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Abstract
Annually large volumes of fresh water laden with sediment are washed down the Daintree
River in North Queensland into the Great Barrier Reef lagoon. To investigate the effects
of land runoff on bioerosion, samples of recently killed colonies of Porites were
laid at 6 sites on a cross shelf transect from Snapper Island at the entrance to the
river to Osprey Reef, approximately 328 km from the river mouth out in the Coral Sea.
Rates and agents of bioerosion were determined over 4 years and inshore sites exhibited
significantly lower rates of total bioerosion than the other sites. Offshore sites
experienced high rates of bioerosion primarily due to grazing and internal bioerosion
by macroborers such as sponges and bivalves was also important at some of these sites.
Inshore sites were covered in heavy layers of silt which inhibited colonization and
growth of microborers, primarily algae. This resulted in lower levels of grazing than
at offshore sites. However the activity of macroborers (primarily sponges and bivalves)
was often high at these sites. The macroboring communities differed between sites
and over time and it is hypothesised that these site differences were due to different
levels of terrestrial runoff. These results are compared with those from French Polynesia
where contaminated terrestrial runoff greatly influenced rates and agents of bioerosion.
However other factors such as overfishing may also play an important role in some
locations.