John T. Lehman

Reconstructing the rate of accumulation of lake sediment: The effect of sediment focusing

Abstract Use of accumulation rates of pollen or minerals to infer regional history is complicated by nonuniform deposition of lake sediment. Sediment focusing, direction of sediment to the deepest part of a basin, can introduce a discrepancy between changes in accumulation rates measured directly from sediment cores and actual changes in influx of sediment or pollen to a lake. This difference depends on values and temporal variation of the ratio of mean depth to maximum depth in a basin as it fills. Several models of sediment accumulation show how measurements from a single core can be transformed to yield basinwide influx rates, and how the distortion due to sediment focusing can be assessed. Basins shaped like hyperboloids or frustums may introduce much greater distortions than basins conforming to ellipsoid or sinusoid shapes.

Microscale patchiness of nutrients in plankton communities.

Autoradiography was used to identify the presence of nutrient patches produced by zooplankton. Algal cells which encounter patches of phosphorus-33 released by swimming animals accumulate more label than cells that do not enter the patches. Differential labeling of algae does not occur when turbulence in the fluid is increased by stirring. Nutrient patchiness at the scale of millimeters or less in nature probably influences the course of competition and coexistence among the phytoplankton.

Palearctic predator invades North American Great Lakes

SummaryBythotrephes cederstroemii Schoedler, a predatory freshwater zooplankter (Crustacea: Cladocera), was first found in the Laurentian Great Lakes in December 1984. The first individuals were from Lake Huron, followed in 1985 with records from Lakes Erie and Ontario. By late August, 1986 the species had spread to southern Lake Michigan (43°N). Bythotrephes has not previously been reported from North America, but has been restricted to a northern and central Palearctic distribution. Its dramatic and widespread rise in abundance in Lake Michigan was greatest in offshore regions. Bythotrephes appears to be invading aggressively, but avoiding habitats presently occupied by glacio-marine relict species that became established in deep oligotrophic North American lakes after the Wisconsin glaciation. Because it is a voracious predator its invasion may lead to alterations in the native zooplankton fauna of the Great Lakes. It offers the chance to study how invading plankton species join an existing community. Judging from its persistence and success in deep European lakes, Bythotrephes may now become a permanent member of zooplankton communities in the Nearctic.

Mixing Patterns and Plankton Biomass of the St. Lawrence Great Lakes under Climate Change Scenarios

Abstract This study is part of an assessment of potential effects of climate change on the St. Lawrence Great Lakes. Its purpose is to investigate potential future lake mixing patterns and primary production. Nested physical and biological models were applied to seasonal mixed layer depth, heat content, primary productivity, and to algal biomass measured as particulate chlorophyll. Two independent second generation General Circulation Models provided scenarios for future conditions of cloud cover, air temperature, humidity, and winds. The climate variables were used to force heat balance and surface mixed layer models for Lakes Superior, Michigan, Huron, Erie, and Ontario. Physical models of heat balance and mixed layer dynamics were coupled with a model of primary biological production and growth of phytoplankton. Simulated climate conditions were for time periods centered at 1975, 2030, 2050, and 2090. Climate projections from both GCMs lead to elevated mixed layer and bottom temperatures in all five lakes by as much as 5°C during this century. Both GCMs point to longer duration of thermal stratification in the five lakes, stronger stability of stratification, and deeper daily mixing depths during peak thermal stratification. For Lake Erie, no striking differences in algal biomass are likely according to climate projections of either model, but for the other lakes, either the duration of nutrient limitation of algal growth is projected to increase, or light limitation caused by deeper mixing is projected to limit the development of algal biomass.

ENHANCED TRANSPORT OF INORGANIC CARBON INTO ALGAL CELLS AND ITS IMPLICATIONS FOR THE BIOLOGICAL FIXATION OF CARBON1,2

Kinetics of uptake of inorganic carbon by the freshwater green alga Chlamydomonas reinhardtii Dang. suggest that rates of fixation may be enhanced at low tensions of CO2 by transport of bicarbonate from the cell surface to the chloroplast. Results are evaluated in the context of models that treat diffusion and reaction of dissolved inorganic carbon across a 3 dimensional finite boundary layer, and they are consistent with the claim that CO2 alone is the substrate used during carbon fixation. An alternative hypothesis, which presumes that both CO2 and bicarbonate are used as substrates, yields predictions which are inconsistent with the data. Instead, bicarbonate seems to act only as a vehicle for the transport of inorganic carbon into the cell, thereby adding its flux to that of CO2, and enhancing rates of synthesis that would otherwise be restricted by uptake of CO2 alone.

