S.W. Christensen

Effects of Acidification on Biological Communities in Aquatic Ecosystems

Extensive information is available on the effects of acidification on biological communities in aquatic ecosystems. Whole-system experiments, meso-cosm experiments, and field surveys have demonstrated major shifts in species composition and decreases in species richness with increasing acidity. Sensitive species may be lost even at moderate levels of acidity. For example, effects on important zooplankton predators, such as Mysis relicta and Epischura lacustris, occur at pH 5.6 to 5.9; acid sensitive mayfly and stonefly species (e.g., Baetis lapponicus) are affected at levels near 6.0; and sensitive fish species, such as the fathead minnow (Pime-phales promelas), experience recruitment failure and extinction at pH 5.6 to 5.9. At the same time, however, acid tolerant species may appear or increase in abundance, resulting in little or no overall decrease in standing crop. Attached filamentous algae, especially Mougeotia spp., often become visibly more abundant in acidified lakes and streams. Many ecosystem level processes (primary productivity, rates of decomposition, and nutrient concentrations) also appear relatively robust and unaffected at least at moderate levels of acidification (pH 5.0 to 5.6). Specific mechanisms for effects of acidification on aquatic biota have been studied particularly for fish communities. The toxicity of acidic waters to fish is determined primarily by pH, calcium, and inorganic monomeric aluminum. Fish population declines associated with acidification may result most commonly from recruitment failure. Relatively few quantitative models are available for predicting biological responses to acidification; most deal with effects on fish populations. All these models require further development and testing prior to their use for regional assessments of effects from acidic deposition.1

Stability and return times of Leslie matrices with density-dependent survival: applications to fish populations

Abstract A Leslie matrix population model is used to predict the stability and times of return to equilibrium of fish populations following perturbations. The model is compensatory, having density dependence in the young-of-the-year (y-o-y) survival term. We first present conditions under which a unique equilibrium point exists for a population. Then, through stability analysis, we derive conditions that the y-o-y survival function must satisfy for stability to be assured. It is demonstrated that the dominant eigenvalue of the linearized Leslie matrix can be used to calculate, to good approximation, the time of return to equilibrium of the population following a perturbation. The return times of five model fish populations are compared over a range of assumed strengths of the compensatory mechanism. The model of the Hudson River striped bass population, in which the peak egg contribution occurs in age class 7, has a slower return time than that of an Atlantic menhaden population, where the peak egg contribution is in age class 2.

Individual-based modeling of populations with high mortality: A new method based on following a fixed number of model individuals

Abstract Individual-based modeling of populations that undergo high mortality can be problematic Large numbers of model individuals must be followed to ensure adequate numbers of survivors at the end of the simulation, but following large numbers of individuals can require excessive computer memory and computational time. Following a sample of individuals from the population partially addresses these problems for short-term simulations but not for model applications requiring long-term predictions. In this paper, we describe a resampling algorithm that permits the long-term simulation of populations undergoing high mortality. A fixed number of model individuals are followed, with each representing some number of identical population individuals. As each model individuals dies, a donor individual by the donor individual is adjusted to represent the loss of individuals due to mortality. The dead individuals attributes are then replaced with those of the donor individual. The high accuracy, reduced memory requirement, and comparable computational costs of the resampling algorithm are demonstrated using an individual-based population model of young-of-the-year striped bass. Differences between predictions without and with resampling were

Linking life history theory, environmental setting, and individual-based modeling to compare responses of different fish species to environmental change

Abstract We link life history theory, environmental setting, and individual-based modeling to compare the responses of two fish species to environmental change, Life history theory provides the framework for selecting representative species, and in combination with information on important environmental characteristics, it provides the framework for predicting the results of model simulations. Individual-based modeling offers a promising tool for integrating and extrapolating our mechanistic understanding of reproduction, growth, and mortality at the individual level to population-level responses such as size-frequency distributions and indices of year-class strength. Based on the trade-offs between life history characteristics of striped bass Morone saxatilis and smallmouth bass Micropterus dolomieu and differences in their respective environments, we predicted that young-of-year smallmouth bass are likely to demonstrate a greater compensatory change in growth and mortality than young-of-year striped bas...

On the Ability to Detect the Influence of Spawning Stock on Recruitment

Abstract Simulated observations of spawning stock size, recruitment, and two random environmental variables were obtained from a density-independent Leslie matrix model. Recruitment to Age 1 was directly proportional to population fecundity but strongly influenced by the effects of the random environmental variables. The simulated observations were subjected to multiple regression analysis which detected the influence of the random environmental variables but did not reliably detect the influence of spawning stock. These results indicate that multiple regression is unreliable in detecting the influence of stock on recruitment when annual variations in recruitment are primarily due to environmental factors.

