Heterotrophic microbial activity in shallow aquifer sediments of Long Island,New York

Bacterial numbers and activities (as estimated by glucose uptake and total thymidine incorporation) were investigated at two sites in Long Island, New York aquifer sediments. In general, bacterial activities were higher in shallow (1.5–4.5 m below the water table or BWT), oxic sediments than in deep (10–18 m BWT), anoxic sediments. The average total glucose uptake rates were 0.18 ± 0.10 ng gdw^–1 h^–1 in shallow sediments and 0.09 ± 0.11 ng gdw^–1 h^–1 in deep sediments; total thymidine incorporation rates were 0.10 ± 0.13 pmol gdw^–1 h^–1 and 0.03 ± 0.03 pmol gdw^–1 h^–1 in shallow and deep sediments, respectively. Incorporation of glucose was highly efficient, as only about 10% of added label was recovered as CO₂. Bacterial abundance (estimated from acridine orange direct counts) was 2.5 ± 2.0 × 10⁷ cells gdw^–1 and 2.0 ± 1.3 × 10⁷ cells gdw^–1 in shallow and deep sediments, respectively. These bacterial activity and abundance estimates are similar to values found in other aquifer environments, but are 10- to 1000-fold lower than values in soil or surface sediment of marine and estuarine systems. In general, cell specific microbial activities were lower in sites from Connetquot Park, a relatively pristine site, when compared to activities found in sites from Jamesport, which has had a history of aldicarb (a pesticide) contamination. To our knowledge, this is the first report of bacterial activity measurements in the shallow, sandy aquifers of Long Island, New York.Correspondence to: D.G. Capone     

Benthic bacterial biomass and production in the Hudson River estuary

Bacterial biomass, production, and turnover were determined for two freshwater marsh sites and a site in the main river channel along the tidally influenced Hudson River. The incorporation of [methyl-³H]thymidine into DNA was used to estimate the growth rate of surface and anaerobic bacteria. Bacterial production at marsh sites was similar to, and in some cases considerably higher than, production estimates reported for other aquatic wetland and marine sediment habitats. Production averaged 1.8–2.8 mg C·m^–2·hour^–1 in marsh sediments. Anaerobic bacteria in marsh sediment incorporated significant amounts of [methyl-³H]thymidine into DNA. Despite differences in dominant vegetation and tidal regime, bacterial biomass was similar (1×10³±0.08 mg C·m^–2) inTrapa, Typha, andNuphar aquatic macrophyte communities. Bacterial abundance and productivity were lower in sandy sediments associated withScirpus communities along the Hudson River (0.2×10³±0.05 mg C·m^–2 and 0.3±0.23 mg C·m^–2·hour^–1, respectively).     

Measurement of bacterial growth rates in subsurface sediments using the incorporation of tritiated thymidine into DNA

Microbial growth rates in subsurface sediment from three sites were measured using incorporation of tritiated thymidine into DNA. Sampling sites included Lula, Oklahoma, Traverse City, Michigan, and Summit Lake, Wisconsin. Application of the thymidine method to subsurface sediments required (1) thymidine concentrations greater than 125 nM, (2) incubation periods of less than 4 hours, (3) addition of SDS and EDTA for optimum macromolecular extraction, and (4) DNA purification, in order to accurately measure the rate of thymidine incorporation into DNA. Macromolecule extraction recoveries, as well as the percentage of tritium label incorporated into the DNA fraction, were variable and largely dependent upon sediment composition. In general, sandy sediments yielded higher extraction recoveries and demonstrated a larger percentage of label incorporated into DNA than sediments that contained a high silt-clay component. Reported results also indicate that the acid-base hydrolysis procedure routinely used for macromolecular fractionation in water samples may not be routinely applicable to the modified sediment procedure where addition of SDS and EDTA are required for macromolecule extraction. Growth rates exhibited by subsurface communities are relatively slow, ranging from 5.1 to 10.2×10⁵ cells g^–1 day^–1. These rates are 2–1,000-fold lower than growth rates measured in surface sediments. These data lend support to the supposition that subsurface microbial communities are nutritionally stressed.     

