The role of Gloeotrichia echinulata in the transfer of phosphorus from sediments to water in Lake Erken

The abundance of Gloeotrichia echinulata colonies in the sediments of Lake Erken and their phosphorus content were investigated to determine the contribution of Gloeotrichia colonies to total sediment phosphorus. Moreover, the potential size of the algal inoculum and the migration to the water during summer were estimated.The surplus phosphorus content of the resting colonies in the sediment was about 45% of total phosphorus, which maximized at 8.5 µg P (mg dw)^–1 or 81 ng P colony^–1. The C:P ratio (by weight) in the early colonies appearing in the lake water was 50:1, while the ratio stabilized at 150 during the major migration period. The internal supply of surplus phosphorus was used during the pelagic growth of the colonies.The internal phosphorus loading to the epilimnion of Lake Erken due to Gloeotrichia migration could, from the measurements of the increase in particulate epilimnetic phosphorus, be estimated at 40 mg P m ^–2 or 2.5 mg P m^–2 d^–1 in late July and early August. Determination of the number of colonies in the sediment before and during the migration verified this value to be a conservative estimate of the internal phosphorus loading due to Gloeotrichia migration to the epilimnion in Lake Erken.The sediment P content calculated from the P concentration in early epilimnion colonies resulted in a value of 35 µg P (g dw)^–1 as a maximum. This corresponds to only 3% of the total phosphorus content in Lake Erken sediment.     

Volumetric and aerial rates of heterotrophic bacterial production in epi- and hypolimnia: the role of nutrients and system morphometry

Epilimnetic and hypolimnetic bacterial production (BP) were measured once in summer, by the incorporation of [³H] - Leucine in each of 14 Quebec (Canada) lakes varying in nutrient content and morphometry. The epilimnetic and hypolimnetic BP were evaluated at two scales: the common per unit volume and areal (m^–2) scale. The per unit volume scale epilimnetic BP was best predicted by total phosphorus (TP, r ²=0.63), and by water residence time (WRT r ²=0.57), with WRT serving as a surrogate for the nutrient and organic matter supply from the catchments. Total phosphorus and lake mean depth (Z _m) together explained 79% of the variation in epilimnetic BP (l^–1). In contrast, hypolimnetic BP (l^–1) was neither linked to nutrients (TP or TN) or dissolved organic carbon (DOC) but only to measures of lake morphometry and best of all to hypolimnetic thickness (Zh; r ²=0.74). With increased Zh, there is an increased dilution of settling organic particles and their nutrients, resulting in a decrease in BP per litre. Conversely, when BP is expressed in areal units (m^–2), hypolimnetic production increases with increasing hypolimnetic thickness. Water column thickness is a master variable, which together with Chl a (abundance of particles) determines hypolimnetic BP at the whole system scale even though the trophic status is the best single indicator of epilimnetic BP on a volumetric scale. Conclusions drawn invariably change with the scale of investigation. Moreover, it is clear that lake morphometry has a major impact on BP. A comparison of whole water column integrated BP with literature derived estimates of the equivalent sediment production (m^–2) below suggests that if the estimated sediment rates are not complete technique artefacts, they are likely to be an order of magnitude higher than the water column rates (m^–2) at the maximum depth sampling sites. The relative importance of the sediments could be expected to rise with a decline in the maximum depth of lakes, characterized by progressively thinner hypolimnia. The present findings point to both a primarily allocthonous fuelling of sediment production and an uncoupling of water and sediment BP.     

Interactions between sediment and water in a shallow and hypertrophic lake: a study on phytoplankton collapses in Lake Søbygård,Denmark

