Effect of mineral suspended matter on the productive characteristics of bacterioplankton and the degradation of labile organic materials

The effect of mineral particulate matter on the population of bacterioplankton, its aggregation, and productive characteristics was studied in model experiments with different concentrations of particulate kaolin and the same concentration of organic substance (sodium humate). It was found that the presence of mineral particulate matter stimulated the aggregation of bacterioplankton, improved bacterial production, and extended the productive period of bacterioplankton. The integral specific production of aggregated bacterioplankton was higher than that of free-swimming bacterioplankton. The energy metabolic coefficient K2 of bacterioplankton in the presence of mineral particulate matter was higher than in its absence.     

Effect of suspended mineral matter on production characteristics of bacterio- and phytoplankton

The effect was determined of organo-mineral detritus (OMD), one of the components of suspended mineral matter in aquatic ecosystems, on the production characteristics of bacterioplankton (bacterial production P _b and destruction of organic matter R _b, as well as bacterial growth efficiency BGE). The relation was determined between these parameters and the ratio of the content of suspended mineral matter M to the total organic carbon content (M/TOC). More active utilization of organic matter by bacterioplankton in the presence of OMD resulted in its positive effect on specific production characteristics of the phytoplankton.     

Relationships between the biomass and production of bacterio- and phytoplanktonic communities

Quantitative ratios of the biomasses of bacterio- and phytoplankton, interrelation of their production characteristics, and association of the functional characteristics with environmental factors were studied for Lake Khanka, the Yenisei River, and the Krasnoyarsk Reservoir. The ratio between the biomasses of bacterioplankton (B_b) and phytoplankton (B_p) in these water bodies was shown to vary within the range exceeding three orders of magnitude. Bacterioplankton biomass was relatively stable and varied from sample to sample by an order of magnitude. In more than 50% of the samples (total sample number, 495), bacterioplankton biomass exceeded that of the phytoplankton. The average B_b/B_p ratios for Lake Khanka, Yenisei River, and Krasnoyarsk Reservoir were 5.1, 2, and 1.4, respectively. Increased B_b/B_p ratios were found to correlate with elevated specific (per unit biomass) phytoplankton production. This finding indicated additional supply of biogenic elements to phytoplankton due to their recycling by bacterial communities. The ratio between bacterioplankton and phytoplankton production for Lake Khanka varied from year to year (0.07 to 0.76). For the Yenisei River and the Krasnoyarsk Reservoir these ratios were on average 0.19 and 0.27, respectively. According to the literature data for other water bodies, bacterial production may reach from 10 to over 100% of the primary production. The equilibrium density of bacterioplankton (maximal density of the population) in Lake Khanka was ~1.5 times higher than in the Yenisei River and the Krasnoyarsk Reservoir due to higher content of suspended mineral matter and associated organo-mineral detritus in the lake. The interaction between dissolved organic compounds sorbed on the surface of mineral particles results in chemical alteration of biochemically stable substrate into compounds which may be assimilated by aquatic microorganisms.     

δ15N of zooplankton species in subarctic lakes in northern Sweden: effects of diet and trophic fractionation

1. To assess the use of stable nitrogen isotopes (δ¹⁵N) for reconstructing trophic relationships in planktonic food webs, crustacean zooplankton species and pelagic dissolved and particulate matter were analysed in 14 subarctic lakes in northern Sweden. The lakes are situated along an altitudinal gradient and show a substantial variation in nutrient content and energy mobilization by bacterioplankton and phytoplankton. 2. The δ¹⁵N of dissolved and particulate matter was comparatively low, suggesting efficient N recycling and low losses of depleted N from the pelagic zone of these unproductive lakes. 3. Copepods had a systematically higher δ¹⁵N than cladocerans, with an average difference of 3.1–4.9‰ within lakes, implying different trophic positions of the two groups. Comparisons of nitrogen pools and energy fluxes suggest that the low cladoceran δ¹⁵N was a result of feeding on bacteria. 4. The difference in δ¹⁵N between copepods and cladocerans declined with decreasing bacterioplankton production among lakes, due either to increasing trophic isotope fractionation or decreasing relative importance of bacteria in the diet of cladocerans.     

