Sediment Total Phosphorus Research Articles

Environmental changes in Chaohu Lake (southeast, China) since the mid 20th century: The interactive impacts of nutrients, hydrology and climate

Abstract Chaohu Lake, the fifth largest freshwater lake in the Yangtze floodplain, is faced with multiple stresses from anthropogenic disturbances and climate change. To explore the ecological changes in Chaohu Lake since the mid 20th century, we examined diatoms, geochemical indicators and particle size in 210 Pb-dated sediment core from the lake. Diatom succession revealed that the lake had switched to a eutrophic state since the late 1970s. Redundancy analysis using limnological data, hydrological and meteorological variables showed that sedimentary total phosphorus (TP) and total organic carbon (TOC), annual mean temperature, annual mean wind velocity, and water-level amplitude (WLA) were five significant factors influencing diatom succession. Diatom assemblages from 1950 till 1978 were driven by WLA and wind. The establishment of Chaohu Dam baffled hydrological connectivity between the lake and the Yangtze River in 1962, and reducing water exchange-induced flow. Meanwhile, weak wind velocity reduced the wind-induced flow in the 1960s. Due to the weak hydrodynamic intensity, the dominant species ( Aulacoseira granulata , a species with high sinking rate) became less important during this period. From 1979 till 2006, diatom assemblages were mainly driven by TP, TOC and temperature, which were highly correlative. Increasing nutrient loading promoted the blooms of eutrophic species (e.g., Cyclostephanos dubius ). In addition, rising temperature would indirectly influence diatom assemblages by mediating nutrient release process. As a consequence, multiple stresses in concert have caused the lake switch to a further eutrophic state indicated by prominent increases in more eutrophic species (e.g., Stephanodiscus parvus ) since 2000. This study provided information on complex trajectories of aquatic ecosystem shifts driven by increasing nutrient loading, hydrological alteration and climate warming in the Yangtze floodplain lake.

Historic primary producer communities linked to water quality and hydrologic changes in the northern Everglades

The northern Everglades Water Conservation Areas have experienced recent ecological shifts in primary producer community structure involving marl periphyton mats and dense Typha-dominated macrophyte stands. Multiple investigations have identified phosphorus (P) as a driver of primary producer community structure, but effects of water impoundment beginning in the 1950s and changes in water hardness [e.g., (CaCO3)] have also been identified as a concern. In an effort to understand pre-1950, primary producer community structure and identify community shifts since 1950, we measured pigment proxies on three sediment cores collected in Water Conservation Area-2A (WCA-2A) along a phosphorus enrichment gradient. Photosynthetic pigments, sediment total phosphorus content (TP), organic matter, total organic carbon and nitrogen were used to infer historic primary producer communities and changes in water quality and hydrology regulating those communities. Excess 210Pb was used to establish historic dates for the sediment cores. Results indicate the northern area of WCA-2A increased marl deposition and increased algal abundance ca. 1920. This increase in (presumably) calcareous periphyton before intensive agriculture and impoundment suggest canal-derived calcium inputs and to some extent early drainage effects played a role in initiating this community shift. The northern area community then shifted to Typha dominance around 1965. The areas to the south in WCA-2A experienced increased marl deposition and algal abundance around or just prior to 1950s impoundment, the precise timing limited by core age resolution. Continued increases in algal abundance were evident after 1950, coinciding with impoundment and deepening of canals draining into WCA-2A, both likely increasing water mineral and nutrient concentrations. The intermediate site developed a Typha-dominated community ca. 1995 while the southern-most core site WCA-2A has yet to develop Typha dominance. Numerous studies link sediment TP >650 mg P/kg to marsh habitat degradation into Typha-dominance. The northern and intermediate cores where Typha is currently support this previous research by showing a distinct shift in the sediment record to Typha dominance corresponding to sediment TP between 600 and 700 mg P/kg. These temporal and spatial differences are consistent with modern evidence showing water-column gradients in mineral inputs (including Ca, carbonates, and phosphorus) altering primary producer community structure in WCA-2A, but also suggest hydroperiod has an effect on the mechanisms regulating periphyton development and Typha dominance.

