Biotic habitats as refugia under ocean acidification

Abstract

Habitat-forming organisms have an important role in ameliorating stressful conditions and may be of particular relevance under a changing climate. Increasing CO2 emissions are driving a range of environmental changes, and one of the key concerns is the rapid acceleration of ocean acidification and associated reduction in pH. Such changes in seawater chemistry are anticipated to have direct negative effects on calcifying organisms, which could, in turn, have negative ecological, economic and human health impacts. However, these calcifying organisms do not exist in isolation, but rather are part of complex ecosystems. Here, we use a qualitative narrative synthesis framework to explore (i) how habitat-forming organisms can act to restrict environmental stress, both now and in the future; (ii) the ways their capacity to do so is modified by local context; and (iii) their potential to buffer the effects of future change through physiological processes and how this can be influenced by management adopted. Specifically, we highlight examples that consider the ability of macroalgae and seagrasses to alter water carbonate chemistry, influence resident organisms under current conditions and their capacity to do so under future conditions, while also recognizing the potential role of other habitats such as adjacent mangroves and saltmarshes. Importantly, we note that the outcome of interactions between these functional groups will be context dependent, influenced by the local abiotic and biotic characteristics. This dependence provides local managers with opportunities to create conditions that enhance the likelihood of successful amelioration. Where individuals and populations are managed effectively, habitat formers could provide local refugia for resident organisms of ecological and economic importance under an acidifying ocean.

Role of hydrodynamics in shaping chemical habitats and modulating the responses of coastal benthic systems to ocean global change

“…These chemical fluctuations can be beneficial or detrimental to the physiological performance of the associated organisms living on the engineer species or in understory (Bergstrom et al, 2019; Cornwall et al, 2013; Garrard et al, 2014; Semesi et al, 2009). Experiencing these fluctuations can therefore modulate an organism's efficiency in coping with ongoing ocean global change such as ocean acidification (Falkenberg et al, 2021; Kapsenberg & Cyronak, 2019; Wahl et al, 2018). This has recently generated much interest in characterizing these chemical habitats and in assessing the responses of “associated species” (see Box 1).…”

“…Chemical habitats related to diffusion and community‐boundary layers could act to buffer organisms against certain ocean global change‐related stressors over time, creating ‘refugia’ (see Box 1) from unfavorable surrounding seawater conditions like reduced pH due to ocean acidification or hypoxia due to water deoxygenation (Bulleri et al, 2018; Falkenberg et al, 2021; Hurd, 2015; Laffoley & Baxter, 2019). The existence of refugia is primarily dependent on the local hydrodynamics as chemical habitats where fluctuations eclipse bulk seawater conditions can only be shaped when the water flow allows the build‐up of boundary layers.…”

“…Temporal and spatial fluctuations in chemical conditions in shallow waters mainly result from the interaction between biological activity and hydrodynamic forcing (Cyronak et al, 2018; Kapsenberg & Hofmann, 2016; Saderne et al, 2013; Wahl et al, 2018). Biological activity is the main driver of chemical variations in highly productive systems that are dominated by ‘engineer species’ (see Box 1) such as seagrass meadows, kelp forests, coral reefs or mollusc beds (Falkenberg et al, 2021). At high population densities, engineer species have the capacity to strongly modulate the chemical and physical conditions of their surrounding waters (Gutiérrez et al, 2003, 2011; Hurd et al, 2014; Kregting et al, 2013; Pujol et al, 2019; Turk et al, 2015; Unsworth et al, 2012).…”

Seaweeds influence oyster microbiota and disease susceptibility

“…These considerations are particularly applicable to the development of integrated multi‐trophic aquaculture, co‐culture or any other practice that relies on increasing biological diversity to increase resilience of marine ecosystems to global change (see e.g. Falkenberg et al, 2021; Groner et al, 2018). We however suggest that future research should evaluate these interactions in natural field condition where oysters, OsHV‐1 and seaweeds coexist.…”

Shark teeth can resist ocean acidification

“…diurnal/seasonal cycles of pH and total alkalinity created by seagrasses or macroalgae, Camp et al, 2016; Challener et al, 2016; Murie & Bourdeau, 2020). Constant exposure to this fluctuation can enhance acclimatization capacity and hence resistance of marine organisms to ocean acidification (Li et al, 2021; Ramajo et al, 2019; Wahl et al, 2018); therefore, seagrass beds and macroalgal forests are regarded as refugia that ameliorate the impact of ocean acidification (Falkenberg et al, 2021). Given the global intensity of fishing effort for sharks (Queiroz et al, 2019), however, their long‐term threat may come from current‐day fishing practices, and reshuffling of food webs with less energy flowing to higher trophic levels (Nagelkerken et al, 2020; Ullah et al, 2018).…”