Spectral Analysis of Ocean Variability at Helgoland Roads, North Sea: A Time Series Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.The study relies on data from a single site in the southeastern North Sea, which is influenced by both river discharge and open North Sea conditions, resulting in unique local hydrological features. However, the authors do not sufficiently explain the scientific rationale for selecting this specific site. Can the observed variability be extended to other coastal systems?

2.All time series are averaged to monthly values. This pre-processing likely erases short-term critical events, such as the rapid, episodic blooms of diatoms or Acartia spp., potentially missing key links between nutrient pulses and biological responses.

3.The authors often interpret cross-wavelet coherence as causal relationships. However, correlation does not equal causation, leading to risks of over-interpreting the data.

4.Significance tests for wavelet analyses assume a red-noise background, but biological parameters frequently show non-red-noise distributions (e.g., spiky fluctuations from blooms). The authors do not address this mismatch, which may make the identified "significant periodicities" unreliable.

5.While the study identifies multiple ecological patterns, it lacks analysis of underlying drivers. Whether nutrient fluctuations stem from natural factors or anthropogenic activities. Without clarifying these mechanisms, the final recommendation of "sustained high-frequency monitoring" is overly vague and lacks practical operability.

6. The author’s affiliation format does not fully comply with academic standards. Additionally, the figures are of low quality.

Author Response

Comment 1.

The study relies on data from a single site in the southeastern North Sea, which is influenced by both river discharge and open North Sea conditions, resulting in unique local hydrological features. However, the authors do not sufficiently explain the scientific rationale for selecting this specific site. Can the observed variability be extended to other coastal systems?

Response:

Introduction has been revised to clarify the scientific rationale for selecting Helgoland Roads (HR). The section now highlights HR’s representativeness as a long-term sentinel site for coastal ecosystems under combined climatic and anthropogenic pressures and situates it within the context of global coastal observatories. This broader framing demonstrates how variability patterns observed at HR are comparable to those in other temperate coastal systems.

Comment 2.

All time series are averaged to monthly values. This pre-processing likely erases short-term critical events, such as the rapid, episodic blooms of diatoms or Acartia spp., potentially missing key links between nutrient pulses and biological responses.

Response:

We retained monthly aggregation to focus on interannual–decadal variability explaining the reason in Methods 2.2. However, a new section has been added in 2.4 and Discussion 4.3 noting that short-lived bloom dynamics may not be fully resolved and recommending complementary high-frequency sampling for future analyses.

Comment 3.

The authors often interpret cross-wavelet coherence as causal relationships. However, correlation does not equal causation, leading to risks of over-interpreting the data.

Response:

A new section has been in Results 3.5 to clarify that cross-wavelet coherence identifies co-variability, not causation.

Comment 4.

Significance tests for wavelet analyses assume a red-noise background, but biological parameters frequently show non-red-noise distributions (e.g., spiky fluctuations from blooms). The authors do not address this mismatch, which may make the identified "significant periodicities" unreliable.

Response:

This limitation is now acknowledged in Discussion 4.4, where the limitations of red-noise assumptions were noted by clarifying that our significance assessment relied on the analytical framework of the WaveletComp package, not AR(1)-based modeling. We now explicitly discuss this limitation and suggest future use of adaptive spectral methods to better address non-red-noise dynamics.

Comment 5.

While the study identifies multiple ecological patterns, it lacks analysis of underlying drivers. Whether nutrient fluctuations stem from natural factors or anthropogenic activities. Without clarifying these mechanisms, the final recommendation of "sustained high-frequency monitoring" is overly vague and lacks practical operability.

Response:

We added a dedicated subsection in Discussion 4.3 synthesizing natural (e.g., climatic oscillations, river discharge) and anthropogenic (e.g., nutrient management, eutrophication control) drivers. This section integrates recent literature on nutrient regulation in the German Bight.

Comment 6.

The author’s affiliation format does not fully comply with academic standards. Additionally, the figures are of low quality.

Response:

All author affiliations have been reformatted to match journal standards. Figure 3-5 were redrawn in high-resolution.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Studying the physical, hydrographic, biological, and other characteristics of the water column is an important task of oceanology, or in other words, the science of the ocean. These parameters are particularly variable in coastal areas. This is precisely the topic of the manuscript under review. The work is professionally executed, written in clear scientific and technical language, accompanied by good illustrations and key references on the subject. However, in order for the article to be accepted for publication, the author needs to make some fairly substantial revisions. Below are the main observations that the author is advised to take into account.

