Light response characteristics of net CO2 exchange in brackish wetland plant communities

Summary Photosynthetic responses to incident photon flux density (400–700 nm; PPFD) was studied in a grass community consisting of Spartina patens and Distichlis spicata and a mixed community having the two grasses and a sedge, Scirpus Olneyi. Net community CO₂ exchange and incident PPFD were monitored from dawn to dusk in a large open gas exchange system, and a hyperbolic light response model was fit to the data for each day. Light response curves from five growing seasons were evaluated for seasonal trends in the compensation value, initial slope, and maximum net CO₂ exchange rate calculated from the model at PPFD=1670 mol m^-2s^-1.All response curves were curvilinear. Data from approximately 30% of the 113 days studied fit saturation curves which occurred primarily in spring and fall. Approximately 5% of all curves constructed required a different response curve for the morning and afternoon. These occurred during mid-summer and were interpreted to be evidence of water stress.The compensation flux density was very high early in the growing season, but rapidly decreased and during the months June, July and August, it averaged near 100 and 120 mol m^-2s^-1 in the mixed and grass communities. The initial slope and maximum net CO₂ exchange rate increased from early May to maxima in July and declined thereafter. Mid-summer mean values for the mixed and grass communities respectively were 34.3±10.3 mmol mol^-1 and 39.1±9.1 mmol mol^-1 for the initial slope and 20.3±4.2 mol m^-2s^-1 and 23.0±3.8 mol m^-2s^-1 for maximum net CO₂ exchange. Daytime respiration accounted for approximately 20% of maximum gross photosynthesis in both communities.Photosynthetic efficiency, CO₂ assimilated per unit total incident solar radiation, was approximately 4.1% and 4.7% at dawn or dusk and 2.3% and 2.6% at midday for the mixed and grass community. Gross photosynthesis, maximum photosynthesis plus midday respiration, accounted for 2.7% and 3.0% of total incident solar radiation in the mixed and grass communities.     

Planktonic phosphorus pool sizes and cycling efficiency in coastal and interior British Columbia lakes

1. Limnologists have long acknowledged the importance of phosphorus (P) in determining the organism biomass and productivity of lake ecosystems. Despite a relatively large number of studies that have examined P cycling in lake ecosystems, there remain several substantial methodological issues that have impeded our understanding of P cycling in limnetic plankton communities. Two critical issues confronting ecologists are (1) a lack of precise measurements of the dissolved inorganic phosphorus (PO) and (2) accurate or complete measurements of dissolved P regeneration rates by plankton communities. 2. Here, we examine patterns of epilimnetic planktonic P pool sizes and turnover rates in eight lakes in British Columbia, Canada over a 2‐year period. We determine the concentrations and turnover times of P in various planktonic compartments (dissolved and various planktonic size fractions), using recently developed methods for estimating phosphate concentration and planktonic regeneration rates. 3. The pico‐ and nanoplankton size fraction (0.2–20 μm) played a central role in planktonic P cycling in lakes examined by this study. On average across lakes, pico‐ and nanoplankton contained >60% of the planktonic P, accounted for >90% PO uptake, and contributed 50% of the plankton community dissolved P regeneration rate. 4. PO concentrations determined by steady state bioassays (ssPO) were extremely low (87–611 pmol L⁻¹) and were 2–3 orders of magnitude less than simultaneously measured colorimetric soluble reactive phosphorus estimates. Lake ssPO concentrations increased linearly with total phosphorus (TP), and the slope of this relationship was approximately 1, indicating that PO remained a consistent proportion of the TP pool across a range of TP concentrations. 5. Turnover rates of the total planktonic P pool and the <20 μm pool became more rapid with increasing lake TP, indicating that, according to this metric, planktonic P cycling efficiency increased with TP concentrations. We also detected a significant relationship between particulate phosphorus (PP) <20 μm turnover time and seston N : P ratios, with PP <20 μm turnover times becoming slower with increasing seston N : P. These findings suggest that long‐standing conceptual models of nutrient cycling that predict slower cycling rates and decreasing cycling efficiency with increasing TP concentrations require further empirical examination. We postulate that patterns in lake P turnover and cycling efficiency are a result of complex interactions between plankton biomass and composition, and the ratios of multiple nutrients (C, N, P), rather than solely a function of the TP pool.     

