Reef shallowing is a critical control on benthic foraminiferal assemblage composition on nearshore turbid coral reefs

J.A Johnson,C.T Perry,S.G Smithers,K.M Morgan,S.A Woodroffe

doi:10.1016/j.palaeo.2019.109240

Abstract

Declining water quality represents a major threat to the coral reefs of Australia's Great Barrier Reef (GBR). Due to their close proximity to river point sources, reefs situated on the inner-shelf of the GBR are widely perceived to be most susceptible to the increased discharge of nutrients and sediments from coastal catchments, many of which have been modified since European settlement. However, the impact and magnitude of water quality change on the GBR's inner-shelf reefs remains unclear. Much of the uncertainty can be attributed to a paucity of long-term records with which to assess ecological and environmental change over appropriate reef-building timescales. Here, we present benthic foraminiferal palaeo-records from three proximal nearshore turbid-zone reefs located within the central region of the GBR. Bayesian age-depth modelling was used to identify the core intervals corresponding to the timing of European settlement in the region (c. 1850 CE), enabling the investigation of: (1) the composition and variability of benthic foraminiferal assemblages during reef shallowing towards sea level; and (2) whether any signal of increased nutrient and sediment inputs, as the result of historical land-use change, can be discerned on the most nearshore reefs of the central GBR. Multivariate analyses identified two assemblage groups, delineated by a significant increase in the relative abundance of Pararotalia spp. up-core. Our results suggest that post-European settlement associated increases in nutrient and sediment inputs are unlikely to have driven the observed shifts in benthic foraminiferal assemblage composition. Rather, we interpret changes in the composition of benthic foraminiferal assemblages as being linked to changes in hydrodynamic energy, light availability and the carbonate content of reef-matrix sediments during reef shallowing towards sea level. Our findings support the hypothesis that nearshore turbid-zone reefs have a higher resistance to increased nutrient and sediment inputs than those located further offshore, towards the inner/mid-shelf boundary of the GBR.

Highlights

Benthic foraminifera are valuable biological indicators of environmental change, with respect to sea level (e.g., Gehrels, 2000; Woodroffe, 2009a) and water quality (e.g., Alve, 1995; Frontalini and Coccioni, 2011)
A single core was selected for analysis from the ‘mature’ reef of Offshore Paluma Shoals (OPS; OPSPC2) that is emergent at LAT; and the ‘juvenile’ reefs of Offshore Paluma Shoals-C (OPS-C; OPS-C-PC1) and Offshore Paluma Shoals-D (OPS-D; OPS-D-PC1) that are both fully submerged at LAT (Fig. 1)
Palaeoecological records established from reef cores recovered from the Paluma Shoals reef complex (PSRC) enabled the investigation of long-term foraminiferal assemblage composition and variability within a nearshore turbid-zone reef setting on the central Great Barrier Reef (GBR)

Summary

Introduction

Benthic foraminifera are valuable biological indicators (or bioindicators) of environmental change, with respect to sea level (e.g., Gehrels, 2000; Woodroffe, 2009a) and water quality (e.g., Alve, 1995; Frontalini and Coccioni, 2011). The utility of benthic foraminifera as bioindicators is largely due to their high taxonomic diversity and abundance; narrow ecological specificity; excellent preservation potential; and relatively short life spans (Coccioni, 2000; Schönfeld et al, 2012). Due to their ability to capture both long-term and short-term environmental change, the application of benthic foraminifera as bioindicators in tropical coral reef environments is a subject of growing interest (Renema, 2018). Deteriorating water quality is considered a major threat to the ecological health of Australia's Great Barrier Reef (GBR). Reported collapses of hard coral communities have been attributed to regional deteriorations in water quality (Roff et al, 2013; Clark et al, 2017), and are often associated with increased macro-algal cover on reefs (De'ath and Fabricius, 2010), Crown-of-Thorns seastar (Acanthaster planci) outbreaks

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