Elsevier

Marine Chemistry

Volume 238, 20 January 2022, 104064
Marine Chemistry

Seasonal changes in seawater calcium and alkalinity in the Sargasso Sea and across the Bermuda carbonate platform

https://doi.org/10.1016/j.marchem.2021.104064Get rights and content

Highlights

  • Two-year time-series of seawater Ca and TA across the Bermuda carbonate platform

  • High precision Ca measurements using a novel spectrophotometric titration system

  • Seasonal fluctuations in Ca and TA measurements relative to offshore waters

  • Ca and TA measurements showed good agreement over similar spatiotemporal scales.

  • Joint Ca and TA anomalies may provide more robust estimates of NEC.

Abstract

Ocean acidification may shift coral reefs from a state of net ecosystem calcification (+NEC) to net ecosystem dissolution (–NEC). Changes in NEC are typically inferred from either measured or calculated total alkalinity (TA) or the dissolved calcium (Ca) to salinity ratio relative to a reference value. The alkalinity anomaly technique has historically been the primary method to estimate NEC due to the greater analytical challenges and uncertainty associated with dissolved Ca measurements in seawater. However, this method assumes that changes in salinity-normalized TA are exclusively the result of calcification and dissolution processes. In many cases, this assumption is valid, but in some environments additional processes can significantly influence seawater TA (e.g., nutrient fluxes and redox processes). Seawater Ca is unaffected or less sensitive to these processes, and therefore, Ca and TA anomalies can be used to estimate absolute or relative changes in NEC with greater confidence. Here, we present a two-year time series of monthly seawater Ca and TA measurements across the Bermuda carbonate platform and the nearby Bermuda Atlantic Time-series Study (BATS) location offshore. High precision Ca measurements (±6 μmol kg−1) were conducted using an improved spectrophotometric titration system and showed mostly good agreement with changes in TA over the same spatial and temporal scales. Ca and TA measurements across the Bermuda platform showed seasonal fluctuations relative to offshore waters, with +NEC during summer months and near-zero or possible –NEC (net dissolution) during winter months. These seasonal patterns were most pronounced at the inshore locations with the longest residence times (10+ days), which allow stronger biogeochemical signals to develop relative to the offshore source water. Although obtaining high accuracy and precision Ca measurements remains challenging, parallel measurements of Ca and TA from both inshore and offshore waters over a multi-annual timescale could strengthen the validity of predictions for when and where a reef system, such as the Bermuda platform, may shift from +NEC to –NEC.

Introduction

Coral reefs are among the most biologically diverse and economically valuable ecosystems, but their future is threatened by a multitude of local and global anthropogenic perturbations (Hughes and Connell, 1999). One of these perturbations is ocean acidification, the lowering of ocean pH and saturation state with respect to calcium carbonate (CaCO3) minerals (e.g., ΩAr for aragonite), resulting from oceanic uptake of anthropogenic carbon dioxide (CO2; Doney et al., 2009; Bates et al., 2014). Ocean acidification is expected to negatively affect organismal CaCO3 production (Kroeker et al., 2010; Chan and Connolly, 2013; Kroeker et al., 2013) while increasing CaCO3 destruction through enhanced bioerosion (Wisshak et al., 2012; Schönberg et al., 2017) and dissolution (Eyre et al., 2014). It has been suggested that these changes could shift coral reefs from a state of net calcification to net dissolution (Kleypas et al., 1999; Hoegh-Guldberg et al., 2007; Andersson et al., 2009; Silverman et al., 2009; Dove et al., 2013; Eyre et al., 2018). Positive net ecosystem calcification (+NEC) occurs when the production of CaCO3 outpaces biologically and geochemically driven CaCO3 dissolution (–NEC; Kleypas et al., 2001; Eyre et al., 2014).

Several studies have predicted that many coral reefs could transition from net calcification to net dissolution when atmospheric pCO2 levels equal or exceed 500 μatm (Yates and Halley, 2006; Hoegh-Guldberg et al., 2007; Andersson et al., 2009; Silverman et al., 2009). Negative net ecosystem calcification (–NEC; net dissolution), however, has already been documented at night and during periods of decreased calcification in some reef systems (Bates et al., 2010; Yeakel et al., 2015; Muehllehner et al., 2016; Bates, 2017). It has been suggested that because of low surface seawater [CO32−] and ΩAr during wintertime in Bermuda, this reef system may already experience periods of near-zero and negative NEC (Bates et al., 2010). Seasonal net dissolution or even annual net erosion have also been observed in other reef systems, such as the Florida Reef Tract (Muehllehner et al., 2016). The fact that certain reefs already undergo periods of net dissolution on seasonal or annual timescales raises concerns for the future function and persistence of these ecosystems. It also highlights the urgency to improve predictions for when and where a reef may experience periods of net dissolution, the duration of these periods, and what this would mean for the ecosystem as a whole.

