Patterns and sources of variation in Daphnia phosphorus content in nature

It has recently been shown that Daphnia can vary in the phosphorus (P)-content of their body tissues, but the relative importance of genetic versus environmental causes for this variation is unexplored. We measured variation in P-content (as % body mass) of Daphnia from eight lakes and conducted experiments to contrast three sources of variation: interspecific variation, clonal variation and phenotypic plasticity. Daphnia P-content decreased with increasing seston C:P ratio across lakes. This relationship reflected both inter- and intraspecific variation. Daphnia parvula and D. dubia exhibited high P-content and were found in shallow lakes with low C:P seston, whereas D. pulicaria had low P-content and was found in deep, stratified lakes having high C:P seston. Populations of D. dentifera spanned this lake gradient and exhibited P-content that was negatively related to seston C:P. Evidence for phenotypic plasticity came from experiments with D. pulicaria and D. dentifera collected from a lake with P-deficient seston and fed a P-sufficient diet in the laboratory. In addition, populations of D. dentifera differed in P-content even after 7 d of feeding on P-sufficient resources, suggesting within-species clonal variation. However, mesocosm experiments revealed broad and surprisingly continuous variation in the P-content of individual clones of D. pulex (range 1.54–1.05%) and D. mendotae (1.51–1.07%) over a gradient in dietary C:P. The broad range in P-content exhibited by individual clones, acclimated for generations, suggests that variation in Daphnia P-content from laboratory experiments needs to be interpreted with caution. These results also show that phenotypic variation in response to environment can be a larger source of variation in P-content than genetic differences within or among species.     

Ecological effects of low-level phosphorus additions on two plant communities in a neotropical freshwater wetland ecosystem

We conducted a low-level phosphorus (P) enrichment study in two oligotrophic freshwater wetland communities (wet prairies [WP] and sawgrass marsh [SAW]) of the neotropical Florida Everglades. The experiment included three P addition levels (0, 3.33, and 33.3 mg P m^–2 month^–1), added over 2 years, and used in situ mesocosms located in northeastern Everglades National Park, Fla., USA. The calcareous periphyton mat in both communities degraded quickly and was replaced by green algae. In the WP community, we observed significant increases in net aboveground primary production (NAPP) and belowground biomass. Aboveground live standing crop (ALSC) did not show a treatment effect, though, because stem turnover rates of Eleocharis spp., the dominant emergent macrophyte in this community, increased significantly. Eleocharis spp. leaf tissue P content decreased with P additions, causing higher C:P and N:P ratios in enriched versus unenriched plots. In the SAW community, NAPP, ALSC, and belowground biomass all increased significantly in response to P additions. Cladium jamaicense leaf turnover rates and tissue nutrient content did not show treatment effects. The two oligotrophic communities responded differentially to P enrichment. Periphyton which was more abundant in the WP community, appeared to act as a P buffer that delayed the response of other ecosystem components until after the periphyton mat had disappeared. Periphyton played a smaller role in controlling ecosystem dynamics and community structure in the SAW community. Our data suggested a reduced reliance on internal stores of P by emergent macrophytes in the WP that were exposed to P enrichment. Eleocharis spp. rapidly recycled P through more rapid aboveground turnover. In contrast, C. jamaicense stored added P by initially investing in belowground biomass, then shifting growth allocation to aboveground tissue without increasing leaf turnover rates. Our results suggest that calcareous wetland systems throughout the Caribbean, and oligotrophic ecosystems in general, respond rapidly to low-level additions of their limiting nutrient.     

Chemical speciation of saliva containing remineralizing solutions and their predicted effectiveness for subsurface lesion remineralization

Abstract

A computer modelling approach to estimate the chemical speciation which occurs in saliva and in subsurface lesion remineralizing solutions has been used to correlate chemical speciation data with published findings related to remineralization of dental enamel. The work also provides a means of selecting improved carrier ligands for the ions involved in remineralization and for optimising tooth protection and repair.     

