Long-term variation in light intensity on a coral reef
An important goal of coral reef science is to understand the roles played by environmental conditions in determining benthic community structure. Pursuit of this goal typically involves testing for associations between community structure and environmental conditions, and in recent years, attention has focused on temperature and seawater pH. Such analyses are helpful in projecting coral reef community structure into the future, but their efficacy depends on the extent to which changes in community structure are correctly associated with causative processes, and whether future conditions are a subset of those experienced in the past. Here, we describe variation from 2014 to 2017 in one environmental factor, photosynthetically active radiation (PAR), at 19-m depth on the reefs of St. John, US Virgin Islands, and evaluate the extent to which this factor could cause changes in benthic community structure. The transmission of downwelling surface PAR to 19-m depth (TPAR-19) varied from 2.0 to 40.0% among days, and from 4.8 to 15.8% among months within a year, and was lower and more variable in the autumn versus other seasons. Monthly TPAR-19 was inversely associated with monthly rainfall, suggesting that runoff affected seawater clarity. The monthly diffuse attenuation coefficient (Kd-PAR) varied 1.7-fold from 0.097 to 0.164 m−1 and was within the range expected for coral reefs. Given the high temporal variation in underwater light intensity, and its importance to foundational reef taxa, understanding of the response of coral reefs to temperature and pH (and other factors) will remain incomplete without closer attention to this physical environmental condition.
KeywordsDiffuse attenuation coefficient PAR Caribbean Scleractinia Climate change
This research was funded by the US National Science Foundation (to PJE DEB 13-50146 and OCE 14-15268) and was completed under permits issued by the Virgin Islands National Park (most recently VIIS-2017-SCI-0031 and VIIS-2017-SCI-0010). We thank A. Martinez, S. Zimmermann, H. Nelson, and A. Potter for assistance with instrument deployment, V. Powell, S. Prosterman, and R. Lockhart for on-site logistical support, the staff of the Virgin Islands Environmental Resource Station (VIERS) for making our visits to St. John productive and enjoyable, and two anonymous reviewers whose comments improved an early draft of this paper. This is contribution number 274 of the CSUN marine biology program.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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