Summer sedimentation in six shallow arctic lakes

The movement of sediment between the lake bottom and water column of shallow lakes can be sizeable due to the large potential for resuspension in these systems. Resuspended sediments have been shown to alter phytoplankton community composition and elevate water column production and nutrient concentrations. We measured the summer sedimentation rates of two lakes in 2003 and six lakes in 2004. All lakes were shallow and located in the Alaskan Arctic. In 2004, turbidity, light attenuation, total sediment:chlorophyll a mass in the sediment traps, and thermal stratification were also measured in each of the lakes. The sediment:chlorophyll a mass was much greater than if the sediment was derived from phytoplankton production in all of the lakes, indicating that the source of the sedimenting material was resuspension and allochthonous inputs. Consistent with these findings, the temporal variation in sedimentation rate was synchronous between most lakes, and sedimentation rate was positively related to wind speed and rainfall suggesting that sedimentation rate was strongly influenced by landscape-scale factors (e.g., wind and rain events). Two of the lakes are located on deposits of loess that accumulated during past glacial periods. These two lakes had sedimentation rates that were significantly greater and more variable than any of the other lakes in the study, as well as high turbidity and light attenuation. Our results indicate that sedimentation in these shallow arctic lakes is supported primarily by allochthonous inputs and resuspension and that landscape-scale factors (e.g., weather and geology) impact on the transport of materials between the lake bottom and water column. Handling editor: J. Saros     

Estimation of export production in the coastal Baltic Sea: effect of resuspension and microbial decomposition on sedimentation measurements

Hydrography, nutrient concentrations, primary production and sedimentation of particulate matter were studied during spring, late summer and autumn in the coastal area of the northern Baltic Sea, SW Finland. Vernal phytoplankton productivity peak and biomass maximum in early May were followed by high sedimentation rates of organic matter at the end of May. In summer, sedimentation rates of organic material were generally low. The decay rates of organic carbon in the sediment traps, estimated by measuring oxygen uptake of settled organic matter, varied between 0.005 to 0.08 d^–1 and were on average 0.02 d^–1. Decomposition of organic matter inside the sediment traps was mainly controlled by temperature, while also organic contents of settled material were significant. Microbial decomposition decreased sedimentation rates of organic carbon and nitrogen on average by 11% and 15%, respectively, during the whole study period of ca. 6 months. Resuspension of organic matter from sediment surface was estimated to contribute ca. 17 and 24% of the total sedimentation of organic carbon during spring and summer, respectively. Export production (i.e. primary sedimentation of organic carbon corrected by decomposition) was estimated to be 32% of the net primary production during the whole productive season and 42% in spring when the flux of primary settling material was greatest. Sedimentation of the spring bloom was the major annual supply of organic matter to the benthos (>80% of the total primary sedimentation).     

Sedimentation in arctic Canada: Particulate organic carbon flux to a shallow marine benthic community in Frobisher bay

Summary Sediment traps designed and constructed by the authors were deployed on the bottom during short periods of time at a depth of 33 m in upper Frobisher Bay. When the results were compared with data obtained from a trap set at the compensation depth (20 m), resuspension of particulate organic carbon was estimated at 25% of the sediment. The annual sedimentation cycle was typified by consistently low winter rates, with variable maximum rates and loads occurring in August following the peak of primary production in the water column. Mean annual sediment flux was found to be 26 g C m^-2 y^-1, representing 31–53% of the range of estimates of carbon fixed in total annual primary production. Relating this to estimated zoobenthic production indicated a benthic conversion efficiency of 53%, not accounting for loss to sediment or benthic primary production. Increased sedimentation following the spring bloom in August appears to stimulate reproduction in some benthic species.     

