Streamwater Phosphorus Research Articles

Nutrient dynamics in stream water and groundwater in riparian zones of a mesoscale agricultural catchment with intense seasonal pumping

Water and soil in agricultural areas are usually enriched with nutrients from excessive fertilizations. Riparian zones, the interfaces between terrestrial and aquatic systems, play crucial roles in delivery of the nutrients to groundwater and surface water. We evaluated spatiotemporal variations of water flow and nutrients in a mesoscale agricultural catchment along the riparian zones of the main stream for dry and rainy season by hydrologic monitoring and measurements of hydrochemical parameters and water stable isotopes. Gaining stream conditions were dominant in the mountainous upper reaches with steep topographic gradient while groundwater level in the lower basin considerably varied seasonally due to intense groundwater pumping to aid in heating greenhouses in winter season, which resulted in losing stream conditions during the dry season. Compared to shallow groundwater, deep groundwater had higher nitrate concentration derived from dry farmland in hilly terrains and induced recharge due to the groundwater pumping. Nitrate concentration was significantly higher in groundwater than that in stream water, indicating that groundwater is a major source of nitrate in stream water considering mainly gaining stream conditions. Compared to stream water, dissolved phosphorus concentration was higher in shallow groundwater recharged from paddy fields and floodplain areas with reducing conditions, evaporation signature of water stable isotopes, and lower nitrate concentration. Nutrient flux estimates for the catchment revealed that nitrate in stream water is considerably contributed by groundwater discharge and shallow groundwater in reducing conditions is a significant source of dissolved phosphorus in stream water, which was more pronounced in the rainy season. This study demonstrated that intense agricultural activities in riparian zones strongly affect stream-aquifer interactions and nutrient delivery to the stream, as well as groundwater. These findings can contribute to understanding hydrological and biogeochemical processes of nutrients in agricultural catchments and establishing an effective management of water use and nutrient application.

A Method for Estimating the Risk of Dam Reservoir Silting in Fire-Prone Watersheds: A Study in Douro River, Portugal

Forest fires are an increasing problem over recent decades. The fires, among other consequences, lead to an increase in the soil vulnerability to water erosion and a consequent increase in sedimentation rates. When barriers are present, such as dams or weirs, there is an amplified risk of sediment and ash deposition in their reservoirs, causing siltation. Thus, there is an interest in studying in more detail the risk of siltation of barriers and reservoirs in the Douro River watershed following wildfires. A detailed barrier inventory was lacking for the Douro River, hampering the identification of siltation-prone areas. In order to fill in this gap, an extensive inventory of barriers in the Douro river basin was carried out for the present study. The result was an abundant and reliable dataset on the Douro River barriers, which allowed a prognosis on the watershed siltation risk. The method for calculating the siltation risk relied on the relationship between the frequency of forest fires, the erosion risk and the frequency of reservoirs. The sub-basins with the greater siltation risk are the Tâmega, Corgo, Sousa and Paiva river basins. Most reservoirs with the highest siltation risk were from small dams. The modelling results were compared with stream connectivity and concentrations of stream water phosphorus (associated with the sediments that flow into the rivers due to the fires). With regard to connectivity, only two reservoirs were at high risk of sedimentation due to fires, so the categories of connectivity risk and fire-based sedimentation risk are probably not related. With regard to risk of high phosphorus loadings, in 8 basins the upper classes for fire-based erosion risk coincided with the upper class for phosphorus loadings suggesting that high phosphorus loading could be associated with fire-based erosion. This study works as a simple but reliable example on the assessment and mapping of siltation risk in stream networks intersected by abundant barriers. It allowed for identifying barriers that can accumulate a large quantity of fine sediments and ashes, interfering with water quality and soil erosion as well as with the storage capacity of the respective barriers.

