Spatial Structure and Temporal Dynamics in Clear Lake, CA: The Role of Wind in Promoting and Sustaining Harmful Cyanobacterial Blooms

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript provides abundant observational data indicating that wind was a dominant factor affecting Lake Clear’s chemistry and biology, resulting in dramatic spatial heterogeneity of phytoplankton biomass and cyanotoxin in the eastern and southeastern Arms of Lake Clear. The manuscript is clearly structured, and the results are well discussed. This is a well-written manuscript. I have only a few minor comments as follows.

Line 165: In Figure 1A, the labels of site number is not clear, which should be larger. The Upper Arm, Oaks Arm, and Lower Arm can also be labeled in Figure 1A.

Line 540-555: This paragraph partially overlaps with earlier descriptions in other sections.

It is better to stay focused on discussion of the data or major findings instead of instruments or specific methods used.

The influence of wind on in promoting and sustaining harmful cyanobacterial blooms is mainly concluded from indirect analyses, such as trends in physicochemical or biological parameters. I am wondering if there were any approximate statistical analyses that can enhance the links or relationships between cyanobacterial blooms and winds. If possible, lag correlation or multivariate analyses or models might help identify potential time-lagged relationships between wind speed and chlorophyll a concentration.

While part of the discussion is about the potential for wind-induced internal phosphorus release, the manuscript does not include direct measurements or data on internal phosphorus fluxes, e.g., across the sediment-water interface, to address this point. The relationship between water column total phosphorus concentrations (actually should be internal phosphorus release fluxes) and wind speed is not obvious. Alternatively, it may be because relevant data have not been presented or analyzed in a way that clearly illustrates this connection, so that the potential role of wind in promoting and sustaining harmful cyanobacterial biomass is not easily identifiable. As a small suggestion, it might be interesting, the authors might consider examining whether any strong wind events occurred during the study period and exploring possible corresponding changes in phosphorus concentrations or algal biomass, which could help clarify the potential influence of wind on bloom dynamics.

Author Response

This manuscript provides abundant observational data indicating that wind was a dominant factor affecting Lake Clear’s chemistry and biology, resulting in dramatic spatial heterogeneity of phytoplankton biomass and cyanotoxin in the eastern and southeastern Arms of Lake Clear. The manuscript is clearly structured, and the results are well discussed. This is a well-written manuscript. I have only a few minor comments as follows.

 

Line 165: In Figure 1A, the labels of site number is not clear, which should be larger. The Upper Arm, Oaks Arm, and Lower Arm can also be labeled in Figure 1A.

 

Figure 1 has been redrawn to improve legibility and address the issues raised (new line 171).

Line 540-555: This paragraph partially overlaps with earlier descriptions in other sections.

It is better to stay focused on discussion of the data or major findings instead of instruments or specific methods used.

Most of the paragraph has been deleted, leaving only sufficient text to identify the direction of the remainder of the Discussion (new line 560).

The influence of wind on in promoting and sustaining harmful cyanobacterial blooms is mainly concluded from indirect analyses, such as trends in physicochemical or biological parameters. I am wondering if there were any approximate statistical analyses that can enhance the links or relationships between cyanobacterial blooms and winds. If possible, lag correlation or multivariate analyses or models might help identify potential time-lagged relationships between wind speed and chlorophyll a concentration.

Yes, this is a great idea, and one we have reconsidered.  We had originally decided that perhaps our available study time (one month of temporally dense Wirewalker information) might be too sparse for a reasonable analysis. Additionally, we observed that the vertical distribution of chlorophyll (our primary metric for phytoplankton biomass carried by the Wirewalker) was quite unusual in Clear Lake.  We observed very high concentrations of chlorophyll close to the bottom of the water column in the lake (see Figure 8E).  This was unexpected and almost certainly, in part, a consequence of dead or dying cells accumulating at or near the bottom. This phenomenon undermined our attempt to try to correlate wind and chlorophyll because the feature is not a consequence of phytoplankton growth, wind speed or direction, but rather sinking-related accumulation (which at times clearly overwhelmed the signal from phytoplankton in the upper water column).

However, based on this Reviewer’s comments, we reconsidered using our dissolved oxygen information, and that has been very insightful. For the analysis, we examined the relationship between wind speed, direction and the dissolved oxygen concentrations below 4 m in the water column (averaged hourly), since our premise was that wind speed and/or direction affected the development and persistence of hypoxic waters in the lower water column.  This new analysis revealed a positive correlation between wind speed and dissolved oxygen concentration in the deep water of the lake, with a lag of 5 hours.  That is, dissolved oxygen concentrations in the lower water column increased approximately five hours after wind speed increased. That finding is consistent with the premise of wind stress acting to mix water vertically in the lake, reducing deep water anoxia. Also, a lag of 5 hours yielded in a negative correlation between wind-direction deviation from the long axis of the lake and dissolved oxygen. That is, a change in wind direction away from the long axis of the lake reduced wind stress on the lake surface which in turn resulted in more quiescent conditions in deeper waters of the lake and therefore increased hypoxic conditions. We have incorporated this information into the Discussion of our manuscript, and we thank this reviewer for bringing the analysis to our attention  (revised text beginning new line 758).

