Coupling between Marine Plankton and Freshwater Flow in the Plumes off a Small Estuary

Freshwater discharge from rivers is a powerful forcing agent in coastal ecosystems. It not only generates strong ecological effects in estuaries, but also drives the dynamics of nearshore marine waters where prominent river plumes form biogeochemical hot spots in coastal seas worldwide. Large plumes from major rivers exert important controls on pelagic processes. The majority of estuaries are smaller, however, and the importance of the smaller plumes they generate is unknown. We measured the degree of coupling between freshwater flow and inshore zooplankton in such a plume from a subtropical estuary on the east coast of Australia. Flow regimes encompassed long periods of low freshwater input, punctuated by pulsed freshets that initiated the formation of buoyant, lower‐salinity plumes in the nearshore marine zone. Plumes stimulated phytoplankton biomass in the receiving waters, and ultimately changes in zooplankton assemblages. Zooplankton responded strongly to river discharge: (1) in the absence of substantial freshwater flows and plumes, zooplankton was broadly similar in density and biomass across the estuarine‐marine gradient; (2) freshets that generated significant plumes strongly modified hydrological conditions and lowered zooplankton in the estuarine and nearshore waters, and (3) after the initial freshet, zooplankton in the residual plume was at a higher density in nearshore than shelf waters. We demonstrate that coupling between riverine and coastal pelagic systems operates in small plumes, but that there is substantial temporal variance linked to fluctuations in freshwater delivery. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Effect of Temperature Gradients on Stress Development During Cryopreservation via Vitrification

This study addresses the problem of thermal stress development in bulky specimens during cryopreservation via vitrification (vitreous means glassy in Latin). While this study is a part of an ongoing effort to associate the developing mechanical stress with the relevant physical properties of the cryopreserved media and to its the thermal history, the current paper focuses exclusively on the role of temperature gradients. Temperature gradients arise due to the high cooling rates necessary to facilitate vitrification; the resulting non-uniform temperature distribution leads to differential thermal strain, possibly resulting in cracking. The cooling rate is assumed constant on the outer surface in this study, and the material properties are assumed constant. It is demonstrated that under these assumptions, mechanical stress develops only when the temperature distribution in the specimen approaches thermal equilibrium at a cryogenic storage temperature. It is shown that the maximum possible stresses for a given cooling rate can be computed with a simple thermo-elastic analysis; these stresses are associated with cooling to sufficiently low temperatures and are independent of the variation of viscosity with temperature. Analytic estimates for these stresses are obtained for several idealized shapes, while finite element analysis is used to determine stresses for geometries used in cryopreservation practice. Stresses that develop under a wider range of storage temperatures are also studied with finite element analysis, and the results are summarized with suitable normalizations. It is found that no stresses arise if cooling ceases above the set-temperature, which defines the transition from viscous-dominated to elastic-dominated behavior; the set-temperature is determined principally by the dependency of viscosity upon temperature. Strategies for rapidly reaching low temperatures and avoiding high stresses are inferred from the results.     

Seasonal succession of phytoplankton in an ice-free pond warmed by a thermal power plant

In a pond receiving warmed cooling waters from a thermal power plant, the physical and chemical properties of the water, phytoplankton, periphyton and zooplankton were monitored on a weekly sampling schedule. In winter the phytoplankton growth was limited by poor light conditions. In mid-February a rapid phytoplankton growth started, simultaneously with increasing light energy, high nutrient concentrations and small herbivorous zooplankton populations. The increase of phytoplankton biomass was stopped by lack of free nutrients and silica at the end of March. From May until August the phytoplankton standing crop was mainly regulated by herbivorous zooplankton. The autumnal maximum of phytoplankton occurred with decreasing zooplankton populations, increasing nutrient concentrations, a turbulence favourable for diatoms and high water temperature.     

Impact assessment of nuclear and thermal power plants on zooplankton in cooling ponds

The current state of assessment of nuclear and thermal power plant toxic effects on zooplankton entrained in the water supply hardware is reviewed. Common research approaches and their inherent problems are considered and particular emphasis is given to specific features of cooling reservoirs and their ecosystems. Complex analysis of zooplankton mortality in various conditions and areas of a cooling reservoir is presented. Our data from field and laboratory studies allow major factors resulting in damage to and mortality of zooplankton to be determined and suggest that, among all plankton species in the cooling reservoir, zooplankton is the most sensitive group to entrainment-induced stress. Our results clearly demonstrate that both field observations and laboratory studies are required to reliably estimate power plant impact on zooplankton and to predict mortality of organisms.     

