High Soluble Reactive Phosphorus Research Articles

Mechanisms underlying episodic nitrate and phosphorus leaching from poorly drained agricultural soils.

Poorly drained depressions within tile-drained croplands can have disproportionate environmental and agronomic impacts, but mechanisms controlling nutrient leaching remain poorly understood. We monitored nitrate and soluble reactive phosphorus (SRP) leaching using zero-tension soil lysimeters across a depression to upland gradient over 2 years in a corn-soybean (Zea mays L.-Glycine max [L.] Merr.) field in Iowa. We also measured stable isotopes (δ15N and δ18O) of nitrate to examine its sources and transformations. SRP concentrations peaked during winter and early spring after phosphorus (P) fertilization (mean=3mg P L-1), with highest values in the depression, and SRP was relatively stable thereafter (mean=0.3mg P L-1) irrespective of periods of high soil moisture that led to widespread iron (Fe) reduction across the field. During a near-average precipitation year, nitrate stable isotopes indicated direct leaching of fertilizer nitrate within days of application, followed by nitrification of fertilizer ammonium and several weeks of denitrification in depressional soils. Nevertheless, nitrate concentrations remained high (mean=28mg N L-1) in the depression despite strong isotopic evidence for denitrification (>48% N removal). During a wet year, nitrate concentrations were lower in the depression than upland and nitrate isotopes were highly variable, consistent with nearly complete nitrate removal by denitrification in the depression and significant denitrification in upland soils. We conclude that poorly drained depressional soils can potentially decrease nitrate leaching via denitrification under sustained wet conditions, but they inconsistently denitrify and are vulnerable to high nitrate and SRP losses when soils are not saturated, especially following fertilization.

Spatiotemporal investigation of geochemical and hydrological controls on release of soluble reactive phosphorus from the shallow aquifer of a riparian zone.

Phosphorus (P) that accumulates in agricultural riparian zones can be released under certain hydrological and biogeochemical conditions, thereby limiting the effectiveness of these zones in reducing P loads from field to stream. The study objective was to explore factors that may be contributing to, or limiting, high soluble reactive phosphorus (SRP) concentrations in the shallow aquifer of an alluvial upland riparian zone located in a continental climate. Field investigations including porewater sampling from six vertical nests, soil sampling, and continuous soil moisture, groundwater table, and redox measurements were conducted over 19 months. Porewater SRP concentrations were generally low in the aquifer considering all sampling times (median=14.7µg/L; interquartile range [IQR]=11.1µg/L, 287 samples). The overall low SRP may be due to low reducible labile soil P (median=21.1 µgP/g dw, IQR=10.9 µgP/g dw, 21 samples). However, high SRP concentrations(>52µg/L, 95% quartile) did occur intermittently in space and time with no clear spatial or temporal patterns. Analyses indicate that most high concentrations were likely not associated with factors previously reported to influence SRP release in riparian aquifers, including redox conditions, pH, and soil drying and wetting. Further, data indicate that internally released or externally supplied SRP may undergo rapid (re-) sequestration within the aquifer, limiting its vertical or horizontal transport. The study findings highlight the complexity of P behavior in riparian zones and the need for caution when assessing the effectiveness of conservation practices and in interpreting potential impacts of subsurface water quality on stream water quality when monitoring locations are distant from the stream.

Nutrient structure dynamics and microbial communities at the water-sediment interface in an extremely acidic lake in northern Patagonia.