Short-term changes in phytoplankton and zooplankton biomass have occurred 1–3 times every summer for the past 5 years in the shallow and hypertrophic Lake Søbygård, Denmark. These changes markedly affected lake water characteristics as well as the sediment/water interaction. Thus during a collapse of the phytoplankton biomass in 1985, lasting for about 2 weeks, the lake water became almost anoxic, followed by rapid increase in nitrogen and phosphorus at rates of 100–400 mg N M^–2 day^–1 and 100–200 mg P m^–1 day^–1. Average external loading during this period was about 350 mg N m^–2 day^–1 and 5 mg P m^–2 day^–1, respectively.Due to high phytoplankton biomass and subsequently a high sedimentation and recycling of nutrients, gross release rates of phosphorus and nitrogen were several times higher than net release rates. The net summer sediment release of phosphorus was usually about 40 mg P m^–2 day^–1, corresponding to a 2–3 fold increase in the net phosphorus release during the collapse. The nitrogen and phosphorus increase during the collapse is considered to be due primarily to a decreased sedimentation because of low algal biomass. The nutrient interactions between sediment and lake water during phytoplankton collapse, therefore, were changed from being dominated by both a large input and a large sedimentation of nutrients to a dominance of only a large input. Nitrogen was derived from both the inlet and sediment, whereas phosphorus was preferentially derived from the sediment. Different temperature levels may be a main reason for the different release rates from year to year.     

Phosphorus in the sediment of the Loosdrecht lakes and its implications for lake restoration perspectives

In 1984 the external phosphorus load of the shallow eutrophic Loosdrecht lakes was reduced from 3.3 to 1.0 mg m^–2 d^–1. The effect of phosphorus release from the sediment on lake restoration was investigated. Diffusive release under aerobic conditions (20 °C) decreased from 1 mg m^–2 d^–1 in 1984 to 0.3 mg m^–2 d^–1 in 1990. The generation of inorganic phosphorus due to mineralization during summer equals 3 mg m^–2 d^–1, which is much higher than the measured rate of diffusive release. Despite that, the phosphorus release is hardly stimulated by anaerobic conditions, which indicates that only a small amount of phosphorus is adsorbed by ferric iron in the top sediment layer. This apparent discrepancy is probably caused by the uptake of inorganic phosphorus uptake during resuspension and the loss of inorganic phosphorus with downward seepage.The estimated removal of phosphorus due to downward seepage of 0.8 mg m^–2 d^–1 agrees well with the average phosphorus retention in the lake. This indicates that sediment burial and diagenesis are unimportant mechanisms for withdrawing phosphorus from the nutrient cycle.Between 1982 and 1991 the total phosphorus content of the upper 2 cm of the sediment decreased from 0.94 to 0.60 g kg^–1 DW. At present, about 20% of total phosphorus in this layer is potentially bioavailable, but largely incorporated in easily degradable organic matter. This pool is much smaller in deeper layers. Based on the estimated and measured rates and pool sizes, the annual average phosphorus cycle in the lakes was modelled to evaluate the effects of various restoration measures. The main predictions of the model are: 1) further reduction of the external load may cause a gradual decrease of the total phosphorus concentration in the lake water; 2) dredging and iron addition, without reduction of the external load, may give a rapid improvement followed by a slow return to the present situation; and 3) reduction of the external load, combined with a cut off of downward seepage will not improve the water quality.     

Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

Production rate ofEichhornia crassipes (Mart.) Solms in river Ganga water

The production rate ofEichhornia crassipes was stimulated by water of the river Ganga and by prevailing environmental conditions. It was highest in October (4.76 g.m^–2.d^–1) and was positively correlated with ammonia nitrogen and total phosphorus in the water but negatively correlated with total alkalinity and transparency. The average annual production of 14.13 t.ha^–1.a^–1 is equivalent to the average production of 0.067 t.ha^–1.a^–1 phosphorus and 0.40 t.ha^–1.a^–1 nitrogen. The concentrations of total nitrogen and total phosphorus of the plant varied seasonally. They decreased with increasing production rate in summer and monsoon.     

