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5Diatoms as indicators of
long-term
environmental change in
rivers, fluvial lakes, and
impoundments
EUAN D. REAVIE AND MARK B. EDLUND
5.1 Introduction
Natural succession of rivers, such as sediment filling, migra-
tion, and the development of abandoned river channels into
terrestrial systems, tends to occur over geological timescales.
In recent centuries rivers have been used as water supplies for
domestic, agricultural, and industrial activities, and for naviga-
tion, fisheries and water power. These anthropogenic activities
have accelerated changes in river systems through pollution,
habitat destruction, non-native species introductions, hydro-
logic manipulation and other physical disturbances. In addi-
tion to the obvious physical impacts, human activities have
had numerous deleterious impacts on water quality and biotic
communities inhabiting rivers (Smol, 2008).
The anthropogenic nutrient and particulate loads carried
by rivers have markedly increased over the past few cen-
turies, causing an overall increase in organic matter flux. How-
ever, one of the most significant manipulations of rivers has
been damming, resulting in impounded aquatic systems that
plainly contrast their previous conditions. Dam construction
has reduced organic flux in many regions (Meade et al., 1990),
and it is estimated that seven times the natural volume of rivers
is stored in the world’s reservoirs (V¨
or¨
osmarty et al., 1997).
Dams have also changed the global silica cycle by storing large
amounts of biogenic silica in reservoir deposits and preventing
its delivery to oceans (Humborg et al., 2000). Enhanced diatom
productivity fueled by excess nutrients in the world’s rivers
has further increased the trapping efficiency of silica within
impoundments (Triplett et al., 2008). If attempts are made to
understand changing conditions in rivers or achieve ecologi-
cal management, conceptualization of long-term conditions is
essential (Reid & Ogden, 2006). Because diatoms are sensitive
to a wide range of conditions, they are particularly suited to
applications in rivers that can be subject to complex physical,
chemical and biological shifts (see Stevenson et al., this vol-
ume). In rare cases, long-term monitoring data may be used to
reconstruct the ecological history of lotic systems. For instance,
Berge (1976) investigated acidification through the compari-
son of old (1949) and recent (1975) diatom samples collected
from Norwegian streams. More recently, Van Dam & Mertens
(1995) used water quality and benthic diatom collections from
1974, 1981, and 1990 to provide a 16-year acidification trend for
streamsintheNetherlands.Theynotedthat many of the histori-
calenvironmentaldatawereinadequatetosupporttheirresults,
which is an expected consequence when older monitoring data
are sought for aquatic management (Smol, 2008). In the 1980s,
river researchers started to explore long-term paleoecological
analyses of diatoms in hopes that, like many similar, highly
successful applications in lakes, they could provide baseline
data and clear indications of natural and artificial ecological
transitions. As will be discussed in this chapter, trial-and-error
was the norm for these early studies, but significant advance-
ments in diatom applications in river paleoecology have been
achieved.
The use of diatoms as monitoring tools in river systems is
treatedinStevensonetal. (this volume). Indeed, numerousstud-
ies have applied diatoms in lotic settings, largely focusing on
modern diatom ecology and littoral habitats, with the primary
objective of monitoring environmental quality. In this chapter
we aim to illustrate how diatom tools are used to reveal long-
term changes in rivers (including former and recurrent river
systems),particularlythroughtheanalysisofdiatomremainsin
sediments. In many cases it will be clear that diatom indicators
developed from assessments of modern materials can be used
in paleoecological contexts, demonstrating that uniformitarian
principles are critical to understanding river ecology. Diatom
paleolimnological applications are also given treatment in
86
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Diatoms as indicators of long-term environmental change in rivers, fluvial lakes, and impoundments 87
several other chapters in this volume, but river ecosystems
necessitate very different considerations than similar studies
on more physically stable systems such as lakes.
While they may seem obvious from a limnological stand-
point, the differences between river and lake systems are worth
discussing in terms of diatoms and paleolimnology. In contrast
to lakes, obtaining suitable stratigraphically dated sediment
cores from rivers can be problematic. Acquiring continuous
lotic sedimentary records can be difficult because of constant
or frequent reworking of riverine material, and so it is often nec-
essary to collect material from more stable environments such
as fluvial lakes, former channels (e.g. oxbows), bays, lagoons,
deltas, backwaters, and floodplains. Although sites near river
channels might be considered to have the most unreliable sed-
imentary records due to frequent scouring, such sites may also
be used if they are very carefully selected.
Earlypaleoecological investigations in rivers were performed
on the Rhˆ
one and Rhine rivers in the 1980s (Bravard et al., 1986;
Klink, 1989). These studies focused, respectively, on the use of
cladoceran and insect remains in the sediments of abandoned
channels to formulate objectives for mitigating and predicting
the impacts of massive civil engineering projects in the rivers.
