Fringing and nearshore coral reefs of the Great Barrier Reef: episodic Holocene development and future prospects

Abstract

The Holocene growth of fringing and nearshore reefs on the GBR is examined. A review of data from 21 reefs indicates that most grow upon Pleistocene reef, boulder, and gravel, or sand and clay substrates, with no cored examples growing directly over rocky headlands or shores. Dated microatolls and material from shallow reef-flat cores indicate that fringing and nearshore reefs have experienced several critical growth phases since the mid-Holocene: (1) from initiation to 5500 YBP, optimum conditions for reef and reef-flat growth prevailed; (2) from 5500–4800 YBP, reef-flat progradation stalls in almost 50% of the reefs examined; (3) of reefs prograding post-4800 YBP, approximately half ceased active progradation around 3000–2500 YBP; (4) reefs prograding to present do so at rates well below mid-Holocene rates; (5) a group of nearshore reefs has established since 3000 YBP, in conditions traditionally considered poor for reef establishment and growth. Importantly, many of the reefs that appear to have grown little for several millennia are veneered by well-developed coral communities. Although local conditions no doubt exert some influence over these growth patterns, the apparent synchronicity of these growth and quiescent phases over wide geographical areas suggests the involvement of broader scale influences, such as climate and sea-level change. Recognition and understanding these phases of active and moribund reef growth provides a useful longer term context in which to evaluate reported current declines in fringing and nearshore reef condition.