Lake Caviahue (37° 50 'S and 71° 06' W; Patagonia, Argentina) is an extreme case of a glacial, naturally acidic, aquatic environment (pH ~ 3). Knowledge of the bacterial communities in the water column of this lake, is incipient, with a basal quantification of the bacterioplankton abundance distribution in the North and South Basins of Lake Caviahue, and the described the presence of sulfur and iron oxidizing bacteria in the lake sediments. The role that bacterioplankton plays in nutrient utilization and recycling in this environment, especially in the phosphorus cycle, has not been studied. In this work, we explore this aspect in further depth by assessing the diversity of pelagic, littoral and sediment bacteria, using state of the art molecular methods and identifying the differences and commonalties in the composition of the cognate communities. Also, we investigate the interactions between the sediments of Lake Caviahue and the microbial communities present in both sediments, pore water and the water column, to comprehend the ecological relationships driving nutrient structure and fluxes, with a special focus on carbon, nitrogen, and phosphorus. Two major environmental patterns were observed: (a) one distinguishing the surface water samples due to temperature, Fe2+, and electrical conductivity, and (b) another distinguishing winter and summer samples due to the high pH and increasing concentrations of N-NH4+, DOC and SO42-, from autumn and spring samples with high soluble reactive phosphorus (SRP) and iron concentrations. The largest bacterial abundance was found in autumn, alongside higher levels of dissolved phosphorus, iron forms, and increased conductivity. The highest values of bacterial biomass were found in the bottom strata of the lake, which is also where the greatest diversity in microbial communities was found. The experiments using continuous flow column microcosms showed that microbial growth over time, in both the test and control columns, was accompanied by a decrease in the concentration of dissolved nutrients (SRP and N-NH4+), providing proof that sediment microorganisms are active and contribute significantly to nutrient utilization/mobilization.

Spatio-temporal variability of porewater phosphorus concentrations in streambed sediments of an agricultural stream

Streambed sediment and groundwater represent important stores of soluble reactive phosphorus (SRP) in freshwater catchments yet their contributions to watershed SRP loads remain unclear. This study evaluates the spatio-temporal variability and geochemical and hydrologic controls on porewater SRP in the streambed along an agricultural reach, including the influence of groundwater-surface water interactions. High porewater SRP (>1000 µg/L) and high available sediment-bound SRP were observed in low groundwater discharge zones, specifically in the shallow (≤0.1 m) streambed sediments. Porewater SRP concentrations were linked to iron and possibly manganese redox cycling with more-reducing conditions consistently coinciding with high SRP. Porewater SRP varied temporally but distinct seasonal changes across the stream reach were not observed. A shallow oxic cap that may limit SRP release to the stream was observed in the very shallow streambed but its presence was spatially and temporally discontinuous. Finally, data indicate P-rich shallow streambed sediments are the likely source of the observed high SRP porewater concentrations in the streambed rather than incoming groundwater from the adjacent fields. Overall, the findings highlight the complexity of SRP dynamics in agricultural streambeds and provide insight into processes governing the timing and location of SRP loading streams.

Variabilidad estructural de la comunidad de macrófitas en un gradiente pluviométrico en la provincia de Chubut

Mallines are Patagonian wetlands characterized by permanent waterlogging on its soils, sustaining small shallow lakes that hosts a vast biodiversity. In these ecosystems, aquatic plants (i.e. macrophytes) develop different life forms adapted to humid conditions, allowing them to regulate the physical and chemical conditions of the mallín. A distinctive characteristic of Patagonia is the exponential west-east decrease in rainfall, which generates three biozones: forest, ecotone and steppe. These biozones have been typified using landform plants, but whether macrophytes assemblages are related to each biozone or not has not been evaluated yet. The aim of this work is to compare the attributes of the macrophyte and wetlands landform plants community in each biozone. During the 2013-2014 austral Summer, 20 shallow lakes were sampled in the west province of Chubut, assessing macrophyte richness and physical and chemical water variables. Macrophytes were classified according to their life forms, origin, and its diversity was calculated. A total of 62 taxa were determined, most of them corresponding to the Cyperaceae, Poaceae and Ranunculaceae families. The steppe exhibited alkaline pH values, and higher conductivity and soluble reactive phosphorus (SRP) than the ecotone and the forest. High SRP values favored the abundance of macrophytes and macroalgae. However, three sites were eutrophic, exhibiting high chlorophyll a and total phosphorus values. Those sites showed higher abundances of macroalgae and emerging macrophytes. The sampled shallow lakes would act as reservoirs of macrophyte diversity at both local and regional scales. Thus, understanding the relationships among macrophytes and environmental variables could be useful for the mallines biodiversity conservation plans.

