Twenty Years of Marine Carbon Cycle Observations at Devils Hole Bermuda Provide Insights into Seasonal Hypoxia, Coral Reef Calcification, and Ocean Acidification

Abstract

Open-ocean observations have revealed gradual changes in seawater carbon dioxide (CO 2 ) chemistry resulting from uptake of atmospheric CO 2 and ocean acidification (OA), but, with few long-term records (>5 years) of the coastal ocean that can reveal the pace and direction of environmental change. In this paper, observations collected from 1996 to 2016 at Harrington Sound, Bermuda, constitute one of the longest time-series of coastal ocean inorganic carbon chemistry. Uniquely, such changes can be placed into the context of contemporaneous offshore changes observed at the nearby Bermuda Atlantic Time-series Study (BATS) site. Onshore, surface dissolved inorganic carbon (DIC) and partial pressure of CO 2 (pCO 2 ; >10% change per decade) have increased and OA indicators such as pH and calcium carbonate (CaCO 3 ) saturation state ( ) decreased from 1996 to 2016 at a rate of two to three times that observed offshore at BATS. Such changes, combined with reduction of total alkalinity over time, reveal a complex interplay of biogeochemical processes influencing Bermuda reef metabolism, including net ecosystem production (NEP = gross primary production-autotrophic and heterotrophic respiration) and net ecosystem calcification (NEC = gross calcification-gross CaCO 3 dissolution). These long-term data show a seasonal shift between wintertime net heterotrophy and summertime net autotrophy for the entire Bermuda reef system. Over annual time-scales, the Bermuda reef system does not appear to be in trophic balance, but rather slightly net heterotrophic. In addition, the reef system is net accretive (i.e., gross calcification > gross CaCO 3 dissolution), but there were occasional periods when the entire reef system appears to transiently shift to net dissolution. A previous 5-year study of the Bermuda reef suggested that net calcification and net heterotrophy have both increased. Over the past 20 years, rates of net calcification and net heterotrophy determined for the Bermuda reef system have increased by ∼30%, most likely due to increased coral nutrition occurring in concert with increased offshore productivity in the surrounding subtropical North Atlantic Ocean. Importantly, this long-term study reveals that other environmental factors (such as coral feeding) can mitigate against the effects of ocean acidification on coral reef calcification, at least over the past couple of decades.

Autonomous Optofluidic Chemical Analyzers for Marine Applications: Insights from the Submersible Autonomous Moored Instruments (SAMI) for pH and pCO2

“…A more complete interpretation of these data will be presented in a future manuscript. Although we had limited data for validation, the absorbances and pCO 2 returned to the levels found when the instruments were deployed, completing the annual cycle (most biogeochemical species have similar year to year levels, e.g., Bates, 2017). This observation supports the long-term stability of the SAMI-CO 2 measurements.…”

Historical Trends in pH and Carbonate Biogeochemistry on the Belize Mesoamerican Barrier Reef System

“…It is also likely that the enhanced NEP in the forereef since the 1980s contributes to negating the open ocean OA signal evident in the backreef (BR-06) pH record. Similarly, the larger-than-expected pH change recorded in the backreef (BR-06) pH record, given historical changes in atmospheric pCO 2 (Figure 2), appears to be largely a consequence of enhanced NEC in the backreef-as observed at other backreef localities on shorter time scales (e.g., Bates, 2017).…”

“…For instance, the apparent increase in NEC in the backreef relative to the forereef could be caused by either an enhancement of calcification in the backreef (relative to the forereef) or an increase in the residence time of the water on the reef so the impact of calcification is proportionally enhanced (e.g., DeCarlo et al, 2017). Similarly, the larger-than-expected pH change recorded in the backreef (BR-06) pH record, given historical changes in atmospheric pCO 2 (Figure 2), appears to be largely a consequence of enhanced NEC in the backreef-as observed at other backreef localities on shorter time scales (e.g., Bates, 2017). Castillo et al (2011) observed that rates of linear extension in S. siderea decreased in the forereef and increased in the backreef of the southern MBRS since the early 1900s.…”

“…Unfortunately, such monitoring of the "health" of coral reefs has only recently begun and instrumental records longer than~5 years are rare. Those longer-term records that do exist tend to be characterized by considerable gaps in sampling (e.g., Silverman et al, 2012Silverman et al, , 2014, with the longest continual record thus far restricted to the last 20 years (Bates, 2017). As a consequence, the full impact of the acidification that has occurred over the last 100 years (~0.1 pH units in the open ocean; Bates et al, 2014), and the ability (or lack thereof) of reef systems to modulate their carbonate biogeochemistry against this apparent threat, remains largely unknown.…”

Biodiversity and Distribution of Benthic Foraminifera in Harrington Sound, Bermuda: The Effects of Physical and Geochemical Factors on Dominant Taxa

“…Patton's Basin is located directly southwest of Trunk Island and directly northwest of Devil's Hole, with a depth range of 17-24 meters; North Basin is directly northeast of Trunk Island and directly west of Hall's Island, with a depth range of 15-20 meters; Middle Ridge is directly south of North Basin and directly southeast of Trunk Island, with a depth range of 10-15 meters; South Basin is directly southeast of Middle Ridge, directly west of Shark Hole, and northeast of Devil's Hole, with a depth range of 17-22 meters. According to the modified version of Bates (2017)'s bathymetric map, Harrington Sound's profile is highly complex, and it was important to account for steep depth-and-slope changes during the sample-collection process. As seen in Figure 2, sample sites 1, 3, and 5 were around Trunk Island, site 4 was east of Flatt's Inlet, site 6 was east of Green Bay Cave, site 7 was collected in the northern section of Patton's Basin, site 10 was collected in the southern section of Patton's Basin/neighboring the northern section of Devil's Hole, site 8 was collected in the northern section of South Basin/neighboring the southern section of Middle Ridge, and site 2 was collected offshore of the old post office and in Middle Ridge.…”