the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Oct 2024
Fine-scale variability in iceberg velocity fields and implications for an ice-associated pinniped
Abstract. Icebergs and sea ice found in proglacial fjords serve as important habitat for pinnipeds in polar and subpolar regions. Environmental forcings can drive dramatic changes in fjord ice coverage, with implications for pinniped distribution, abundance, and behavior. To better understand how pinnipeds respond to changes in iceberg habitat, we combine (i) iceberg velocity fields over hourly to monthly timescales, derived from high-rate time-lapse photogrammetry of Johns Hopkins Glacier and Inlet, Alaska, with (ii) aerial photographic surveys of harbor seals (Phoca vitulina richardi) conducted during the pupping (June) and molting (August) seasons. Iceberg velocities typically followed a similar diurnal pattern: flow was weak and variable in the morning and strong and unidirectional in the afternoon. The velocity fields tended to be highly variable in the inner fjord, across a range of timescales, due to changes in the strength and location of the subglacial outflow plume, whereas in the outer fjord the flow was more uniform and eddies consistently formed in the same locations. During the pupping season, seals were generally more dispersed across the slow moving portions of the fjord (with iceberg speeds 0.2 m s−1.. In contrast, during the molting season the seals were increasingly likely to be found on fast moving icebergs in or adjacent to the glacier outflow plume. Use of slow moving icebergs during the pupping season likely provides a more stable ice platform for nursing, caring for young, and avoiding predators. Periods of strong glacier runoff and/or katabatic winds may result in more dynamic and less stable ice habitat, with implications for seal behavior and distribution within the fjord.
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RC1: 'Comment on egusphere-2024-2950', Anonymous Referee #1, 08 Nov 2024
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This is an interesting and well-written study on the biophysics of a tidal glacier fjord habitat for harbor seals in southeast Alaska. The approach is clever as well as an unique and novel application for pinniped habitat studies. Although sample sizes precluded the authors from detailed statistical analysis of links between seals and iceberg velocity within the fjord, there are some interested descriptive analyses and figures presented here.
The bulk of the results focus on describing iceberg movements and velocity within the fjord, which represents considerable effort and resources in itself. That said, it would be helpful though to have a bit more framing on the ecological significance and context of this glacial iceberg habitat for harbor seals in the Introduction. Although the authors did not consider it here, I wonder if there’s an ability to estimate iceberg size and shape from any of the imagery used in this study, in terms of iceberg selection by seals and how fjord velocity is related to availability of suitable habitat. I recommend considering these kinds of aspects in the Discussion or possibly adding a section on potential next steps or additional research. There are also 9 figures, and it might be worthwhile to move a couple to supplementary materials and also introduce some quantitative analysis to more precisely link environmental conditions and diel patterns to iceberg velocity. In my line comments, I provide suggestions to improve clarity and details of the study.
Line comments
23-31: While this paragraph lays out glacial ice as important habitat for harbor seals in Glacier Bay and that surveys are conducted during pupping and molting season, there could be additional details about the ecological function and purpose that glacial ice plays for harbor seals (i.e. why is it important habitat?). This is unique habitat for harbor seals and so additional introduction to the ecology of this ecosystem would be useful, particularly for readers unfamiliar with this habitat.
24 (and later): I deeply appreciate the inclusion and normalization of Tlingit place names here and throughout. Another option might be to put the Tlingit name first in the sentence and then parenthetically the western name, noting hereafter referred to as. For example: “In Sit’ Eeti Geeyi (or what is currently known as and hereafter referred to as Glacier Bay),…” Otherwise, please indicate Tlingit in the parentheses for readers who may be unfamiliar with the region, e.g.: Glacier Bay (Sit’ Eeti Geeyi in Tlingit).
71-74: It’s unclear how often (if at all) conditions affected camera imagery, but presumably fog, rain, or other conditions could affect visibility and therefore the ability to detect and track icebergs in images. Please clarify if this is the case and how it was dealt with if so.
90-91: Figure 2b seems to contradict the text here for flights in 2022 (although this line is referring to 2019), with several image footprints that appear to overlap, presumably where the aircraft is turning near the glacier terminus. Please clarify in the next paragraph about 2022 flights if the differences in survey methods between years introduced the potential for overlapping images and therefore double or multi-counting of seals.
104: please clarify what kind of shapefiles were created. Was it points with a single location for seals? Or if it was polygons or something else, provide more detail about how these were created.
105-106: In other words…two sets of shapefiles, specifically for pups as well as non-pups, were created during pupping season in both years of aerial surveys? Only non-pup shapefiles, in each year of surveys, were created during the molting season?
Please also include parenthetically which months or time period were considered pupping and molting seasons – was this the same period in 2019 and 2022?
143 and Fig 3: This is such an interesting figure, and tells so much information. However, I’m a little confused about how the “prominent, persistent” eddies were identified. The text suggests that the placement of the eddies outlined in the boxes were persistent across days/time, but I don’t see each of these 3 locations with the eddies for each day in Fig 3. So maybe I’m missing across what time scale an eddy was considered to be persistent.