Benthic Phosphorus Dynamics in the Gulf of Finland, Baltic Sea

Benthic fluxes of soluble reactive phosphorus (SRP) and dissolved inorganic carbon (DIC) were measured in situ using autonomous landers in the Gulf of Finland in the Baltic Sea, on four expeditions between 2002 and 2005. These measurements together with model estimates of bottom water oxygen conditions were used to compute the magnitude of the yearly integrated benthic SRP flux (also called internal phosphorus load). The yearly integrated benthic SRP flux was found to be almost 10 times larger than the external (river and land sources) phosphorus load. The average SRP flux was 1.25 ± 0.56 mmol m−2 d−1 on anoxic bottoms, and −0.01 ± 0.08 mmol m−2 d−1 on oxic bottoms. The bottom water oxygen conditions determined whether the SRP flux was in a high or low regime, and degradation of organic matter (as estimated from benthic DIC fluxes) correlated positively with SRP fluxes on anoxic bottoms. From this correlation, we estimated a potential increase in phosphorus flux of 0.69 ± 0.26 mmol m−2 d−1 from presently oxic bottoms, if they would turn anoxic. An almost full annual data set of in situ bottom water oxygen measurements showed high variability of oxygen concentration. Because of this, an estimate of the time which the sediments were exposed to oxygenated overlying bottom water was computed using a coupled thermohydrodynamic ocean–sea and ecosystem model. Total phosphorus burial rates were calculated from vertical profiles of total phosphorus in sediment and sediment accumulation rates. Recycling and burial efficiencies for phosphorus of 97 and 3%, respectively, were estimated for anoxic accumulation bottoms from a benthic mass balance, which was based on the measured effluxes and burial rates.

Spatial distribution characteristics of phosphorus forms in sediment of Lake Hongfeng, Guizhou Province

选取贵州省红枫湖这一典型的亚深水型人工湖泊作为对象,研究了6个代表性湖区沉积物柱芯磷的赋存形态,结合水体温度和溶解氧现场监测结果,探讨了该类型湖泊沉积物内源磷释放风险.研究结果表明,红枫湖沉积物总磷含量普遍较高,表层沉积物总磷含量均值大于1500 mg/kg,明显高于下部沉积物.NaOH-SRP和rest-P是沉积物磷的主要赋存形态.NH₄Cl-P和BD-P含量表层最高,随深度增加明显降低,rest-P含量在垂向上变化不大.人为活动是造成沉积物总磷及赋存形态空间分布差异的主要原因,工业废水和网箱养鱼活动等大大增加了沉积物NaOH-SRP含量.红枫湖深水湖区底部水温多在14.5~23.5℃之间,随季节变化底部水温差异明显,溶解氧含量通常不高于1.5 mg/L,整体处于季节性缺氧或厌氧状态.以NaOH-SRP为主的沉积物磷赋存形态和深水缺氧环境大大增加了红枫湖沉积物内源磷释放的风险,在未来的研究中亟待加强对该类型湖泊沉积物内源磷释放机制、控制因素及治理措施的研究.;Lake Hongfeng is a man-made deep lake. Phosphorus of sediment samples that were collected from representative six water areas were analyzed. Water temperature and dissolved oxygen were investigated four times in the field by multi-parameter water quality observation analyzer YSI. The results showed that total phosphorus in sediment was generally high. The average value of total phosphorus in surface sediment was more than 1500 mg/kg, significantly higher than that in the bottom sediments. NaOHSRP and rest-P were the main forms of phosphorus in sediment. NH₄Cl-P, BD-P and HCl-P contents decreased with depth, and rest-P contents were stabile in vertical. Human activities (e. g. chemical waste water and artificial fish) greatly increased NaOHSRP contents in sediment. Temperature of bottom water was between 14.5-23.5℃, and the maximum temperature differences was 9.9℃ in different seasons. The bottom water was seasonal anoxic or anaerobic, which may result in changes of sediment phosphorus forms. Risk of endogenous phosphorus release increased due to anoxic condition of sediment-water interface in deep lake.

