Seawater acidification more than warming presents a challenge for two Antarctic macroalgal‑associated amphipods

Abstract

Elevated atmospheric pCO 2 concentrations are triggering seawater pH reductions and seawater temperature increases along the western Antarctic Peninsula (WAP). These factors in combination have the potential to influence organisms in an antagonistic, additive, or synergistic manner. The amphipods Gondogeneia antarctica and Paradexamine fissicauda represent prominent members of macroalgal-associated mesograzer assemblages of the WAP. Our primary objective was to investigate amphipod behavioral and physiological responses to reduced seawater pH and elevated temperature to evaluate potential cascading ecological impacts. For 90 d, amphipods were exposed to combinations of seawater conditions based on present ambient (pH 8.0, 1.5°C) and predicted end-of-century conditions (pH 7.6, 3.5°C). We recorded survival, molt frequency, and macroalgal consumption rates as well as change in wet mass and proximate body composition (protein and lipid). Survival for both species declined significantly at reduced pH and co-varied with molt frequency. Consumption rates in G. antarctica were significantly higher at reduced pH and there was an additive pH−temperature effect on consumption rates in P. fissicauda. Body mass was reduced for G. antarctica at elevated temperature, but there was no significant effect of pH or temperature on body mass in P. fissicauda. Exposure to the pH or temperature levels tested did not induce significant changes in whole body biochemical composition of G. antarctica, but exposure to elevated temperature resulted in a significant increase in whole body protein content of P. fissicauda. Our study indicates that while elevated temperature causes sub-lethal impacts on both species of amphipods, reduced pH causes significant mortality.

Ocean warming and acidification alter Antarctic macroalgal biochemical composition but not amphipod grazer feeding preferences

“…Desmarestia anceps and D. menziesii and their associated amphipods were collected by hand with mesh bags in March 2013 (see techniques in Huang et al 2007) using SCUBA at depths ranging from 5 to 30 m at 3 sites within 3.5 km of the US Palmer Station, Anvers Island, on the WAP (64°46' S, 64°03' W). The macroalgae and amphipods were processed using the methods of Schoenrock et al (2015) and Schram et al (2016b). In brief, amphipods Gondogeneia antarctica were care fully sorted for similar-sized adults and were maintained in 4 l plastic Nalgene bottles fitted with fine-mesh screening.…”

“…Previous studies have demonstrated that prominent representatives of macroalgae−mesograzer as semblages along the Antarctic Peninsula exhibit contrasting responses to chronic exposure to OWA (Schoenrock et al 2015, Schram et al 2016b. For example, 2 of the most conspicuous benthic canopy-forming macro algae, D. menziesii and D. anceps, ex hibited few responses when exposed to OWA, with no significant changes detected in photosynthetic para meters, overall growth, or concentrations of chlorophyll a (Schoenrock et al 2015).…”

“…The only significant change was noted in the levels of phloro tannins, which increased with exposure to reduced pH (Schoen rock et al 2015). In contrast, 2 common amphi pod mesograzers exhibited significant physiological responses to OWA (Schram et al 2016b). Early exposure impacts included sub-lethal changes in physiology and compensatory feeding on a preferred alga under conditions of reduced pH, followed over time by > 50% mortality under conditions of either reduced pH or a combination of reduced pH and elevated temperature (Schram et al 2016b).…”

“…In contrast, 2 common amphi pod mesograzers exhibited significant physiological responses to OWA (Schram et al 2016b). Early exposure impacts included sub-lethal changes in physiology and compensatory feeding on a preferred alga under conditions of reduced pH, followed over time by > 50% mortality under conditions of either reduced pH or a combination of reduced pH and elevated temperature (Schram et al 2016b). Such varied responses indicate the potential for shifts in macroalgae−mesograzer tro phic dynamics with predicted OWA.…”

“…Our aims were to (1) evaluate the potential interaction strength between these species when exposed to OWA and (2) determine whether chronic OWA exposure makes an unpalatable alga more palatable to a common generalist grazer through a reduction in the efficacy of chemical feeding deterrents or changes in biochemical compo-sition (food quality). Based on the general resistance to OWA exhibited by D. anceps and D. menziesii (Schoen rock et al 2015) and the sensitivity of G. antarctica (Schram et al 2016b), we hypothesized that amphipods would exhibit compensatory feeding of chemically defended D. anceps and D. menziesii to offset elevated metabolic demands.…”