Factors controlling bacterial production in marine and freshwater sediments

We collected benthic bacterial production data measured by ³H thymidine incorporation (TTI) (25 studies), frequency of dividing cells (FDC) (3 studies), dark-C0₂ assimilation (1 study) and ³H-adenine uptake (2 studies) from the literature, which included 18 marine, 6 river, and 2 lake studies. In all of the studies that used the TTI method, ³H-DNA was isolated and incubations were carried out at in situ temperatures. Most of the researchers also determined ³H-DNA extraction efficiencies and isotope dilution, thus interpretable estimates of bacterial production were used in the analysis. In marine sediments, bacterial production rates were linked to bacterial biomass, bacterial abundance, sediment organic matter, temperature, and sediment chlorophyll a, with these variables explaining between 40% and 68% of the variation in production rates. Simple relationships between production and bacterial biomass or bacterial abundance, or between production and sediment organic matter, were improved by also including temperature in the analysis of marine sediments. Sediment organic matter explained an appreciable fraction (58%) of the observed production in freshwater sediments. Temperature was the most powerful predictor of the observed variability in specific growth rates (r ² = 0.48 and r ² = 0.58) in marine and freshwater sediments, respectively. Thus, bacterial production and specific growth rates are most closely linked to substrate supply and temperature in marine and freshwater sediments. Offprint requests to: B. C. Sander.     

Bacterial Diversity in Shallow Oligotrophic Marine Benthos and Overlying Waters: Effects of Virus Infection,Containment, and Nutrient Enrichment

Little is known of the factors shaping sediment bacterial communities, despite their high abundance and reports of high diversity. Two factors hypothesized to shape bacterial communities in the water column are nutrient (resource) availability and virus infection. The role these factors play in benthic bacterial diversity was assessed in oligotrophic carbonate–based sediments of Florida Bay (USA). Sediment–water mesocosm enclosures were made from 1-m diameter clear polycarbonate cylinders which were pushed into sediments to 201 cm sediment depth enclosing 80 L of water. Mesocosms were amended each day for 14 d with 10 µM NH ₄ ⁺ and 1 µM PO ₄ ^3– . In a second experiment, viruses from a benthic flocculent layer were concentrated and added back to flocculent layer samples which were collected near the mesocosm enclosures. Photosynthesis by microalgae in virus-amended incubations was monitored by pulse-amplitude modulated (PAM) fluorescence. In both experiments, bacterial diversity was estimated using automated rRNA intergenic spacer analysis (ARISA), a high-resolution fingerprinting approach. Initial sediment bacterial operational taxonomic unit (OTU) richness (236 ± 3) was higher than in the water column (148 ± 9), where an OTU was detectable when its amplified DNA represented >0.09% of the total amplified DNA. Effects on bacterial diversity and operational taxonomic unit (OTU) richness in nutrient-amended mesocosms may have been masked by the effects of containment, which stimulated OTU richness in the water column, but depressed OTU richness and diversity in sediments. Nutrient addition significantly elevated virus abundance and the ratio of viruses to bacteria (p < 0.05 for both) in the sediments, concomitant with elevated bacterial diversity. However, water column bacterial diversity (in unamended controls) was not affected by nutrient amendments, which may be due to rapid nutrient uptake by sediment organisms or adsorption of P to carbonate sediments. Addition of live viruses to benthic flocculent layer samples increased bacterial OTU diversity and richness compared with heat-killed controls; however, cluster analyses showed that the community structure in the virus-amended mesocosms varied greatly between replicates. Despite the strong effects upon eubacterial communities, photosynthesis of co-occurring protists and cyanobacteria was not significantly altered by the presence of virus concentrates. This study supports the hypothesis that nutrient availability plays a key role in shaping sediment bacterial communities, and also that viruses may regulate the abundance of the dominant competitors and allow less dominant organisms to maintain or increase their abundance in a community due to decreased competition for resources.     