Evaluating soil and nutrients (C, N, and P) loss in Chinese Torreya plantations

Improper land-use changes may lead to a loss of soil resources and cause environmental pollution. Chinese Torreya plantation (hereafter CTP) is an important cash tree plantation for nuts production in the mountainous areas of subtropical China. The increasing development of CTPs, to increase seed production, can result in the complete erasure of local natural vegetation.In this study, the vulnerability to soil erosion, loss of soil organic carbon (SOC) and nutrients in CTPs due to land-use change were evaluated. The results indicated that the rates of diffusive soil erosion in the young CTPs with extreme precipitation were about six-fold higher than with the natural vegetation. At sites with a similar slope, there was no significant difference in soil erosion levels between the young and old CTPs. The old CTPs did not hold significantly higher levels of SOC and soil total nitrogen (STN) in their topsoil when compared with the young CTPs. The natural mixed broadleaved subtropical forests lost about 35% of their SOC and 25% of their STN after they were converted into CTPs, but the CTPs had higher soil total phosphorus. The C: N ratios at the different sites were close to 11:1, but the N: P ratios were diverse. There were high levels of organic carbon, nitrogen and phosphorus in stream water. Adequate coverage of natural vegetation within or around the CTPs should be maintained to decrease soil erosion and nutrient loss. Suggestions to develop CTPs while protecting the environment are discussed. Overall, it was determined that aspects of the current management practices and strategies for developing CTPs should be changed to decrease soil erosion and nutrient loss.

Spring stimulates leaf decomposition in moderately eutrophic streams

In forested headwater streams, decomposition of allochthonous organic matter is a fundamental process driven by aquatic microbes and invertebrate shredders. We examined how season and eutrophication affect leaf decomposition and the associated decomposer communities by immersing leaves of a late deciduous species (Quercus robur) in five streams in Portugal along a gradient of eutrophication in autumn and spring. We found hump-shaped relationships between leaf decomposition and total nitrogen and phosphorus in stream water in both seasons. Leaf decomposition and shredder biomass were higher during spring in streams with moderate levels of eutrophication. Fungal sporulation and biomass were stimulated at moderate levels of eutrophication and inhibited at low or high levels of eutrophication. Fungal assemblage composition shifted between seasons and along the gradient of eutrophication. Tricladium chaetocladium increased its contribution to total conidial production in spring, while Dimorphospora foliicola was dominant in the most eutrophic streams where Articulospora tetracladia was almost absent. Invertebrate shredders were the primary decomposers of leaves in streams with moderate levels of eutrophication, particularly in the warmest season. Although the presence of late deciduous plant species, such as oak, in the riparian corridors may help to mitigate food depletion to freshwater decomposers in spring, our results suggest that moderate eutrophication can accelerate decomposition further reducing litter standing stocks in the warmer seasons.

Leaf-litter stoichiometry is affected by streamwater phosphorus concentrations and litter type

The stoichiometric ratios of organisms and their food resources can influence C and nutrient dynamics in aquatic ecosystems. Several investigators have quantified linkages between nutrient enrichment and consumer stoichiometry for stream detritivores, but very few have systematically quantified the effect of P enrichment on leaf-litter stoichiometry. Here, we examine the potential stoichiometric changes of 2 species of leaf litter subjected to varying levels of P enrichment in laboratory microcosms and mixed species across a natural P gradient of streams in the Ozark Highlands Region, Arkansas, USA. Leaf-litter %P content increased and C∶P ratios decreased with increasing levels of P enrichment and with increasing lability of the leaf species. In the laboratory study, C∶P of maple and oak leaves in the control treatment was ∼2500, whereas this ratio decreased to 500 and 1000 in the high-P treatments, respectively. Total P (TP) was inversely related to leaf-litter C∶P along the natural P gradient of streams in the Ozarks. Our results add to the growing body of information on the potential bottom-up effects of anthropogenic nutrient loading in streams and the influence of water-column nutrients and leaf quality on this response.

Hydrochemical processes in lowland rivers: insights from in situ, high-resolution monitoring

