Intratidal Variability of Water Quality in the Damariscotta River, Maine

Round 1

Reviewer 1 Report

This manuscript presents the water quality dynamics of the Damariscotta River Estuary in midcoast Maine. Using a comprehensive dataset, the author aims to examine the temporal variabilities of turbidity, salinity, pH and oxygen conetration as a feature of tidal dynamics in the estuary. The main message is importatnt and can be concluded as that the interaction between tides and topography results in different hydrodynamical features in the lower and upper estuary which in turn significantly affect the dynamics of sediments and cholophyll concentration. Overall, I like this study very much. I think it is a well written manuscript and the analyses are generally appropriate. So I would like to recommend the paper for publication in the present form.
 

Author Response

Reviewer 1:

This manuscript presents the water quality dynamics of the Damariscotta River Estuary in midcoast Maine. Using a comprehensive dataset, the author aims to examine the temporal variabilities of turbidity, salinity, pH and oxygen conetration as a feature of tidal dynamics in the estuary. The main message is importatnt and can be concluded as that the interaction between tides and topography results in different hydrodynamical features in the lower and upper estuary which in turn significantly affect the dynamics of sediments and cholophyll concentration. Overall, I like this study very much. I think it is a well written manuscript and the analyses are generally appropriate. So I would like to recommend the paper for publication in the present form.

Dear Reviewer 1,

We thank you for your enthusiastic assessment of our submission. We look forward to our research becoming available to the scientific community through publication in Water.

Sincerely,

Brandon Lieberthal

Reviewer 2 Report

The paper presents analyses the behaviour of the tides along the Damariscotta river in order to characterise the dynamics of the estuary and demonstrate that variations of the tides considerably impacts quality of the water in the estuary. Hence, the paper demonstrates that the irregularities of the estuary result in asymmetric dynamics.

The paper is well written and draws important conclusions to understand the relationship between tide dynamics and water quality. Therefore, once the comments included in the attached document are addressed, the manuscript can be accepted for its publication.

Comments for author File: Comments.pdf

Author Response

Reviewer 2:

The paper presents analyses the behaviour of the tides along the Damariscotta river in order to characterise the dynamics of the estuary and demonstrate that variations of the tides considerably impacts quality of the water in the estuary. Hence, the paper demonstrates that the irregularities of the estuary result in asymmetric dynamics.

The paper is well written and draws important conclusions to understand the relationship between tide dynamics and water quality. Therefore, once the comments included in the attached document are addressed, the manuscript can be accepted for its publication.

Dear Reviewer 2,

We thank you for your assessment of our submission, and we appreciate your general comments. We have addressed each of your comments below, and we made revisions accordingly.

General comments

The impact of climate change in water quality is mentioned several times in the Introduction of the manuscript. However, an important effect of climate change that has recently been discovered and is directly linked with the ocean resources is ignored in this paper: wave energy resource variations over the last decades. Wave energy has significantly increased in some areas of the North Atlantic Ocean and, which is particularly interesting, the extreme events are more powerful and more frequent. See the following references for further information:

[A] Penalba, M., Ulazia, A., Ibarra-berastegui, G., Ringwood, J., & Seenz, J. (2018). Wave energy resource variation off the west coast of Ireland and its impact on realistic wave energy converters' power absorption. Applied Energy, 224 (2018), 205-219. https://doi.org/10.1016/i.apenergy.2018.04.121 

[B] Reguero, B. G., Losada, I. J., & Mendez, F. J. (2019). A recent increase in global wave power as a consequence of oceanic warming. Nature Communications, 10 (205), 1-14. https://doi.org/10.1038/s41467-018-08066-0

This is definitely an interesting avenue of research and worth further exploration, in particular for studies in coastal Maine. The Damariscotta estuary is a relatively short (30 km) and narrow (1 km at the mouth) estuarine system that does not provide significant fetch for waves to develop, especially those that are large enough to be utilized by Wave Energy Conversion (WEC) devices to harness energy. This makes the sheltered estuary environment of the Damariscotta (and other estuaries) excellent places to house floating aquaculture platforms. On the other hand, the state of Maine and the University of Maine are investing heavily in offshore wind turbine development in the Gulf of Maine, in an effort to reduce Maine’s dependence on imported fossil fuels. However, this is outside the scope of the paper.

