B. J. Cosby

Aggregating Fine‐Scale Ecological Knowledge to Model Coarser‐Scale Attributes of Ecosystems

As regional and global scales become more important to ecologists, methods must be developed for the application of existing fine-scale knowledge to predict coarser-scale ecosystem properties. This generally involves some form of model in which fine-scale components are aggregated. This aggregation is necessary to avoid the cumulative error associated with the estimation of a large number of parameters. However, aggregation can itself produce errors that arise because of the variation among the aggregated components. The statistical expectation operator can be used as a rigorous method for translating fine-scale relationships to coarser scales without aggregation errors. Unfortunately this method is too cumbersome to be applied in most cases, and alternative methods must be used. These alternative methods are typically partial corrections for the variation in only a few of the fine-scale attributes. Therefore, for these methods to be effective, the attributes that are the most severe sources of error must be identified a priori. We present a procedure for making these identifications based on a Monte Carlo sampling of the fine-scale attributes. We then present four methods of translating fine-scale knowledge so it can be applied at coarser scales: (1) partial transformations using the expectation operator, (2) moment expansions, (3) partitioning, and (4) calibration. These methods should make it possible to apply the vast store of fine-scale ecological knowledge to model coarser-scale attributes of ecosystems.

Time scales of catchment acidification

Recent research had focused on certain chemical processes in catchment soils. These processes were probable keys to the responses of surface water quality to acid deposition and included anion retention, weathering, of minerals as a source of base cations and aluminum, and alkalinity generation by dissociation of carbonic acid with the subsequent exchange of hydrogen ions for base cations. The unanswered question was how quickly and to what extent these processes controlled surface water quality changes in response to changes in atmospheric deposition of sulfuric acid. This paper discussed the quantitative model used to estimate patterns and absolute time scales of surface water acidification. The model was based on mathematical representations of those processes that were thought the preliminary catchment controls on acidification. The model was called MAGIC (model of acidification of groundwater in catchments). 16 references, 2 figures.

Ecological effects of nitrogen and sulfur air pollution in the US: what do we know?

Four decades after the passage of the US Clean Air Act, air-quality standards are set to protect ecosystems from damage caused by gas-phase nitrogen (N) and sulfur (S) compounds, but not from the deposition of these air pollutants to land and water. Here, we synthesize recent scientific literature on the ecological effects of N and S air pollution in the US. Deposition of N and S is the main driver of ecosystem acidification and contributes to nutrient enrichment in many natural systems. Although surface-water acidification has decreased in the US since 1990, it remains a problem in many regions. Perturbations to ecosystems caused by the nutrient effects of N deposition continue to emerge, although gas-phase concentrations are generally not high enough to cause phytotoxicity. In all, there is overwhelming evidence of a broad range of damaging effects to ecosystems in the US under current air-quality conditions.

Influence of net freshwater supply on salinity in Florida Bay

An annual water budget for Florida Bay, the large, seasonally hypersaline estuary in the Everglades National Park, was constructed using physically based models and long-term (31 years) data on salinity, hydrology, and climate. Effects of seasonal and interannual variations of the net freshwater supply (runoff plus rainfall minus evaporation) on salinity variation within the bay were also examined. Particular attention was paid to the effects of runoff, which are the focus of ambitious plans to restore and conserve the Florida Bay ecosystem. From 1965 to 1995 the annual runoff from the Everglades into the bay was less than one tenth of the annual direct rainfall onto the bay, while estimated annual evaporation slightly exceeded annual rainfall. The average net freshwater supply to the bay over a year was thus approximately zero, and interannual variations in salinity appeared to be affected primarily by interannual fluctuations in rainfall. At the annual scale, runoff apparently had little effect on the bay as a whole during this period. On a seasonal basis, variations in rainfall, evaporation, and runoff were not in phase, and the net freshwater supply to the bay varied between positive and negative values, contributing to a strong seasonal pattern in salinity, especially in regions of the bay relatively isolated from exchanges with the Gulf of Mexico and Atlantic Ocean. Changes in runoff could have a greater effect on salinity in the bay if the seasonal patterns of rainfall and evaporation and the timing of the runoff are considered. One model was also used to simulate spatial and temporal patterns of salinity responses expected to result from changes in net freshwater supply. Simulations in which runoff was increased by a factor of 2 (but with no change in spatial pattern) indicated that increased runoff will lower salinity values in eastern Florida Bay, increase the variability of salinity in the South Region, but have little effect on salinity in the Central and West Regions.