These data from the Netherlands illustrated that paleolimno-
logical information was not limited to lake systems, and it was
not surprising that down-core diatom studies in rivers were
soon to follow. The sections that follow describe case studies
using diatoms in paleoecological assessments in extant and
former rivers. In a relative sense, river paleolimnology is still
in its infancy, but, using the literature to date, we summa-
rize methods and recommendations from scientists who have
overcome the often daunting problems associated with lotic
systems.
Some ambiguity surrounds the definition of this chapter
as one that deals with diatoms in river systems. Most lakes
are part of a flow-through network of some kind, with the
possible exception of certain lake types such as headwaters
and perched lakes. Hence, some limnologists might dispute
whether a particular lake is a closed or fluvial lake. Take the
Rideau Canal system (Canada) as an example: a series of lakes,
rivers, and locks connecting the Ottawa River to Lake Ontario
(Legget, 1975). A casual limnological assessment of many of
the Rideau Canal lakes would conclude that they are typical
temperate, freshwater lakes with few or no detectable river
characteristics. But, because the majority of these lakes have
major inflows and outflows, one might define them as fluvial.
Christie & Smol (1996) performed a diatom-paleolimnological
analysis on one of these lakes, and such a study could well apply
to this chapter as well. Although studies on lotic systems have
a unique set of concerns and strategies, it is worth noting that
diatom-based methods used in lakes, rivers, and other aquatic
systems are dictated by the unique properties of each system.
5.2 Are the fluvial diatoms allochthonous
or autochthonous?
Several researchers have touted the value of nearshore diatoms
as indicators of river condition using periphytic assemblages
collected from rocks (e.g. Lavoie et al., 2008), plants (e.g. Reavie
& Smol, 1998a), sediments (e.g. Tibby, 2004), and artificial sub-
strates (e.g. Hoagland et al., 1982). These researchers have the
advantage of knowing that the diatom species they encounter
probably lived at or near their sample locations. These locally
derived species, termed autochthonous, make ideal indicator
organisms because they can be calibrated to the water quality
and other environmental conditions present at their respective
sample locations. Furthermore, if there is sample contamina-
tion from diatoms that have been carried from elsewhere (i.e.
allochthonous taxa), one may make assumptions to discern
living from dead diatoms; for instance by examining whether
the cells contain cytoplasmic contents, or if they are old, empty
frustules that may have been carried great distances. Identifying
the source of sedimented diatoms from rivers and impound-
mentscanbeproblematicascell contents often degrade rapidly;
new frustules may look identical to frustules that have been
resuspended in the river for several years. One might assume
that highly fragmented or dissolved valves are allochthonous,
but that assumption has been shown to be incorrect at times
(Beyens & Denys, 1982), and such a consideration is probably of
little use in most studies. It has been suggested that planktonic
diatoms in sediments are by definition allochthonous and so
should not be used in paleolimnological studies (Simonsen,
1969), but as summarized in case studies below, phytoplank-
tonic diatoms can provide critical information on large spatial
scales in rivers. Until more advanced methods are developed
to identify allochthonous diatom remains in river sediments,
researchers should assume that at least some of the ecologi-
cal interpretation that arises from these assemblages represent
conditions up-gradient from the sample location and that a
subjective assessment of the allochthonous component of an
assemblage may be necessary.
5.3 Dealing with uncertain
temporal profiles
Sediment dating usually has little to do with diatoms, but this
chapter would not be complete without some background
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88 Euan D. Reavie and Mark B. Edlund
on how researchers have obtained suitable temporal pro-
files for paleoecological analyses in river systems. Even with
detailed hydrological and bathymetric data, any paleolimno-
logical investigation on a river demands an exploratory period
to find suitable core sites. Good planning (e.g. historical aerial
photos), determination, and fastidious groundwork appear to
have prevailed in such studies to date, as we are unaware of
river paleolimnology studies that had to be abandoned due to
“uncorable” conditions. Unlike lake studies, where the sin-
gle deepest point in a basin is typically suited to an excellent
temporal record (Charles et al., 1991), the shrewd river pale-
olimnologist will have time, effort and budget set aside to
identify, and discard if necessary, sediment cores from areas
with poor temporal records. Carignan et al. (1994) performed
isotopic dating on several cores collected from fluvial lakes
throughout the St. Lawrence River. Several of these cores had
monotonous or erratic isotope profiles, indicating that they
had been homogenized, scoured, or otherwise physically dis-
turbed. However, select cores had strong exponential isotope
decay profiles, and additional analyses allowed for exceptional
resolution in nutrient, trace-metal, and organic contaminant
trends for the previous 50 years.
