Use of the [14C]Leucine Incorporation Technique To Measure Bacterial Production in River Sediments and the Epiphyton

Bacterial production is a key parameter for the understanding of carbon cycling in aquatic ecosystems, yet it remains difficult to measure in many aquatic habitats. We therefore tested the applicability of the [¹⁴C]leucine incorporation technique for the measurement of bulk bacterial production in various habitats of a lowland river ecosystem. To evaluate the method, we determined (i) extraction efficiencies of bacterial protein from the sediments, (ii) substrate saturation of leucine in sediments, the biofilms on aquatic plants (epiphyton), and the pelagic zone, (iii) bacterial activities at different leucine concentrations, (iv) specificity of leucine uptake by bacteria, and (v) the effect of the incubation technique (perfused-core incubation versus slurry incubation) on leucine incorporation into protein. Bacterial protein was best extracted from sediments and precipitated by hot trichloroacetic acid treatment following ultrasonication. For epiphyton, an alkaline-extraction procedure was most efficient. Leucine incorporation saturation occurred at 1 μM in epiphyton and 100 nM in the pelagic zone. Saturation curves in sediments were difficult to model but showed the first level of leucine saturation at 50 μM. Increased uptake at higher leucine concentrations could be partly attributed to eukaryotes. Addition of micromolar concentrations of leucine did not enhance bacterial electron transport activity or DNA replication activity. Similar rates of leucine incorporation into protein calculated for whole sediment cores were observed after slurry and perfused-core incubations, but the rates exhibited strong vertical gradients after the core incubation. We conclude that the leucine incorporation method can measure bacterial production in a wide range of aquatic habitats, including fluvial sediments, if substrate saturation and isotope dilution are determined.     

Use of the [(14)C]leucine incorporation technique to measure bacterial production in river sediments and the epiphyton.

Bacterial production is a key parameter for the understanding of carbon cycling in aquatic ecosystems, yet it remains difficult to measure in many aquatic habitats. We therefore tested the applicability of the [(14)C]leucine incorporation technique for the measurement of bulk bacterial production in various habitats of a lowland river ecosystem. To evaluate the method, we determined (i) extraction efficiencies of bacterial protein from the sediments, (ii) substrate saturation of leucine in sediments, the biofilms on aquatic plants (epiphyton), and the pelagic zone, (iii) bacterial activities at different leucine concentrations, (iv) specificity of leucine uptake by bacteria, and (v) the effect of the incubation technique (perfused-core incubation versus slurry incubation) on leucine incorporation into protein. Bacterial protein was best extracted from sediments and precipitated by hot trichloroacetic acid treatment following ultrasonication. For epiphyton, an alkaline-extraction procedure was most efficient. Leucine incorporation saturation occurred at 1 microM in epiphyton and 100 nM in the pelagic zone. Saturation curves in sediments were difficult to model but showed the first level of leucine saturation at 50 microM. Increased uptake at higher leucine concentrations could be partly attributed to eukaryotes. Addition of micromolar concentrations of leucine did not enhance bacterial electron transport activity or DNA replication activity. Similar rates of leucine incorporation into protein calculated for whole sediment cores were observed after slurry and perfused-core incubations, but the rates exhibited strong vertical gradients after the core incubation. We conclude that the leucine incorporation method can measure bacterial production in a wide range of aquatic habitats, including fluvial sediments, if substrate saturation and isotope dilution are determined.     

Seasonal Dynamics of Benthic and Planktonic Algae in a Nutrient‐Rich Lowland River (Spree,Germany)

We studied chlorophyll a (chl. a), biovolume and species composition of benthic algae and phytoplankton in the eutrophic lower River Spree in 1996. The chl. a concentration was estimated as 3.5 (2.7–4.5) µg/cm² for epipsammon, 9.4 (7.4–11.9) µg/cm² for epipelon and 6.7 (5.7–7.8) µg/cm² for the epilithon (median and 95% C. L.). The mean total biomass of benthic algae was significantly higher (6.0 µg chl. a/cm²) than the areal chl. a content of the pelagic zone (1.6 µg chl. a/cm²). Although certain phytoplankton taxa were abundant in the periphyton, benthic taxa generally dominated the assemblages. Seasonal dynamics of benthic algae were probably controlled by light and nitrate supply (sand), discharge fluctuations (sand, mud) and invertebrate grazing (stones). This paper shows the importance of benthic algae even in phytoplankton‐rich lowland rivers with sandy or muddy sediments. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A River's Liver – Microbial Processes within the Hyporheic Zone of a Large Lowland River

