Bacterioplankton heterotrophic activity in relation to the phytoplankton compartment in a recently formed reservoir

Seasonal and spatial variations in bacterial abundance, biomass and potential heterotrophic activity in a recently flooded reservoir were measured for two consecutive years, in conjunction with phytoplankton biomass (chlorophyll‐a concentration) and activity (primary production). The mean value of primary production remained constant between the two study years, while those of the chlorophyll‐a concentrations, bacterial abundance, bacterial biomass and bacterial heterotrophic activity decreased. The observed trends in the bacterial variables were linked to changes in the relative importance of allochthonous dissolved organic matter, in addition to selective grazing activity on bacterioplankton. Multivariate regression analyses identified bacterial abundance (29%) and temperature (17%) as dominant correlates of the bacterial potential heterotrophic activity. We concluded that, in a new reservoir, organic matter, other than that from phytoplankton, might be of great importance for bacterioplankton metabolism. Furthermore, grazing activity on bacteria by metazoa in a new reservoir represents, on occasion, an important trophic link between the top consumer and otherwise unavailable dissolved organic carbon sources. Finally, even if little energy is transferred to larger consumers, the microbial route is still important in re‐mineralizing organic matter in Sep Reservoir.     

Dynamics of Lake Michigan Phytoplankton: Mechanisms Controlling Epilimnetic Communities

Lake Michigan epilimnetic algal and particulate organic carbon sedimentation rates, determined from moored and floating sediment traps, and algal loss rates due to crustacean zooplankton grazing, determined from manipulation experiments with natural assemblages, were estimated during 1983 and 1984 field seasons. Sedimentation was the most important algal loss in spring when colonial diatoms and calanoid copepods dominated the plankton. Grazing was the most important loss during summer when phytoflagellates and Daphnia spp. dominated. Comparisons between algal community production and loss due to sedimentation and grazing suggest that natural mortality may be significant, but it does not dominate control. An hypothesis is offered for overall control of phytoplankton dynamics in Lake Michigan.     

Experimental evidence for density‐dependent effects and the importance of algal production in determining population growth rates of riverine zooplankton

Food limitation effects on life history traits of lake zooplankton have been well documented but few studies have examined linkages between population growth rates and food resources in riverine environments. In rivers, allochthonous inputs of particulate organic matter may mitigate food limitation effects allowing density‐independent mechanisms associated with washout (discharge) and feeding interference (turbidity) to assume greater importance. We experimentally manipulated densities of commonly occurring riverine zooplankton (Bosmina longirostris and cyclopoid copepods) within 2000 l mesocosms containing ambient or algal‐enriched food resources. The experiment was repeated through time (July, August, September) to represent the range of zooplankton densities and food resource levels observed in the Ohio River during warm‐water, low‐flow conditions. High growth rates and low sensitivity to density‐dependent effects were observed during July when particulate organic carbon (POC) and chlorophyll concentrations were highest. Lower growth rates and stronger response to density‐dependent effects were observed during August and September experiments when POC and chlorophyll concentrations were lower. Direct manipulations of algal abundance resulted in higher growth rates when gains in chlorophyll were accompanied by increases in the edible size fraction (September experiment). Algal C concentrations were found to be a significant predictor of variation in population growth rates for Bosmina but not cyclopoids. Algal C concentrations in the Ohio River rarely fell below experimentally derived minimum food thresholds but were often below saturation thresholds suggesting that population growth rates were constrained by autochthonous food resources despite the prevalence of allochthonous carbon. Copyright © 2005 John Wiley & Sons, Ltd.     