Relating sediment phosphorus mobility to seasonal and diel redox fluctuations at the sediment–water interface in a eutrophic freshwater lake

Relationships between phosphorus cycling and redox conditions in the sediments of eutrophic Missisquoi Bay, Lake Champlain were investigated over diel and seasonal timescales in three consecutive summers (2007–2009), one of which (2007) did not experience a cyanobacteria bloom for the first time in a decade. Sediment extraction data showed that reactive phosphorus (RP) is strongly correlated to reactive iron (RFe), suggesting that the mobility of a large portion (30–40%) of the total sediment phosphorus pool is associated with nanocrystalline iron oxide minerals that may be influenced by redox conditions. RP concentrations in the top sediments increased throughout 2007 but decreased throughout 2008; calculations indicate that ~ 1 mm of sediment could account for the increased total dissolved phosphorus observed in overlying water. Redox conditions were measured over 24 h in situ within sediment cores and at the sediment–water interface (SWI) at different stages of each season using voltammetry. SWI redox conditions became progressively more reduced across the season and overnight and were significantly more reducing in the presence of a bloom. Soluble RP and cyanobacteria cell counts measured at five depths through the water column strongly correlated with the most reducing conditions at the SWI. Observations suggest that redox controlled nutrient flux between the sediments and the water column is variable over diel and seasonal cycles. Cyanobacteria blooms most significantly affect SWI redox conditions, suggesting that blooms may enhance RP flux from sediments, setting up a positive feedback loop that can propagate and sustain blooms in shallow freshwater systems.

Eutrophication of Tenkiller Reservoir, Oklahoma, from nonpoint agricultural runoff

Tenkiller Ferry Reservoir, a large (51.6 km2) US midcontinent reservoir in Oklahoma, switched from oligo-mesotrophic prior to 1975 to eutrophic by 1986, evidenced by changes in phytoplankton taxa, chlorophyll (Chl), total phosphorus (TP), transparency, and areal hypolimnetic oxygen deficit. External TP loading increased 2.5-fold between 1974 and 2004, mainly as nonpoint loading from disposal of an annual (2001–2004) average 406,818 metric tons (mt) of untreated poultry litter on watershed pastures, which added an annual (2001–2006) average 4120 mt of TP. Phosphorus runoff from litter, estimated as 5 % of applied, was 63 % of external loading to the Illinois River from 2001– 2004, 71 % after the 2004 waste water treatment plant upgrade. The 9 % load decrease from the upgrade did not affect Chl. Sediment TP release, seldom determined for reservoirs, accounted for 16 % of annual external plus internal TP load. Trophic state graded from riverine to lacustrine zone. In wet summers with low residence times, the lacustrine zone was eutrophic; in dry summers it was mesotrophic. Transition and riverine zones were always eutrophic or hypereutrophic. Trophic state assessment ideally requires multiple sampling years in all reservoir zones. A similar Oklahoma reference reservoir, Broken Bow, with modest watershed poultry activities and low inflow TP concentrations, was oligo-mesotrophic. Rehabilitation of Tenkiller Reservoir requires large reductions of TP loading by ceasing watershed litter disposal, changes in watershed management practices, and application of in-reservoir procedures. [Supplemental materials are available for this article. Go to the publisher's online edition of Lake and Reservoir Management to view the supplemental file.]

Phosphorus in sediments and pore waters of selected Polish dam reservoirs

Abstract This study aimed to examine and explain the spatial distribution of total phosphorus in sediments and orthophosphates in interstitial water in four Polish dam reservoirs with varying hydrological and geochemical characteristics. The results are linked to physicochemical parameters of sediments: organic matter and calcium carbonates content, pH, redox potential and conductivity. In two of the studied reservoirs hydrological dynamics were a driving force in phosphorus circulation, despite the big difference in their mean water retention times, and interactions with Ca and Fe were of secondary importance In other reservoirs with complicated shorelines and reservoir beds, due to water flow disturbance the hydrological impact was not as pronounced. Geochemical processes, such as precipitation of phosphorus associated with Ca+2, Fe(III) or in complexed with humic substances became more significant.

