[Coral-List] Reefs cooled by internal wave bores

[Mike Field]

Colleagues,

For those of you interested in changing sea surface temperatures and possible beneficial effects  from internal wave bores, we have a recent paper in Coral Reefs that may be of interest. The title and abstract are below; if you are interested and don't have access to the journal, please contact the lead author (cstorlazzi at usgs.gov) to obtain a pdf.

Mike



Rapid fluctuations in flow and water-column properties in Asan Bay, Guam: Implications for selective resilience of coral reefs in warming seas


Hydrodynamics and water-column properties were investigated off west-central Guam from July 2007 through January 2008. Rapid fluctuations, on time scales of 10s of min, in currents, temperature, salinity, and acoustic backscatter were observed to occur on sub-diurnal frequencies along more than 2 km of the fore reef but not at the reef crest. During periods characterized by higher sea-surface temperatures, weaker wind forcing, smaller ocean surface waves, and greater thermal stratification, rapid decreases in temperature and concurrent rapid increases in salinity and acoustic backscatter coincided with onshore-directed near-bed currents and offshore-directed near-surface currents. During the study, these cool-water events, on average, lasted 2.3 hours and decreased the water temperature 0.57°C, increased the salinity 0.25 PSU, and were two orders of magnitude more prevalent during the summer season than the winter. During the summer season when the average satellite-derived sea-surface temperature anomaly was +0.63°C, these cooling events, on average, lowered the temperature 1.14°C along the fore reef but only 0.11°C along the reef crest. The rapid shifts appear to be the result of internal tidal bores pumping cooler, more saline, higher-backscatter oceanic water from depths greater than 50 m over cross-shore distances of 100s of m into the warmer, less saline waters at depths of 20 m and shallower. Such internal bores appear to have the potential to buffer shallow coral reefs from predicted increases in sea-surface temperatures by bringing cool, offshore water to shallow coral environments. These cooling internal bores may also provide additional benefits to offset stress such as supplying food to thermally-stressed corals, reducing stress due to ultraviolet radiation and/or low salinity, and delivering coral larvae from deeper reefs not impacted by surface thermal stress. Thus the presence of internal bores might be an important factor locally in the resilience of select coral reefs facing increased thermal stress.

Storlazzi, C.D., Field, M.E., Cheriton, O.M., Presto, M.K., and Logan, J.B., 2013. “Rapid fluctuations in flow and water-column properties in Asan Bay, Guam: Implications for selective resilience of coral reefs in warming seas” Coral Reefs, v. 32, p. 949-961






********************
Michael E. Field
Senior Scientist Emeritus
U.S. Geological Survey 
Pacific Coastal and Marine Science  Center
400 Natural Bridges Dr.
Santa Cruz, CA 95060

831-212-1251
mfield at usgs.gov

http://coralreefs.wr.usgs.gov