Seasonality of chlorophyll and nutrients in Lake Erken – effects of weather conditions

The effect of submerged macrophytes on interactions among epilimnetic phosphorus, phytoplankton, and heterotrophic bacterioplankton has been acknowledged, but remains poorly understood. Here, we test the hypotheses that the mean summer phytoplankton biomass (chlorophyll a): phosphorus ratios decrease with increased macrophyte cover in a series of nine lakes. Further, we test that both planktonic respiration and bacterioplankton production increase with respect to phytoplankton biomass along the same gradient of increasing macrophyte cover. Increased macrophyte cover was associated with a lower fraction of particulate phosphorus in epilimnia, with total particulate phosphorus declining from over 80% of total phosphorus in a macrophyte free lake to less than 50% in a macrophyte rich lake. Phytoplankton biomass (chlorophyll a) too was lower in macrophyte dominated lakes, despite relatively high levels of total dissolved phosphorus. Planktonic respiration and bacterioplankton production were higher in macrophyte rich lakes than would be expected from phytoplankton biomass alone, pointing to a subsidy of bacterioplankton metabolism by macrophyte beds at the whole lake scale. The results suggest that the classical view of pelagic interactions, which proposes phosphorus determines phytoplankton abundance, which in turn determines bacterial abundance through the production of organic carbon, becomes less relevant as macrophyte cover increases.     

Pathways of soil organic matter formation from above and belowground inputs in a Sorghum bicolor bioenergy crop

When aboveground materials are harvested for fuel production, such as with Sorghum bicolor, the sustainability of annual bioenergy feedstocks is influenced by the ability of root inputs to contribute to the formation and persistence of soil organic matter (SOM), and to soil fertility through nutrient recycling. Using ¹³C and ¹⁵N labeling, we traced sorghum root and leaf litter‐derived C and N for 19 months in the field as they were mineralized or formed SOM. Our in situ litter incubation experiment confirms that sorghum roots and leaves significantly differ in their inherent chemical recalcitrance. This resulted in different contributions to C and N storage and recycling. Overall root residues had higher biochemical recalcitrance which led to more C retention in soil (27%) than leaf residues (19%). However, sorghum root residues resulted in higher particulate organic matter (POM) and lower mineral associated organic matter (MAOM), deemed to be the most persistent fraction in soil, than leaf residues. Additionally, the overall higher root‐derived C retention in soil led to higher N retention, reducing the immediate recycling of fertility from root as compared to leaf decomposition. Our study, conducted in a highly aggregated clay‐loam soil, emphasized the important role of aggregates in new SOM formation, particularly the efficient formation of MAOM in microaggregate structures occluded within macroaggregates. Given the known role of roots in promoting aggregation, efficient formation of MAOM within aggregates can be a major mechanism to increase persistent SOM storage belowground when aboveground residues are removed. We conclude that promoting root inputs in S. bicolor bioenergy production systems through plant breeding efforts may be an effective means to counterbalance the aboveground residue removal. However, management strategies need to consider the quantity of inputs involved and may need to support SOM storage and fertility with additional organic matter additions.     

Evaluating decline parameters of rotifer Brachionus plicatilis populations as an interstitial water toxicity bioassay

Population decline of the rotifer Brachionus plicatilis has been evaluated as a sensitive and reliable bioassay for assessing toxicity of marine sediment interstitial water. Three environmental conditions were examined using both interstitial and seawater cultures: (a) adverse effects from dissolved contaminant concentrations; (b) effects from particulate organic matter; and (c) increase or decrease of bacterial biomass from addition of mixtures of antibiotics. Three different parameters were measured to characterize decline: (i) time required for 50% of inoculated population to die (TL₅₀); (ii) curve of the decline rate of Brachionus plicatilis (µ _BP and (iii) graphical area contained below plot of the egg:female ratio curve against time (A _BPM ).Results indicated that, for each of the different assays, the three parameters of the Brachionus plicatilis population decline test were sensitive to concentrations of contaminants dissolved in interstitial waters (principally: total ammonia, Cu, Cr and alkylbenzenesulphonates) but not to particulate organic matter. Nevertheless, the presence or absence of mixed antibiotics with the contaminants may influence the Brachionus plicatilis population decline test, principally by retarding the hatching of eggs. Based on these results, Brachionus plicatilis is confirmed as an appropriate organism for use as an indicator of interstitial water contamination, using either decline rate, TL₅₀ or both parameters. The presence of particulate matter has no effect on these parameters, but the bacterial population may be an influence, although to a lesser extent than the toxicants.     

