Seasonal and spatial variability in Lake Michigan sediment small-subunit rRNA concentrations

We have used molecular biological methods to study the distribution of microbial small-subunit rRNAs (SSU rRNAs), in relation to chemical profiles, in offshore Lake Michigan sediments. The sampling site is at a depth of 100 m, with temperatures of 2 to 4 degrees C year-round. RNA extracted from sediment was probed with radiolabeled oligonucleotides targeting bacterial, archaeal, and eukaryotic SSU rRNAs, as well as with a universal probe. The coverage of these probes in relation to the present sequence database is discussed. Because ribosome production is growth rate regulated, rRNA concentrations are an indicator of the microbial populations active in situ. Over a 1-year period, changes in sedimentary SSU rRNA concentrations followed seasonal changes in surface water temperature and SSU rRNA concentration. Sedimentary depth profiles of oxygen, reduced manganese and iron, and sulfate changed relatively little from season to season, but the nitrate concentration was approximately fivefold higher in April and June 1997 than at the other times sampling was done. We propose that sediment microbial SSU rRNA concentrations at our sampling site are influenced by seasonal inputs from the water column, particularly the settling of the spring diatom bloom, and that the timing of this input may be modulated by grazers, such that ammonia becomes available to sediment microbes sooner than fresh organic carbon. Nitrate production from ammonia by autotrophic nitrifying bacteria, combined with low activity of heterotrophic denitrifying bacteria in the absence of readily degradable organic carbon, could account for the cooccurrence of high nitrate and low SSU rRNA concentrations.     

Biogeochemical silica mass balances in Lake Michigan and Lake Superior

Silica budgets for Lake Michigan and Lake Superior differ in several respects. Mass balance calculations for both lakes agree with previous studies in that permanent burial of biogenic silica in sediments may be only about 5% of the biogenic silica produced by diatoms. Because dissolution rates are large, good estimates of permanent burial of diatoms can not be obtained indirectly from the internal cycle of silica (silica uptake by diatoms and subsequent dissolution) but must be obtained from the sediment stratigraphy. The annual net production of biogenic silica in Lake Michigan requires 71% of the winter maximum silica reservoir which must be maintained primarily by internal cycling in this large lake whereas the comparable silica demand in Lake Superior is only 8.3%. The greater silica demand in Lake Michigan is the result of phosphorus enrichment which has increased diatom production. It is hypothesized that steady-state silica dynamics in Lake Michigan were disrupted by increased diatom production between 1955 and 1970 and that a new steady state based on silica-limited diatom production developed after 1970. Mass balance calculations for Lake Michigan show in contrast with previous work that the hypothesized water column silica depletion of 3.0 g · m^–3 could have occurred even though 90% or more of the biogenic silica production is recycled.     

Historic Trends in Lake Michigan Silica Concentrations

Historic data on soluble silica in the offshore waters of Lake Michigan were compiled and analyzed to determine whether the data supported a decrease in silica concentration which had been hypothesized previously on the basis of other studies. Although the data base was limited and no data were obtained for offshore waters (depths >40 m) prior to 1954, the available data support the conclusion that the silica concentration decreased rapidly after 1954. The thesis is developed that rapid silica depletion occurred in the 15-year period from 1954 to 1969 when the winter maximum concentration decreased from approximately 4.4 to 1.4 mg SiO₂ · 1⁻¹ and the summer minimum decreased from approximately 2.2 to <0.2 mg SiO₂ · 1⁻¹. The decrease in silica concentration is attributed to increased production and sedimentation of diatoms that resulted from increased anthropogenic phosphorus loading. Total phosphorus in offshore waters was probably <10 μg P · 1⁻¹ during the period of rapid silica depletion. The rapid decrease in silica concentration with relatively small phosphorus enrichment demonstrates that the tight coupling of biological and geochemical processes drastically affected the biogeochemistry of silica in a large, aquatic system within a relatively short period of time.     