Influence of surface geology on phosphorus export in coastal forested headwater catchments in Akita, Japan

ABSTRACT Surface geology is a key factor regulating the geographical patterns of P export; however, geographically extensive data on P export mechanisms from natural coastal mountain regions are lacking. We compared stream-water characteristics in forested headwater catchments with different types of surface geology (marine sedimentary rocks [MSRs] and igneous rocks [IRs]) and evaluated the factors regulating soluble reactive phosphorus (SRP) concentrations in stream water. Geographically extensive stratified sampling was conducted: stream-water and sediment samples were obtained from 66 MSR catchments and 55 adjacent IR catchments along the coastal forested mountain regions in Akita, Japan. SRP and major cations and anions in stream waters were analyzed. To evaluate the effect of ionic composition on SRP, sediments were extracted with NaCl and CaCl2 solutions and the SRP concentrations in the extracts were measured. Stream SRP was significantly higher in MSR catchments (median 23 µg L−1) than in IR catchments (median 8 µg L−1) (P < 0.001), and tended to be higher in MSR catchments with younger rocks. Dissolved inorganic nitrogen (DIN) did not differ between catchments with different geologies; therefore, DIN/SRP was significantly lower in MSR catchments (median 18), close to the Redfield ratio. On a Piper diagram, high-SRP stream waters were of the NaCl water type, and stream SRP was significantly correlated with Na+ (r = 0.642, P < 0.0001). In IR catchments, stream waters had relatively higher Ca2+ and Mg2+ concentrations. SRP in the sediment extracts was significantly higher in MSR catchments and samples obtained by NaCl extraction than in IR catchments and CaCl2-extracted samples. Exchangeable Na+ in sediments was significantly higher in MSR catchments, decreasing Ca2+ in the liquid phase by an ion-exchange reaction, which is likely also key in maintaining high SRP levels. We conclude that MSR catchments in coastal Akita are a significant P source for streams even in headwater forests, and that site-specific NaCl water types and higher exchangeable Na+ concentrations in sediments result in higher SRP concentrations in stream waters. These results have critical implications for P cycles in freshwater ecosystems influenced by Na+.

Nutrients regeneration pathway, release potential, transformation pattern and algal utilization strategies jointly drove cyanobacterial growth and their succession

In order to better understand the contribution of nutrients regeneration pathway, release potential and transformation pattern to cyanobacterial growth and succession, 7 sampling sites in Lake Chaohu with different bloom degree were studied every two months from February to November 2018. The carbon, nitrogen (N) and phosphorus (P) forms or fractions in surface, interstitial water and sediments as well as extracellular enzymatic activities, P sorption, specific microbial abundance and community composition in sediments were analyzed. P regeneration pathway was dominated by iron-bound P desorption and phosphorus-solubilizing bacteria solubilization in severe-bloom and slight-bloom area respectively, which both resulted in high soluble reactive phosphorus (SRP) accumulation in interstitial water. However, in severe-bloom area, higher P release potential caused the strong P release and algal growth, compared to slight-bloom area. In spring, P limitation and N selective assimilation of Dolichospermum facilitated nitrate accumulation in surface water, which provided enough N source for the initiation of Microcystis bloom. In summer, the accumulated organic N in Dolichospermum cells during its bloom was re-mineralized as ammonium to replenish N source for the sustainable development of Microcystis bloom. Furthermore, SRP continuous release led to the replacement of Dolichospermum by Microcystis with the advantage of P quick utilization, transport and storage. Taken together, the succession from Dolichospermum to Microcystis was due to both the different forms of N and P in water column mediated by different regeneration and transformation pathways as well as release potential, and algal N and P utilization strategies.

Phosphorus characteristics and microbial community in the sediment-water-algal system during algal growth.

