Updated Absolute Gravity Rate of Change Associated With Glacial Isostatic Adjustment in Southeast Alaska and Its Utilization for Rheological Parameter Estimation

In Southeast Alaska (SE-AK), rapid ground uplift of up to 3 cm/yr has been observed associated with post-Little Ice Age glacial isostatic adjustment (GIA). Geodetic techniques such as global navigation satellite system (GNSS) and absolute gravimetry have been applied to monitor GIA since the last 1990s. Rheological parameters for SE-AK were determined from dense GNSS array data in earlier studies. However, the absolute gravity rate of change observed in SE-AK was inconsistent with the ground uplift rate, mainly because few gravity measurements from 2006 to 2008 resulted in imprecise gravity variation rates. Therefore, we collected absolute gravity data at six gravity points in SE-AK every June in 2012, 2013, and 2015, and updated the gravity variation rate by reprocessing the absolute gravity data collected from 2006 to 2015. We found that the updated gravity variation rate at the six gravity points ranged from −2.05 to −4.40 μGal/yr, and its standard deviation was smaller than that reported in the earlier study by up to 88 %. We also estimated the rheological parameters to explain the updated gravity variation rate, and their optimal values were determined to be 55 km and 1.2 × 10^19 Pa s for lithospheric thickness and upper mantle viscosity, respectively. These optimal values are consistent with those independently obtained from GNSS observations, and this fact indicates that absolute gravimetry can be one of the most effective methods in determining sub-surface structural parameters associated with GIA accurately. Moreover, we utilized the gravity variation rates for estimating the ratio of gravity variation to vertical ground deformation at the six gravity points in SE-AK. The viscous ratio values were obtained as −0.168 and −0.171 μGal/mm from the observed data and the calculated result, respectively. These ratios are greater (in absolute) than those for other GIA regions (−0.15 to −0.16 μGal/mm in Antarctica and Fennoscandia) because glaciers in SE-AK have melted more recently than in other regions.

Shallow-emerged coral may warn of deep-sea coral response to thermal stress

In the Gulf of Alaska, commercially harvested fish species utilize habitats dominated by red tree corals (Primnoa pacifica) for shelter, feeding, and nurseries, but recent studies hint that environmental conditions may be interrupting the reproductive lifecycle of the corals. The North Pacific has experienced persistent and extreme thermal variability in recent years and this pattern is predicted to continue in coming decades. Recent discovery of deep-water emerged coral populations in Southeast Alaska fjords provided opportunity for detailed life-history studies and comparison to corals in managed habitats on the continental shelf. Here we show that sperm from deep colonies develops completely, but in shallow colonies, sperm development is prematurely halted, likely preventing successful production of larvae. We hypothesize that the divergence is due to differing temperature regimes presently experienced by the corals. Compared to deep populations below the thermocline, shallow populations experience much greater seasonal thermal variability and annual pulses of suspected near-lethal temperatures that appear to interrupt the production of viable gametes. The unique opportunity to comprehensively study emerged populations presently affected by thermal stress provides advance warning of the possible fate of deep corals in the Gulf of Alaska that will soon experience similar ocean conditions.

Leveraging eDNA metabarcoding to characterize nearshore fish communities in Southeast Alaska: Do habitat and tide matter?

Nearshore marine habitats are critical for a variety of commercially important fish species, but assessing fish communities in these habitats is costly and time-intensive. Here, we leverage eDNA metabarcoding to characterize nearshore fish communities near Juneau, Alaska, USA, a high-latitude environment with large tidal swings, strong currents, and significant freshwater input. We investigated whether species richness and community composition differed across three habitat types (sand beaches, eelgrass beds, and rocky shorelines) and between high and low tides. Additionally, we tested whether replication of field samples and PCR reactions influenced either species richness or composition. We amplified a 12S mitochondrial locus in our samples and identified 188 fish amplicon sequence variants (ASVs), corresponding to 21 unique taxa, with approximately half of these resolved to single species. Species richness and composition inferred from eDNA differed substantially among habitats, with rock habitats containing fewer taxa and fewer overall detections than sand and eelgrass habitats. The effect of tide was more subtle and suggested a habitat-tide interaction, with differences in taxa between tides largely isolated to sand habitats. Power analyses indicated that additional field sampling is useful to detect subtle changes in species richness such as those due to tide. PCR replicates typically identified a small number of additional taxa. The most notable result from our study was that shore morphology appeared to substantially influence community structure. Rocky shorelines sloped quickly into deep water, while sand and eelgrass habitats descended much more gradually. We hypothesize that differences in taxa observed among habitats were largely due to lack of mixing between bottom and surface water, providing further evidence that eDNA transport is minimal and that many marine eDNA detections are derived from highly localized sampling locations. We suggest that future studies could explore the extent to which habitat and nearshore physical processes influence eDNA detections.

Is the modern-day dieback of yellow-cedar unprecedented?

In Southeast Alaska, many stands of yellow-cedar (Callitropsis nootkatensis D. Don; Oerst. ex D.P. Little; hereafter: ‘YC’) contain numerous standing, dead snags. Snag-age estimates based on morphology have been used to support the interpretation that a warming climate after ca. 1880 triggered unprecedented YC dieback. Here we present new estimates of YC snag longevity by cross-dating 61 snags with morphologies that suggest they stood dead for extended periods. All but four of these snags have lost their outermost rings to decay, so we estimate when they died using a new method based on wood-ablation rates measured in six living trees that display partial cambial dieback. Results indicate that ~59% of YC snags that lost their branches to decay (Class 5 snags) have remained standing for > 200 years, and some for as long as 450 years (snag longevity mean ± SD: 233 ± 92 years). These findings, along with supporting evidence from historical photos, dendrochronology, and snag-morphology surveys in the published literature suggest that episodes of YC dieback also occurred before 1880 and before significant anthropogenic warming began. The roles played by climate change in these earlier dieback events remain to be further explored.