Can export of organic matter from estuaries support zooplankton in nearshore,marine plumes?

Marine and terrestrial ecosystems are connected via transfers of nutrients and organic matter in river discharges. In coastal seas, such freshwater outflows create prominent turbidity plumes. These plumes are areas of high biological activity in the pelagos, of which zooplankton is a key element. Conceptually, the increased biomass of zooplankton consumers in plumes can be supported by two alternative trophic pathways—consumption of fresh marine phytoplankton production stimulated by riverine nutrients, or direct trophic subsidies through the uptake of terrestrial and estuarine organic matter flushed to sea. The relative importance of these two pathways has not been established previously. Isotopic tracing (carbon and nitrogen) was used to measure the extent of incorporation of marine versus terrestrial matter into mesozooplankton consumers in the plumes off a small estuary in eastern Australia. Replicate zooplankton samples were taken during baseflow conditions with minimal freshwater influence to the sea, and during pulsed discharge events that generated turbidity plumes in coastal waters. Food sources utilized by zooplankton differed among locations and with the strength of freshwater flow. Terrestrial and estuarine carbon only made a sizeable contribution (47%) to the carbon demands of zooplankton in the lower estuary during pulsed freshwater flows. By contrast, in plumes that developed in nearshore marine waters, phytoplankton supplied up to 90% of the dietary carbon of zooplankton feeding in the plumes. Overall, it was “fresh” carbon, fixed by marine phytoplankton, the growth of which became stimulated by fluvial nutrient exports, that dominated energy flows in plume regions. The trophic role of terrestrial and estuarine organic exports was comparatively minor. The trophic dynamics of plankton in small coastal plumes is closely linked to variations in freshwater flow, but this coupling operates mainly through the enhancement of in-situ phytoplankton production rather than cross-boundary transfers of organic matter to marine food webs in the pelagos.     

The plankton community of an acid blackwater South Carolina power plant impoundment

Robinson Impoundment, an acid blackwater system in the South Carolina sandhills, serves as a cooling pond for a 854 MW power plant. It is divided by a causeway into an upper and lower impoundment. The upper impoundment is narrow, shallow, exhibits higher flow, and much more periphytic and rooted aquatic macrophyte vegetation. The lower impoundment is wide and deep with many sandy shoreline areas and little macrophyte vegetation.The zooplankton and phytoplankton communities of both the lower and upper impoundment were investigated over a three-year period (1980–1982) in an effort to determine power plant discharge effects on the plankton. Primary productivity rates were low, characteristic of an oligotrophic system. The phytoplankton community was dominated by the Bacillariophyceae and Chlorophyceae in cell density and the Chlorophyceae in species richness. The zooplankton community was dominated numerically by copepods, rotifers, and cladocerans, respectively. Principal zooplankton species includedDiaptomus mississippiensis, Bosmina longirostris, Diaphanosoma brachyurum, Keratella americana, K. cochlearis, Pompholyx sulcata, andConochiloides coenobasis. The rotifers were dominant in species richness, while crustacean zooplankton species richness was usually quite low. Statistical analyses performed using phytoplankton and zooplankton population variables indicated that in most cases upper impoundment densities were significantly lower (p < 0.05) than lower impoundment densities which were likely a function of the lotic upper impoundment habitat. No significant differences were found between the discharge area and the rest of the lower impoundment stations suggesting that localized power plant discharge effects did not occur during the study. Peak midsummer thermal discharge (36.0 ° C) may have caused temporary reductions inD. mississippiensis and several rotifers. Thermal stimulation during summer may have caused population increases for the filter-feeding cladocerans,B. longirostris andD. brachyurum.     