Deposition,resuspension, and decomposition of particulate organic matter in the sediments of Lake Itasca,Minnesota, USA

Sediment traps were used to investigate the settling, resuspension, and decomposition of particulate organic matter in Lake Itasca, MN (USA). Traps were deployed in the epilimnion and hypolimnion of the deepest basin during June, 1988, sampled twice during stratified conditions (August, September) and once after the lake had mixed (October). The downward flux of particulate material increased from summer to fall. The net sedimentation of organic matter ranged from 0.6 to 2.3 g m^–2 d^–1 at 4 m and increased to 2.1 to 3.2 g m^–2 d^–1 two meters above the bottom sediment indicating that resuspended sediment was at least 33% of the settling mass during all periods. The C:N ratios of captured particles (6.8–9.5) were between the ratios of plankton (5.8 to 6.8) and the sediments (9.9 to 10.2) but smaller than the ratios of terrestrial organic materials (13.5 to 222). The monosaccharide compositions of the entrapped particles were similar to plankton samples and different from the distinct composition of the sediments. Capture of rebound particles similar to the primary flux and not decomposition may have been responsible for this similarity. Total monosaccharide concentrations were lower in the sediments than in entrapped particles. Individual sugars exhibited different patterns of accumulation in the sediments. Glucose was lowest in sediments when the relative concentrations were compared to those in source materials and entrapped particles. In contrast, sediments had the highest rhamnose and fucose concentrations. Bacterial biomass could only account for small portions of these sugars in the sediment. The distinct monosaccharide composition of resuspended sediments was not strongly recorded in materials captured by the sediment traps even after the lake had mixed.     

Long-term Changes and Seasonal Development of Phytoplankton in a Strongly Stratified,Hypertrophic Lake

Changes in the phytoplankton community of the hypertrophic, sharply stratified Lake Verevi have been studied over eight decades. Due to irregular discharge of urban wastewater, the trophic state of the lake has changed from moderately eutrophic to hypertrophic. We found that the trophic state in summer increased in the 1980s and remained at a hypertrophic level since then. Planktothrix agardhii was recorded first in the 1950s and became the dominant species in the 1980s, forming biomass maxima under the ice and in the metalimnion during the vegetation period. In summer 1989, P. agardhii contributed almost 100% of the phytoplankton biomass. Generally, the highest biomass values occurred in the metalimnion. In spring, when P. agardhii was less numerous, diatoms and cryptophytes prevailed. In springs 2000 and 2001 different diatoms dominated – Synedra acus var. angustissima (18.6 g m⁻³) and Cyclostephanos dubius (9.2 g m⁻³), respectively. In recent years, the spring overturn has been absent. In the conditions of strong thermal stratification sharp vertical gradients of light and nutrients caused a large number of vertically narrow niches in the water column. During a typical summer stage, the epilimnion, dominated by small flagellated chrysophytes, is nearly mesotrophic, and water transparency may reach 4 m. The lower part of the water column is hypertrophic with different species of cryptophytes and euglenophytes. A characteristic feature is the higher diversity of Chlorococcales. Often, species could form their peaks of biomass in very narrow layers, e.g. in August 2001 Ceratium hirundinella (18.6 g m⁻³) was found at a depth of 5 m (the lower part of the metalimnion with hypoxic conditions), Cryptomonas spp. (56 g m⁻³) at 6 m (with traces of oxygen and a relatively high content of dissolved organic matter) and euglenophytes (0.6 g m⁻³) at 7 m and deeper (without oxygen and a high content of dissolved organic matter).     

Sedimentary losses of phosphorus in some natural and artificial Iowa lakes

Phosphorus sedimentation in four natural and four artificial Iowa lakes was measured by using sediment traps to determine if sedimentary phosphorus losses were greater in artificial lakes than in natural lakes and the limnological factors influencing phosphorus loss rates. Mean phosphorus sedimentation rates ranged from 13.3 to 218 mg · m^–2 day^–1. Although phosphorus sedimentation rates for the natural lakes as a group did not differ significantly from the rates for artificial lakes, there were significant differences among individual lakes. Phosphorus sedimentation rates also varied significantly during different seasons at different locations within a lake and at different depths within a location. Despite the variance, phosphorus sedimentation rates were strongly correlated with inorganic sediment concentrations and inorganic matter sedimentation rates, thus suggesting that inorganic sediments influence phosphorus sedimentation rates. When Iowa data were combined with data from published studies, mean sedimentation rates were directly correlated with mean chlorophyll a concentrations of the lakes. These data strongly suggest that sedimentation rates as measured by sediment traps are strongly influenced by the trophic status of a lake. Though sedimentation rates were higher in the more productive lakes, it is suggested that these rates represent only gross sedimentation rates rather than net sedimentation rates because of resuspension and resedimentation of bottom sediments.     

