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1. Growth and longevity of the Antarctic scallop Adamussium colbecki under annual and multiannual sea ice

   https://doi.org/10.1017/S0954102020000322  

   Cronin, Kelly E.; Walker, Sally E.; Mann, Roger; Chute, Antonie S.; Long, M. Chase; Bowser, Samuel S. (December 2020, Antarctic Science)

   null (Ed.)

   Abstract Ecosystem engineers such as the Antarctic scallop ( Adamussium colbecki ) shape marine communities. Thus, changes to their lifespan and growth could have far-reaching effects on other organisms. Sea ice is critical to polar marine ecosystem function, attenuating light and thereby affecting nutrient availability. Sea ice could therefore impact longevity and growth in polar bivalves unless temperature is the overriding factor. Here, we compare the longevity and growth of A. colbecki from two Antarctic sites: Explorers Cove and Bay of Sails, which differ by sea-ice cover, but share similar seawater temperatures, the coldest on Earth (-1.97°C). We hypothesize that scallops from the multiannual sea-ice site will have slower growth and greater longevity. We found maximum ages to be similar at both sites (18–19 years). Growth was slower, with higher inter-individual variability, under multiannual sea ice than under annual sea ice, which we attribute to patchier nutrient availability under multiannual sea ice. Contrary to expectations, A. colbecki growth, but not longevity, is affected by sea-ice duration when temperatures are comparable. Recent dramatic reductions in Antarctic sea ice and predicted temperature increases may irrevocably alter the life histories of this ecosystem engineer and other polar organisms. 

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2. HIGH VARIABILITY OF SUBANNUAL GROWTH BANDS IN THE ANTARCTIC SCALLOP ADAMUSIUM COLBECKI

   https://doi.org/10.1130/abs/2018AM-324369  

   CRONIN, K. E; WALKER, S. E; BOWSER, S. S (October 2018, Geological Society of America Abstracts with Programs)

   null (Ed.)

   The Antarctic scallop Adamussium colbecki may be a crucial paleoenvironmental proxy for Antarctic sea ice during the Holocene. Sea ice can melt annually or persist for multiple years, with implications for the diet and growth of this ecosystem engineer. Subtle growth variations under each sea ice regime could be analyzed using striae (surficial concentric ridges) that putatively form fortnightly in juveniles. Previous work described alternating groups of widely spaced striae (summer) and narrowly spaced striae (winter). Each group may have 12 striae, or a pair of wide and narrow groups (cycle) may have ~ 28; both scenarios suggests approximate tidal (lunar) periodicity in striae formation. However, consistency of striae formation (total striae per valve and group) must be assessed in different environments, as factors such as sea ice or temperature could affect striae growth. We examined striae number, groups, and cycles in juvenile growth (< 50 mm) using scallops collected from two sites in western McMurdo Sound, Antarctica, that differ by sea-ice cover: Explorers Cove (EC) and Bay of Sails (BOS). Both sites have similar summer temperatures (-1.97°C), but EC has multi-annual sea ice whereas BOS has annual sea ice. We predict that annual melt and subsequent phytoplankton blooms likely induce a stronger environmental control than lunar periodicity. Thus, BOS scallops should have equal striae in wide and narrow groups, whereas EC should have fewer striae per wide group and fewer total striae as summer food availability would be greater at BOS and EC valves may cease growth in lower nutrient conditions. Median striae per wide or narrow group was similar at both sites (~12) and median total striae did not differ significantly between sites (EC: 188.5; BOS:183), suggesting striae formation is unaffected by sea ice. Similar median cycles per valve (~5), corroborate previous work that A. colbecki are ~ 5 years old at 50 mm shell height, and ~ 12 striae per group supports lunar periodicity of formation. However, striae per group varied widely (EC: 3–41; BOS 3–38) and 55% of valves had > 182 total striae and 30% had > 208, indicating ages of 7+ and 8+ yrs assuming fortnightly striae formation. Individual striae and group/cycle data contradict each other, calling into question consistent fortnightly striae formation in juvenile A. colbecki. 

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3. OXYGEN AND CARBON ISOTOPES IN ADAMUSSIUM COLBECKI: δ13C PROXY FOR SEA-ICE PERSISTENCE?

   https://doi.org/doi: 10.1130/abs/2020AM-357341  

   Cronin, K.; Walker, S.E.; Gillikin, D.P.; Bowser, S.S. (October 2020, Geological Society of America Abstracts with Programs)

   null (Ed.)

   The Antarctic scallop Adamussium colbecki is a promising proxy for sea-ice persistence and can potentially resolve subannual seawater conditions characteristic of annual and multiannual sea ice. Alternating groups of widely- and narrowly-spaced striae (small ridges on valve surfaces) are thought to indicate seasonal growth differences: wide groups in summer, narrow groups in winter. Shell oxygen (δ18Os) and carbon (δ13Cs) in striae groups may therefore reflect seasonal seawater conditions. We expect lower δ18Os in wide summer striae groups under both annual and multiannual sea ice if glacial meltwater mixes through the water column. We also expect higher δ13Cs in wide striae groups under annual sea ice but not under multiannual sea ice, as phytoplankton blooms post seaice breakout enrich seawater δ13CDIC. Scallops were collected from two sites in western McMurdo Sound (Ross Sea) located ~30 km apart: Explorers Cove (EC) has multiannual sea ice and Bay of Sails (BOS) has annual sea ice. Adults were collected live by divers at 9–18 m depth in 2008 from EC and BOS. Additional juveniles (< 2 yrs) were collected from EC in 2016. Two adults each from EC and BOS and two 2016 juveniles were serially sampled for stable isotopes. δ13Cs decreases over ontogeny due to metabolic effects; the linear trend was removed to enable seasonal comparison. Detrended residuals are referred to as δ13Cs det. Mean δ18Os (~3.7‰) is not different in narrow and wide striae groups under either annual or multiannual sea ice, suggesting negligible glacial meltwater mixing at depth and minimal seasonal temperature change at both sites. δ18Os values are within expected equilibrium range and decrease over ontogeny, suggesting increased growth during warmer temperatures in older scallops. In contrast, mean δ13Cs det is ~1‰ higher in wide summer striae groups than narrow winter striae groups under annual sea ice at BOS, but not different between striae groups under multiannual sea ice in EC adults. δ13Cs det is also higher in wide summer striae groups from 2016 EC juveniles, however sea ice broke out at EC in 2015, so juveniles experienced annual-like sea-ice conditions. Seasonal differences in δ13Cs suggest that carbon isotopes coupled with striae width in A. colbecki may be a good proxy for sea-ice persistence in Antarctica both in modern and fossil assemblages. 