Influence of soft rush (Juncus effusus) on phosphorus flux in grazed seasonal wetlands

Livestock significantly affect wetland soils and vegetation but their impacts on wetland nutrient dynamics are poorly understood. We set up a full factorial laboratory experiment to assess the effects of Juncus effusus, grazing exclusion, and flooding on P flux from intact cores collected from seasonal wetlands in cattle pastures in south Florida. We collected intact cores from Juncus tussocks and plant interspaces inside and outside 4-year grazing exclosures in five replicate wetlands. We incubated the cores for 50 days under continuous flooding or weekly 1-day flooding cycles and measured P concentrations in surface and pore water. Grazing exclosures had less Juncus (17%) and bare ground (2%) than adjacent grazed areas (Juncus, 48%; bare ground, 12%), but did not affect P fluxes. Initial fluxes of soluble reactive P (SRP) were much higher in cores with Juncus (242 ± 153 mg P m⁻² day⁻¹) than without Juncus (14 ± 20 mg P m⁻² day⁻¹). In weekly flooded cores P fluxes fell to 19.7 ± 13.4 mg P m⁻² day⁻¹ in cores with and 2.7 ± 2.6 in cores without Juncus. The strong effect of Juncus on P flux was an indirect effect of cattle grazing, but 4 years of grazing exclusion did not have a significant effect on P fluxes.     

Soluble reactive phosphorus (SRP) transport and retention in tropical, rain forest streams draining a volcanic landscape in Costa Rica: in situ SRP amendment to streams and laboratory studies

Soluble reactive phosphorus (SRP) transport/retention was determined in two rain forest streams (Salto, Pantano) draining La Selva Biological Station, Costa Rica. There, SRP levels can be naturally high due to groundwater enriched by geothermal activity within the surfically dormant volcanic landscape, and subsequently discharged at ambient temperature. Combined field and laboratory approaches simulated high but natural geothermal SRP input with the objective of estimating the magnitude of amended SRP retention within high and low SRP settings and determining the underlying mechanisms of SRP retention. First, we examined short-term SRP retention/transport using combined SRP-conservative tracer additions at high natural in situ concentrations. Second, we attempted to observe a DIN response during SRP amendment as an indicator of biological uptake. Third, we determined SRP release/retention using laboratory sediment assays under control and biologically inhibited conditions. Short-term in situ tracer-SRP additions indicated retention in both naturally high and low SRP reaches. Retention of added SRP mass in Upper Salto (low SRP) was 17% (7.5 mg-P m⁻² h⁻¹), and 20% (10.9 mg-P m⁻² h⁻¹) in Lower Salto (high SRP). No DIN response in either nitrate or ammonium was observed. Laboratory assays using fresh Lower Salto sediments indicated SRP release (15.4 ± 5.9 μg-P g dry wt.⁻¹ h⁻¹), when incubated in filter sterilized Salto water at ambient P concentration, but retention when incubated in filter sterilized river water amended to 2.0 mg SRP l⁻¹ (233.2 ± 5.8 μg-P g dry wt.⁻¹ h⁻¹). SRP uptake/release was similar in both control- and biocide-treated sediments indicating predominantly abiotic retention. High SRP retention even under biologically saturated conditions, absence of a DIN response to amendment, patterns of desorption following amendment, and similar patterns of retention and release under control and biologically inhibited conditions all indicated predominantly abiotic P flux.     

蚯蚓活动对红壤磷素主要形态及有效磷含量的影响

通过盆钵生物试验和采用Hedley磷素分级方法研究了秉氏环毛蚓(Pheretima pingi)对红壤磷素主要形态和红壤有效磷含量的影晌.结果表明,在蚯蚓和有机物料(稻草、花生秸、油菜秸)的共同作用下,经过100d培养后土壤有效磷含量显著提高.统计分析结果表明,花生秸接种蚯蚓处理与不接种蚯蚓处理和油莱秸接种蚯蚓处理与不接种蚯蚓处理之间的土壤有效磷含量差异均达显著水平.采用Hedley磷素分级法测得的树脂磷由原土的痕量增加到10.5～17.8 mg·kg^-1,NaHCO₃溶解态磷由原土的14.5 mg·kg^-1增加到23.5～53.6 mg·kg^-1,微生物细胞磷由原土的1.0 mg·kg^-1增加到6.8～9.7 mg·kg^-1,土壤有机磷含量由原土的37.9 mg·kg^-1增加到50.7～59.3 mg·kg^-1,而土壤中活性最低的残留磷含量则有所降低.蚯蚓活动对红壤磷素具有较强的活化作用.     

