There is growing concern about the effects of ocean acidification (OA) on coral reefs, with many studies indicating decreasing calcium carbonate production and reef growth. However, to accurately predict how coral reefs will respond to OA, it is necessary to characterize natural carbonate chemistry conditions, including the spatiotemporal mean and variability and the physical and biogeochemical drivers across different environments. In this study, spatial and temporal physiochemical variability was characterized at two contrasting reef locations in Bocas del Toro, Panama, that differed in their benthic community composition, reef morphology, and exposure to open ocean conditions, using a combination of approaches including autonomous sensors and spatial surveys during November 2015. Mean and diurnal temporal variability in both physical and chemical seawater parameters were similar between sites and sampling depths, but with occasional differences in extreme values. The magnitude of spatial variability was different between the two sites, which reflected the cumulative effect from terrestrial runoff and benthic metabolism. Based on graphical vector analysis of TA–DIC data, reef metabolism was dominated by organic over inorganic carbon cycling at both sites, with net heterotrophy and net calcium carbonate dissolution dominating the majority of observations. The results also highlight the potentially strong influence of terrestrial freshwater runoff on surface seawater conditions, and the challenges associated with evaluating and characterizing this influence on benthic habitats. The Bocas del Toro reef is a unique system that deserves attention to better understand the mechanisms that allow corals and coral reefs to persist under increasingly challenging environmental conditions.
Coral reef community composition and ecosystem function may change in response to anthropogenic ocean acidification. However, the magnitude of acidification on reefs will be modified by natural spatial and temporal variability in seawater CO2chemistry. Consequently, it is necessary to quantify the ecological, biogeochemical, and physical drivers of this natural variability before making robust predictions of future acidification on reefs. In this study, we measured temporal and spatial physiochemical variability on a reef flat in Kāne‘ohe Bay, O‘ahu, Hawai‘i, using autonomous sensors at sites with contrasting benthic communities and by sampling surface seawater CO2chemistry across the reef flat at different times of the day during June and November. Mean and diurnal temporal variability of seawater CO2chemistry was more strongly influenced by depth gradients (~ 0.5–10 m) on the reef rather than benthic community composition. Spatial CO2chemistry gradients across the reef flat reflected the cumulative influence from benthic metabolism, bathymetry, and hydrodynamics. Based on graphical assessment of total alkalinity–dissolved inorganic carbon data, reef metabolism in November was dominated by organic carbon cycling over inorganic carbon cycling, while these processes were closely balanced in June. Overall, this study highlights the strong influence of depth on reef seawater CO2chemistry variability through its effects on benthic biomass to seawater volume ratio, seawater flow rates, and residence time. Thus, the natural complexity of ecosystems where a combination of ecological and physical factors influence reef chemistry must be considered when predicting ecosystem biogeochemical responses to future anthropogenic changes in seawater CO2chemistry.
more » « less- NSF-PAR ID:
- 10461587
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 64
- Issue:
- 3
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 913-934
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
null (Ed.)Spatial and temporal carbonate chemistry variability on coral reefs is influenced by a combination of seawater hydrodynamics, geomorphology, and biogeochemical processes, though their relative influence varies by site. It is often assumed that the water column above most reefs is well-mixed with small to no gradients outside of the benthic boundary layer. However, few studies to date have explored the processes and properties controlling these multi-dimensional gradients. Here, we investigated the lateral, vertical, and temporal variability of seawater carbonate chemistry on a Bermudan rim reef using a combination of spatial seawater chemistry surveys and autonomous in situ sensors. Instruments were deployed at Hog Reef measuring current flow, seawater temperature, salinity, pH T , p CO 2 , dissolved oxygen (DO), and total alkalinity (TA) on the benthos, and temperature, salinity, DO, and p CO 2 at the surface. Water samples from spatial surveys were collected from surface and bottom depths at 13 stations covering ∼3 km 2 across 4 days. High frequency temporal variability in carbonate chemistry was driven by a combination of diel light and mixed semi-diurnal tidal cycles on the reef. Daytime gradients in DO between the surface and the benthos suggested significant water column production contributing to distinct diel trends in pH T , p CO 2 , and DO, but not TA. We hypothesize these differences reflect the differential effect of biogeochemical processes important in both the water column and benthos (organic carbon production/respiration) vs. processes mainly occurring on the benthos (calcium carbonate production/dissolution). Locally at Hog Reef, the relative magnitude of the diel variability of organic carbon production/respiration was 1.4–4.6 times larger than that of calcium carbonate production/dissolution, though estimates of net organic carbon production and calcification based on inshore-offshore chemical gradients revealed net heterotrophy (−118 ± 51 mmol m –2 day –1 ) and net calcification (150 ± 37 mmol CaCO 3 m –2 day –1 ). These results reflect the important roles of time and space in assessing reef biogeochemical processes. The spatial variability in carbonate chemistry parameters was larger laterally than vertically and was generally observed in conjunction with depth gradients, but varied between sampling events, depending on time of day and modifications due to current flow.more » « less
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The interplay between ocean circulation and coral metabolism creates highly variable biogeochemical conditions in space and time across tropical coral reefs. Yet, relatively little is known quantitatively about the spatiotemporal structure of these variations. To address this gap, we use the Coupled Ocean Atmosphere Wave and Sediment Transport (COAWST) model, to which we added the Biogeochemical Elemental Cycling (BEC) model computing the biogeochemical processes in the water column, and a coral polyp physiology module that interactively simulates coral photosynthesis, respiration and calcification. The coupled model, configured for the north-shore of Moorea Island, successfully simulates the observed (i) circulation across the wave regimes, (ii) magnitude of the metabolic rates, and (iii) large gradients in biogeochemical conditions across the reef. Owing to the interaction between coral net community production (NCP) and coral calcification, the model simulates distinct day versus night gradients, especially for pH and the saturation state of seawater with respect to aragonite (Ω
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