PONE-D-24-14571Trends and drivers of hypoxic thickness and volume in Northern Gulf of Mexico: 1985-2018PLOS ONE

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Reviewer #1: Yes

Reviewer #2: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The authors derive a geospatial model for predicting hypoxic area, volume and thickness based on real-time data from multiple monitoring programs within the notorious besieged northern Gulf of Mexico hypoxic region. The model utilizes a space-time geostatistical framework and implements a rank-based inverse normal transformation to improve estimates of hypoxic thickness, an important metric typically overlooked. The authors are experts within the arena of geospatial modeling and their geostatistical methodology represents a sound alternative approach. Lack of correlation between hypoxic thickness and the other hypoxic metrics of area and volume, as well as a progressive increasing trend in thickness over the last three decades within the region underscore the importance of hypoxic thickness as an informative metric. The explicit space-time dimensions of the geospatial framework used constitute a novel aspect relative to other approaches, such as biogeochemical process models. Another strength of the model is the validation performed using actual data to identify potential biases precluding prediction accuracy. Potential environmental drivers are also evaluated. Using multiple regression to analyze output from the model, spring nitrogen loading was shown to be the primary driver of hypoxia within the system, in addition to several subsidiary drivers involving meteorological patterns. Subregions within the northern GoM sometimes responded differently to the same meteorological drivers. One of the biggest benefits of the study is the heuristic value in comparison with other approaches aimed at understanding the evolution of hypoxia within the northern Gulf of Mexico.

Reviewer #2: PONE reviewer comments Matli & Obenouer

This analysis extends prior analyses of area and volume to include thickness as a metric with which to compare annual and monthly trends and to support resource management decisions. Thickness as a metric is more difficult to understand compared to area (a synthetic number) and volume (also a synthetic number) compared to thickness, which as a synthetic number, is more difficult to understand because it is compiled from multiple hydrographic profiles across a broad range of depths and geographic locations and represents a unified metric that seems less useful as a management tool, either in reduction of nutrient inputs or fishery-related resources.

There is also a mixture of geographically different monitoring cruises, especially differences between TAMU and EPA compared to each other and to LUMCON. Additionally, LUMCONT is a one- or two-day cruise to specific transects contained within the larger LUMCON MC station locations during the shelfwide cruises. It is not clear how these various hydrographic profiles become a monthly or annual metric for statistical analyses.

Many stated results, e.g., hypoxia is more likely to occur in July and August than in June, is already well known. Winds from the east in spring disperse nutrients and fresh water from the Mississippi River in the spring season support the phytoplankton biomass that eventually falls to the sea bed and is respired.

Many general terms, e.g., For volume, the corresponding results are 0, 2, 11, 10, and 1 years, respectively, indicating that peak volume tends to lag peak area, at least on average

…..taken together, these results suggest both patterns as well as aberrations in the seasonal….

There is little doubt that a tremendous amount of consistent, strong analytical methods, and a thorough understanding of the metrics provided are correct. These are trademarks of these co-authors.

There needs to be a more concise presentation of the results and their interpretation.

Small things that caught my eye:

Inconsistent use of ….hypoxia… as a noun, and …..hypoxic…..as an adjective ……hypoxia metrics, when considered along with other riverine inputs and meteorological factors in multiple regression models. Hypoxic volume,….

L36 …..the size of the hypoxic zone

L 63, Louisiana Universities Marine Consortium

L 80, thickness of hypoxia across the Gulf hypoxia region from May, second hypoxia should be hypoxic, and so on through the manuscript.

L 101, change to ….sampling frame….

Fig. 1, a mixture of font styles

L 172, multiple cruises in each of the years analyzed, but the geographic footprints of those cruises were quite different.

L 175 et seq. Selection of ‘seasons’ does not appear to be consistent, but rather the months in which there were cruises.

L186, Mississippi and Atchafalaya rivers

L 212, a lag of 12 days

L 261, is there a reference for this statement ROMS vs FVCOM?