Relationships Between Bacterial Populations and Oxygen Levels in the Central Basin of Lake Erie

Studies were made to ascertain the relationships between bacteria and particulate organic carbon, particulate nitrogen, and the depletion of oxygen from the hypolimnetic waters of the central basin of Lake Erie. Heterotrophic bacterial populations, dissolved oxygen, particulate organic carbon, and total particulate nitrogen were determined during May, August, and September. Data indicate 1) a relationship between heterotroph populations and dissolved oxygen; 2) epilimnion oxygen concentrations of 9 mg/L were associated with bacterial levels of ?4 × 10³ colonies per mL; 3) bacterial densities started to increase from the thermocline downward; 4) hypolimnion oxygen levels of ?l mg/L were associated with bacterial densities of approximately three times the epilimnion values; 5) bacterial oxygen uptake rate in the hypolimnion varied from 1.5 to 7.4 × 10^?10 mg/L/hr; 6) particulate organic carbon and bacteria showed similar distribution patterns in the water column; 7) particulate nitrogen remained unchanged at 0.1 mg/L in the water column showing no relationship to the other parameters measured.     

Estimation of bacterial production through the coupling of (methyl 3H) Thymidine incorporation and several parameters over a Microcystis aeruginosa bloom in a hypereutrophic reservoir

The coupling of the changes in bacterial quantitative and metabolic aspects during Microcystis aeruginosa bloom conditions together with several environmental parameters was studied in the hypereutrophic Villerest reservoir, France. Bacterial abundance varied from 5.20 to 21.28 × 10⁶ bacteria-mL^?1, while bacterial biomass ranged between 75 and 507 μg C mL^?1. These results confirmed the highly eutrophic status of the Villerest reservoir. The relative quantitative importance of attached bacteria increased as Microcystis proliferated. Methyl-³H incorporation and D-(U-¹⁴C) glucose uptake varied from 8.99 to 60.57 × 10⁶ cells-mL^?1, and 0.016 to 1.587 μg CL^?1 h^?1, respectively. Correlations between several abiotic and biotic parameters showed that phytoplankton regulated bacterial growth by releasing organic carbon which is directly uptaken by heterotrophic bacteria. In the hypolimnion, the sedimentation of decaying algae represented a substantial substrate for bacterial growth.     

Activity and abundance of bacterioplankton in three diverse lowland water courses

The hypothesis that the activity and abundance of bacterioplankton were similar in two low-level fenland water courses, but were different from those in a high-level main river, was rejected. Mean values of V_max for glucose mineralization, V_max per bacterium, abundance of free-living and total directly counted bacteria, abundance of colony-forming units and percentage of culturable bacteria were similar in the low-level Beverley and Barmston Drain and in the River Hull, but were less in the low-level Holderness Drain. The less favourable regime in the Holderness Drain was related to intermittently acid conditions. Temporal variation by bacterioplankton in all three water courses was, however, also related to environmental variables other than pH, notably temperature, chlorophyll, biochemical oxygen demand, current velocity, total suspended solids and particulate organic matter. The activity and abundance of bacterioplankton appeared to have increased in the River Hull between 1975–1976 and 1992–1994.     

Diet of Invasive Adult White Perch (Morone americana) and their Effects on the Zooplankton Community in Missisquoi Bay,Lake Champlain

White perch (Morone americana) became established in Missisquoi Bay, Lake Champlain in the mid 1990s. Since that time, cyanobacteria blooms have become common in summer. Although introduced planktivorous fish often impact plankton communities through a reduction in Daphnia density, such effects can be difficult to predict in an opportunistic species such as white perch. In this study, we examined the extent of zooplanktivory exhibited by adult white perch in Missisquoi Bay. Adult white perch were collected from Missisquoi Bay on ten dates in spring and summer of 2005. White perch diet consisted of large numbers of Daphnia on dates when Daphnia densities exceeded 20 individuals/L and when Daphnia comprised more than 50% of the zooplankton assemblage. When Daphnia densities were below these threshold values, adult white perch diet consisted predominantly of benthic prey. Our results show that white perch feed on large numbers of Daphnia in Missisquoi Bay and select Daphnia over other zooplankton taxa when they are abundant. It is likely that adult white perch grazing in Missisquoi Bay has contributed to a reduction in Daphnia density which in turn may be contributing to summertime cyanobacteria dominance in this bay.     