Influence of internal loading on phosphorus concentration in shallow lakes before and after reduction of the external loading

An analysis of data from 49 shallow lakes showed, that the parameters of empirical models between phosphorus loading and concentration in the lake (e.g. Vollenweider type of relations) differ significantly for lakes without or with a reduced external loading. For lakes without a reduction of the external loading the summer phosphorus concentration is determined by the external phosphorus loading and the hydraulic loading. For these lakes the classical models suffice; deviations between calculations and measurements are partly due to errors made in the determination of the loading.In contrast, for lakes where the external loading was reduced, the measured internal loading explains most of the variation in the summer lake concentration. The external loading is of minor importance and the classical models cannot be applied. The internal loading measured before reduction of the external loading is not useful in predicting the concentration afterwards. Instead of the internal loading, the sediment composition can be used. The advantage of using sediment composition is that these variables are easier to determine and vary less in time. The most promising variable is the ratio between total P and total Fe in the sediment.Abbreviations: Q_s hydraulic loading (m y^-1) - hydraulic retention time (y) - L_ext external phosphorus loading (gP m^-2 y^-1) - L_int internal phosphorus loading (gP m^-2 y^-1) - P_lake phosphorus concentration in the lake (gP m^-3) - P_inlet phosphorus concentration in the inlet water (gP m^-3) - P_sed phosphorus content on the sediment (gP kg^-1 d.w.) - Fe_sed iron content of the sediment (gFe kg^-1 d.w.) - Y dependent variable multiple regression calculations - X1, X2 independent variables multiple regressions calculations - a, a1 constants - a2, b constants     

Alkaline phosphatase activity of coral reef benthos

Alkaline Phosphatase (AP-ase) activity was measured for a variety of benthic algae and a community of reef organisms. Algae with epiphytic bacteria showed a higher AP-ase activity than algae without bacteria (11.6 mol P g^-1 h^-1 vs. 1.9mol P g^-1h^-1). AP-ase activity associated with the benthos was estimated to be in the range of 10–100 mmol P m^-2d^-1, at least 1000 Cold greater than reported activity in the water column. Enzyme activity of reef benthos at saturated organic phosphate (P) substrate concentrations was sufficiently high that P uptake from organic substrates could be as fast as inorganic P uptake. Organic P compounds may be important in P recycling, but there is no evidence that organic P represents a significant new source of P to coral reefs.     

Nitrogen and phosphorus requirements for raising mycorrhizal seedlings of Leucaena leucocephala in containers

A greenhouse study was undertaken to determine the nitrogen and phosphorus fertilization requirements for raising mycorrhizal seedlings in soil in containers. Seedlings of Leucaena leucocephala were grown for 40 days in dibble tubes containing fumigated or nonfumigated soil uninoculated or inoculated with Glomus aggregatum. The soil was fertilized with NH₄NO₃ solution to obtain 25–200 mg N kg^-1 soil, and with a KH₂PO₄ solution to establish target soil solution P concentrations of 0.015–0.08 mg P l^-1. At the end of 40 days, seedlings were transplanted into pots containing 5-kg portions of fumigated soil. Posttransplant vesicular arbuscular mycorrhizal fungal (VAMF) effectiveness, measured as pinnule P content, plant height, shoot dry weight and tissue N and P concentrations, was significantly increased by pretransplant VAMF colonization in both soils. The best posttransplant mycorrhizal colonization and mycorrhizal growth responses were observed if the nonfumigated pretransplant soil was amended with 50 mg N kg^-1 soil and 0.04 mg P l^-1 or if the fumigated pretransplant soil was amended with 100 mg N kg^-1 soil and 0.04 mg P 1^-1. There was no relationship between NP ratios of nutrients added to the pretransplant soil medium and shoot NP ratios observed after transplanting. Shoot NP ratio was also not correlated with root colonization level.Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 4025     

External and internal factors regulating metabolic rates of an estuarine benthic community

