Coral Records of Sediment Input to the Fringing Reef of Moloka‘i, Hawai‘i
Pollution to coral reef ecosystems is perceived to be widespread. Anecdotal evidence suggests the threat is increasing. However, without continuous measurements of sediment concentration and coral cover, it is difficult to evaluate the quantitative impact of sediment concentrations on coral ecosystems over many decades. A new study of coral geochemistry provides one method to close this gap.
Because terrestrial runoff carries sediment, nutrients, and pollutants, there are considerable threats to the health and resilience of corals. Increased sediment concentrations in the water and on the seabed may bury coral in place, reduce its ability to recruit (add new corals to the population), reduce the amount of light available for photosynthesis, and promote harmful algal growth—ultimately leading to changes in species composition and reduced coral growth.
The watersheds of the Hawaiian Islands have experienced land-use changes over the past 200 years. Cultivation, overgrazing, and wildfires have exposed fine volcanic sands, silts, and clays to erosion. The fringing reef off the south coast of Moloka‘i is widely recognized as one of the most impacted large coral ecosystems in Hawai‘i. Anecdotal evidence suggests the level of sediment on the reef has increased over the past several decades. This has prompted several U.S. Geological Survey (USGS) studies aimed at understanding the dynamics of sediment transport and its impact on coral reef health. See "The Coral Reef of South Moloka‘i, Hawai‘i—Portrait of a Sediment-Threatened Fringing Reef," U.S. Geological Survey Scientific Investigations Report 2007-5101, edited by Michael Field, Susan Cochran, Josh Logan, and Curt Storlazzi.
To test the hypothesis that sedimentation has increased over time, the present study used barium-to-calcium (Ba/Ca) ratios as a measure of sediment input to the reef on timescales of decades to centuries. Ba is desorbed and incorporated into the coral skeleton upon contact with seawater in amounts that depend on the concentration of clay and silt particles in the water column. Because Ba is sourced from the fine-grained components of terrestrial soil, it is a proxy for the amount of land-based sediment in the coral environment.
Working with Malcolm McCulloch and Stacy Jupiter (Australian National University), USGS scientists Nancy Prouty, Mike Field, Curt Storlazzi, and Josh Logan drilled and recovered coral cores from four sites along the fringing reef of the south-shore of Moloka‘i from living scleractinian (stony) corals (Porites lobata). The cores were analyzed for their chemical content to determine if the level of sediment on the reef has increased over the last several decades.
The Effective River End-Member (EREM) concentration was calculated to gain information about the dissolved or solid Ba concentration in the contributing rivers. The coral Ba concentration was used to calculate the Ba EREM concentrations from the closest point of river discharge, the Kawela Gulch. This value was consistent with upstream Kawela storm runoff samples collected in November 2007 following peak river discharge. Baseline Ba EREM calculations were also consistent with dissolved Ba concentrations in seawater measured offshore from the Kawela Gulch during a nonstorm sampling event. Comparable values of the coral calculated Ba EREM concentrations relative to both empirical storm and baseline Ba concentrations suggest that coral Ba/Ca ratios proportionately record nearshore dissolved Ba concentrations off the South Moloka‘i reef flat.
Second-order processes such as resuspension, however, can significantly alter the barium budget in the nearshore environment. Field observations by USGS scientists Curt Storlazzi, Kathy Presto, and Mike Field and academic colleague Andrea Ogston (University of Washington) suggest that during windy afternoons and evenings, suspended sediment concentrations can be in excess of those reported to be injurious to corals. This is particularly true during storms and under swell conditions when waves propagating onto the reef flat during high tide resuspend large amounts of sediment and subsequently alter the marine Ba budget.
This study shows that controls on coral Ba/Ca ratios are a combination of sediment supply (upslope sources), sediment accommodation space (storage capacity), and resuspension. For example, coral Ba/Ca ratios were found to vary inversely to growth rate if sediment supply was relatively constant. If sediment storage capacity is sufficient (for example, via mangrove forest) and allows for periods of "recovery" when turbidity levels are reduced, then coral Ba/Ca values return to baseline values and overall growth rates are not reduced. Therefore, variability in coral Ba/Ca along the south shore of Moloka‘i can be explained by differences in sinks as well as sources.
For example, coral Ba/Ca variability at ‘Umpipa‘a, a coring site on the western side of the reef flat, is sensitive to pulses of sediment discharge associated with nearby coastal modification and episodic releases of sediment from the mangroves when its carrying capacity is breached. In particular, Ba/Ca ratios increased significantly in the mid-1930s and were accompanied by elevated Fe/Ca and Al/Ca ratios during that time, as well as a reduction in growth rate. Historical aerial photographs show a rapid mangrove expansion from 1915 to 1940, when shoreline progradation rates (rates at which the shoreline was being built seaward) reached a maximum of 27 m/year (read more about mangroves and their effects on coastal change in chapter 16 of "The Coral Reef of South Moloka‘i, Hawai‘i—Portrait of a Sediment-Threatened Fringing Reef"). Accelerated progradation rates from 1915 to 1940 are not surprising because, by the 1920s, much of the Ho‘olehua Saddle was set aside as Hawaiian Homestead lands and was opened to farming. With the muds enriched in metals such as Fe and Ba, progradation of the shoreline most likely acted as a mobile mud-belt source to the reef.
With this information, a theoretical model can be constructed indicating gradients of sediment loading (source) and sediment storage (sink) and relative response of coral growth. This information can help resource managers calculate trajectories of coral reef health related to future land-use change and climate change, because both processes will affect the supply and storage of sediment to the reef flat.
The complete reference for the new publication is: Prouty, Nancy G., Field, Michael E., Stock, Jonathan D., Jupiter, Stacy D., and McCulloch, Malcolm D., in press, Coral Ba/Ca records of sediment input to the fringing reef of the southshore of Moloka‘i, Hawai‘i over the last several decades: Marine Pollution Bulletin, 10.1016/j.marpolbul.2010.05.024 [http://dx.doi.org/10.1016/j.marpolbul.2010.05.024].
in this issue:
Sediment Input to the Fringing Reef of Moloka‘i, Hawai‘i