Differential responses of zooplankton assemblages to environmental variation in temporary and permanent ponds

Permanent and temporary wetlands in Mediterranean shrublands represent unique repositories of biodiversity, which are increasingly threatened by human-induced habitat loss. The zooplankton of a permanent (P1) and a temporary pond (T35) in the Natural Reserve of Castelporziano, a rare residual stretch of such a shrubland in Central Italy (Latium), was investigated to: (1) expand and deepen knowledge of these endangered freshwater habitats, which represent a crucial component of Mediterranean biodiversity; (2) identify environmental controls regulating the development of zooplankton communities of each environment; and (3) highlight differences in the adaptive responses of the zooplankton community in relation to the different ecological conditions experienced by permanent and temporary habitats. Despite summer desiccation in T35, the two ponds exhibited a relative homogeneity in hydrological and physico-chemical dynamics. Zooplankton assemblages contained 41 total taxa, of which 32 were found in P1 and 28 in T35. Out of the 41 taxa identified, 22 (>50%) were exclusively present in one of the two ponds. On a yearly basis, the community dynamics of P1 seemed to be conditioned by physical and chemical factors and by hydrological cycle characteristics, while the community of T35 responded to algal blooms, food competition and predator/prey equilibria rather than correlating to abiotic factors. The main differences amongst zooplankton assemblages were observed over short time scales and occurred both within and between seasons, highlighting the role of some structural taxa that dominated the average composition of the community throughout the year, and the importance of “quick-response” taxa in determining the short-term composition and structure variation of pond zooplankton. A year-round cyclic community succession peculiar to each pond is described.     

The role of zooplankton in the intrabiocoenotic phosphorus cycle and factors affecting phosphorus excretion in a lake

The temporal and spatial excretion rates of specific size groups of limnetic zooplankton were studied by measuring changes in soluble reactive phosphorus following incubation. Animals were collected in the epilimnion and hypolimnion of Stonehouse Pond, New Hampshire from August, 1972 to July, 1973 and separated into > 0.308 and < 0.308 (mm) size groups. Temporal excretion rates varied considerably within groups, however, similar patterns were observed in both strata. Peak excretion rates were observed in the spring and fall with a low in winter months. In addition, smaller animals excreted at higher rates and excretion rates of both size groups in the epilimnion exceeded those in the hypolimnion.The phosphorus uptake of natural seston relative to the amount released by zooplankton is considered using mass balance equations and kinetic analyses. During the spring of 1973 excretion rates increased, however, this increase was offset by an even greater rise in the rate of phosphorus uptake by the seston. This suggests that during this period the phosphorus excretions of zooplankton were not sufficient to meet the amount of phosphorus being removed by the seston.The experimental work was based on a thesis submitted to the Graduate School of the University of New Hampshire in partial fulfillment of the requirements for the degree of Doctor of Philosophy.     

Phosphorus limitation and excess carbon in zooplankton

Organisms rely on a series of chemical reactions, which are constrained by the availability of key chemical elements, such as carbon (C), nitrogen (N), and phosphorus (P). Ecological stoichiometry provides a tool for analyzing how the balance of elements required by organisms affects food-web dynamics. Ecological stoichiometric theory suggests that the balance between supply and demand of elements is determined by the conversion efficiency from resources to organisms.Autotrophs and heterotrophs commonly face unequal access to and uptake of elements. The stoichiometric variability of autotrophs is based on their ability to maintain the balance of elements required for growth. This creates a challenge for their grazers. Phytoplankton can adjust their P content to ambient nutrient concentrations, while zooplankton cannot store excess nutrients. Ecological stoichiometric theory thus suggests that zooplankton have relatively fixed stoichiometry compared with phytoplankton.Nutrient limitation is common in aquatic systems. Stoichiometric imbalances between phytoplankton and zooplankton mean that zooplankton rarely find optimal food sources, and phytoplankton production is in excess. P availability potentially limits zooplankton growth, because of the high C:P ratio in phytoplankton relative to zooplankton demand. Based on the Liebig minimum principle, organisms are normally limited by a single nutrient, while everything else is in excess. Under P deficiency, excess C cannot be allocated to zooplankton somatic growth, and the net intake of C must balance the C:P ratio of zooplankton. Thus, when zooplankton encounter nutritionally imbalanced foods the elements in excess are released in order to maintain homeostasis. Excess C, released by zooplankton results in two biochemical challenges: (1) to sequester the limiting element and (2) to either store or dispose of the element in surplus.Zooplankton must resort to various physiological solutions to cope with these challenges. As a first option, zooplankton can reduce their C assimilation efficiency but maintain their P assimilation efficiency. Alternatively, after assimilation, excess C may be stored in C-rich compounds. Finally, assimilated excess C could also be disposed of through respiration or extracellular release. Excess C released by zooplankton reduces C transfer efficiency and sequestration in aquatic ecosystems.In aquatic ecosystems, C sequestration largely depends on the balance between uptake and demand for key nutrient elements. These feedback mechanisms have arisen only because organisms must obey stoichiometric rules at the cell and body levels, which greatly constrain the range of element values in ecosystems. Thus, the fate of C in ecosystems is determined by the absolute and relative demands for N and P of each organism. Limiting elements are utilized for growth and transferred in food chains with high efficiency, while non-limiting elements must be disposed of. Therefore, low C:P phytoplankton communities subject to high turnover rates and high productivity are selectively channeled into zooplankton. When zooplankton face high C:P foods, excess C is returned to the environment. Hence, nutrient-deficient phytoplankton constitute poor food, influencing the entire food web and adversely affecting secondary production at all levels.Excess C processed by zooplankton has far-reaching implications for ecosystem food-web functioning and C sequestration. Studies of the fate of excess C in zooplankton would increase the understanding of energy flow and material cycling in aquatic ecosystems. This paper reviews the reasons for P limitation and excess C in zooplankton, principal routes for the disposal of excess C, and the ecological effects of this. In addition, the paper aims to provide insight and a theoretical foundation for related studies in China.     