Effects of environmental changes on the succession of diatom assemblage during the last 50 years in Lake Chaohu

Ecosystem in shallow lakes is generally subjected to the effects of multiple stressors,including nutrient,hydrology and climate.It is important to identify the impacts of multi-stressors for understanding the mechanism of environmental change in the lakes.Based on sedimentary proxies(i.e.,210Pb and 137Cs dating,diatom,geochemical indicators and particle size) from Chaohu sediment core and documentary data(i.e.,hydrology,climate,population and agriculture) in Chaohu basin,this study estimated quantitatively the effects of nutrient,hydrology and climate on diatom succession since 1950 using the redundancy analysis.Before 1978 diatom assemblages were dominated by Aulacoseira granulata,followed by an abrupt increase in eutrophic species(i.e.Cyclostephanos dubius).Since 2000 diatom assemblages were characterized by C.dubius,with increases in eutrophic species(e.g.,Cyclostephanos tholiformis and Stephanodiscus parvus).Results showed that sedimentary total phosphorus,total nitrogen,annual mean temperature,annual mean wind speed and annual water level amplitude were five significant variables,explaining solely 6.5%,4.7%,4.5%,4.7% and 3.0% of the variances in diatoms respectively.Therefore,nutrient input was the most important factor on diatom succession.Meanwhile,changes in climate and hydrological conditions also imposed an effect on diatom succession in Lake Chaohu during the last 50 years.

Accumulation of phosphorus in coastal marine sediments: relationship to benthic and diffusive fluxes

Sedimentary phosphorus was characterized in sediment cores from 3 coastal ecosystems of the Gulf of Cadiz. High spatial variability was observed in total phosphorus (from 445 to 20291 μg g.sed -1 ) and in the other phosphorus phases studied. This variability correlates with the proximity of the 10 sampling stations to sources of urban and/or industrial effluent in the zone. The benthic and diffusive fluxes were measured concurrently with sediment collection at these stations. The measured values of benthic fluxes range between –14 and 6 mmol m -2 d -1 . Generally, stations that showed increased interstitial phosphate concentrations with increasing depth were characterized by positive values in phosphate benthic fluxes and by high percentages of reactive forms of sedimentary phosphorus. Negative benthic fluxes were associated with stations receiving more anthropogenic matter, which showed progressively decreasing phosphate concentrations in the interstitial water with depth. In these anthropogenic areas, the non-reactive forms of phosphorus (those associated with ferric oxyhydroxide and authigenic carbonate fluorapatite) are abundant, and reach values exceeding 75% of total phosphorus in sediment.

Phosphorus Speciation in the Surface Sediment and River Water from the Orontes (Asi) River, Turkey

The paper presents the results of the distribution of phosphorus bioavailability (total, organic, and inorganic phosphorus) in surface sediment and river water from the Orontes (Asi) River (Samandag, Antakya, Turkey) in March 2008. Speciation of inorganic phosphorus was carried out using a method on sequential extractions, with each releasing four forms of inorganic phosphorus-loosely sorbed phosphorus, phosphorus bound to aluminum (P-AL), phosphorus bound to iron (P-Fe), and phosphorus bound to calcium (P-Ca). Total phosphorus concentrations are, on average, 3.724 mg/L and 312.30 mg/kg dry weight in the river water and surface sediment, respectively. The interstitial water of the surface sediment contains less than 1% of the surface sediment's total phosphorus pool. The most abundant form of inorganic phosphorus is P-Ca. Relative abundances of the remaining phosphorus forms both in surface sediment and river water follow the order P-Al > loosely bound-P > P-Fe.