Cultivation of algae and nutrient removal in a waste heat utilization process

A process providing a beneficial use for waste heat and excess nutrients in the cooling waters of nuclear reactors and fossil-fueled power generating plants has been developed. The process involves the cultivation of selected strains of thermotolerant microalgae in heated discharge waters and the subsequent harvesting of the algal biomass for nutrient removal, recovery of energy and fertilizer, and extraction of high value products. The design of such a process is presented for a large cooling reservoir receiving a discharge of 10⁹1^?1 d^?1 of secondary cooling water containing 100 μg 1^?1 of available P and 400 μg 1^?1 of available N. Based on this nutrient load, with a 1% P content in the algal biomass and a productivity of 10 g m^?2 d ^?1, a 100 ha region would be needed for the process. Hydraulic barriers (submerged plastic curtains) would isolate the 100 ha algal production area “cultivation zone” in the influent end of the reservoir to create a hydraulic and thermal environment conductive to the selective growth of filamentous, thermotolerant, nitrogen-fixing, blue-green algae. The algal culture would be inoculated into the thermal plume and harvested near the distal barrier of the cultivation zone with rotating, backwashed, fine mesh screens (“microstrainers”). A portion of the harvested biomass would be recycled to the inoculation site to maintain a dense culture. This process could mitigate both thermal and nutrient loadings on receiving bodies of water.     

Plankton composition, abundance and dynamics in a severely stressed cooling reservoir

The effects of severe thermal stress imposed by the intermittentoperation of a nuclear reactor on plankton abundance and dynamicswere investigated in Pond C, a cooling reservoir on the SavannahRiver Site in South Carolina, USA. Temperatures in Pond C rangedup to 58°C during reactor operation. The thermal effluenteliminated zooplankton from regions where the temperature exceeded45°C, reduced zooplankton abundance by 1–3 ordersof magnitude and typically halved the number of taxa. Reactoroperation also reduced phytoplankton biovolume, often by >70%.During intermittent reactor operation, the rotifer Filinia longisetadominated the zooplankton and two cladocerans of the genus Moinawere abundant. These species were not abundant during extendedreactor shutdowns. The success of Filinia and Moina was dueprimarily to their tolerance of high temperatures. Sparse phytoplanktonprobably limited some zooplankton taxa, although other taxa,such as Filinia may have utilized bacterial resources. Reactoroperation may have intensified predation on crustacean zooplanktonwhen fish were concentrated in refuge areas with zooplankton.Processes by which zooplankton repopulated the reservoir afterreactor shutdown were inferred from zooplankton distributionpatterns, and population growth and birth rates. Repopulationtypically occurred within a few days due to rapid growth ofpopulations from refuge areas within the reservoir and colonistsbrought in through a tributary canal. Mechanisms of zooplanktonrepopulation in Pond C suggest that refuges or colonizationcorridors should be maintained when the re-establishment ofcommunities following cessation of stresses is desired.     

Elastic-plastic finite elements analysis of transient and residual stresses in ceramo-metal restorations

Transient and residual stresses occurring in partially fixed dental prostheses after the firing process can be calculated with elastic or elastic-plastic finite element analyses (FEA). In this study, firstly, the mechanical and thermal properties at various temperatures of the materials used in a porcelain fused metal (PFM) system were obtained by experimental and literature studies. The effects of viscoelastic and viscoplastic behaviours of the dental porcelain at the elevated temperatures were reflected onto its elastic properties. The equivalent heat transfer coefficients were determined experimentally by measuring temperatures and the results were supplied as input to the 3D finite elements analysis. It has been observed that the maximum stresses occur within a short time period after cooling begins and that stresses decrease during the cooling process and remain at a constant value at the end of cooling; these are the thermal residual stresses.     

Modelling the influence of copepod behaviour on faecal pellet export at high latitudes

The contribution of faecal pellet (FP) production by zooplankton to the downward flux of particulate organic carbon (POC) can vary from <1 % to more than 90 % of total POC. This results from varying degrees of interception and consumption, and hence recycling, of FPs by zooplankton in the upper mixed layers, and the active transport of FP to depth via diel vertical migration (VM) of zooplankton. During mid-summer at high latitudes, synchronised diel VM ceases, but individual zooplankton may continue to make forays into and out of the surface layers. This study considers the relative importance of different VM behaviours on FP export at high latitudes. We focussed on copepods and parameterised an individual-based model using empirical measures of phytoplankton vertical distribution and the rate of FP production, as a function of food availability. FP production was estimated under three different behaviours common to high-latitude environments (1) no VM, (2) foray-type behaviour and (3) synchronised diel VM. Simulations were also made of how each of these behaviours would be observed by an acoustic Doppler current profiler (ADCP). The model found that the type of copepod behaviour made a substantial difference to the level of FP export to depth. In the absence of VM, all FPs were produced above 50 m, where the probability of eventual export to depth was low. In foray-type scenarios, FP production occurred between 0 and 80 m, although the majority occurred between 30 and 70 m depth. Greatest FP production in the deeper layers (>70 m) occurred when diel VM took place. Simulated ADCP vertical velocity fields from the foray-type scenario resembled field observations, particularly with regard to the occurrence of positive anomalies in deeper waters and negative anomalies in shallower waters. The model illustrates that active vertical flux of zooplankton FP can occur at high latitudes even when no synchronised VM is taking place.     