L 300, …too little variance…

L 336, 2000 was a drought year, little stratification

L 354 – 356, these sentences are contradictory. Overall, these results suggest that most of the variability in hypoxic volume can be attributed to changes in hypoxic area. At the same time, variability in hypoxic thickness, which is only modestly correlated to area, is also important.

L 372-373, is there a conclusion here?

Fig. 5, symbols are difficult to see.

L 388-389, after reading this paragraph is there any merit to determining hypoxia thickness?

L 447, ability to constraint uncertainty…. Change to …constrain…or chloropigments

L 449 “…..Future work in improving this model can focus on incorporating covariates that capture short term changes in system conditions….” Reviewer, of these limitations, it would take dynamic measurements of conductivity, temperature, DO, Chla or chloropigments, possibly turbidity, through the water column with some type of profiler on a buoy that could capture the hypoxic thickness. Or, some type of AUV that can change position, lower raise probes, move to a new position, over some periodicity that would capture short-term variability suitable for the models. OR, several AUVs. Any of these would be prohibitively expensive and not likely to be funded by NOAA research programs, especially for a ‘thickness’ indicator that is not as informative as area or volume, for resource management.

S—1, should the journal Environmental science and technology be capitalized here and elsewhere?

Northern Gulf of Mexico is not a specific geographic location, and should be represented as …..northern Gulf of Mexico.

(National Geophysical Data Ceter, 2001) should be ….Center….

Additional details on the development of these equations is provided in Obenour et al., (2013). Should be ….are…

SI—3 Rosette-mounted samplers, however, are on large rigs with other …suggest ‘frames’ rather than rigs…. ‘rig’ also used in manuscript text.

Figures SI—6 Summer-wide daily estimates of area, volume, and thickness with the 95% CI of area, and volume for 1997-2000 Question: why are area values daily and volume and thickness are continuous (if, I am reading this graphic correctly)?

Table S9c. Candidate variables used in Summer Average regressions (MR indicates Mississippi River, AR indicates Atchafalaya River, W indicates western shelf). Capitalize “River” If Mississippi and Atchafalaya rivers is used, ‘river’ is not capitalized as it is not a geographic location.

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Reviewer #1: Yes: Chet F. Rakocinski

Reviewer #2: No

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Attachments

Attachment

Submitted filename: PONE reviewer comments Matli and Obenour.docx

Trends and drivers of hypoxic thickness and volume in northern Gulf of Mexico: 1985-2018

PONE-D-24-14571R1

Dear Dr. Matli,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Academic Editor

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Reviewer #2: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

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4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: Matli PONE Hypox thickness and volume of hypoxia nGoMx

Some thoughts came to mind as I re-read this manuscript, not particularly with the quality of the manuscript, but with some of the results. I can see why thickness is less tractable than area or volume because the thickness feature is a result of winds piling up water masses to the east at the time when Gulf coastal winds shift from the east to from the west. [Also noted in the last sentence of the abstract.] There are also summer conditions where winds from the north may prevail or from the south, or a switching in the prevailing winds during the research cruise. These several conditions would influence the volume (not so much the total) and thickness.

….on average, there is a seasonal lag in peak hypoxic volume and thickness compared to hypoxic area…..

Are there any suggestions about the formation of hypoxia beginning in the sediments with respiration of organic matter and then migrating up into the lower water column, or a combination of this along with the respiration of settled phytoplankton below the pycnocline?

Another reason that hypoxic volume may not be considered a standard metric for the nGoMx hypoxic zone is that the system is open on the westward and southern sides, or disjunct among two areas, and therefore more difficult to estimate.

Not important, and an editor will catch these, but a space before a period in L64.

L100 delete comma after et al. Same for line 134. This may be a punctuation issue for the journal. There is usually not a comma after et al. before a date. No more marked.

L217, meaning of ‘severe’ Again line 221, and elsewhere, L229, L348 Is this a relative value of volume or thickness?

Pointing out differences in the ‘east’ and ‘west’ portions of the study area is useful.

Conclusion, mixture of past and present tenses.

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Reviewer #2: No

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Submitted filename: Matli PONE Hypoxia thickness and volume nGoMx.docx