Ecology and distribution of diatoms in Biscayne Bay, Florida (USA): Implications for bioassessment and paleoenvironmental studies

The spatial and temporal distribution of planktonic, sediment-associated and epiphytic diatoms among 58 sites in Biscayne Bay, Florida was examined in order to identify diatom taxa indicative of different salinity and water quality conditions, geographic locations and habitat types. Assessments were made in contrasting wet and dry seasons in order to develop robust assessment models for salinity and water quality for this region. We found that diatom assemblages differed between nearshore and offshore locations, especially during the wet season when salinity and nutrient gradients were steepest. In the dry season, habitat structure was primary determinant of diatom assemblage composition. Among a suite of physicochemical variables, water depth and sediment total phosphorus (STP) were most strongly associated with diatom assemblage composition in the dry season, while salinity and water total phosphorus (TP) were more important in the wet season. We used indicator species analysis (ISA) to identify taxa that were most abundant and frequent at nearshore and offshore locations, in planktonic, epiphytic and benthic habitats and in contrasting salinity and water quality regimes. Because surface water concentrations of salts, total phosphorus, nitrogen (TN) and organic carbon (TOC) are partly controlled by water management in this region, diatom-based models were produced to infer these variables in modern and retrospective assessments of management-driven changes. Weighted averaging (WA) and weighted averaging partial least squares (WA-PLS) regressions produced reliable estimates of salinity, TP, TN and TOC from diatoms ( r 2 = 0.92, 0.77, 0.77 and 0.71, respectively). Because of their sensitivity to salinity, nutrient and TOC concentrations diatom assemblages should be useful in developing protective nutrient criteria for estuaries and coastal waters of Florida.

Spatial and temporal distributions of nitrogen, phosphorus and heavy metals in the intertidal sediment of the Chang jiang River Estuary in China

Spatial and temporal distributions of nutrients (N and P) and heavy metals (Cu, Zn, Pb and Cd) in the intertidal sediments at Dongtan wetland of the Changjiang River (Yangtze River) Estuary were examined by laying one transect running seaward to landward during January, April, June, August and October of 2005. Total nitrogen (TN) and heavy metal concentrations in the intertidal sediment showed an increasing trend from open bare flat to the high marsh, with the maximum concentrations in the high marsh and the minimum values in open bare flat. In contrast, sediment total phosphorus (TP) concentrations changed little among sampling sites. One-way ANOVA found that there were significantly temporal variations of nutrients and heavy metals concentrations in the intertidal sediment (P < 0.05). The concentrations of heavy metals generally showed an increasing trend from January to October. As compared with other large estuaries, heavy metal contamination in the intertidal sediment of the Changjiang River Estuary was relatively low.

Abundance and Chemical Speciation of Phosphorus in Sediments of the Mackenzie River Delta, the Chukchi Sea and the Bering Sea: Importance of Detrital Apatite

Utilizing a sequential extraction technique this study provides the first quantitative analysis on the abundance of sedimentary phosphorus and its partitioning between chemically distinguishable phases in sediments of the Bering Sea, the Chukchi Sea and the Mackenzie River Delta in the western Arctic Ocean. Total sedimentary phosphorus (TSP) was fractionated into five operationally defined phases: (1) adsorbed inorganic and exchangeable organic phosphorus, (2) Fe-bound inorganic phosphorus, (3) authigenic carbonate fluorapatite, biogenic apatite and calcium carbonate-bound inorganic and organic phosphorus, (4) detrital apatite, and (5) refractory organic phosphorus. TSP concentrations in surface sediments increased from the Chukchi Sea (18 μmol g−1 of dried sediments) to the Bering Sea (22 μmol g−1) and to the Mackenzie River Delta (29 μmol g−1). Among the five pools, detrital apatite phosphorus of igneous or metamorphic origin represents the largest fraction (~43%) of TSP. The second largest pool is the authigenic carbonate fluorapatite, biogenic apatite as well as CaCO3 associated phosphorus (~24% of TSP), followed by the Fe-bound inorganic phosphorus, representing ~20% of TSP. The refractory organic P accounts for ~10% of TSP and the readily exchangeable adsorbed P accounts for only 3.5% of TSP. Inorganic phosphorus dominates all of phosphorus pools, accounting for an average of 87% of the TSP. Relatively high sedimentary organic carbon and total nitrogen contents and low δ13C values in the Mackenzie River Delta together with the dominance of detrital apatite in the TSP demonstrate the importance of riverine inputs in governing the abundance and speciation of sedimentary phosphorus in the Arctic coastal sediments.