Nutrients in precipitation and the phytoplankton responses to enrichment in surface waters of the Albemarle Peninsula,NC, USA after the establishment of a large-scale chicken egg farm

Nitrogen (N) and phosphorus (P) released in waste from animal feeding operations can stimulate phytoplankton biomass production in local receiving waters. Changes in weekly wet atmospheric N and P were measured from 2005 to 2008 at monitoring stations located 0.8, 7.9, and 10.3 km downwind from a new chicken egg production facility on the Albemarle Peninsula, North Carolina. After this farm began operating, there was a significant doubling in mean wet NH₄ ⁺ concentrations (465–1,022 μg l⁻¹) at 0.8 km with no change at the other sites. To measure the phytoplankton responses to nutrient enrichment, we conducted seasonal N and P enrichment bioassays from 2006 to 2008 on nearby Phelps Lake and Alligator River. These low-nutrient waters responded to nutrient additions, with the highest increases in phytoplankton primary productivity (¹⁴C uptake) and biomass (chlorophyll a) occurring in the combined N and P treatments suggesting co-limitation of N and P. Although we did not find an increased nutrient signal in precipitation at sites >0.8 km from the farm, there is the potential for atmospheric deposition of N to these and other waters located N/NE of the farm given prevailing winds and distance that NH₄ ⁺ aerosols can travel. Furthermore, surface runoff from the farm may impact receiving waters downstream (e.g., Pungo and Pamlico Rivers). In order to prevent excessive phytoplankton productivity and biomass both N and P inputs should be carefully assessed and potentially controlled in these nutrient-sensitive waters.     

Occurrence and pathogenicity of Naegleria fowleri in artificially heated waters.

The occurrence of pathogenic Naegleria fowleri in thermal discharges, recipient waters, and cooling towers of eight power plants located in western Pennsylvania was investigated for 2 years in conjunction with several environmental measurements. Pathogenic N. fowleri was detected in one cooling tower and in the discharge, receiving waters, or both of five of eight localities. The occurrence of this organism was related to elevated temperatures, but no significant correlation was found for other biological and chemical parameters. Laboratory experiments on the effect of pH on pathogenic N. fowleri documented 100% survival at a range from 2.1 to 8.15. Higher pH reduced or killed the amoebae. No case of human primary amoebic meningoencephalitis occurred during the study.     

Occurrence and pathogenicity of Naegleria fowleri in artificially heated waters

The occurrence of pathogenic Naegleria fowleri in thermal discharges, recipient waters, and cooling towers of eight power plants located in western Pennsylvania was investigated for 2 years in conjunction with several environmental measurements. Pathogenic N. fowleri was detected in one cooling tower and in the discharge, receiving waters, or both of five of eight localities. The occurrence of this organism was related to elevated temperatures, but no significant correlation was found for other biological and chemical parameters. Laboratory experiments on the effect of pH on pathogenic N. fowleri documented 100% survival at a range from 2.1 to 8.15. Higher pH reduced or killed the amoebae. No case of human primary amoebic meningoencephalitis occurred during the study.     

Ethylene Biosynthesis during Aerenchyma Formation in Roots of Maize Subjected to Mechanical Impedance and Hypoxia

Germinated maize (Zea mays L.) seedlings were enclosed in modified triaxial cells in an artificial substrate and exposed to oxygen deficiency stress (4% oxygen, hypoxia) or to mechanical resistance to elongation growth (mechanical impedance) achieved by external pressure on the artificial substrate, or to both hypoxia and impedance simultaneously. Compared with controls, seedlings that received either hypoxia or mechanical impedance exhibited increased rates of ethylene evolution, greater activities of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, ACC oxidase, and cellulase, and more cell death and aerenchyma formation in the root cortex. Effects of hypoxia plus mechanical impedance were strongly synergistic on ethylene evolution and ACC synthase activity; cellulase activity, ACC oxidase activity, or aerenchyma formation did not exhibit this synergism. In addition, the lag between the onset of stress and increases in both ACC synthase activity and ethylene production was shortened by 2 to 3 h when mechanical impedance or impedance plus hypoxia was applied compared with hypoxia alone. The synergistic effects of hypoxia and mechanical impedance and the earlier responses to mechanical impedance than to hypoxia suggest that different mechanisms are involved in the promotive effects of these stresses on maize root ethylene biosynthesis.     

