Millennium-scale records of benthic foraminiferal communities from the central Great Barrier Reef reveal spatial differences and temporal consistency
Understanding long-term community dynamics and the ways in which they respond to major disturbances is a central management theme within coastal marine ecosystems. River outputs from the Queensland coastline directly affect inshore marine communities of the Great Barrier Reef (GBR), Australia. Of these, the Burdekin River exports the highest volume of terrestrial runoff. Following European settlement in the mid-19th century, over three quarters of the native vegetation from the Burdekin catchment were cleared for agricultural purposes. Despite such extensive historical catchment modification, the impact of these changes on the inshore GBR is largely unknown, primarily due to the paucity of long-term ecological data. To assess the effects of modern land-use change on inshore reef environments and to establish an historical baseline of community structure, we examined the sedimentary geochemistry and benthic foraminiferal assemblages of eight sediment cores collected from two coral reefs situated inside (Pandora) and outside (Havannah) an inner-shelf sediment prism formed during the Holocene. Foraminiferal community structure was reconstructed from the past millennium, and the time series was constrained using U-series dating of coral fragments within the cores. Environmental records were reconstructed using stable carbon isotopes (δ13C) and elemental C:N ratios from bulk sediment samples. Non-parametric analysis of community structure in benthic foraminifers indicated no change in community structure through time at either reef. Despite this apparent ecological persistence through time, significant differences in foraminiferal community structure were observed between the two reefs. The communities were clearly characterized by different functional groups; heterotrophic genera were persistent within, and symbiont-bearing genera were persistent outside, the Holocene inshore sediment wedge. We found no difference in the source of organic matter (interpreted from δ13C values) either between reefs or through time, yet elemental C:N ratios indicated a difference in the amount of organic matter between reefs. The influence of the Holocene inshore sediment wedge was demonstrated by the dissimilarity in sedimentary C:N ratios between the two reefs.