Seawater pH Predicted for the Year 2100 Affects the Metabolic Response to Feeding in Copepodites of the Arctic Copepod Calanus glacialis

Abstract

Widespread ocean acidification (OA) is transforming the chemistry of the global ocean, and the Arctic is recognised as a region where the earliest and strongest impacts of OA are expected. In the present study, metabolic effects of OA and its interaction with food availability was investigated in Calanus glacialis from the Kongsfjord, West Spitsbergen. We measured metabolic rates and RNA/DNA ratios (an indicator of biosynthesis) concurrently in fed and unfed individuals of copepodite stages CII-CIII and CV subjected to two different pH levels representative of present day and the “business as usual” IPCC scenario (RCP8.5) prediction for the year 2100. The copepods responded more strongly to changes in food level than to decreasing pH, both with respect to metabolic rate and RNA/DNA ratio. However, significant interactions between effects of pH and food level showed that effects of pH and food level act in synergy in copepodites of C. glacialis. While metabolic rates in copepodites stage CII-CIII increased by 78% as a response to food under present day conditions (high pH), the increase was 195% in CII-CIIIs kept at low pH—a 2.5 times greater increase. This interaction was absent for RNA/DNA, so the increase in metabolic rates were clearly not a reaction to changing biosynthesis at low pH per se but rather a reaction to increased metabolic costs per unit of biosynthesis. Interestingly, we did not observe this difference in costs of growth in stage CV. A 2.5 times increase in metabolic costs of growth will leave the copepodites with much less energy for growth. This may infer significant changes to the C. glacialis population during future OA.

Contrasting physiological responses to future ocean acidification among Arctic copepod populations

“…But these changes depended on food level and no clear response could be concluded. The lack of response of C. glacialis CVs in the present study is corroborated by a recent study in the Kongsfjord (Thor et al., ) and has also been shown to last during longer term incubations where metabolic rates remained equal in C. glacialis CVs and C. hyperboreus CVs and females incubated at pH F (free scale pH) 8.13 and 7.26 for 62 days (Hildebrandt, Niehoff & Sartoris, ). Metabolic rates of CVs increased linearly across a range from pH T 8.02 to pH T 7.16 in a study on culture reared C. finmarchicus applying reaction norm statistics similar to the present study (Pedersen et al., ), whereas a later study found no effects between pH T 7.92 and pH T 7.51 in wild caught C. finmarchicus CVs and females (Runge et al., ).…”

“…Also in a previous study, Thor et al. observed significant changes in the metabolic reaction to feeding at pH T 7.73 compared to pH T 8.11 in early copepodite stages (CII‐CIII) but no changes in CVs (Thor et al., ). Hildebrandt and colleagues found a similar lack of response of ingestion and metabolism in C. glacialis CVs (Hildebrandt et al., , ).…”

No maternal or direct effects of ocean acidification on egg hatching in the Arctic copepod Calanus glacialis

“…pH T was then calculated according to Clayton and Burne [ 43 ] with a dye-addition correction [ 44 ] and adjusted from measurement temperature to in situ temperature according to Gieskes [ 45 ]. We did not measure a second carbonate chemistry variable as in our previous OA studies [ 17 , 32 , 33 , 46 – 49 ], and we were unable to calculate the full carbonate chemistry, which is otherwise recommended [ 50 ].…”

“…As a consequence, much attention has been given C . glacialis and its possible future in a changing Arctic [ 32 – 36 ].…”

Setting the scene

“…Arctic copepods, such as Calanus glacialis, are less affected by increased seawater pCO 2 , even at the younger life stages (Bailey et al 2017). However, lowered pH may increase metabolic cost for this species at the expense of growth performance (Thor et al 2016).…”