Fixation of phosphorus in lake sediments using iron(III)chloride: experiences,expectations

After a reduction of the external phosphorus loading to a lake, an internal loading from the sediments may delay the improvement of the water quality. The accepted method to combat internal loading is careful dredging of the upper sediment layers (Cooke et al., 1986), but this method is costly and time consuming. Addition of phosphorus binding agents to the sediments might offer an alternative. In the Netherlands the use of aluminum compounds, the most common phosphorus binding agent, for water quality improvement purposes is not favoured. Therefore a sediment treatment with a solution of iron(III)chloride was tested. Iron was chosen because it is considered to be a natural binder of phosphate. 100 g m^–2 of Fe³⁺ were added to the sediments of the shallow (1.75 m average depth) and eutrophic Lake Groot Vogelenzang (The Netherlands) in October and November 1989. The iron(III)chloride solution was diluted 100 times with lake water and mixed with the surface sediments with a water jet.Following the addition the concentrations of total phosphorus (Fig. 1), chlorophyll-a and suspended solids decreased. This improvement of the water quality lasted for three months. After this time the total phosphorus concentration increased again, but remained at a lower level than in spring and summer of 1989. The phosphorus release rate from the sediments as measured from intact sediment cores decreased from 4 to 1.2 mg P m^–2 d^–1 (n = 5), and the bioavailability of the sediment phosphorus, as measured with bioassays, decreased from 34 to 23% (n = 5) shortly after the treatment. One year after the treatment the release rate was increased to 3 mg P m^–2 d^–1 (n = 5). Before treatment, the lake was thought to have a residence time of over one year. However, the chloride added to the lake disappeared according to a dilution rate of 0.03 d^–1 or a retention time of about 35 days. A high external loading due to rapid flushing with phosphorus-rich water from surrounding lakes possibly prevented a more durable improvement in water quality. Another possibility is that the iron addition has lost its phosphate binding capacity due to reduction or binding with other anions like carbonate or sulphide. Therefore the suitability of the method to reduce internal loading and especially the long term availability of added iron to bind phosphorus needs additional proof.The treatment of the 18 ha area of Lake Groot Vogelenzang took three weeks. The operational costs were about US$ 125000. This is fast and cheap compared to dredging. Application of the technique is limited to those cases where the sediments are not polluted with micro-pollutants and the water depth need not be increased.     

Variability in the water chemistry of shallow ponds in southeast England, with special reference to the seasonality of nutrients and implications for modelling trophic status

The variability in water chemistry of samples taken on a monthly basis (October 1990–December 1991) from 31 shallow, artificial ponds in southeast England was examined. The survey revealed great within-year variation in the concentrations of nutrients. Total phosphorus (TP) concentrations displayed no overall marked seasonal pattern, although many sites experienced summer peaks with a simultaneous increase in soluble reactive phosphorus (SRP) concentrations, indicating that sediment P release occurred. SRP and nitrate (NO₃–N) concentrations displayed a marked seasonal pattern similar to that observed in deep, stratifying lakes, with 55% and 94% of the sites surveyed, respectively, experiencing a decline in concentrations in the spring, maintaining low levels throughout summer and the highest levels occurring in winter. Dissolved silica (SiO₂–(Si) also displayed a marked seasonal distribution with a spring decline in concentrations associated with diatom growth, followed by an increase in the summer.The study demonstrated that intra-annual variability in nutrient concentrations is high and tends to be greatest in the most enriched waters. Thus, a high sampling frequency is required to provide representative annual mean data. Furthermore, annual means rather than winter–spring means provide more appropriate estimates of TP and SRP in these waters, owing to the importance of internal cycling of nutrients in summer. The findings are especially relevant to sampling strategy design and the averaging of seasonal water chemistry data for use in predictive models of lake trophic status.     

Nitrogen and phosphorus accumulation and cycling by Nymphoides peltata (Gmel.) O. Kuntze (Menyanthaceae)