Significance of bacterial biomass in the nutrition of a freshwater isopod (Lirceus sp.)

The quantitative significance of bacterial biomass in the nutrition of detritivores remains equivocal. We have used tritiated thymidine to specifically label stable macromolecules in natural assemblages of sediment-associated and detritus-associated bacteria. This material was presented to the isopod (Lirceus sp.) and incorporation of bacterial biomass measured. The isopod incorporated roughly 1 ng bacterial carbon (mg wet wt.)^-1 h^-1 from leaf discs and about 6 ng mg^-1 h^-1 from sediment. Calculation of grazing rate from changes in cell counts yields grazing rates from 2.3–17.9 ng C mg^-1 h^-1. Even the maximum grazing rate, which is an overestimate of C assimilated, represents only 14.7% of C respired by the isopod.     

Effects of Toxic Substances on Natural Bacterial Assemblages Determined by Means of [3H]Thymidine Incorporation

The effects of 3,5-dichlorophenol, 2,4-dinitrophenol, and potassium dichromate on natural bacterial assemblages were examined by means of [³H]thymidine incorporation into trichloroacetic acid-insoluble material. Results from a large number of coastal marine and freshwater samples suggest the following. (i) The effects of the three toxicants included reductions in the bacterial cell number as well as changes in rates of [³H]thymidine incorporation and in [³H]thymidine incorporation per cell. The concentrations that inhibited [³H]thymidine incorporation by 50% ranged from 3 to 11 mg liter⁻¹ for 3,5-dichlorophenol, 6 to 10 mg liter⁻¹ for 2,4-dinitrophenol, and 21 to 123 mg liter⁻¹ for potassium dichromate, with a tendency to higher values in bacterial assemblages from more eutrophic environments. (ii) The effects of 3,5-dichlorophenol and potassium dichromate determined by [³H]leucine incorporation into bacterial protein were similar or larger than those obtained from [³H]thymidine incorporation. (iii) Two to four hours of exposure to the toxicants was necessary before stable maximum effects were found in [³H]thymidine incorporation. (iv) Storage of natural environmental samples should be avoided, since tests with water stored for 1 to 3 days sometimes produced results different from results obtained from in situ tests. (v) The effects of 3,5-dichlorophenol, 2,4-dinitrophenol, and potassium dichromate on natural bacterial assemblages were relatively constant during periods with different growth rates in the assemblages, during various periods of the year, and between samples from freshwater and marine localities. With some precautions, [³H]thymidine incorporation can be used as a quick and sensitive method for determining the effects of toxicants on aquatic bacterial assemblages from natural environmental samples.     

High concentrations of viruses in the sediments of Lac Gilbert, Québec

Viruses were found to be very abundant in the top layer of the sediments of Lac Gilbert, Québec. Viruses were extracted from the sediments using pyrophosphate buffer, and viruses from the diluted extracts were pelleted onto grids and enumerated using transmission electron microscopy. Viral abundance in the sediments ranged from 6.5 × 10⁸ to 1.83 × 10¹⁰ ml^–1, which is 10- to 1,000-fold greater than the number observed in the water column. This increase corresponds well with the 100- to 1,000-fold increase in bacterial abundance in the sediments. Viral abundance differed significantly among the surface sediment samples taken at different bottom depths and among samples taken at different depths of the water column. Viral abundance also varied significantly between the oxic and anoxic zones of the water column and the sediments. The virus-to-bacteria ratio varied greatly among the different sediment sites but not among depths in the water column. Viral abundance in the water column was related to bacterial abundance and chlorophyll concentration, whereas viruses in the sediments were most abundant in sediments with high organic matter content. Elevated viral abundance and their erratic distribution in the sediments suggest that viruses might play an important role in sediment microbial dynamics. Correspondence to: Roxane Maranger     