Caremustbeexercisedindating, as isotopic supplies to rivers
are rarely dominated by atmospheric sources, and with land-
use changes the delivery and sources of radioisotopes likely
have changed over time (Gell et al., 2005a), in direct violation
of most common dating models (e.g. constant rate of supply,
CRS; Triplett et al., 2009). Researchers are encouraged to use all
available dating tools and test multiple cores when analyzing
riverine sediments. A combination of 210Pb, 137 Cs, accelerator
mass spectrometry (AMS) 14C, pollen, and magnetic suscep-
tibility was critical for dating and correlating sediment cores
from the natural impoundments of Lake Pepin and Lake St.
Croix in the Upper Mississippi River basin (Engstrom et al.,
2009; Triplett et al., 2009). Only with well-dated cores were
diatom studies in the St. Lawrence and Mississippi basin per-
formed, as described in case studies below.
5.4 Natural fluvio-lacustrine systems
Retrospective diatom applications in fluvial lakes have become
more common in the last decade. A fluvial lake is somewhat
arbitrarily described as an open-water widening of a river with a
surface area large enough that it may be considered a lake, but
with river-like flow characteristics. Because of their lake-like
properties, fluvial lakes are valuable sources of sedimentary
records that may otherwise be unavailable in the high-flow
regions of a river. Some fluvial lakes are naturally impounded
lakes formed by processes such as tributary deltas, dune block-
age,landslides,andlavaflows.Unlike artificial impoundments,
which largely reflect human interventions, natural impound-
ments allow for investigations of processes in rivers that have
occurred over longer geologic timescales. The same princi-
ples are involved when applying diatoms in investigations of
natural impoundments or fluvial lakes and we describe these
applications in the context of several case studies on large
transboundary rivers.
5.4.1 St. Lawrence River case study
Asaneconomicallyimportantwaterwayconnecting the Atlantic
Ocean to the Laurentian Great Lakes, the St. Lawrence River,
which is also an international boundary between Canada
and the United States, has been subjected to considerable
anthropogenicdisturbance.Rapidagriculturalexpansion in the
late nineteenth and early twentieth centuries increased nutri-
ent loads. Canalization and rapid shoreline industrialization
occurred in the early twentieth century, and in the 1950s the St.
Lawrence Seaway was constructed, resulting in dramatic mod-
ifications to the hydrologic regime of the river. The Seaway is
a series of 15 shipping locks which reduced the flow rate and
amplitude of water-level fluctuations. Not surprisingly, these
hydrologic changes resulted in significant changes to sedimen-
taryregimesandnutrientcycles, and provided a highlyfavorable
environment for aquatic macrophytes.
As mentioned previously, significant isotope work identi-
fied several sites in the St. Lawrence River containing con-
formable sedimentary profiles (Carignan et al., 1994). Many
investigations followed these findings, including a diatom-
based paleolimnological study on several cores from the
river’s fluvial lakes (Reavie et al., 1998). In particular, a core
from Lake Saint-Franc¸ois, a fluvial lake between the cities
of Cornwall and Montr´
eal (Canada), contained strong shifts
in diatom assemblages related to industrialization and other
human activities (Figure 5.1). Simultaneously, studies were
being performed on the modern diatom assemblages of the
St. Lawrence River to determine the chemical and habitat
characteristics of the diatom species (O’Connell et al., 1997;
Reavie & Smol, 1997, 1998a, b). One of these studies (Reavie
& Smol, 1997) used modern diatom assemblages collected
from rocks, macrophytes, and the macroalga Cladophora K¨
utz.
to develop a diatom-based habitat model. As it is generally
known which taxa are planktonic, a robust habitat inference
model was developed that was subsequently used to infer shifts
in littoral habitats based on diatom assemblages in sediment
cores.
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Diatoms as indicators of long-term environmental change in rivers, fluvial lakes, and impoundments 89
var.
var.
Figure 5.1 Diatom microfossil profiles of common species in a core
from Lake Saint-Franc¸ois, St. Lawrence River. Major periods of local
development are indicated on the right. (Modified from Reavie et al.
(1998).)
ThecoreprofilefromLakeSaint-Franc¸ois (Figure 5.1) indi-
cated a notable shift from benthic species (e.g. Navicula minima
Grunow) in the late twentieth century to epiphytic taxa such
as Cocconeis placentula var. euglypta Ehrenb., a taxon shown to
dominate the modern assemblages living on macrophytes and
Cladophora. These shifts were largely attributed to hydrologic
modifications that resulted in stabilization of littoral areas and
an augmented standing crop of macrophytes. The mid 1900s
also experienced an increase in planktonic taxa that require
high levels of nutrients such as Stephanodiscus Ehrenb. and Frag-
ilaria crotonensis Kitton, reflecting the increased nutrient load
and stagnation of flow. While this study provided a clear eco-
logical history of the river, it also showed that diatom-based
paleolimnological studies can be a valuable addition to moni-
toring programs in large rivers.
