Appendix C: Paleolimnological Sampling
(Sedimented Diatoms)

Diatoms and chrysophytes preserved in lake sediments are integrators of lake history and make it possible to infer changes in other biotic assemblages (Charles et al. 1994, Dixit et al. 1992). Environmental variables, such as alkalinity, aluminum, dissolved organic carbon (DOC), salinity, nickel, conductivity, calcium, total nitrogen, total phosphorus, Secchi transparency, and "trophic state," have been inferred using diatom-based predictive models (Charles et al. 1994, Dixit et al. 1992, Fritz 1990).

Sedimented diatoms samples are not subject to short-term temporal variability as are other assemblages such as phytoplankton. A sediment surface sample contains an integrated record of 1 to 3 years, and sediment cores can be calibrated with other information (e.g., varves, known contamination events, radioisotopes, pollen) to obtain time series with resolution of up to 1 to 10 years.

The diatom fossil record can aid in establishing reference conditions. Surface sediments represent contemporary lake conditions and usually integrate the assemblage over 1 or more years (Dixit et al. 1992), whereas presettlement conditions can be characterized by sediment cores of 0.5m to 1.0m depths (Charles et al. 1994). Dating sediment cores is possible using pollen or 210Pb (radon decay product).

Paleolimnological analysis, part of the EMAP protocol (USEPA 1994b), requires development of a data set that associates current environmental conditions with current surficial diatom assemblages. Present-day associations are used to infer past conditions based on fossil diatom assemblages in deeper sediment layers. The calibration process is usually done in two steps. First is an evaluation of species-environment relationships and statistical determination of which environmental characteristics can be inferred using predictive models. Currently, this is done using Canonical Correspondence Analysis (CCA) (Dixit et al. 1992, Jongman et al. 1987, ter Braak 1986) frequently using the computer program CANOCO (ter Braak 1986). The second step is to develop the predictive models and calculate error estimates (Birks et al. 1990, Charles and Smol l994). The computer program WACALIB (Line et al. 1994) is an efficient way to perform these calculations. Further documentation of these methods is in Charles and Smol (1994), Charles et al. (l994), Birks et al. (1990), and ter Braak and Juggins (1993).

Level of Effort

Analysis of an individual paleolimnological subsample (section of a core) generally requires 1 to 4 hours,which is similar to other diatom and phytoplankton analyses. In order to infer ancient conditions, a calibration data set is necessary, consisting of surface sediment samples from 50 or more impaired and unimpaired lakes. Standard diatom metrics, as well as similarity indices, can be calculated (Table C-1).

Methods

Field sampling - For sedimented diatoms can be relatively fast. Field methods outlined below are the same as those used in EMAP (USEPA 1994b)

Sample location - Sediment samples are obtained from or near the deepest area of the lake. A single core is sufficient.

Sample collection - The procedure from the EMAP manual (USEPA 1994b) is followed for diatom analysis and is quoted here: Sediment cores are collected from the deep, central area of a lake (at or near the index site) using a modified K-B gravity corer (Glew 1989). Core samples are extruded from the corer and subsectioned immediately after collection. A core of at least 45 cm length is desired. The top and bottom lcm intervals are collected from each core and placed in separate sealable plastic bags along with wet paper towels to prevent desiccation. These intervals are sectioned and removed from the core using an apparatus described by Glew (1988). The sample bags are labeled with either standardized adhesive labels or identification information is written on each bag using a permanent ink marking pen. The bags prepared from a single core sample are placed in a sealed container for storage and transport. Samples are kept at 4°C until shipment. Detailed procedures for collecting and preparing core samples are presented in the field operations manual.

Sample analysis - Analytical methods are the same as for sedimented diatoms (Section 6.3) summarized in Table C-2.

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Appendix A ~ Appendix B ~ Appendix C ~ Appendix D
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