Use of Radiolabelled Thymidine and Leucine To Estimate Bacterial Production in Soils from Continental Antarctica

Tritiated thymidine incorporation (TTI) into DNA was used to examine bacterial production in two soil types from the Robertskollen group of nunataks in northwestern Dronning Maud Land, providing the first estimates of bacterial production in soil habitats on the Antarctic continent. Although estimates of bacterial productivity in soils near to bird nests (344 (plusmn) 422 ng of C g [dry weight](sup-1) h(sup-1)) were higher than those for soils from beneath mosses (175 (plusmn) 90 ng of C g [dry weight](sup-1) h(sup-1); measured by TTI at 10(deg)C), these differences were not significant because of patchiness of bacterial activity (P > 0.05). TTI- and [(sup14)C]leucine ([(sup14)C]Leu)-derived estimates of bacterial production were similar when incubations of 3 h were used, although incubations as short as 1 h were sufficient for measurable uptake of radiolabel. Dual-label incorporation of [(sup3)H]thymidine ([(sup3)H]TdR) into DNA and [(sup14)C]Leu into protein indicated that TTI did not reflect bacterial production of in situ assemblages when incubations were longer than 3 h. Isotope dilution analysis indicated that dilution of the specific activity of exogenously supplied [(sup3)H]TdR by de novo synthesis of TdR precursor could be limited by additions of [(sup3)H]TdR at a concentration of 1 nmol per ca. 115 mg of soil. TTI exhibited a psychrotrophic response to variation in temperature, with a temperature optimum of ca. 15(deg)C and a Q(inf10) value for 0 to 10(deg)C of 2.41.     

Dynamics of bacterioplankton in a mesotrophic French reservoir (Pareloup)

Bacterioplankton abundance, biomass and production were studied at a central station (35 m depth) from April 1987 to September 1988 in a mesotrophic reservoir. Bacterial production was calculated by the (³H) thymidine method.For the water column, integrated estimates of bacterioplankton abundance ranged from 2.3 10⁹ to 4.6 10⁹ cells l^–1, and carbon biomass from 0.037 to 0.068 mg C l^–1; the thymidine incorporation rates ranged from 0.8 to 17.2 picomoles l^–1 h^–1, leading to net bacterial production estimates of less than 0.7 µg C l^–1 d^–1 in winter to 18 µg C l^–1 d^–1 in summer. About 55% of the production occurred in the euphotic layers.Over the year, the bacterial carbon requirement represented 90% of the autotrophic production for the whole lake. It was five times lower than autotrophic production in spring, but twice as high in summer. This important temporal lack of balance suggests that not all the spring primary production products are consumed immediately and/or that other carbon sources probably support bacterial growth in summer.     

Bacterial productivity in the water column and sediments of the Georgia (USA) coastal zone: Estimates via direct counting and parallel measurement of thymidine incorporation

Three methods of estimating bacterial productivity were compared using parallel samples of Atlantic Ocean water (within 0.25–15 km of the Georgia coast). The frequency-of-dividing cells (FDC) method and the [³H]thymidine incorporation method gave results which were strongly correlated (r=0.97), but the FDC estimates were always higher (X2 to X7) than the [³H]thymidine estimates. Estimates of bacterial productivity ranged from 2–4×10⁸ cells·l^–1·h^–1 at 0.25 km from shore to 1–9×10⁷cells·l^–1·h^–1 at 15 km. A method involving incubation of 3-m filtrates and direct counting gave results that could not be easily translated into estimates of bacterial productivity. Application of the FDC method to sediment samples gave high productivity estimates, which could be not reconciled with productivity estimates based on sediment oxygen uptake.     