Phosphorus (P) characteristics in eutrophic lakes change during algal growth. Furthermore, algae have a significant relationship with the microbial communities of lake sediments. This study addressed the influence of algal growth and soluble reactive phosphorus (SRP) concentrations on P characteristics within the sediment-water-algal (SWA) system. Results indicated that the SWA system simulating a high algal bloom level (SWA-HAB) had a correspondingly high SRP concentration (258.9 μg L-1), and that algal growth promoted a high SRP concentration in the overlying water. The high SRP concentration in overlying water could support algal growth, resulting in a high chlorophyll a (Chl-a) concentration (285.23 mg L-1). During algal growth, the P release flux was high in sediments from the high-SRP SWA system, with the highest P release flux measured at 0.982 mg m-2 day-1. Furthermore, microbial community abundance had a significant relationship with Chl-a concentrations in overlying water (p < 0.05) and increases with algae growth.

Sediment phosphorus fraction and release potential in the major inflow rivers of Chaohu Lake, Eastern China

Surface sediment samples from nine inflow rivers of Chaohu Lake in Eastern China were analyzed by the sequential extraction procedure and the zero equilibrium phosphorus concentration (EPC0) was determined to evaluate the behavior of phosphorous at the water–sediment interface. The results reveal that substantial amounts of P had accumulated in river sediments from agricultural runoff and domestic sewage. For most of the sample sites, the total P content was higher in sediments from urban rivers than in those from rural rivers. On average, the reactive inorganic P consisted of slightly more than 60% of the total P, with Fe-P being the dominant P form. Compared with rural sediments, the Al-P contents were significantly higher in urban sediments, while the Res-P contents were significantly lower. The differences can be attributed to the differential discharge of P from watersheds with contrasting land uses and to the dramatic changes in the ambient conditions of urban rivers. A higher soluble reactive phosphorus (SRP) value of the overlying water relative to the EPC0 value in the mouth of the urban rivers of Chaohu Lake indicates that bottom sediments still play a role in adsorbing phosphorus from the water column before it reaches Chaohu Lake.

Carbon and nutrient export regimes from headwater catchments to downstream reaches

Abstract. Excessive amounts of nutrients and dissolved organic matter in freshwater bodies affect aquatic ecosystems. In this study, the spatial and temporal variability in nitrate (NO3−), dissolved organic carbon (DOC) and soluble reactive phosphorus (SRP) was analyzed in the Selke (Germany) river continuum from three headwaters draining 1–3 km2 catchments to two downstream reaches representing spatially integrated signals from 184–456 km2 catchments. Three headwater catchments were selected as archetypes of the main landscape units (land use × lithology) present in the Selke catchment. Export regimes in headwater catchments were interpreted in terms of NO3−, DOC and SRP land-to-stream transfer processes. Headwater signals were subtracted from downstream signals, with the differences interpreted in terms of in-stream processes and contributions from point sources. The seasonal dynamics for NO3− were opposite those of DOC and SRP in all three headwater catchments, and spatial differences also showed NO3− contrasting with DOC and SRP. These dynamics were interpreted as the result of the interplay of hydrological and biogeochemical processes, for which riparian zones were hypothesized to play a determining role. In the two downstream reaches, NO3− was transported almost conservatively, whereas DOC was consumed and produced in the upper and lower river sections, respectively. The natural export regime of SRP in the three headwater catchments mimicked a point-source signal (high SRP during summer low flow), which may lead to overestimation of domestic contributions in the downstream reaches. Monitoring the river continuum from headwaters to downstream reaches proved effective to jointly investigate land-to-stream and in-stream transport, and transformation processes.

