The effects of flow and temperature over multiple time scales on scleractinian growth and photo-physiology in Moorea, French Polynesia

Includes bibliographical references (leaves 71-78)Water flow has wide-ranging effects on coral photophysiology (light-adapted quantum yield, QY) with modest enhancement increasing QY and larger enhancement reducing QY. These opposing results suggest a threshold effect in QY along a flow continuum. This study addresses the broad effects of flow and temperature, and the time-scale over which they act on the growth and photophysiology of scleractinian corals. In the first facet of this study, I sought to reconcile the aforementioned opposing trends of the flow effects on corals by first comparing the growth of massive Porites and branching Porites irregularis among 3 lagoon areas differing in water motion to determine if flow affects growth with a threshold effect. Growth of massive Porites responded with a threshold response to flow, whereas growth of P. irregularis increased from low (8.0 emfs) to medium flow (19.0 emfs) and further increased from medium to high flow (31.0 emfs). Second, I used a flume to test the hypothesis that a flow-by-temperature interaction may explain the contrasting effects of flow on QY on the top of massive Porites. Here, QY displayed a threshold response peaking at 23 emfs at 28.3???C, but not at 31.1???C. Third, I tested for intra-colony variation in response of QY to flow and temperature. For massive Porites, QY of downstream surfaces did not vary with flow at 28.3???C, but at 31.1 ???C, QY increased to 33 emfs, subsequently decreasing to 43 emfs. QY of upstream surfaces was variable at 28.3???C though showing an initial increase with flow and a decrease under high flow; at 31.1???C, QY on upstream surfaces increased steadily to 43 emfs. For P. irregularis, QY on upstream surfaces at ambient temperature demonstrated a threshold effect with flow, whereas QY of downstream surfaces remained constant with flow. At high temperature, QY on upstream surfaces increased to 23 emfs and subsequently declined slightly. QY on downstream surfaces increase from 0 emfs to 14.5 emfs and subsequently decreased slightly at 33 emfs. In the second facet of this study, I tested for the effects of flow on coral photophysiology over ecologically-relevant medium-term (6 hr) and long-term (31 d) time periods in the flume and field, respectively, to see if the pattern observed initially can be extended through time. In both the medium- and long-term studies, photophysiological performance was higher in high flow than in low flow. Together, these results first indicate the potential for threshold effects of flow on growth of Porites in the field. In addition, they demonstrate a threshold effect of flow on photophysiology at ambient temperature in a flume and the results show intra-colony variation in response to temperature and flow. Finally, they demonstrate that under field and laboratory conditions of high light and temperature, enhanced flow maintains elevated levels of photophysiology. Thus, these results emphasize the strong possibility of conflicting flow effects on multiple facets of scleractinian physiology based on the interactive effects of flow speed, temperature and time-scale