Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 293

large populations, but there was no clear evidence that the communities were
supported by local primary production or that resources were provided by eolian
input. The presence of collembolans suggested that algae could be the base of the
web food, but no infaunal invertebrates were detected in soils. Considering the
pattern of succession, Kaufmann (2001) concluded that decomposers, like
dipteran larvae and millipedes, might have facilitated ecological succession
where well-established communities existed. The presence of vegetation made
no significant difference to the colonization pattern of pioneering invertebrates,
which suggests a strong predatory or scavenging component to the pioneer
assemblage and possible allogenic inputs.
2.3 Postglacial Colonization of Freshwater Environments
Biodiversity in glacial lakes and streams is low compared with typical temper-
ate and tropical freshwater environments. Biodiversity increases from deep
to shallow lakes and from upstream to downstream settings ( Likens, 2010 ).
Fishes, when present, are the dominant predator of the glaciolacustrine ecosys-
tem, while the primary benthic community is composed chiefly of worm-like
organisms (e.g., oligochaetes, nematodes, rotifers, nematomorphs, and insect
larvae), crustaceans, diminutive molluscs, bacteria, algae, and a few aquatic
vascular plants (in photic zones).
As only cold-adapted or cold-tolerant species can inhabit these settings, pro-
ductivity is low and food webs are simple ( Likens, 2010 ). Glacial lakes are typ-
ically oligotrophic to ultraoligotrophic, and the development of a pioneer
community cannot always depend on planktonic primary producers due to large
amounts of suspended glacial rock flour and low nutrient density. In some arctic
lacustrine food webs, the benthos is the primary source of organic carbon
( Sierszen et al., 2003 ). Despite low productivity, benthic organisms provide
other animals, such as predators, access to environments otherwise too harsh
to occupy.
For those glacial freshwater environments linked to the coastline by river
systems, offshore freshwater refugia located on exposed continental shelves
can play a role in successful recolonization. In addition to terrestrial
freshwater habitats, coastal-marine areas may have been available to
freshwater fish as glacial meltwaters altered ocean water salinity nearshore
( Power, 2002 ). Power (2002) postulated that during postglacial extensions of
fish ranges, proglacial lakes would have been uninhabitable and were too much
covered by ice during most of the year. Using Arctic rivers as analogs for what
fish may have encountered during range extension (high sediment loads, unsta-
ble bed conditions, and frequent meltwater flooding), Powers (2002) argued that
proglacial conditions would have been difficult to overcome for migrating fish.
These extreme conditions would have required the ability to exploit a wide range
of niches, as primary productivity would have beenmuch reduced. In one study of
a recently (within the past 40 years) deglaciated area in Alaska, it was found