Microbioerosion - Trace Fossils as Indicators of Sedimentary Environments - page 230

of Sieblos/Rh¨n, Germany, Radtke (1998) reports further ichnotaxa. Again
touching upon a fossil marine-brackish paleoenvironment, Radtke (2007) iden-
tifies 10 ichnotaxa in a drill-core from the Oligocene Pechelbronn Formation
from the Mainz Basin, Germany.
Only very limited information on microendolith occurrences is at hand from
hypersaline settings as, for instance, the hypersaline lagoon Ahivadolimni on
Milos Island, Greece ( Pantazidou et al., 2006 ).
Bioerosion by microendoliths is not limited to marine waters but also takes
place in pure freshwater environments such as lakes or below terrestrial soils on
limestone bedrock (see Schneider and Le Campion-Alsumard, 1999 for a
review). A number of freshwater bioeroding cyanobacteria have been described
over the years (e.g., Anagnostidis and Pantazidou, 1988; Seeler and Golubic,
1991 ), but an ichnotaxonomical assignment of their traces is still lacking.
Further research in this direction would be highly promising and could yield
key ichnotaxa for identifying fossil lacustrine paleoenvironments. Likewise,
by means of intensified research on hypersaline microbioerosion, stenohaline
euendoliths and their traces might be identified that could pinpoint paleoenvir-
onments with unusually high salinities.
4.4 Paleotrophodynamics
The interplay of dissolved inorganic nutrients and particulate organic matter in
water with an abundance and diversity of bioerosion agents is complex and only
partially understood. This is because these environmental factors have different
effects on the various types of bioeroding biota and the epibiont cover which, in
turn, may inversely affect each other. Moreover, nutrients and organic matter
influence other controlling factors which are then difficult to separate in experi-
mental studies and the geological record alike—such as the negative effect of
eutrophication promoting plankton blooms on the availability of light for pho-
toautotrophic microendoliths. At the same time, high plankton abundance pro-
motes the activity of planktivorous macroborers such as recorded by, for
instance, the global correlation of plankton primary production and the abun-
dance of boring bivalves ( Highsmith, 1980 ).
Hallock and Schlager (1986) and Hallock (1988) compiled the knowledge
on the role of nutrient availability on bioerosion and thereby highlighted the
important consequences on the balance between carbonate accumulation and
degradation in recent carbonate buildups, and the implications of this interplay
on the fossil record.
With respect to microbioerosion, Chazottes (1996) and Chazottes et al.
(2002) experimentally studied eutrophicated versus pristine reef transects at
R ´ union Island and concluded that the influence of reef-water quality on bioero-
sion is chiefly governed by the composition of the epibiont cover. Grazing was
lowest and microbioerosion highest in the nutrient-enriched transects where
macroalgae and encrusting algae flourished, whereas pristine reef areas