The title of this post, a phrase often
associated with architecture, is a concise way of saying that the appearance or aesthetics of a functional object
often is constrained by its ultimate use. Synthetic sediment can be thought of
in this way. Its composition and to a
lesser extent its appearance may be constrained by its specific
application. This might be a good
working assumption for those developing synthetic sediment: decide a batch’s
functionality and work backwards to develop a formulation to achieve that
functionality.
We think up many possible uses or functions for sediment in
a laboratory setting: aquarium decoration, substrate for testing interactions
between chemicals and sediment components, substrate for growing aquatic plants
or culturing aquatic macro-invertebrates, a clean reference sediment to control
test variables associated with physical properties of test sediment, a
chemical-spiked positive control to confirm the observed response to a test
sediment, and others. Each of these
uses, and others we might imagine, requires a different “minimum” level of
detail or complexity of the sediment substrate; a substrate cannot be too
complex for a given use, but complexity less than the minimum will not
work. We could think of natural sediment
as having the highest level of complexity, so any sample of natural sediment will
work for any use. The accuracy of that
statement might be debatable, but for our purposes the complexity of natural
sediment is the benchmark to shoot for and to which synthetic substrate
complexity is compared.
The first response might be to develop one,
universally-complex synthetic substrate and use it for all possible functions
and applications. In practice, that
might be an ambitious approach because of the difficulty in assuring that the
complexity of the synthetic material matches the natural. Secondly, replicating that level of
complexity by multiple labs in multiple locations throughout the world would
require a lot of time and expense by the participating labs. For many applications, such effort would be
over-kill. Lastly, regional differences
in natural sediment characteristics, or organism differences in micro-habitat
requirements, would make a single universal synthetic substrate inappropriate. This is one of the drawbacks with current
synthetic sediment formulations.
So thinking of synthetic substrates as use-specific can
simplify how specific labs might approach formulating it. Recipes and components (type, number) would be
specific to end-use, regional environment, data quality objectives and organism
habitat needs. To apply this approach
requires a clear understanding of what components can or should be used for
which end-use substrate. This means
knowing how each individual component affects and contributes to the
characteristics of the whole, and how each interacts with the other components.
Most of my work with synthetic sediment so far is focused on
researching, documenting and testing how individual components affect
characteristics of the whole material.
This knowledge will help end-users of synthetic sediment select
appropriate components and recipes for their specific needs. Some components have little influence on
substrate characteristics; large particle-size sand (silica; quartz) is an
example. Most have a strong affect on
the characteristics of the whole material.
Examples include small particle-size geochemical clay (layered
alumina-silicate minerals), geochemical silt (simple oxides, simple silicates, carbonates,
feldspar and granite minerals), all types of organic matter, and mineral
components like metal-sulfides. One set
of experiments, presented at a technical conference in Long Beach, California,
demonstrated how the type and amount of mineral oxides included in a mixture
can have a profound effect on pore water pH and on the cohesion and compaction
properties of whole substrates. You can find the summary of that presentation on page 313 of the 2012 conference abstract book, found online HERE.
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