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4. THE ANTARCTIC SCALLOP ADAMUSSIUM COLBECKI AS A PROXY FOR SEA-ICE DURATION IN ANTARCTICA

   https://doi.org/10.1130/abs/2019AM-340824  

   CRONIN, K. E.; WALKER, S. E.; GILLIKIN, D. P.; BOWSER, S. S. (October 2019, Geological Society of America Abstract with Programs)

   null (Ed.)

   Sea ice is critical in structuring Antarctic marine ecosystems, controlling disturbance and primary productivity. Sea ice either melts annually or persists for multiple years, but variability in sea-ice duration is poorly understood prior to satellite images. The Antarctic scallop Adamussium colbecki, with its circum-Antarctic distribution and Holocene fossil history, may be a proxy for sea-ice duration. Previous work on A. colbecki links some trace elements to ice melt and productivity. Further, increments between growth bands (striae) are thought to vary seasonally. To evaluate A. colbecki suitability as a sea-ice proxy, we tested correspondence between growth and trace elements known to represent sea ice or productivity at two sites in western McMurdo Sound: Explorers Cove (EC) with multiannual sea ice and Bay of Sails (BOS) with annual sea ice. Trace element signals should be dampened or absent at EC, whereas those from BOS should cycle annually. One A. colbecki shell each from EC and BOS were collected live in 12 m of water. Trace elements previously linked to ice melt (Mn/Ca, Fe/Ca, and Pb/Ca), metabolism (Mg/Ca), and primary productivity (Ba/Ca, Li/Ca) were sampled from interstrial increments using an LA-ICP-MS along the central axis from umbo to last striae. Interstrial distances (ISDs) were measured and compared to trace elements using wavelet coherence analysis. Coherence (covariance between ISD and trace elements) exceeding 95% significance are reported here. Results indicate that ISD and trace elements only cohere during episodic sea-ice melt at EC and cohere throughout adult growth at BOS. All EC trace element concentrations display a common pattern: cyclic growth followed minimal variation in early adult ontogeny, with intermittent variation resuming later in adult growth. In contrast, trace elements from the BOS scallop exhibit strong cyclic behavior throughout ontogeny. ISD coheres with trace elements at EC for short strial sequences (5-30) twice in adult growth, corresponding to partial sea ice melts at EC during 1999 and 2002. Conversely, BOS trace elements cohere with ISD for long (20-140) strial sequences during adult growth, indicating annual sea-ice melt. Results indicate that A. colbecki archives sea-ice duration, thus its fossil record can be used to investigate past variability. 

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5. Nitrogen and oxygen isotopes in the shell of the Antarctic scallop Adamussium colbecki as a proxy for sea-ice cover in Antarctica.

   https://doi.org/10.1130/abs/2019AM-338279  

   Camarra, S.; Puhalski, E.; Gillikin, D.; Cronin, K.; Walker, S.E. (October 2019, Geological Society of America Abstracts with Programs)

   null (Ed.)

   Adamussium colbecki is a large thin-shelled scallop common in Antarctic waters and well represented in the fossil record. Shell oxygen (δ18Os) and nitrogen isotopes in carbonate bound organic matter (δ15NCBOM) have the potential to record sea ice state over time. To test this hypothesis we will analyze A. colbecki shells from Explorers Cove and Bay of Sails, western McMurdo Sound, Antarctica. These sites have different sea ice states: persistent (multiannual) sea ice at Explorers Cove and annual sea ice (that melts out every year) at Bay of Sails. Six adults shells collected at these sites in 2008 (3 from each site) and two juveniles collected in 2016 from Explorers Cove will be serially sampled for δ18Os values from the growing shell margin to the umbo. We hypothesize that melting glacial ice will pulse freshwater with low δ18O values into the system, which will be recorded in as larger amplitude pulses in shells from the Bay of Sails, but as dampened pulses in Explorers Cove. Carbonate bound organic material will be sampled for δ15NCBOM values. Recent studies illustrated that δ15NCBOM values provide a similar proxy as soft tissue δ15N values (Gillikin et al., 2017, GCA, 200, 55–66, doi: 10.1016/j.gca.2016.12.008). The organic content of the shell is low and the shells are thin, so δ15NCBOM values will be more time averaged than δ18Os values. Nevertheless, sea-ice organic N should have higher δ15N values compared to open water organics due to nitrate draw down in the ice (Fripiat et al., 2014, Global Biogeochem. Cycles, 28, 115–130, doi:10.1002/2013GB004729). Thus we expect large differences between Explorers Cove with persistent sea ice cover and Bay of Sails where the sea ice melts out every year. We posit that oxygen and nitrogen isotopes in A. colbeckishells have a high potential to record sea ice cover. 

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