Fish population losses from Adirondack Lakes: The role of surface water acidity and acidification

Changes over time in the species composition of fish communities in Adirondack lakes were assessed to determine (1) the approximate numbers offish populations that have been lost and (2) the degree to which fish population losses may have resulted from surface water acidification and acidic deposition. Information on the present-day status offish communities was obtained by the Adirondack Lakes Survey Corporation, which surveyed 1469 Adirondack lakes in 1984–1987 (53% of the total ponded waters in the Adirondack ecological zone). Two hundred and ninety-five of these lakes had been surveyed in 1929–1934 during the first statewide biological survey; 720 had been surveyed in one or more years prior to 1970. Sixteen to 19% of the lakes with adequate historical data appeared to have lost one or more fish populations as a result of acidification. Brook trout and acid-sensitive minnow species had experienced the most widespread effects. Populations of brook trout and acid-sensitive minnows had been lost apparently as a result of acidification from 11% and 19%, respectively, of the lakes with confirmed historical occurrence of these taxa. By contrast, fish species that tend to occur primarily in lower elevation and larger lakes, such as largemouth and smallmouth bass and brown trout, have experienced little to no documented adverse effects. Lakes that were judged to have lost fish populations as a result of acidification had significantly lower; pH and, in most cases, also had higher estimated concentrations of inorganic aluminum and occurred at higher elevations than did lakes with the fish species still present. No other lake characteristics were consistently associated with fish population losses attributed to acidification. The exact numbers and proportions of fish populations affected could not be determined because of limitations on the quantity and quality of historical data. Lakes for which we had adequate historical data to assess long-term trends in fish communities were significantly larger and deeper and have higher pH than do Adirondack lakes in general; thus, fish communities adversely affected by acidification and acidic deposition may be underrepresented in this study.

Maryland Striped Bass: Recruitment Declining below Replacement

Abstract A mathematical technique was developed to examine interrelationships among first-year survival rates, adult fecundity, and adult survival of striped bass Morone saxatilis based on indices of year-class strength. Application of this technique to striped bass in Maryland waters of the Chesapeake Bay provided evidence for reduced survival in the life cycle. If adult fecundity and survival have remained constant, first-year survival declined significantly from 1969 to 1983, and averaged less than that needed for replacement for the last 10 years. Treatment of the individual spawning grounds separately indicated that the downward trend in survival for the pooled data was the result of declines in the upper bay and, to a lesser extent, in the Choptank River. Alternatively, if first-year survival and adult fecundity were assumed to have remained constant, an annual decline of about 1.9% in adult survival would have been required to produce the observed trend in the pooled year-class data. This would be ...

An Empirical Methodology for Estimating Entrainment Losses at Power Plants Sited on Estuaries

Abstract A model based on empirically derived age-, time-, and space-variant entrainment susceptibility data may be used for estimating conditional entrainment mortality of aquatic organisms, particularly fish and shellfish, caused by operation of one or more power plants on an estuary. Model application requires knowledge of the morphometry of the water body, the power-plant flow rates, the probability of entrainment survival, and the duration, distribution, and abundance of entrainable age-groups. A novel feature of the model is that organism distribution and movement within the model are defined by information derived from field samples rather than by hydrodynamic principles and equations.

An Overview of ARM Program Climate Research Facility Data Quality Assurance.

We present an overview of key aspects of the Atmospheric Radiation Measurement (ARM) Program Climate Research Facility (ACRF) data quality assurance program. Processes described include instrument deployment and cali- bration; instrument and facility maintenance; data collection and processing infrastructure; data stream inspection and as- sessment; problem reporting, review and resolution; data archival, display and distribution; data stream reprocessing; en- gineering and operations management; and the roles of value-added data processing and targeted field campaigns in speci- fying data quality and characterizing field measurements. The paper also includes a discussion of recent directions in ACRF data quality assurance. A comprehensive, end-to-end data quality assurance program is essential for producing a high-quality data set from measurements made by automated weather and climate networks. The processes developed dur- ing the ARM Program offer a possible framework for use by other instrumentation- and geographically-diverse data col- lection networks and highlight the myriad aspects that go into producing research-quality data.

Potential importance of spatial and temporal heterogeneity in pH, Al, and Ca in allowing survival of a fish population: A model demonstration

Abstract Discrepancies usually exist between the results of bioassay studies and the status of natural fish populations. One cause of such discrepancies is the spatial and temporal heterogeneity in the stressing factors or contaminant concentrations in natural systems. To provide a means of relating laboratory and field bioassay results to natural populations, we have modified an existing Monte Carlo mathematical model to track the movement and survival of fish in a body of water, represented by a two-dimensional grid, subject to acidification stress. We assumed that the fish (adults or the less mobile and generally more sensitive early life stages) are able to sense realistic gradients of pH, aluminium (Al), and calcium (Ca) (or alkalinity) and to move to reduce the chemical stress. We have also considered the effects of food availability and habitat preference on fish movement. Each fish takes one spatial ‘step’ at a time, the direction being randomly selected but also biased by the above factors. A function for accumulation and repair of damage is implemented within the model to relate mortality to the variable exposure history the fish accumulate by moving in the chemically heterogeneous environment. Using the model, we evaluated the influence of the strength and sensitivity of fish avoidance behavior, the presence or absence of the refuge, and the timing of multiple pulses on fish survival. The results highlight the potential importance of a chemically heterogeneous natural environment in allowing a population to survive under average water-quality conditions that laboratory bioassays suggest should prohibit survival. Laboratory and field behavior experiments, including characterization of water chemistry on a microscale in the field, are suggested to explore this phenomenon further.