Studies on Gull Lake, Michigan

The seasonal and depth distribution of phytoplankton in Gull Lake, a moderately large, temperate, dimictic, recreational lake in southern Michigan, U.S.A., is described and discussed. Diatoms (Fragilaria crotonensis, Cyclotella michiganiana, Asterionella formosa, and Synechococcus) grow mainly in summer. Diatoms grew under the winter ice, and objections are lodged to the classic explanation of seasonal growth in temperate lakes, and to a recent hypothesis concerning the `paradox of the plankton'. Two raphe-bearing large diatom species were considered to be truly planktonic. A Synechococcus species was similar to that described previously only from Loch Leven.     

Lake circulation and sediment transport in Lake Myvatn

Lake circulation and sediment transport in Lake Myvatn have been calculated using AQUASEA, a numerical model developed by Vatnaskil Consulting Engineers. The goal of the modelling was to calculate changes in sediment transport within the lake due to changes in lake bathymetry caused by diatomite mining. The model uses the Galerkin finite element method and consists of a hydrodynamic flow model and a transport-dispersion model. The flow model is based on the shallow water equations and the wave equation. The transport model is based on the conservation of mass for suspended sediment. The model was calibrated against measurements performed during the summer of 1992. These included measurements of water elevation, current velocity, wave height, and concentration of suspended sediment. After calibration, the model was run for different mining scenarios to determine their impact on the sediment transport in the lake.     

Depuration of PCBs in the Lake Michigan Ecosystem

A suite of physical processes controls the rate at which polychlorinated biphenyl (PCB) concentrations change in Lake Michigan fauna. To the extent that these processes are homogenous in space, predator and forage fish species are exposed to the same water column and sediment concentrations of PCB in Lake Michigan, whereas exposure due to diet varies with trophic position. We can think of observed PCB concentrations in a particular species as realizations of an unknown process consisting of many sources of variability. Because all species are utilizing the resources of the same ecosystem, a component of variability of PCB concentration is shared by all organisms in the system, whereas other components are not. Individual species were modeled with parameters that were estimated as the overall, or reference mean and a shared rate of change, plus an individual species offset from that reference mean and shared rate of change. This approach allows species-specific differences to be manifest via the offset, whereas the shared components of variability are known with greater precision than would be possible if we considered only a single species. The objective of this work is to determine the extent to which observed declines in PCB concentrations in various species in the Lake Michigan ecosystem are similar and synchronous. A high degree of similarity and synchrony between species would imply a high degree of spatial homogeneity of the physical processes controlling PCB concentrations in Lake Michigan fauna. A lack of synchrony suggests that other factors, such as food web interactions or spatial patterns, may be important. Received 9 March 1999; accepted 29 February 2000.     

Recent geologic development of Lake Michigan (U.S.A.)

The stresses placed on Lake Michigan since the advent of industrialization require knowledge of the sedimentology of the whole lake in order to make informed decisions for environmental planning. Sediment accumulation rates are low: areas of the lake receiving the most sediment average only 1 mm a^?1; deep-water basins average 0.1 to 0.5 mm a^?1; and large areas are not receiving any sediment. Sediment was deposited rapidly (typically 5 mm a^?1), in the form of rock flour, during the deglaciation of both Lake Michigan and Lake Superior Basins. Then the rate of accumulation decreased by 80–90% and has remained relatively constant since final deglaciation. Because active sedimentation occurs mostly in the deep water areas of the lake, the sediment remains undisturbed and contains a record of the chemical history of the lake.     

The phytoplankton of spring Lake,Michigan

Summary Spring Lake in Michigan U.S.A. is a highly productive body of water, containing many rare and unknown phytoplanktonic algae. A list of 283 species identified and their general distribution during the eight months of collection is brought.     

