Novel methodology for in situ carbon dioxide enrichment of benthic ecosystems

نویسندگان

  • Justin E. Campbell
  • James W. Fourqurean
چکیده

Future climate change will likely represent a major stress to shallow aquatic and coastal marine communities around the world. Most climate change research, particularly in regards to increased pCO2 and ocean acidification, relies on ex situ mesocosm experimentation, isolating target organisms from their environment. Such mesocosms allow for greater experimental control of some variables, but can often cause unrealistic changes in a variety of environmental factors, leading to “bottle effects.” Here we present an in situ technique of altering dissolved pCO2within nearshore benthic communities (e.g., macrophytes, algae, and/or corals) using submerged clear, open-top chambers. Our technique utilizes a flow-through design that replicates natural water flow conditions and minimizes caging effects. The clear, open-top design additionally ensures that adequate light reaches the benthic community. Our results show that CO2 concentrations and pH can be successfully manipulated for long durations within the open-top chambers, continuously replicating forecasts for the year 2100. Enriched chambers displayed an average 0.46 unit reduction in pH as compared with ambient chambers over a 6-month period. Additionally, CO2 and HCO3 – concentrations were all significantly higher within the enriched chambers. We discuss the advantages and disadvantages of this technique in comparison to other ex situ mesocosm designs used for climate change research. *Corresponding author: E-mail: [email protected] Acknowledgments We thank William Anderson at the FIU Stable Isotope Lab and Peter Swart at the University of Miami Stable Isotope Lab for invaluable laboratory assistance with our isotope samples. Rene Price and Pamela Sullivan helped with the alkalinity measurements. Patrick Rice and the Florida Keys Community College provided logistical support. Rebecca Bernard, Jeana Drake, Pamela Parker, Bryan Dewsbury, and Sat Gavassa provided assistance in the field. This work was supported by the National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research Program under Grant No. DBI-0620409, and an FIU Graduate School Doctoral Evidence Acquisition Fellowship. This is contribution number 513 from the Southeast Environmental Research Center at FIU. DOI 10.4319/lom.2011.9.97 Limnol. Oceanogr.: Methods 9, 2011, 97–109 © 2011, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS Much of climate change research within marine systems has been directed toward understanding how long-term and pervasive changes in oceanic pCO2 can serve as a stressor on both benthic and water column organisms, and the processes they regulate. For example, it is expected that increases in dissolved CO2 concentrations are likely to reduce long-term calcification rates across many coral reef ecosystems (Kleypas et al. 1999; Langdon et al. 2000). A variety of calcifying marine organisms; microalgae (Riebesell et al. 2000; Zondervan et al. 2001), macroalgae (Jokiel et al. 2008; Kuffner et al. 2008; Gao and Zheng 2010), and corals/invertebrates (Langdon et al. 2000; Langdon and Atkinson 2005; Shirayama and Thornton 2005; Gazeau et al. 2007; Lombard et al. 2010) have all displayed declines in CaCO3 production with experimentally elevated pCO2. Additional studies have highlighted interactive effects between temperature and pCO2 on the calcification rates of marine organisms (Martin and Gattuso 2009; RodolfoMetalpa et al. 2010). Such studies clearly demonstrate implications for the resilience of coral reefs under increasing anthropogenic and climatic pressures. However, a majority of these experiments have been conducted within artificial indoor mesocosms, which can isolate target organisms from realistic natural conditions, and can fail to account for environmental variation (Hall-Spencer et al. 2008; Kleypas and Yates 2009; Hendriks et al. 2010). Whereas some studies have begun using outdoor mesocosm facilities (Riebesell et al. 2007; Jokiel et al. 2008) and field manipulations along natural pH gradients (Cigliano et al. 2010; Dias et al. 2010), we know relatively little in regards to in situ responses of organisms to altered pCO2 and pH conditions. Several studies have additionally suggested that altered pCO2 within coastal environments may have the