Feeding behaviour,efficiency and food preference in yabbies <Emphasis Type="Italic">Cherax</Emphasis><Emphasis Type="Italic"> destructor</Emphasis>

The influence of body size on the consumption of live zooplankton (Daphnia spp.) by freshwater crayfish was examined using yabbies (Cherax destructor) ranging from 5 to 45 g. Food preference between live zooplankton and inert pellets was also assessed under experimental conditions. In experimental tanks, yabbies of four size classes (<15, 15–24.9, 25–34.9 and 35–45 g) were presented with live Daphnia. All yabbies were held in separate tanks with five animals per size class. In yabbies less than 15 g, the feeding mode on zooplankton involved rapid searching and probing with the first two pairs of walking legs. Once a prey was located, the chelae on the end of these walking legs would grasp the zooplankton and then rapidly move it towards the mouthparts. Yabbies larger than 25 g tended to use their walking legs to push the Daphnia nearer to their third maxilliped which would then force or scoop the zooplankton towards the mouthparts. A short-term feeding trial showed that there was no significant difference between size classes in regards to zooplankton consumption (P > 0.05). Capture efficiency of live Daphnia by yabbies less than 15 g was significantly lower (76%, P = 0.008) than the three larger size classes (93.6%). Yabbies less than 15 g consumed a significantly (P < 0.001) higher percentage (5.2%) of their body weight than the other size classes (1.1%, 0.8%, and 0.6%, respectively). In the presence of both live zooplankton and a pellet diet, yabbies spent significantly (P = 0.005) more time feeding on zooplankton (85%) than on inert pellets (15%). This was the first study to quantify zooplankton consumption by yabbies and the results provide insights into understanding the trophic role of freshwater crayfish in structuring zooplankton communities and the husbandry management of crayfish farming. Handling editor: S. I. Dodson     

Feeding ecology of three Astyanax species (Characidae, Tetragonopterinae) from a floodplain lake of Mogi-Guaçú River, Paraná River Basin, Brazil

Synopsis The feeding ecology of three characids (A. fasciatus, A. bimaculatus and A. schubarti) was studied monthly during 1988 in Lake Inferñao, a major floodplain lake of the Mogi-Guaçú River in the State of São Paulo. River flooding directly influenced the diet of the omnivorous A. bimaculatus and A. fasciatus which responded to maximum inundation (March) by consuming predominantly allochthonous insects. In contrast, A. schubarti was less influenced by the river flood cycle on qualitative changes in diet and relied basically on aquatic macrophytes and periphytic algae. The importance of zooplankton in the diet of the three species was low, and may be attributed to its low density in the water column (< 1 ind l^–1). Ontogenetic diet changes were evident for the three species. For A. fasciatus and A. bimaculatus the importance of zooplankton was high at the early stages, decreasing with size. A. schubarti at younger stages consumed diversified items which gradually decreased quantitatively up to the size class of 65 mm; from this size onwards, the diet became restricted to the consumption of periphytic algae and macrophytes.     

Sorption and bioavailability of phosphorus during the drainage period of flooded-drained soils

Summary Changes in P sorption and bioavailability were studied with 4 soils previously flooded and drained as occurs in rice-based cropping systems. Phosphorus sorption was measured at 15 and 119 days after drainage and the bioavailability of added and native soil-P was determined at 9, 16, 30, 45, 70 and 135 days in both flooded-drained and unflooded soils. The P sorptivity and bonding energy of sorption increased under flooded-drained soil conditions. At 119 days after drainage the P sorptivity and bonding energy of sorption decreased as compared to 15 days after drainage. The P sorptivity of the flooded-drained soils, however, did not reach the same levels as existed in the soils prior to flooding. The bioavailability of P during the drainage period remained low and did not measurably change up to 70 days after drainage. At 135 days after drainage the bioavailability of P increased significantly, but did not reach the level found in the corresponding unflooded soils.     