Partitioning of food resources by sculpins in Lake Michigan

Synopsis Resource partitioning was studied in two benthic Lake Michigan fishes, the deepwater sculpin,Myoxocephalus thompsoni, and the slimy sculpin,Cottus cognatus, that exhibit nearly disjunct distributions along a hypolimnetic depth gradient. Fish were collected in an area of sympatry over two 24 h periods. These sculpins exhibited food segregation—slimy sculpins ate primarilyPontoporeia affinis, deepwater sculpins ate bothP. affinis andMysis relicta — and their depth segregation was associated with the reported abundance of these prey. Different feeding behaviors may be responsible for this association, since slimy sculpins consumed intermediate size ranges ofPontoporeia and deepwater sculpins consumedPontoporeia in a pattern similar to the ambient size distribution. Neither fish showed a daily activity cycle, and both appear food-limited.     

In situ predatory behavior ofMysis relicta in Lake Michigan

Selectivity coefficients (W) and predation rates on Lake Michigan zooplankton were determined forMysis relicta during spring through fall using anin situ method. W values indicated the following ranked order of prey preference: Cladocera > copepod copepodites and copepod nauplii > adult diaptomids and cyclopoids. With few exceptions, W values for different prey categories remained fairly constant despite greatly changing relative abundances of prey. Predation rates and prey selectivity were similar in most cases to those determined in laboratory studies. Ingestion rates (percent dry body weight · day^–1) were correlated to total prey biomass (r = 0.38) and to effective prey biomass (r = 0.85), where the weighting factors were overall mean selectivity coefficients for the different prey categories. This result suggested that seasonally varying composition of prey caused much of the variation in ingestion rates among experiments. Feeding trials performed at the same depth with daytime and nighttime assemblages of zooplankton indicated that Cladocera may escape heavyMysis predation at night by migrating from the metalimnetic-hypolimnetic interface into the epilimnion.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.     

Long-Term Patterns of Protozoan Colonization in Douglas Lake, Michigan

Long-term patterns of noninteractive and interactive protozoan colonization of polyurethane foam (PF) artificial substrates in Douglas Lake, Michigan, were examined for a 14-yr period. Species-time data were fitted to the Mac Arthur-Wilson equilibrium model, S =?eq(1 - e^Gt), and examined through time from 1969-1982. Comparisons were made to historical water chemistry measurements. No long-term changes in water chemistry were evident. Similarly, equilibrium species number (?eq) and colonization rate (G) oscillated about a mean through time. Protozoan colonization of PF substrates appeared stable for extended periods and showed modest variation from year to year. Examination of 7-yr-old substrates in 1982 revealed little difference from young (< 50 days) substrates. Previous reports of senescence of artificial substrate communities may have been due to habitat loss within the substrates. No evidence existed for chemical or biological degradation of the lake.     

Bacteria,Archaea, and Crenarchaeota in the Epilimnion and Hypolimnion of a Deep Holo-Oligomictic Lake

In a deep, subalpine holo-oligomictic lake, the relative abundance of Archaea and Crenarchaeota, but not that of Bacteria, increases significantly with depth and varies seasonally. Cell-specific prokaryotic productivity is homogeneous along the water column. The concept of active Archaea observed in the deep ocean can therefore be extended to a deep oxic lake.The abundance, activity, and community composition of epilimnetic and hypolimnetic prokaryotes have been less thoroughly investigated in deep lakes than in oceans. Strong evidence that the depth gradient plays a role in modulating the balance between the domains of Bacteria and Archaea has been found in various marine systems (8, 12, 13, 20). It has been shown that the percentage of Bacteria in the deep marine hypolimnion decreases (up to 5,000 m) while, conversely, the percentage of Archaea increases. The percentage of Crenarchaeota is also higher in the mesopelagic zone than in surface waters (10).Although Archaea have been found in a variety of freshwater habitats (3), little has thus far been published on differentiating between Bacteria, Archaea, and Crenarchaeota in the hypolimnion of deep lakes. An exception is a study of the high-altitude ultraoligotrophic Crater Lake (21, 22), where group I marine Crenarchaeota were observed in deep-water populations (22). This study and another study of various lakes from three continents (9) are based on summer sampling, making it impossible to ascertain the effects of temporal variability on the vertical distribution of Archaea and Crenarchaeota, as has been done for marine systems and shallow lakes (for examples, see references 8 and 11).Our primary objective was to follow variations in the relative abundance of Bacteria, Archaea, and Crenarchaeota found in the hypolimnetic waters of a deep holo-oligomictic lake with a permanent oxic hypolimnion and compare them with those in the epilimnetic assemblages. We used the catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) technique and compared the data thus obtained with prokaryotic productivity.     