5.4.2 The Upper Mississippi River – Lakes Pepin and
St. Croix case study
The Mississippi River drains 40% of the contiguous United
States and is the largest and most economically important
river system in North America. The Mississippi River, with
its culturally enhanced burden of phosphorus, nitrogen, and
sediment, has been linked to coastal eutrophication, in partic-
ular the annual formation of the Gulf of Mexico “Dead Zone”
(Rabalais et al., 2003). Modern nutrient exports to the Gulf are
significantly derived from the Upper Mississippi River (Alexan-
der et al., 2000; Rabalais et al., 2002); however, only recently
have retrospective studies addressed the role that the Upper
Mississippi and its tributaries have had in regulating land-use
impacts and mass transport from the continental to coastal
environments.
Two natural impoundments in the Upper Mississippi basin,
Lake Pepin on the Mississippi River and Lake St. Croix on
the St. Croix River, have been the focus of intense paleoeco-
logical investigation (Figure 5.2). The lakes formed approxi-
mately 10.3 ka after drainage from proglacial lakes Agassiz and
Duluth receded. Deltaic fans from the Chippewa, Vermillion
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90 Euan D. Reavie and Mark B. Edlund
BSi (mg cm−2 yr −1
)
% Benthic % Steph.
Fragilaria hantzschii (10^6 m^3/ yr) (1000s t/yr)
% Planktonic DI-TP (µg l −1
) Sedimentation P load Glauconite (%) % PEB
Diatoxanthin % Planktonic DI-TP (µg l −1
) P load (t yr −1
) Sedimentation Point source pop'n (1000s)
(1000s t yr −1
) P (t yr −1
)
Figure 5.2 Diatom and biogeochemical profiles in sediment cores
and historical reconstructions from Lake St. Croix and Lake Pepin
(Upper Mississippi River basin) and the Gulf of Mexico. Lakes St. Croix
and Pepin proxies include biogenic silica (BSi, mg cm−2yr−1), dia-
toxanthin (nmol g−1organic matter), percent diatom groups, diatom-
inferred total phosphorus (DI-TP, μgL
−1, ppb), total phosphorus load
to each lake (TP load, tonnes yr−1), total sedimentation (tonnes yr−1
or m3yr−1), basin population, and point-source phosphorus load-
ings (tonnes yr−1). Lake St. Croix proxies adapted and modified from
Edlund et al. (2009a,b) and Triplett et al. (2009); Lake Pepin proxies
adapted and modified from Engstrom et al. (2009). Gulf of Mexico
proxies that are indicative of anoxia include percent glauconite on
coarse grains (modified from Rabalais et al., 2007) and PEB index (%
low-oxygen-tolerant benthic foraminifers; modified from Osterman
et al., 2008). Portions of figure previously published in the Journal of
Paleolimnology are used with kind permission of Springer Science and
Business Media.
and Cannon rivers were deposited to form a series of river-
ine lakes in the Mississippi River valley, the former creating
Lake Pepin (Blumentritt et al., 2009). Lake St. Croix is a nar-
row, 37 km long lake with four major basins (10–22 m deep)
along the Minnesota–Wisconsin border. The riverine character
of the lakes is obvious in their short residence times (days
to weeks), but both lakes retain full post-glacial sediment
records that were used to reconstruct ecological change and
historical phosphorus and sediment loads using a combination
of diatom-inferred phosphorus reconstructions and whole-
lake mass balance techniques (Eyster-Smith et al., 1991; Blu-
mentritt et al., 2009).
In both natural impoundments, the diatoms indicated dra-
matic ecological changes from clear-water benthic forms to
planktonic dominance in the last 200 years (Figure 5.2; Edlund
et al., 2009a; Engstrom et al., 2009). Diatom-inferred total
phosphorus increased in both impoundments following Euro-
American settlement, with especially large increases after AD
1950 (Edlund et al., 2009a; Engstrom et al., 2009). Historical
phosphorus mass balances indicated that phosphorus load-
ing to each impoundment had also increased rapidly after
World War II in response to growing populations and increased
point- and non-point-source loadings (Figure 5.2; Edlund et al.,
2009b; Engstrom et al., 2009; Triplett et al., 2009). Whole-basin
sedimentation patterns differed between the lakes. Sedimenta-
tion rates in Lake Pepin continue to increase (Engstrom et al.,
2009), whereas sedimentation rates peaked in Lake St. Croix
during the 1960s, but remain threefold higher than background
levels (Triplett et al., 2009).
