Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter^?1 d^?1 and decreased thereafter to a level of approximately 124 μg C liter^?1 d^?1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml^?1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.     

Annual Changes of Abundance and Biomass of Planktonic Ciliates in the Gdańsk Basin,Southern Baltic

Seasonal changes in the species composition, abundance and biomass of planktonic ciliates were determined every 2–3 weeks at two sites of 30 m depth and one location of 105 m depth in the southwestern Gdańsk Basin between January 1987 and January 1988. A total of 40 ciliate taxa were observed during this period. Autotrophic Mesodinium rubrum dominated ciliate abundance and biomass: maximal values of 50 · 10⁻¹ ind. ^1-1 and 65 μg C 1⁻¹ were recorded. The annual mean biomass of M. rubrum comprised 6 to 9% of the annual mean phytoplankton biomass. The highest abundances and biomasses of heterotrophic ciliates were noted at all stations in the spring and summer in the euphotic zone with maximum values of 28 · 10³ ind. 1⁻¹ and 23 μg C 1⁻¹. Three ciliates assemblages were distinguished in the epipelagic layer: large and medium-size non-predatory ciliates, achieving peak abundance in spring and autumn; small-size microphagous ciliates and epibiotic ciliates which were abundant in summer, and large-size predacious ciliates dominating in spring. Below 60 m, a separate deep-water ciliate community composed of Prorodon-like ciliates and Metacystis spp. was found. The ciliate biomass in the 60–105 m layer was similar to the ciliate biomass in the euphotic zone. The heterotrophic ciliate community contributed 10 to 13% to the annual mean zooplankton biomass. The potential annual production of M. rubrum comprised 6 to 9% of the total primary production. Carbon demand of non-predatory ciliates, calculated on the basis of their potential production, was estimated to be equivalent to 12–15% of the gross primary production.     

Microbial plankton across Drake Passage

We determined biomass and activity of microbial plankton across the Polar Front (PF) in Drake Passage during January 1994. Temperature was around 0°C south and between 3 and 5°C north of the PF. Both biomass and activities of microorganisms were significantly lower in the Antarctic waters south of the PF than in the sub-Antarctic waters north of it. Thus, values of chlorophyll a, integrated between 0 and 200 m, reached 150 mgm⁻² north, but only 25 mg m⁻² south of the PF. Likewise, bacteria varied between 10¹⁴ and 4×10¹³ cells m⁻². However, the abundance of heterotrophic nanoflagellates was extremely low throughout Drake Passage (around 3×10¹⁰ cells m⁻²). Bacterial doubling times were long (mean of 25 days). Bacterivory was estimated from the abundance of predators and prey and from temperature. The grazing impact on bacterioplankton biomass was insignificant (less that 0.05% per day) and low on bacterial heterotrophic production (15% per day). Neither biomass nor the activities of microorganisms were found to increase at the PF. The microbial food web was uncoupled and the bacteria did not seem to be controlled by predation.     

Spatial variations of size-fractionated Chlorophyll, Cyanobacteria and Heterotrophic bacteria in the Central and Western Pacific