Little is known on microbial activities in the sediments of large lowland rivers despite of their potentially high influence on biogeochemical budgets. Based on field measurements in a variety of sedimentary habitats typical for a large lowland river (Elbe, Germany), we present results on the abundance and production of sedimentary bacteria, the potential activity of a set of extracellular enzymes, and potential nitrification and denitrification rates. A diving bell was used to access the sediments in the central river channel, enabling us to sample down to 1 m sediment depth. Depth gradients of all measures of microbial activity were controlled by sediment structure, hydraulic conditions, as well as by the supply with organic carbon and nitrogen. Microbial heterotrophic activity was tightly coupled with the availability of carbon and nitrogen, whereas chemolithotrophic activity (nitrification rate) was related to the available surface area of particles. In the central bed of the river, bacterial production and extracellular enzyme activity remained high down to the deepest sediment layers investigated. Due to the large inner surface area and their connectivity with the surface water, the shifting sediments in the central channel of the river were microbially highly active There, vertically integrated bacterial production amounted to 0.95 g C m⁻³ h⁻¹, which was 2.9 to 5.5 times higher than in the nearshore habitats. We conclude that carbon and nitrogen cycling in the river is controlled by the live sediments of the central river channel, which thus represent a “liver function” in the river's metabolism.     

Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM)

The major proportion of heterotrophic activity in running waters islocalized on the solid surfaces of sediments in the benthic and hyporheic zoneand is dominated by microorganisms. However, this assertion is based on thestudies of small streams, and little is known about the microbial metabolism oforganic matter in river ecosystems. We therefore explored the relationshipsbetween bacterial abundance and production and the gradients of organic matterquality and quantity in sediments of a sixth-order lowland river (Spree,Germany). We found vertical gradients of detrital variables (particulateorganicmatter (POM), particulate organic carbon (POC), nitrogen (PN), and protein) andof bacterial variables (abundance, production, turnover time, and proportion ofbacterial carbon in total POC) in two different sediment types. These gradientswere steeper in stratified sediments than in the shifting sediments. Detritalvariables correlated strongly with bacterial abundance and production. The bestcorrelation was found for detrital variables indicating substrate quantity andquality (r_S = 0.90 for PN with abundance). Although bacterialbiomasscomprised only 0.7% of the POC (1.9% of PN, 3.4% of the protein) in sediments,the turnover of sedimentary organic carbon was fast (median = 62d), especially in the shifting sediments. Our findings demonstratethat sediment dynamics significantly foster organic carbon metabolism in riversystems. Thus, these sediments, which are typical for lowland rivers, stronglyinfluence the metabolism of the whole ecosystem.     

Bacterioplanktonic biomass and production in the river Meuse (Belgium)

This paper presents results of bacterial biomass determination by epifluorescence microscopy after acridine orange staining and ³H-thymidine incorporation measurements in the river Meuse. Bacterial production is calculated from thymidine incorporation using an experimental conversion factor (0.5 10¹⁸ bacterial cells produced per mole of thymidine incorporated into macromolecules). Seasonal variations of bacterial biomass and production at two stations are presented. Biomass ranges between 0.05 mgC · 1⁻¹ (in winter) and 0.8 mgC · 1⁻¹ (in summer). The variations of bacterial production seem to be closely linked to those of primary production; values lower than 1 μgC · 1⁻¹ · h⁻¹ are found in winter and high values (> 5 μgC 1⁻¹ · h ⁻¹) in summer. Longitudinal profiles in the Belgian course of the river show important increase of biomass and production from upstream to downstream. Bacterial growth yield (Y) has been determined (Y = 0.3) in order to calculate bacterial carbon uptake from bacterial production.     

Fluxes of bacteria and organic matter into a blackwater river from river sediments and floodplain soils

