Heteroplasmy in the mitochondrial genome offers a rare opportunity to track the evolution of a newly arising maternal lineage in populations of non-model species. Here, we identified a previously unreported mitochondrial DNA haplotype while assembling an integrated database of DNA profiles and photo-identification records from humpback whales in southeastern Alaska (SEAK). The haplotype, referred to as A8, was shared by only two individuals, a mature female with her female calf, and differed by only a single base pair from a common haplotype in the North Pacific, referred to as A-. To investigate the origins of the A8 haplotype, we reviewed n = 1,089 electropherograms (including replicate samples) of n = 710 individuals with A- haplotypes from an existing collection. From this review, we found 20 individuals with clear evidence of heteroplasmy for A-/A8 (parental/derived) haplotypes. Of these, 15 were encountered in SEAK, four were encountered on the Hawaiian breeding ground (the primary migratory destination for whales in SEAK) and one was encountered in the northern Gulf of Alaska. We used genotype exclusion and likelihood to identify one of the heteroplasmic females as the likely mother of the A8 cow and grandmother of the A8 calf, establishing the inheritance and germ-line fixation of the new haplotype from the parental heteroplasmy. The mutation leading to this heteroplasmy and the fixation of the A8 haplotype provide an opportunity to document the population dynamics and regional fidelity of a newly arising maternal lineage in a population recovering from exploitation.

Understanding the ecosystem efects of ocean warming is increasingly important as marine heatwaves become more common and increase in severity. Here, we used Glacier Bay National Park long-term monitoring data (1985–2020) to investigate a sudden, sharp decline in humpback whale reproductive success and survival following the onset of the 2014–2016 Northeast Pacifc marine heatwave (PMH). Oceanographic data confrm a persistent warm-water anomaly in 2015–2016 in Glacier Bay, months later than the PMH was documented in the North Pacifc. We assessed changes in demographic parameters pre- and post-PMH using whale and calf counts and multi-state closed population capture–recapture models. Non-calf abundance decreased by 56% between 2013 and 2018, followed by increases in 2019–2020. The predicted proportion of females in the population declined in 2015–2017 (0.40–0.44). For 5 years during and after the heatwave (2015–2019) calf production was far lower than historic levels (0.041 calves per adult female, in contrast to 0.27 pre-PMH). Calf survival dropped tenfold beginning with calves born in 2013 (0.396–0.032) and midsummer calf losses occurred at an unprecedented rate starting in 2014. Non-calf survival declined from 0.982 pre-PMH to 0.899 post-PMH, lower than any value reported for this species. We surmise that documented changes to the forage fsh and zooplankton prey base during and after the PMH were the main driver of reduced humpback whale survival and reproductive success. Humpback whale abundance and productivity in southeastern Alaska will likely take years to recover from the PMH, assuming a return to favorable feeding conditions. Our work highlights this population’s continued vulnerability as the climate warms into previously unobserved states.

We combine a glacier outburst flood model with a glacier flow model to investigate decadal to centennial variations in outburst floods originating from ice-dammed marginal basins. Marginal basins can form due to the retreat and detachment of tributary glaciers, a process that often results in remnant ice being left behind. The remnant ice, which can act like an ice shelf or break apart into a pack of icebergs, limits a basin’s water storage capacity but also exerts pressure on the underlying water and promotes drainage. We find that during glacier retreat there is a strong, nearly linear relationship between flood water volume and peak discharge for individual basins, despite large changes in glacier and remnant ice volumes that are expected to impact flood hydrographs. Consequently, peak discharge increases over time as long as there is remnant ice remaining in a basin, and peak discharge begins to decrease once a basin becomes ice-free. Thus, similar size outburst floods can occur at very different stages of glacier retreat. We also find that the temporal variability in outburst flood magnitude depends on how the floods initiate. Basins that connect to the subglacial hydrological system only after reaching flotation depth yield greater long-term variability in outburst floods than basins that are continuously connected to the subglacial hydrological system (and therefore release floods that initiate before reaching flotation depth). Our results highlight the importance of improving our understanding of both changes in basin geometry and outburst flood initiation mechanisms in order to better assess outburst flood hazards and their impacts on landscape and ecosystem evolution.

Fjords are conduits for heat and mass exchange between tidewater glaciers and the coastal ocean, and thus regulate near-glacier water properties and submarine melting of glaciers. Entrainment into subglacial discharge plumes is a primary driver of seasonal glacial fjord circulation; however, outflowing plumes may continue to influence circulation after reaching neutral buoyancy through the sill-driven mixing and recycling, or reflux, of glacial freshwater. Despite its importance in non-glacial fjords, no framework exists for how freshwater reflux may affect circulation in glacial fjords, where strong buoyancy forcing is also present. Here, we pair a suite of hydrographic observations measured throughout 2016–2017 in LeConte Bay, Alaska, with a three-dimensional numerical model of the fjord to quantify sill-driven reflux of glacial freshwater, and determine its influence on glacial fjord circulation. When paired with subglacial discharge plume-driven buoyancy forcing, sill-generated mixing drives distinct seasonal circulation regimes that differ greatly in their ability to transport heat to the glacier terminus. During the summer, 53%–72% of the surface outflow is refluxed at the fjord's shallow entrance sill and is subsequently re-entrained into the subglacial discharge plume at the fjord head. As a result, near-terminus water properties are heavily influenced by mixing at the entrance sill, and circulation is altered to draw warm, modified external surface water to the glacier grounding line at 200 m depth. This circulatory cell does not exist in the winter when freshwater reflux is minimal. Similar seasonal behavior may exist at other glacial fjords throughout Southeast Alaska, Patagonia, Greenland, and elsewhere.