Differential zooplankton feeding behaviors,selectivities, and community impacts of two planktivorous fishes

Synopsis We examined the feeding behaviors and selectives of two common planktivorous fishes, pumpkinseeds Lepomis gibbosa and fathead minnows Pimephales promelas in the laboratory. Ingestion rates for both pumpkinseeds and fathead minnows feeding on zooplankton increased as a function of fish length. Pumpkinseeds fed on zooplankton strictly as particulate feeders, with preferences increasing as a function of zooplankton body size regardless of taxonomic identity. Preferences were highest for large Daphnia, intermediate for intermediate-sized copepods, and lowest for small Ceriodaphnia. Fathead minnows displayed the ability to use both particulate-feeding and filter-feeding behaviors. Differential preferences tended to reflect both zooplankton size and taxon, being highest for large, slow-swimming Daphnia, intermediate for small Ceriodaphnia, and lowest for faster-swimming copepods. These differences in prey capture behaviors and preferences of the two fishes are reflected in the zooplankton taxonomic composition of small ponds containing each fish type. The crustacean zooplankton assemblages in ponds containing both pumpkinseeds and fathead minnows were dominated by copepods. Cladocerans were rare. In ponds containing pumpkinseeds, but no fathead minnows, cladocerans were abundant, generally accounting for up to 80% of total crustacean zooplankton biomass. These results suggest that the type of planktivore, and not simply the presence or abundance of planktivores in a system, can determine zooplankton community structure.     

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m⁻² lake surface area d⁻¹ with EPC0 values ranging between 180 and 270 μg P L⁻¹. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.     

Prospects for phosphorus recovery from poultry litter

Land disposal of poultry litter is an environmental concern often associated to excess phosphorus (P) in soils and potential water pollution in regions with intense poultry production. Although poultry litter can be moved off the farm and traded as fertilizer, its transportation becomes less economical with increasing distances from the farm. Thus, new litter management alternatives are needed to reduce the environmental impact of P litter application to land. This paper summarizes established and emerging alternative technologies in the U.S. that facilitate handling, concentration, and transporting of litter P. Furthermore, it examines the potential integration of technologies into poultry litter management systems that could reduce poultry litter volume and increase P content in litter byproducts. The adoption of alternative technologies may encourage new opportunities to produce bio-energy, fertilizer, and other valuable P byproducts from poultry litter while reducing environmental impact and promoting sustainable poultry production.     

Evaluation of phosphorus distribution and bioavailability in sediments of a subtropical wetland reserve in southeast China

The phosphorus (P) fractions and bioavailable P in the sediments from the Quanzhou Bay Estuarine Wetland Nature Reserve were investigated using chemical extraction methods for the first time to study the distribution and bioavailability of P in the reserve sediments. A hypothesis was presented suggesting that the bioavailable P in the sediments could be evaluated using the P fractions. The total phosphorus (TP), inorganic phosphorus (IP), organic phosphorus (OP), non-apatite phosphorus (NAIP), and apatite phosphorus (AP) contents in the sediments were in the ranges of 303.87–761.59 mg kg⁻¹, 201.22–577.66 mg kg⁻¹, 75.83–179.16 mg kg⁻¹, 28.86–277.90 mg kg⁻¹, and 127.36–289.94 mg kg⁻¹, respectively. The water soluble phosphorus (WSP), readily desorbable phosphorus (RDP), algal available phosphorus (AAP), and NaHCO₃ extractable phosphorus (Olsen-P) contents in the sediments were in the ranges of 0.58–357.17 mg kg⁻¹, 80.77–586.75 mg kg⁻¹, 1.09–24.12 mg kg⁻¹, and 54.96–676.82 mg kg⁻¹, respectively. The correlation analysis results showed that the NAIP was the major component of the bioavailable P and that the impact of the AP on the bioavailable phosphorus may be minimal. Due to the low TP content in the sediments of the Quanzhou Bay Estuarine Wetland Nature Reserve, the potential pollution risks of P in the sediments may not be very high. The results also show that the bioavailable P concentrations in the sediments of the Quanzhou Bay Estuarine Wetland Nature Reserve could not be evaluated by measuring the P fractions and that the hypothesis was untenable.     