In 1980, the seasonal changes in nitrogen and phosphorus concentration of various plant parts of Nymphoides peltata (Gmel.) O. Kuntze, together with aspects of nitrogen and phosphorus cycling by this species were studied in an oxbow lake of the river Waal (The Netherlands). The nitrogen and phosphorus stores of the water, seston, sediment and macrophyte compartments were assessed each month.The underground Nymphoides structures had high nitrogen and phosphorus concentrations before and after the main growing season, while during summer the aboveground plant parts had high nutrient contents. Nymphoides peltata accumulated maximum amounts of nitrogen (334 mmol m⁻²) and phosphorus (56.6 mmol m⁻²) in July. The upper layers of the bottom appeared to be an enormous nutrient reservoir (94–99% of total) of which the largest part was not directly available to Nymphoides. Nutrient uptake from the sediments by N. peltata is suggested by the fact that the bottom and/or interstitial water of the sample station devoid of rooted macrophytes, contained higher concentrations of nitrogen and phosphorus than that of the Nymphoides stands. The annual flux of nutrients from Nymphoides to the detritus compartment was estimated to be ca. 1200 mmol nitrogen and 164 mmol phosphorus per m² of littoral. During breakdown of the detritus there was a relatively fast net conversion of organically bound nitrogen and phosphorus to inorganic forms, especially at higher temperatures.Nymphoides has the potential to function as an important nitrogen and phosphorus pump, which regenerates sediment nutrients.     

Release of sedimentary nitrogen and phosphorus in polder ditches of a low-moor peat area

The release of N and P from the sediment of two ditches, one (A) dominated by filamentous algae and the other (B) by water-lilies, was estimated by core and enclosure experiments. The release rates for ditch A tended to be higher than those for ditch B. Sediment cores covered by a filamentous algae layer released about 1.5 times more N and P than those from which the layer had been removed. During the incubation of the cores in the dark at 20°C for 2–3 weeks, about 10% of the N in the filamentous algae layer was mineralized. The mineralization could be described as a first-order reaction with a rate constant of about 0.2 d^–1. On average the cores of ditches A and B released about 40 mg mineral N and 3 mg.m^–2.d^–1 soluble reactive phosphorus. Defining the release from the sediment in the enclosures as the net increase of N and P in the water phase and in the vegetation minus the input, a negative net release,i.e. net accumulation of N and P in the sediment, was found over the summer half of the year. The negative values were due to the significant N and P input, resulting from pumping ditch water into the enclosures in order to compensate for downward seepage. From the enclosure experiments a downward seepage rate of 14 mm.d^–1 and an external load of about 6 g.m^–2 total N and 0.6 g.m^–2 total P during the summer half of the year —i.e. 33 mg.m^–2.d^–1 N and 3 mg.m^–2.d^–1 P. respectively — was calculated for the ditches. Tentative gross release rates — based on the sum of the positive net release of N and P into the water phase over 1–2 weeks intervals and the net increase of N and P in the vegetation — converted to 20°C and allowing for underestimation of the primary production by a factor of 5, amounted to 58 mg mineral N and 7 mg.m^–2.d^–1 soluble reactive phosphorus during the summer half of the year. Combining the rates estimated by cores and enclosures and converting them to rates at the mean water temperature during the summer half of the year, the release of mineral N and soluble reactive phosphorus roughly amounted to 40 and 4 mg.m^–2.d^–1, respectively. The release rates as well as the external load indicated a relatively low eutrophication of the ditches.     

Carbon,phosphorus and nitrogen budgets of the littoral Equisetum belt in an oligotrophic lake

Stores and flows of carbon, phosphorus and nitrogen in a littoral Equisetum stand were studied in 1978–1980 in the oligotrophic, mesohumic lake Pääjärvi, southern Finland. The major carbon and nutrient stores were sediment and Equisetum. The seasonal cycle of the macrophyte vegetation had a profound influence on the whole littoral ecosystem. In spring, when only dead remains of Equisetum were present above ground, there were few differences in nutrient, chlorophyll a and zooplankton concentrations between the littoral and the open lake; phytoplankton and epiphytes were the major producers.In early June, when new shoots of Equisetum reached the water surface, water exchange between the littoral and the open lake started to diminish, and the characteristic features of a closed macrophyte zone gradually developed: by August the P, Chl a and zooplankton concentrations in the littoral were 5–10 times those in the open lake. From late June until autumn Equisetum was overwhelmingly dominant both in biomass and in production.The measured total primary production and respiration values indicated a high rate of internal cycling of carbon and nutrients. The daily P requirements of plant growth exceeded the total P stored in the water by a factor of 2–4, and also exceeded the release of nutrients in excretion. High N:P ratios in the water (total 10–64, inorganic 18–171) suggested that P was probably always the limiting nutrient.The P content of the annual production of Equisetum in Pääjärvi was 2.3% of the mean annual P load, and 5.3% of the mean total P storage in the water volume of the lake.     