CHANGE IN THE ACID-BASE STATUS OF AN APPALACHIAN MOUNTAIN CATCHMENT FOLLOWING FOREST DEFOLIATION BY THE GYPSY MOTH

Infestation by the gypsy moth (Lymantria dispar) can alter biogeochemical conditions in affected catchments. Stream-water concentration data obtained over the period of 1980–1993 for White Oak Run, a stream in Shenandoah National Park, Va., indicate that change in catchment acid-base status is associated with forest defoliation by the moth larva. Stream-water concentration changes following defoliation included increasing concentrations of strong-acid anions, base-cations, and hydrogen ion, as well as decreasing concentrations of acid-neutralization capacity (ANC) and sulfate. The largest change was in the concentration of nitrate; annual discharge-weighted mean concentrations increased from predefoliation levels consistently less than 5 μeq L−1 to postdefoliation levels greater than 50 μeq L−1. An intensification of acidification was indicated by record-high hydrogen ion concentrations and record-low ANC concentrations. The long-term biogeochemical implications of the infestation are uncertain due to the nonlinearity of the observed responses and unknown patterns of recovery and recurrence.

Changes in acidification of lochs in Galloway, southwestern Scotland, 1979–1988: The MAGIC model used to evaluate the role of afforestation, calculate critical loads, and predict fish status

Abstract Decades of acid deposition in the Galloway area, southwestern Scotland, have resulted in acidification of surface waters and damage to fish. In the period since 1980, however, acidic deposition has decreased substantially. A survey of 50 lochs conducted in 1979 and repeated in 1988 reveals major changes in water chemistry over this 9 year period. Together these two data sets separated in time by 9 years and covering a period of relatively large and rapid change in acid deposition offer a valuable basis for the evaluation of acidification models. Concentrations of SO 4 in the lochs were on the average 42% lower in 1988 relative to 1979. The decline is readily explained by the large and rapid decline in sulphate concentrations in precipitation in the area. Concentrations of non-marine base cations decreased from 155 to 90 μequiv1 −1 and acid neutralising capacity (ANC) increased. The change in ANC was due mostly to decreased concentrations of Al. pH levels showed no systematic change from 1979 to 1988. The regional changes in water chemistry over the period 1979–1988 are corroborated by regular measurements at several of these lochs over this 9 year period. MAGIC (Model for Acidification of Groundwater In Catchments) successfully reproduces the major changes in water chemistry observed over the period 1979–1988. Both calibration to the 1979 data with prediction of 1988, and calibration to the 1988 with reconstruction of 1979 give close fits to the observations. The model provides a means by which the future impact of acidic deposition and afforestation can be evaluated. A fish response function coupled to MAGIC provides the basis for evaluation of past and future fish status in the region. MAGIC predictions under different acid deposition and forestry scenarios indicate that if acidic deposition is held constant at 1988 levels, afforestation causes further acidification of the lochs. Acidic deposition emerges as the major cause of soil and water acidification in the Galloway region, although forestry practices can exacerbate the effects.

Nitrogen, organic carbon and sulphur cycling in terrestrial ecosystems: linking nitrogen saturation to carbon limitation of soil microbial processes

Elevated and chronic nitrogen (N) deposition to N-limited terrestrial ecosystems can lead to ‘N saturation’, with resultant ecosystem damage and leaching of nitrate (NO3−) to surface waters. Present-day N deposition, however, is often a poor predictor of NO3− leaching, and the pathway of the ecosystem transition from N-limited to N-saturated remains incompletely understood. The dynamics of N cycling are intimately linked to the associated carbon (C) and sulphur (S) cycles. We hypothesize that N saturation is associated with shifts in the microbial community, manifest by a decrease in the fungi-to-bacteria ratio and a transition from N to C limitation. Three mechanisms could lead to lower amount of bioavailable dissolved organic C (DOC) for the microbial community and to C limitation of N-rich systems: (1) Increased abundance of N for plant uptake, causing lower C allocation to plant roots; (2) chemical suppression of DOC solubility by soil acidification; and (3) enhanced mineralisation of DOC due to increased abundance of electron acceptors in the form of

A preliminary model of long-term changes in stream acidity in southwestern Scotland

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