Organic carbon biodegradability and heterotrophic bacteria along a combined sewer catchment during rain events

The aim of this study was the evaluation of fluxes of organic carbon and biodegradability of waste waters during rain events in order to better predict the impacts of combined sewer overflows (CSO). The combined sewer system studied was located in a densely populated urban catchment adjacent to Paris. Five subcatchments of different size, have been monitored during several rain events for suspended solids, conductivity, ammonium, chemical and biological oxygen demand. In addition, two of these subcathments have been evaluated for dissolved and particulate organic carbon, their biodegradable fractions, and the total bacterial biomass. Several indexes, indicate a lower degradability of waste water during dry weather at the downstream stations, accompanied by an increase of bacterial size and bacterial biomass. A further decrease of biodegradable organic carbon related to rain intensity can be seen at all stations. Our results show a higher proportion of refractory organic carbon during rain events than during dry weather with a possible contribution of the in-sewer sediments to the bacterial wet weather flux.     

Behavioral response of Lake Michigan Daphnia mendotae to Mysis relicta

We performed laboratory experiments to determine if Mysis relicta induce changes in the behavior of Daphnia mendotae collected from Lake Michigan. Laboratory results indicate that Daphnia perceived Mysis kairomones and responded by changing their vertical position in cylinders. Experiments using different resource levels, and two procedures to examine the potential effects of the chemical cues from Mysis or from particulate matter or bacteria associated with capture and defecation of prey, suggest that Daphnia detect Mysis via a chemical cue. This is the first laboratory study that we are aware of that indicates that a zooplankton species from the Great Lakes responds behaviorally to an invertebrate predator. Our findings support the hypothesis that changes in vertical distribution of zooplankton associated with changes in invertebrate predator density, observed in previous Great Lakes studies, is due to behavioral responses to reduce predation risk. It is important to understand and quantify such responses, because predator-induced changes in prey behavior represent trait-mediated interactions that can potentially strongly affect prey growth rates, and indirectly affect resources, competitors, and predators of the prey.     

Dissolved and particulate organic carbon concentrations in stream water and relationships with land use in multiple-use watersheds of the Han River (Korea)

This study examines temporal variation in dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations in streams in the Han River watershed, Korea. On days without significant antecedent rain, DOC and POC concentrations ranged from 0.87 to 3.23 mg C/L and 0.24 to 2.92 mg C/L, respectively. Following rain events, both DOC and POC concentrations were higher. Soil and compost had δ¹³C values similar to stream δ¹³C-DOC and δ¹³C-POC. These results demonstrate the importance of studies using tracer approaches and the value of research on sources of organic carbon transported in streams in multiple use monsoonal watersheds.     

Lake restoration in Denmark

Lake restoration in Denmark has involved the use of several different restoration techniques, all aiming to improve lake water quality and establishing clear‐water conditions. The most frequently used method, now used in more than 20 lakes, is the reduction of zooplanktivorous and benthivorous fish (especially roach (Rutilus rutilus) and bream (Abramis brama)) with the objective of improving the growth conditions for piscivores, large‐sized zooplankton species, benthic algae and submerged macrophytes. Piscivore stocking (mainly Esox lucius (pike)), aiming especially at reducing the abundance of young‐of‐the‐year fish, has been used in more than 10 lakes and frequently as a supplement to fish removal. Hypolimnetic oxidation, with oxygen and nitrate, has been undertaken in a few stratified lakes and sediment dredging, with the purpose of diminishing the internal phosphorus loading, has been experimented with in one large, shallow lake. Submerged macrophyte implantation has been conducted in some of the biomanipulated lakes to increase macrophyte abundance and distribution. Overall, the results from lake restoration projects, in the mainly shallow Danish lakes, show that external nutrient loading must be reduced to a level below 0.05–0.1 mg P L^–1 under equilibrium conditions to gain permanent effects on lake water quality. By using fish removal, at least 80% of the fish stock should be removed over a period of not more than 1–2 years to obtain a substantial effect on lower trophic levels and to avoid regrowth of the remaining fish stock. Stocking of piscivores requires high densities (>0.1 individuals m^–2) if an impact on the plankton level is to be obtained and stocking should be repeated yearly until a stable clear‐water state is reached. The experiments with hypolimnetic oxygenation and sediment dredging confirm that internal phosphorus loading can be reduced. Experience from macrophyte implantation experiments indicates that protection against grazing by herbivorous waterfowl may be useful in the early phase of recolonization.     

