Seaweed fails to prevent ocean acidification impact on foraminifera along a shallow‐water CO₂ gradient

Abstract

Ocean acidification causes biodiversity loss, alters ecosystems, and may impact food security, as shells of small organisms dissolve easily in corrosive waters. There is a suggestion that photosynthetic organisms could mitigate ocean acidification on a local scale, through seagrass protection or seaweed cultivation, as net ecosystem organic production raises the saturation state of calcium carbonate making seawater less corrosive. Here, we used a natural gradient in calcium carbonate saturation, caused by shallow-water CO2 seeps in the Mediterranean Sea, to assess whether seaweed that is resistant to acidification (Padina pavonica) could prevent adverse effects of acidification on epiphytic foraminifera. We found a reduction in the number of species of foraminifera as calcium carbonate saturation state fell and that the assemblage shifted from one dominated by calcareous species at reference sites (pH ∼8.19) to one dominated by agglutinated foraminifera at elevated levels of CO2 (pH ∼7.71). It is expected that ocean acidification will result in changes in foraminiferal assemblage composition and agglutinated forms may become more prevalent. Although Padina did not prevent adverse effects of ocean acidification, high biomass stands of seagrass or seaweed farms might be more successful in protecting epiphytic foraminifera.

Effect of temperature rise and ocean acidification on growth of calcifying tubeworm shells (<i>Spirorbis spirorbis</i>): an in situ benthocosm approach

“…In the DBL of F. vesiculosus, pH was found to increase by up to 1.5 units from dark conditions to bright daylight (Spilling et al, 2010;Wahl et al, 2016). Consequently, this surface boundary layer of the algae can potentially provide a shelter from ocean acidification during daylight (Hendriks et al, 2014;Pettit et al, 2015).…”

Macroalgal responses to ocean acidification depend on nutrient and light levels

“…are indicators of high water quality in the Mediterranean (Bermejo et al, 2013) and since Padina spp. tolerate loss of external calcification as CO 2 levels increase (Johnson et al, 2012;Pettit et al, 2015).…”

Antarctic crustacean grazer assemblages exhibit resistance following exposure to decreased pH

“…Though not a focus of the present study, changes in light availability may also have influenced the overall response of assemblages to reduced seawater pH. However, pH buffering capacity has been demonstrated to be constrained in extreme, low-pH regimes such as ocean acidification-resistant macroalgae and calcifying foraminiferal assemblages at naturally acidified CO 2 vent sites (Pettit et al 2015). Despite the potential buffering effects of an association with benthic macroalgae, the crustacean grazers examined in the present study are exposed to seasonal variation in seawater pH and associated carbonate chemistry parameters .…”