1. I would recommend specifying the geographical location of the Helgoland Roads (HR) station, whose data was used for analysis, in the title of the article.
2. In the introduction, the authors focus on the North Sea and, in particular, on the HR station located between the islands of Helgoland and Düne. Meanwhile, the issues addressed in the article are well known and have long been studied in many areas of the World Ocean. The article would benefit if the authors provided a brief overview of such studies.
3. The location of the HR station is very specific. It is the tidal channel of the German Bight. In addition, the oceanographic characteristics are influenced by terrigenous material from the Elbe River. The author should pay more attention to the hydrography, sedimentation, bathymetry, and other oceanographic features of the station's location.
4. The work is based on data processing from 1968 to 2017. It is necessary to specify what measuring equipment and methods were used during this period. What generalizations and data analysis were performed in previous years. How did the authors compare data from different time periods and what is the margin of error of such a comparison?
5. Line 117. With all due respect to the cited work (17), I would recommend providing more recent references. Overall, the processing section is written very briefly, with virtually no description of the processing procedure. This makes it impossible to verify the accuracy of the results.
6. The results are presented in a relatively complete manner and with a large number of graphs. At the same time, there is no generalization of all this diversity of data. In my opinion, this is the main shortcoming of the work. Thus, the main patterns calculated using mathematical methods are quite obvious and, based on a qualitative analysis, which I believe has been conducted repeatedly in the past (I repeat that the work lacks an analysis of previous studies on this topic).
7. The lack of in-depth analysis is also evident in the Conclusions. The list includes fairly obvious things, such as the connection between salinity and the discharge of the Elbe River, the reduction of organic matter due to anthropogenic factors, Nutrients decreased with human management, etc. I would recommend that the author use spectral analysis to develop a generalized model of oceanographic development in the studied area over a long period of observation.

Author Response

Comment 1.

I would recommend specifying the geographical location of the Helgoland Roads (HR) station, whose data was used for analysis, in the title of the article.

Response:

The title has been revised to:

“Spectral analysis of ocean variability at Helgoland Roads, Wadden Sea: a time series study.”

Comment 2.

In the introduction, the authors focus on the North Sea and, in particular, on the HR station located between the islands of Helgoland and Düne. Meanwhile, the issues addressed in the article are well known and have long been studied in many areas of the World Ocean. The article would benefit if the authors provided a brief overview of such studies.

Response:

The Introduction now includes a comparative overview of global time-series programs following the reviewer’s suggestion. These examples contextualize HR within global coastal monitoring networks.

Comment 3.

The location of the HR station is very specific. It is the tidal channel of the German Bight. In addition, the oceanographic characteristics are influenced by terrigenous material from the Elbe River. The author should pay more attention to the hydrography, sedimentation, bathymetry, and other oceanographic features of the station's location.

Response:

We revised Methods 2.1 to describe bathymetry, tidal regime, sediment composition, and Elbe River influence, citing.

Comment 4.

The work is based on data processing from 1968 to 2017. It is necessary to specify what measuring equipment and methods were used during this period. What generalizations and data analysis were performed in previous years. How did the authors compare data from different time periods and what is the margin of error of such a comparison?

Response:

This is now detailed in Methods 2.2, which documents instrumentation transitions (thermometers → CTDs; manual → automated nutrient analyzers), calibration overlap, and estimated uncertainty ranges (±3 % for physical, ±5–8 % for biogeochemical data).

Comment 5.

Line 117. With all due respect to the cited work (17), I would recommend providing more recent references. Overall, the processing section is written very briefly, with virtually no description of the processing procedure. This makes it impossible to verify the accuracy of the results.

Response:

We replaced outdated citation for spectral analysis with more recent methodological works and expanded Methods 2.4 to describe MTM implementation, adaptive weighting, and AR (1) background modeling.

Comment 6.

The results are presented in a relatively complete manner and with a large number of graphs. At the same time, there is no generalization of all this diversity of data. In my opinion, this is the main shortcoming of the work. Thus, the main patterns calculated using mathematical methods are quite obvious and, based on a qualitative analysis, which I believe has been conducted repeatedly in the past (I repeat that the work lacks an analysis of previous studies on this topic).

Response:

The Results section has been restructured to integrate numerical synthesis and cross-variable comparisons (Figs. 3–8). A new Section 3.5 (“Synthesis of multiscale variability”) summarizes dominant multi-decadal patterns and their ecological significance. The Conclusion was rewritten to highlight generalized system behavior and practical implications for coastal monitoring.

Comment 7.

The lack of in-depth analysis is also evident in the Conclusions. The list includes fairly obvious things, such as the connection between salinity and the discharge of the Elbe River, the reduction of organic matter due to anthropogenic factors, Nutrients decreased with human management, etc. I would recommend that the author use spectral analysis to develop a generalized model of oceanographic development in the studied area over a long period of observation.

Response:

We incorporated multi-taper spectral, wavelet, and cross-wavelet analyses to identify dominant temporal patterns and scale-dependent interactions over the full observation period. These results are synthesized in the Discussion (Section 4.3) and summarized in the Conclusions, where we now present a generalized conceptual model of oceanographic development in the German Bight.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

No more comments

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors, thank you for your work. I am satisfied with the results of the review and believe that the manuscript can be published in its present form.