A Chlorophyll Biomass Time-Series for the Distributed Biological Observatory in the Context of Seasonal Sea Ice Declines in the Pacific Arctic Region

Round 1

Reviewer 1 Report

 

This paper presents in situ chlorophyll concentrations measured at the Pacific Arctic DBO sites since 2000. They interpret their results with respect to sea ice cover and conclude that while sea ice has declined since 2000, there has been no significant change in chlorophyll over that same time period.  They are careful to note that this may be due to the restricted area over which their data were collected and the fact that they were looking only at July. They also present some October data for one of the DBO lines and show that there is evidence of increased nutrient upwelling (or mixing) and higher chlorophyll, suggesting the possibility of a fall bloom.

 

Because they are presenting DBO data, I was confused about where the data they presented came from.  After reading the paper, I can only speculate that these were data collected by the authors themselves and do not include chlorophyll data collected by others that have been incorporated into the larger DBO dataset.  This seems like an odd choice if their goal was to see if chlorophyll concentration in the Pacific Arctic has in the last 20 years.  Wouldn’t a larger dataset be better for testing this hypothesis?

 

I was surprised to see that the paper doesn’t actually include a plot showing changes in chlorophyll each year over time.  The authors include extensive tables containing all the chlorophyll data, which is great but would be better in a supplement or appendix, but no year-by-year plot, and more importantly, the statistics that go with it.  Figure 2 comes closest to this, but the data are binned into multiple year bins of varying length, so it is impossible to see the year-to-year changes.

 

Despite the extensive data in the tables, the results sections are very short, with each only consisting of a single paragraph of about 5-10 lines.  At a minimum I would have liked to see the data broadly described with respect to local conditions of temperature, salinity, mixed layer depth, and nutrient concentration.  This was done for the October data but not for anything else.  Consequently, there is quite a bit of variability in the chlorophyll data but none of it is explained.  Even much of the data that are presented are not described in the text.  For example, section 3.2. on DBO 2 says nothing about the maximum chlorophyll concentrations or the depth of the chlorophyll max. Because of this extreme brevity, the paper reads like more of a data report than a fleshed-out scientific paper.

 

Surprisingly, the Discussion was even shorter, consisting of a single paragraph.  Furthermore, there wasn’t a single citation in the entire section.  There was no attempt to compare their results to previous studies or to use previous studies as context for their observations.  For example, Lewis et al. (2020) reported that despite large sea ice declines in the Arctic over the past few decades, there had been no increase in mean chlorophyll concentration over much of the Arctic (consistent with the authors’ observations) until the last decade.  They even provide a map of changes in satellite chlorophyll over time where it can be seen that the Chukchi Sea has some areas with significant increases in chlorophyll and some areas that have declined.  It would be interesting to see how the authors might incorporate this kind of information into their story.  It would also be nice to hear what the authors thought the implications, if any, of an unchanging chlorophyll concentration were for the larger ecosystem.  Surely, some Discussions go on for far too long, but the 19 lines here are not sufficient for a full-fledged scientific paper.

 

Other comments

Line 24.  Change to “should not be expected”

Line 70. Change to “as a result of”

Line 102. Change to “Data from a fall cruise in October 2020 are” (data is a plural word)

Line 108.  So all the chlorophyll data used in this study were analyzed in the same way?  So no data from other groups were used then, correct?

Line 172-174. Figure 4 does not show this.  It is a spatial plot of depth integrated chlorophyll.  I didn't see any time series plots, other than maybe Figure 2

Line 204.  Why is this text here (October 2020 Norseman II cruise, and on).

Line 220-221. Doesn’t “or simply viable cells being brought into sufficient light in the presence of inorganic nutrients” also represent new production?

Line 225-227.  Many studies have shown that production increases as sea ice has retreated. Only recently has chlorophyll also increased significantly over time (Lewis et al. 2020).  I don’t know that anyone is assuming that chlorophyll concentrations will increase as sec ice retreats.  Primary production maybe, but not chlorophyll concentration.  If the authors know of any, they should be discussed here.

 

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Review of: A Chlorophyll Biomass Time-Series for The Distributed Biological Observatory in The Context of Seasonal Sea Ice Declines in The Pacific Arctic Region

By: Lee W. Cooper and Jacqueline M. Grebmeier

 

Long term chl a data of the Arctic Pacific collected during July at a time series stations are analyzed for trends. This is interesting since the Arctic experiences pronounce changes that have affected productivity of the Arctic. However, no clear trends were observed over time in this study for the month of July. At first sight this is somewhat surprising, given the remote sensing observations over this region that indicate increased chl a concentrations on an annual basis. The lack of a comparison with remote sensing chl a for July is the greatest weakness of this paper. It is unclear if remote sensing shows increased chl a over the past 20 years for the month of July in the area of the time series stations. A line up of in situ and remote sensing chlorophyll a would make this paper much more interesting.

Surface chl a is not presented, which could be compared with remote sensing.

Methods are lacking at several points. For example how was depth integrated chl a calculated at each station? What were the sample depths for chl a? How were the time series of chl a analyzed for trends?

Nutrient concentrations are not shown for the time series.

Figure 2 shows averages over 6 years for comparison. Why? Is this a convenient time interval? Why not all data points? Were 6 year averages tested for trends? This is not specified. How were the data tested for trends?

Sea ice declines at these stations not shown.

 

Tables 1-5 can be presented in a supplement. A much shorter table can be presented in the ms, showing average values.

 

Figures 6 and 7: all show salinity as a heading, but salinity is not shown. Would be better to position graphs of July and October next to each other to facilitate comparison.

 

After addressing these issues (revision) the paper would become interesting for publication in Geosciences.

Author Response

please see the attachment

Author Response File: Author Response.docx