From Bacteria to Fish: Ecological Consequences of Seasonal Hypoxia in a Great Lakes Estuary

The occurrence of bottom-water hypoxia is increasing in bodies of water around the world. Hypoxia is of concern due to the way it negatively impacts lakes and estuaries at the whole ecosystem level. During 2015, we examined the influence of hypoxia on the Muskegon Lake ecosystem by collecting surface- and bottom-water nutrient samples, bacterial abundance counts, benthic fish community information, and performing profiles of chlorophyll and phycocyanin as proxies for phytoplankton and cyanobacterial growth, respectively. Several significant changes occurred in the bottom waters of the Muskegon Lake ecosystem as a result of hypoxia. Lake-wide concentrations of soluble reactive phosphorus (SRP) and total phosphorus increased with decreasing dissolved oxygen (DO). Bacterial abundance was significantly lower when DO was less than 2.2 mg L−1. Whereas there were no drastic changes in surface chlorophyll a concentration through the season, phycocyanin increased threefold during and following a series of major wind-mixing events. Phycocyanin remained elevated for over 1.5 months despite several strong wind events, suggesting that high SRP concentrations in the bottom waters may have mixed into the surface waters, sustaining the bloom. The fish assemblage in the hypolimnion also changed in association with hypoxia. Overall fish abundance, number of species, and maximum length all decreased in catch as a function of bottom DO concentrations. The link between hypoxia and wind events appears to serve as a positive feedback loop by continuing internal loading and cyanobacterial blooms in the lake, while simultaneously eroding habitat quality for benthic fish.

Fate of Phosphorus in a Grass Buffer Zone Under Concentrated Flow Condition

The grass buffer zone (GBZ) is commonly used a best management practice for the control of agricultural pollutants. However, the sorption and removal mechanisms of pollutants within GBZ have not been clearly identified. We studied changes in soluble reactive phosphorus (SRP) from dairy house wastewater in the GBZ, after the wastewater application to the field was stopped. The results showed that Morgan soil test P levels in the saturated area were low compared to the soils in the relatively drier area. High SRP concentrations &gt;4 mg/L were observed in monitoring wells near the discharge points and in the downslope area. The SRP concentration as high as 9.4 mg/L near the discharge point in the saturated area was caused by the highly reduced soil conditions, while the SRP concentration in the downslope area was likely due to the topographic convergence. The SRP concentrations for the wells in the GBZ decreased &lt;10% of the influent dairy house wastewater concentration once the soil was dry after an application has been intentionally shut down. Therefore, we concluded that the GBZ may play a significant role to remove P from wastewater under the condition that the soil stays aerobic.

Detrital stoichiometry as a critical nexus for the effects of streamwater nutrients on leaf litter breakdown rates

Nitrogen (N) and phosphorus (P) concentrations are elevated in many freshwater systems, stimulating breakdown rates of terrestrially derived plant litter; however, the relative importance of N and P in driving litter breakdown via microbial and detritivore processing are not fully understood. Here, we determined breakdown rates of two litter species, Acer rubrum (maple) and Rhododendron maximum (rhododendron), before (PRE) and during two years (YR1, YR2) of experimental N and P additions to five streams, and quantified the relative importance of hypothesized factors contributing to breakdown. Treatment streams received a gradient of P additions (low to high soluble reactive phosphorus [SRP]; ~10-85 µg/L) crossed with a gradient of N additions (high to low dissolved inorganic nitrogen [DIN]; ~472-96 µg/L) to achieve target molar N:P ratios ranging from 128 to 2. Litter breakdown rates increased above pre-treatment levels by an average of 1.1-2.2x for maple, and 2.7-4.9x for rhododendron in YR1 and YR2. We used path analysis to compare fungal biomass, shredder biomass, litter stoichiometry (nutrient content as C:N or C:P), discharge, and streamwater temperature as predictors of breakdown rates and compared models containing streamwater N, P or N + P and litter C:N or C:P using model selection criteria. Litter breakdown rates were predicted equally with either streamwater N or P (R2 = 0.57). In models with N or P, fungal biomass, litter stoichiometry, discharge, and shredder biomass predicted breakdown rates; litter stoichiometry and fungal biomass were most important for model fit. However, N and P effects may have occurred via subtly different pathways. Litter N content increased with fungal biomass (N-driven effects) and litter P content increased with streamwater P availability (P-driven effects), presumably via P storage in fungal biomass. In either case, the effects of N and P through these pathways were associated with higher shredder biomass and breakdown rates. Our results suggest that N and P stimulate litter breakdown rates via mechanisms in which litter stoichiometry is an important nexus for associated microbial and detritivore effects.

