Tropical coastal environments: coral reefs and mangroves - Environmental Sedimentology - page 338

(b)

(a)

20 km

Dominant grains
Halimeda
Mollusc
Coral

1963

Biscayne
Bay

N

South Florida

1989

Gulf of
Mexico

Florida Bay

20 km

Case Fig. 9.3 (a) Simplified map showing the main geomorphological features of South Florida. (b) Sediment data collected in
1963 and 1989 from the Middle Keys area showing greater abundance of coral within the sediments in 1989. (Adapted from Lidz
& Hallock 2000.)

bioeroders (borers). This has, in large part, been linked to increased nutrient levels in nearshore
waters as a result of increased sewage discharge. This stimulates phytoplankton production
and in turn rates of bioerosion by suspension-feeding boring organisms.
In a more localized study around Key Largo, Cockey et al. (1996) also identified increasing
nutrient levels in nearshore waters as a likely cause of temporal changes in the composition of
foraminiferal assemblages within reef-related sediments. Samples from 1959 to 1961 indicated
a predominance of larger, symbiont-bearing Soritidae foraminifera, which comprised some
50 - 80% of the species identified. Samples recovered from the same sites in 1991-92, however,
showed an overwhelming dominance of smaller, heterotrophic Miliolidae and Rotaliidae.
These studies illustrate the close linkages that exist between the reef community and production
of carbonate sediment. Major changes in the abundance of either carbonate producers or
degraders can result in detectable shifts in the abundance of carbonate sediment constituents
over decadal time-scales. These studies illustrate the potential of sedimentary data as geo-
indicators of environmental change.

Relevant reading

Cockey, E., Hallock, P. & Lidz, B.H. (1996) Decadal-scale changes in benthic foraminiferal assemblages off Key
Largo, Florida. Coral Reefs 15 , 237- 48.
Lidz, B.H. & Hallock, P. (2000) Sedimentary petrology of a declining reef ecosystem, Florida reef tract (U.S.A).
Journal of Coastal Research 16 , 675-97.
Perry, C.T. (1996) The rapid response of reef sediments to changes in community structure: implications for
time-averaging and sediment accumulation. Journal of Sedimentary Research 66 , 459- 67.
Szmant, A.M. & Forrester, A. (1996) Water column and sediment nitrogen and phosphorous distribution
patterns in the Florida Keys, USA. Coral Reefs 15 , 21- 42.