Increase in pH stimulates mineralization of ‘native’ organic carbon and nitrogen in naturally salt-affected sandy soils

Large amounts of terrestrial organic C and N reserves lie in salt-affected environments, and their dynamics are not well understood. This study was conducted to investigate how the contents and dynamics of ‘native’ organic C and N in sandy soils under different plant species found in a salt-affected ecosystem were related to salinity and pH. Increasing soil pH was associated with significant decreases in total soil organic C and C/N ratio; particulate (0.05–2 mm) organic C, N and C/N; and the C/N ratio in mineral-associated (<0.05 mm) fraction. In addition, mineral-associated organic C and N significantly increased with an increase in clay content of sandy soils. During 90-day incubation, total CO₂-C production per unit of soil organic C was dependent on pH [CO₂-C production (g kg⁻¹ organic C) = 22.5 pH – 119, R ² = 0.79]. Similarly, increased pH was associated with increased release of mineral N from soils during 10-day incubation. Soil microbial biomass C and N were also positively related to pH. Metabolic quotient increased with an increase in soil pH, suggesting that increasing alkalinity in the salt-affected soil favoured the survival of a bacterial-dominated microbial community with low assimilation efficiency of organic C. As a result, increased CO₂-C and mineral N were produced in alkaline saline soils (pH up to 10.0). This pH-stimulated mineralization of organic C and N mainly occurred in particulate but not in mineral-associated organic matter fractions. Our findings imply that, in addition to decreased plant productivity and the litter input, pH-stimulated mineralization of organic matter would also be responsible for a decreased amount of organic matter in alkaline salt-affected sandy soils.     

Ecosystem Changes Following Restoration of a Buckthorn‐Invaded Woodland

Shifts in plant‐community composition following habitat degradation and species invasions can alter ecosystem structure and performance of ecosystem services. In temperate North American woodlands, invasion by aggressive Eurasian shrubs has produced dense thickets with depauperate understory vegetation and increased rates of litter decomposition and nutrient cycling, attributes that could impair storage of carbon as soil organic matter (SOM). It is important to know if such impairment has occurred and, if so, the extent to which restoration can return this service. We used an oak‐woodland restoration chronosequence in northeastern Illinois to contrast structural and functional attributes of unrestored areas dominated by Rhamnus cathartica (common buckthorn) with areas that had undergone buckthorn removal and ongoing, active management for less than 1 to 14 years. With increasing age, restored areas had higher understory plant diversity and cover (p < 0.0001 and 0.005, respectively) and higher litter mass (p = 0.018). These structural differences were associated with some evidence of reduced soil erosion (p = 0.027–0.135) but greater soil CO₂ efflux (p = 0.020–0.033). Total particulate organic matter (POM) in the soil increased with restoration age, which was driven by increases in the slow‐turnover, mineral‐associated SOM fraction. However, variance was high and relationships were only weakly significant (p = 0.082 and 0.083 for total POM and mineral‐associated SOM, respectively). Our results suggest that, in addition to better documented biodiversity benefits, beneficial changes to ecosystem properties and processes may also occur with active, long‐term restoration of degraded woodlands.     

Is the copepod egg production in a highly turbid estuary (the Gironde, France) a function of the biochemical composition of seston?

The influence of the biochemical composition of particles originating from surface waters of the Gironde estuary on egg production rates of Eurytemora affinis zooplanktonic population was studied. In the high turbidity zone, suspended particulate matter had a low nutritional quality because the easily available organic fraction represented less than 15% of the overall particulate organic matter. In waters located seaward of the high turbidity zone, a slight increase in nutritional quality was observed. As a result, the sum of easily extractable organic macromolecules represented 15 to 33% of the overall particulate organic matter. The present study suggests that the low egg-production rate of Eurytemora affinis, occurring in the high turbidity zone, results from combined effects of temperature and bad feeding conditions in the area. Low copepod production can be explained by little phytoplankton growth due to light limitation and, therefore, restricted food availability, as well as difficulties in food selection, non-living particle may being dominant.     