Abstract. This paper introduces new insights into the hydrochemical functioning of lowland river systems using field-based spectrophotometric and electrode technologies. The streamwater concentrations of nitrogen species and phosphorus fractions were measured at hourly intervals on a continuous basis at two contrasting sites on tributaries of the River Thames – one draining a rural catchment, the River Enborne, and one draining a more urban system, The Cut. The measurements complement those from an existing network of multi-parameter water quality sondes maintained across the Thames catchment and weekly monitoring based on grab samples. The results of the sub-daily monitoring show that streamwater phosphorus concentrations display highly complex dynamics under storm conditions dependent on the antecedent catchment wetness, and that diurnal phosphorus and nitrogen cycles occur under low flow conditions. The diurnal patterns highlight the dominance of sewage inputs in controlling the streamwater phosphorus and nitrogen concentrations at low flows, even at a distance of 7 km from the nearest sewage treatment works in the rural River Enborne. The time of sample collection is important when judging water quality against ecological thresholds or standards. An exhaustion of the supply of phosphorus from diffuse and multiple septic tank sources during storm events was evident and load estimation was not improved by sub-daily monitoring beyond that achieved by daily sampling because of the eventual reduction in the phosphorus mass entering the stream during events. The results highlight the utility of sub-daily water quality measurements and the discussion considers the practicalities and challenges of in situ, sub-daily monitoring.

Water Quality Functions of a 15‐Year‐Old Riparian Forest Buffer System1

Newbold, J. Denis, Susan Herbert, Bernard W. Sweeney, Paul Kiry, and Stephen J. Alberts, 2010. Water Quality Functions of a 15‐Year‐Old Riparian Forest Buffer System. Journal of the American Water Resources Association (JAWRA) 46(2):299‐310. DOI: 10.1111/j.1752‐1688.2010.00421.xAbstract: We monitored long‐term water quality responses to the implementation of a three‐zone Riparian Forest Buffer System (RFBS) in southeastern Pennsylvania. The RFBS, established in 1992 in a 15‐ha agricultural (row crop) watershed, consists of: Zone 1, a streamside strip (∼10 m wide) of permanent woody vegetation for stream habitat protection; Zone 2, an 18‐ to 20‐m‐wide strip reforested in hardwoods upslope from Zone 2; and Zone 3, a 6‐ to 10‐m‐wide grass filter strip in which a level lip spreader was constructed. The monitoring design used paired watersheds supplemented by mass balance estimates of nutrient and sediment removal within the treated watershed. Tree growth was initially delayed by drought and deer damage, but increased after more aggressive deer protection (1.5 m polypropylene shelters or wire mesh protectors) was instituted. Basal tree area increased ∼20‐fold between 1998 and 2006, and canopy cover reached 59% in 2006. For streamwater nitrate, the paired watershed comparison was complicated by variations in both the reference stream concentrations and in upslope groundwater nitrate concentrations, but did show that streamwater nitrate concentrations in the RFBS watershed declined relative to the reference stream from 2002 through the end of the study in early 2007. A subsurface nitrate budget yielded an average nitrate removal by the RFBS of 90 kg/ha/year, or 26% of upslope subsurface inputs, for the years 1997 through 2006. There was no evidence from the paired watershed comparison that the RFBS affected streamwater phosphorus concentration. However, groundwater phosphorus did decline within the buffer. Overland flow sampling of 23 storms between 1997 and 2006 showed that total suspended solids concentration in water exiting the RFBS to the stream was on average 43% lower than in water entering the RFBS from the tilled field. Particulate phosphorus concentration was lower by 22%, but this removal was balanced by a 26% increase in soluble reactive phosphorus so that there was no net effect on total phosphorus.

Streamwater phosphorus and nitrogen across a gradient in rural–agricultural land use intensity

This paper provides an overview of the impacts of rural land use on lowland streamwater phosphorus (P) and nitrogen (N) concentrations and P loads and sources in lowland streams. Based on weekly water quality monitoring, the impacts of agriculture on streamwater P and N hydrochemistry were examined along a gradient of rural–agricultural land use, by monitoring three sets of ‘paired’ (near-adjacent) rural headwater streams, draining catchments which are representative of the major geology, soil types and rural/agricultural land use types of large areas of lowland Britain. The magnitude and timing of P and N inputs were assessed and the load apportionment model (LAM) was applied to quantify ‘continuous’ (point) source and ‘flow-dependent’ (diffuse) source contributions of P to these headwater streams. The results show that intensive arable farming had only a comparatively small impact on streamwater total phosphorus (TP loads), with highly consistent stream diffuse-source TP yields of ca. 0.5 kg-P ha −1 year −1 for the predominantly arable catchments with both clay and loam soils, compared with 0.4 kg-P ha −1 year −1 for low agricultural intensity grassland/woodland on similar soil types. In contrast, intensive livestock farming on heavy clay soils resulted in dramatically higher stream diffuse-source TP yields of 2 kg-P ha −1 year −1. The streamwater hydrochemistry of the livestock-dominated catchment was characterised by high concentrations of organic P, C and N fractions, associated with manure and slurry sources. Across the study sites, the impacts of human settlement were clearly identifiable with effluent inputs from septic tanks and sewage treatment works resulting in large-scale increases in soluble reactive phosphorus (SRP) loads and concentrations. At sites heavily impacted by rural settlements, SRP concentrations under baseflow conditions reached several hundred μg-P L −1. Load apportionment modelling demonstrated significant ‘point-source’ P inputs to the streams even where there were no sewage treatment works within the upstream catchment. This indicates that, even in sparsely populated rural headwater catchments, small settlements and even isolated groups of houses are sufficient to cause significant nutrient pollution and that septic tank systems serving these rural communities are actually operating as multiple point sources, rather than a diffuse input.

