The role of periphyton in mediating the effects of pollution in a stream ecosystem.

The effects of pollutants on primary producers ramify through ecosystems because primary producers provide food and structure for higher trophic levels and they mediate the biogeochemical cycling of nutrients and contaminants. Periphyton (attached algae) were studied as part of a long-term biological monitoring program designed to guide remediation efforts by the Department of Energy's Y-12 National Security Complex on East Fork Poplar Creek (EFPC) in Oak Ridge, Tennessee. High concentrations of nutrients entering EFPC were responsible for elevated periphyton production and placed the stream in a state of eutrophy. High rates of primary production at upstream locations in EFPC were associated with alterations in both invertebrate and fish communities. Grazers represented >50% of the biomass of invertebrates and fish near the Y-12 Complex but <10% at downstream and reference sites. An index of epilithic periphyton production accounted for 95% of the site-to-site variation in biomass of grazing fish. Analyses of heavy metals in EFPC periphyton showed that concentrations of zinc, cadmium, copper and nickel in periphyton decreased exponentially with distance downstream from Y-12. Zinc uptake by periphyton was estimated to reduce the concentration of this metal in stream water approximately 60% over a 5-km reach of EFPC. Management options for mitigating eutrophy in EFPC include additional reductions in nutrient inputs and/or allowing streamside trees to grow and shade the stream. However, reducing periphyton growth may lead to greater downstream transport of contaminants while simultaneously causing higher concentrations of mercury and PCBs in fish at upstream sites.

The contribution of crassulacean acid metabolism to the annual productivity of two aquatic vascular plants.

Net annual productivity and annual carbon budgets were determined for populations of Littorella uniflora var. americana and Isoetes macrospora in a mesotrophic and oligotrophic lake in northern Wisconsin, to assess the contribution of Crassulacean Acid Metabolism (CAM) to annual productivity of the species in their natural environment. Nocturnal carbon accumulation (CAM), daytime uptake of external CO2 via the C3 mechanism, and refixation of endogenously generated CO2 from daytime respiration were the sources of carbon income. CAM activity as diurnal acid rhythms reached maxima of 89 to 182 µeq·g-1 leaf fresh weight for the various populations.Maximum rates of daytime 14C uptake ranged from 0.56 to 1.46 mg C·g-1 leaf dry wt.·h-1 for the study populations. Refixation of daytime respired CO2 averaged 37% for the four populations. Carbon loss was due largely to "dark" respiration, during the day and night. Nocturnal carbon accumulation, daytime CO2 uptake and 24-h dark respiration were of similar magnitude, indicating dark respiration was equivalent to ∼50% of gross photosynthesis.Net annual production was measured for each population by following leaf turnover. Turnover rates for the Littorella populations were 1.56 and 1.72·yr-1, and for the Isoetes populations, 0.85 and 1.00·yr-1. Measured net annual productivity and calculated net annual productivity (based on carbon exchange) agreed within an average of 12% for the four populations. While CAM activity was greater for the more productive population of each species, the results suggest that the contribution of CAM to annual productivity is greater for the less productive population of each species. CAM contributed 45 to 55% of the annual carbon gain for the study populations.

Seasonal diurnal acid rhythms in two aquatic crassulacean acid metabolism plants.

The diurnal change in titrable acidity in two aquatic CAM plants, Littorella uniflora var. americana (Fern.) Gl. and Isoetes macrospora Duriev., growing at two sites, was monitored at biweekly intervals for two years during the ice-free period. Both plants exhibited the classic pattern of CAM acitivity, with deacidification 60 to 90% complete by noon. The maximum diurnal acid rhythms observed were 169±7, and 154±20 µeq·g-1 fresh weight for the two Littorella populations, and 182±9 and 133±16 µeq·g-1 fresh weight for the two Isoetes populations. The seasonal pattern of the diurnal acid rhythm was correlated with temperature and light. The maximum activity occurred in midsummer, with negligible activity under ice cover. This pattern was similar to that of terrestrial CAM plants from non-arid environments. Comparison of CAM activity for populations of the same species indicates that the magnitude of CAM activity is closely related to plant productivity, and appears to be related to light and perhaps CO2 availability. In these plants, CAM extends the diel period of carbon accumulation and contributes 40 to 50% of the annual carbon gain. The prolonged period of carbon acquisition and effective conservation of respired CO2 via CAM is of paramount importance in the growth and productivity of these plants in oligotrophic environments.