Spatial and temporal variations in nutrients in water and riverbed sediments at the mouths of rivers that enter Lake Hachiro, a shallow eutrophic lake in Japan

We evaluated the spatial and temporal variations in dissolved inorganic nitrogen (DIN=NO3−+NO2−+NH4+) and soluble reactive phosphorus (SRP) concentrations and the processes that control them in water and riverbed sediments at the mouths of five main rivers (MTN, BBM, IKW, TYK, and BFM) that enter Lake Hachiro, a shallow eutrophic lake in Japan. At the river mouths, we sampled water monthly and analyzed the nutrient concentrations and dissolved N2O and CH4 concentrations. We also sampled riverbed sediments to determine the SRP release potential, inorganic P fractions, and denitrification potential (dNp). Riverine SRP concentrations showed large spatial and temporal variations, with values increasing from summer to autumn in the three southernmost rivers (IKW, TYK, and BFM). High dissolved CH4 concentrations were observed in association with high SRP concentrations, indicating that SRP was released from the sediments under hypoxic conditions. In contrast, NO3− concentrations did not differ among the rivers and decreased during summer, likely owing to denitrification, thereby decreasing DIN/SRP ratios to a level suitable for the formation of summer algal or cyanobacterial blooms in the three southernmost rivers. Sediment incubation demonstrated high SRP release rates and high dNp values in the sediments of these rivers due to their higher iron (Fe)-bound P and organic matter contents. The riverine SRP concentration was significantly correlated with dissolved CH4 and sediment dNp in positive, and DIN/SRP in negative. In conclusion, differences in Fe-bound P and sediment organic matter content suggested to cause spatial and temporal variations in water quality, with high SRP release and denitrification under hypoxic summer conditions, resulting in lower DIN/SRP ratios that may trigger algal or cyanobacterial blooms in this shallow eutrophic lake.

Comparison of quartz sand, anthracite, shale and biological ceramsite for adsorptive removal of phosphorus from aqueous solution

The choice of substrates with high phosphorus adsorption capacity is vital for sustainable phosphorus removal from waste water in constructed wetlands. In this study, four substrates were used: quartz sand, anthracite, shale and biological ceramsite. These substrate samples were characterized by X-ray diffractometry and scanning electron microscopy studies for their mineral components (chemical components) and surface characteristics. The dynamic experimental results revealed the following ranking order for total phosphorus (TP) removal efficiency: anthracite > biological ceramsite > shale > quartz sand. The adsorptive removal capacities for TP using anthracite, biological ceramsite, shale and quartz sand were 85.87, 81.44, 59.65, and 55.98 mg/kg, respectively. Phosphorus desorption was also studied to analyze the substrates' adsorption efficiency in wastewater treatment as well as the substrates' ability to be reused for treatment. It was noted that the removal performance for the different forms of phosphorus was dependent on the nature of the substrate and the adsorption mechanism. A comparative analysis showed that the removal of particulate phosphorus was much easier using shale. Whereas anthracite had the highest soluble reactive phosphorus (SRP) adsorptive capacity, biological ceramsite had the highest dissolved organic phosphorus (DOP) removal capacity. Phosphorus removal by shale and biological ceramsite was mainly through chemical adsorption, precipitation or biological adsorption. On the other hand, phosphorus removal through physical adsorption (electrostatic attraction or ion exchange) was dominant in anthracite and quartz sand.

Elevated Dissolved Phosphorus in Riparian Groundwater along Gaining Urban Streams