Modelling of Spirulina platensis growth in fresh water using response surface methodology

The influence of nutrient addition on the growth rate of Spirulina platensis in the Mangueira Lagoon water was studied in order to investigate the feasibility of using this water for biomass production. The addition of urea and sodium bicarbonate was studied through surface response methodology, over concentration ranges from 0.0 to 0.01170 M, and 0.0–19.70 gl^–1 respectively. The growth of Spirulina platensis in Mangueira Lagoon water with no addition of nutrients was carried out and compared with the biomass growth after nutrient addition. The results indicated that the optimal level of nutrients was 0.00585 M urea and without the addition of sodium bicarbonate. The biomass concentration was 1.4 gl^–1 in 780 h of cultivation and the doubling time (t _d) was 3.85 days. In 300 h, the biomass concentration in the medium without nutrient addition was 0.9 gl^–1, with a doubling time of 3.80 days.     

Pore water phosphorus and iron concentrations in a shallow,eutrophic lake — indications of bacterial regulation

Peak pore water SRP and iron(II) concentrations were found during summer in surface sediments in the shallow and eutrophic L. Finjasjön, Sweden, and the concentrations generally increased with water depth. The SRP variation in surface sediments (0–2 cm) was correlated with temperature (R² = 0.82–0.95) and iron(II) showed a correlation with sedimentary carbon on all sites (R² = 0.42–0.96). In addition, sedimentary Chla, bacterial abundances and production rates in surface sediments (0–2 cm) varied seasonally, with peaks during spring and fall sedimentation. Bacterial production rates were correlated with phosphorus and carbon in the sediment (R² = 0.90–0.95 and R² = 0.31–0.95, respectively), indicating a coupling with algal sedimentation. A general increase in sediment Chla and bacterial abundances towards sediments at greater water depth was found. Further, data from 1988–90 reveal that TP and TFe concentrations in the lake were significantly correlated during summer (R² = 0.81 and 0.76, in the hypolimnion and epilimnion, respectively). The results indicate that the increase in pore water SRP and Fe(II) in surface sediments during summer is regulated by bacterial activity and the input of organic matter. In addition, spatial and temporal variations in pore water composition are mainly influenced by temperature and water depth and the significant correlation between TP and TFe in the water suggests a coupled release from the sediment. These findings support the theory of anoxic microlayer formation at the sediment-water interface.     

Control of phosphorus loading and flushing as restoration methods for Lake Veluwe,The Netherlands

As a result of high nutrient loading Lake Veluwe suffered from an almost permanent bloom of the blue-green algaOscillatoria agardhii Gomont. In 1979, the phosphorus loading of the lake was reduced from approx. 3 to 1 g P.m^–2.a^–1. Moreover, since then the lake has been flushed during winter periods with water low in phosphorus. This measure aimed primarily at interrupting the continuous algal bloom. The results of these measures show a sharp decline of total-phosphorus values from 0.40–0.60 mg P.l^–1 (before 1980) to 0.10–0.20 mg P.l^–1 (after 1980). Summer values for chlorophylla dropped from 200–400 mg.m^–3 to 50–150 mg.m^–3.The increase in transparency of the lake water was relatively small, from summer values of 15–25 cm before the implementation of the measures to 25–45 cm afterwards. The disappointing transparency values may be explained by the decreasing chlorophylla and phosphorus content of the algae per unit biovolume. Blue-green algae are gradually loosing ground. In the summer of 1985 green algae and diatoms dominated the phytoplankton for the first time since almost 20 years. To achieve the ultimate water quality objectives (transparency values of more than 100 cm in summer), the phosphorus loading has to be reduced further.     