Phosphorus and organic matter enrichment in snowmelt and rainfall–runoff from three corn management systems

The physical nature of sediment transport favors the movement of smaller and less dense materials, typically resulting in greater (enriched) sediment total phosphorus (TP) and organic matter (OM) concentrations than those of the parent soil. In rainfall–runoff, residue cover significantly influenced P enrichment, with the median TP enrichment ratio (TP ER) for corn-grain (1.4) being significantly greater ( p = 0.007) than that for corn-silage (0.8) and manured corn-silage (1.0) fields, which were similar. While greater P enrichment was observed in snowmelt compared to rainfall–runoff, attributable to correspondingly high OM ER values, TP ER was unaffected by treatment differences ( p = 0.60). The OM enrichment was much higher (~ 10×) during snowmelt (9.7) compared to rainfall–runoff (0.97). Low flow rates over frozen surfaces may have favored enhanced transport of OM and clay-sized particles in snowmelt. Crop residue contributions from corn-grain systems favor greater transport of organic P forms (i.e., low dissolved reactive P/total dissolved P ratios) whereas inorganic P dominance was observed in silage fields. Additional intra-event runoff data generated for fields with different crop-row orientations (contour vs. up-and-down the slope) provided more insights into the temporal dynamics of P enrichment process. The TP ER values were generally inversely related to flow rate and soil loss. This study addresses crucial knowledge/information gaps on the role played by the mode of runoff generation (rainfall–runoff vs. snowmelt) and field roughness conditions (due to crop residue, crop-row orientations) on P enrichment from row-cropped agricultural systems.

Surficial sediment phosphorus fractions along a biogeochemical gradient in Nyanza (Winam) Gulf, northeastern Lake Victoria and their possible role in phosphorus recycling and internal loading

Different phosphorus fractions and metal element composition of surficial sediments were measured on three occasions in 2005 and 2006 along a transect between Nyanza Gulf and offshore Lake Victoria, in order to assess the potential for sediments to contribute to the water column P concentrations in Lake Victoria. Total phosphorus (TP), apatite phosphorus (AP), inorganic phosphorus (IP) and organic phosphorus (OP) increased in sediments along the gulf towards the main lake while the non-apatite inorganic phosphorus (NAIP) increases were less defined. The longitudinal gradient of sediment TP and its fractions in Nyanza Gulf is a result of high rates of terrigenous input and resuspension and transport of the light, phosphorus rich inorganic and organic matter towards the main lake. TP in the sediment ranged from 812.7 to 1,738 mg/kg dry weight (DW) and was highest in the Rusinga Channel, the exchange zone between the gulf and the main lake. AP was the most important TP fraction, contributing between 35 and 57.3% of TP. Ca content in the sediment was strongly associated with TP and AP in the sediment (r2 = 0.92 and 0.98, respectively) in the gulf and the channel, indicating the importance of apatite in controlling P availability in these zones. In the gulf and the Rusinga Channel, the less bioavailable apatite phosphorus dominated, whereas in the deeper main lake OP was the major fraction illustrating the importance of anaerobic release of P from sediments and acceleration of internal P loading in the main lake.

Influence of the hyporheic zone on the phosphorus dynamics of a large gravel‐bed river, Garonne River, France

AbstractPhosphorus (P) concentrations in sediments and in surface and interstitial water from three gravel bars in a large river (Garonne River, southern France) were measured daily, downstream of a wastewater treatment plant for a city of 740 000 inhabitants (Toulouse). Measurements were made of vertical hydraulic gradient (VHG), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP) and total phosphorus (TP) in water and of three extractable forms of phosphorus (water extractable, NaOH extractable and H2SO4 extractable) in hyporheic sediments from the gravel bars. Dissolved phosphorus was the major contributor to TP (74–79%) in both interstitial and surface waters on all sampling dates, and in most cases surface water P concentrations were significantly higher than interstitial concentrations. Hyporheic sediment TP concentrations ranged between 269 and 465 µg g−1 and were highest in fine sediment fractions. Acid‐extractable P, a non‐bioavailable form, represented at least 95% of sediment TP. A positive relationship was observed between VHG and TP in two of the gravel bars, with wells that were strongly downwelling having lower TP concentrations. These results suggest that in downwelling zones, hyporheic sediments can trap surface‐derived dissolved P, and that much of this P becomes stored in refractory particulate forms. Bioavailable P is mainly present in dissolved form and only occupies a small fraction of total P, with particulate P comprising the majority of total P. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Variability in the Mobilization of Sediment and Phosphorus across 13 European Soils

