes assemblage was highly variable during the high water phase and moderately stable during low water phase in both lagoons, with more stability or less variability in Cardonales than Arhuacas. Also, there were significant differences in the fish assemblages between the two lagoons, mainly during low waters (ANOSIM; p < 0.001). The species that contributed most to the mean dissimilarity between the lagoons were Hypostomus argus, Aphanotorulus ammophilus, Potamorhina altamazonica, Prochilodus mariae, Loricaria gr. cataphracta, Oxydoras sifontesi, Hydrolycus armatus, Hyphopthalmus edentatus and Pterodoras rivasi. The last four species were more commonly collected in Los Cardonales. Also, the species of small size (mainly SL < 5 cm) such as Rhinosardinia amazonica, Moenkhausia sp. 1 "lepidura", Moenkhausia sp. 2, Aphyocharax alburnus, Characidium sp. 1, Moenkhausia sp. 3, Exodon paradoxus and Roeboides dientonito contributed to the mean dissimilarity among the beach and aquatic vegetation habitats. The patterns of the species assemblage organization were related to the dynamics of the floods. Non-random (i. e., deterministic) associations between species caused by the selection of habitats and/or to biological interactions, apparently were more common during low waters when there is an increased density of fish and so the biotic interactions are intensified. Stochastic associations, on the other hand, seemed to be more common during high water when species are more dispersed.

Numerical analysis of effects of tidal variations on storm surges and waves

This study examines the effects of tides on surges, wave setups and waves, in terms of tidal amplitudes and phases, by using a coupled numerical model of Surge, WAve and Tide (called as SuWAT). The SuWAT model, composed of depth integrated nonlinear shallow water equations and Simulating WAves Nearshore (SWAN) model, is able to simultaneously run with an arbitrary number of nested domains by using the Message Passing Interface. The results for an idealized case indicate that surge and wave setup are increased in the phase of low water and decreased in the high water phase; on the other hand, waves change in a reverse manner. Such changes are enhanced by large tidal variations. The conventional method (e.g., surge plus tide independently) has the possibility of overestimation for the total water level. The hindcast results for Typhoon Ewiniar in 2006 show that the run with tides is more accurate 10% than that without tides in coastal areas of Korea. The nested scheme improves the accuracy up to 40% for the prediction of water levels in the simulations. It is shown that the present coupled model, SuWAT, is capable of predicting both water levels and waves under storm events with reasonable accuracy against the observations.

Variação espacial e temporal de larvas de Odonata associadas com macrófitas aquáticas em duas lagoas da planície de inundação do alto rio Paraná, Brasil

Samples of aquatic macrophytes were collected on Guarana ( Eichhornia azurea ) and Patos ( E. crassipes and E. azurea ) Lakes from March 1992 to February 1993. On Guarana Lake the highest density and diversity were registered during the high water phase with dominance of Telebasis and Acanthagrion, while on Patos Lake, highest density and diversity were observed during the low water phase. The dominant taxa on Patos Lake were Telebasis, Acanthagrion, Neoneura (?), Coryphaeschna adnexa, Miathyria, Diastatops intensa and Erythemis . DCA and ANOVA differentiated Patos Lake mainly, because of the high abundance of C. adnexa, Miathyria, D. intensa and Erythemis which may be due to morphology of E. crassipes that shelters larger number of invertebrates. Water level variation of rivers influenced concentration of dissolved oxygen and pH. This variation was related the monthly fluctuation of larvae density. Difference between lakes shown in DCA analysis was chiefly due to variation of Odonata density.