1. The Ogeechee River, in south-eastern Georgia, U.S.A, is a blackwater river with an extensive floodplain that is inundated regularly during winter months. Previous studies have shown that low to moderate bacterial production rates cannot support the relatively high suspended bacteria concentrations observed (10^7?-10⁸ cells ml^?1), suggesting an allochthonous source of bacteria. We report the results of a combination of field and flume experiments which demonstrate that river sediments and floodplain soils are significant sources of suspended bacteria during seasonal flooding. Benthic bacteria are also entrained by normal discharges. There are sizeable fluxes of POC and DOC from river sediments and floodplain soils. 2. Bacterial, POC and DOC fluxes (14, 250, and 790 mg Cm^?2 h^?1, respectively) were substantial when water was percolated upward through floodplain soils. 3. Simulation of overland flow using a flume demonstrated further fluxes of bacteria and POC from floodplain soils (up to 61 and 10700 mg Cm^?2h^?1, respectively) and river sediments, but did not yield additional DOC from floodplain soils. 4. These laboratory results are supported by experiments in which we measured significant increases in concentrations of bacteria and DOC in a downstream direction in (i) the main river channel during a winter flood in 1986, and (ii) a floodplain slough (channel side-arm) which re-entered the main channel 800m from its initial divergence. Inputs of bacteria and DOC from the surrounding floodplain were estimated to be up to 3500 kg DOC h^?1, and 4000 kg bacteria Ch^?1 over a 50-km reach. 5. These previously unmeasured fluxes of organic carbon help to explain the high concentrations of suspended bacteria in the Ogeechee River.     

Nitrogen removal in coastal sediments of the German Wadden Sea

Although sediments of the German Wadden Sea are suspected to eliminate a considerable share of nitrate delivered to the SE North Sea, their denitrification rates have not been systematically assessed. We determined N₂ production rates over seasonal cycles (February 2009–April 2010) at two locations with two sediments types each, the first site (Meldorf Bight) receiving nitrate during all seasons from the Elbe river plume, and a second site on the island of Sylt, where nitrate is depleted during summer months. In sediments from the Sylt site, N₂ production ranged from 15 to 32 μmol N₂ m^?2 h^?1 in the fine sand station and from 7 to 13 μmol N₂ m^?2 h^?1 in the coarse sand station; N₂ production was not detected when nitrate was depleted in May and July of 2009. N₂ production in the Meldorf Bight sediments were consistently detected at higher rates (58–130 μmol N₂ m^?2 h^?1 in the very fine sand station and between 14 and 30 μmol N₂ m^?2 h^?1 in the medium sand station). Analysis of ancillary parameters suggests that major factors controlling N₂ production in coastal sediments of the German Wadden Sea are the nitrate concentrations in the overlying water, the ambient temperature, and the organic matter content of the sediment. Extrapolating our spot measurements to the zone of nitrate availability and sediment types, we estimate an annual nitrogen removal rate around 16 kt N year^?1 for the entire northern sector of the German Wadden Sea area. This corresponds to 14% of the annual Elbe river nitrogen load.     

Biofilm development and extracellular enzyme activities on wood in billabongs of south-eastern Australia

• 1 The accrual of organic matter, chlorophyll a and bacteria, and the activities of various extracellular enzymes were studied during biofilm formation on River Red Gum (Eucalyptus camaldulensis) wood submerged in two temperate Australian billabongs for 24 weeks over summer and winter of 1989–90.
• 2 Peak organic matter content of the biofilm ranged from 0.7 to 3.3mg AFDW cm^?2, chlorophyll a content from 1.3 to 4. 2μg cm^?2 and bacterial abundance from 18 × 10⁶ to 94 × 10⁶ cells cm^?2. Most variation in organic matter content, chlorophyll a content and bacterial abundance in the biofilms couid be attributed to the duration of immersion (28–48% of variation) and to the interaction between site and submergence period (11–12%). Differences between sites and between seasons were less important in explaining total variation.
• 3 Alkaline phosphatase, aminopeptidase and [3-D-glucosidase activities, determined per unit substratum surface area, were up to 138 ± 26 nmol cm^?2h^?1, 113 ± 1 nmol cm^?2h^?1 and 9.3 ± 2.2 nmol cm^?2h^?1, respectively. Activities of these three enzymes determined per unit organic biomass were up to 203 ± 25, 157 ± 13, and 16 ± 2.1 nmol mg¹ AFDW h^?1 respectively. Enzyme activities expressed on an area- or biomass-specific basis responded differently to the effects of season, site and duration of substratum exposure.
• 4 Few consistent relationships could be established between the activity of a given enzyme system and the activity of other enzymes, nor with the various biomass parameters, such as total organic matter content, chlorophyll a content or bacterial abundance.
• 5 We suggest that submerged wood of the River Red Gum is an important site for biofilm development in lentic systems in south-eastern Australia, and thus as a food resource for grazing invertebrates and for transformations of various nutrients and organic matter.