Unravelling the determinants of stream midge biodiversity in a boreal drainage basin

Summary1. Few extensive lotic studies have examined patterns in the biodiversity of non‐biting midges (Diptera: Chironomidae) along major environmental gradients. Our aim was to fill this gap by describing patterns in species diversity, assemblage composition and distributions of midges across a boreal drainage basin.2. We found that the diversity of midges, as measured by rarefied species richness, Fisher’s α and Pielou’s evenness, responded positively to stream size in regression analysis. By contrast, species density was most strongly correlated to a gradient in suspended solids and phosphorus in stream water, as well as macrophyte cover. Spatial variables were not significantly correlated with species diversity.3. Midge assemblage composition was best explained by a model incorporating five composite environmental gradients in canonical correspondence analysis. The environmental gradients were stream size, macrophyte cover, alkalinity, nitrogen and suspended solids. Spatial variables did not overcome the effects of environmental gradients on assemblage composition.4. Cluster analysis divided the 27 study sites into four groups with relatively similar midge assemblages. These groups were statistically significant in multi‐response permutation procedure, and 15 of the 49 midge taxa recorded varied significantly among the groups in indicator value analysis. Discriminant function analysis showed that stream size, macrophyte cover and habitat structure predicted 66.7% of sites into correct groups.5. The information provided by the present analyses may be of considerable importance in conservation planning at the drainage basin level. The fact that species diversity and assemblage composition varied primarily along the stream size gradient suggests that sites belonging to the different size classes (first to fifth order) are needed to conserve the biodiversity of midges. The other environmental gradients should also be considered in conservation planning, because they explained significant amounts of variability in midge assemblage composition.

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water phosphorus over storm events

Summary Concentrations of suspended particulate matter (SPM), NO 3 -N and P fractions: PO 4 -P, dissolved organic P (DOP), particulate P (PP) and bioavailable exchangeable P were examined over 5 storm events in two nested agricultural catchments in NE Scotland: a (51 km 2 ) catchment and its headwater (4 km 2 ). NO 3 -N showed anticlockwise hysteresis for all storms in both catchments. In contrast, the headwater showed strong clockwise hysteresis of SPM, dissolved and particulate P concentrations, but which weakened through summer to spring. Less pronounced hysteresis of P forms in the larger catchment was attributed to a combination of factors: a less energetic system, nutrient leaching from the floodplain, a point source of a small sewage treatment works and the occurrence of coarser soil and sediment parent materials with less P adsorption and transport capacity. The headwater exhibited a strong ‘first flush’ effect of sediment and dissolved P, particularly following dry conditions, received a significant transfer of readily-solubilized organic P from the surrounding soils in late summer and after manure applications in winter, and was the likely cause of large sediment associated P signals observed in the 51 km 2 catchment. Our results suggest that steeper gradient headwaters should be targeted for riparian improvements to mitigate soil erosion from headwater fields. The efficiency of riparian erosion controls is also dependant on the size of the store of fine sediment material within the stream channel and this may be large.

