Assessing the impacts of ocean acidification, global warming and terrestrial runoff on the cross-shelf variability of coral calcification in the central Great Barrier Reef

Abstract

Ocean acidification and thermal stress due to anthropogenic greenhouse gas emissions present significant, potentially interacting, threats to the future of coral reefs. Coastal reef environments, as in the case of the Great Barrier Reef (GBR), can also be exposed to terrestrial stressors. This thesis evaluates the combined effects of ocean acidification, rising temperatures and river inputs on the calcification of Porites corals along a transect across-shelf the central GBR, north of Townsville. Calcification rates were obtained for 41 long-lived Porites corals from 7 reefs, in an inshore to offshore transect across the central GBR. The boron isotope composition (d11B) of selected cores was used to reconstruct annual and sub-annual changes in seawater pH in inner-shelf and mid-shelf environments. These unique seawater pH records are integrated with sea-surface temperature, river discharge and rainfall records to assess the nature and cause of seasonal, interannual, decadal and long-term (̃50 years) trends in coral calcification. Significant across-shelf differences in the temporal variability and long-term evolution of coral calcification are documented and can be related to local and global-scale changes in environmental conditions and water quality. Corals in the mid-shelf and outer-reef regions of the GBR exhibit an increase in calcification of 10.9% (1.1% S.E.) and 11.1% (3.9% S.E.) respectively since ̃1950 which are associated to the rise in sea-surface temperatures. However, calcification rates of mid-shelf corals show a decline of 3.3% (0.9% S.E.) over the recent period (1990-2008). This may indicate that a thermal optimum for calcification has been reached. Calcification rates in inner-shelf reefs over 1930-2008 display a long term trend of decreasing calcification of 4.6% (1.3% S.E.). The interannual-decadal component of variation is modulated by wet and dry periods, particularly during the last ̃40 years. The negative effects of bleaching on coral growth are evident in inshore reefs, and are particularly strong during 1998, with a significant recovery occurring after 3 years. This translate to constant calcification rates of 1.1% (2.0% S.E.) for the inner-shelf reefs over 1990-2008. These results highlight the need to consider regional differences in environmental factors when assessing and predicting changes in the GBR. Sub-annual and annual variation in the d11B of inner-shelf corals record seasonal and interannual seawater pH changes of up to 0.5 pH units. This variability is overlain on a long-term decrease of 0.02 pH units per decade, consistent with estimates of surface seawater acidification due to rising atmosphere CO2 levels. Sub-annual low pH values occur in summer and partly reflect the effects of higher temperatures and increased calcification (a source of CO2). Higher d11B (pH) values are observed in wet years when nutrients supplied by river run-off promote extensive phytoplankton blooms that take up CO2 and increase seawater pH. Decreased calcification of inner-shelf corals during large flood events, despite higher pH conditions, may reflect increased shading, turbidity, sedimentation and/or competition for carbon. The complex interactions between processes that can affect coral calcification, particularly in coastal zones, need to be considered when predicting the future of coral reefs in warmer and more acidic oceans.