Ecological effects of sulfur dioxide,fluorides, and minor air pollutants: recent trends and research needs

The regulation of the emissions of 'traditional' primary air pollutants (fluorides, sulfur dioxide) has changed the pattern of exposure of ecological systems, with greatly reduced exposure close to sources, but with a smaller effect in some remote areas. Measurements show that recovery is occurring at some sites, in fresh water chemistry (reduced acidity) and in sensitive biota (sustainable fish populations). However, the pattern of change in exposure has not always been simply related to emission reductions. An understanding of responses to recent changes will improve our predictions of the response to future emission changes, both locally and globally. As exposure to 'traditional' pollutants is reduced, the potential for other pollutants to have effects becomes more evident. In the aqueous phase, we need to understand the role of soluble and suspended organics, but this also means explicit recognition of the possibility of phase exchange, and the role of photolytic reactions on plant, soil, and water surfaces. Do highly reactive free radicals in the atmosphere, formed by the action of sunlight on volatile organic compounds, have direct effects on plants? Organic compounds and heavy metals may be bioactive as gases and particles, but for many potentially toxic compounds, the experimental evidence for biotic response is very limited. To evaluate the potential effects of pollutants, we need to understand the pathways by which airborne pollutants enter and react within ecosystems. For vegetation, we have to consider bidirectional fluxes, and distinguish among uptake through stomata, through leaf surfaces, or through roots. There are several challenges for the future. (1) Can we devise experiments that permit exposure of vegetation to gases, particles, and/or aqueous pollutants at 'realistic' concentrations? (2) Can we include the potential interactions with photolytically derived free radicals, and the dynamics of exchange? (3) How do we allow for responses to pollutant mixtures, or the simultaneous exposure to pollutants in gas, particle, and aqueous phases? The recognition of the importance of the dynamic exchange of pollutants between phases will be the key to the development of effective experimental approaches to evaluating cause-effect relationships between pollutant mixtures and ecosystem responses.     

Seasonal profiles of leaf ascorbic acid content and redox state in ozone-sensitive wildflowers

Cutleaf coneflower (Rudbeckia laciniata L.), crown-beard (Verbesina occidentalis Walt.), and tall milkweed (Asclepias exaltata L.) are wildflower species native to Great Smoky Mountains National Park (U.S.A.). Natural populations of each species were analyzed for leaf ascorbic acid (AA) and dehydroascorbic acid (DHA) to assess the role of ascorbate in protecting the plants from ozone stress. Tall milkweed contained greater quantities of AA (7-10 micromol g(-1) fresh weight) than crown-beard (2-4 micromol g(-1) fresh weight) or cutleaf coneflower (0.5-2 micromol g(-1) fresh weight). DHA was elevated in crown-beard and cutleaf coneflower relative to tall milkweed suggesting a diminished capacity for converting DHA into AA. Tall milkweed accumulated AA in the leaf apoplast (30-100 nmol g(-1) fresh weight) with individuals expressing ozone foliar injury symptoms late in the season having less apoplast AA. In contrast, AA was not present in the leaf apoplast of either crown-beard or cutleaf coneflower. Unidentified antioxidant compounds were present in the leaf apoplast of all three species. Overall, distinct differences in antioxidant metabolism were found in the wildflower species that corresponded with differences in ozone sensitivity.     

Atmospheric nitrogen compounds-issues related to agricultural systems

Workable pollution abatement policies and effective legislation must be based on sound science. However, despite many years of research, there are still uncertainties about the effects of atmospheric nitrogen compounds on crops and other vegetation. This paper reviews the current state of knowledge of the main compounds, focussing on the concentrations and combinations of pollutants that occur in rural areas. The sources, concentrations and effects of oxidised, reduced and organic nitrogen compounds are considered in turn, then the effects of deposited nitrogen on ecosystems are discussed. Research priorities on the effects of deposited nitrogen in Europe and the USA are compared. Finally, the review leads to a list of issues for discussion and recommendations for research.     

Regulation of growth in Laminaria digitata: use of in-vivo nitrate reductase activities as an indicator of nitrogen limitation in field populations of Laminaria spp.

The seasonal variation in growth rate of a population of Laminaria digitata (Huds.) Lamour growing at Arbroath, Scotland was studied between August 1981 and September 1982, and was found to follow the biphasic annual cycle typical of this genus. Growth rates were maximum (0.3 cm cm^-1 mo^-1) in early June and minimum (0.05 cm cm^-1 mo^-1) between September and January. An analysis of the relationship between the seasonal changes in environmental factors (inorganic nitrogen concentrations, irradiance and temperature) with those of growth rate and the accumulation or mobilisation of cellular reserves of carbohydrates and nitrate, indicated that growth was nitrogen-limited between June and October and light-limited (with a possible co-involvement of temperature) for the remainder of the year. These conclusions were supported by the seasonal changes in the ratio of actual: potential in-vivo nitrate reductase activities in L. digitata, thus confirming the suitability of this technique for monitoring the occurrence of nitrogen limitation in Laminaria spp. The seasonal changes in blade nitrate reductase activities closely followed those of growth rate, with maximum activities [0.3 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] being present in late May and minimum levels [0.01 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] occurring between November and March. The correlation observed between nitrate reductase activities and growth rate is consistent with the ability of Laminaria spp. to store excess inorganic nitrogen, available during winter and early spring, as NO ₃ ^- , and with the requirement to conserve enzyme protein during the summer period of nitrogen limitation.     