Geographic and vertical variations of size-fractionated (0.2–1μm, 1–10 μm, and >10 μm) Chlorophyll a (Chl.a) concentration, cyanobacteria abundance and heterotrophic bacteria abundance were investigated at 13 stations from 4^°S, 160^°W to 30^°N, 140^°E in November 1993. The results indicated a geographic distribution pattern of these parameters with instances of high values occurring in the equatorial region and offshore areas, and with instance of low values occurring in the oligotrophic regions where nutrients were almost undetectable. Cyanobacteria showed the highest geographic variation(ranging from 27×10³ to 16,582×10³cell l^-1), followed by Chl.a (ranging from 0.048 to 0.178μg l^-1), and heterotrophic bacteria (ranging from2.84×10³ to 6.50 ×10⁵ cell l^-1). Positive correlations were observed between nutrients and Chl.a abundance. Correspondences of cyanobacteria and heterotrophic bacteria abundances to nutrients were less significant than that of Chl.a. The total Chl.a was accounted for 1.0–30.9%, 35.9–53.7%, and 28.1–57.3% by the >10μm, 1–10 μm and 0.2–1 μm fractions respectively. Correlation between size-fractionated Chl.a and nutrients suggest that the larger the cell size, the more nutrient-dependent growth and production of the organism. The ratio of pheophytin to chlorophyll implys that more than half of the >10 μm and about one third of the 1–10 μm pigment-containing particles in the oligotrophic region were non-living fragments, while most of the 1–10 μm fraction was living cells. In the depth profiles, cyanobacteria were distributed mainly in the surface layer, whereas heterotrophic bacteria were abundant from surface to below the euphotic zone. Chl.a peaked at the surface layer (0–20 m) in the equatorial area and at the nitracline (75–100 m) in the oligotrophic regions. Cyanobacteria were not the principle component of the picoplankton. The carbon biomass ratio of heterotroph to phytoplankton was greater than 1 in the eutrophic area and lower than 1 in oligotrophic waters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Distribution of small phytoflagellates along an Arctic fjord transect

Phytoflagellates < 10 μm substantially contribute to the abundance, biomass and primary production in polar waters, but information on the distribution of specific groups is scarce. We applied catalysed reporter deposition‐fluorescence in situ hybridization to investigate the distribution of total phytoflagellates and of eight specific groups along a 100 km transect west off Kongsfjorden (Spitsbergen) from 29 to 31 July 2010. Phytoflagellates contributed to > 75% of the depth‐integrated abundance and biomass of total eukaryotes < 10 μm at all stations. Their depth‐integrated abundance and biomass decreased along the transect from 1.5 × 10¹² cells m^?2 (6.6 × 10¹² pgC m^?2) at the outermost station to 1.7 × 10¹⁰ cells m^?2 (4.7 × 10¹⁰ pgC m^?2) at the innermost station. Chlorophytes contributed to the total abundance of phytoflagellates with a range from 13% to 87% (0.7–30.5 × 10³ cells ml^?1), and predominated in open waters. The contribution of haptophytes was < 1–38% (10–4500 cells ml^?1). The other groups represented < 10%. The temperature and salinity positively correlated with the total abundance of phytoflagellates, chlorophytes, haptophytes, bolidophytes and pelagophytes. Cryptophytes, pedinellids and pavlovophytes were negatively associated with the nutrient concentrations. The community composition of phytoflagellates changed along the transect, which could have implications on food web dynamics and biogeochemical cycles between the open ocean environment and Kongsfjorden investigated here.     

A Vigorous Specialized Microbial Food Web in the Suboxic Waters of a Shallow Subtropical Coastal Lagoon

To examine the extent of the microbial food web in suboxic waters of a shallow subtropical coastal lagoon, the density and biomass of bacteria and protozooplankton were quantified under different dissolved oxygen (DO) levels. In addition, bottom waters of a stratified site were compared with bottom waters of a homogeneous site under periods of high and low biological oxygen production/consumption in the lagoon. At the stratified site, microbial biomass decreased with oxygen decline, from oxia to suboxia, with a recovery of the initial total biomass after a 20-day period of persistent suboxia. A peak in density and biomass of purple sulfur bacteria (PSB) (90 μg C L(-1)) occurred in the suboxic waters 20 days prior to the peak in biomass of ciliates >50 μm (Loxophyllum sp. of 150 μm) (160 μg C L(-1)), demonstrating a top down biomass control. Ciliates >50 μm were positively correlated with PSB and bacteriochlorophyll a (photosynthetic pigment of PSB). Total protozoan biomass reached 430 μg C L(-1) in the suboxic waters of the stratified site, with ciliates >50 μm accounting for 90% of the total ciliate biomass and of 55 % of biomass of protozoa. At the homogeneous site, total protozoan biomass was only 66 μg C L(-1), where flagellates and ciliates <25 μm were the dominant microorganisms. Therefore, as light is available for primary producers in the bottom waters of shallow stratified coastal lagoons or estuaries, one can expect that high primary production of PSB may favor a specialized microbial food web composed by larger microorganisms, accessible to zooplankton that tolerate low DO levels.     

