Environmental Engineering Reference
In-Depth Information
increased sediment trapping by aquatic macrophytes, themselves promoted by
accelerated nutrient fluxes. The separation of these allochthonous, river-
derived indicators from those produced within the wetland enables assessment
of both the condition of the river and the individual wetland to be made.
The chronic nature and cause of diffuse pollution often allows it to be
underrated as a driver of change. Humans experiencing pollution are, natur-
ally, alert to changes that occur over short time frames such as black water or
anoxia events that lead to fish kills. In many instances, this ecological catas-
trophe is predisposed due to the antecedent conditions brought on by diffuse
sources over decades. This degraded state is often not recognised as the che-
mical shifts are difficult to perceive and the short-term focus of the observer
denies them a credible baseline against which to assess condition. Rarely were
ecological monitoring programmes established early enough, and equally as
rarely were early monitoring programmes funded for long enough, for a strong
temporal perspective to be acquired to put present conditions in context. This
record of change can be accessed retrospectively however. By virtue of the
retention, in archives of wetland sediment, of biological indicators of wetland
condition over time, an extended record, even beyond intensive human settle-
ment, can be generated. These palaeoecological studies can provide lessons
of the past lost to memory and provide the benefits of hindsight otherwise
unavailable to restoration scientists and practitioners. The strongest know-
ledge base, as Davis (
1989
) identified, and Bennion and Battarbee (
2007
) reiter-
ated recently, is the splicing of long-term studies with palaeoecological studies,
so that the past and the present can cooperate to enable us to identify baseline
condition(s) and the natural variation around it, the timing and nature of
impact, the responsiveness of the system and the present trajectory of change.
Study areas
Two study regions are selected here to reveal the insights gained from palaeo-
limnological approaches to wetland condition assessment. The first, the MDB,
is subject to the impacts of long-term land development for primary industry.
Sites in the lower catchment are illustrated here to reveal the complex inter-
action of increases in the flux of salts, sediments and nutrients in a highly
abstracted water resource under a drying climate. Two coastal wetlands outside
the basin are discussed to reveal the impact of water diversions on wetland
condition. The second region, the upper Mississippi River Basin, focuses on two
in-channel lakes that have accumulated sediments since the last deglaciation.
Held up by natural impoundments, these wetlands represent a unique oppor-
tunity to evaluate the changing condition of the river per se without having to
distinguish allochthonous from autochthonous evidence.
The MDB covers 1.073
10
6
km
2
and ranges from montane landscapes in the
southeast that, in winter-spring, provide for most of the catchment runoff, to
