Bacterial biomass in surface sediments of a headwater stream was measured as a function of dissolved organic carbon (DOC) flux and temperature. Bacterial biomass was estimated using epifluorescence microscopic counts (EMC) and ATP determinations during exposure to streamwater containing 1,788μg DOC/liter and after transfer to groundwater containing 693μg DOC/liter. Numbers of bacteria and ATP concentrations averaged 1.36×10(9) cells and 1,064 ng per gram dry sediment, respectively, under initial DOC exposure. After transfer to low DOC water, biomass estimates dropped by 53 and 55% from EMC and ATP, respectively. The decline to a new steady state occurred within 4 days from ATP assays and within 11 days from EMC measures. A 4°C difference during these exposures had little effect on generation times. The experiment indicated that 27.59 mg/hour of natural DOC supported a steady state bacterial biomass of approximately 10μg C/g dry weight of sediment (from EMC determinations). Steady state bacterial biomass estimates on sediments that were previously muffled to remove organic matter were approximately 20-fold lower. The ratio of GTP∶ATP indicated differences in physiological condition or community composition between natural and muffled sediments.

Landscape-scale alterations that accompany urbanization may negatively affect the population structure of wildlife species such as freshwater turtles. Changes to nesting sites and higher mortality rates due to vehicular collisions and increased predator populations may particularly affect immature turtles and mature female turtles. We hypothesized that the proportions of adult female and immature turtles in a population will negatively correlate with landscape urbanization. As a collaborative effort of the Ecological Research as Education Network (EREN), we sampled freshwater turtle populations in 11 states across the central and eastern United States. Contrary to expectations, we found a significant positive relationship between proportions of mature female painted turtles (Chrysemys picta) and urbanization. We did not detect a relationship between urbanization and proportions of immature turtles. Urbanization may alter the thermal environment of nesting sites such that more females are produced as urbanization increases. Our approach of creating a collaborative network of scientists and students at undergraduate institutions proved valuable in terms of testing our hypothesis over a large spatial scale while also allowing students to gain hands-on experience in conservation science.

Observations of natural and manipulated populations were used to investigate the regulation of population sizes and community structure in a group of soil amoebae, the cellular slime molds (Dictyosteliida). Correlations of slime mold abundance and distributional patterns with soil bacteria in the field suggest that food supply is a potential factor in the regulation of these species' numbers. One species, Dictyostelium mucoroides, responds most visibly to seasonal changes, spring and fall being peak seasons. When total slime mold number are partitioned into active and encysted forms, amoebae account for as much as 51% and 24% of the population in the fall and spring, respectively, vs. only 10—12% of the population during the summer and winter months. Large additions of various bacteria to field plots caused significant increases in D. mucoroides numbers. Moreover, the ability of this species to respond to a second addition of bacteria, made several days later, depended on its density. High—density populations failed to respond to additional food, whereas those which had already returned to base levels showed increases. These findings support the hypotheses that cellular slime molds are food limited in nature, and that community diversity is due, at least in part, to differential resource utilization by the species in nature.

When forested riparian zones are cleared for agriculture or development, major changes can occur in the stream temperature regime and consequently in ecosystem structure and function. Our main objective was to compare the summer temperature regimes of streams with and without forest canopy cover at multiple sites. The secondary objective was to identify the components of the stream heat budget that had the greatest influence on the stream temperature regime. Paired stream reaches (one forested and one non-forested or 'open') were identified at 11 sites distributed across the USA and Canada. Stream temperature was monitored at the upstream and downstream ends of 80 to 130-m-long reaches during summer, and five variables were calculated to describe the stream temperature regime. Overall, compared with forested reaches, open reaches tended to have significantly higher daily mean (mean difference = 0.33 ± 1.1°C) and daily maximum (mean difference = 1.0 ± 1.7°C) temperatures and wider daily ranges (mean difference = 1.1 ± 1.7°C). Mean and maximum daily net heat fluxes in open reaches tended to be greater (or less negative) than those in forested reaches. However, certain sites showed the opposite trends in some variables because of the following: (i) Daily mean and maximum temperatures were biased by differences in inflow temperature between paired reaches and (ii) inputs of cold groundwater exerted a strong influence on temperature. Modelling and regression results suggested that within sites, differences in direct solar radiation were mainly responsible for the observed differences in stream temperature variables at the daily scale.

Dissolved organic carbon (DOC) dynamics were measured in a second-order stream located in a pasture in southeastern Pennsylvania. In situ measurements made at six stations in early summer showed a diel patern of net DOC release from the streambed communities with predawn minima and afternoon maxima. When an 18-m section of the stream was covered with black plastic for 1 mo to exclude algal growth and measures were repeated in late summer, a net removal of DOC occurred in the covered section, while in an adjacent uncovered section DOC was released. When a bovine manure extract was introduced into the covered section, the DOC removal rate increased by more than an order of magnitude. Jewel-weed (Impatiens capensis L.) extract also elicited an elevated rate of DOC removal. When these extracts were added individually to microcosms, removal rates in the dark were 57 and 65% of the in situ uptake estimates for the manure extract and leachate, respectively. Abiotic controls showed that nearly all of the DOC removal was biotic uptake. DOC flux measurements in a third-order section of White Clay Creek, located on an adjacent watershed, also revealed a net DOC release from the uncovered streambed. Introduction of a bovine manure extract elicited a DOC removal rate an order of magnitude less than in the pasture stream. A companion microcosm experiment yielded an uptake rate that was 178% of the in situ estimate.