Effects of Macrobenthos on Sediment Structure and Contaminants
Presiding: S J Bentley, Louisiana State University; L J Thibodeaux, Louisiana State University; D D Reible, University of Texas at Austin
B13A-01 INVITED 13:30h
Through A Glass Darkly: Observations On The Chasm Between Basic And Applied Research In Bioturbation
Concepts about the structure and function of marine ecoystems have always been limited by either the artificial conditions of laboratory studies or the biases imposed by the sampling technology employed in the field. Given the same system, same sediments, same animals, and same concerns, basic research generally asks very different questions compared to those of applied research. Looking back over the past 150 years of research in benthic ecology provides a sobering insight that, despite the use of new technologies in bioturbation research, we are still traveling down the same roads leading to biased judgments. While it may be impossible to escape Heisenberg's reality, some rules of the road will be presented to help bridge the gap between basic and applied research in bioturbation processes.
B13A-02 13:45h
A Magnitite Tracer Protocol for Seasonal Measurement of Bed Sediment Biodiffusion Coefficients
Anticipating its use in contaminant remediation this paper describes the development of a particle tracer technique for making short-term,in situ measurements of aquatic bed sediment biodiffusion coefficients. The bioturbation process in the upper sediment layers of streams, lakes, estuaries and the marine environment moves particles and porewater. When present, organic chemcals, metals, colloids, etc., are transported across these layers and exchanges may occur at the sediment water interface. Fickian biodiffusion coefficients that characterize such particle movements,Db (cm2/yr), are used for assessing chemical diagenesis rates and contaminant fluxes and are specific to each site. Chemical fate and transport models, developed for tracking contaminants in aquatic systems, requite a method of measuring in situ Db numerical values. The protocols should be able to detect seasonal and other time or position dependent variations in the coefficients for improved model predictions. Magnetite, a tracer with a long history of use in scientific studies of porous media, particle transport, etc., was chosen. Unlike most tracers it is inexpensive, naturally occuring and readiy available. The steps of the protocol include deployment and retrieval of the tracer, magnitite sepatation and measurement, mathematical model interpretation, an statistical treatment of data. Application of the techinque was tested on South Capitol Lake, a manmade, freshwater lake located in Baton Rouge, Louisiana. The surficial sediment in the lake was found to contain a fairly large population of oglichaete worms in abundances of approximately 18,000 per/m2. Field deployments were conducted in December 2001, January and February 2002, giving biodiffusion coefficients of 0.95, 0.46 and 0.66 cm2/yr. Although the protocol was capable of in situ measurements, testing over one or more calendar years at this and other sites will be needed to determine if the protocol can be used to detect changes in Db with season of the year.
B13A-03 14:00h
Scaling of Microbial Competence for Sediment Remediation
Reliable characterization of the spatial distribution of sediment site attributes, such as contaminant concentrations or microbial activity depends on how well sampled values represent all values throughout the entire study site. In addition to the reliability of samples described statistically, the physical scale of the samples may further introduce uncertainties. Whereas geostatistical tools have been developed to interpolate the attribute values in space, these do not explicitly take into account the uncertainties associated with the various scales (field, lab, mesocosm) at which the data have been collected. Hence, a model to evaluate uncertainties arising from the various sampling scales, is required to properly sample and interpret data from large sites such as contaminated sediments. Here, we describe a statistical model to optimize the reliability of sampled data on a multi-scale basis. The model not only serves as a tool to evaluate relationships over different scales by their covariances, but also make further use of these covariances as basis for a precision-optimized estimator. Unlike conventional geostatistic tools which are based on the point-to-point spatial structures, the multi-scale model introduces a new framework for spatial analysis in which regional values at different scales are anchored by the correlations of each other. The model is developed using least-squares optimization for estimations of different scales, by which the estimation variance can be evaluated for the estimation of all scales combined. The estimation by the new model is expected to have less unfavored smoothing effect than the conventional kriging approaches in the neighborhood of sampled locations. Preliminary results from a comparison to indicator kriging of a spatial dioxin dataset from the Passaic River indicate that both estimation models agree with regions with lower values, while the multi-scale model preserves the features in the regions where hot-spot measurements exist. Information on the smallest scale appropriate for the dataset is also honored by using the multi-scale model, while kriging approaches gives artificial extrapolation at the near-distance variation despite the sampling scheme of the data set. Uncertainties introduced by sampling equipments can subsequently be analyzed by the multi-scale model after the evaluation of estimation uncertainties completes, to meet the practical end of the model developed. Two evaluation tools for spatial estimation models, cross-validation and jackknifing, are performed on both the multi-scale model and the conventional kriging approach in order to assess the competence of the developed model. Both evaluation tools are similar in concept in that subsamples are removed from the original data set to be estimated by the rest of data points, while focusing differently on either the overall estimation performance or the regional estimation capability. The comparison, using the Passaic River dataset, will be used to inform the applicability and objectivity for both models.