Contribution of the water-sediment interface to the transformation of biogenic substances: application to nitrogen compounds

Up to now the water-sediment interface (WSI) has not been considered by limnologists and oceanographers as a theoretical frontier between two phases, but as a variable layer comprising the overlying bottom water, the superficial sediment, and the bioturbed sediment. This heterogeneous benthic boundary layer forms a non-equilibrated assemblage of an aqueous phase, a mineral phase influencing the exchanges, and a dissolved gaseous phase regulating biological activity, detrital organic matter present as dissolved or adsorbed compounds and particles, and a living benthic community dominated by microorganisms. Biological activity and the equilibrium between these components are linked to depth, bottom currents, primary production and allochtonous inputs of organic matter. The settling rate of particulate matter in the water column, which depends on both the size and density of particles, is also an important factor. Settled organic matter is broken down mainly by heterotrophic microorganisms. In the first stage, dissolved or particulate matter is depolymerized by exoenzymes linked to the bacterial membrane. Other heterotrophic organisms, like flagellates and ciliates, contribute to the activity of bacterial populations or to a modification of the physical and chemical characteristics of organic matter like macrofauna. Bacterial activity in the WSI is one to two orders of magnitude higher than in overlying bottom water, depending on the density of the bacterial population, on the amount of available organic matter, and on the presence of predators grazing on bacteria and macrofauna and increasing oxygen and dissolved substrates. The WSI is now considered as a sink for particulate matter and biopolymers, and/or as a sink or source for organic monomers and NH₄ ⁺ depending on their concentrations. As a result of its contribution to mineral and organic cycles, the WSI is a crossroad for exchanges between aqueous and sediment layer. During the last decade, intensive work has been performed in this field, but progress has been slow due to the heterogeneity of the layer and the adsorptive properties of the mineral fraction.     

The issue of outwelling in the Guadiana River estuary (Portugal): some findings and research suggestions in the context of recent evidence

The “Outwelling Theory” states that salt marshes play a major role in exporting production to adjacent estuarine and coastal ecosystems. However, it has been found that some marshes act as net importers instead of net exporters of organic matter and nutrients. Once we include mangroves and refine the analysis to comprehend bacterioplankton, organic and stable isotope tracers, the picture became, more complex, making room for a revival of the outwelling idea. The exchanges between the Castro Marim salt marsh and the main estuary were tentatively established determining periodically, in a selected cross-section, the concentrations of TSS, FSS, VSS, NH₄, NO₂, NO₃, N_Kjeldhal, SiO₄, PO₄, TDP, Chlorophyll a and Pheopigments, measuring their fluxes along tidal cycles and computing the corresponding budgets. Apparently, the sedimentary behaviour of the marsh will be close to equilibrium during the period of study. However, it will import mainly inert matter and export mainly organic matter in the same period. Moreover, extrapolating these results to the entire Guadiana salt marshes, the exchanges of sediment do not seem to be significant. Particularly, the marshes will not trap a significantly amount sediment transported by the main river (0.5%). It also seems to follow, that in a general way, the Guadiana salt marshes might have a more significant role than was anticipated in the system economy of OM and nutrients and their outwelling to coastal waters, assuring outputs that could amount to something like 6% of the river load of N, 1.2% of the river load of P, and 20-57% of the river load of TOC, for an average year, and 42% of the river load of N and 35% of the river load of P in a dry year. These findings suggest that a more detailed investigation, over an extended period of time, is certainly worthwhile.     

Carbon sequestration under Miscanthus: a study of 13C distribution in soil aggregates

The growing of bioenergy crops has been widely suggested as a key strategy in mitigating anthropogenic CO₂ emissions. However, the full mitigation potential of these crops cannot be assessed without taking into account their effect on soil carbon (C) dynamics. Therefore, we analyzed the C dynamics through four soil depths under a 14‐year‐old Miscanthus plantation, established on former arable land. An adjacent arable field was used as a reference site. Combining soil organic matter (SOM) fractionation with ¹³C natural abundance analyses, we were able to trace the fate of Miscanthus‐derived C in various physically protected soil fractions. Integrated through the whole soil profile, the total amount of soil organic carbon (SOC) was higher under Miscanthus than under arable crop, this difference was largely due to the input of new C. The C stock of the macroaggregates (M) under Miscanthus was significantly higher than those in the arable land. Additionally, the C content of the micro‐within macroaggregates (mM) were higher in the Miscanthus soil as compared with the arable soil. Analysis of the intramicroaggregates particulate organic matter (POM) suggested that the increase C storage in mM under Miscanthus was caused by a decrease in disturbance of M. Thus, the difference in C content between the two land use systems is largely caused by soil C storage in physically protected SOM fractions. We conclude that when Miscanthus is planted on former arable land, the resulting increase in soil C storage contributes considerably to its CO₂ mitigation potential.     