Summary The structure and metabolism of a soft-sediment estuarine macrofaunal community were measured over an annual cycle at two depth-contours in mesohaline Chesapeake Bay. Additional data for plankton productivity and respiration, as well as seston and sediment organics are also summarized for these communities. Benthic community respiration ranged from 0.24–3.38 g O₂ m^-2 d^-1, and significant differences were detected between the two depths. Similarly, macroinfaunal standing stocks reached 11.2 and 32.3 g (ash free) m^-2 for 3 m and 6 m depth communities, respectively, and both exhibited mid-summer declines in abundance. Inferences drawn from these data facilitated a partitioning of benthic community respiration into macrofaunal and meiofaunal/microbial components with a residual term, much of which could be explained statistically by interactions between these two components. A multi-variate statistical model developed from these data matched benthic respiration measurements within 1–2 S.E. Mass-balances of organic carbon were estimated for water column and benthos at the two depthcontours for early and late summer, as well as for an entire, time-weighted year. These various analyses led to the tentative conclusions that this benthic community was regulated by such internal factors as macrofaunal/meiofaunal grazing and microbial gardening, and by external factors such as temperature and predation by nekton. However, it appears that the ultimate control for this community was the supply of energy from organic carbon.Contribution No. HPEL-1206, USASupported by grants with the Maryland Department of Natural Resources (PS-72-02(77-78)), J.A. Mihursky, Coordinating Principal Investigator     

The role of bacteria in the nutrient exchange between sediment and water in a flow-through system

The contribution of bacteria to phosphorus (P) and nitrogen (N ) release from, or retention in, sediment was studied in a flow-through system. Live and formaldehyde-killed sediment communities were incubated in 25-liter bottles with a continuous flow of P- or P + N-enriched water. Sediment bacteria in the killed communities were inhibited by adding formaldehyde (final concentration 0.04% v/v) to the sediment before the start of the experiment. Bacterial activity in the live sediments measured with [³H]thymidine and [¹⁴C]leucine incorporation techniques did not change essentially during the experiment period (7–8 days). Chemical mechanisms were found to be of principal importance in PO₄-P retention in the sediment. In the live samples, the net retention of PO₄-P was lower than in the killed samples, which was likely due to the reduced O₂ conditions in the sediment as a consequence of bacterial mineralization. In total P exchange, however, bacteria increased the retention rate by recycling dissolved organic P in the sediment. In the live communities the retention of N was very efficient, and all the introduced NH₄ -N and NO₃-N was immobilized by sediment bacteria. Nitrogen enrichment, however, did not alter the P exchange rates. The gradual emergence of bacterial activity (and grazing) in the killed communities, subsequent to the dilution of formaldehyde concentration, enhanced the release of PO₄-P and NH₄-N from sediment.     

Phosphate uptake capacity of summer phytoplankton of the Loosdrecht Lakes in relation to phosphorus loading and irradiance

Summer populations of the phytoplankton of the Loosdrecht Lakes were enclosed in laboratory scale enclosures (LSE), supplied with 7.5 g P.l^–1.d^–1 and 105 g P.l^–1.d^–1, respectively. The maximum initial phosphate uptake rate (V_m) was related to irradiance and primary production. At phosphate uptake saturating light-irradiance V_m values up to 4 times the V_m values in the dark were measured.The phosphate uptake capacity per unit dry weight remained more or less constant throughout the experiments in the LSE receiving the lower amount of phosphorus, and declined in the LSE receiving the higher amount of phosphorus. Within the range of V_m values measured (<10 g P.mg DW^–1.h^–1 or 1,3 g P. g chla ^–1.h^–1), the growth rate of the phytoplankton was not influenced by alterations in phosphorus availability.     

Distribution of carbon,nitrogen and phosphorus in a moss community-soil system developed on a cold desert in Antarctica

The distributions of carbon, nitrogen and phosphorus in a moss community-soil system developed on a naked region in Rundvågskollane (69⁰50'S, 39⁰09'E), East Antarctica, were investigated in order to analyze the flow of matter in an Antarctic terrestrial ecosystem.

1. The moss community was formed from many moss blocks of different sizes and was composed ofBryum pseudotriquetrum (Hedw.) Gaertn., Meyer et Scherb.,Ceratodon purpureus (Hedw.) Brid. andGrimmia lawiana J. H. Willis. The surface of the community was covered with cyanobacteria.
2. It was estimated that nitrogen fixed by cyanobacteria flowed from these organisms to the moss and that little nitrogen was transported within the moss body.
3. A large amount of phosphorus existed in the soil. The moss community had a high phosphorus content although the amount itself was relatively little due to the small phytomass.
4. It seems that absorption of phosphorus from soil is difficult for moss, because of the paucity of water necessary for the movement of phosphorus and the suppressed growth of moss due to the arid conditions.