Estimates of NEC can be calculated from the difference in measured or calculated total alkalinity (TA) (i.e., the alkalinity anomaly technique; Smith and Key, 1975), or the dissolved calcium (Ca) to salinity (S) ratio (Chisholm and Gattuso, 1991) relative to a reference value based on the following reaction:CaCO3+H2O+CO2Ca2++2HCO3where dissolution of one mole of CaCO3 produces one mole of Ca and two moles of TA with TA defined as (Dickson, 1981):TA=HCO3+2CO32+BOH4+OH+HPO42+2PO43+H3SiO4+NH3+HSH+FHSO4HFH3PO4+other minor acidsminor bases

The reverse reaction of Eq. (1) represents CaCO3 formation, which removes one mole of Ca and two moles of TA. To calculate absolute rates of NEC in coral reef environments (typically expressed in mol m−2 time−1), knowledge of seawater density, water column depth, and residence time are required to accurately integrate measured changes in TA or Ca across space and time (Smith and Key, 1975; Langdon et al., 2010). Nonetheless, comparisons of salinity-normalized TA (nTA) or Ca (nCa) values (i.e., normalized to a constant salinity to account for chemical changes due to freshwater cycling; e.g., Friis et al., 2003; Jiang et al., 2014) between inshore and offshore locations can be used to infer relative changes in NEC on different temporal and spatial scales assuming constant residence time and depth conditions (Yeakel et al., 2015).

The alkalinity anomaly technique has by far been the dominant method to measure CaCO3 dissolution and precipitation in coral reefs (Smith and Key, 1975; Kinsey, 1978; Smith and Kinsey, 1978; Gattuso et al., 1999), mostly due to the analytical challenges and uncertainty associated with Ca measurements. In order to use TA as a proxy for carbonate dissolution and precipitation, however, it is commonly assumed that these processes are primarily responsible for modifying seawater TA and that other processes are negligible (Smith and Key, 1975; Chisholm and Gattuso, 1991). This assumption is likely valid for many coral reef environments because the influence from CaCO3 production/dissolution on TA is largely dominant, but it is not valid in environments where other processes significantly affect the various proton acceptors or donors that constitute TA (see Eq. (2); Feely et al., 2002; Chung et al., 2003; Feely et al., 2004; Carter et al., 2014; Carter et al., 2021; Steiner et al., 2021). For example, TA is unaffected by the exchange of CO2 during photosynthesis and respiration, but the release and uptake of inorganic nutrients during these processes can significantly influence TA in environments where organic carbon cycling is significantly greater than CaCO3 production/dissolution, including open ocean environments and highly productive coastal regions (Brewer and Goldman, 1976; Goldman and Brewer, 1980; Kanamori and Ikegami, 1982). Similarly, processes within sediments, groundwater and certain anoxic systems can also modify alkalinity, such as anaerobic oxidation of organic matter (Emerson and Hedges, 2003; Thomas et al., 2009; Hu and Cai, 2011; Mackenzie et al., 2011) and the associated redox dynamics of various compounds, such as nitrate, iron, manganese or sulfur (Burdige, 1993). Numerous studies have shown that TA contributions from dissolved organic matter (DOM) can also be significant in organic-rich environments such as coastal waters, algal blooms, estuaries, rivers and mangroves (Cai et al., 1998; Hernández-Ayon et al., 2007; Muller and Bleie, 2008; Kim and Lee, 2009; Hunt et al., 2011; Yang et al., 2015).

