Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 454

3. Assemblage diversities in brackish-water strata are substantially lower than
in equivalent marine strata ( Fig. 2 ). Trace-fossil diversities are considerably
lower in brackish-water strata than in marine units. Lower behavioral diver-
sity may be ascribed to the importance of ethologically flexible feeding
strategies needed to rapidly harvest the rich food resources in dynamic
sedimentary conditions (see 1). Another consideration is that, for modern
settings, these robust ethologies are mostly used by opportunistic, com-
monly brackish-water tolerant animals (e.g., nereid polychaetes, the bivalve
Macoma balthica , arthropods such as Corophium volutator , and the lug-
worm Arenicola marina ). As such, the traces produced by animals employ-
ing these generalist behaviors are rapidly and widely distributed in
marginal-marine settings.
4. Common presence of high population densities. Although the diversity of
trace fossils is lower in brackish-water settings, trace fossils are commonly
present in high population densities, that is, with high bioturbation indices
(BI) ( Fig. 2 A-D; MacEachern and Gingras, 2007; Pemberton et al., 1982 ).
This characteristic is exemplified by the occurrence of mono-ichnospecific
trace-fossil assemblages in highly burrowed strata (i.e., BI

¼

4 or 5). There
are at least two likely reasons for this pattern. First, due to the presence of
tidal transport from the marine realm and low overall energies in many
brackish-water settings, food resources are comparably abundant on and
in the sediment. As well, favorable physiological or behavioral adaptations
by some animals enable them to outcompete others under stressful condi-
tions (see 3). The successful animals are thereby able to flourish in the envi-
ronment, with minimal interference from competitors.
5. Brackish-water environments tend to promote infaunal over epifaunal life-
styles. One reason for this may be that living within the sediment provides
protection against high-frequency (daily or semi-daily) salinity fluctuations
( Chapman, 1981; Knox, 1986 ). Interstitial waters show much more uniform
salinities throughout the tidal cycle because they are buffered from the more
variable surface waters by the sediment body. Infaunal lifestyles are also a
consequence of the abundance of food resources in such regimes, encourag-
ing deposit-feeding within the sediment ( Gingras et al., 1999 ).
6. Longitudinal trends in brackish-water settings are indicative of landward
freshening of the depositional waters ( Fig. 3 ). Although not specifically out-
lined by Pemberton et al. (1982) , it is derivative of their work (2 and 3).
A similar trend was identified by Howard et al. (1975) and Howard and
Frey (1975) from their work on the Ogeechee River estuary. Hauck et al.
(2009) showed correspondence between the function ([maximum burrow
size observed]

[diversity of macroscopic infauna]) and mean salinity
within the modern estuary, Kouchibouguac Bay, New Brunswick, Canada.
Similarly, Gingras et al. (1999) document a progressive diminution and
diversity reduction at Willapa Bay, Washington. This work suggests that
both animal sizes and burrow diversities show a crude relationship to