     

Predator-induced bottom-up effects in oligotrophic systems

Five treatments (replication n=2) were applied to mesocosms in an oligotrophic lake (TP=6–10 µg 1^-1) to assess the effects of fish on planktonic communities. The treatments were: (1) high fish (30 kg ha^–1 Lepomis auritus, Linnaeus), (2) low fish (10 kg ha^–1), (3) high removal of zooplankton, (4) low removal of zooplankton and (5) control. Total phosphorus, chlorophyll a, zooplankton biomass, and species richness decreased from high fish > low fish > control > low removal > high removal treatments. The fish treatments were dominated by crustacean zooplankton, while rotifers outnumbered the other zooplankters in the removal treatments. Calculations of zooplankton grazing rates suggested that clearance rates seldom exceeded 2% of the enclosure volume d^–1 and were unlikely to have had much influence on phytoplankton biomass. Calculations from a phosphorus bioenergetics model revealed that when fish were present, their excretion rates were higher than the rates ascribed to zooplankton. Diet analysis showed that the fish derived most of their energy from the benthos and periphyton, and that fish excretion and egestion made significant contributions to the very oligotrophic pelagic phosphorus pool. In the absence of fish, zooplankton excretion was highest in the control treatments and lowest in the zooplankton removal treatments. Our results suggest that in oligotrophic systems, planktivorous fish can be significant sources of phosphorus and that fish and zooplankton induced nutrient cycling have significant impacts on planktonic community structure.     

Growth rates and the salinity response of an Antarctic ice microflora community

Summary An ice microflora community collected from the bottom of seasonal pack-ice off the Amery Ice Shelf, Antarctica, was grown at salinities which varied from 11.5 to 34. The response exhibited by the community and by individual species was characterized by an initial lag phase-adaptation period followed by a short period of exponential growth. Doubling rates based on changes in chlorophyll a had a range from 0.05 to 0.23 day^-1 during the time required to reach maximum chlorophyll a concentration and a range of 0.04 to 0.42 day^-1 during a period of exponential growth. Exponential growth rates of individual species ranged from 0.2 to 1.0 doublings day^-1. Growth occurred at all salinities above 11.5. Community growth rates increased with increasing salinity, and the growth-salinity response of most species was shifted toward higher salinities suggesting that this Antarctic ice microalgal community was adapted to the ambient salinity regime: 34.     

What factors control planktonic ciliates during summer in a highly eutrophic lake?

A mesocosm experiment in 24 enclosures (6 m³) started at the end of June 1996 in a highly eutrophic shallow lake, Lake Köyliönjärvi (SW Finland). The original factorial design with nutrient, fish and macrophyte treatments was lost due to strong winds causing leakages. However, after the walls were made leak-proof again on July 11, the planktonic communities developed in divergent ways. On July 31 there was a tenfold variation in total crustacean biomass among the enclosures and the lake (40.2–417.5 g C l^–1), and chlorophyll a varied from 9.5 to 67.0 g l^–1. Here, the single-day data on the 25 planktonic communities is analysed by means of correlation and factor analysis in order to identify factors controlling the protozoans, with particular emphasis on ciliates. The data set comprised: total phosphorus, nitrogen, chlorophyll, bacteria, autotrophic picoplankton, heterotrophic flagellates, abundance and species composition of ciliates, phytoplankton and metazooplankton. The results indicate that although the total ciliate abundance (ranging from 16.2 to 95.0 ind l^–1) was controlled by food resources, the observed differences in ciliate community structure could be attributed partly to differential predation by metazooplankton. The effect of Daphnia cucullata, the dominant daphnid cladoceran, was stronger than that of other metazoans.     