Resultsofthesepaleolimnologicalanalyseshavebeeninstru-
mental in determining that both rivers suffer from nutrient
impairment and that the Mississippi is further impaired for
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Diatoms as indicators of long-term environmental change in rivers, fluvial lakes, and impoundments 91
turbidity. Background nutrient and sedimentation rates have
guided development of nutrient and sedimentation targets as
federal and interstate agencies institute remediation policies
(Edlund et al., 2009b). Lastly, these studies clearly establish
a historical linkage among increased nutrient and sediment
loads, their ecological impacts, and mass transport from the
Upper Mississippi drainage with coastal eutrophication and
anoxia in the Gulf of Mexico (Figure 5.2; Rabalais et al., 2007;
Osterman et al., 2008; Edlund et al., 2009a; Engstrom et al.,
2009).
5.5 Backwaters and floodplain lakes
Geomorphologic evolution of a river is a dynamic process
driven by downcutting, erosion, and sediment deposition that
results in channel movement, channel abandonment, and cre-
ation of floodplain features such as wetlands, meanders, and
oxbow lakes. Floodplain water bodies have been subject to pale-
oecological analysis to address research questions on geolog-
ical and human impact timescales. On geologic scales, Gaiser
et al. (2001) investigated an oxbow wetland near the Savannah
River (South Carolina, United States) to reconstruct the hydro-
logic environment over the last 5200 years. Diatoms recorded
an initial period of mid-Holocene lake-like conditions from
4600–3800 BP. A shift to aerophilic taxa followed when the
wetland became a temporary pond during the late Holocene
that was subject to periodic drying until a nearby reservoir con-
structed in 1985 permanently flooded the wetland.
Sch¨
onfelder & Steinberg (2002) studied a 4000-year record
of long-term human impact using sediments from paleome-
anders and oxbow lakes in Germany’s rivers Havel and Spree.
Diatoms were used to reconstruct historical nutrient dynamics
and baseline conditions and showed that both rivers had long
been moderately eutrophic systems and that increased intensity
of land use (e.g. deforestation) raised nutrient levels especially
in the last 800–1000 years. More intense human impacts were
noted in post 1800s diatom assemblages.
Southeast Australia’s river systems (Murray, Yarra, Dar-
ling) have numerous wetland and billabong habitats in their
floodplains. To aid interpretation of paleoecological stud-
ies in billabongs, Reid & Ogden (2009) recently developed a
powerful diatom-based model by relating surface-sediment
diatom assemblages to nutrients and adjacent human activ-
ities such as farming. Numerous paleolimnological studies
in Australian rivers have evaluated changes on human and
geological timescales. For instance, Thoms et al. (1999) and
Reid et al. (2007) were able to track declines in submerged
macrophytes through the decline of periphytic diatoms in sed-
iment profiles, indicating that since European settlement tur-
bidity had increased due to anthropogenic soil erosion. Thoms
et al. (1999) further used diatoms and other biological and geo-
chemical proxies on Murray River billabongs to set benchmark
conditions and show that ecological changes began with early
post-European settlement and before flow regulation. Post-
European impacts were also identified in a billabong in the
Yarra River floodplain by using diatom fossils to track signif-
icant erosion and nutrient enrichment (Leahy et al. 2005). In
particular, a shift from an assemblage dominated by Cyclotella
stelligera (Cleve & Grunow) Van Heurck to a diverse assemblage
of epipelic and aerophilous species marked the transition from
low-nutrient, pre-settlement conditions to a post-settlement
period of significant soil erosion. Gell et al. (2005a) used
diatom and other records to identify post-European changes in
salinity, pH, turbidity, and nutrients, especially along the reg-
ulated river systems. Critical to the interpretation of sediment
records were cautious approaches to dating and an understand-
ing of connectivity between floodplain habitats and the main
rivers.
Billabongs have also been the subject of long-term climate
studies.A∼5000yeardiatom-basedpaleolimnological analysis
of Tareena Billabong (Australia) indicated that it was naturally
a freshwater system, and several thousand years of inferred
shifts in sedimentation and turbidity reflected climate-related
changes in river influence (Gell et al. 2005b). In order to deter-
mine if recent pH changes in the Goulburn River were within
thenaturalrangefortheriver,Tibbyet al. (2003) analyzed
the diatoms from a sediment core collected from a fluvial bil-
labong. Using a diatom-based pH model, ditom-inferred pH
indicated an over 3000-year period of stable pH, and that ion
concentrations shifted outside the natural range shortly follow-
ing European arrival in the area.
