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Paleolimnology Homepage

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

July 15, 2010

Introduction

Effective management of aquatic resources requires long-term environmental data. The job of the paleolimnologist is to analyze and interpret the diverse information contained in the sedimentary records of lakes, wetlands, reservoirs, and some parts of rivers. This history is archived in a surprisingly complete repository beneath their deep waters. Every second of every day, sediments are accumulating. Incorporated in these sediments is a record of the organisms that lived in and around the lake, as well as proxy data related to processes occurring in the lake, the composition of the lake’s water, the conditions in its watershed, and the air above it!

Most environmental studies are done “after-the-fact”, or after a problem has already been identified (such as an acidification of a lake or development of algal blooms or fish kills due to deepwater anoxia). It is often difficult to effectively assess and treat these problems without some historical knowledge of how and when the problems originally developed.

Typical questions that a paleolimnologist might address may include: Why did the lake lose its deepwater oxygen, or was it naturally anoxic? Did the lake naturally have large algal blooms? If so, then perhaps mitigation efforts are fruitless as this is the lake’s “natural state”. At what point in time, and at what level of nutrient enrichment, did eutrophication symptoms become a problem? All these as well as many other questions need to be considered in an historical context.

Applications:

Paleolimnological approaches are now being used in enlightened societies to tackle a large suit of environmental and management questions. Although most of the work thus far has been related to lake eutrophication and acidification, these powerful approaches can be used to study problems such as the introduction of exotic species, taste and odor complaints, erosion problems, the extirpation of populations, tracking contaminant trajectories, consequences of water level changes and impoundments, ozone depletion and resultant increases in ultraviolet radiation, and the repercussions of climatic change.

Methodology:

The most commonly used biological indicators in paleolimnology are the glass cell walls of diatoms, a major group of algae consisting of over 10,000 species. Diatoms are a very successful group of organisms that live in virtually every body of water. They are characterized by having cell walls made up of two glass or siliceous halves, called valves. The structure and sculpturing of these valves are species specific, so that a trained phycologist can identify diatom species simply by studying (using high-power light microscopes) the valve structure.

As each species of diatom has its own characteristic environmental requirements (e.g., some live in acidic or alkaline waters, some thrive in either low or high nutrient levels in lakes, some like to live on aquatic macrophytes, and so on), by reconstructing past diatom populations, paleolimnologists can accurately infer past lake conditions.

For a number of reasons, diatoms (class Bacillariophyceae) are the most widely used group of paleolimnological indicators, and they are particularly well suited to studies of lake eutrophication. Diatoms have restricted nutrient requirements and they are sensitive indicators of lake trophic status. The siliceous cell walls (valves) of diatoms are abundant and well preserved in most lake sediments, and can usually be identified to the specific or subspecific level. Sediment diatom assemblages are composed of a large number of taxa, and therefore contain considerable ecological information.

Other biological indicators commonly used in paleolimnology include the scales and cysts of chrysophyte algae, shells and body parts of animals, and the chitinous exoskeletons of a large number of invertebrates. Included in the latter group are, for example, the preserved head capsules of midge larvae (chironomids), which are excellent indicators of deepwater oxygen levels.

Approach

(Hall, R.I. and Smol, J.P. 1996. Paleolimnological assessment of long-term water-quality changes in south-central Ontario lakes affected by cottage development and acidification. Can. J. Fish. Aquat. Sci. 53:1-17)

An effective approach to studying diatom communities with respect to lake water quality is to analyze the surface-sediment diatoms from a set of lakes with complete and reliable limnological data. The surface sediments, e.g., top 1-cm interval, contain an integration of diatoms from a variety of lake habitats that have accumulated during the past few years. These surface sediments contain a wealth of information that can then be used to infer past water-quality conditions, on the basis of the percent abundances of sedimentary diatom taxa.

Detailed assessments of anthropogenic impacts on lake water quality require continuous temporal records of diatom assemblages. However, it is logistically prohibitive to analyze complete sediment cores for a large population of lakes. For large-scale studies aiming to provide regional estimates of limnological change, the most effective approach is to analyze microfossils in present-day and preindustrial sediment core samples. Differences between preindustrial and present-day inferences estimate water-chemistry changes caused by anthropogenic activity and also provide information on background or reference conditions.

This approach has been successfully used to provide regional estimates of lake water chemistry changes in the Adirondacks, New York (e.g., pH, acid-neutralizing capacity (ANC), monomeric [Al] and [DOC]), Sudbury, Ontario (pH, total [Al], [Ni], [Ca]), and the northeastern U.S.A. (pH, [TP], [Cl], Secchi depth).

Sediment cores:

In the case of the 54 lakes in south-central Ontario, sediment cores were collected from the deepwater regions using a Glew gravity corer fitted with a 6.6 cm internal diameter Lucite tube. To minimize stratigraphic disturbance, sediment cores were sectioned at the lake shore immediately after collection using a vertical extruder.

Present-day (0-1 cm interval) and preindustrial (1 cm thick interval from >20 cm core depth, and usually >25 cm) sediment samples were removed from the cores. Confidence that the core bottom samples represented preindustrial (i.e., pre-1850) conditions in all lakes was expressed, because ²¹⁰Pb sediment dating from large numbers of south-central Ontario lakes indicates that a sediment depth of 15 to 20 cm almost always represents the pre- 1850 period. ²¹⁰Pb dating from other Precambrian Shield regions indicates that preindustrial sediment intevals are usually located within the top 20 cm (e.g., Adirondacks).

Shortcomings of this sediment coring methodology:

The sediment core top and bottom comparison approach can only estimate water chemistry changes that have occurred between two discrete points in time: the present day and pre-1850. This top-bottom approach cannot infer trends that have occurred during the intervening period. Human activities may have caused more severe water-quality deterioration in the past. For example, the most severe perturbations in many of the lakes may have occurred during the initial land clearance activities of European immigrants. Additional research is required to investigate trends through time and the impacts of these activities on lake water quality.

Acknowledgements

A significant amount of the content of our web pages in paleolimnology are derived/excerpts from various published papers authored by past and/or present researchers at the PEARL, Queen's University, Kingston, Ontario!

A state-of-the-art textbook in paleolimnology:- Smol, J.P. 2008. Pollution of Lakes and Rivers: A Paleoenvironmental Perspective. 2nd ed. Blackwell Publishing, Oxford. x,383 pp. ISBN-13: 978-1-4051-5913-5.

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