Cell wall anionic polymers and peptidoglycan of Actinoplanes philippinensis VKM Ac-647

The cell wall of Actinoplanes philippinesis VKM Ac-647 harbours several carbohydrate-containing anionic polymers. (1) The main polymer of the wall is of a poly(glycosylglycerol phosphate) nature. Its monomeric units — O--d-mannopyranosyl-(14)--d-galactopyranosyl-(11)-glycerol monophosphates — are connected by phosphodiester bonds involving the hydroxyl groups at glycerol C3 and galactose C6. There also are chains without mannosyl substitutents. The teichoic acid structure has been established by chemical analysis and with ¹H and ¹³C NMR spectroscopy. This is the first finding of a teichoic acid with mannosyl residues in a bacterial cell wall. (2) The phosphorylated mannan contains mannose and 2-O-methylmannose. Its core chain has -1,2; -1,3; and -1,6 substitutions as revealed by ¹³C NMR spectroscopy.The peptide unit of the peptidoglycan contains no l-alanine, instead of which position 1 is occupied by glycine; and diaminopimelic acid is represented, besides its meso- (or DD) form, by small amounts of its LL isomer.Abbreviations Gro glycerol - Gro2P glycerol-2 phosphate - APT attached-proton-test - P_tot total content of phosphorus - P_lab phosphorus mineralized in 7 min at 100°C - P_NA phosphorus of nucleic acids - P_stab stable phosphorus - T trace amounts     

Total and active microbial communities and <Emphasis Type="Italic">phoD</Emphasis> as affected by phosphate depletion and pH in soil

Background and Aims

Soil microbial communities contribute to organic phosphorus cycling in a variety of ways, including secretion of the PhoD alkaline phosphatase. We sampled a long-term grassland fertilization trial in Switzerland characterized by a natural pH gradient. We examined the effects of phosphate depletion and pH on total and active microbial community structures and on the structure and composition of the total and active phoD-harboring community.

Methods

Archaeal, bacterial and fungal communities were investigated using T-RFLP and phoD-harboring members of these communities were identified by 454-sequencing.

Results

Phosphate depletion decreased total, resin-extractable and organic phosphorus and changed the structure of all active microbial communities, and of the total archaeal and phoD-harboring communities. Organic carbon, nitrogen and phosphorus increased with pH, and the structures of all total and active microbial communities except the total fungal community differed between the two pH levels. phoD-harboring members were affiliated to Actinomycetales, Bacilliales, Gloeobacterales, Planctomycetales and Rhizobiales.

Conclusions

Our results suggest that pH and associated soil factors are important determinants of microbial and phoD-harboring community structures. These associated factors include organic carbon and total nitrogen, and to a lesser degree phosphorus status, and active communities are more responsive than total communities. Key players in organic P mineralization are affiliated to phyla that are known to be important in organic matter decomposition.

     

Land-use influence on stream water quality and diatom communities in Victoria, Australia: a response to secondary salinization

Diatom communities were analyzed in 39 streams located in drainages with varied land-use practices throughout Victoria, Australia. Thirteen water quality parameters were also measured in each stream. Most streams had low HCO₃ ^1- concentrations (low buffering capacity) with >90% of the waters dominated by Na¹⁺ and Cl^1-. Phosphate concentrations ranged from 0.003 to 2.0 mg/L. Diatom communities (245 taxa) were strongly correlated with land-use practices, i.e. historic clear cutting, and secondary salinization. Streams influenced by heavy irrigation practices and dryland farming had reduced species diversity and richness compared to systems with low to moderate land use. A nonmetric multidimensional ordination of diatom communities in the 39 streams was conducted. An ANOSIM on the ordination showed that diatom communities in upland watersheds with native forest canopies and low salinization, lowland streams in watersheds with cleared forest canopies, moderate agricultural utilization and salinization, and lowland streams in areas with high irrigation and salinization were all significantly different (p<0.001) from one another. Community ordination techniques showed that both specific conductance (salinity) and phosphorus interacted to determine stream diatom community structure in drainages with high secondary salinization. Drainages with low to moderate agricultural activity and low nutrients, but with a wide range of salinities showed strong associations with the diatom taxa Amphora coffeaeformis, Cymbella pusilla and Entomoneis paludosa, whereas, streams in regions with heavy agricultural practices and high phosphorus had Bacillaria paradoxa, Nitzschia hungarica, N. frustulum and Aulacoseira granulata as numerically important diatoms. In contrast, Rhizosolenia eriensis, Frustulia rhomboides, Eunotia pectinatus and Tabellaria flocculosa were strongly associated with upland streams with fast current, relatively low O-PO₄ ^3- concentrations, low pH, low salinity, and low temperature. In general, the diatom communities in saline streams (3 mS) were similar to those previously reported in saline lakes in Victoria.     