5.6 Artificial impoundments
Artificial impoundments change the characteristics of water
bodies from those of rivers to lakes, impacting physical, chem-
ical, and biological properties (Baxter, 1977; Friedl & W¨
uest,
2002). Damming typically increases residence time of water up-
gradient of the dam, resulting in temperature increases, stratifi-
cation changes, materials retention, and an increase in primary
production in the newly formed reservoir. Further, tailwater
reaches below dams are also modified, sometimes reduced to
lower-grade rivers or streams, and in cases of hydroelectric
dams, these down-gradient reaches can be subject to discharge
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92 Euan D. Reavie and Mark B. Edlund
fluctuations resulting from dam operations. Impoundments
and tailwaters also serve as primary sites for introduction and
establishment of invasive species including the diatom Didymo-
sphenia geminata (Lyngb.) M. Schmidt (see Spaulding et al., this
volume). In this section we summarize selected case studies
of diatom applications for evaluating the impacts of artificial
impoundment.
A natural aging process occurs in lakes due to nutrient and
sediment loading and gradual filling of a basin. With rare
exceptions (Melcher & Sebetich, 1990), reservoirs tend to fill
up with sediment more rapidly due to more efficient trapping
of suspended particles and nutrients in these artificial lacus-
trine basins (Benson, 1982). When a river is dammed and a new
lake is formed, lotic benthos typically perish and are replaced
by plankton. For instance, Zeng et al. (2007) observed signif-
icant increases in planktonic diatoms and other algal taxa in
reservoirs following dam construction in the Yangtze River
(China), and Wehr & Thorp (1997) noticed reductions in ben-
thic species up-gradient of navigation dams in Ohio rivers.
Diatom-paleolimnological analyses from artificial impound-
ments have mainly focused on reconstructing ecological shifts
since damming. Hall et al. (1999) used diatom microfossils
and supporting algal pigment analyses to describe the long-
term impacts of impoundment in two southern Saskatchewan
(Canada) impoundments. Sedimentary profiles from one reser-
voirhadhighconcentrationsofeutrophicdiatoms(e.g.Stephan-
odiscus species) after damming in 1967, followed by olig-
otrophication denoted by increases in lower-nutrient taxa such
as Tabellaria Ehrenb. ex K¨
utz. Hall et al. emphasized that,
contrary to what is typically assumed, reservoir ontogeny
does not always lead to eutrophication due to the multitude
of environmental factors that must be considered for each
system.
Unlike a physically stabilized reservoir, the downstream por-
tion often has non-uniform currents, higher turbidity, and
increased potential for bank erosion (Ward & Stanford, 1979).
Ecological changes are particularly common downstream of
hydroelectric dams, where short-term fluctuations in flow are
the typical result of operation. Although not as common, stud-
ies of diatoms to infer ecological changes downstream of dams,
weirs, and other storages indicate that r-selected species domi-
nate due to frequent physical disturbance of the aquatic system
(Silvester & Sleigh, 1985). In a case from impounded rivers
in Colorado (Zimmerman & Ward, 1984), it was noted that,
compared to unregulated rivers, diatoms were extirpated in
favor of filamentous green algae at sample locations below
impoundments. Further complexity of diatom colonization
below a hydroelectric dam in the Colorado River was explored
by Peterson (1986), who observed that variations in water shear
stresses resulted in variations in the attached diatom commu-
nities, a result of colonization ability and immigration rates
among the diatom taxa. For instance, the epiphyte Cocconeis
pediculus Ehrenb. was identified as a likely “poor immigrant” in
high shear habitats. Such findings are important to river moni-
toring programs that rely on understanding diatom community
dynamics and species composition.
Significant multidisciplinary work has been performed on
theimpactsoftheIronGateIdamandreservoironnutrientflow
through the Danube River (Serbia, Romania). The Iron Gate I
dam, built in 1972, was expected to decrease nutrient and sed-
iment loads downstream to the Black Sea. Unlike the majority
of diatom-based paleoecological studies, Teodoru et al. (2006)
used biogenic silica from sediment cores to track the change in
diatoms following dam installation. Dissolved silica is taken up
by diatoms to create biogenic silica (BSi) as the primary compo-
nent of their frustules. Downcore trends in BSi can be used to
track historic changes in diatom production (Schelske 1999).
Using BSi from sediment cores collected from the Iron Gate I
reservoir, Teodoru et al. (2006) noted an increasing trend over
the previous 20 years, suggesting enhanced diatom growth.
However, despite previous assumptions to the contrary, the
inferred environmental impacts of Iron Gate I were minor
compared to changes occurring due to smaller dams in the
Danube’s headwaters and coastal pollution. Although the Iron
Gate I impoundment is the largest in the Danube, it was not
playing a dominant role in decreasing silica loads downstream.
The work of Teodoru et al. not only demonstrated the power of
diatom studies in reservoirs, but further that these studies can
put environmental impacts in a holistic, quantitative perspec-
tive for a large river.