Microbial Population Dynamics in the Sediments of a Eutrophic Lake (Aydat,France) and Characterization of Some Heterotrophic Bacterial Isolates

The bacterial populations of anoxic sediments in a eutrophic lake (Aydat, Puy-de-Dôme-France) were studied by phospholipid fatty acid analysis (PLFA) and also by culturing heterotrophic bacteria under strictly anaerobic conditions. The mean PLFA concentrations of prokaryotes and microeukaryotes were 5.7 ± 2.9 mgC g^–1 DS and 9.6 ± 6.7 mgC g^–1 DS, respectively. The analysis of bacterial PLFA markers was used to determine the dynamics of the Gram-positive and Gram-negative species of anaerobic bacteria, Clostridiae, and sulfate-reducing bacteria. Throughout the sampling period the concentrations of i15:0 (from 20 nmol g^–1 DS to 130 nmol g^–1 DS), markers of Gram-positive bacteria, were higher than those for Gram-negative bacteria. The dynamics of Clostridiae (Cy15:0) paralleled those of sulfate-reducing bacteria that were marked by i17:17. Partial 16S rDNA sequencing and the physiological study of the various fermenting strains, whose abundance in the superficial sediment layer was 1.1 ± 0.4 × 10⁶ cells mL^–1, showed that all the isolates belonged to the Clostridiae and related taxa (Lactosphaera pasteurii, Clostridium vincentii, C. butyricum, C. algidixylanolyticum, C. puniceum, C. lituseburense, and C. gasigenes). All the isolates were capable of metabolizing a wide range of organic substrates.     

The effect of water depth on bacterial numbers,thymidine incorporation rates and C:N ratios in northeast Atlantic surficial sediments

The effect of water depth on bacterial biomass and their ability to synthesise DNA, by measuring their rate of [³H]-thymidine incorporation, was investigated in the northeast Atlantic at three sites of varying water depth (1100–3580 m) and sediment characteristics. Thymidine incorporation rates (y) in surficial sediments varied between 0.028 and 1.44 pmol h^–1 g^–1 and showed an exponential relationship with depth (x) according to the equation y= 2.05e^–0.0011x (r=0.9830 for n=7, P<0.001). However, this relationship failed when a layer of phytodetritus was found overlying the surface sediment and [³H]-thymidine incorporation rates increased by 80–339%. In contrast, bacterial numbers varied between 1.09 and 11.96 × 10⁸ cells g^–1 (dry weight) and showed no significant relationships with water depth or sediment POC/TN content. Significant exponential relationships were also found between water depth (x) and the POC (y ₁) and total nitrogen (TN, y ₂) content of surficial sediments according to the following equations: where y ₁ = 719e^–0.0003x (r=0.8700 for n=9, P<0.01) and y ₂ = 76e^–0.0002x(r=0.7582 for n=9 P<0.02). These relationships were irrespective of the presence or absence of an overlying layer of phytodetritus. This suggests that the POC and TN content of these surficial deep sea sediments is directly related to the flux of material through the water column, which significantly impacts bacterial production.     

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.     

Bacterial production in freshwater sediments: Cell specific versus system measures

Estimates of bacterial production based on total trichloroacetic acid (TCA)-precipitable [methyl-³H]thymidine incorporation and frequency of dividing cell (FDC) techniques were compared to sediment respiration rates in Lake George, New York. Bacterial growth rates based on thymidine incorporation ranged from 0.024 to 0.41 day^–1, while rates based on FDC ranged from 1.78 to 2.48 day^–1. Respiration rates ranged from 0.11 to 1.8mol O₂·hour^–1·g dry weight sediment^–1. Thymidine incorporation yielded production estimates which were in reasonable agreement with respiration rates. Production estimates based on FDC were 4- to 190-fold higher than those predicted from respiration rates.     