Measurement and prediction of sedimentation in small Swedish lakes

Sediment traps were placed in 29 small lakes in south and central Sweden at 2 m below the surface of the lakes and at 2 m above the lake bottom. Traps were exposed for approximately 120 days during the summer months before collection. Rates of sedimentation in both top and bottom traps were compared to 32 catchment, morphometric and water column parameters in an attempt to identify the processes which influence sediment accumulation. Using only lake water pH, maximum lake depth (D_max) and lake surface area (A_o), 67% of the variance in the bottom trap sedimentation rates was explained. Only pH and A_o were useful predictors for the top traps. Using the bottom traps as a measure of gross sedimentation and the top traps as a measure of net sedimentation (plus periphyton growth in the traps), resuspension was separated from net sedimentation in the bottom traps. Resuspension calculated from these data is compared with more conventional methods of calculation.     

Effects of different macrophyte growth forms on sediment and P resuspension in a shallow lake

The effects of floating-leaved, submerged and emergent macrophytes on sediment resuspension and internal phosphorus loading were studied in the shallow Kirkkojärvi basin by placing sedimentation traps among different plant beds and adjacent open water and by sediment and water samples. All the three life forms considerably reduced sediment resuspension compared with non-vegetated areas. Both among submerged (Ceratophyllum demersum, Potamogeton obtusifolius, Ranunculus circinatus) and emergent (Typha angustifolia) plants, resuspension rate was on average 43% of that in the adjacent open water, while within floating-leaved plants (Nuphar lutea) the corresponding value was 87%. The effects of submerged and emergent vegetation increased in the course of the growing season together with increasing plant density. Among floating-leaved vegetation, such seasonal trend in resuspension effects was not observed. Compared with the non-vegetated area, floating-leaved, submerged and emergent plants reduced internal phosphorus loading on average by 21, 12 and 26 mg m⁻² d⁻¹, respectively. The effects of floating-leaved plants on resuspension-mediated internal phosphosrus loading were thus comparable to the effects of the other two life forms.     

Water and Nutrient Mass Balance of the Partly Meromictic Temperate Lake Verevi

Mass balances of total nitrogen and total phosphorus were calculated for Lake Verevi (area 0.126 km², maximum depth 11 m, mean depth 3.6 m), a sharply stratified small lake located in South Estonia within the borders of the town Elva. The lake has up to 10 small inflows but only three of them are nearly permanent. Accidental overflows from near-by oxidation ponds during high floods have been the major source of the nutrient load of the lake in the past. L. Verevi receives a significant part of its inflow from groundwater, which is difficult to measure. In dry years the outflow is temporary. During summer the lake is sharply thermally and chemically stratified. The spring turnover is often incomplete even in homothermal conditions, thus giving the lake some meromictic features. The influx of nitrogen exceeded the outflux at any supposed proportion (20%, 50%, 80%) of surface runoff. The lake retained 45–90% of the nitrogen influx by sedimentation and/or by denitrification. The largest nitrogen losses with loss rates more than 10 kg N d⁻¹ occurred in May and June. The calculated phosphorus retention rate became strongly negative during mixing periods. From June to November, phosphorus release from the sediment exceeded sedimentation by 205 kg in 1991 and by 79 kg in 1993. Earlier stagnation and absence of a full spring turnover in the 2000 has slowed down the recovery of the lake because less phosphorus is flushed out. However, the stronger stratification and significantly smaller phosphorus content in the epilimnion limits biological activity and as a result improves the water quality of the surface layer.     