The objectives of this study were to examine the variability in mobilization and transport of primary particles and associated total phosphorus (TP) in sediments eroded by overland flow from 13 European arable soils and to consider the empirical support for more process-based alternatives to modeling phosphorus (P) transfers. The 13 soils were subjected to simulated rainfall in laboratory experiments. Rainfall was applied to a soil flume (0.5 x 0.25 m) for 30 min at intensity of 60 mm h(-1), and all overland flow generated during this period was collected. Two simulations were performed 5 d apart. The soils generated a wide range of overland flow (13.3-26.9 mm) and sediment (1.1-16.9 g). The sediments from the experiments were enriched with medium silt particles (6-20 mum). Except for one soil in the second simulation, all of the study soils produced overland flow sediments (OFS) that were enriched with P (TP 976-3884 mg kg(-1), P enrichment ratio 0.92-4.42). Sediment TP was positively correlated (P < 0.05) with the sediment clay contents for both simulation events, and sediment total P enrichment was negatively correlated with soil TP. Attempts to reconstruct the TP content of sediments from TP analysis of the soil particle size fractions yielded significant (P < 0.05) relationships between reconstructed and measured TP values. However, the reconstructed TP values were consistently lower than the measured sediment TP contents, indicating that further improvements to measurement techniques are required if physically based modeling of P transport in overland flow is to be successful at small scales.

The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large, shallow lake

Distinct horizontal water column concentration gradients of nutrients and chlorophyll a (Chl a) occur within large, shallow, eutrophic Lake Taihu, China. Concentrations are high in the north, where some of the major polluted tributaries enter the lake, and relatively low in the south, where macrophytes generally are abundant. It is not clear, however, whether these water column concentration gradients are similarly reflected in spatial heterogeneity of nutrient concentrations within the bottom sediments. The main objective of this study was therefore to test if horizontal and vertical variations in the phosphorus and nitrogen content in bottom sediments of Lake Taihu are significantly related to (1) horizontal variations in overlying water column nutrient concentrations and (2) other sediment geochemical constituents. We measured the concentration of total phosphorus (TP) and total nitrogen (TN) in surficial sediments (0–2 cm) and TP, TN and Chl a concentrations in water column samples, collected from 32 sites in 2005. In 2006 sediment, TP, TN, carbon, iron and manganese concentrations were measured vertically at 2 cm intervals, extending to a depth of approximately 20 cm, at an additional eight sites. Linear correlation analysis revealed that surficial sediment TP concentrations across the 32 stations were related significantly, though weakly, to annual mean water column concentrations of TP, TN as well as Chl a. Correlations of surficial sediment TN with water column variables were, however, not significant (P > 0.05). Amongst the geochemical variables tested, the vertical variability of sediment TP concentrations was most strongly related to sediment manganese and carbon concentrations. A multiple stepwise linear regression revealed that the combination of sediment manganese and carbon concentrations explained 91% of the horizontal variability in sediment TP concentrations and 65% of the vertical variability.