Geochemical cycling of silver in marine sediments along an offshore transect

Although there have been many surface water and water column silver (Ag) analyses in the ocean, the absence of high resolution pore water and solid phase Ag profiles has hampered our understanding of its oceanic geochemical cycling. This manuscript presents pore water and solid phase profiles of Ag along an offshore transect in the northeast Pacific off the coasts of Washington/Oregon states, USA. Pore water Ag concentrations are uniformly low (< 0.3 nmol kg − 1 ) in profiles from sediments that have low bottom water oxygen concentrations, have shallow oxygen penetration depths (O 2,pen < 1 cm) and underlie short water columns (< 500 m water depth). The solid phase Ag concentrations at these sites are also low (< 1 μmol kg − 1 ). This is in contrast to sediments from intermediate water depths (~ 2000 m) that have similar oxygen penetration depths (O 2,pen < 1 cm), but have elevated pore water Ag concentrations (0.7 nmol kg − 1 ) at the sediment–water interface and higher solid phase Ag concentrations (4–8 μmol kg − 1 ). At sites from ~ 3000 to 4000 m water depth, where O 2,pen > 1 cm, pore water Ag concentrations reach extremely high concentrations in the top 5 cm (8–24 nmol kg − 1 ). High concentrations in pore waters provide evidence for a flux of Ag from ocean sediments, but the more oxidizing nature of these sediments precludes appreciable solid phase Ag accumulation in the top 30 cm (< 2 μmol kg − 1 ). The accumulation of Ag in sediments is not simply dependent on redox conditions; more oxidizing sediments do not accumulate solid phase Ag, and neither do more reducing sediments from shallow water depths. Only a sufficiently long water column will result in additional delivery of Ag to sediments by scavenging onto settling particles, and result in Ag accumulation in sediments where O 2,pen < 1 cm. Although upward Ag fluxes from sediments underlying shorter water columns are small (0.02–0.07 nmol cm − 2 y − 1 ), calculated fluxes increase for sediments underlying longer water columns and are largest for the more oxidizing sediments (2–5 nmol cm − 2 y − 1 ). Calculated fluxes of pore water Ag to the solid phase at these more oxidizing stations are inconsistent with measured solid phase Ag concentrations and suggest that the pore water profiles represent non-steady state conditions. Clearly, the early diagenesis of Ag is a highly dynamic process and more research is required to fully understand Ag cycling in sediments in continental margin locations.

Spatial and seasonal variations in the stable carbon isotopic composition of methane in stream sediments of eastern Amazonia

The stable carbon isotopic composition of methane (δ13C-CH4) gas bubbles formed in the sediments of three Amazonian streamswas determined over a 5-yr period. The study sites were two ‘várzea’ floodplain (A¸cu and Maicá) and one ‘terrafirme’ (Jamaraquá) streams. The δ13C of sedimentary organic matter (SOM) from the surrounding vegetation and bottom sediments were also determined. The mean δ13C value ofSOMwas lower in the terra-firme (-29.6‰) than in the várzea stream (-23.8‰) as a result of less C4 plant deposition in the former. The δ13C-CH4 values varied systematically both seasonally and spatially among the sites during all 5 yr of the study, in association with changes during hydrologic cycle. Overall, the variation in values of δ13C-CH4 during the high water phase covered a narrower range of values, -63 to -56‰. Contrastively, during the low water phase the δ13C-CH4 values for várzea and terra-firme streams are different and are in direct opposition. At this phase, the δ13C-CH4 at terra-firme stream is at least 20‰ depleted of 13C compared to várzea streams. Changes in organic matter sources, water levels and associated microbial degradation processes control the observed seasonal and spatial variations in net stable carbon isotopic composition of methane.

Abundance and diversity of phytoplankton in the Paraná River (Argentina) 220 km downstream of the Yacyretá reservoir

Patterns in the temporal composition, abundance and diversity of the phytoplankton community of the Paraná river prior to and after the initial filling phase of the Yacyretá reservoir are analyzed. The study site is located 220 km downstream from the Yacyretá reservoir and 30 km downstream from the confluence with the Paraguay river. Because both rivers remain separate and unmixed at the study site, we compared the possible effects of the impoundment on both river banks (left and right banks) in hydrological periods with similar duration and magnitude of the low and high water phases. Physical and chemical conditions measured on the right bank (water from the Paraguay river) were similar at both periods (pre and post-impoundment) whereas conductivity, pH and orthophosphate concentration increased on the left bank (water from the High Paraná river and Yacyretá reservoir) after the impoundment. Changes in phytoplankton density and diversity were observed only in samples collected from water flowing from the reservoir (left bank). The density of Chlorophyceae (Chloromonas acidophila, Chlamydomonas leptobasis, Choricystis minor, Chlorella vulgaris, Scenedesmus ecornis, Monoraphidium minutum, M. contortum and M. pusillum) and Cryptophyceae (Rhodomonas minuta, Cryptomonas marssonii and C. ovata) increased while Cyanophyceae (Cylindrospermopsis raciborskii, Raphidiopsis mediterranea and Planktolyngbya subtilis) and Bacillariophyceae (Aulacoseira granulata and its bioforms) decreased compared to previous studies conducted on the left bank of the Paraná river. Phytoplankton collected from the right bank of the river did not differ in pre and post- impoundment samples because they originate from the Paraguay river, which remains relatively unaffected by human activities.