Phototrophic picoeukaryotes of Onega Bay,the White Sea: Abundance and species composition

The abundance, biomass, and composition of phototrophic picoeukaryotes (PPE, cell size less than 3 μm) were studied in Onega Bay of the White Sea in June 2015. The highest PPE abundance and biomass were registered in the 0–5-m water layer. In the bay, in the 0–5-m water layer, the average abundance and biomass varied from 0 to 36.8 × 10⁴ cell/L and from 0 to 117 μg С/m³, respectively. The Illumina sequencing of V4 region of 18S rRNA gene revealed eight classes of PPE. Mamiellophyceae dominated both by number of reads and by operational taxonomic units. The green algae Bathycoccus prasinos, Ostreococcus tauri, and Micromonas pusila, as well as diatoms Skeletonema marinoi and Minidiscus trioculatus, were identified to the species level.     

Effects of environmental stressors and their interactions on zooplankton biomass and abundance in a large eutrophic lake

We assessed long-term impacts of multiple stressors and their interaction on the zooplankton community of the large, eutrophic, cyanobacteria-dominated Lake Peipsi (Estonia, Russia). Stressor dataset consisted in time series (1997–2018) of temperature, nutrients, pH, water transparency, phytoplankton biomass and taxonomic richness. The best predictors were selected with random forests machine-learning algorithms and the subsequent models were constructed with generalized linear modeling. We also aimed to identify graphical thresholds representing non-linear, marked responses of abundance or biomass to stressors. Temperature was the dominant stressor for explaining zooplankton abundance and biomass, followed by cyanobacteria biomass, total nitrogen concentration and water transparency. The effect of water temperature was positive, whereas the effect of cyanobacteria became negative after their biomass exceeded a threshold of?~?2 mg l^?1. However, the two stressors together had antagonistic effects on zooplankton, causing a decrease in biomass and abundance. For zooplankton, critical thresholds of total nitrogen (~?700 μg l^?1), total phosphorus (~?70 μg l^?1), and water transparency (~?1.4 m) after which zooplankton metrics changed drastically, were determined. These findings show that although lake warming alone could be positive for zooplankton, the necessity of reducing interacting stressors that influence harmful cyanobacteria growth and biomass, especially nitrogen loads, must be considered.

     

Picophytoplankton abundance in the Velikaya Salma strait,White Sea

In July-August 2009, the abundance of picophytoplankton (Pico) in the Velikaya Salma strait varied from 3.4 × 10⁶ to 19.4 × 10⁶ cells/L, while its biomass (B) was 0.8–3.3 mg C/m³. In August 2010, Pico abundance was significantly higher (up to 216 × 10⁶ cells/L and 36.8 mg C/m³). Pico consisted mainly of cyanobacteria. It constituted 13 (2009) to 28% (2010) of the total phytoplankton biomass. In April 2010, Pico numbers varied from 0.1 × 10⁶ to 0.22 × 10⁶ cells/L and its biomass was 0.05–0.28 mg C/m³. Picoeukaryotes were predominant. Pico constituted not more than 2.7% of the phytoplankton biomass. In the ice column, the integrated Pico abundance was 430 × 10⁶ cells/m² and the integrated biomass was 365 μg C/m².     