Multivariate Analysis of Paired Watershed Data to Evaluate Agricultural Best Management Practice Effects on Stream Water Phosphorus

Quantification of the effects of management programs on water quality is critical to agencies responsible for water resource protection. This research documents reductions in stream water phosphorus (P) loads resulting from agricultural best management practices (BMPs) implemented as part of an effort to control eutrophication of Cannonsville Reservoir, a drinking water supply for New York City. Dairy farms in the upstate New York reservoir basin were the target of BMPs designed to reduce P losses. A paired watershed study was established on one of these farms in 1993 to evaluate changes in P loading attributable to implementation of BMPs that included manure management, rotational grazing, and improved infrastructure. Intensive stream water monitoring provided data to calculate P loads from the 160-ha farm watershed for all runoff events during a two-year pre-treatment period and a four-year post-treatment period. Statistical control for inter-annual climatic variability was provided by matched P loads from a nearby 86-ha forested watershed, and by several event flow variables measured at the farm. A sophisticated multivariate analysis of covariance (ANCOVA) provided estimates of both seasonal and overall load reductions. Statistical power and the minimum detectable treatment effect (MDTE) were also calculated. The results demonstrated overall event load reductions of 43% for total dissolved phosphorus (TDP) and 29% for particulate phosphorus (PP). Changes in farm management practices and physical infrastructure clearly produced decreases in event P losses measurable at the small watershed scale.

Non-Point-Source Impacts on Stream Nutrient Concentrations Along a Forest to Urban Gradient

We conducted statistical analyses of a 10-year record of stream nutrient and sediment concentrations for 17 streams in the greater Seattle region to determine the impact of urban non-point-source pollutants on stream water quality. These catchments are dominated by either urban (22-87%) or forest (6-73%) land cover, with no major nutrient point sources. Stream water phosphorus concentrations were moderately strongly (r2=0.58) correlated with catchment land-cover type, whereas nitrogen concentrations were weakly (r2=0.19) and nonsignificantly (at alpha<0.05) correlated with land cover. The most urban streams had, on average, 95% higher total phosphorus (TP) and 122% higher soluble reactive phosphorus (SRP) and 71% higher turbidity than the most forested streams. Nitrate (NO3), ammonium (NH4), and total suspended solids (TSS) concentrations did not vary significantly with land cover. These results suggest that urbanization markedly increased stream phosphorus concentrations and modestly increased nitrogen concentrations. However, nutrient concentrations in Seattle region urban streams are significantly less than those previously reported for agricultural area streams.

A Daily Time Series Analysis of StreamWater Phosphorus Concentrations Along an Urban to Forest Gradient

During a 1-year period, we sampled stream water total phosphorus (TP) concentrations daily and soluble reactive phosphorus (SRP) concentrations weekly in four Seattle area streams spanning a gradient of forested to urban-dominated land cover. The objective of this study was to develop time series models describing stream water phosphorus concentration dependence on seasonal variation in stream base flows, short-term flow fluctuations, antecedent flow conditions, and rainfall. Stream water SRP concentrations varied on average by +/- 18% or +/- 5.7 microg/L from one week to another, whereas TP varied +/- 48% or +/- 32.5 microg/L from one week to another. On average, SRP constituted about 47% of TP. Stream water SRP concentrations followed a simple sine-wave annual cycle with high concentrations during the low-flow summer period and low concentrations during the high-flow winter period in three of the four study sites. These trends are probably due to seasonal variation in the relative contributions of groundwater and subsurface flows to stream flow. In forested Issaquah Creek, SRP concentrations were relatively constant throughout the year except during the fall, when a major salmon spawning run occurred in the stream and SRP concentrations increased markedly. Stream water SRP concentrations were statistically unrelated to short-term flow fluctuations, antecedent flow conditions, or rainfall in each of the study streams. Stream water TP concentrations are highly variable and strongly influenced by short-term flow fluctuations. Each of the processes assessed had statistically significant correlations with TP concentrations, with seasonal base flow being the strongest, followed by antecedent flow conditions, short-term flow fluctuations, and rainfall. Times series models for each individual stream were able to predict approximately 70% of the variability in the SRP annual cycle in three of the four streams (r2 = 0.57-0.81), whereas individual TP models explained approximately 50% of the annual cycle in all streams (r2 = 0.39-0.59). Overall, time series models for SRP and TP dynamics explained 82% and 76% of the variability for these variables, respectively. Our results indicate that SRP, the most biologically available and therefore most important phosphorus fraction, has simpler and easier-to-predict seasonal and weekly dynamics.