153: The order of figure numbering is hard to follow – it doesn’t seem like figures are presented in the order in which they are referenced in the text. In this case, the text jumps from Fig 5 to Fig 8.
Figs. 4 and 7: there are a lot of figures in this paper, and I wonder about some that could be moved to supplementary material because they are not primary contributors to the key points. These figures may be good options for the supplement. These figures are also challenging to line up the environmental data with the velocity data, and I wonder about providing statistics to support the statements about correlations between environmental data and velocity, which can then be used to reinforce the visual interpretation as well as may be able to be summarized more succinctly in a table for example.
179-190: The use of CCDFs is an interesting approach for describing seal iceberg velocity presence, given the small sample size in the number of days sampled. Yet Fig 8 provides a nice visual representation showing seals affiliation with plumes, and it raises the question of whether iceberg velocity is the correct predictor of seals in the fjord and hauled out on icebergs. Another potential interesting predictor, not considered here, would be proximity to plumes.
Fig. 8: during the pupping period, can pups on ice be distinguished from the adults on ice? It’s mentioned at line 105 that there were different shapefiles for pups and non-pups, so if possible, it would be interesting to describe if there are differences in use of icebergs, and relative to the plume for pups (or mom-pup pairs) and non-pups.
Fig 9: Please include a description of what the y-axis, P(V>v), means in the caption, including V and v.
286: Are the aerial survey seal data also publicly available? Please include references for users interested in seal data.
Citation: https://doi.org/10.5194/egusphere-2024-2950-RC1 -
RC2: 'Comment on egusphere-2024-2950', Anonymous Referee #2, 09 Nov 2024
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General Comments:
This paper explores pinniped distribution in an iceberg habitat by examining the relationship between glacial fjord velocity and harbor seal distribution. Key takeaways include how seal distribution is influenced by glacial velocity and outflow plums; seals are more likely to be found on the edges of plumes, in relatively slower moving water during the pupping season, and relatively faster moving water during the molting season. It is also discussed how adult harbor seals are not likely to be energetically impacted by swimming against glacier currents, but recently weaned pups may be negatively impacted if they have to spend more time in the water due to changing glacial habitat conditions. This paper gives important and novel insight into how changes in the glacial ecosystem due to climate change (i.e. increased precipitation and glacial calving) may impact harbor seal distribution and behavior. This information improves our understanding of how ice-associated marine mammal species may be influenced by ongoing ecological shifts, which is vital to inform future conservation efforts.
I believe there are aspects not discussed that would enhance the paper, particularly a more detailed exploration of potential future research directions. Should more effort be made to survey and conduct this research for the month of July? Since the paper mentions that the mean velocity in July is greater compared to June and August, would this give valuable insights into how glacial surface currents impact distribution? What future statistical analyses could be done to add to these results?
Furthermore, given that other studies have found that the haulout behavior of ice-associated pinnipeds is impacted by environmental variables (ex. Hamilton et al., 2014 published in Plos One), I also think there needs to be more of a discussion about potential covariates that may impact seal distribution rather than solely glacial fjord velocity and plumage area, especially since seals are more likely to be seen while hauled out vs. when swimming (Line 172). Glacial fjord velocity and plumage area are only a part of the bigger picture. Lastly, there is no discussion of why seals may select for recently calved icebergs (Line 175 & Figure 8); is it because they are larger, more stable, etc.?
Specific Comments:
Line 19: Clarify “given the dramatic changes in ice coverage”; is this referring to the overall reduction in circumpolar ice concentrations? Seasonality shifts?
Line 24: Add “...(Sít’ Eetí Geeyi in the Tlingit language)” ; no mention of the Tlingit people is given throughout the paper
Line 56: How much overlap between photos?
Line 63: What does an initial guess mean in this context?
Line 105-106: Provide clarification; does this mean pups were not included in the data processing for the molting season? Or all seals in imagery were processed as non-pups?
Figure 3: Why were these days selected to show the velocity fields? Should the aerial survey day velocity streamlines also be included in this figure?
Figure 8: Define time frame for “recently” calved icebergs.
Line 216: It was not clear that 2019 was used to represent the pupping year and 2022 to represent the molting year due to small sampling size until I read the caption under Figure 9. Potentially make that more clear here, earlier in the paper.
Lines 211-214: Seals on slower moving icebergs during the pupping season does not explain why males are on slower moving ice; thoughts on this would add to the discussion.
Lines 217- 243: Much of the discussion focuses on the potential energetic demands of glacial surface currents on harbor seals, but this is the first time energetic demands are mentioned. Incorporating this idea earlier, perhaps in the introduction, would enhance the paper's overall cohesiveness.
Technical Corrections:
Line 85: Repetitive with above; potentially cut
Line 145: Place “Each column in Figure 5…” sentence in figure 5 caption
Line 204-207: Move this paragraph to the first paragraph of discussion (line 192-197) to improve flow
Line 219: “Between” is written twice
Figures are not always in the order they appear in the text. For example, Fig 8 is mentioned before Fig 7
Citation: https://doi.org/10.5194/egusphere-2024-2950-RC2
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