Finally, dam removal has been suggested as a way to restore
the ecological integrity of a river, and diatom-based monitor-
ing data have been used to track the expected recovery fol-
lowing dam decommissioning. Thomson et al. (2005) stud-
ied the upstream and downstream algal assemblages before,
during, and after the removal of a small Pennsylvania dam.
Abundance and taxonomic composition of diatoms and other
algae were evaluated. Although they acknowledged that every
dam removal situation would be different, the authors found
that diatom abundance and species richness declined dramat-
ically following removal due to the sudden massive flux of sed-
iments downstream. However, they also suggested that such
impacts as indicated by the diatom communities are likely to
be short-lived, and so dam removal would have a net ecological
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Diatoms as indicators of long-term environmental change in rivers, fluvial lakes, and impoundments 93
benefit. To our knowledge, no paleoecological application has
used diatoms to track recovery following decommissioning of
adam.
5.7 Terminal lagoons, deltas,
and estuaries
While identifying depositional basins for river paleolimnol-
ogy is a challenge, river mouths are an ideal place to look for
diagnostic diatom-based records. Deltas, riverine wetlands and
receiving lagoons are likely to contain microfossil material that
may have been carried great distances by the upgradient river,
and so those microfossils may be used to provide an integrated
picture of the river’s ecological condition. To minimize topical
overlap, we refer the reader to Cooper et al. (this volume) for
a discussion of diatom applications in estuarine river mouths,
with particular attention to work performed on Chesapeake
Bay (Cooper, 1995). Further, refer to Trobajo and Sullivan (this
volume) for a summary of the ways that diatoms are used to
reconstruct the histories of coastal environments.
One modern diatom-based model was designed with the
intent of using the model equations to infer environmental
conditions in the Mackenzie Delta lakes (Hay et al., 1997). Delta
floodplain lakes allow for paleolimnological investigations of
watersheds due to close interaction between floodplain lakes
and rivers. In the case of Hay et al. (1997), a training set of
floodplain lakes was used to develop a diatom model that can
reconstruct the frequency and magnitude of flooding and other
river influences on these lakes. The frequency and duration of
lakefloodingbyariverhavestrongcontroloverthediatom com-
munity through hydrological and chemical shifts. For instance,
frequently flooded lakes tend to be turbid and so favor tolerant
phytoplanktonic diatom species (e.g. Asterionella Hassall and
Aulacoseira Thwaites), whereas infrequently flooded lakes are
typically transparent in the summer and support dense macro-
phyte populations; so those lakes tend to contain rich assem-
blages of benthic and epiphytic diatom species (e.g. Navicula
Bory). Further, Hay et al. identified a relationship between the
structure of diatom assemblages and methane (CH4),avari-
able which reflects decomposition of organic material, espe-
cially in non-flushed lakes with high macrophyte densities. The
authors acknowledged that diatoms were unlikely to be directly
responding to methane concentrations, but that the methane
variable captured diatom community changes that are asso-
ciated with flooding. A regression model was developed to
infer sub-ice winter methane concentrations from sedimentary
diatom assemblages.
The Hay et al. (1997) model was applied to down-core diatom
assemblages in eight Mackenzie Delta lakes (Michelutti et al.,
2001). Not surprisingly, the lake with the most common river-
ine influence had high densities of planktonic species and very
low diatom-inferred methane (DI-CH4), whereas lakes with
rare or no connections to rivers had higher modern DI-CH4.
The Michelutti et al. (2001) sediment cores each represented a
few hundred years of accumulated material, but an important
future application of this model may be to generate regional
climate proxy data that can provide a record to evaluate warm-
ing scenarios in the Arctic. The importance of such work is
underscored by more recent studies on the Slave River Delta,
which flows into Great Slave Lake, northern Canada (Sokal
et al., 2008), and on the Peace-Athabasca Delta (PAD), which
connects to the Slave River (Wolfe et al., 2005). Wolfe et al.
(2005) evaluated diatom profiles from a small lake in the PAD
to explore the affects of flow regulation and climatic variability
on lake ecology. In concord with several ancillary indicators
(dendrochronology, magnetic susceptibility, isotopes, organic
content, plant and invertebrate macrofossils), they were able to
reconstruct riverine influences based on past accumulations of
Fragilaria pinnata Ehrenb., a diatom known to be carried in tur-
bid river water. Clear trends related to dry and wet conditions
were reconstructed for the last 300 years.
Terminal floodplain lakes in Australia have also been sub-
jects of diatom paleoecological studies. Fluin et al. (2007) used
diatoms to explore the degree of historical connectivity between
two adjacent terminal lagoons of the Murray River. Compari-
son of the relative numbers of riverine diatoms in sedimentary
profiles revealed that the two systems evolved independently
due to geomorphic separation. MacGregor et al. (2005) used
diatom records from terminal basins of the Lower Snowy River
to describe the Holocene record. For instance, diatom-inferred
salinity indicated that Lake Curlip was a saline system (∼7000
BP) which became brackish as sea levels dropped over subse-
quent millennia and the diatom record reflected greater riverine
influence. As is typical in similar systems in Australia, human
intervention in the forms of land clearance and hydrological
manipulation resulted in dramatic changes to water quality in
the lake, as indicated by a shift to a brackish and nutrient-
tolerant diatom flora over the last century.