Microplankton dynamics in a perennially ice-covered Antarctic lake – Lake Hoare

1. Temporal and spatial variation in planktonic abundance, biomass and composition were determined in Lake Hoare (McMurdo Dry Valleys, Antarctica) over two summer seasons (1996–97 and 1997–98). 2. Phototrophic nanoflagellates (PNAN) dominated planktonic biomass, with a mean monthly biomass ranging between 27.3 and 40.4 μg C L^?1. The deep chlorophyll maximum was mainly composed of cryptophytes (>87% of total PNAN biomass) and varied in depth between 6 and 12 m. 3. Maximum bacterial concentration was 11.8 × 10⁵ cells mL^?1. Bacterial abundance showed relatively little temporal variation, with the exception of a drop in numbers that occurred in late November of both years studied. 4. Ciliates were the most successful heterotrophic protozoan group, with a mean monthly biomass (1.2–3.2 μg C L^?1) being typically at least double that of heterotrophic nanoflagellate (HNAN) biomass (0.1–0.7 μg C L^?1). 5. Microbial processes within this lake appear to be dominated by bottom up control. The relative importance of allochthonous inputs into the lake (from the ice‐cover and stream flow) and autochthonous recycling (by microzooplankton regeneration) are considered. 6. Results from a horizontal transect indicate that the permanence of the main sample hole may have enhanced planktonic biomass over a relatively small spatial scale.     

Adult aquatic insects: Potential contributors to riparian food webs in Australia's wet–dry tropics

Abstract Changes in the abundance and biomass of aquatic and terrestrial aerial insects with distance (mid‐stream, 0, 10–15 and 160 m) from lowland streams were examined across the dry season landscape in Kakadu National Park, northern Australia. Malaise traps and sticky intercept traps were used to sample the insects at four streams, spaced over an area of 1650 km². Malaise and intercept catches were dominated by Diptera (flies and midges), both numerically and by biomass. Chironomid midges were the most abundant taxon, making up 43.4 and 51.0% of the malaise and intercept trap catches, respectively. However, most chironomids were small (less than 3 mm body length), contributing 34.9% to intercept trap biomass, but only 5.2% in malaise traps. Ceratopogonid midges and caddisflies (Trichoptera) accounted for most of the remaining adult aquatic insects. Major terrestrial components were Diptera and Hymenoptera in malaise traps and Coleoptera and Diptera in intercept traps. The total abundance and biomass of insects were much greater over streams and along the water's edge than in riparian (10–15 m) and savanna (160 m) habitats primarily because of the presence of large numbers of adult aquatic insects. The abundance and biomass of terrestrial insects in malaise traps showed no relationship with distance, but intercept trap catches suggested slightly greater abundances over the water and at the water's edge. The great abundance of aquatic insects relative to terrestrial insects close to streams suggests that they have the potential to be an important component of the diets of riparian insectivores, and predation may be an important pathway by which aquatic nutrients and energy are moved into terrestrial food webs.     

Copepod grazing in a summer cyanobacteria bloom in the Gulf of Finland

Blooms of the cyanobacteria Nodularia spumigena and Aphanizomenon flos-aquae dominated the phytoplankton assemblages of the western Gulf of Finland and the eastern side of the northern Baltic Sea in late July–August, 1992. The bloom overlapped the peak seasonal contributions of the dominant mesozooplankton herbivores in the region, the copepods Acartia bifilosa and Eurytemora affinis and the cladoceran Bosmina longispina maritima. Using radio-labelling techniques; the copepods were offered one of the cyanobacteria, Nodularia, as well as the 10–54 µm fraction of the natural phytoplankton assemblage. In general, incorporation rates of the labelled phytoplankton into the copepods declined with increasing contributions of the cyanobacteria. For both copepods, incorporation was inversely related to total phytoplankton biomass, whether measured as chlorophyll, total cells or cyanobacteria biomass. The very low rates for Acartia (< 0.8 µl [copepod h]^–1) indicated that this copepod was likely starving in the cyanobacteria bloom, consistent with the generally poor condition of the animal observed in the laboratory. The other major mesozooplanktor, B. longispina maritima, ingested substantially more cyanobacterial biomass than the two copepods, based on HPLC-identified cyanobacteria-specific pigment echinenone in the gut. Bloom carbon provided < 1% and < 4% of the daily rations for Acartia and Eurytemora, respectively. Total copepod demand in the cyanobacteria blooms was trivial, < 1% of bloom biomass consumed daily. These results suggest that copepod herbivory is relatively unimportant in dissipating summer cyanobacteria blooms in the Gulf of Finland.     