Ecosystem-Level Effects of Zebra Mussels (Dreissena polymorpha): An Enclosure Experiment in Saginaw Bay,Lake Huron

We examined the short-term effects of zebra mussels (Dreissena polymorpha) on ecosystem processes in late August 1991 in Saginaw Bay, Lake Huron. Four 1,600-L enclosures, made of Fabreen with a diameter of 1 m, a depth of 2 m, and closed at the bottom, were used to enclose natural plankton communities. These communities were dominated by diatoms with some chlorophytes, chrysophytes, and cyanophytes. Phytoplankton growth was limited by P-availability. Two enclosures were held as controls, and zebra mussels encrusting unionid shells were suspended in two of the enclosures: one enclosure (HZ) contained approximately four-fold greater numbers of mussels than the other (LZ). The concentration of suspended particles, chlorophyll, and algal biomass in HZ and LZ declined over a 6-day interval. Diatom numbers declined more than other taxa. Phytoplankton growth rates in HZ and LZ increased to near μ_max; there was no apparent change in photosynthetic parameters a or P_max scaled for chlorophyll. Soluble reactive P (SRP) increased significantly (p < 0.05) in HZ but not LZ. Dissolved organic P (DOP) and ammonium ion were elevated; dissolved organic carbon (DOC) was unchanged in HZ and LZ. The rate of phosphate uptake by bacteria and algae declined to less than 2% of controls; this rate decrease could not be explained simply by grazing losses or isotope dilution. The rate of ammonium regeneration by the plankton and the potential rate of ammonium uptake by the plankton did not differ significantly in HZ or LZfrom the control enclosures. Our findings indicate that the zebra mussel can have significant short-term effects on phytoplankton abundance, water transparency, water chemistry and phosphorus dynamics. We propose a model of zebra mussel effects that suggests high densities of zebra mussels may indirectly alter and control those processes that are rate-limited or concentration-limited by nutrient availability.     

Stable carbon and nitrogen isotopic signatures of organic matter sources in near-shore areas of Lake Victoria,East Africa

Stable isotope ratios of organic carbon and nitrogen (δ¹³C and δ¹⁵N) were measured in suspended particulate matter (SPM) and sediment in the near-shore areas of Tanzanian waters of Lake Victoria (LV) to identify the various sources of organic matter in different areas of the lake. Spatial variations in δ¹³C, δ¹⁵N, and C/N ratios in SPM and sediments in inshore areas were due to differences in the proportions and sources of autochthonous or allochthonous matter. Watershed characteristics, such as urbanization, and lake characteristics, such as algal blooms, also immensely influenced the stable isotope signal of the organic matter in sediments. Stable C and N ratios showed that inshore areas in bays without pronounced fluvial input were dominated by autochthonous planktonic organic matter; conversely, for bays receiving significant fluvial inflows, the composition of organic matter depended on the extent of preservation of shoreline vegetation. Signals resulting from input of anthropogenic organic waste as a result of urbanization were distinguishable from natural sources. This work demonstrates the potential to discriminate between input sources of organic material into lakes using stable isotope signals in sediment and suspended particulate matter.     

Heterotrophic Bacterioplankton Dynamics at a Site off the Southern Shore of Lake Superior