Spatial Variability of Soil Properties in the Shortgrass Steppe: The Relative Importance of Topography, Grazing, Microsite, and Plant Species in Controlling Spatial Patterns

We conducted a study to evaluate the relative importance of topography, grazing, the location of individual plants (microsite), and plant species in controlling the spatial variability of soil organic matter in shortgrass steppe ecosystems. We found that the largest spatial variation occurs in concert with topography and with microsite-scale heterogeneity, with relatively little spatial variability due to grazing or to plant species. Total soil C and N, coarse and fine particulate organic matter C and N, and potentially mineralizable C were significantly affected by topography, with higher levels in toeslope positions than in midslopes or summits. Soils beneath individual plants (Bouteloua gracilis and Opuntia polyacantha) were elevated by 2–3 cm relative to surrounding soils. All pools of soil organic matter were significantly higher in the raised hummocks directly beneath plants than in the soil surface of interspaces or this layer under plants. High levels of mineral material in the hummocks suggest that erosion is an important process in their formation, perhaps in addition to biotic accumulation of litter beneath individual plants. Over 50 y of heavy grazing by cattle did not have a significant effect on most of the soil organic matter pools we studied. This result was consistent with our hypothesis that this system, with its strong dominance of belowground organic matter, is minimally influenced by aboveground herbivory. In addition, soils beneath two of the important plant species of the shortgrass steppe, B. gracilis and O. polyacantha, differed little from one another. The processes that create spatial variability in shortgrass steppe ecosystems do not affect all soil organic matter pools equally. Topographic variability, developing over pedogenic time scales (centuries to thousands of years), has the largest effect on the most stable pools of soil organic matter. The influence of microsite is most evident in the pools of organic matter that turn over at time scales that approximate the life span of individual plants (years to decades and centuries).     

Pigment patterns in suspended matter from the Elbe estuary,Northern Germany

In the Elbe hardly anything is known about the actual fate of phytoplankton and the resultant pigment composition of suspended matter. As part of a longterm study on the role of suspended matter in the cycles of nitrogen and associated oxygen consumption processes in the Elbe estuary in northern Germany (‘Sonderforschungsbereich 327 Elbe’ project) and the characterization of estuarine and coastal water bodies with regard to the transport of pollutants such as heavy metals, we have started to characterize suspended matter with regard to the pigments present therein. This will allow us to study phytoplankton turnover and to determine the role of phytoplankton decay mechanisms in the mobilization, binding, and transport of pollutants associated with particulate matter. Our first data on the pigment composition of suspended matter from the Elbe and particularly over the region of Hamburg harbour, obtained using a High Performance Liquid Chromatography (HPLC) method, is presented. The pigment concentrations varied considerably over the summer months. Values up to 250 mg l⁻¹ of chlorophyll a were observed upstream of Hamburg. We found that on its course through Hamburg the concentrations (μg g⁻¹ of particulate matter) of all pigments in the Elbe drop to under half those found upstream of Hamburg. The pigment concentrations in the turbidity maximum were significantly lower than in the rest of the estuary due to the decline of algae as a result of inadequate light conditions and the salinity gradient. The highest particulate matter concentrations were found in the turbidity maximum and just downstream of Hamburg. The highest particulate organic carbon values were found just downstream of Hamburg. The presence of of fucoxanthin and the chlorophyllsc andb, and the changes in pigment patterns were indicative for the dominant algal classes and the phytoplankton succession in the suspended matter. The ratio of chlorophylla to lutein was found to be a possible indicator of phytoplankton breakdown.     

CO2 Saturation and Trophic Shift Induced by Microbial Metabolic Processes in a River-Dominated Ocean Margin (Tropical Shallow Lagoon,Chilika, India)

An effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO₂ (pCO₂) and CO₂ air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L^?1 d^?1 with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L^?1 d^?1 and from 14.1 to 389.4 µg C L^?1 d^?1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the high pCO₂ (10,134 µatm) and CO₂ air–water flux (714 mm m^?2 d^?1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO₂ supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period.     