Phytoplankton of Lake Bol’shie Shvakshty (Belarus) during the Shift of the Ecosystem from a Macrophyte–Weakly Eutrophic to a Phytoplankton–Hypereutrophic State

Changes in the quantitative characteristics and functioning of phytoplankton in Lake Bol’shie Shvakshty have been assessed. The changes are evoked by the introduction of herbivorous fishes into the lake and the resulting disturbance of ecological balance in the ecosystem and the shift of the lake into a hypertrophic state from a weakly eutrophic state. Human interference has caused the cyanobacteria density (abundance) and biomass values in the overall phytoplankton composition to strongly exceed (3.5 × 10⁹ cells/L and above 68 μg/L chlorophyll-а, respectively) the threshold value for safe recreational use of water bodies (20 million cells/L and 10 μg/L chlorophyll-а) established by the World Health Organization (WHO). The lake can be assigned to the third level of hazard to human health within the classification proposed by the WHO, as the cyanobacteria density is higher than 100 million cells/L and chlorophyll-а content is higher than 50 μg/L. MC-producing Microcystis species were identified among the cyanobacteria that has propagated in the lake in recent years, and five microcystin variants, including the highly toxic MC-LR, have been detected in the water.     

Seasonal dynamics of the planktonic community in Lake Druzhby, Princess Elizabeth Land, Eastern Antarctica

1. The temporal abundance and composition of the plankton of a continental Antarctic lake (Lake Druzhby) situated in the Vestfold Hills, Eastern Antarctica was investigated from December 1992 to December 1993. The system was dominated by microbial plankton (cyanobacteria, heterotrophic bacteria and protozoans) with few metazoans. 2. Chlorophyll a concentrations ranged between 0.15 and 1.1 μg l^–1 and showed highest levels from late winter to spring. 3. Heterotrophic bacteria ranged between 75 and 250 × 10⁶ l^–1 with highest abundances in late winter/spring. Mean bacterial biovolumes showed considerable seasonal variation (0.05–0.31 μm³). Largest biovolumes occurred in summer and this was the time of highest community biomass. 4. Heterotrophic nanoflagellates reached highest abundances in late summer (maximum 14 × 10⁵ l^–1). Their mean biovolume also exhibited considerable seasonal variation, ranging between 1.77 and 27.0 μm³, with largest size resulting in community biomass peaking in early summer. Ciliated protozoa were poorly represented and sparse. Phototrophic nanoflagellates were sparse in this lake; instead the phototrophic plankton was dominated by a small rod-shaped cyanobacterium which constituted the largest carbon pool in the system. It was common throughout the year, its biomass peaking in autumn. Its presence is discussed in relation to lake morphometry and light climate. 5. Heterotrophic flagellate grazing rates ranged from 6.78 bacteria cell^–1 day^–1 at 2 °C to 11.8 bacteria cell^–1 day^–1 at 4 °C. They remove around 2% of the bacterial carbon pool per day during summer and winter. 6. Nutrient levels were low and recorded in pulses. Dissolved and particulate organic carbon were also low, usually less than 3 mg l^–1 and 600 μg l^–1, respectively. The carbon pools were derived from autochthonous sources. This lake system is driven by bottom-up forces and lacks top-down control, which fits into the picture currently seen for continental Antarctic lakes.     

Iron‐mediated suppression of bloom‐forming cyanobacteria by oxine in a eutrophic lake

