Guidelines for Measuring Changes in Seawater pH and Associated Carbonate Chemistry in Coastal Environments of the Eastern United States.

Abstract

These guidelines are written for a variety of audiences ranging from shellfish growers interested in monitoring pH with inexpensive equipment to citizen monitoring groups to advanced chemistry laboratories interested in expanding existing capabilities. The purpose is to give an overview of available sampling, analytical and data reporting approaches that will contribute to the usefulness of coastal acidification measurements for both the needs of those intending to monitor as well as those of other interested stakeholders along the Atlantic seaboard of the US. The state of the science, including recommended best practices, is rapidly evolving, so certain sections may be either too sparse or too detailed. Thus, we encourage users of the guidelines to begin with a careful review of the detailed contents listing and to take note of references to other guidelines available in the open literature. Coastal and estuarine systems hold significant economic and recreational importance for communities along the Atlantic seaboard. These systems support finfish, bivalve, crustacean and seabird populations and play vital roles in water quality and the cycling of nitrogen and carbon. However, seawater pH and other characteristics of coastal carbonate chemistry are changing through a process known as coastal acidification, which is a fundamentally similar but more complex version of ocean acidification. Coastal acidification has the potential to disrupt the species composition and ecological functioning of coastal biological communities and threaten commercially important aquatic life. As in the open ocean, the carbonate system in coastal waters consists of the major forms of inorganic carbon present in seawater, which are carbon dioxide, bicarbonate and carbonate. Although there are numerous groups interested in monitoring pH or other carbonate parameters in coastal waters, there is little available guidance on how these groups can best utilize or expand their existing capabilities. Coastal acidification differs somewhat from ocean acidification, which is a global process that involves a reduction in the pH of the ocean (see section below on the seawater carbonate system). It is caused primarily by carbon dioxide from the atmosphere entering the ocean. Coastal acidification is a more localized, further reduction in pH. It is primarily driven by high levels of respiration (typically by bacteria involved in decomposition), which releases carbon dioxide into the water. Coastal acidification is often fueled by nutrients entering the water from land, stimulating phytoplankton blooms that subsequently decompose on or near the seabed. Coastal acidification happens in coastal waters because that is where high nutrient levels and algal blooms occur [http://www.necan.org/]. In the past few decades, only half of the CO2 released by human activity, including fossil fuel emissions, land use change and cement production, has remained in the atmosphere; of the remainder, about 30% has been taken up by the ocean and 20% by the terrestrial biosphere (Khatiwala et al., 2009; Sabine et al., 2004). The evidence for decreasing pH in the open ocean is unequivocal (Caldeira and Wickett, 2003; Doney et al., 2009), as is the evidence for negative effects on many marine organisms when these chemical changes are simulated under controlled laboratory conditions (e.g., Kroeker et al., 2013; Talmage and Gobler, 2009). However, scientists are just beginning to test the severity of these effects in ocean and coastal ecosystems where an organism’s chemical environment is only one of many ecological factors affecting its fitness. There are many clear cases of extreme biological sensitivity to acidification among economically important coastal organisms such as shellfish and corals, but the biological responses of many other species are variable and difficult to predict (Kroeker et al., 2010). For example, many types of marine plants and algae may be harmed by lower pH (i.e., higher acidity) but may also benefit from increases in the carbon dioxide they require for photosynthesis (Riebesell, 2004). Thus, although species composition may change in the future, neither the details nor the ecosystem level consequences (e.g., food production) are predictable (Grear et al., 2017). The continued study of these effects needs to be accompanied by a clear understanding of how coastal carbonate chemistry varies through space and time. A number of methods have been described for the coastal current and upwelling zones of the US west coast (e.g., McLaughlin et al., 2014; McLaughlin et al., 2013). While coastal upwelling occurs on the east coast, deep water upwelling does not strongly influence acidification in the short term (Wang et al., 2013). Thus, observations from the mid- and outer-shelf may be less comparable to the 2 | M e a s u r i n g C h a n g e s i n C o a s t a l C a r b o n a t e C h e m i s t r y inshore environment than on the west coast. Moreover, many coastal organisms have sensitive estuarine and nearshore life stages that coincide with mid and late summer extremes in dissolved oxygen, pH, and other characteristics of the carbonate system and are thus expected to be especially vulnerable (Wallace et al., 2014). These issues raise concern about coverage in the nearshore environments that fall outside of areas covered by the major federal observing programs (e.g., ECOMON and GOMECC) and which tend to be too infrequent to capture either seasonal or more frequent excursions in carbonate chemistry. The decrease in pH in the open ocean during the industrial age has been on the order of 0.1 to 0.2 pH units per century (Caldeira and Wickett, 2003), which translates to more than a 25% increase in the concentration of hydrogen ions. Relative to coastal environments, pH in the open ocean is generally less dynamic in terms of diurnal and seasonal variations (Hofmann et al., 2011), which has made open ocean trends easier to distinguish from background variability. In addition, the ocean is extremely important to the global carbon cycle, so scientists have been taking highly precise measurements of the carbonate system in the open ocean for decades. However, due to greater variability of pH in the coastal environment, a trend of similar magnitude would require a larger number, and longer time-series, of samples to detect (Keller et al., 2014). This creates a unique challenge for coastal monitoring because current best practices for handling and analyzing samples for carbonate system parameters are expensive, and therefore possibly not feasible for the high frequency and spatially extensive sampling that would be necessary to detect decadal and spatial trends in the coastal environment. For example, while pH is easy to measure with handheld meters or multi-function autonomous sensors that use glass membrane pH electrodes, chemical oceanographers often question the value of these measurements for the study of carbonate chemistry, including acidification (Re´rolle et al., 2012). Although this criticism is sometimes unwarranted because of differences in study goals (e.g., see the “climate vs. weather quality” discussion below; Newton et al., 2014), accepted protocols are unlikely to change without an improved understanding of coastal acidification, and until issues relating to appropriate pH scales, calibration standards, instrument drift, and indirect pH estimation are further refined or agreed upon by the research community. These guidelines are meant to be a resource for learning about and performing measurements of the seawater carbonate system, especially as they relate to coastal acidification. The intended audience includes scientists in academic, government, and non-government organizations including those involved in citizen science and shellfish management. Many such organizations are already monitoring or beginning to monitor components of the seawater carbonate system that may be partially or completely sufficient for assessing coastal acidification. For example, specific organizations in the northeast are examining coastal acidification as a potential cause for recent declines in shellfish abundance. Other organizations study coastal carbonate chemistry and acidification as part of a broader interest in coastal carbon cycles. Clearly, there is a wide diversity of rationales and capabilities for monitoring acidification in the coastal environment. Numerous publications exist in the peer-reviewed and online “gray” literature that describe recommended practices for measuring and calculating the various components of the seawater carbonate system. Most of these resources are written by and for oceanographic researchers and place less emphasis on informing, for example, the expansion of an existing shellfish or nutrient monitoring program to include coastal acidification parameters. In addition, the available resources (e.g., Dickson et al., 2007; McLaughlin et al., 2014; Riebesell et al., 2010) tend to be generalized to accommodate a wide variety of instrument and laboratory 1 . I n t r o d u c t i o n | 3 configurations. This creates a challenge for the new investigator and slows the rate at which new monitoring efforts can be implemented. Thus, this document will attempt to address the unique issues and some of the available solutions for measuring the seawater carbonate system in coastal and estuarine environments. This includes alternatives and clarifications of existing best practices that will make them suitable for typical environments of the US east coast. Because the study of ocean and coastal acidification is changing rapidly, these guidelines are not meant to be prescriptive, but are intended to facilitate the development of compatible datasets for sharing insights and experiences from the present community of investigators interested in coastal acidification. Although these guidelines are intended to apply throughout the east coast, much of the nearshore research in the eastern US has been conducted in the northeast. There are likely to be solutions that we have not covered here, so continued communication through acidification monitoring networks will be critical. Before describing specific methods, we provide an overview of coastal acidification, the seawater carbonate system, and ecological considerations that normally affect study design. There is a vast literature on these topics for the ocean and a growing one for the coasts. For a recent overview of the state of the science in the US northeast, see Gledhill et al. (2015) and other resources on the Northeast Coastal Acidification Network (NECAN) website (http://www.necan.org). For other eastern US coastal regions, see ongoing developments on the SOCAN (http://secoora.org/socan) and MACAN websites (http://midacan.org). Although the seawater carbonate system is described later in greater detail, our overview begins with the summary in Figure 1-1.