Seasonal variability of calcium carbonate precipitation and dissolution in shallow coral reef sediments

Shallow, permeable calcium carbonate (CaCO₃) sediments make up a large proportion of the benthic cover on coral reefs and account for a large fraction of the standing stock of CaCO₃. There have been a number of laboratory, mesocosm, and in situ studies examining shallow sediment metabolism and dissolution, but none of these have considered seasonal variability. Advective benthic chambers were used to measure in situ net community calcification (NCC) rates of CaCO₃ sediments on Heron Island, Australia (Great Barrier Reef) over an annual cycle. Sediments were, on average, net precipitating during the day and net dissolving at night throughout the year. Night dissolution rates (−NCC_NIGHT) were highest in the austral autumn and lowest in the austral winter driven by changes in respiration (R) and to a lesser extent temperature and Ω_arag/pH. Similarly, precipitation during the day (+NCC_DAY) was highest in March and lowest in winter, driven primarily by benthic net primary production (NPP) and temperature. On average, sediments were net precipitating over a diel cycle (NCC_24h) but shifted to net dissolving in July and December. This shift was largely caused by the differential effects of seasonal cycles in organic metabolism and carbonate chemistry on NCC_DAY and NCC_NIGHT. The results from this study highlight the large variability in sediment CaCO₃ dynamics and the need to include repeated measurements over different months and seasons, particularly in shallow reef systems that can experience large swings in light, temperature, and carbonate chemistry.