1. Published studies show that cyanobacteria have higher Fe requirements than eukaryotic algae. To test whether Fe availability can affect formation of a cyanobacterial bloom, a strong Fe chelator, oxine (8‐hydroxyquinoline, C₉H₇NO), was added to enclosures in eutrophic Lake 227 in the Experimental Lakes Area (ELA) (northwestern Ontario). 2. Aphanizomenon schindlerii growth was suppressed, and growth of eukaryotic chlorophytes significantly promoted in enclosures to which oxine had been added. Significant eukaryotic growth did not occur in enclosures treated with ammonium, suggesting that N supplied by degradation of oxine was not responsible for eukaryotic success in the oxine enclosures. 3. In situ Fe²⁺ measurements were unreliable because of interference from high concentrations of dissolved organic compounds. However, oxine rapidly promoted oxidation of Fe²⁺ to Fe³⁺ in deionised water, suggesting that rapid removal of Fe²⁺ also occurred in the oxine‐treated enclosures. 4. In batch cultures, 10 μm Fe and 10 μm oxine (a 1 : 1 ratio) completely inhibited the growth of the cyanobacteria Synechococcus sp. and Anabaena flos‐aquae and the chlorophytes Pseudokirchneriella subcapitata and Scenedesmus quadricauda. Increasing Fe 10‐fold to 100 μm Fe completely and partially reversed oxine inhibition in the two chlorophytes but could not overcome inhibition of the cyanobacteria, indicating that inhibition was Fe‐mediated at least in the eukaryotes. Since oxine binds Fe³⁺ in a 1 : 3 ratio (Fe : oxine), inhibition at a 1 : 1 ratio indicates that not all of the Fe is bound, and a mechanism involving Fe other than chelation was at least partly responsible for inhibition. 5. Collectively, the enclosure and laboratory results suggest that the outcome of competition between cyanobacteria and eukaryotic algae in the oxine‐treated enclosures in Lake 227 was likely a result of decreased availability of Fe, especially Fe²⁺. 6. The results suggest that remediation methods that dramatically restrict the supply rate of Fe²⁺ could reduce the relative abundance of cyanobacteria in eutrophic systems.     

Factors influencing the abundance and metabolic capacities of microorganisms in Eastern Coastal Plain sediments

The abundance and metabolic capacities of microorganisms residing in 49 sediment samples from 4 boreholes in Atlantic Coastal Plain sediments were examined. Radiolabeled time-course experiments assessing in situ mirobial capacities were initiated within 30 min of core recovery. Acetate (1-¹⁴C- and³H-) incorporation into lipids, microbial colony forming units, and nutrient limitations were examined in aliquots of subsurface sediments. Water-saturated sands exhibited activity and numbers of viable microorganisms that were orders of magnitude greater than those of the low permeability dense clays. Increased radioisotope utilization rates were observed after 6–24-h incubation times when sediments were amended with additional water and/or nutrients. Supplements of water, phosphate, nitrate, sulfate, glucose, or minerals resulted in the stimulation of microbial activities, as evidenced by the rate of acetate incorporation into microbial lipids. Additions of water or phosphate resulted in the greatest stimulation of microbial activities. Regardless of depth, sediments that contained >20% clay particles exhibited lower activities and biomass densities, and greater stimulation with abundant water supplementation than did sediments containing >66% sands and hydraulic conductivities > 200 μm sec.^?1.     

PHOTOSYNTHESIS IN AN EXTREME SHADE ENVIRONMENT: BENTHIC MICROBIAL MATS FROM LAKE HOARE, A PERMANENTLY ICE-COVERED ANTARCTIC LAKE

We investigated the composition of benthic microbial mats in permanently ice-covered Lake Hoare, Antarctica, and their irradiance vs. photosynthetic oxygen exchange relationships. Mats could be subdivided into three distinct depth zones: a seasonally ice-free “moat” zone and two under-ice zones. The upper under-ice zone extended from below the 3.5 m thick ice to approximately 13 m and the lower from below 13 m to 22 m. Moat mats were acclimated to the high irradiance they experienced during summer. They contained photoprotective pigments, predominantly those characteristic of cyanobacteria, and had high compensation and saturating irradiances (E_c and E_k) of 75 and 130 μmol photons·m⁻²·s⁻¹, respectively. The moat mats used light inefficiently. The upper under-ice community contained both cyanobacteria and diatoms. Within this zone, biomass (as pigments) increased with increasing depth, reaching a maximum at 10 m. Phycoerythrin was abundant in this zone, with shade acclimation and efficiency of utilization of incident light increasing with depth to a maximum of 0.06 mol C fixed·mol⁻¹ incident photons under light-limiting conditions. Precipitation of inorganic carbon as calcite was associated with this community, representing up to 50% of the carbon sequestered into the sediment. The lower under-ice zone was characterized by a decline in pigment concentrations with depth and an increasing prevalence of diatoms. Photosynthesis in this community was highly shade acclimated and efficient, with E_c and E_k below 0.5 μmol·m⁻²·s⁻¹ and 2 μmol·m⁻²·s⁻¹, respectively, and maximum yields of 0.04 mol C fixed·mol⁻¹ incident quanta. Carbon uptake in situ by both under-ice and moat mats was estimated at up to 100 and 140 mg·m⁻²·day⁻¹, based on the photosynthesis–irradiance curves, incident irradiance, and light attenuation by ice and the water column.     