B13A-04 14:15h
Bioturbation and Bioaccumulation: Defining Access and Availability at the Sediment-water Interface
Traditionally, the transport and fate of hydrophobic contaminants at the sediment-water interface has largely been assumed to be controlled by sediment erosion and resuspension processes. Many sediment contaminants, however, lie in stable depositional environments where exposure and risk is defined by bioturbation, the mixing activities of benthic organisms, and bioaccumulation, the uptake of contaminants by these organisms, causing both direct toxic effects and indirect effects through the food chain. The observational field evidence indicating the importance of these processes has recently been reported by Thibodeaux (2005). This presentation will focus on efforts to understand and quantify these processes through experiments and modeling. Estimation of the depth and intensity of organism mixing processes and the relationship of desorption phenomena to accumulation in these organisms will be discussed. The coupling of physicochemical and biological processes will be emphasized. Pore water concentrations will be identified as a key indicator of exposure and risk. Deterministic models showing promise at predicting the dynamics of pore water concentrations and stochastic models reproducing some key characteristics of benthic community behavior will be developed.
B13A-05 14:30h
Aquatic Plants and Animals as Ecosystem Engineers
Studies on aquatic plants and animals focus on population dynamics, the structure of communities and the part played by organisms in food webs and other ecosystem processes. As Lawton and Jones point out in "Linking Species and Ecosystems", less attention is given to the role of organisms as ecosystem engineers, modifying the environment in which they live. Yet plants can have a profound effect on their surroundings, altering flow patterns and trapping large amounts of organic and inorganic material. Animals also affect aquatic ecosystems in many ways, both in building structures such as tubes and shelters, and in their feeding. For example, detritus feeders often produce large numbers of faecal pellets (and pseudofaeces in bivalves) and these are very different in size to the materials ingested. Pellets are deposited in masses over the bed of streams, lakes and the sea and therefore effect a translocation of nutrients. The action of plants and animals in altering their environment is likely to be a significant process in all water bodies, from both small to large scale.
B13A-06 14:45h
Effects of Crayfish and Fish on Sediment Accumulation and Macroinvertebrate Community Structure in a Temperate Stream
A variety of large taxa can act as ecosystem engineers in stream communities. These taxa are often found in the same stream. We used an enclosure/exclosure experiment to evaluate the individual and combined effects of a crayfish and a benthic-feeding fish on sediment accumulation and macroinvertebrate abundance in a temperate stream. There were 5 treatments in the experiment: 1) central stoneroller enclosure, 2) crayfish enclosure, 3) crayfish plus central stoneroller enclosure, 4) manually brushing the substrate, and 5) a control. The brushed treatment allowed us to separate the consumptive effects of the crayfish and central stonerollers from the indirect effects resulting from their bioturbation. The control contained significantly more sediment than the other 4 treatments indicating that both crayfish and central stonerollers acted as ecosystem engineers. Reduction in sediment abundance had negative effects on the abundance of 2 genera while three taxa were more abundant in sediment-free substrata. Macroinvertebrate abundance in the brushed treatment was not different from the 3 treatments with crayfish or central stonerollers suggesting that the main effects of consumers in this experiment were indirect. Our results demonstrate that both crayfish and central stonerollers can influence lotic community structure by acting as ecosystem engineers.