Ocean acidification decreases grazing pressure but alters morphological structure in a dominant coastal seaweed

Abstract

Ocean acidification driven by anthropogenic climate change is causing a global decrease in pH, which is projected to be 0.4 units lower in coastal shallow waters by the year 2100. Previous studies have shown that seaweeds grown under such conditions may alter their growth and photosynthetic capacity. It is not clear how such alterations might impact interactions between seaweed and herbivores, e.g. through changes in feeding rates, nutritional value, or defense levels. Changes in seaweeds are particularly important for coastal food webs, as they are key primary producers and often habitat-forming species. We cultured the habitat-forming brown seaweed Fucus vesiculosus for 30 days in projected future pCO2 (1100 μatm) with genetically identical controls in ambient pCO2 (400 μatm). Thereafter the macroalgae were exposed to grazing by Littorina littorea, acclimated to the relevant pCO2-treatment. We found increased growth (measured as surface area increase), decreased tissue strength in a tensile strength test, and decreased chemical defense (phlorotannins) levels in seaweeds exposed to high pCO2-levels. The herbivores exposed to elevated pCO2-levels showed improved condition index, decreased consumption, but no significant change in feeding preference. Fucoid seaweeds such as F. vesiculosus play important ecological roles in coastal habitats and are often foundation species, with a key role for ecosystem structure and function. The change in surface area and associated decrease in breaking force, as demonstrated by our results, indicate that F. vesiculosus grown under elevated levels of pCO2 may acquire an altered morphology and reduced tissue strength. This, together with increased wave energy in coastal ecosystems due to climate change, could have detrimental effects by reducing both habitat and food availability for herbivores.

Climate Change Increases Susceptibility to Grazers in a Foundation Seaweed

“…Susceptibility to grazing in F. vesiculosus and other fucoids can be affected by the morphology (e.g., toughness) and/or nutrient content (e.g., nitrogen content) of the thallus, as well as the presence of chemical defense compounds (e.g., phlorotannins). Changes in tissue toughness in F. vesiculosus were found by Rothäusler et al (2017) in response to reduced salinity and warming, as well as by Kinnby et al (2021) who found that seaweeds became more fragile under elevated pCO 2 . However, none of these previous studies found a correlation between tissue toughness and susceptibility to grazing (Rothäusler et al, 2017;Kinnby et al, 2021).…”

“…Changes in tissue toughness in F. vesiculosus were found by Rothäusler et al (2017) in response to reduced salinity and warming, as well as by Kinnby et al (2021) who found that seaweeds became more fragile under elevated pCO 2 . However, none of these previous studies found a correlation between tissue toughness and susceptibility to grazing (Rothäusler et al, 2017;Kinnby et al, 2021). Raddatz et al (2017) found a weak negative correlation between susceptibility to grazing and C:N ratio in F. vesiculosus, however, this was not related to changes in climate variables (pCO 2 or temperature).…”

“…Values are averages ± SD for ambient levels as well as the average ± SD across all the manipulated treatments, n = 4,120 for temperature and salinity, n = 30 for pCO 2 and pH NBS . (Kinnby et al, 2021). The pCO 2 was monitored using a LI-850 CO 2 /H 2 O gas analyzer (Li-Cor).…”

“…It provides habitat and food for invertebrates, fish, and other algal species in rocky intertidal areas in both sheltered and exposed rocky shorelines (Bergström et al, 2005;Forslund and Kautsky, 2013). Evidence from previous studies show that changes to a single environmental factor can have negative (reduced salinity, Nygård and Dring, 2008;Connan and Stengel, 2011a;increased temperature, Mensch et al, 2016) or positive (reduced salinity, Rugiu et al, 2018;Kinnby et al, 2021) effect on growth. Changes to multiple climate factors can cause tissue necrosis (reduced salinity and increased temperature, Takolander et al, 2017), reduce growth (reduced salinity and increased temperature, Rugiu et al, 2018) or mitigate the effects of changes to a single variable (increased temperature and elevated pCO 2 , Mensch et al, 2016).…”

“…Phlorotannins are a group of polyphenolic metabolites that can occur in high concentrations in brown seaweeds, especially in fucoids. These compounds have several functions, including UV protection and chemical defense against grazers (Pavia et al, 1997;Toth and Pavia, 2007), and have been experimentally shown in F. vesiculosus to decrease when exposed to lower salinity (Connan and Stengel, 2011b;Kinnby et al, 2020) and elevated pCO 2 (Kinnby et al, 2021). Climate change induced alterations in chemical defenses and overall biochemical composition of seaweed biomass can have consequences for ecological interactions such as grazing pressure on the seaweeds.…”