     

Exploitation of sediment bacterial carbon by juveniles of the amphipod Monoporeia affinis

1. Carbon budget parameters were measured for young-of-the-year Monoporeia affinis in a combined field and laboratory (microcosm) study, designed to quantify the role of sediment bacteria as a carbon source for juvenile amphipods. Special emphasis was placed on the stimulative effects of amphipod activity (foraging, feeding, bioturbation) on sediment bacterial production and abundance by including the carbon thus generated in carbon budget calculations. 2. Amphipod production was clearly higher at lower densities, suggesting strong intraspecific interactions. Negative production was recorded at amphipod densities of 10000 and 20000 ind. m^?2. Negative production was not accompanied by a decrease in amphipod total lipid content, however, probably due to the lack of easily mobilized lipids in juvenile amphipods. Amphipod respiration rate was 0.45 μg O₂ ind.^?1 h^?1, or O.15 μg C ind.^?1 h^?1. Sediment bacterial carbon content averaged 1.31 and 0.90 mg g^?1 DW under field and laboratory conditions, respectively. 3. Bacterial carbon was not quantitatively important for Monoporeia. Due to higher bacterial abundance and production in natural, stratified sediment, assimilation of bacterial carbon was highest for the field population, providing 6.3% of the amphipods' carbon requirement. In microcosm populations, bacterial carbon accounted for between 1.7 and 5.2% of overall amphipod carbon demand, increasing with amphipod density and bioturbation. 4. Ingestion rate, rather than the quantity of bacterial carbon in the sediment, was found to limit absorption of bacterial carbon from the sediment.     

Spatiotemporal Variation of Bacterial Assemblages in a Shallow Subtropical Coastal Lagoon in Southern Brazil

A study on the bacterioplankton of Conceição Lagoon (27°34′ S–48°27′ W), Southern Brazil, was carried out in July 2005 (austral winter) and January 2006 (austral summer) to characterize the bacterial spatiotemporal distribution and to determine the heterotrophic and photoautotrophic bacterial dominance in hypoxic/oxic stratified waters. Bacterial abundance increased significantly (p?5 (winter) to 3.21?×?10⁶ cells mL^?1 (summer), heterotrophic coccus/rod-shaped (HCR) cells from 7.00?×?10⁴ to 3.60?×?10⁶ cells mL^?1, and heterotrophic filamentous (HF) bacteria from 2.90?×?10³ to 2.74?×?10⁵ cells mL^?1. Bacterial biovolumes also increased in summer with mean biovolumes of CCY ranging from 0.38 to 1.37 μm³, HCR cells from 0.31 to 1.12 μm³, and HF from 3.32 to 11.34 μm³. Principal component analysis showed that salinity, temperature, and light were the abiotic factors that better explained the temporal variability of bacterial assemblages. Bacterial heterotrophy dominated in the lagoon, excepted by the southern and part of central sector in January 2006, when autotrophic-dominated microbial community occurred. Spatially, bacterial assemblages were influenced by nutrient gradient, oxygen, and salinity with a positive relationship between biovolumes and nutrients and a negative relationship between abundance of coccus cyanobacteria and nutrients. area revealed a singular temporal pattern with hypoxic bottom waters in winter and oxygen-rich waters appearing in summer related with the availability of light and predominant microbes. Thus, oxygen consumption/production is likely to be regulated by the amount of light reaching the bottom, stimulating the production of oxygen by oxygenic phototrophs.     

Vertical zonation and production rates of epiphytic algae on Phragmites australis

1. The vertical distribution of chlorophyll in epiphyton on Phragmites australis showed a peak in the middle sections of the submerged parts. Just below the water surface and above the sediment, chlorophyll concentrations were much less. 2. During winter and early spring, loosely attached diatoms were predominant just below the water surface and on the middle sections of the Phragmites plants. Near the bottom, adnate diatoms, parenchymatous thalli of chlorophytes and cyanobacteria were abundant. 3. At high photosynthetic active radiation (PAR) during May, filamentous species of Ulpthricophyceae and Zygophyceae developed dense populations on the middle sections of the stems. 4. Primary production rate was proportional to chlorophyll concentration although production maxima were recorded above the biomass maxima. 5. Where illumination was low, the chlorophyll-specific rate of photosynthesis (P^B rate) decreased proportionally with the vertical decrease of PAR in the littoral zone, independent of the chlorophyll concentration on the stems. 6. When illumination was high, the P^B rate decreased as biomass increased, and was independent of surface radiation. 7. The dependence of primary production rate on chlorophyll concentration produced a saturation curve with a maximum production at 4.6 μgC cm^?2h^?1.     