A nested catchment approach for defining the hydrological controls on non-point phosphorus transport

Stream water phosphorus (P) concentrations for three nested pastureland catchments located in southern Ireland, having areas of 0.14, 2.11, and 15.24 km 2, were monitored for the purpose of defining the hydrological controls on the P transport. Concentration–discharge ( C– Q) relationships were developed for total phosphorus (TP) and its two components, dissolved phosphorus (DP) and particulate phosphorus (PP), where Q is the area-normalized discharge. An analysis of covariance statistical test was applied to determine if any differences in C– Q relationships existed between sub-catchments. Significant differences ( p<0.01) between sites were found for the TP and PP C– Q relationships, but not for the DP C– Q relationships, indicating that phosphorus transport in its sediment-bound particulate form was alone responsible for the differences in overall P fluxes between sites. The intermediate-scale catchment possessed the highest discharge-specific PP and suspended solid concentrations. An analysis of catchment topography was undertaken in an attempt to explain the observed differences in the C– Q relationships for the individual sub-catchments. The distributions of the ln(A s / tan β) topographic index, where A s is the upslope contributing area per unit contour width and β is the local slope, were first considered in order to test the conceptual model of variable saturated area control on the overland PP transport. Next, the distributions of the length–slope ( LS) factor from the universal soil loss equation were evaluated as an index of erosion to describe PP transport. Neither topographic descriptor was found to satisfactorily conform to the C– Q observations, indicating that local catchment sediment delivery factors or in-stream channel processes are likely to be responsible for the observed differences.

Landscape variation in phosphorus concentration and effects on detritus‐based tropical streams

Landscape‐scale variation in streamwater phosphorus (P) concentration can affect aquatic food webs. Such variation occurs naturally in streams at La Selva Biological Station in Costa Rica due to spatially variable inputs of geothermally modified groundwater. We examined effects of this gradient on detrital food web components at 16 stream sites. The Michaelis‐Menten model provided a good fit of the relationship between soluble reactive phosphorus (SRP) and leaf decay rate, fungal biomass, and invertebrate biomass, indicating that these variables were controlled by P concentration and that half‐saturation constants were relatively low (7—13 µg L−21 SRP). In a subsequent short‐term (3 week) whole‐stream P enrichment study, we found no effect of P addition on leaf decay rate or on biomass or density of invertebrates. However, laboratory tests of P, N, and Ca concentrations on mass loss of leaves showed detectable stimulation by both N and P after 3 weeks. A fourth study assessed the relative contribution of invertebrate consumption versus P concentration in determining decay rates among streams. The majority of variation was due to P concentration (71%), compared to effects of invertebrates (3%) or invertebrate × P interactions (14%). Overall, we found that a landscape‐scale natural gradient in P concentration influenced decay rates of organic matter and biomass of consumers, providing evidence that benthic detrital food webs can be limited from the bottom up by nutrients. Microbial processes appeared to be most important in driving differences in organic matter decay among sites, but invertebrates also contributed to elevated decay rates at high‐P sites.

NITROGEN AND PHOSPHORUS IN WATER AS RELATED TO ENVIRONMENTAL SEVI'ING IN NEBRASKA1

ABSTRACT: Spatial distributions of nitrogen and phosphorus in water were related to environmental setting as part of a regional water‐quality assessment of the Central Nebraska Basins. The environmental settings (Sandhills, Loess Hills, Glaciated Area, and Platte Valley) were characterized by different concentrations of nitrogen and phosphorus in ground water and stream water. Statistically significant differences in nitrate concentrations in both ground‐water and stream‐water samples were related to regional distributions of cropland and rangeland. Nitrate concentrations were larger, especially in shallow ground water, in environmental settings dominated by cropland and associated fertilizer use than in settings dominated by rangeland. Similarly, total‐nitrogen and nitrate concentrations were relatively large in selected streams draining primarily cropland. Comparative concentrations of phosphorus in stream water on the basis of environmental setting were similar to those of nitrogen, although the largest phosphorus concentrations probably relate to wastewater discharge into small streams. Nitrogen and phosphorus concentrations in much of the Platte River apparently reflected the quality of water entering the study unit from upstream and limited base‐flow contributions from within the Platte Valley itself.