Geographical specificity of the river’s regime in Bulgaria

This paper presents the types of river regime based on the three characteristics and two categories. The types are isolated from the number of cases with a high water and low water phases. The subtypes were detached from the duration of a high water and low water phases. There are three types (and seven subtypes) of river regime in Bulgaria. Mildness-Continental type is with spring high water phase, summer-autumn low water phase and transitional water during winter. Continental-Mediterranean type is with winter-spring high water phase and summer-autumn phase of low flow. Transitional type is with spring-summer high water phase and increase in flow during November and December, steady low water phase from July (August) until October and with the winter low water phase.

Vertical distribution of phytoplankton in a clear water lake of Colombian Amazon (Lake Boa, Middle Caquetá)

Vertical distribution of phytoplankton is highly influenced by physical and chemical factors, but the knowledge about these aspects remain unknown in Colombian Amazon lakes. In this work, the relations between the physico-chemical variables and the vertical distribution of phytoplankton community of a clear water lake (Lake Boa) are analyzed. Samples were taken at every 30 cm from surface to bottom in the center of the lake at different times of the hydrological cycle. Phytoplankton transformed from a diluted community dominated by cyanobacteria in high waters to one with elevated abundance of coccal chlorophytes in low waters. In the low water phase, the vertical distribution of Cryptomonas spp., Peridinium cf. umbonatum and Chlamydomonas spp. had significant statistical associations with the redox potential (RP), reactive phosphorus (PO4), and hydrogen sulfide (H2S) (p ≤ 0.05). During the flood stage the conditions were more varied and significant associations were detected between the vertical arrangement of Botryococcus braunii, Oscillatoria splendida, Dinobryon serturlaria, Euglena acus, and Trachelomonas volvocina with conductivity, dissolved oxygen (DO), pH, temperature, dissolved organic carbon (DOC), PO4, RP, and ammonia (NH4). The vertical structure of the Lake Boa’s phytoplankton responds to annual changes produced by the flood pulse, and to the vertical environmental variations, which are stronger during the high water phase.

Hydrological seasonality and benthic algal biomass in a Neotropical floodplain river

Our study evaluated spatiotemporal variation of physicochemical parameters and benthic and sestonic algal biomass in littoral areas of the main channel and floodplain lakes of the Cinaruco River, a lowland tributary of the Orinoco River in Venezuela. The Cinaruco is characterized by a highly predictable annual flood pulse, high transparency, and extremely low conductivity and suspended sediment load. During 2002 and 2003, 10 sites in the main channel and floodplain lagoons were sampled on 12 occasions. Correspondence analysis based on water physicochemical variables and algal biomass showed that ˜80% of the variation in the multivariate space among sites and sampling dates was explained by 2 axes. During the high-water periods, river and lagoon sites showed high similarity in water variables and algal biomass. Conductivity, SiO3, and chlorophyll a concentrations (both benthic and sestonic) were low during the high-water phase. On the other hand, water variables and algal biomass differed between river and lagoon sites during the low-water period. The absence of flow in lagoons and consistently low algal biomass (sestonic and benthic) in river sites were the most important features of the spatial variability between main channel and lagoon sites during low-water phases. Benthic chlorophyll a was highly uniform at small spatial scales and significantly heterogeneous at large spatial scales. The annual flood regime of the Cinaruco, which drives the concentrations of dissolved materials and affects material interchanges between aquatic and terrestrial systems, also appears to be responsible for creating strong patterns of seasonal and spatial variation in benthic algal crops.