The possibility of predator avoidance by Lake Michigan zooplankton

Low densities of Diaptomus ashlandi, Diacyclops thomasi, and Daphnia galeata mendotae were measured at depths where Mysis relicta formed nighttime aggregations. Calculations suggest that mysid predation can not account for the rarity of prey animals at these depths, which further suggests that the prey avoided the mysids. Unlike D. galeata mendotae, Daphnia pulicaria was common in mysid aggregations. The somewhat larger size of D. pulicaria may reduce its vulnerability to mysid predation, and consequently may explain the vertical distribution differences between the two congeners. Vertical distributions of Limnocalanus macrurus and copepod nauplii showed no obvious relationships to the mysid distributions. These were the only two taxa with distributions that were correlated with chlorophyll a concentrations. All crustacean taxa were rare in the epilimnion at night when sonar recorded a dense fish school.     

Lake sediment records of erosional processes

Accumulations of sediment at the beds of lakes, estuaries and reservoirs provide partial records of materials transported from the surrounding water catchment areas. Physical, chemical, biological and magnetic analyses, with data for accumulation rates, have been used in a range of environmental settings to infer the rate, form, cause and source of erosion. This paper is a brief review of these studies from a hydrological perspective, setting sediment studies within a lake-catchment system. The need for long term erosional records is discussed in terms of the type of erosional data which may be obtained. Alternative approaches to studying short and long term erosion are assessed with regard to their cost-effectiveness and their levels of precision and accuracy. Finally, some suggestions are made about how these erosion records may be used to model hydrological, pedological and geomorphological processes, thus linking together long term, short term and contemporary timescales of process operation.     

Diatoms in sediment cores in Utah Lake,Utah, U.S.A.

Diatoms from sediment cores taken from Provo Bay, Geneva and midlake sites in Utah Lake, Utah County, Utah were studied. Algal communities from strata at four centimeterintervals of the cores were analyzed. This study has demonstrated that the diatom flora in Utah Lake has changed through time and that the diatom communities of Provo Bay are floristically unique from those of the main body of the lake.     

Seasonal and Spatial Variability in Lake Michigan Sediment Small-Subunit rRNA Concentrations

We have used molecular biological methods to study the distribution of microbial small-subunit rRNAs (SSU rRNAs), in relation to chemical profiles, in offshore Lake Michigan sediments. The sampling site is at a depth of 100 m, with temperatures of 2 to 4°C year-round. RNA extracted from sediment was probed with radiolabeled oligonucleotides targeting bacterial, archaeal, and eukaryotic SSU rRNAs, as well as with a universal probe. The coverage of these probes in relation to the present sequence database is discussed. Because ribosome production is growth rate regulated, rRNA concentrations are an indicator of the microbial populations active in situ. Over a 1-year period, changes in sedimentary SSU rRNA concentrations followed seasonal changes in surface water temperature and SSU rRNA concentration. Sedimentary depth profiles of oxygen, reduced manganese and iron, and sulfate changed relatively little from season to season, but the nitrate concentration was approximately fivefold higher in April and June 1997 than at the other times sampling was done. We propose that sediment microbial SSU rRNA concentrations at our sampling site are influenced by seasonal inputs from the water column, particularly the settling of the spring diatom bloom, and that the timing of this input may be modulated by grazers, such that ammonia becomes available to sediment microbes sooner than fresh organic carbon. Nitrate production from ammonia by autotrophic nitrifying bacteria, combined with low activity of heterotrophic denitrifying bacteria in the absence of readily degradable organic carbon, could account for the cooccurrence of high nitrate and low SSU rRNA concentrations.     