Findings of low concentrations of dissolved phosphorus in groundwater in large surveys [e.g., United States Geological Survey's National Water-Quality Assessment (NAWQA) Program ( Dubrovsky, N. M.; et al. The Quality of Our Nation's Water: Nutrients in the Nation's Streams and Groundwater, 1992-2004 . U.S. Geological Survey Circular 1350; USGS : Reston, VA , 2010 . ); >5000 wells] support the common perception that groundwater is generally of little importance for transporting phosphorus. Here, we address whether this applies to urban riparian settings, where discharging groundwater may potentially contribute to urban stream syndrome and downstream eutrophication problems. This survey study includes 665 samples of groundwater collected along gaining stream reaches at six urban sites. Considering the combined sample set, 27% had soluble reactive phosphorus (SRP) concentrations >0.1 mg L(-1), which is more than double that determined in the NAWQA Program (12%), while for individual sites the range was 12-52%, excluding one site with consistently low SRP (0%). None of the sites showed significant correlation between SRP and the artificial sweetener acesulfame, a promising wastewater indicator, including two with known wastewater contamination (but the lowest SRP). Rather, high SRP concentrations were associated with geochemically reducing conditions. This could mean that natural aquifer or stream sediment materials were a primary contributor of the elevated SRP observed in this study.

Hydrochemistry, ostracods and diatoms in a deep, tropical, crater lake in Western Mexico

Crater lakes are sensitive to natural and anthropogenic environmental changes and it is important to define and understand their current status in order to evaluate past changes related to climate variability and future ones associated with modern global change. We sampled lake waters, surface sediments and sediment trap samples (dry vs wet season) in lake Santa Maria del Oro (SMO), Western Mexico (21°22’58’’N, 104°34’48’’W, 750 m asl). Its present condition was assessed in terms of: i) thermal and oxygen stratification patterns, ii) hydrochemistry and nutrient status, iii) carbonate precipitation (dry vs wet season), and iv) spatial (depth) and seasonal (dry vs wet season) distribution of ostracod and diatom communities. Our results indicate that this 65-m deep lake is warm monomicitic, with a stable thermal stratification for most of the year (thermocline 16-24 m, metalimnetic gradient up to 7ºC). The water column is thermally homogeneous from late January to early March. Dissolved oxygen is vertically homogeneous only in January, when deoxygenation occurs throughout the water column. This is the first report of such a situation in a Mexican lake. Santa Maria del Oro has slightly alkaline, [HCO3–]-[Cl–] and [Na+]&gt;[Mg2+]&gt;&gt;[Ca2+] waters. Although it is a freshwater system, total dissolved solids, electrical conductivity (EC) and [Cl–] indicate that the lake has undergone evaporative concentration. Aragonite precipitation, occurring during the wet and warm part of the year, favours Ca2+ depletion and Cl– enrichment of lake waters. This is a mesotrophic lake with relatively high soluble reactive phosphorus and silica levels. Nitrogen reaches its highest values during winter mixing (January), but becomes the limiting nutrient during stratification. Ostracod and diatom assemblages differ in specific regions of the lake. Ostracods are more diverse and abundant in littoral areas with dense vegetation, where the community is mainly formed by Potamocypris variegata, Cypridopsis vidua and Darwinula stevensoni. Diatoms show high diversity and low dominance in the littoral areas, where benthic, periphyticspecies (Amphora coffeaeformis, Planothidium delicatulum and Hippodonta lunemburgensis) are dominant. Deeper (&gt;10 m) areas show low diversity and high dominance, and sediment trap data indicate a diatom bloom during winter mixing, dominated by Aulacoseria ganulata and Nitzschia amphibia.

Impact of water quality on bacterioplankton assemblage along Cértima River Basin (central western Portugal) assessed by PCR–DGGE and multivariate analysis