Volumetric and aerial rates of heterotrophic bacterial production in epi- and hypolimnia: the role of nutrients and system morphometry

Epilimnetic and hypolimnetic bacterial production (BP) were measured once in summer, by the incorporation of [³H] - Leucine in each of 14 Quebec (Canada) lakes varying in nutrient content and morphometry. The epilimnetic and hypolimnetic BP were evaluated at two scales: the common per unit volume and areal (m^–2) scale. The per unit volume scale epilimnetic BP was best predicted by total phosphorus (TP, r ²=0.63), and by water residence time (WRT r ²=0.57), with WRT serving as a surrogate for the nutrient and organic matter supply from the catchments. Total phosphorus and lake mean depth (Z _m) together explained 79% of the variation in epilimnetic BP (l^–1). In contrast, hypolimnetic BP (l^–1) was neither linked to nutrients (TP or TN) or dissolved organic carbon (DOC) but only to measures of lake morphometry and best of all to hypolimnetic thickness (Zh; r ²=0.74). With increased Zh, there is an increased dilution of settling organic particles and their nutrients, resulting in a decrease in BP per litre. Conversely, when BP is expressed in areal units (m^–2), hypolimnetic production increases with increasing hypolimnetic thickness. Water column thickness is a master variable, which together with Chl a (abundance of particles) determines hypolimnetic BP at the whole system scale even though the trophic status is the best single indicator of epilimnetic BP on a volumetric scale. Conclusions drawn invariably change with the scale of investigation. Moreover, it is clear that lake morphometry has a major impact on BP. A comparison of whole water column integrated BP with literature derived estimates of the equivalent sediment production (m^–2) below suggests that if the estimated sediment rates are not complete technique artefacts, they are likely to be an order of magnitude higher than the water column rates (m^–2) at the maximum depth sampling sites. The relative importance of the sediments could be expected to rise with a decline in the maximum depth of lakes, characterized by progressively thinner hypolimnia. The present findings point to both a primarily allocthonous fuelling of sediment production and an uncoupling of water and sediment BP.     

Primary Production of Phytoplankton in a Strongly Stratified Temperate Lake

Lake Verevi (12.6 ha, maximum depth 11.0 m, mean depth 3.6 m) is a strongly eutrophic and stratified lake. Planktothrix agardhii is the most characteristic phytoplankton species in summer and autumn, while photosynthesizing sulphur bacteria can occur massively in the metalimnion. Primary production (PP) and chlorophyll a concentration (Chl a) were seasonally studied in 1991, 1993, 2000, and 2001. Vertical distribution of PP was rather complex, having usually two peaks, one at or near the surface (0–1 m), and another deeper (at 3–7 m) in the metalimnion. The values of dark fixation of CO₂ in the metalimnion were in most cases higher than those in the upper water layer. Considering the average daily PP 896 mg C m⁻² and yearly PP 162 mg C m⁻², Secchi depth 2.34 m, and epilimnetic concentrations of chlorophyll a (19.6 mg m⁻³), total nitrogen and total phosphorus (TP, 52 mg m⁻³) in 2000, L. Verevi is a eutrophic lake of a ‘good’ status. Considering the total amounts of nutrients stored in the hypolimnion, the average potential concentrations in the whole water column could achieve 1885 mg m⁻³ of TN and 170 mg m⁻³ of TP reflecting hypertrophic conditions and a ‚bad’ status. Improvement of the epilimnetic water quality from the 1990s to the 2000s may have resulted from incomplete spring mixing and might not reflect the real improvement. A decreased nutrient concentration in the epilimnion has supported the establishment of a ‘clear epilimnion state’ allowing light to penetrate into the nutrient-rich metalimnion and sustaining a high production of cyanobacteria and phototrophic sulphur bacteria.     