Depth- and current-induced effects on wave propagation into the Altamaha River Estuary, Georgia

A study of sea surface wave propagation and its energy deformation was carried out using field observations and numerical experiments over a region spanning the midshelf of the South Atlantic Bight (SAB) to the Altamaha River Estuary, GA. Wave heights on the shelf region correlate with the wind observations and directional observations show that most of the wave energy is incident from the easterly direction. Comparing midshelf and inner shelf wave heights during a time when there was no wind and hence no wave development led to an estimation of wave energy dissipation due to bottom friction with corresponding wave dissipation factor of 0.07 for the gently sloping continental shelf of the SAB. After interacting with the shoaling region of the Altamaha River, the wave energy within the estuary becomes periodic in time showing wave energy during flood to high water phase of the tide and very little wave energy during ebb to low water. This periodic modulation inside the estuary is a direct result of enhanced depth and current-induced wave breaking that occurs at the ebb shoaling region surrounding the Altamaha River mouth at longitude 81.23°W. Modelling results with STWAVE showed that depth-induced wave breaking is more important during the low water phase of the tide than current-induced wave breaking during the ebb phase of the tide. During the flood to high water phase of the tide, wave energy propagates into the estuary. Measurements of the significant wave height within the estuary showed a maximum wave height difference of 0.4 m between the slack high water (SHW) and slack low water (SLW). In this shallow environment these wave–current interactions lead to an apparent bottom roughness that is increased from typical hydraulic roughness values, leading to an enhanced bottom friction coefficient.

Gradiente litorâneo-limnético do fitoplâncton e ficoperifíton em uma lagoa da planície de inundação do Rio Mogi-Guaçu

O estudo visou analisar a estrutura e dinâmica do fitoplâncton e ficoperifiton durante um ciclo hidrologico no gradiente litorâneo-limnetico na Lagoa do Diogo, uma lagoa marginal da planicie de inundacao do Rio Mogi-Guacu (Estado de Sao Paulo). Foram realizadas quatro coletas durante o ciclo hidrologico (enchente, cheia, vazante e seca) em tres locais (agua aberta, interface limnetico-litorânea e dentro do banco de macrofitas aquaticas) para o fitoplâncton e na regiao litoral para o perifiton associado a Eichhornia azurea Kunth. Biomassa (clorofila-a) e densidade das classes de algas foram analisadas para ambas comunidades. Analise de componentes principais separou as duas maiores fases hidrologicas: chuvosa e seca. A primeira foi associada as maiores temperaturas, nivel de agua, menores valores de nitrato e as maiores densidades de Euglenophyceae, Cryptophyceae e Chrysophyceae e aos maiores teores de clorofila-a. O maior regime de perturbacoes nas epocas de enchente e cheia favoreceu grupos oportunistas, representados pelos fitoflagelados. Houve, ainda, a separacao do fitoplâncton da regiao litoral nas epocas de cheia, vazante e seca, indicando a influencia do gradiente espacial do sistema. A comunidade perifitica apresentou tendencia distinta da fitoplanctonica, com maiores valores de biomassa e densidades durante os periodos de vazante e seca. O regime hidrologico foi considerado a principal funcao de forca sobre a estrutura do fitoplâncton e perifiton, bem como sobre a interacao e o intercâmbio de algas entre essas comunidades na lagoa marginal.

Abundance and composition of Rotifera in an abandoned meander lake (Lago Amapá) in Rio Branco, Acre, Brazil

The rotifer community in Lago Amapá (abandoned meander lake of the Rio Acre floodplain) was investigated with respect to seasonal changes. Sampling was performed at three collection stations, generally weekly, during the dry season between May 8, 1997 and October 31, 1997, and during the rainy season between January 2, 1998 and February 24, 1998. The density and composition of rotifer species (48 taxons of rotifers) were determined, and their diversity and abundance were observed to be greater, respectively, at high-water and low-water. Anova was performed, by means of the F test, to test for seasonal differences in environmental variables and rotifers, showing that there was a highly statistically significant difference for pH and transparency (p &lt; 0.01). Pearson's correlation was used to show that rotifer numbers during the low-water phase correlated negatively with electrical conductivity (r = -0.8824, p &lt; 0.05) and during the high-water phase correlated negatively with depth (r = -0.7513, p &lt; 0.05). Seasonal changes, caused by an increase in water levels and the low diversification of niches, influenced the composition and abundance of the animal group studied.

