Sub-weekly coral linear extension measurements in a coral reef

Abstract

Coral growth rates are often used as a metric of coral health and are measured extensively in the laboratory under controlled conditions to better understand the potential impacts of future climate change scenarios. However, in the field, corals live in dynamic environments, which can be subjected to multiple types of stressors that can not be mimicked in the laboratory. Furthermore, the temporal scales over which many environmental conditions can vary in the reef, such as extreme temperature anomalies, tidal fluctuations, and point source pollution events are far shorter than most field estimates of coral growth, which are generally at annual or seasonal scales. To measure the impact to coral growth of environmental variables that vary on time scales of less than a year or a few months requires developing new growth measurement techniques. With the goal of measuring coral growth at sub-weekly scales in the field, we developed a technique to measure 5-day linear extension growth rates. We tested our approach on colonies of Acropora hyacinthus living in a shallow back-reef ecosystem with routine extreme daily fluctuations in temperature, pH, and dissolved oxygen saturation. Using serial skeletal staining and petrographic thin sectioning we measured linear extension in A. hyacinthus during three consecutive 5-day growth periods that had differing amounts of environmental variability. At our field site in American Samoa, the second growth period had the largest tidal swings, resulting in higher variability: within a day temperature ranged up to 5.4 °C (reaching a maximum of 31.9 °C) and pH ranged up to 0.41 units (with a minimum of pH 7.78). We tested whether corals are able to maintain even linear extension rates across these short periods of time or not. After confocal microscopy analysis of stained skeletal samples we found that linear extension rates were similar across the three growth periods. Our fine-scale measurements suggest that during periods with different magnitudes of tidally driven environmental variability, but constant mean conditions, short-term linear extension growth rates remain consistent.