Seasonal and interannual variation of bacterial production in lowland rivers of the Orinoco basin

1. We examined the influence of hydrologic seasonality on temporal variation of planktonic bacterial production (BP) in relatively undisturbed lowland rivers of the middle Orinoco basin, Venezuela. We sampled two clearwater and two blackwater rivers over 2 years for dissolved organic carbon (DOC), chlorophyll, phosphorus and bacterial abundance to determine their relationship to temporal variation in BP. 2. Dissolved organic carbon concentration was greater in blackwater (543–664 μm ) than in clearwater rivers (184–240 μm ), and was generally higher during periods of rising and high water compared with low water. Chlorophyll concentration peaked (3 μg L^?1) during the first year of study when discharge was lowest, particularly in blackwater rivers. Soluble reactive phosphorus (SRP) was very low in the study rivers (<3.8 μg L^?1) and concentration increased during low water. 3. Average BP was higher in clearwater (0.20–0.26 μg C L^?1 h^?1) than in blackwater rivers (0.14–0.17 μg C L^?1 h^?1), although mean bacterial abundance was similar among rivers (0.6–0.8 × 10⁶ cells mL^?1). 4. Periods of higher chlorophyll a concentration (low water) or flushing of terrestrial organic material (rising water) were accompanied by higher BP, while low BP was observed during the period of high water. 5. Interannual variation in BP was influenced by variations in discharge related to El Niño Southern Oscillation events. 6. Seasonal variation in BP in the study rivers and other tropical systems was relatively small compared with seasonal variation in temperate rivers and lakes. In addition to the low seasonal variation of temperature in the tropics, low overall human disturbance could result in less variation in the inputs of nutrients and carbon to the study rivers compared with more disturbed temperate systems.     

Growth, production and losses of phytoplankton in the lowland River Spree: carbon balance

1. Phytoplankton carbon assimilation and losses (exudation, dark carbon losses) as well as oxygen release and dark community respiration were measured regularly for 2 years at four stations along the lower Spree (Germany). Carbon balance of river phytoplankton was estimated using measured assimilation, metabolic losses and variations in algal carbon along a stretch of river. 2. The light/dark bottle method was modified to simulate vertical mixing. 3. Waxing and waning of phytoplankton populations dominated the load of particulate organic carbon as well as the oxygen budget of the river. 4. Phytoplankton assimilated 310–358 g C m^?2 yr^?1. A mean value of 586 mg C m^?3 day^?1 was fixed in photosynthesis, with 16.7 mg C being exuded during the day and 20.1 mg lost at night. The measured dark respiration was equivalent to only 28% of the daily gross oxygen production of the plankton community. Phytoplankton washed from upstream lakes and reservoirs was not measurably damaged by turbulent transport. 5. In spring, 18–22% of assimilated carbon was used for net biosynthesis of phytoplankton along the river course. At this time, the carbon balance of this part of the Spree was dominated by autochthonous net production. During summer, however, total carbon losses exceeded the intensive carbon assimilation. The decline of algal biomass along the river course in summer was not explicable by measurable physiological losses. The importance of sedimentation and grazing losses is discussed.     

Viruses in bottom sediments of a Mesotrophic Reservoir (Rybinsk Reservoir,Upper Volga)

The total number and morphological and size composition of viriobenthos, number of bacteria infected by viruses, and burst size, as well as virus-induced mortality, abundance, and production of bacteriobenthos, have been estimated in bottom sediments of the Rybinsk Reservoir. The total number of viriobenthos in the reservoir varies within (1.1–10.9) × 10⁹ (on average, (5.9 ± 0.6) × 10⁹) particles/cm³; the total number of virus-to-bacteria ratio ranges within 0.2–2.1 (on average 0.9 ± 0.3). A weak positive correlation is found between the abundance of benthic viruses and the abundance and production of benthic bacteria. In most surveyed parts of the reservoir, infected benthic bacteria were not found or the portion of visibly infected bacterial cells (FVIC) did not exceed 0.5% of the total abundance of bacteriobenthos (N _B). A comparatively high infection of bacteria by viruses was recorded in bottom sediments only at one deep-water station, where FVIC was 2.5 of N _B.     