Evolution of the paradigm regarding the relative importance of allochthonous and autochthonous sources of organic matter in aquatic systems has rekindled interest in the role of bacteria in energy transfer. The development of material budget calculations characterizing conditions of net heterotrophy requires knowledge of spatiotemporal dynamics in the bacterial community. Here we present results from three years of measurement of bacterioplankton abundance and production at three locations on the south shore of Lake Superior. In general, bacterial numbers (0.63 × 10⁶ ± 0.29 × 10⁶ cells·mL⁻¹) and production (0.037 ± 0.055 mgC·m⁻³·hr⁻¹) were comparable to those reported previously for Lake Superior and were consistent with the system's place along the trophic gradient. Interannual differences in abundance and production were apparently related to the timing and magnitude of seasonal phytoplankton dynamics. There was no inter-transect variation or systematic nearshore-offshore gradients in bacterial activity despite substantial differences in proximity to sources of terrigenous materials (between transects) and in phytoplankton biomass (with distance offshore). The most striking signals in bacterial activity were those evidenced by peaks in bacterial production associated with the deep chlorophyll maximum and with the decline in the phytoplankton community with the approach to turnover. It is hypothesized that bacterioplankton activity in Lake Superior exists in a near steady state, fueled by labile organic matter produced through phytoplankton excretion and the photolytic processing of refractory terrigenous materials. Superimposed on this rather stable signal are peaks in bacterial production apparently related to senescence of the phytoplankton community and, perhaps, the generation of nutrients by the grazing community.     

Bacterioplankton production in a water column of Lake Biwa

Seasonal production of bacterioplankton in a water column of Lake Biwa was examined based on cell‐specific growth rates at above (surface layer) and below the thermocline (deep layer). The growth rates were estimated by incubating bacterioplankton in situ with a dilution technique. The rates ranged from 0.05 per day in winter to 0.89 per day in summer, generally with much higher rates in the surface layer than in the deep layer. In an entire water column, bacterial production (in terms of carbon [C]) ranged from 0.217 to 0.811 gC/m² per day with a mean of 0.451 gC/m² per day, which, on average corresponded to 43% of primary production. However, no significant correlation was detected between bacterial production and primary production rates. Although the bacterial production rate correlated positively to water temperature, surface and deep layer rates were comparative for some dates because the deep layer shared a large fraction of the water column. These results suggest that, although specific bacterial activity was low in the deep layer due to the low temperature, bacteria in that layer play substantial roles in consumption of organic matter and material flows in Lake Biwa.     

The distribution and turnover of benthic organic matter in a lowland river: influence of hydrology,seston load and impoundment

The standing stocks of benthic organic matter (BOM) and some of its biochemical compounds (particulate organic carbon, particulate nitrogen, protein, and chlorophyll a) were investigated in the upper 12 cm of the sediments at several sampling sites in a free‐flowing and in an impounded river section of the lowland River Spree (Germany). Several environmental variables were recorded, in order to determine their potential influence on the organic matter content in the sediments. It was shown that the organic matter content in the upper sediment layers was mainly determined by the local hydrological regime, which was best reflected by the particle size distribution at the sampling sites. For the deepest investigated sediment strata, no influencing factors could be determined. The amount and quality of suspended matter was of minor importance to sediment composition. Also seasonal effects could not be detected. The impounded river section exhibited less seasonal variation in organic matter storage, as well as shorter carbon turnover lengths, longer carbon turnover times and lower rates of annual organic matter recycling than the free‐flowing river section, which indicates that changes in river hydraulics result in alterations to the organic matter budget. These alterations could be related to the changed mesohabitat composition and to the higher standing stocks of organic matter in the impounded river reach. We conclude that both morphological and hydrological alterations affect the structure and function of running water ecosystems because of their negative effect on the organic matter cycling. Copyright © 2002 John Wiley & Sons, Ltd.     

Toward Resurrection Ecology: Daphnia mendotae and D. retrocurva in the Coastal Region of Lake Superior,among the First Successful Outside Invaders?