Production and Vertical Flux of Attached Bacteria in the Hudson River Plume of the New York Bight as Studied with Floating Sediment Traps

We investigated the growth and vertical flux of attached bacteria with floating sediment traps in the Hudson River Plume of the New York Bight during the spring diatom blooms. Traps were floated at the base of the mixed layer (ca. 10 m) for 1-day periods. After recovery, we measured bacterial abundance and rates of [methyl-³H]thymidine incorporation in the trap samples. The vertical flux of attached bacteria was estimated with a model formulated to distinguish between bacterial accumulation in traps due to in situ growth and that due to vertical flux. Attached bacterial flux ranged from 0.6 × 10¹¹ to 2.0 × 10¹¹ cells m⁻² day⁻¹, and attached bacterial settling rates of 0.1 to 1.0 m day⁻¹ were observed during periods of vertical particulate organic carbon flux ranging from 254 to 1,267 mg of C m⁻² day⁻¹. In situ growth of bacteria in sediment traps was unimportant as a source of bacterial increase when compared with vertical flux during our study. The vertical flux of attached bacteria removed 3 to 67% of the total daily bacterial production from the water column. Particulate organic carbon is not significantly mineralized by attached bacteria during its descent to the sea floor in the plume area during this period, when water temperature and grazing rates are at their annual minima.     

Sedimentation and distribution of gamma-emitting radionuclides in bottom sediments of southern Lake Päijänne,Finland, after the Chernobyl accident

The rates of sedimentation of fallout nuclides were determined by means of sediment traps during 28 months after the Chernobyl accident in southern Lake Päijänne, Finland. The spatial distribution of the radionuclide content of the lake bottom was studied on 35 sediment cores in winters 1987/88 and 1988/89. The results were compared with simultaneous observations of the radionuclide content of the water and seston. The role of different transfer mechanisms in the elimination of radionuclides from the water column is discussed.The values recorded for the flux to the lake sediments were on average of the same order of magnitude as the initial deposition on the lake surface (Cs-137 65 kBqm^–2). The radionuclide flux to the sediments was rapid during the first months after the accident. After that the elimination of dissolved nuclides from the water mass became significantly slower. The highest flux rate was that of Ce-144 and the lowest that of Rh-106 (Ru-106). Of the radiocesium, about half of the initial inventory was transferred to the sediments after the first observation year.The content of radionuclides showed considerable spatial variation on the lake bottom (Cs-137 7–280 kBqm^–2). Direct adsorption of radiocesium explains unexpectedly high concentrations on shallow erosion bottoms. There was a clear tendency for the concentrations to increase with depth, as a result of the focusing effect. Sediment resuspension had a significant impact on the total flux of radionuclides to deepwater sediments. Estimates were made of the resuspended flux of radionuclides.     

Characteristics and mechanisms of the hydroponic bio-filter method for purification of eutrophic surface water

The hydroponic bio-filter method (HBFM) was adopted to purify eutrophic surface water. The average removal efficiency of total nitrogen (TN) and total phosphorus (TP) was 16.8% and 30.8%, respectively, at the hydraulic loading rate (HLR) of 3.0 m³ (m² d)⁻¹. The mass removal rate of TN and TP accordingly reached 1.0 and 0.1 g (m² d)⁻¹ separately. The sedimentation of particulate nitrogen and phosphorus played a major role in removal of nitrogen and phosphorus, which contribute 62.2% and 75.9%, respectively. The optimal HLR of HBFM ranged from 3.0 to 4.0 m³ (m² d)⁻¹. The sediment in midstream reached a maximum nitrification potential of 4.76 × 10⁻⁶ g (g h)⁻¹, while upstream it reached a maximum denitrification potential of 8.1 × 10⁻⁷ g (g h)⁻¹. The distribution of nitrification potential corresponded to the ammonium-oxidizing bacteria density. The key for improving nitrogen removal efficacy of HBFM system consisted of changing the nitrification/denitrification region distribution and accordingly enhancing the denitrification process. The sum of dissolved nitrogen removed by denitrification and plant assimilation was nearly equal to the amount released by sediment. Secateur length of Nasturtium officinale had some effect on its uptake rate. The length of cut should be less than 10 cm at a time. The harvesting frequency of once a month for N. officinale had no influence on nitrogen and phosphorus removal.     