Predictive models for phosphorus retention in wetlands

The potential of wetlands to efficiently remove (i.e., act as a nutrient sink) or to transform nutrients like phosphorus under high nutrient loading has resulted in their consideration as a cost-effective means of treating wastewater on the landscape. Few predictive models exist which can accurately assess P retention capacity. An analysis of the north American data base (NADB) allowed us to develop a mass loading model that can be used to predict P storage and effluent concentrations from wetlands. Phosphorus storage in wetlands is proportional to P loadings but the output total phosphorus (TP) concentrations increase exponentially after a P loading threshold is reached. The threshold P assimilative capacity based on the NADB and a test site in the Everglades is approximately 1 g m^–2 yr^–1. We hypothesize that once loadings exceed 1 g m^–2 yr^–1 and short-term mechanisms are saturated, that the mechanisms controlling the uptake and storage of P in wetlands are exceeded and effluent concentrations of TP rise exponentially. We propose a One Gram Rule for freshwater wetlands and contend that this loading is near the assimilative capacity of wetlands. Our analysis further suggests that P loadings must be reduced to 1 g m^–2 yr^–1 or lower within the wetland if maintaining long-term low P output concentrations from the wetlands is the central goal. A carbon based phosphorus retention model developed for peatlands and tested in the Everglades of Florida provided further evidence of the proposed One Gram Rule for wetlands. This model is based on data from the Everglades areas impacted by agricultural runoff during the past 30 years. Preliminary estimates indicate that these wetlands store P primarily as humic organic-P, insoluble P, and Ca bound P at 0.44 g m^–2 yr^–1 on average. Areas loaded with 4.0 g m^–2 yr^–1 (at water concentrations>150 g·L^–1 TP) stored 0.8 to 0.6 g m^–2 yr^–1 P, areas loaded with 3.3 g m^–2 yr^–1 P retained 0.6 to 0.4 g m^–2 yr^–1 P, and areas receiving 0.6 g m^–2 yr^–1 P retained 0.3 to 0.2 g m^–2 yr^–1. The TP water concentrations in the wetland did not drop below 50 g·L^–1 until loadings were below 1 g m² yr^–1 P.     

Production and nutrient dynamics of plant communities on a sub-Antarctic Island

Summary Studies of plant standing crop and of the nutrient concentrations in precipitation, soils and plants have enabled an assessment of the inter- and intra-system nutrient flows for five plant communities at Marion Island (46°54S, 37°45E). These communities, which are representative of those occupying more than 90% of the island's lowland (below 300m above sea level) were: a fjaeldmark on a rocky plateau (dominated by the cushion plant Azorella selago), an open fernbrake and closed fernbrake (both dominated by the fern Blechnum penna-marina) and two mire-grasslands (on very wet peats and dominated by graminoid and bryophyte species). Annual net primary production (ANP) at the five communities was high and substantial quantities of nutrients were taken up annually by the vegetation. N (6.5 to 24.8 g m^-2 year^-1) was the element taken up from the soil in the largest quantities, despite the fact that instantaneous values of available N pools were exceptionally low (0.003 to 0.69 g m^-2 to 25 cm depth). Either K (3.5 to 9.9 g m^-2 year^-1) or Ca (1.7 to 9.7 g m^-2 year^-1) was taken up in the second largest amount. Net quantities of nutrients translocated into the annual aboveground growth of vascular plants were, except for K and Na, greater than the seasonal mean standing stocks in the aerial biomass. Net translocation estimates ignored leaching losses from the biomass. Nutrient turnover times in the total (living plus dead, above- and belowground) vegetation were between 1 and 4 years, lower than for most Northern Hemisphere tundra communities. The quantities of nutrients in circulation were mostly less than 3% of their total pool (plants plus soil) sizes, except for K (13 to 26%) and, in four of the communities, Mg (6 to 15%). Precipitation inputs of N, K, Ca and Mg were considerably lower than the amounts required in the ANP. No P occurred in the precipitation. Biological fixation of N was much less than the precipitation input. The vascular plant species appear to be less efficient in conserving N through back-translocation from senescing photosynthetic tissue than are most plants of similar life forms from northern hemisphere tundra and tundra-like areas. Only 11 to 30% of the N taken up into the annual aboveground growth was back0translocated before or during senescence. Back-translocations of P (39 to 71%) and K (71 to 965) were greater. However, all of these estimates ignore leaching losses. Despite the apparently poor ability of the plants to back-translocate N, the total nutrient costs of the aboveground ANP at the five sites (14 to 32 mg per g m^-2 ANP) were very much in the lower part of the range reported for a wide variety of vegetation types. Nutrient costs of the ANP for the miregrassland communities were especially low, mainly because of low requirements for Ca and Mg. In view of the small soluble and available pools of some nutrients (especially N and P) and the substantial nutrient requirement for the ANP, it is concluded that net nutrient mineralization in decomposition and nutrient absorption by the vegetation are closely coupled.     