In Lake Superior, almost all embayment and coastal species depend on overwintering stages. Diapausing eggs from sediments provide a long-term record of species presence in addition to furnishing individuals for genetic characterization and experimental studies (resurrection ecology). Here we begin to clarify the historical development of present day species distributions in Lake Superior by examining species composition in time and space around the Keweenaw Peninsula. Keweenaw Bay sediments document the relatively recent increased abundance (perhaps arrival) of Daphnia mendotae and D. retrocurva in coastal assemblages, two species previously assumed to be characteristic of late summer assemblages. Ephippial eggs are confirmed to species by hatching experiments and genetic characterization. The timing of D. mendotae and D. retrocurva appearance coincides roughly with initial ship traffic through the St. Marys River, suggesting colonization from the lower Great Lakes at that time. An alternative hypothesis is that run-off from forest clearance altered coastal waters and encouraged Daphnia development. Eutrophication and increased abundance of planktivorous fishes, particularly perch and smelt, may also have contributed to Daphnia species succession in the Keweenaw Waterway, whereas recent interactions with Bythotrephes in Keweenaw Bay may be pushing assemblages towards dominance of D. mendotae and Holopedium. Contact between D. mendotae and resident D. dentifera in the Keweenaw Waterway led to hybridization. In the waterway, hybrids were more common in the past, yet hybridization continues in small ponds and embayments near regions of species contact.     

Removal of dissolved nitrogen, phosphorus and carbon from stormwater by biofiltration mesocosms.

Biofiltration systems are becoming a popular stormwater treatment device in water sensitive urban design for the removal of fine particulate and dissolved pollutants from stormwater. However, there is limited published data on the effectiveness of these systems for nutrient removal. We constructed biofiltration mesocosms to assess nutrient removal (nitrogen, phosphorus and carbon) under experimental conditions. Different types of media were compared (gravel, sand, and sandy-loam) in vegetated and non-vegetated mesocosms (six treatments in total). Five plant species were used. Vegetated sand and vegetated sandy-loam provided the best overall treatment. Vegetated mesocosms were very effective in removing nitrogen (63-77% removal) and phosphorus (85-94% removal) from synthetic stormwater, and removed substantially more nutrients than the non-vegetated treatments. All treatments removed a substantial portion of the carbon from the stormwater (28-66%). When flushed with tap water, nitrogen and phosphorus were retained by the vegetated mesocosms, but leached from the non-vegetated mesocosms. Plant growth was most vigorous in the sandy-loam media, indicating that this is a good growth media, even without the addition of organic matter.     

Ecological Interactions Between Bythotrephes cederstroemi and Leptodora kindtii and the Implications for Species Replacement in Lake Michigan

The zooplankton predator, Leptodora kindtii, declined in abundance at an offshore reference station in Lake Michigan in the mid-1980s following the invasion of another zooplankton predator, Bythotrephes cederstroemi. Both predators feed largely on daphnid prey and it was observed that densities of three Daphnia species declined in abundance at the reference station following the Bythotrephes invasion. Circumstantial evidence would suggest that the native predator, Leptodora, was competitively suppressed by Bythotrephes. However, results of laboratory experiments presented here show that Bythotrephes will readily attack and eat Leptodora when the predators are maintained under concentrated densities, even if alternative prey are available for the Bythotrephes. There was no evidence in these experiments that Leptodora attacked or ate Bythotrephes. These results imply that predation by Bythotrephes on Leptodora may alternatively account for the collapse of Leptodora in offshore Lake Michigan. In this note I discuss evidence in support of competition and predation as alternative hypotheses to explain the pattern of species replacement, Bythotrephes for Leptodora, observed in Lake Michigan. The existing data are not definitive and tempt further inquiry.     

Stable states,buffers and switches: An ecosystem approach to the restoration and management of shallow lakes in The Netherlands

Lake restoration in the Netherlands has been focused mainly on the control of external P loading from point sources. However, this approach did not result in the water quality desired. The algae-dominated turbid water state may be extremely stable, and then additional measures are necessary to remove certain ‘blockages’ such as: the persistent bloom of Oscillatoria algae, the P release from the lake sediments, and the abundance of fish, preventing zooplankton and submerged macrophytes from developing . This paper addresses: (1) the need for an ecosystem approach, (2) the resistance of shallow lake ecosystems to changes in nutrient loading, (3) the concept of stable states, buffers and switches, and (4) the perspectives for lake restoration in the Netherlands. Priority should be given to fighting the Oscillatoria blooms. Winter flushing with water low in TP and algae proved to be an effective tool for reducing these blooms. Reduction of planktivorous fish, such as bream and roach, could enhance the top-down control of algae through the grazing by zooplankton, particularly by the large Daphnia species.