Additionally, we examined data recently published (Swann et al., 2024). That study examined seasonal peaks in total phosphorus over a three-year period that actually straddled the time when our study was conducted (their study was performed 2019-2021, ours in 2020). Although that study lacked the exceptionally high temporal resolution that our study has provided, seasonal phosphorus concentrations in Swann et al. peaked during the summer and early fall, and demonstrated that periods of general anoxia were characterized by temporally intermittent periods of high and low dissolved oxygen (as we have demonstrated on a diel time scale). Overall, these findings are highly consistent with our study which was performed over a much shorter time scale (one month). Together, the data demonstrate the pivotal role of wind in summertime anoxic development and upward nutrient flux. A short description of the findings of Swann et al. (2024) has also been incorporated into the Discussion of our manuscript (revised text beginning on line 699). We feel that these new pieces of information strengthen our conclusions.

While part of the discussion is about the potential for wind-induced internal phosphorus release, the manuscript does not include direct measurements or data on internal phosphorus fluxes, e.g., across the sediment-water interface, to address this point. The relationship between water column total phosphorus concentrations (actually should be internal phosphorus release fluxes) and wind speed is not obvious. Alternatively, it may be because relevant data have not been presented or analyzed in a way that clearly illustrates this connection, so that the potential role of wind in promoting and sustaining harmful cyanobacterial biomass is not easily identifiable. As a small suggestion, it might be interesting, the authors might consider examining whether any strong wind events occurred during the study period and exploring possible corresponding changes in phosphorus concentrations or algal biomass, which could help clarify the potential influence of wind on bloom dynamics.

Another excellent critical comment.  It is unfortunate that we were not able to make concomitant phosphorus measurements during the study. This has been clearly stated in the Discussion now, but these measurements were simply beyond the scope of our project. However, the relationship between anoxia in the hypolimnion of the lake and phosphorus release from the sediments is very well-established for Clear Lake and other lakes. We have modified the text in the Discussion to make this correlation clear (i.e. anoxic/hypoxic events and the release of phosphorus into the overlying water column) (revised text beginning new line 699). We also expanded the discussion to the work of De Palma-Dow et al. (2022) and Swann et al. (2024) which make the case strongly. Additionally, data from Swann et al. (and from our own study) were analyzed to examine the relationship between wind, anoxia and phosphorus release from the sediments, as noted in our response to the previous comment by this Reviewer.  We hope that this makes the case clearer to the reader.

Reviewer 2 Report

Comments and Suggestions for Authors

The article presents new valuable information on the spatial heterogeneity of phytoplankton biomass and several biologically relevant chemical parameters on time scales relevant to the initiation or prolongation of cyanoHABs in Clear Lake, California, USA in August 2020. The study was well designed and carefully implemented, using advanced methodologies, specific devices and the automated sensing approaches. Specifically, the unique insight into the processes leading to blooms was revealed by the water column profiler, which demonstrated rapid development (within a few hours) of suboxic conditions during periods of calm weather. The results of the study are clearly described, analyzed in sufficient details and illustrated by a reasonable number of figures, including the supplementary materials.

Meanwhile, I still have some general and specific comments that require the authors’ attention prior to further processing of the submission.

General comments

1. I believe that it is important to compare the results of this study (which was carried out 5 years ago!) with the current situation in Clear Lake in terms of lake’s chemistry, physics and biological parameters and their spatial (including vertical) and temporal dynamics, to be able to confirm the persistence of the discovered patterns. This would substantiate their application for the lake’s ecosystem modelling and management.
2. The article would benefit from a deeper comparative analysis of the data from Clear Lake and other similar waterbodies experiencing cyanoHABs globally.
3. In the context of this study, it would be relevant and beneficial to discuss, at least briefly, the available publications on the role of abiotic stability and chaotic plankton dynamics in HABs formation.

Specific comments

Fig. 1, Line 167: The “regional location” of Clear Lake is shown in Fig. 1 D (not in “B”; please correct).

Fig. 1: It would be good to show and name clearly the Upper, Oaks and Lower Arms of the lake – either in the map in “A” or in the additional scheme within Fig. 1.

L 242, 244, 246, 248: “spp.” should not be italicized.

L 246: Delete the dot after the closing bracket.