Interactions between sediment and water in a shallow and hypertrophic lake: a study on phytoplankton collapses in Lake Søbygård,Denmark

Short-term changes in phytoplankton and zooplankton biomass have occurred 1–3 times every summer for the past 5 years in the shallow and hypertrophic Lake Søbygård, Denmark. These changes markedly affected lake water characteristics as well as the sediment/water interaction. Thus during a collapse of the phytoplankton biomass in 1985, lasting for about 2 weeks, the lake water became almost anoxic, followed by rapid increase in nitrogen and phosphorus at rates of 100–400 mg N M^–2 day^–1 and 100–200 mg P m^–1 day^–1. Average external loading during this period was about 350 mg N m^–2 day^–1 and 5 mg P m^–2 day^–1, respectively.Due to high phytoplankton biomass and subsequently a high sedimentation and recycling of nutrients, gross release rates of phosphorus and nitrogen were several times higher than net release rates. The net summer sediment release of phosphorus was usually about 40 mg P m^–2 day^–1, corresponding to a 2–3 fold increase in the net phosphorus release during the collapse. The nitrogen and phosphorus increase during the collapse is considered to be due primarily to a decreased sedimentation because of low algal biomass. The nutrient interactions between sediment and lake water during phytoplankton collapse, therefore, were changed from being dominated by both a large input and a large sedimentation of nutrients to a dominance of only a large input. Nitrogen was derived from both the inlet and sediment, whereas phosphorus was preferentially derived from the sediment. Different temperature levels may be a main reason for the different release rates from year to year.     

The role of phytoplankton in the removal of arsenic by sedimentation from surface waters

The role of phytoplankton in the removal of arsenic (As) by particle adsorption and sedimentation was investigated in Moira Lake, Canada. Sampling water and suspended particles over one year illustrated significant variation in As partitioning between particulate and aqueous phases, but failed to establish a correlation between the partition coefficient, K _d, and indicators of phytoplankton biomass. A highly significant inverse logarithmic relationship was noted between K _d and the concentration of suspended particles (log K _d = 5.1 – 1.4 log SS; p = 0.0001) in an apparent demonstration of the particle concentration effect (O' Connor & Connolly, 1980).Particle deposition, measured by means of sediment traps, appeared to include a substantial component of resuspended surficial sediment making sediment trap results unreliable for quantifying the removal of substances from the water column. The As concentration of particles from deep traps deployed during late summer and early fall exceeded the As concentrations of suspended particles and surficial sediment, and may indicate that a highly contaminated nepheloid layer acts as a temporary sink for As.     

Vertical distribution of organic constituents in an Antarctic lake: Lake Fryxell

Vertical distribution of organic constituents, i.e. total organic carbon (TOC), hydrocarbons, fatty acids and hydroxy acids in water and sediment samples from Lake Fryxell (77° 35 S, 163° 15 E) of southern Victoria Land, Antarctica were studied to elucidate their features in relation to stratification of the lake waters and likely distribution of microorganisms. The TOC content of the surface water (5.0 m; just below the ice cover of 4.50 m thickness) was 1.4 mg l^–1. It increased markedly with depth and attained a maximum value of 21.7 mg C l^–1 at a depth of 17.5 m, but decreased to the bottom (13.3 mg C l^–1). The high TOC content of the anoxic bottom layers (> 15 m) is attributable to the concentration of refractory organic substances over long periods following the degradation of labile organic constituents. Hydrocarbons were not found in the water column, but the major constituent of the bottom sediment was n-C_{29 : 2} alkene. Total concentrations of fatty acids in the oxic layers ( 10 m) were highest at 10.0 m and much higher than those in the anoxic layers (> 10 m), probably reflecting the phytoplankton population. The content of branched (iso and anteiso) fatty acids and 3-hydroxy acids in the anoxic layers were much greater than those in the oxic layers which would seem to reflect the distribution of bacterial abundance. The differences of organic composition between the water column and sediments imply that sinking dead organisms were quickly degraded in the lake bottom. Also, the composition of microorganisms in the water column must be very different from that in the sediments.     