The information on bacterial community composition (BCC) in Portuguese water bodies is very scarce. Cértima River (central western Portugal) is known to have high levels of pollution, namely organic. In the present work, the BCC from a set of 16 water samples collected from Cértima River Basin and its main tributaries was characterized using 16S rDNA-denaturing gradient gel electrophoresis, a culture-independent molecular approach. Molecular data were related to environmental parameters through multivariate analysis to investigate potential impact of water pollution along the river. Principal component analysis using environmental data showed a water quality gradient from more pristine waters (at the mountain tributaries) to waters with increasingly eutrophic potential (such as Fermentelos Lake). This gradient was mainly defined by factors such as organic and inorganic nutrient sources, electrical conductivity, hydrogen carbonate concentration, and pH. Molecular results showed variations in BCC along Cértima River Basin but in the main river section, a Bacteroidetes phylotype (Flavobacterium sp.) proved to be dominant throughout the river course. Multivariate analysis suggests that spatial variation of BCC along the Cértima River Basin depended mainly on parameters such as Chl a, total suspended solid (TSS), total organic carbon, electrical conductivity, and HCO[Formula: see text] levels. Bacteroidetes phylotypes were all related to higher electrical conductivity and HCO[Formula: see text] levels although some of these were also correlated with high SO[Formula: see text] and others with high soluble reactive phosphorus, nitrate, TN, and Kjeld-N levels. The Gammaproteobacteria occurrence was correlated with high SO[Formula: see text] levels. One of the Betaproteobacteria phylotypes showed to correlate with low redox potential (E(h)) and high temperature, pH, TSS, and Chl a levels while another one showed a negative correlation with Chl a values.

Seasonal modulation of bacterioplankton community at a temperate eutrophic shallow lake

At the present time, there is still a lack of information about environmental parameters modulating variations on bacterial diversity in temperate lakes, particularly from Portugal. Fermentelos Lake (Central Portugal) is a shallow water body that sustains an important wetland area. The strong nutrient inputs from agriculture and industrial runoffs have led to its current eutrophic status. The present work aimed to understand which factors modulate the seasonal bacterioplankton diversity at this lake using 16S rRNA PCR-denaturing gradient gel electrophoresis (DGGE) and multivariate analysis. Environmental data demonstrated eutrophic features throughout all samples with nitrate concentrations reaching 12.0 mg N (NO3−) l−1 in March 2006, while the highest conductivity (609 μS cm−1), soluble reactive phosphorus (0.37 mg l−1), total suspended solids (87.2 mg l−1) and chlorophyll a (286.6 μg l−1) levels were recorded in August 2007. Over the past two decades there was a general increase in nitrate, pH and conductivity levels at this lake, suggesting the eutrophication process is still in progress. Multivariate analysis showed that summer versus winter DGGE patterns could be established for bacterial assemblages and were mainly defined by water temperature and chlorophyll a. Actinobacteria were dominant throughout the study period although a general preference for higher temperature, pH, total suspended solids, conductivity, soluble reactive phosphorus (SRP) and chlorophyll a levels was observed. The highest concentrations of nitrogen sources were related to Bacteroidetes and phototrophic eukaryote (cryptophycean) dominance. The expansion of Betaproteobacteria, Alphaproteobacteria and Cyanobacteria phylotypes was generally associated to high temperature, pH, conductivity and SRP values.

Sources and transport of algae and nutrients in a Californian river in a semi‐arid climate

Summary1. To elucidate factors contributing to dissolved oxygen (DO) depletion in the Stockton Deep Water Ship Channel in the lower San Joaquin River, spatial and temporal changes in algae and nutrient concentrations were investigated in relation to flow regime under the semiarid climate conditions.2. Chlorophyll‐a (chl‐a) concentration and loads indicated that most algal biomass was generated by in‐stream growth in the main stem of the river. The addition of algae from tributaries and drains was small (c.15% of total chl‐a load), even though high concentrations of chl‐a were measured in some source waters.3. Nitrate and soluble‐reactive phosphorus (SRP) were available in excess as a nutrient source for algae. Although nitrate and SRP from upstream tributaries contributed (16.9% of total nitrate load and 10.8% of total SRP load), nutrients derived from agriculture and other sources in the middle and lower river reaches were mostly responsible (20.2% for nitrate and 48.0% for SRP) for maintaining high nitrate and SRP concentrations in the main stem.4. A reduction in nutrient discharge would attenuate the algal blooms that accelerate DO depletion in the Stockton Deep Water Ship Channel. The N : P ratio, in the main stem suggests that SRP reduction would be a more viable option for algae reduction than nitrogen reduction.5. Very high algal growth rates in the main stem suggest that reducing the algal seed source in upstream areas would also be an effective strategy.