Abstract
Pteropods have been hailed as the “canary in the coal mine” for ocean acidification, however, questions remain about their life history, habitat, and the environmental parameters that the isotopic composition of their shells reflect. In order to use pteropods as recorders of ocean chemistry, it is first necessary to understand where they calcify and how this may change through the year, whether this signal is affected by dissolution, and if shells are retained in the subfossil, and eventually fossil, record. Here we create the first annual record of the stable isotopic composition of shells of the pteropodHeliconoides inflatusin the Cariaco Basin, Venezuela utilizing samples and data from the CARIACO time series. Sixty-fourH. inflatusspecimens from 17 sediment trap samples between November 1996 and April 1998, and 22 specimens from the late Holocene-aged CAR2000-MC-2 core were analyzed for shell condition (an assessment of the amount of dissolution that a shell has experienced), size, and carbon and oxygen isotopic composition. Carbon isotopic measurements of juveniles (< 1mm) were more variable than those in adults (>1 mm), suggesting juvenile pteropods likely have a higher growth rate, and therefore different metabolic vital effects, and a more varied diet than adult pteropods.H. inflatuswas found to have an apparent calcification depth of 51.2 ± 34.0 m, suggesting they calcify at the shallowest part of their diurnal migration in the mixed layer (10–35 m in the Cariaco Basin).H. inflatusshell calcification will therefore only be impacted by changes in water chemistry at mixed layer depths. The shell condition did not impact the stable isotopic composition of the shells in either the sediment trap or core sample, suggesting the potential for using the isotopic composition of pteropod shells as oceanographic proxies when they are preserved. Comparisons between sediment trap and core sample show a 0.5°C warming that is marginally significant and a significant 0.45‰ decrease in δ13C between the late Holocene and the late 1990's. These measurements reflect changes in oceanic conditions linked to anthropogenic fossil fuel emissions known as the Suess effect, and lay the groundwork for establishing pteropods as paleoceanographic proxies in the future.
Highlights
Understanding the impact of ocean acidification on epipelagic ecosystems is inherently difficult
There was no correlation between the amount of time pteropod shells spent in the collecting cups and their shell condition (p = 0.507, R2 = 0.011) (Supplementary Figure 3), similar to the findings of another sediment trap study in the Cariaco Basin (Oakes and Sessa, 2020)
Based on comparison between the calculated δ18Oarag of seawater (LeGrande and Schmidt, 2006) and measured δ18O of pteropod shells, we found an apparent calcification depth (ACD) of 51.2 ± 34.0 m for H. inflatus in the Cariaco Basin (Figure 5)
Summary
Understanding the impact of ocean acidification on epipelagic ecosystems is inherently difficult. To gain a more holistic view of the impact of ocean acidification on marine biota, recent studies have proposed the development of organisms as biological indicators of human-induced changes to ocean chemistry (Bednaršek et al, 2017, 2019; Gaylord et al, 2018; Marshall et al, 2019) Marine calcifiers, such as foraminifera and mollusks, are widely used as proxies for ocean chemistry. Planktic foraminifera have lifespans on the scale of weeks to months, and their blooms are often seasonal (Tedesco and Thunell, 2003; Jonkers and Kucera, 2015) The chemistry of their shells records a snapshot of ocean conditions. Benthic marine mollusks are most abundant in coastal settings, and their larvae are found in surface waters, their lack of larval diagnostic features and the limited understanding of their early life stages restricts their utility for deciphering upper ocean conditions (Parker et al, 2013; León et al, 2019)
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