Abundance, encystment and sedimentation of acantharia during autumn 1990 in the East Greenland Sea

The abundance and sedimentation of acantharia and their cystswas recorded in the water column and sediment traps in the EastGreenland Sea in August-September 1990. Although acanthariaconstituted <1% of total suspended particulate organic carbon(POC) in the water column, up to 90% (average 55%) of the POCsedimenting in 100 m was present in the form of acanthariancysts during a 9 day drift experiment. Rapid dissolution ofstrontium sulphate, of which their shells and spines are constructed,was evidenced by their disappearance with depth in the watercolumn, maximum dissolution occurring between 500 and 1000 mwater depth. Mass encystment and sedimentation of this singlegroup of sarcodine protozoa can have dramatic effects on, themeasurement of particulate fluxes in the open ocean, and maybe a recurrent phenomenon in the eastern North Atlantic.     

Decomposition of Blue-Green Algal (Cyanobacterial) Blooms in Lake Mendota, Wisconsin

Decomposition of natural populations of Lake Mendota phytoplankton dominated by blue-green algae (cyanobacteria) was monitored by using oxygen uptake and disappearance of chlorophyll, algal volume (fluorescence microscopy), particulate protein, particulate organic carbon, and photosynthetic ability (¹⁴CO₂ up-take). In some experiments, decomposition of ¹⁴C-labeled axenic cultures of Anabaena sp. was also measured. In addition to decomposition, mineralization of inorganic nitrogen and phosphorus were followed in some experiments. Decomposition could be described as a first-order process, and the rate of decomposition was similar to that found by others using pure cultures of eucaryotic algae. Nitrogen and phosphorus never limited the decomposition process, even when the lake water was severely limited in soluble forms of these nutrients. This suggests that the bacteria responsible for decomposition can obtain all of their key nutrients for growth from the blue-green algal cells. Filtration of lake water through plankton netting that removed up to 90% of the algal biomass usually did not cause a similar decrease in oxygen demand, suggesting that most of the particulate organic matter used for respiration of the decomposing bacteria was in a small-particle fraction. Short-term oxygen demand correlated well with the particulate chlorophyll concentration of the sample, and a relationship was derived that could be used to predict community respiration of the lake from chlorophyll concentration. Kinetic analysis showed that not all analyzed components disappeared at the same rate during the decomposition process. The relative rates of decrease of the measured parameters were as follows: photosynthetic ability > algal volume > particulate chlorophyll > particulate protein. Decomposition of ¹⁴C-labeled Anabaena occurred at similar rates with aerobic epilimnetic water and with anaerobic sediment, but was considerably slower with anaerobic hypolimnetic water. Of the various genera present in the lake, Aphanizomenon and Anabaena were more sensitive to decomposition than was Microcystis. In addition to providing a general picture of the decomposition process, the present work relates to other work on sedimentation to provide a detailed picture of the fate of blue-green algal biomass in a eutrophic lake ecosystem.     

Gliding motility of Peloploca spp., and their distribution in the sediment and water column of a eutrophic lake

Observations indicating gliding motility in the gas-vacuolate, filamentous organism Peloploca were made using microcapillary tubes. Tubes containing semi-solid agar, incubated in sediment cores gave good enrichments of Peloploca spp. The organisms, which had the form of helical bundles of filaments, were seen to corkscrew through the agar at up to 2–3 m s^-1.The vertical distribution of Peloploca spp. in the sediment and water column of a eutrophic lake was examined periodically during summer stratification. The organisms were confined to anoxic conditions in the sediment prior to stratification. With increasing anoxia in the hypolimnion, the population shifted upwards in the sediment, and towards the end of stratification, in the most reducing conditions, appeared in the lower hypolimnion. Anaerobically incubated sediment cores also showed the movement of the Peloploca population from sediment into the overlying water.It is suggested that the gliding motility and helical morphology of Peloploca bundles enable them to migrate through sediment in response to oxygen and Eh gradients, in addition to their use of gas vacuoles to adjust their position in the water column. The taxonomic implications of gliding motility in Peloploca spp. are discussed.