Phase III – Coral Reefs and Global Change – A historical perspective spanning the Western Indian Ocean

Coral reef ecosystems are increasingly under threat due to climate change, warming oceans, ocean acidification, and terrestrial influences, such as sedimentation and eutrophication. This study proposes to examine the spatial and temporal environmental changes affecting coral reef ecosystems in the Western Indian Ocean to provide a broader management context and to allow conservation scientists to identify environments where corals are expected to survive climate change and ensure that management and conservation actions are focused on these key areas. The project had three specific objectives: (i) 1) To reconstruct sea surface temperature, river runoff and nutrient export on historical time scales by geochemical monitoring of massive coral growth banding spanning the western Indian Ocean, (ii) To study the effect of ocean acidification on coral growth and calcification on a large-scale for nearshore and open ocean reefal systems throughout the western Indian Ocean, and (iii) To quantify the effects of long-term climate and land-cover changes on reefal ecosystems by integrating remote sensing and hydrological modelling with coral data for 100 years.

The project was implemented in four countries of the WIO, namely Mauritius, Madagascar, Tanzania, and Kenya, except for objective 3 which was implemented on Madagascar. The targeted beneficiaries of this project included the scientific community, conservationists, and decision and policy makers.

To achieve the objectives, coral cores were drilled to reconstruct environmental changes in various reef complexes about natural climate change and anthropogenic influences. Several biological (growth rate, density, calcification) and environmental parameters (luminescence, trace elements, nutrients, isotopes) in massive coral skeletons of the species Porites sp. were examined in all four countries. Due to a lack of observational long-term hydrological data for Madagascar, river discharge and sediment yields in the NE and SW regional watersheds were simulated using a grid-based spatial water balance model. STREAM (Spatial Tools for River Basins, Environment and Analysis of Management Options) was used to simulate monthly river discharge. A configuration of the STREAM model was developed and run for the entire Madagascar for 1950-2006. Sediment yield per unit area was computed using the Non-Point Source Pollution and Erosion Comparison Tool (N-SPECT) developed by the National Oceanic and Atmospheric Administration (NOAA), which was run for each year from 1950-2006 

The study found a strong SST and SSS relationship between the Ifaty site and the Agulhas Channel (AC) core region. Model data confirm the physical link in SST between the MC and the AC core regions and return flow region. Further, the results show that 76.3% of the particles released reach the AC in 3 years, with a median transit time between the two sites of 240 days. Both the coral composite SST and stalagmite AT indicate the period between 1690 and 1740 as the coolest on record. In addition, both records indicate relatively high temperatures during the late 18th and 19th centuries, a cool period in the early 20th century, and a warming towards the end of the 20th century. Results suggest regional differences in the main environmental drivers of reef sedimentation (i.e. on annual time-scales, precipitation, river flow and sediment load) explained the variability in coral proxies of river discharge for the northeast region, while El Niño-Southern Oscillation (ENSO) and air and sea surface temperature being the best predictors in the southwest region. A comparison of the Sr/Ca records and growth parameters of two Porites lutea colonies, using two gridded SST datasets, off N E Madagascar showed strong seasonality and weak positive ENSO anomalies on a slow 43-year warming trend at significantly different rates. During positive ENSO events, the calcification rates of the two corals were negatively correlated, while skeletal density anomalies were opposite. The study found positive Pacific Decadal Oscillation (PDO) phases to be associated with increased Indian Ocean temperatures and rainfall in eastern Madagascar, while precipitation in southern Africa and eastern Australia declines, suggesting that PDO has important implications for future multidecadal variability of African rainfall. 

This project produced models for Madagascar for the period 1950-2006 to simulate monthly river discharge and sediment yields. The models suggest that regional land use management is far more critical than climate change in influencing sedimentation in nearshore ecosystems in Madagascar. The two models can also be applied to watersheds in East Africa and Mauritius, and this can form the scope of future work by WIOMSA.

Spin-offs from this project include two projects:  i) Netherlands Science Foundation ALW1PJ/09050: CLIMATCH – Climatic and anthropogenic change in seasonal river runoff and impacting cyclones resolved by novel spectral geochemistry of giant corals in Indian Ocean catchments (2011-2014), and ii) MASMA Integrative project: “Evaluating current responses and projecting the effects of climate change on WIO coral reef ecosystems from historical environmental variability”.