Biodiversity and extracellular enzymatic activity of heterotrophic bacterial communities in Bardawil Lagoon,Egypt

The biodiversity of heterotrophic viable bacteria (209 isolates) in the hypersaline Bardawil Lagoon, Egypt, was studied. Composition and extracellular activities of viable culturable heterotrophic bacteria (VCHB) in the water and in non-colonised and seagrass-colonised sediments of Bardawil Lagoon were determined bimonthly during 1997 and 1998. The average ± SD total Kjeldhal nitrogen was 1.69 ± 0.44 mg l^?1 in water, 335.95 ± 19.22 mg kg^?1 in colonised sediments, and 215.5 ± 16.0 mg kg^?1 in non-colonised sediments. Exoenzymatic bacterial activity (glycosidase) presented a seasonal trend with average values of 1.02 ±0.16 μM cm^?3 min^?1 in colonised sediment samples and was 0.36 ± 0.27 μM cm^?3 min^?1 in non-colonised sediments. Mean of VCHB was 4 017 ± 565 cfu g^?1 and 1 195 ± 242 cfu g^?1 for colonised and non-colonised sediments, respectively. Bacterial isolates from Bardawil Lagoon water and sediments yielded a wide diversity of VCHB: a total of 209 different species, belonging to 13 genera from the water and 12 genera from the sediments.     

Influence of hydrological seasonality on bacterioplankton in two neotropical floodplain lakes

Seasonal fluctuation in river stage strongly affects the ecological functioning of tropical floodplain lakes. This study was conducted to assess the influence of hydrological seasonality on bacterial production and abundance in two floodplain lakes of the Autana River, a blackwater river in the Middle Orinoco basin, Venezuela. Water samples for nutrient chemistry, chlorophyll a, and microbiological determinations were collected in two floodplain lakes and in the mainstem of the river during 1997–98. DOC and chlorophyll a concentrations were similar between mainstem and lake sites during high water when river and lakes were well connected but became different during the period of low water when the interaction was minimal. Higher values of bacterial production were observed in the floodplain lakes (0.62–1.03 g C l^–1 h^–1) compared to the mainstem sites (0.17–0.19 g C l^–1 h^–1) during the period of low water, while during the period of high water river and lake sites showed similar levels (0.04 g C l^–1 h^–1). Bacterial numbers followed bacterial production in the floodplain lakes, reaching higher numbers during the period of low water (1.41–2.40 × 10⁶ cells ml^–1). Availability of substrate and inorganic nutrients, pH, and inputs and losses of bacterial cells could be determining the observed seasonal patterns in bacterial production and abundance. The Autana lakes exhibited a strong seasonal pattern in the chemical and biological conditions, showing higher productivity during the lentic phase that lasted between 5 and 6 months.     

Seasonal variation in composition and production of planktonic communities in the lower River Rhine

1. The composition and activity of phytoplankton, zooplankton and bacterioplankton in the lower River Rhine were measured in 1990 as part of an international biological inventory of the river. A seasonal study was carried out on two stations: one in the river mouth (km 1019) and one at the German/Dutch border (km 863). 2. High densities of phytoplankton (with up to 140 μg chlorophyll a 1-^?1) and occasional depletion of dissolved silicate were observed at the upstream station. Phosphate concentrations were also lowered during blooms. 3. Phytoplankton blooms, dominated by a few species of centric diatoms, declined one order of magnitude during downstream transport. During non-bloom conditions (low) algal densities were maintained during transport, or increased slightly, indicating the suitability of the river reach for algal growth. 4. Bacterial cell number and production (measured by the ³H-thymidine method) showed a broad summer maximum with activity peaks (0.5 nK < M thymidine h^?1) coincident with declining phytoplankton blooms. Winter values of bacterial production (0.02–0.05 n < Mh^?1) were substantial, probably as a result of allochthonous input of organic matter. 5. Rotifers and crustaceans made up the greater part of the zooplankton biovolume, but at the upstream station the contribution of Dreissena larvae and rhizopods was also substantial. High zooplankton biovolumes, of over 500 × 10⁶μm³1-^?1, were observed only during the phytoplankton spring bloom. 6. Quantitative relationships between the high phytoplankton production (2.1–3.4 gCm^?2 day^?1), the high bacterial substrate uptake (0.5–1gCm^?2 day^?1), and grazing were analysed for the growing season 1990. Algal grazing by metazoan herbivores was substantial only during spring, while the role of phagotrophic microplankton and cell lysis were indicated as major factors responsible for the downstream decline of phytoplankton blooms in the lower Rhine.     

Benthic carbon is inefficiently transferred in the food webs of two eutrophic shallow lakes

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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