Copper nutrition of wheat in relation to nitrogen and phosphorus fertilization

Summary In a pot culture study, copper addition to soil increased the crop yield only in presence of nitrogen. The latter increased the utilization of both native as well as applied copper but more that of applied. It also minimised the adverse effect of applied phosphorus on copper utilization. Phosphorus at the rate 45 ppm had the tendency of decreasing copper uptake by wheat if applied without nitrogen or with its low level.     

Influence of hydropattern and vegetation type on phosphorus dynamics in flow-through wetland treatment systems

Phosphorus (P) flux from wetland soil can be a significant factor affecting overall wetland treatment performance. The purpose of our study was to quantify the effects of water level drawdown on P exchange between surface water and organic soil in a constructed wetland. We used 12 fiberglass mesocosms filled with 30 cm of peat soil to quantify nutrient exchange between surface water and organic soil in a wet-dry-wet cycle. Six mesocosms were planted with emergent macrophytes and six mesocosms were maintained free of emergent vegetation. We evaluated four treatments including continuously and intermittently flooded treatments, both with and without emergent macrophytes. Each treatment was replicated three times and every mesocosm was plumbed to monitor flow volumes and water chemistry. Effluent P concentrations were similar for all four treatments prior to first drawdown period. However, upon re-flooding, all intermittently flooded tanks exhibited a three to fourfold increase in surface water P concentration, which lasted for a period of up to ten weeks. The magnitude of nutrient flux to surface water and the time period over which P release took place were season dependent, with longer duration of high nutrient flux during dry-season drawdowns. Results of repeated measures analysis indicated that hydropattern was the dominant factor affecting P-flux to overlying surface water, while presence or absence of emergent vegetation had no significant influence on effluent concentrations. Organic and particulate phosphorus fluxes were substantially higher in treatments lacking emergent macrophytes, subsequent to the dry-season drawdowns. Intermittently flooded treatments with no emergent vegetation generated the most dissolved and particulate phosphorus. Our results indicate that maintaining saturated soil is sufficient to retain stored P, while plants played no significant role in P retention for a wetland receiving P-loading rate on the order of 0.1 g week⁻¹ during a wet-dry-wet cycle.     

Relationship between hypolimnetic phosphorus and iron release from eleven lakes in Maine, USA

We studied five eutrophic (high phosphorus) and six mesotrophic/oligotrophic (low phosphorus) lakes in Maine, USA, all of which are dimictic and develop anoxic hypolimnia during stratification. The lakes were sampled during the stratified period from May to September 1999. Late summer hypolimnetic total phosphorus (P) concentrations in the high-P lakes ranged from 185 to 460ppb; epilimnetic total P increased up to 30ppb from the spring to the fall overturn. During the same period, the low-P lakes had hypolimnetic total P concentrations in the range of 6–19ppb.Individual high-P lakes demonstrated strong temporal correlations between aqueous hypolimnetic dissolved Fe and total P concentrations (R ²0.88) with an average molar Fe:P ratio of 11.9±4.2. For the combined data, the high-P lakes exhibited strong correlation between hypolimnetic Fe and P concentrations (R ²=0.82). The low-P lakes, however, did not show a good correlation between the hypolimnetic Fe and P concentrations. Among the low-P lakes two lakes had hypolimnetic Fe fluxes comparable to the Fe fluxes of the high-P lakes. These two lakes had considerably higher hypolimnetic Fe:P ratios than all other lakes studied here. There were no significant differences in surface sediment Fe(III) or P fractions that correlated with the differences in the relationship between aqueous concentrations of Fe and P in these two outlier low-P lakes. A model for the generation of hypolimnetic acid neutralization capacity (ANC) was developed based on microbially-catalyzed reduction of Fe(III) hydroxide, Mn(IV) oxide and sulfate. Reduction of Fe(III) hydroxide was the most important contributor to the increase in the hypolimnetic ANC in all high-P and the two outlier lakes. Assuming that all hypolimnetic P was due to the reduction of Fe(III) hydroxide by bacteria and sulfide, average summer hypolimnetic P flux for each lake was predicted using the sediment reducible Fe(III):P ratio. The observed and predicted average P fluxes in the high-P lakes corresponded reasonably, suggesting that in these lakes internal P release is closely related to the reduction of Fe(III) hydroxide. Other release or sequestration mechanisms may operate for the release and availability of P in the low-P lakes.