Trophic state and water turn-over time in six choked coastal lagoons in Brazil

Comparison of total phosphorus and chlorophyll-a concentration, nutrient loading, and water turn-over time in six shallow choked lagoons along the coast of the state of Rio de Janeiro, Brazil, established that water turn-over time is related to the trophic state of the lagoons with additional anthropogenic nutrient loading affecting this relationship. Turnover time was calculated as a flushing half-life from rainfall, evaporation, runoff, and tidal exchange data, and trophic state was calculated from the quantity and quality of dissolved inorganic nutrients, total phosphorus, and chlorophyll-a standing stock. Flushing half-life of the lagoons ranged between 1 and 27 days, annual phosphorus areal loading from 3 to 18 mg m^–2d^–1, and chlorophyll-a standing stock from 6 to 160 mg M^–2     

Effect of an aquatic herbicide on sediment nutrient flux in a freshwater marsh

Three concentrations of the herbicide simazine were added to in situ macrophyte-free enclosures with and without sediment contact. Changes in the concentrations of total ammonia, total reactive phosphorus, and silicon were monitored, and net sediment flux was calculated from the difference in nutrient concentration between bottomed and unbottomed enclosures. Rates of sediment release for all three nutrients were unaltered by 0.1 mg · l^–1 simazine in relation to a control, whereas rates were increased proportionally at 1.0 and 5.0 mg · l^–1. These results suggest that increases in dissolved nutrients commonly observed following herbicide treatment of shallow waters may not be attributable solely to macrophyte decay, byt may also involve a complex interaction of biotic and abiotic sediment nutrient exchange processes.Contribution Number 103 from the University of Manitoba Field Station, Delta Marsh, Canada     

Physicochemical limnology of the Tongue River Reservoir,Montana

A one year physicochemical survey was conducted on the Tongue River Reservoir, a run of the river impoundment in southeastern Montana. The Tongue River was the only significant inflow and supplied 93, 96 and 97% of the nutrient, major ion and water inputs to the impoundment. Heat advected from inflowing water accounted for 17% of the energy gained during the summer heating cycle. The annual nutrient load to the reservoir from the river was 20.2 g m^–2 total nitrogen (TN) and 3.8 g m^–2 total phosphorus (TP). Due to the absence of reducing conditions at depth and the complex seasonal pattern of water movement through the reservoir, 99% of the TN load was discharged but 49% of the TP load was retained in the reservoir.     

The 1994 experimental opening of the Bonnet Carre Spillway to divert Mississippi River water into Lake Pontchartrain, Louisiana

A diversion of Mississippi River water into Lake Pontchartrain, Louisiana, USA by way of the Bonnet Carre Spillway has been proposed as a restoration technique to help offset regional wetland loss. An experimental diversion of Mississippi River water into Lake Pontchartrain was carried out in April 1994 to monitor the fate of nutrients and sediments in the spillway and Lake Pontchartrain. Approximately 6.4×10⁸ m³ of Mississippi River water was diverted into Lake Pontchartrain over 42 days. As water passed through the Bonnet Carre Spillway, there were reductions in total suspended sediment concentrations of 82–83%, nitrite+nitrate (NO_x) of 28–42%, in total nitrogen (TN) of 26–30%, and in total phosphorus (TP) of 50–59%. 3.9±1.1 cm of accretion was measured in the spillway. Nutrient concentrations at the freshwater plume edge in Lake Pontchartrain compared to the Mississippi River were lower for NO_x (44–81%), TN (37–57%), and TP (40–70%), and generally higher for organic nitrogen (−7–57%). The Si:N ratio generally increased and the N:P ratio decreased from the river to the plume edge. Nutrient stoichiometric ratios indicate water at the plume edge was not silicate limited, suggesting conditions favoring diatomic phytoplankton.     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

Studying the phosphorus release from the Loosdrecht Lakes sediments,using a continuous flow system

Phosphorus release from the Loosdrecht Lakes sediments was studied, using a continuous flow reactor. The summer release maxima were 4 mg P.m^–2.d^–1 in 1984 and 1.4 mg P.m^–2.d^–1 in 1985. Temperature and downward seepage controlled release rates to a great extent, the pH of the overlying water being only of minor importance. From these results it could be concluded that release processes might be driven by mineralization of particulate organic phosphorus in the sediment. Pore water studies in the sediments of the release reactor confirmed this hypothesis. From the profiles phosphorus dissolution rates were calculated.