Littoral-pelagial interchange and the decomposition of dissolved organic matter in a polyhumic lake

The small, polyhumic lake, Mekkojärvi (southern Finland), is bordered by a moss vegetation zone (Warnstorfia and Sphagnum species) which provides a habitat-rich and productive environment for many planktonic and periphytic animals. Impacts of moss on the metabolism of bacterioplankton, phytoplankton and zooplankton in polyhumic water were investigated in laboratory throughflow systems. Growing Warnstorfia (together with epiphytic algae and bacteria) suppressed the production of planktonic algae but had no clear effect on leucine uptake, and hence bacterial production, or on the decomposition of humic substances. Phenol uptake and mineralization rates, however, were lower in the littoral water than in the pelagial water. Excretion of organic carbon by Warnstorfia algae or Daphnia longispina (the predominant crustacean in the pelagial water) provided only a minor contribution to bacterial production; therefore, a major contribution had to be from humic substances. A bacterial production efficiency of 31–38% could account for the microbial respiration in the water. The results indicated that bacterial, or detrital matter (originating largely from the littoral zone), could not obviate the need for algal food, and that a great deal of particulate matter in the water was poor or useless food for Daphnia. In all, the bulk of dissolved organic matter in Lake Mekkojärvi was biochemically highly recalcitrant. Our results indicate that humic substances (from watershed or littoral area) which, through bacterial degradation, enter the planktonic food web of the lake are mainly lost through respiration by microorganisms.     

Spatial distribution of bacterioplankton production across the Weddell-Scotia Confluence during early austral summer 1988–1989

Summary We applied two methods to measure bacterio-plankton production, the [³H]-thymidine (TTI) and the [³H]-leucine (LEU) incorporation into cold trichloro-acetic acid precipitate. Both methods gave similar results of the distribution of production in time and space (r ²=0.82, n=66). Using empirically determined conversion factors the TTI gave production values from 21 to 125 mg Cm^–2 day^–1, which are within the range reported earlier from the Southern Ocean. Highest production rates were associated with the open water in the Confluence area (59°S–60°S) and with the Scotia Sea front. Low production rates were recorded from the ice covered areas in the Weddell Sea and in the open Scotia Sea waters. Good correlation on an areal basis was found between bacterioplankton production and other measures of heterotrophy, including ETS (r²=0.93, n=9) and NH₄(r²=0.50, n=21). Good correlation was also found between bacterioplankton and phytoplankton production (r²=0.63, n=19). Bacterioplankton production seems to be driven by products from photosynthesis and heterotrophic processes, most likely grazing, which are tightly coupled to autotrophy. Quantitatively, bacterioplankton production was on an average 11 % of net primary production, which is clearly a lower value than the 30% based on a review from temperate freshwater and marine ecosystems, but is comparable with values reported from the spring period in subarctic ecosystems. In comparison with the measurements of ETS, bacterioplankton contribution to community respiration was also lower than predicted from results from temperate ecosystems. We concluded from these results and the results obtained from microcosm experiments (Bjørnsen and Kuparinen 1991b) that the flux of organic matter to eucaryote heterotrophs via bacterioplankton during spring and early summer periods in the Southern Ocean is of considerable, but not of equivalent importance as in temperate waters.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Collection and characterization of a bulk PM2.5 air particulate matter material for use in reference materials

A contemporary PM_2.5 (particulate matter smaller than 2.5 Μm aerodynamic diameter) aerosol material from an urban site has been collected for the production of a new standard reference material that will be made available for the development of new PM_2.5 air quality standards. Air particulate matter corresponding to the PM_2.5 fraction was collected at an established Environmental Protection Agency monitoring site in Baltimore, Maryland. The air-sampling system that has been constructed for this collection separates fine particles with a cyclone separator and deposits them onto an array of Teflon membrane filters. The fine air particulate material is removed by ultrasonication or by mechanical means and collected for further preparation of standards. The composition of the collected PM_2.5 aerosol, as well as the composition of the deposited PM_2.5 aerosol, are determined by instrumental nuclear activation analysis and other techniques.     

Influence of organic matter and flooding on the chemical and electrochemical properties of sodic soil and rice growth

The influence of organic matter, added in the form ofCasuarina equisetifolia andAcacia nilotica leaves, on the chemical and electrochemical kinetics of a flooded sodic soil and rice growth, was studied in a pot experiment. With the addition of organic matter, not only the peaks of CO₂ production and maximum concentrations of extractable Fe and Mn and other cations occurred earlier, but their concentrations were also significantly higher as compared to the control (no organic matter). The high concentrations of CO₂ and reduced redox potential (Eh) appeared to influence the soil pH, exchangeable sodium percentage (ESP) and the accumulation of cations and to be chiefly responsible for better rice growth. Acacia proved more effective than Casuarina in improving rice yield and the sodic soil.