We cited Reguero and added another background sentence to the beginning of the Introduction: “The Gulf of Maine waters are warming faster than 99% of the world’s ocean [4], which can impact water quality and wave climates [5].”

Apart from the attenuation between Clark Cove and Briar Cove shown in Figure 2 and explained in the text, there is a significant increase between South Bristol and Fort Island for which no explanation is provided. Can the authors explain this effect in the revised version? In addition, I assume the linear representation of the lines in Figure 2 is not realistic. How does such an increase in amplitude ratio affect the surrounding areas of Fort Island (the ones between South Bristol and Fort Island, and Fort Island and Clark Cove)?

The reviewer makes a good point. The amplification of overtides between South Bristol and Fort Island is explained by the two 100 m constrictions around Fort Island. We added the following paragraph to the discussion section as clarification:

“In general, overtides are amplified where the river is narrowest [36]. The high amplification of D4  between South Bristol and Fort Island can be explained by the constrictions around the island, which may enhance overtides via wave reflection [37] and bed stress [38].”

It is true that the linear representation is not realistic. We don’t have finer scale data in the region around Fort Island, but we speculate that the overtide amplification would be a sudden increase, not a linear increase, as the tide passes around Fort Island. Therefore, we replaced Figure 2 with a scatter plot.

In the wet season, do authors identify differences between rain and snow precipitations? Does this different affect on the dynamics?

We thank the reviewer for the comment. We added the following lines to the Introduction.

“Monthly precipitation ranges from ~ 80 mm per month in the dry season to ~ 120 mm per month in the wet season. Snowfall typically begins in late November and ends in early April, with the highest rates of ~ 350 mm per month in January and February.”

In general, we expect discharge from the Damariscotta Lake Dam to affect the dynamics of the tides much more than precipitation. Certain storm events do have a significant effect on water levels, up to an additional 50 cm, but they do not affect the average tidal amplitudes over the course of the season.

In addition, Table 2 shows that there is no consistency between dry and wet seasons for the different variables analysed (turbidity, temperature or even salinity). For example, monthly turbidity in Clark Cove is significantly greater in dry seasons, while the opposite happens in Hog Island. Can the authors clarify this? Is this to be expected?

The mean turbidity is higher in dry season than wet season in both Clark Cove and Hog Island, as we expect. The amplitudes of turbidity are somewhat inconsistent because of the patterns of which sediment is flushed into and out of the estuary throughout a typical tidal cycle. Particularly, higher velocity flood phases results in greater suspended sediment in Hog Island because of its high quantity of stored sediment and reflection off the dam. We do not see this effect in Clark Cove because much of the riverbed is exposed bedrock, and sediment freely flows through the region in both flood and ebb phase. We added a clarifying comment in the Discussion section:

“Because of the high quantity of stored sediment and reflection off the Damariscotta Lake Dam, monthly and semi-diurnal amplitudes of turbidity were higher in the wet season than the dry season in the northern reach.”

Can climate change effects modify these estuarine dynamics? In that case, do you think these potential variations can be estimated/predicted?

We have run analytical models on the Damariscotta river that estimate that by the year 2100, tidal amplitudes may be reduced by 0.5% and tidal currents may be reduced by 7 cm/s. This research however, has not yet been published and is not ready to be cited. The last paragraph of the Discussion section speculates on the effects of climate change on estuarine dynamics, but a more quantitative analysis is beyond the scope of this paper.

Thank you for your consideration.

Sincerely,

Brandon Lieberthal