Phosphorus chemistry and bacterial community composition interact in brackish sediments receiving agricultural discharges

Background

External nutrient discharges have caused eutrophication in many estuaries and coastal seas such as the Baltic Sea. The sedimented nutrients can affect bacterial communities which, in turn, are widely believed to contribute to release of nutrients such as phosphorus from the sediment.

Methods

We investigated relationships between bacterial communities and chemical forms of phosphorus as well as elements involved in its cycling in brackish sediments using up-to-date multivariate statistical methods. Bacterial community composition was determined by terminal restriction fragment length polymorphism and cloning of the 16S rRNA gene.

Results and Conclusions

The bacterial community composition differed along gradients of nutrients, especially of different phosphorus forms, from the estuary receiving agricultural phosphorus loading to the open sea. This suggests that the chemical composition of sediment phosphorus, which has been affected by riverine phosphorus loading, influenced on bacterial communities. Chemical and spatial parameters explained 25% and 11% of the variation in bacterial communities. Deltaproteobacteria, presumptively sulphate and sulphur/iron reducing, were strongly associated to chemical parameters, also when spatial autocorrelation was taken into account. Sulphate reducers correlated positively with labile organic phosphorus and total nitrogen in the open sea sediments. Sulphur/iron reducers and sulphate reducers linked to iron reduction correlated positively with aluminium- and iron-bound phosphorus, and total iron in the estuary. The sulphate and sulphur/iron reducing bacteria can thus have an important role both in the mineralization and mobilization of nutrients from sediment.

Significance

Novelty in our study is that relationships between bacterial community composition and different phosphorus forms, instead of total phosphorus, were investigated. Total phosphorus does not necessarily bring out interactions between bacteria and phosphorus chemistry since proportions of easily usable mobile (reactive) phosphorus and immobile phosphorus forms in different sediments can vary. Our study suggested possible feedbacks between different forms of phosphorus and bacterial community composition.     

Oscillations in density and diversity ofPisidium communities in two biotopes in Southern Poland

Summary From September 1979 to October 1980 in two types of biotopes (in the stream at Kali and in the Biaa Przemsza River near Jaroszowiec, southern Poland)P.amnicum, P.milium, P.subtruncatum, P.nitidum, P.personatum, P.casertanum andP. sp.non det. were found.P.amnicum was accepted as an indicator species for the river community, andP.milium for the stream. The irregular changes of species composition of the sphaeriid community in the stream at Kali are rather accidental and the only temporary occurrence ofP.nitidum, P.personatum andP. sp.non det. in the Biaa Przemsza River might be connected with seasonal migration.The dominance structure of thePisidium community in the stream agrees with the model described by CONNELL (1977) for the communities of large sedentary animals and plants. The dominance relations in the river community are similar to the situation described by CONNELL (1977) as characteristic for small lakes and islands.The density of the Sphaeriidae on the chosen habitat in the Biaa Przemsza River oscillated between 300 and 3.300 ind./m² and it reached almost 130.000 ind./m² in the stream at Kali.The SHANNON-WIENER's diversity index (H) and that of equitability or evenness of species abundances (J) had low values during the whole period of investigation.This study was performed as part of the Problem MRII/15 of the Polish Academy of Sciences.