5.8 Flowing rivers
Of all paleoecological river studies, those based on diatom
remains collected at or near a river channel might be assumed
to be the most problematic, owing to the most physically
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94 Euan D. Reavie and Mark B. Edlund
harsh conditions for sedimentary regimes. Nonetheless, select
studies have been successful in reconstructing historical con-
ditions in these high-flow systems.
5.8.1 Zippel Bay case study
In a recent paleolimnological study in northern Minnesota of a
fluvial bay, diatom remains were used to describe long-term
eutrophication and erosion, the resulting dissolved-oxygen
declines, and other threats to biological integrity (Reavie &
Baratono, 2007). The impacted area comprised two primary
tributaries flowing into a protected fluvial bay; sediment cores
were collected from these three components. Cores from the
tributaries were collected approximately 100 m perpendicular
to the shipping channel in order to minimize disturbance from
boating activities and scouring. The bay core was collected from
the flat, deep basin.
Despite high flow rates in the tributaries, 210Pb dating analy-
sesindicatedthatthecoreswere suitable for retrospective analy-
ses. These tributary profiles were particularly valuable because
they allowed diatom assemblages and their ecological inter-
pretations to be linked with similar trends in the downstream
bay core, enabling isolation of the relative impacts of human
activities in each tributary’s respective catchment. Further, by
analyzing the three profiles relative amounts of allochthonous
and autochthonous diatoms in the bay core could be estimated.
5.8.2 Thames estuary case study
Juggins (1992) undertook a large project aimed at deriving pale-
osalinity estimates for the Thames estuary in central London,
UK. The Thames has been subject to hydrologic modifica-
tion since the fourteenth century. Embankments in particular
have acted to straighten and smooth the banks of the estu-
ary, resulting in an increase in tidal amplitude. Diatoms from
21 archeological samples, dating from the first to the seven-
teenth century, were collected from three sites. In contrast
to higher-resolution lotic paleoecological studies, this work
was less prone to unpredictable temporal shifts in the diatom
assemblages simply because of the long temporal record and
relatively small number of samples that were each likely to con-
tain diatom remains that had been integrated from several years
of deposition.
Diatoms from the Thames archeological sites were used to
track eutrophication, indicated largely by the appearance of
Stephanodiscus species, as far back as the medieval period. Pale-
osalinity estimates were generated from archeological assem-
blages using a weighted-averaging transfer function. The mix
offreshwaterandmarineformsinhistoricsedimentsindicated,
contrary to general beliefs, that the Thames was tidal in central
London during the Roman period. Greater detail on this project
is provided in Juggins & Cameron (this volume).
5.9 Summary
Diatoms are known to be excellent indicators of environmen-
tal conditions in rivers. However, their use in paleoecolog-
ical applications of lotic ecosystems is more recent due to
slower development of techniques to deal with the possibility
of inconsistent temporal profiles in sediment cores. In many
cases, successful applications in rivers have involved selecting
sites with lake-like behavior such as fluvial lakes. Studies have
proven that a researcher with patience and a sufficient bud-
get for exploratory work can successfully apply diatom indi-
cators down-core in more challenging systems. This move by
scientists to tackle problems in rivers is due to an increas-
ing recognition of the environmental losses resulting from
human impacts, such as hydrologic manipulation, pollution,
and eutrophication. Furthermore, the impacts of river regula-
tion have been dramatic, and diatom indicators for impounded
rivers and their reservoirs are needed to track environmental
impacts in these artificial systems.
As in the Thames and Canadian delta work described in this
chapter, the application of riverine diatoms in paleolimnol-
ogy needs not be performed on an extant river. Instead, one
might use the riverine characteristics of diatom taxa to inter-
pret the distant past of an aquatic, or even terrestrial, system
that was once influenced by flowing water. We anticipate that
as socioeconomic concerns grow due to climate changes, such
studies will play an increasing role in defining background and
human-induced changes.
Information on riverine diatom species descriptions and
ecological characterizations is constantly being gathered (see
Stevensonetal.,thisvolume).Theseimportantdatawillsupport
ongoingeffortstodescribelong-termtrends in rivers using sed-
imentary diatom assemblages. Current applications are based
on fairly sparse available literature. The most pressing needs
are to continue taxonomic and ecological characterization of
diatom indicators and refinement of paleoecological methods
that are suited to river applications.
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