Characterization of Lake Michigan coastal lakes using zooplankton assemblages

Zooplankton assemblages and water quality were examined bi-weekly from 17 April to 19 October 1998 in 11 northeastern Lake Michigan coastal lakes of similar origin but varied in trophic status and limnological condition. All lakes were within or adjacent to Sleeping Bear Dunes National Lakeshore, Michigan. Zooplankton (principally microcrustaceans and rotifers) from triplicate Wisconsin net (80 μm) vertical tows taken at each lake's deepest location were analyzed. Oxygen–temperature–pH–specific conductivity profiles and surface water quality were concurrently measured. Bray-Curtis similarity analysis showed small variations among sample replicates but large temporal differences. The potential use of zooplankton communities for environmental lake comparisons was evaluated by means of BIOENV (Primer 5.1) and principal component analyses. Zooplankton analyzed at the lowest identified taxonomic level yielded greatest sensitivity to limnological variation. Taxonomic and ecological aggregations of zooplankton data performed comparably, but less well than the finest taxonomic analysis. Secchi depth, chlorophyll a, and sulfate concentrations combined to give the best correlation with patterns of variation in the zooplankton data set. Principal component analysis of these variables revealed trophic status as the most influential major limnological gradient among the study lakes. Overall, zooplankton abundance was an excellent indicator of variation in trophic status.     

Feeding Habits of Diaptomus ashlandi and Diaptomus sicilis in Lake Michigan

The herbivorous habits of Diaptomus ashlandi and Diaptomus sicilis were studied at a nearshore station in Lake Michigan. Filtering and feeding rates were measured from December 1975 to October 1976. Feeding was measured from the change in concentration in three size fractions of chlorophyll a. D. ashlandi fed only on particles in the smallest size class, while the larger D. sicilis consumed particles in all three size fractions. During the isothermal period (winter and spring), the feeding rates of both species were proportional to temperature or chlorophyll concentration. D. sicilis preferentially ingested particles in the largest size class, when this size class increased significantly during the spring bloom. During the summer the highest feeding rates of D. ashlandi occurred in the epilimnion. The highest feeding rates of D. sicilis were in the metalimnion, where large particles in a subsurface chlorophyll maximum were available for consumption. These results indicated the importance of the depth distributions of herbivores and phytoplankton for feeding.     

Temporal and spatial variability in nearshore bacterioplankton communities of Lake Michigan

The spatial and temporal variability of bacterial communities were determined for the nearshore waters of Lake Michigan, an oligotrophic freshwater inland sea. A freshwater estuary and nearshore sites were compared six times during 2006 using denaturing gradient gel electrophoresis (DGGE). Bacterial composition clustered by individual site and date rather than by depth. Seven 16S rRNA gene clone libraries were constructed, yielding 2717 bacterial sequences. Spatial variability was detected among the DGGE banding patterns and supported by clone library composition. The clone libraries from deep waters and the estuary environment revealed highest overall bacterial diversity. Betaproteobacteria sequence types were the most dominant taxa, comprising 40.2–67.7% of the clone libraries. BAL 47 was the most abundant freshwater cluster of Betaproteobacteria , indicating widespread distribution of this cluster in the nearshore waters of Lake Michigan. Incertae sedis 5 and Oxalobacteraceae sequence types were prevalent in each clone library, displaying more diversity than previously described in other freshwater environments. Among the Oxalobacteraceae sequences, a globally distributed freshwater cluster was determined. The nearshore waters of Lake Michigan are a dynamic environment that experience forces similar to the coastal ocean environment and share common bacterial diversity with other freshwater habitats.