Phenotypic plasticity and mechanical stress: biomass partitioning and clonal growth of an aquatic plant species

Mechanical stresses from wind, current or wave action can strongly affect plant growth and survival. Survival and distribution of species often depend on the plant's capacity to adapt to such stresses, particularly when amplified by climatic variations. Few studies have dealt with plastic adjustments in response to mechanical stress compared to resource stress. We hypothesized that mechanical stress should favor plastic adjustments that result in increased biomass production in zones protected from the stress and that altered growth patterns should be reversible after mechanical stress removal. Here we measured plastic adjustments in morphological traits and clonal architecture for an aquatic clonal species (Berula erecta) under two contrasting mechanical stresses in the field-standing vs. running water. Reversion of the morphological changes was then assessed using transplants in standing water. In the case of mechanical stress, size reduction, biomass reallocation within clones (higher allocations to clonal growth and to belowground organs), and a more compact growth form (reduced spacer lengths) contributed to reducing the damage risk. The removal of mechanical stress induced compensatory growth, probably linked to the production of low density tissues. However, most patterns of dry mass partitioning induced by current stress were not reversed after stress removal.     

Lactose Variability of Escherichia coli in Thermally Stressed Reactor Effluent Waters

Lactose-utilizing and nalidixic acid-resistant populations of Escherichia coli, having an optimum growth temperature of 37°C, were placed in modified diffusion chambers. The chambers were submerged in the epilimnion and hypolimnion of a 1,100-hectare lake (Par Pond) which receives cooling water from a nuclear production reactor. Control chambers were placed in a deep-water reservoir and a Flowing-Streams Laboratory, both of which had comparable temperatures to Par Pond. The populations of E. coli were sampled regularly for up to 3 weeks. Viability of the bacteria was determined by dilution plating to nutrient agar followed by replicate plating onto selective medium to determine lactose utilization and nalidixic acid sensitivity. Initial populations of E. coli were lactose positive but changed to lactose negative in Par Pond when the reactor was operating (i.e., cooling water from the heat exchangers was being discharged to the lake). This alteration occurred most rapidly in the chambers closest to the cooling-water discharge point. Such changes did not occur in a deep-water reservoir, in Par Pond when the reactor was not operating, or in the Flowing-Streams Laboratory. The nalidixic acid-resistant characteristic remained stable regardless of the chambers' placement or reactor operations. Although the reasons for such alterations are unclear, it appears that lactose-negative populations of E. coli are selected for in these reactor effluent waters. The loss of the lactose characteristic prevents the recognition and identification of E. coli in this cooling lake (when the reactor is operating) and may prevent the assessment of water quality based on coliform recognition.     

Hydrobiological consequences of the addition of phosphate precipitants to inlet water of lakes

Summary In three model reservoirs (LUND, 1975) a method reducing bluegreen algal blooms in lakes was studied. Iron or aluminium were added to inlet waters for chemically binding the inflowing phosphorus.The research program, started in 1975, includes intensive monitoring of many chemical and hydrobiological variables, the determination of water and mass balances and since 1977 measurements or primary production rates with¹⁴C. In this paper only the results found in 1977 are discussed. An attempt is made to describe quantitatively how growth rates and changes in biomass are interconnected and how phosphorus precipitation changes these variables.In all reservoirs a large discrepancy was observed between the actual rate of increase in the algal population and the relative production rate. The latter appeared to be higher by one order of magnitude. The relative death rate due to grazing can account for the large difference between these growth rates only when selective grazing of zooplankton on phytoplankton is assumed.It can be concluded that treatment of inlet water with AVR, an aluminium salt, is unsuccessful in reducing algal development. Treatment with ironsulphate may be successful, but a reduction of the relative growth rates was not observed. The effects of grazing of zooplankton andDreissena polymorpha need further investigation.     