TWO-DIMENSIONAL NUMERICAL ANALYSIS OF SUSPENDED SOLID IN HIJI RIVER ESTUARY

On the basis of the results of field measurements taken by the authors in 2001, numerical analysis for a suspended solid was carried out by two-dimensional numerical model that includes the aggregation process. The main objectives are to obtain the fundamental data for the effects of turbidity maximum performance of suspended solid on the environment of water quality and the bottom sediment in an estuary. Turbidity maximum occurs in the high water phase at a 6.6km point from the river mouth and the duration of the high concentration is about 1-2 hours. These results agree well with the field measurements. At a 5.5-6.7km point from the river mouth where the aggregation is active, the diameter of the floc is about 100-1, 000 times the diameter of suspended solid at the upper boundary and the effective density of the floc is approximately 13-56kg/m3. It is approximately the same as that of fluid.

Effects of water level, abiotic and biotic factors on bacterioplankton abundance in lagoons of a tropical floodplain (Paraná River, Brazil)

The purpose of this paper was to assess the influence of basic limnological variables on bacterioplankton abundance in twenty lagoons in the Upper Parana River floodplain, Brazil. Twelve abiotic (depth, total nitrogen, total phosphorus, dissolved organic carbon (DOC), water transparency, temperature, dissolved oxygen, alkalinity, pH, turbidity, electrical conductivity and total inorganic carbon), and two biotic (chlorophyll-a and rotifers abundance) limnological factors were used as independent variables to explain bacterial countings. Principal component analysis (PCA) was used to reduce the dimensionality of the abiotic data and chlorophyll-a. Subsequently, multiple regression analysis was used to examine the relationship between bacterial abundance and the principal component axes and rotifer densities. Bacterioplankton abundance fluctuated from 1.8 to 5.9 × 106 cells.ml−1 during the high water phase and from 1.4 to 4.0 × 106 cells.ml−1 during the low water phase and they were significantly higher during the former phase (t=3.06; p=0.007). Bacterial abundance was not significantly correlated with chlorophyll-a and rotifers abundance. On the other hand, it was positively correlated with principal components 1 and 2, indicating that DOC, total nitrogen and phosphorus concentration, temperature and pH are important to explain bacteria abundance. These results indicate that the input of different sources of DOC and inorganic nutrients from the floodplain into the lagoons during the high water phase is more important to explain bacterial abundance than autochthonous phytoplanktonic organic matter or predation by rotifers.

Do Melaleuca ericifolia SM. leaves suppress organic matter decay in freshwater wetlands?

Essential oils of paperbarks, Melaleuca spp., have been shown in laboratory studies to inhibit bacterial activity and slow the rate of cellulose decay. Field (Gippsland Lakes, south-eastern Victoria, Australia) and glasshouse experiments were conducted to test the hypothesis that leaves of Melaleuca ericifolia SM. (the swamp paperbark) suppressed the decay of leaf litter under conditions existing or mimicking those in natural wetlands. Under field conditions, neither brown nor green M. ericifolia leaves, at loadings that would normally occur in a typical paperbark wetland and over a range of leaf-mass ratios, significantly affected the decay rate of two common freshwater macrophytes, Triglochin procerum (water ribbon) and Phragmites australis (common reed). In contrast, glasshouse experiments showed that Melaleuca ericifolia leaves (both intact and ground, and both brown and green) suppressed decay of T. procerum when placed in moist conditions on the sediment surface. However, no inhibitory effect was observed when leaves were flooded. Purified, commercially available Melaleuca essential oil (extracted from Melaleuca alternifolia leaves) decreased the rate at which T. procerum leaves decayed under glasshouse conditions by as much as 43 %. As with entire leaves, the inhibitory effect of purified Melaleuca essential oil was greatest when the leaves were placed in moist, but not submerged, conditions. Experiments designed to test the possibility that the inhibition was due to a simple, physical (coating) effect rather than metabolic inhibition of microbes showed clearly that the effect was not due simply to the hydrophobic nature of essential oils. This study indicates that M. ericifolia leaf litter in freshwater wetlands probably exerts only a small effect on the rate at which leaves from other wetland plant species decay. Any inhibitory effect is likely to be greater after water levels drop (e.g., seasonally in a Mediterranean climate) and remaining leaf material is left moist on the surface of water logged sediments than when material is totally submerged, as during the wetland's high water phase.