Phytate as a novel phosphorus-specific paleo-indicator in aquatic sediments

A reliable geochemical paleo-indicator for phosphorus remains elusive, despite the importance of understanding historical changes in the nutrient status of aquatic ecosystems. We assessed the potential of phytate (salts of myo-inositol hexakisphosphate) as a novel phosphorus-specific paleo-indicator by measuring its concentrations in dated sediments from an embayment in Helsinki, Finland, with a known 200-year history of trophic changes. Phytate was extracted in a solution containing sodium hydroxide and EDTA and detected by solution 31P NMR spectroscopy with spectral deconvolution. Concentrations varied markedly with sediment depth and paralleled previously determined changes in diatom assemblages and geochemical indicators linked to trophic status. In contrast, total sediment phosphorus did not reflect phosphorus inputs to the embayment, presumably due to the mobilization of inorganic phosphate under anoxic conditions during periods of high pollutant loading. Importantly, phytate appeared to be stable in these brackish sediments, in contrast to other organic and inorganic phosphates which declined abruptly with depth. We therefore conclude that phytate represents a potentially important indicator of historical changes in phosphorus inputs to water bodies, although additional studies are required to confirm its stability under conditions likely to be encountered in lakes and coastal ecosystems.

Community Oxygen and Nutrient Fluxes in Seagrass Beds of Florida Bay, USA

We used clear, acrylic chambers to measure in situ community oxygen and nutrient fluxes under day and night conditions in seagrass beds at five sites across Florida Bay five times between September 1997 and March 1999. Underlying sediments are biogenic carbonate with porosities of 0.7–0.9 and with low organic content (<1.6%). The seagrass communities always removed oxygen from the water column during the night and produced oxygen during daylight, and sampling date and site significantly affected both night and daytime oxygen fluxes. Net daily average fluxes of oxygen (−4.9 to 49 mmol m−2 day−1) ranged from net autotrophy to heterotrophy across the bay and during the 18-month sampling period. However, the Rabbit Key Basin site, located in the west-central bay and covered with a dense Thalassia testudinum bed, was always autotrophic with net average oxygen production ranging from 4.8 to 49 mmol m−2 day−1. In November 1998, three of the five sites were strongly heterotrophic and oxygen production was least at Rabbit, suggesting the possibility of hypoxic conditions in fall. Average ammonium (NH4) concentrations in the water column varied widely across the bay, ranging from a mean of 6.9 μmol l−1 at Calusa in the eastern bay to a mean of 0.6 μmol l−1 at Rabbit Key for the period of study. However, average NH4 fluxes by site and date (−240 to 110 μmol m−2 h−1) were not correlated with water column concentrations and did not vary in a consistent diel, seasonal, or spatial pattern. Concentrations of dissolved organic nitrogen (DON) in the water column, averaged by site (15–25 μmol l−1), were greater than mean NH4 concentrations, and the range of day and night DON fluxes (−920 to 1,300 μmol m−2 h−1), averaged by site and date, was greater than the range of mean NH4 fluxes. Average DON fluxes did not vary consistently from day to night, seasonally or spatially. Mean silicate fluxes ranged from −590 to 860 μmol m−2 h−1 across all sites and dates, but mean net daily fluxes were less variable and most of the time contributed small amounts of silicate to the water column. Mean concentrations of filterable reactive phosphorus (FRP) in the water column across the bay were very low (0.021–0.075 μmol l−1); but site average concentrations of dissolved organic phosphorus (DOP) were higher (0.04–0.15 μmol l−1) and showed a gradient of increasing concentration from east to west in the bay. A pronounced gradient in average surficial sediment total phosphorus (1.1–12 μmol g DW−1) along an east-to-west gradient was not reflected in fluxes of phosphorus. FRP fluxes, averaged by site and date, were low (−5.2 to 52 μmol m−2 h−1), highly variable, and did not vary consistently from day to night or across season or location. Mean DOP fluxes varied over a smaller range (−8.7 to 7.4 μmol m−2 h−1), but also showed no consistent spatial or temporal patterns. These small DOP fluxes were in sharp contrast to the predominately organic phosphorus pool in surficial sediments (site means = 0.66–7.4 μmol g DW−1). Significant correlations of nutrient fluxes with parameters related to seagrass abundance suggest that the seagrass community may play a major role in nutrient recycling. Integrated means of net daily fluxes over the area of Florida Bay, though highly variable, suggest that seagrass communities might be a source of DOP and NH4 to Florida Bay and might remove small amounts of FRP and potentially large amounts of DON from the waters of the bay.