Two hundred years of zooplankton vertical migration research

Vertical migration is a geographically and taxonomically widespread behaviour among zooplankton that spans across diel and seasonal timescales. The shorter-term diel vertical migration (DVM) has a periodicity of up to 1 day and was first described by the French naturalist Georges Cuvier in 1817. In 1888, the German marine biologist Carl Chun described the longer-term seasonal vertical migration (SVM), which has a periodicity of ca. 1 year. The proximate control and adaptive significance of DVM have been extensively studied and are well understood. DVM is generally a behaviour controlled by ambient irradiance, which allows herbivorous zooplankton to feed in food-rich shallower waters during the night when light-dependent (visual) predation risk is minimal and take refuge in deeper, darker waters during daytime. However, DVMs of herbivorous zooplankton are followed by their predators, producing complex predator–prey patterns that may be traced across multiple trophic levels. In contrast to DVM, SVM research is relatively young and its causes and consequences are less well understood. During periods of seasonal environmental deterioration, SVM allows zooplankton to evacuate shallower waters seasonally and take refuge in deeper waters often in a state of dormancy. Both DVM and SVM play a significant role in the vertical transport of organic carbon to deeper waters (biological carbon sequestration), and hence in the buffering of global climate change. Although many animal migrations are expected to change under future climate scenarios, little is known about the potential implications of global climate change on zooplankton vertical migrations and its impact on the biological carbon sequestration process. Further, the combined influence of DVM and SVM in determining zooplankton fitness and maintenance of their horizontal (geographic) distributions is not well understood. The contrasting spatial (deep versus shallow) and temporal (diel versus seasonal) scales over which these two migrations occur lead to challenges in studying them at higher spatial, temporal and biological resolution and coverage. Extending the largely population-based vertical migration knowledge base to individual-based studies will be an important way forward. While tracking individual zooplankton in their natural habitats remains a major challenge, conducting trophic-scale, high-resolution, year-round studies that utilise emerging field sampling and observation techniques, molecular genetic tools and computational hardware and software will be the best solution to improve our understanding of zooplankton vertical migrations.     

Environmental Effects of the Strategic Petroleum Reserve Program on Louisiana Continental Shelf Communities

In order to create solution cavities in coastal salt domes tostore the nation's Strategic Petroleum Reserves, it has beennecessary to dispose of up to a million barrels of concentratedbrine daily in coastal waters for several years. The naturalsystem receiving this effluent is characterized by a variablehydrographic regime and high but variable standing crops ofplankton, benthos and nekton. In the summer months the systemis periodically stressed by stratification of the water columnand bottom hypoxia. The brine effluent was found to remain nearthe bottom, achieve very rapid dilution, and impinge upon nomore than 40 km² of bottom. No demonstrable effects were notedon the sediments or phytoplankton populations, but effects wereobserved on the zooplankton, benthos, and nekton. The benthosdisplayed long-term cumulative impacts. Results of this studyare examined to provide a basis for designing future impactstudies in coastal waters.     

Lactose variability of Escherichia coli in thermally stressed reactor effluent waters.

Lactose-utilizing and nalidixic acid-resistant populations of Escherichia coli, having an optimum growth temperature of 37 degrees C, were placed in modified diffusion chambers. The chambers were submerged in the epilimnion and hypolimnion of a 1,100-hectare lake (Par Pond) which receives cooling water from a nuclear production reactor. Control chambers were placed in a deep-water reservoir and a Flowing-Streams Laboratory, both of which had comparable temperatures to Par Pond. The populations of E. coli were sampled regularly for up to 3 weeks. Viability of the bacteria was determined by dilution plating to nutrient agar followed by replicate plating onto selective medium to determine lactose utilization and nalidixic acid sensitivity. Initial populations of E. coli were lactose positive but changed to lactose negative in Par Pond when the reactor was operating (i.e., cooling water from the heat exchangers was being discharged to the lake). This alteration occurred most rapidly in the chambers closest to the cooling-water discharge point. Such changes did not occur in a deep-water reservoir, in Par Pond when the reactor was not operating, or in the Flowing-Streams Laboratory. The nalidixic acid-resistant characteristic remained stable regardless of the chambers' placement or reactor operations. Although the reasons for such alterations are unclear, it appears that lactose-negative populations of E. coli are selected for in these reactor effluent waters. The loss of the lactose characteristic prevents the recognition and identification of E. coli in this cooling lake (when the reactor is operating) and may prevent the assessment of water quality based on coliform recognition.