In contrast to TA, alteration to the conservative behavior of Ca in seawater is more strongly dominated by CaCO3 precipitation and dissolution, although there are certainly exceptions to this in areas affected by hydrothermal activity (de Villiers, 1998) and for certain river and submarine groundwater water inputs (Beckwith et al., 2019). Outside of these exceptions, Ca measurements could add rigor to studies in both coastal and open ocean environments where multiple processes influence TA. Furthermore, in coral reef environments, it could potentially be used to evaluate the relative importance of the formation/dissolution of Mg-calcite minerals (MgxCa1-xCO3) relative to the formation/dissolution of calcite and aragonite minerals by examining the difference between TA and Ca derived NEC rates (Andersson et al., 2007). Established methods for measuring Ca, however, are often limited in application due to lower analytical precision relative to TA (~10 μmol kg−1 for most methods, e.g., Kanamori and Ikegami, 1980; Steiner et al., 2014; He et al., 2020); versus typically 2–2.5 μmol kg−1 for TA, e.g., Dickson et al., 2007) as well as considerable labor and time requirements. Nonetheless, a number of studies have employed joint Ca and TA measurements to validate the alkalinity anomaly technique (Chisholm and Gattuso, 1991), investigate the influence of different reef communities on TA:Ca ratios in mesocosm flumes (Murillo et al., 2014; Gazeau et al., 2015), and assess CaCO3 sediment dissolution in a natural environment exposed to seasonally elevated pCO2 conditions (Andersson et al., 2007). In the open ocean, Ca and TA measurements have been applied to assess the occurrence of excess alkalinity and the influence of Ca input from hydrothermal vent systems (e.g., Kanamori and Ikegami, 1982; de Villiers, 1998; Steiner et al., 2014; Rosón et al., 2016; Steiner et al., 2021). To our knowledge, however, Ca and TA measurements on larger spatial and longer temporal scales in coral reef systems are limited (e.g., Steiner et al., 2014).

Regardless of the challenges and assumptions associated with Ca measurements, joint Ca and TA anomalies have the potential to provide more robust estimates of NEC, parse apart the contribution from different processes to changes in TA, and identify the relative contribution of different mineral phases (i.e., CaCO3 or MgxCa1-xCO3) on both small and large spatial and temporal scales. Here, we evaluate an improved spectrophotometric titration system for seawater Ca measurements, which addresses some of the analytical challenges faced by other methods. We assess the potential of Ca measurements from this system as an additional proxy to infer in situ changes in reef-scale relative NEC using a multi-annual time-series of Ca and TA measurements across the Bermuda carbonate platform and offshore at the Bermuda Atlantic Time-series Station site (BATS; Michaels and Knap, 1996; Steinberg et al., 2001; Lomas et al., 2013).

Section snippets

Study site

The Bermuda carbonate platform is located in the subtropical gyre of the North Atlantic and covers an area of approximately 665 km2 (Fig. 1; Morris, 1977; Bates et al., 2010). The platform is comprised of a range of habitats including a well-developed rim reef and large, exposed fore-reef zones, a central lagoon (<18 m depth) with numerous patch reefs and extensive carbonate sand areas, and small enclosed bays and sounds (Morris, 1977; Bates, 2001). These environments are dominated by various

Offshore variability

Monthly observations from BATS showed seasonal sea surface temperature (SST) variations ranging from 20.0 °C to 28.8 °C with lowest temperatures observed in March or April and seasonal highs in September of each year. Salinity at BATS ranged from 36.29 to 36.70 (Fig. 3). Excluding statistical outliers (defined as values less than Q1 – [1.5*IQR] or greater than Q3 + [1.5*IQR], where Q1 is the 25th percentile, Q3 is the 75th percentile and IQR is the inter-quartile range [Q3-Q1]; Tukey, 1977),

Trends in calcium and total alkalinity

Assuming conservative behavior of Ca and TA in the offshore environment, nCa and nTA were expected to have little variability at BATS. Excluding statistical outliers, the standard deviation (±1σ) of nCa offshore was ±11 μmol kg−1 for the time-series. This is considerably higher relative to offshore nTA, which had a standard deviation of ±4 μmol kg−1 over the same time period. However, the relative standard deviation of nCa was smaller than that of nTA (0.10% and 0.17%, respectively). In

Concluding remarks

To our knowledge, this study represents the first multi-annual time-series of monthly Ca measurements across a coral reef platform. Coupled Ca and TA measurements showed mostly good agreement over the same spatial and temporal scales as well as strong seasonal patterns across the Bermuda platform relative to offshore waters. These findings suggest that the use of long-term calcium measurements in conjunction with alkalinity measurements can provide additional insight into seasonal and

Declaration of Competing Interest

None.

Acknowledgements

We are grateful to Mr. Taylor Wirth and Ms. Stephanie Smith for their assistance with maintaining and troubleshooting the calcium titration system. We also thank Associate Editor Dr. Alfonzo Mucci, Dr. Zvi Steiner and one anonymous reviewer for their constructive feedback that significantly improved a previous version of this manuscript.

Funding

This project was supported by the U.S. National Science Foundation (NSF) OCE 14–16518 (AJA, RJ) and OCE 12–55042 (AJA).

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  • Cited by (3)

    1

    Present address: University of California, Davis Bodega Marine Laboratory, Bodega Bay, CA 94923, USA

    2

    Present Address: University of Hawai'i at Mānoa, Honolulu, HI, 96822

    3

    Present Address: Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA

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