Bacterial productivity in ponds used for culture of penaeid prawns

The quantitative role of bacteria in the carbon cycle of ponds used for culture of penaeid prawns has been studied. Bacterial biomass was measured using epifluorescence microscopy and muramic acid determinations. Bacterial growth rates were estimated from the rate of tritiated thymidine incorporation into DNA. In the water column, bacterial numbers ranged from 8.3×10⁹ 1^–1 to 2.57×10¹⁰ 1^–1 and production ranged from 0.43 to 2.10 mg Cl^–1 d^–1. In the 0–10 mm zone in sediments, bacterial biomass was 1.4 to 5.8 g C m^–2 and production was 250 to 500 mg C m^–2 d^–1. The results suggested that most organic matter being supplied to the ponds as feed for the prawns was actually being utilized by the bacteria. When the density of meiofauna increased after chicken manure was added, bacterial biomass decreased and growth rates increased.     

Does Virus-Induced Lysis Contribute Significantly to Bacterial Mortality in the Oxygenated Sediment Layer of Shallow Oxbow Lakes?

Despite the recognition that viruses are ubiquitous components of aquatic ecosystems, the number of studies on viral abundance and the ecological role of viruses in sediments is scarce. In this investigation, the interactions between viruses and bacteria were studied in the oxygenated silty sediment layer of a mesotrophic oxbow lake. A long-term study (13 months) and a diel study revealed that viruses are a numerically important and dynamic component of the microbial community. The abundance and decay rates ranged from 4.3 × 10⁹ to 7.2 × 10⁹ particles ml of wet sediment⁻¹ and from undetectable to 22.2 × 10⁷ particles ml⁻¹ h⁻¹, respectively, and on average the values were 2 orders of magnitude higher than the values for the overlying water. In contrast to our expectations, viruses did not contribute significantly to the bacterial mortality in the sediment, since on average only 6% (range, 0 to 25%) of the bacterial secondary production was controlled by viruses. The low impact of viruses on the bacterial community may be associated with the quantitatively low viral burden that benthic bacteria have to cope with compared to the viral burden with which bacterial assemblages in the water column are confronted. The virus-to-bacterium ratio of the sediment varied between 0.9 and 3.2, compared to a range of 5.0 to 12.4 obtained for the water column. We speculate that despite high numbers of potential hosts, the possibility of encountering a host cell is limited by the physical conditions in the sediment, which is therefore not a favorable environment for viral proliferation. Our data suggest that viruses do not play an important role in the processing and transfer of bacterial carbon in the oxygenated sediment layer of the environment investigated.     

Annual Bacterioplankton Biomasses and Productivities in a Temperate West Coast Canadian Fjord

Bacterioplankton numbers, biomasses, and productivities, as well as chlorophyll a concentrations and phytoplankton productivities, were assayed from 1 March 1984 to 12 August 1985 through a 250-m-deep seawater column in Howe Sound, a temperate fjord-sound on the southern coast of British Columbia, Canada. Primary production during this 18-month period was 845 g of C m⁻². Bacterial production was assayed over this same period as 193 g of C m⁻² (thymidine incorporation) and 77 g of C m⁻² (frequency of dividing cells). Bacterial productivities per cubic meter were usually greater in the euphotic zone than in deeper aphotic water, but when integrated through the water column, approximately half of the bacterial production occurred in the deeper aphotic portion. Bacterial production occurred throughout the year, although at reduced rates in late fall and early winter; primary production almost ceased during late fall and early winter. Because of this heterotrophic bacterioplankton production was a very large portion of the microbial (bacterial plus phyto-plankton) production at this time. In mid-summer bacterial production was a small proportion of the microbial production. Because of this asynchrony in peaks and troughs of bacterial and phytoplankton production through the year, data comparison is best done over an annual cycle. On this basis the bacterial production in the Howe Sound water column was between 23 and 9% of the phytoplankton production when a bacterial C to biovolume ratio of 0.107 pg of C μm⁻³ was assumed; the corresponding values were 64 and 29% when a ratio of 0.300 pg of bacterial C μm⁻³ was assumed.     

