Macroalgae contribute to nested mosaics of pH variability in a subarctic fjord

Abstract

Abstract. The Arctic Ocean is considered the most vulnerable ecosystem to ocean acidification, and large-scale assessments of pH and the saturation state for aragonite ( arag ) have led to the notion that the Arctic Ocean is already close to a corrosive state. In high-latitude coastal waters the regulation of pH and arag is, however, far more complex than offshore because increased biological activity and input of glacial meltwater affect pH. Effects of ocean acidification on calcifiers and non-calcifying phototrophs occupying coastal habitats cannot be derived from extrapolation of current and forecasted offshore conditions, but they require an understanding of the regimes of pH and arag in their coastal habitats. To increase knowledge of the natural variability in pH in the Arctic coastal zone and specifically to test the influence of benthic vegetated habitats, we quantified pH variability in a Greenland fjord in a nested-scale approach. A sensor array logging pH, O 2 , PAR, temperature and salinity was applied on spatial scales ranging from kilometre scale across the horizontal extension of the fjord; to 100 m scale vertically in the fjord, 10-100 m scale between subtidal habitats with and without kelp forests and between vegetated tidal pools and adjacent vegetated shores; and to centimetre to metre scale within kelp forests and millimetre scale across diffusive boundary layers of macrophyte tissue. In addition, we assessed the temporal variability in pH on diurnal and seasonal scales. Based on pH measurements combined with point samples of total alkalinity, dissolved inorganic carbon and relationships to salinity, we also estimated variability in arag . Results show variability in pH and arag of up to 0.2-0.3 units at several scales, i.e. along the horizontal and vertical extension of the fjord, between seasons and on a diel basis in benthic habitats and within 1 m 3 of kelp forest. Vegetated intertidal pools exhibited extreme diel pH variability of > 1.5 units and macrophyte diffusive boundary layers a pH range of up to 0.8 units. Overall, pelagic and benthic metabolism was an important driver of pH and arag producing mosaics of variability from low levels in the dark to peak levels at high irradiance generally appearing favourable for calcification. We suggest that productive coastal environments may form niches of high pH in a future acidified Arctic Ocean.

Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation?

“…Dense seaweeds, farmed as well as wild, also represent productivity hot-spots with associated high pH during day when photosynthesis reduces CO 2 concentrations (e.g., Middelboe and Hansen, 2007;Krause-Jensen et al, 2015). They may, hence, serve a role in protecting calcifies from projected ocean acidification (Krause-Jensen et al, 2015).…”

“…They may, hence, serve a role in protecting calcifies from projected ocean acidification (Krause-Jensen et al, 2015). Kelp forests support high biodiversity, including calcifiers such as lobsters, crabs, molluscs, and crustaceans (Steneck et al, 2002;Smale et al, 2013) and high daytime pH probably contributes to this effect.…”

Long photoperiods sustain high pH in Arctic kelp forests

“…Hence, it follows that the relative duration of the positive (day) versus the negative (night) period, that is, the photoperiod, should determine the overall effect. We therefore hypothesized that photoperiod should constrain diurnal fluctuations in pH and CO 2 and, hence, the potential role of vegetated habitats as buffers or amplifiers of OA effects ( 6 , 21 ). However, the role of photoperiod in constraining the effects of marine macrophytes on pH and CO 2 remains untested.…”

The ability of macroalgae to mitigate the negative effects of ocean acidification on four species of North Atlantic bivalve

“…A study by Anthony et al (2013) found that within mixed assemblages of turf and fleshy macroalgae, the saturation state of aragonite increased during the daytime. Krause-Jensen et al (2015) reported that macroalgae may provide a refuge for calcifying organisms. Specifically, within a subarctic fjord, macroalgae drove strong diel variability in pH and Ωaragonite, with M. edulis being found to grow in close association with macroalgae, even in tidal 15 pools that became supersaturated and undersaturated between day and night cycles, respectively (Krause-Jensen et al, 2015).…”

“…However, as pointed out by Wilson et al (1990), the accumulation of secondary metabolites and decreased dissolved oxygen associated with the overgrowth of Ulva is often mitigated in high-flow areas, alleviating potential harm to the nearby organisms. Furthermore, it is plausible that other macroalgae that are not known to negatively impact marine life provide similar buffering of carbonate chemistry (Anthony et al, 2013;Krause-Jensen et al, 2015;Wahl et al, 2017).…”