Sizes of bacterial cells in soils determined by cascade filtration technique

This paper studies the number of bacteria in typical chernozem and mountain-meadow soil by the traditional method and the cascade filtration technique. The total number of bacteria in these soils, which was obtained in filters of different diameters during filtering the suspension of a certain amount, is 1.5–5 times higher than that obtained by the traditional method. In the structure of the bacterial biomass in both soils, the biomass of bacterial cells with a diameter of 0.38–0.43 μm was dominating by 8–90%. In the typical chernozem, the biomass of cells with a diameter of 0.17 μm was slightly more than 1%; in the mountain-meadow soil, the percentage of the biomass of cells with a diameter of 0.17 μm increased by 5%. The average volume and diameter of the bacteria in the studied soils were calculated. In typical chernozem, the average volume of bacterial cells was equal to 0.0046 μm³ and the diameter was 0.206 μm. In the mountain-meadow soils, these values were slightly lower, 0.0038 μm³ and 0.194 μm, respectively. The biomass of the bacterial cells, which is usually calculated based on the cell volume of 0.1 μm³, is overestimated by about five times when counting the number on the filters. The percentage of the real biomass of soil bacteria is traditionally much lower than that estimated.     

Arthropod communities in a Japanese cedar (Cryptomeria japonica D. Don) plantation: Abundance,biomass and some properties

Arboreal arthropod communities were censused by insecticidal knockdown in a plantation of Japanese cedar,Cryptomeria japonica, in central Japan from April 1983 to February 1985 at intervals of two months. All arthropods sampled were allocated to higher taxonomic groups or guilds and the composition of the communities was analyzed with respect to abundance, biomass and their seasonal trends. The total densities fluctuated seasonally from 200 to 3500 m⁻²; corresponding biomass values ranged from 7 to 600 mg fresh weight m⁻². Maximum density and biomass generally occurred in summer and were minimum in winter. The consistently dominant guild in terms of abundance were the detritivores, mostly comprising Collembola and oribatid mites, accounting for 36–93% of all arthropods; phytophages, predators and detritivores were major guilds in terms of biomass, showing average proportions of 27%, 23% and 20%, respectively. Biomass ratios of predators to prey were generally high in some canopy communities, suggesting the importance of predation pressure in regulating the population levels of arthropods in forests. Average individual size of predators was approximately proportional to that of prey, irrespective of tree locality. Soil arthropod communities maintained densities approximately 10²–10³ times as large as the corresponding canopy communities throughout the year. The seasonal variations in abundance were much greater in canopy than in soil communities.     

Seasonality,abundance, and biomass of bacteria in a southwestern reservoir

The seasonality, abundance, and biomass of planktonic bacteria was investigated in a south temperate zone reservoir. Epilimnetic samples were collected periodically throughout 1983 from 5 locations within Lake Arlington, TX. Total bacteria were determined from epifluorescence microscopy and averaged 1.1 × 10¹³ cells m^–3 of water. Planktobacteria accounted for 85% of total cell counts and 73% of total bacterial biomass. Cell volumes were substantially larger in winter than in summer and were negatively correlated with temperature. Cell volumes ranged from 0.076 to 0.330 µm³ and averaged 0.160 µm³. The average biovolume corresponded to a sphere 0.670 µm in diameter. Bacterial biomass was high, averaging 172 mg C m^–3 of water and reached seasonal maximum during winter months. Correlation analysis (simple linear and multiple linear) revealed that approximately 50% of the variation in bacterial biomass could be accounted for by variation in temperature and dissolved organic carbon.