Protozoan Grazing and Bacterial Production in Stratified Lake Vechten Estimated with Fluorescently Labeled Bacteria and by Thymidine Incorporation

In stratified Lake Vechten, The Netherlands, protozoan grazing was estimated on the basis of uptake of fluorescently labeled bacteria and compared with bacterial production estimated on the basis of thymidine incorporation. By using a grazer-free mixed bacterial population from the lake in continuous culture, an empirical relationship between cell production and thymidine incorporation was established. Thymidine incorporation into total cold-trichloroacetic-acid-insoluble macromolecules yielded a relatively constant empirical conversion factor of ca. 10¹⁸ (range, 0.38 × 10¹⁸ to 1.42 × 10¹⁸) bacteria mol of thymidine⁻¹ at specific growth rates (μ) ranging from 0.007 to 0.116 h⁻¹. Although thymidine incorporation has been assumed to measure DNA synthesis thymidine incorporation appeared to underestimate the independently measured bacterial DNA synthesis by at least 1.5- to 13-fold, even if all incorporated label was assumed to be in DNA. However, incorporation into DNA was found to be insignificant as measured by conventional acid-base hydrolysis. Methodological problems of the thymidine technique are discussed. Like the cultures, Lake Vechten bacteria showed considerable thymidine incorporation into total macromolecules, but no significant incorporation into DNA was found by acid-base hydrolysis. This applied not only to the low-oxygen hypo- and metalimnion but also to the aerobic epilimnion. Thus, the established empirical conversion factor for thymidine incorporation into total macromolecules was used to estimate bacterial production. Maximum production rates (141 × 10⁶ bacteria liter⁻¹ h⁻¹; μ, 0.012 h⁻¹) were found in the metalimnion and were 1 order of magnitude higher than in the epi- and hypolimnion. In all three strata, the estimated bacterial production was roughly balanced by the estimated protozoan grazing. Heterotrophic nanoflagellates were the major consumers of the bacterial production and showed maximum numbers (up to 40 × 10⁶ heterotrophic nanoflagellates liter⁻¹) in the microaerobic metalimnion.     

Binding of phosphate in sediment accumulation areas of the eastern Gulf of Finland,Baltic Sea

The relationships between P and components binding P were studied by analysing the concentrations of N, P, Fe, Mn, Ca and Al in sediments and pore water along the estuarine transect of the River Neva in August 1995. The high sediment organic matter concentration resulted in low surface redox potential and high pore-water o-P concentration, whereas the abundance of amphipods resulted in high surface redox potentials and low pore-water o-P concentration. However, despite the variation in sediment organic matter and the abundance of amphipods, very reduced conditions and slightly variable concentrations of Tot-P (0.7–1.1 mg g^–1 DW) were observed in the 10–15 cm sediment depth along the estuarine gradient, indicating that the pools of mobile P were largely depleted within the depth of 0–15 cm. Multiple regression analysis demonstrated that organic matter and Tot-Fe concentration of the sediment were closely related to the variation in Tot-P concentration of the sediments (r ² = 0.817, n=32). In addition, the high total Fe:P ratio suggested that there is enough Fe to bind P in sediments along the estuarine gradient. However, low Fe_diss concentrations in the pore water of reduced sediment (redox-potential <–50 mV) indicated efficient precipitation of FeS (FeS and FeS₂), incapable to efficiently bind P. Consequently, the low Fe_diss:o-P ratio (< 1) recorded in pore water in late summer implied that Fe³⁺ oxides formed by diffusing Fe_diss in the oxic zone of the sediments were insufficient to bind the diffusing o-P completely. The measured high o-P concentrations in the near-bottom water are consistent with this conclusion. However, there was enough Fe_diss in pore water to form Fe³⁺ oxides to bind upwards diffusing P in the oxic sediment layer of the innermost Neva estuary and the areas bioturbated by abundant amphipods.