Coral Reef Carbonate Chemistry Variability at Different Functional Scales

Abstract

There is a growing recognition for the need to understand how seawater carbonate chemistry over coral reef environments will change in a high-CO 2 world to better assess the impacts of ocean acidification on these valuable ecosystems. Coral reefs modify overlying water column chemistry through biogeochemical processes such as net community organic carbon production (NCP) and calcification (NCC). However, the relative importance and influence of these processes on seawater carbonate chemistry vary across multiple functional scales (defined here as space, time, and benthic community composition), and have not been fully constrained. Here, we use Bermuda as a case study to assess (1) spatiotemporal variability in physical and chemical parameters along a depth gradient at a rim reef location, (2) the spatial variability of total alkalinity (TA) and dissolved inorganic carbon (DIC) over distinct benthic habitats to infer NCC:NCP ratios [< several km 2 ; rim reef vs. seagrass and calcium carbonate (CaCO 3 ) sediments] on diel timescales, and (3) compare how TA-DIC relationships and NCC:NCP vary as we expand functional scales from local habitats to the entire reef platform (10's of km 2 ) on seasonal to interannual timescales. Our results demonstrate that TA-DIC relationships were strongly driven by local benthic metabolism and community composition over diel cycles. However, as the spatial scale expanded to the reef platform, the TA-DIC relationship reflected processes that were integrated over larger spatiotemporal scales, with effects of NCC becoming increasingly more important over NCP. This study demonstrates the importance of considering drivers across multiple functional scales to constrain carbonate chemistry variability over coral reefs.

Spatiotemporal variability in seawater carbon chemistry for a coral reef flat in Kāne‘ohe Bay, Hawai‘i

“…). The explanation for the observed trends as a function of depth was twofold; first, shallower depth likely resulted in increasing biomass to water volume ratio, and thus, greater modification of seawater chemistry owing to biogeochemical processes (i.e., NCP and NCC) (Falter et al ; Takeshita et al ). Similarly, shallow depths have higher heat gain or loss per volume of water compared to deeper parts of the reef, and consequently, experienced greater temperature extremes and variability (Lowe and Falter ).…”

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…For example, Hofmann et al [55] compiled pH sensor data from ecosystems ranging from Antarctica to tropical coral reefs, and presented distinct biome-specific pH patterns that occur on diel, semi-diel, and stochastic timescales. This led to further investigation into exploring patterns of variability in ecosystems such as kelp forests [57,58], coral reefs [59][60][61][62], seagrass meadows [63,64], continental shelf [65], and upwelling regions [66,67]. Diel pH variability in coral reefs was found to correlate with community structure and net accretion rates [68], suggesting the potential for natural variability to influence impacts of ocean acidification.…”

pH Variability Exacerbates Effects of Ocean Acidification on a Caribbean Crustose Coralline Alga

“…The variable treatments simulated a 0.3 unit diel cycle in pH, with highest pH during peak daylight hours, and lowest pH at the end of the night. This represents daily changes in pH due to photosynthesis during the day and respiration during the night (Rivest et al, 2017;Takeshita et al, 2018). In the amb/variable treatment, pH ranged from 7.65 to 7.95 over 24 h, simulating a moderate diel cycle in pH that is characteristic on some shallow coral reefs (Hofmann et al, 2011;Price et al, 2012;Guadayol et al, 2014).…”

“…Until recently, the majority of perturbation experiments have used this as a basis for simulating OA with constant low pH treatments. However, many benthic organisms, including tropical CCA, inhabit nearshore ecosystems where environmental conditions, including pH, fluctuate over multiple temporal scales (Takeshita et al, 2018). In these habitats, biophysical coupling between metabolism (e.g., photosynthesis and respiration) and physical properties of the water column can cause pH to fluctuate on a diel cycle (Kleypas et al, 2011).…”