A Model for Growth, Development, and Diet Selection by the Invertebrate Predator Bythotrephes cederstroemi

Abstract A growth model is developed to predict C content, respiration, and rates of both embryonic and post-embryonic development by the invertebrate predator Bythotrephes . The model incorporates temperature dependent rates of embryonic and postembryonic development as well as respiration and variations in body mass. Estimated growth efficiencies indicate that this invertebrate predator converts about 25% of its prey biomass into predator mass, thus reducing potential resource availability to plank-tivorous fish. The physiological economics of Bythotrephes with respect to both P and C indicate that copepods can be only a minor component of the predators diet, owing to the high C:P composition of copepod tissues. Mass balance constraints dictate that field-collected populations of Bythotrephes in Lake Michigan consume a prey mixture with bulk tissue stoichiometries less than about 40:1 C:P by mass. The requisite stoichiometry correponds with the elemental composition of Cladocera, particularly Daphnia , which are the dominant Cladocera in offshore regions. Thus an analysis based on conservative elemental budgets of P and C points to Daphnia as the dominant prey of Bythotrephes in Lake Michigan.

Content and turnover rates of phosphorus in Daphnia pulex: effect of food quality

The content of phosphorus in individual Daphnia pulex and the rates at which the element turns over are functions of the P contents of algal cells fed to the zooplankton. Chlamydomonas and Ankistrodesmus were grown in semi-continuous cultures containing 2 µM and 10 µM PO4 to produce differences in cellular P of the algae. Animals reared on the cultures high in P contained 60 percent more P than did animals of equal size that were fed low P cells. Daphnia with high body contents of P turned over that P faster than did the animals fed low P algae. Measured turnover rates imply that large differences exists between rates of gross P assimilation and P excretion for animals fed high and low P sources.

PHOTOSYNTHETIC CAPACITY AND LUXURY UPTAKE OF CARBON DURING PHOSPHATE LIMITATION IN PEDIASTRUM DUPLEX (CHLOROPHYCEAE)1

When cells of Pediastrum duplex Meyen experience phosphorus depletion, their capacity for carbon fixation declines, but sizes and carbon contents of the cells increase several‐fold, an apparent instance of “luxury uptake” of carbon. Maximum rates of uptake of phosphate increase during the same period, and are consequently correlated with the enlarged surface area of the cells. Thus the disadvantage of increased cell volume, which may accelerate sinking speed in nature, is offset by the increased capacity of the cells for nutrient uptake.

Reduced river phosphorus following implementation of a lawn fertilizer ordinance

Abstract Statistical comparisons of 2008 surface water quality data with a historical data set at weekly and subweekly resolution revealed statistically significant reductions in total phosphorus (TP) and a trend of reduction in dissolved phosphorus following implementation of a municipal ordinance limiting the application of lawn fertilizers containing phosphorus. No reductions were seen at an upstream control river site not affected by the ordinance. Nontarget analytes including nitrate, silica and colored dissolved organic matter did not change systematically as did P. The data were analyzed in the context of a statistical model that characterized historical temporal variability and predicted the sampling effort needed to detect changes of specified magnitude. Expected changes of about 25% in monthly mean value were predicted to require weekly samples during the summer for only 1 or 2 years for TP; statistically significant reductions measured after 1 year averaged 28%, or about 5 kg P/day. The lawn fertilizer ordinance was only one component of broader efforts to reduce nonpoint source loading of P, however, so the magnitude of its role in the measured changes remains uncertain.

Whole lake selective withdrawal experiment to control harmful cyanobacteria in an urban impoundment

Different environmental conditions support optimal growth by Aphanizomenon and Microcystis in Ford Lake, Michigan, USA, based on weekly species biovolume and water chemistry measurements from June through October 2005-2007. Experimental withdrawal of hypolimnetic water through the outlet dam was conducted in 2006, with 2005 and 2007 acting as control years, to test theory regarding management of nuisance and toxic cyanobacteria. The dynamics of Aphanizomenon and Microcystis blooms in Ford Lake appear to be driven largely by NO3(-) concentrations, with higher levels shifting the advantage to Microcystis (P<0.0001). Aphanizomenon was most successful with a mean TN:TP ratio (mol:mol) of 48.3:1, whereas Microcystis thrived with a mean ratio of 70.1:1. Withdrawal of hypolimnetic water successfully destabilized the water column and led to higher levels of NO3(-) and the near elimination of the Aphanizomenon bloom in 2006 (P<0.0001). Selective withdrawal did not reduce Microcystis biovolume or microcystin toxicity. Microcystis biovolume and NO3(-) levels were positively correlated with microcystin toxin (P=0.01) and jointly accounted for 30.5% of the variability in the data. Selective withdrawal may be a viable management option for improving water quality under certain circumstances. To fully address the problem of nuisance and toxic algal blooms in Ford Lake, however, an integrated approach is required that targets cyanobacteria biovolume dynamics as well as conditions suited for toxin production.