Fig. 2: Poor indication of panels A-F; please revise.

L 253: Correct a misprint (“Lyngbya”).

L 306: “Materials and Methods” (not vice versa).

L 354: “observations observed…” – please revise.

Fig. 7, L 410: Please check the A and B panels and their explanations in the legend, and correct accordingly.

L 702, 707: Wrong format of references.

 

Author Response

The article presents new valuable information on the spatial heterogeneity of phytoplankton biomass and several biologically relevant chemical parameters on time scales relevant to the initiation or prolongation of cyanoHABs in Clear Lake, California, USA in August 2020. The study was well designed and carefully implemented, using advanced methodologies, specific devices and the automated sensing approaches. Specifically, the unique insight into the processes leading to blooms was revealed by the water column profiler, which demonstrated rapid development (within a few hours) of suboxic conditions during periods of calm weather. The results of the study are clearly described, analyzed in sufficient details and illustrated by a reasonable number of figures, including the supplementary materials.

Meanwhile, I still have some general and specific comments that require the authors’ attention prior to further processing of the submission.

General comments

1. I believe that it is important to compare the results of this study (which was carried out 5 years ago!) with the current situation in Clear Lake in terms of lake’s chemistry, physics and biological parameters and their spatial (including vertical) and temporal dynamics, to be able to confirm the persistence of the discovered patterns. This would substantiate their application for the lake’s ecosystem modelling and management.

To address this request we have added text to the Discussion from additional (recent) studies regarding wind, hypoxia/anoxia and internal phosphorus loading in Clear Lake. Time and spatial scales in those studies (Swann et al. 2024, De Palma et al. 2022, Smith et al. 2023) extend observations in Clear Lake into recent years, and they confirm our conclusions on the interaction of these parameters. This information also addressed two of the main questions raised by Reviewer #1 (please see responses to two major comments from Reviewer #1).  In short, historical records confirm that the behavior of the lake (in terms of chemistry/physics/biology) has remained consistent over decades, and therefore the processes affecting blooms in Clear Lake have been relatively unchanged for decades (revised text beginning on new line 699).

1. The article would benefit from a deeper comparative analysis of the data from Clear Lake and other similar waterbodies experiencing cyanoHABs globally.

An historical analysis of information on Clear Lake was specifically the goal of a previous publication from our group that summarized nearly 70 years of data on the lake (see Smith et. al. 2023 cited in this manuscript; ref #8).  We have tried to avoid overlapping that analysis, but we agree that some perspective here is in order.  We have brought pertinent summary details into the Discussion, as noted for the previous comment from the Reviewer (revised text beginning new line 686). A brief mentions of three other hypereutrophic waterbodies with similar harmful algal bloom problems has also been added into the Discussion, as requested (two regional lakes and one international lake).

1. In the context of this study, it would be relevant and beneficial to discuss, at least briefly, the available publications on the role of abiotic stability and chaotic plankton dynamics in HABs formation.

This is a complex and very wide-ranging request, as the information on environmental stability and plankton dynamics is extremely rich, and often situation- and waterbody-specific (as detailed in Section 4.3). A number of publications cited in the manuscript already address the interplay between abiotic stability (or instability) and plankton dynamics (Small 1963; Rueda et al. 2003; Mantzouki et al. 2018; Xue et al. 2022; Mesman et al. 2022; Wu et al. 2010; Visser et al. 2016). So we’re at a bit of a loss how to expand this topic without significantly increasing the size of the Discussion. However, we have tried to address this request, briefly, in the revised text of the Discussion.

Specific comments

Fig. 1, Line 167: The “regional location” of Clear Lake is shown in Fig. 1 D (not in “B”; please correct).

The figure has been corrected (new line 171).

Fig. 1: It would be good to show and name clearly the Upper, Oaks and Lower Arms of the lake – either in the map in “A” or in the additional scheme within Fig. 1.

Figure 1 has been improved accordingly (new line 171).

L 242, 244, 246, 248: “spp.” should not be italicized.

Corrected (new lines 251-257).

L 246: Delete the dot after the closing bracket.

Corrected (new line 255).

Fig. 2: Poor indication of panels A-F; please revise.

Figure 2 reproduced poorly, not sure why, they appear fine in the submitted figure.  Panel labels have been redone (new line 261).

L 253: Correct a misprint (“Lyngbya”).

Corrected (new line 263).

L 306: “Materials and Methods” (not vice versa).

Corrected (new line 317).

L 354: “observations observed…” – please revise.

Corrected (new line 366).

Fig. 7, L 410: Please check the A and B panels and their explanations in the legend, and correct accordingly.

Corrected (new line 423).

L 702, 707: Wrong format of references.

Reference links corrected and new references added (multiple lines).