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

Abstract

Shallow, permeable calcium carbonate (CaCO 3 ) sediments make up a large proportion of the benthic cover on coral reefs and account for a large fraction of the standing stock of CaCO 3 . 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 CaCO 3 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 CaCO 3 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.

Late Afternoon Seasonal Transition to Dissolution in a Coral Reef: An Early Warning of a Net Dissolving Ecosystem?

“…When comparing NEC reef to NCC sediment , they show similar patterns in monthly and seasonal variation (Stoltenberg et al., 2019). Most importantly, some of the months in which reef NEC was negative before sunset coincide with the months where highest sediment dissolution rates were measured (Stoltenberg et al., 2019).…”

“…When comparing NEC reef to NCC sediment , they show similar patterns in monthly and seasonal variation (Stoltenberg et al., 2019). Most importantly, some of the months in which reef NEC was negative before sunset coincide with the months where highest sediment dissolution rates were measured (Stoltenberg et al., 2019). In December, for example, our estimated contribution of NCC sediment to reef NEC is also among the highest (78%), indicating that the reef‐wide dissolution measured in that month may be a direct result of increased rates of sediment dissolution (Table 1).…”

“…Over a diel cycle, coral reefs and sediments are typically net calcifying (positive NEC) during the day and net dissolving (negative NEC) at night. Generally, daytime calcification exceeds nighttime dissolution, resulting in positive NEC over 24 h (McMahon et al., 2013; Stoltenberg et al., 2019). There are some exceptions to this; for example, in the Gulf of Eilat, Red Sea, the reef maintained positive NEC throughout the night (Silverman et al., 2007) and a barrier reef flat in Hawaii was net dissolving at night in only about half of the measurements (Shamberger et al., 2011).…”

“…There are some exceptions to this; for example, in the Gulf of Eilat, Red Sea, the reef maintained positive NEC throughout the night (Silverman et al., 2007) and a barrier reef flat in Hawaii was net dissolving at night in only about half of the measurements (Shamberger et al., 2011). In contrast, some reefs and reef sediments already show seasonal net dissolution (Eyre et al., 2018; Meléndez et al., 2018; Muehllehner et al., 2016; Stoltenberg et al., 2019). Periods of net dissolution have been attributed to instances of very high cloud cover and turbidity (Takeshita et al., 2016; Yates & Halley, 2006) regional acidification events (Yeakel et al., 2015) or when the reef is net heterotrophic (e.g., negative net ecosystem production; NEP; N. Bates et al., 2009, 2010; McMahon et al., 2013).…”

Meta-Analysis Reveals Reduced Coral Calcification Under Projected Ocean Warming but Not Under Acidification Across the Caribbean Sea

“…Declines in carbonate production rates (Perry et al, 2013) and reduced coral cover (Cote et al, 2005) on Caribbean reefs have shifted these ecosystems to less structurally complex reefs dominated by algae, sponges, and gorgonians (Norstrom et al, 2009). Over the past two decades, ocean warming and acidification have been identified as two of the primary stressors causing this shift and therefore there have been widespread efforts to better understand how coral reefs will likely respond to projected increases in these two human-induced global change stressors (Kleypas et al, 1999;Doney et al, 2009;Enochs et al, 2016;Stoltenberg et al, 2019).…”