ability to impact the functioning of aquatic and marine plant communities (Zimmerman et al. 1997; Short and Neckles 1999; Palacios and Zimmerman 2007; Hall-Spencer et al. 2008; Martin et al. 2008; Kleypas and Yates 2009). External increases in CO2 and HCO3 – concentrations have the ability to increase the diffusive flux of dissolved inorganic carbon (DIC) across leaf boundary layers, and have been shown to increase seagrass production (Hall-Spencer et al. 2008), leaf photosynthetic rates (Durako 1993; Beer and Koch 1996; Invers et al. 1997; Zimmerman et al. 1997), and plant reproductive output (Palacios and Zimmerman 2007). Other studies have additionally demonstrated changes to the seagrass epiphyte community under various CO2 enrichment scenarios, with large declines in biogenic carbonate production, and potential biogeochemical shifts within these shallow, coastal systems (Martin et al. 2008). Submerged macrophytes comprise much of the coastal benthic community around globe and are important contributors to the carbon sink capacity of the world’s oceans (Duarte et al. 2011). Thus, similar to declines in coral reef calcification, changes in oceanic pCO2 may additionally have widespread implications for these productive and economically important ecosystems. Experiments that address CO2-mediated impacts on benthic plants have likewise mainly been restricted to mesocosm designs (Titus et al. 1990; Titus 1992; Zimmerman et al. 1997; Pagano and Titus 2007; Palacios and Zimmerman 2007). Aquarium and mesocosm facilities generally provide optimal conditions to encourage vigorous plant and/or algal growth, thus responses detected within the laboratory may not hold true for natural communities, where alternate resources may become increasingly limiting under elevated CO2 loads. Several terrestrial studies have documented disparities between ex situ and in situ responses in regards to the impacts of CO2 enrichment on plant community dynamics (Ainsworth and Long 2005). Within these systems, CO2 mediated growth responses can be rapidly constrained by the availability of other essential resources, such as water and/or nutrients (Diaz et al. 1993). Many mesocosm pCO2 experiments control seawater carbonate equilibrium via either acid addition (which shifts the relative concentrations of the carbonate species with no increase in total DIC) or CO2 bubbling (which simultaneously increases the abundance of CO2, HCO3 –, and total DIC). Whereas it has been suggested that the latter technique of CO2 enrichment best replicates forecasted changes in seawater carbonate parameters (Hurd et al. 2009; Schulz et al. 2009), few studies have attempted to transition this ex situ methodology toward an in situ design (Barry et al. 2010). Here we describe a novel technique of long-term CO2 enrichment applicable to the study of changes in pCO2 on the productivity and functioning of aquatic and marine benthic communities. Our methodology consists of a submerged array of clear, open-top, flow-through acrylic chambers, which allows for continuous CO2 enrichment over long time periods. Our system utilizes an efficient technique of in situ CO2 bubbling, which maximizes the dissolution and containment of CO2 gas, while minimizing losses to the external water column and atmosphere. Furthermore, the open, flow-through design allows for ample light to reach target organisms (macrophytes, corals, and/or algae), while reducing caging effects. Easy access to the benthic community through the open tops of the enclosures additionally allows for accurate measurements of carbonate parameters (pH and alkalinity), and a variety of biotic processes (plant/algal growth rates, calcification rates, and photosynthetic fluorometric responses). This system provides an inexpensive and efficient technique of in situ pCO2 manipulation around a wide variety of benthic communities to study various climate change/ocean acidification scenarios. Materials and procedures Site description In situ pCO2 manipulation was initiated on 1 Jul 2009 within a shallow, nearshore benthic plant community within the Florida Keys, Florida, USA (24.55° N, 81.75° W). The benthic community was dominated by the seagrass Thalassia testudinum, with lower abundances of the seagrasses Syringodium Campbell and Fourqurean In situ benthic carbon dioxide enrichment

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تاریخ انتشار 2017