Flood response and palaeoecology of the high-latitude, terrigenoclastic influenced coral reefs of Hervey Bay, Queensland, Australia

Abstract

Worldwide, coral reefs are considered to be in a state of decline. While anthropogenic stressors such as land modification, overfishing, coral harvesting, recreational impacts and pollution are known to negatively impact coral reef communities and are of great concern, we have little knowledge of the natural historical range of variability in coral abundance and community structure with which to compare the current state. This is particularly the case for marginal coral reefs, which are not often the subject of research, but can offer unique insights into the natural and anthropogenic stressors of coral reefs. In order to examine the natural and anthropogenic drivers of the taxonomic composition of marginal coral reefs, we examined both modern and historical coral assemblages from the high-latitude, terrigenoclastic influenced coral reefs of Hervey Bay, Queensland, Australia. To understand the potential impacts of modern terrestrial runoff, a likely driver of historical change on these reefs, photo-transects of modern coral communities before and after repeated flooding from nearby rivers were used to assess the spatial and temporal impacts of these disturbances on: coral abundance; coral community structure, and; flood-resistant trait diversity and composition. Water quality testing was carried out during the repeated flooding of 2013 to improve understanding of flood plume conditions and duration. The historical natural range of variation in coral abundance and community structure was examined through the collection of 17 sediment cores and 120 Uranium-Thorium dated samples, which enabled us to generate a precise chronological history of these coral reefs through the Holocene epoch. Flooding resulted in significant changes to coral abundance and community structure and this varied along gradients of terrestrial and riverine exposure. Total hard and soft coral abundance decreased by ~40% after flooding in 2011, with a further decrease by ~28% of the remaining coral after flooding in 2013, for a cumulative decrease of ~56%. Salinity, total suspended solids, total nitrogen and total phosphorus at inshore coral reefs were altered for up to six months relative to pre-flooding baselines as a result of flooding in 2013. Submarine groundwater discharge caused substantial reductions in salinity at these reefs for a further four months. Taxonomic composition, as well as some traits, changed significantly as a result of flooding. Although some flood impacts were expected, for example the relative increase in abundance of flood resistant species in the coral community (e.g. Turbinaria), more changes were expected to have occurred in trait composition as a result of the flooding. This indicated the likely importance of other flood associated stressors, for example hyposalinity, to flood related mortality. Spatial variability in the relative abundance of coral functional traits correlated well with both distance from the mainland and distance from rivers, indicating the importance of terrestrial and riverine stressors to reef coral community composition. Palaeoecological investigations revealed that the coral reefs of Hervey Bay have existed for at least 6500 years. Reef geomorphology varied widely, from solid, vertical relief reefs to simple, non-accreting nearshore coral communities, depending on proximity to the mainland. Historical reef coral diversity was low (13 genera) and coral communities alternated cyclically (~1600 years) between those dominated by Acropora and those dominated by other genera. Modern communities are consistent with this cyclic pattern of change. While the millennial scale periodicity was similar to that of ENSO, it was also similar to the periodicity of Dansgaard-Oeschger, Bond, oceanic tidal and thermohaline cycles. Historical coral abundance and community structure correlated significantly with frequency of ENSO, though the effect on coral community structure varied with distance from mainland. Coral abundance and community structure also correlated with SST, with increased relative abundance of Acropora at higher temperatures, though total coral abundance decreased significantly nearshore with increased SST. A lack of core material from 1200 - 1969 AD, possibly a hiatus in coral growth, prevented a clear assessment of the impacts of European colonisation and modification of local catchments. However, the contrast between currently low and historical high relative abundance of Acropora and the recent replacement with a novel coral community on Four Mile Reef after colonisation, suggests the possibility of anthropogenic influence. It is predicted that the oceans will warm substantially in the future and that high-latitude reefs may play a major role as refugia. The coral reefs of Hervey Bay have persisted for 6.5 millennia and were resilient to past changes in climate, rainfall, sea-level and terrestrial runoff. Assuming that there have been no recent anthropogenic changes to the natural stressors of these reefs, there is potential for the reefs of Hervey Bay to act as a high latitude refuge in the future. Land and reef managers should take heed of the importance of terrestrial runoff, ENSO and ocean temperatures and strive to minimise anthropogenic impacts so that the remaining coral reefs will persist for many more millennia.