Freezing rate and duration determine the physiological response of intertidal fucoids to freezing

Differential thermal analysis (DTA) was used to measure the freezing temperature of nine species of red brown intertidal macroalgae from the coast of Maine, USA in 1991. Using slow and rapid cooling rates approximating those found in the field for Ascophyllum nodosum (L.) Le Jol. we found that, for a given rate, the freezing points of all species were similar: -7.06 to -8.02°C for slow cooling (ca. 0.25°C min^-1) and -3.42 to -4.56°C for rapid cooling (ca. 5.0°C min^-1). In the low shore species, Fucus evanescens C. Ag., photosynthesis was inhibited to a greater extent when plants were frozen or thawed rapidly than after slow freezing or thawing. However, in the upper shore species, F. spiralis (L.), photosynthesis recovered rapidly and completely regardless of freezing rate. Rapidly frozen F. evanescens also experienced greater loss of plasmalemmal integrity, evidenced by a greater loss of cellular contents on re-immersion, than those frozen slowly. Light-limited photosynthesis following freezing was more severely inhibited than light-saturated photosynthesis. Respiration was generally enhanced immediately after freezing, but then declined to rates below those of unfrozen controls within 2 h following re-immersion, with control rates of respiration being achieved after a 24 h recovery period. Our data suggest that the physiological consequences of winter emersion at sub-zero temperatures may vary widely between individual plants of freezing-susceptible species, due to the wide variations in freezing rate associated with microhabitat effects.     

Reclamation of acid waters using sewage sludge

An exhausted sand quarry which had filled with acid water (pH 3) from the oxidation of pyrite was treated with calcium hydroxide to neutralize the water (pH 8), and sewage sludge to prevent further ingress of acid. The water remained neutral for 2 years, an appreciable quantity of base being generated by the reduction of sulphate to sulphide in the anoxic sediment formed by the sewage sludge. After this time the water reverted to acid conditions, chiefly because the lake was too shallow to retain the sewage sludge over a sufficiently large area of its bed. Incubation experiments showed that the sewage sludge had a large capacity for sulphate reduction, which was equally efficient in acid or neutral waters and that the areal rate of consumption was sufficiently fast to neutralize all incoming acid, if at least 50% of the lake bed was covered with sludge. Throughout the course of the field investigations there was no foul smell and the lake was quickly colonized by phytoplankton, macrophytes and insects. Although nutrients associated with the sewage sludge stimulated photosynthesis and so caused the generation of additional organic matter, they were exhausted within two years. To ensure permanent reclamation, phosphate fertilizer could be added once the initial supply has been consumed. Neutralization removed trace metals from the system, presumably due to formation of insoluble oxyhydroxide and carbonates. The solubility of aluminium was apparently controlled by a basic aluminium sulphate (jurbanite).     

Persistent stimulation of CO2 assimilation and stomatal conductance by summer ozone fumigation in Norway spruce

CO(2) assimilation rate, stomatal conductance and chlorophyll content of current and previous years' needles of Norway spruce were measured in May 1988, 205 days after the cessation of ozone fumigation during the summer of 1987. Rates of assimilation were consistently higher for both needle year age classes for ozone fumigated trees in comparison to control trees, although only statistically significant for part of the day for current year's needles. A 26% and 48% stimulation, overall, in mean daily rates of assimilation for current and previous years' needles of ozone fumigated trees was observed. This was due to an enhanced apparent quantum yield and light saturated rate of assimilation of ozone fumigated trees. The temperature response regression of assimilation versus temperature was also greater, such that at any given temperature, assimilation was higher for ozone treated trees than control trees. Stomatal conductance was greater for ozone fumigated trees than the controls, but this was only marginally statistically significant. Moreover, there was a consistent increase in chlorophyll content in both year classes in ozone-treated trees. These results are discussed in relation to a possible long term effect of ozone fumigation upon the processes of conifer winter hardening and spring de-hardening.     

Fluoride loading of a lepidopteran larva (Pieris brassicae) fed on treated diets

Experiments to assess the impact of dietary fluoride on larvae of Pieris brassicae were designed to substantiate field observations that at polluted sites little fluoride is absorbed by insect larvae and that most of the body load is caused by surface contamination. Treatment with fluoride as HF or AlF3 had no effect on pupal weight of Pieris and we conclude that fluoride, at concentrations likely to be found on vegetation at polluted sites, is unlikely to have a direct impact on the growth of chewing herbivores. Fluoride in the gut formed the biggest component of the total body load. A significant amount of fluoride was detected in exuviae, but resulted from surface contamination. When larvae from polluted food were transferred to untreated food it took several days for the fluoride content of the faeces to return to background levels.