Solubility of inorganic phosphorus in stream water as influenced by pH and calcium concentration

Solubility of inorganic phosphorus (P) in aquatic systems is regulated by physico-chemical characteristics of the water column. Laboratory studies were conducted to determine the influence of pH and calcium (Ca) on inorganic P precipitation and solubility relationships in the water column. Water samples from selected streams and canals in south Florida were incubated at 25°C in 2.8 L. continuouslystirred reactors under different PCO 2 levels. Precipitation of soluble P wwas influenced by the initial Ca concentration of the water. Water samples from a drainage canal of the Everglades agricultural area (EAA)(115 mg Cal −1) showed the highest P precipitation rate and Otter Creek water (20 mg Ca l −1) the lowest. Precipitation of inorganic P increased at Ca concentrations above 100 mg l −1 and at pH > 9.0. Solubility of inorganic P in the EAA water decreased above pH 8.5. At pH 9.0 more than 60% of the soluble P at all Ca levels was precipitated. In contrast, P solubility at low Ca levels (20 and 50 mg Ca l −1) was not affected at pH< 9.0. The decrease in P solubility as pH and Ca levels were increased suggests that calcium phosphate mineral formation occurred in the stream and canal water. Calcium phosphate formed in Otter Creek and Dry Lake waters rapidly dissolved as the pH decreased. Most of the soluble P in the Otter Creek (>90%) and Dry Lake (>75%) waters came back into solution; however, about 50% of the soluble P from the EAA canal water remained precipitated. Ion activity product calculations indicated that Otter Creek and Dry Lake waters were at equilibrium with octacalcium phosphate. In contrast, the EAA canal water was supersaturated with respect to beta tricalcium phosphate or with a similar mineral such as whitlockite. High Mg concentrations in the EAA water were apparently responsible for a different P behavior than for the Dry Lake and Otter Creek waters.

Periphyton Response to Longitudinal Nutrient Depletion in a Woodland Stream: Evidence of Upstream-Downstream Linkage

Longitudinal gradients in streamwater nutrient concentrations in Walker Branch are generated as a result of instream nutrient uptake and spatially confined groundwater inputs during the period from November to May. The response of the stream periphyton community to these longitudinal nutrient reductions was determined by measuring periphyton biomass, productivity, species composition, and phosphorus (P) cycling indices at four stations along a longitudinal transect in the stream. Phosphorus cycling indices (chlorophyll-specific phosphatase activity, phosphorus content of periphyton) exhibited significant changes along the longitudinal transect during those times of the year when streamwater soluble reactive phosphorus (SRP) concentrations also decreased along the transect. During the period from June to October, however, neither streamwater phosphorus concentrations nor phosphorus cycling characteristics exhibited longitudinal trends. Regressions between phosphatase activity and streamwater SRP concentration and between phosphorus content and streamwater SRP were highly significant for all data combined, with SRP explaining ≥74% of the variation in phosphatase activity and P content. Measures of periphyton biomass (chlorophyll a, total biovolume), and productivity (areal carbon fixation rate, chlorophyll-specific carbon fixation rate) exhibited no consistent longitudinal patterns, even during the period of longitudinal streamwater phosphorus depletion. Regressions between productivity measures and streamwater SRP concentration for all data combined were significant, but SRP explained ≤56% of the variation in productivity. Periphyton biomass and productivity at all stations along the longitudinal transect appear to be maintained at low levels by high and longitudinally uniform rates of herbivory throughout the year. Algal species composition exhibited some response to longitudinal nutrient depletion. The biovolume and percentage of the blue-green alga Chamaesiphon investiens increased and the percentage of the chlorophyte Stigeoclonium sp. declined longitudinally when nutrients also declined. Our results demonstrate an upstream-downstream biotic linkage in Walker Branch. We show that instream nutrient uptake can reduce the concentrations of nutrients in stream water and thereby influence the structure and functioning of downstream periphyton communities. However, increases in nutrient cycling in response to lower streamwater concentrations can partially compensate for nutrient depletion by upstream organisms, thereby buffering primary productivity in downstream periphyton communities from changes in nutrient supply.