Author's response

egusphere-2025-656

Dear Reviewer,

Thank you very much for your valuable comments. The point-by-point responses to your comments are listed below.

RC2

The manuscript explores differences in water quality and metabolism dynamics in the Lower Bode between an extreme low-flow event and “normal” low-flow events. Metabolism is calculated using high-frequency in-situ data using the diel oxygen model. Calculated total GPP and inferred phytoplankton GPP rates are used to get benthic GPP. Nitrate uptake rates are quantified using a mass balance approach. The paper highlights the role of benthic algae in elevating GPP and gross nitrate uptake during extreme low-flow events. The paper summarizes its findings in a conceptual model how future, probably more common, extreme low-flow conditions will affect stream health. These results are of interest to freshwater ecologists and limnologists interested in future ecosystem changes.

Main points:

• Metabolism model: It would be great to show the diel model you have been using for metabolism calculations here. Also, you discuss later how temperature affects reaction rates using the Arrhenius equation, and you highlight a biological reaction rate increase of 1.7 – 3.3 % using a back-of-the-envelope approach. I wonder how your results would like if you’d account for temperature kinetics in eq. 1; e.g., GPP_P = G_P * C_PHY * ROC * z * Theta^(T2-T1). This would result in lower benthic GPP and maybe more pronounced differences between low-flow and extreme low-flow years.

Response: We appreciate the reviewer's attention to the temperature effects on GPP by phytoplankton in our metabolism model. To clarify, our formulation already explicitly accounts for temperature dependence through the net growth rate term G_P for the phytoplankton, which is dynamically calculated as:

G_P = k_G - k_R – k_D

where for the phytoplankton growth rate k_G incorporates:

• k_G = k_Gmax × X_T × X_L × X_N
• k_Gmax: Maximum growth rate at 20°C, day^-1 (0.5-4.0)

• X_T: Temperature multiplier. We used Additional Temperature Function for phytoplankton growth calculation as:

X_T = exp[-κ₁(T_opt-T)²]   if T ≤ T_opt

      exp[-κ₂(T-T_opt-T)²]   if T > T_opt

• T_opt: optimum temperature for growth, °C(10 – 27)
• κ₁, κ₂: temperature coefficients below and above optimum, 1/°C² (0.005 – 0.04)

• X_L, X_N: Light and nutrient limitation (range: 0-1)

In this case, the T_opt, κ₁, κ₂ are assigned with the values of 13 degree Celsius, 0.02 C^-2 and 0.02 C^-2 respectively. Key points regarding temperature effects on phytoplankton growth are as follows:

1. The temperature response is fundamentally nonlinear, with distinct coefficients below/above T_opt
2. All the above parameters were calibrated against observed chlorophyll-a dynamics (Huang et al., 2022), successfully reproducing seasonal patterns at STF stations and diurnal variability in phytoplankton activity.
3. This formulation provides more biological realism than a simple Arrhenius (Θ) correction because: It captures growth inhibition at supraoptimal temperatures and reflects species-specific thermal optima.

The GPP by benthic algae derives from: GPP_benthic = GPP_total - GPP_phytoplankton. In this case, the temperature effect with temperature effects implicitly incorporated.

We will supplement the details on how the G_P is calculated and the temperature effect on it in the revised version and supplementary materials.

Minor points:

• L45: I don’t think “increased solar radiation” can count as an environmental condition here, wouldn’t the causal connection be reduced cloud cover (as environmental condition) causing less reflection of incoming solar radiation?

Response: Thank you for the comment. In the terminology of water quality modeling, for example in WASP, solar radiation is seen as an environmental condition. Therefore, we followed the common protocol. The solar radiation we mean here is the net solar energy reaching the water surface after accounting for atmospheric absorption and cloud effects. While we agree that reduced cloud cover is indeed the primary meteorological driver of increased solar radiation, from a water quality modeling perspective, it is the resultant radiation flux at the water surface that serves as the direct environmental condition affecting aquatic processes. To avoid confusion, we will change “increased solar radiation” to “near-surface solar radiation”.

• Fig2: So, the red lines in (c) represent then the low-flow conditions right, which are compared to the red lines during 2018 which were the extreme low-flow event? Could you please maybe color them differently and add a legend for clarification? Same for Figure 3

Response: Ok. We will revise the figures as the reviewer suggested. 

• L160: “Key metrics such as […] were analysed using MATLAB.”

Response: Thank you for the comment. We will revise the sentence accordingly.

• L177: Please state C_PHY for consistency here as g C/m3 (which of course wouldn’t affect any results)

Response: Thank you for the comment. Yes, we will state C_PHY for consistency here as g C/m³ in the revised version.

• Eq 2: Is travel time dynamic or constant in your model?

Response: Yes, the travel time in our model is dynamic. It is calculated with the hydrodynamic module in WASP.

• Eq 3: Is I then the input loadings at t, hence all of US + TR?

Response: Yes, it is.

• L210: It’s a bit confusing that delta isn’t explained here but only visually in Fig. 4. I first thought that you mean the difference between two measurements here. Is the O2 deficit between 100% saturation and measured saturation conc.? What is Chl-a accumulation?

Response: We thank the reviewer for highlighting this ambiguity. We recognize these terms require clearer definition. Daily DO delta (DOΔ) is calculated as the difference between the daily DO maximum and minimum. The DO deficit is defined as the difference between 100% DO saturation concentration and measured DO concentration. The Chl-a accumulation is defined as the difference between the downstream Chl-a concentration and the upstream concentration, reflecting net algal growth over the study reach. These descriptions on certain terms will be supplemented to section 2.3 and the caption of Table 1.

L225: What do you mean with “thermal capacity”, capacity related to biota like in Lake 2003 or the specific heat capacity of water (which wouldn’t be affected), or heat storage in reduced volumes?

Response: We thank the reviewer for this important clarification. Here, "thermal capacity" refers specifically to the heat storage potential of the reduced water volume, not the specific heat capacity of water (which remains constant) nor biotic tolerance thresholds. We recognize this could be misinterpreted and will revise the text to "thermal buffering capacity". In addition, the reference by Lake (2003) will be removed to avoid conceptual conflation.

• Table 1: Are the values given for LF and ExLF the averages across the individual seasons? Hence, is ExLF of 2018 compared to the average behavior of all LF’s before? If indeed these are averages, maybe also give standard deviations or quantiles to make full use of your high-frequency data. It’s a bit contradicting that you praise high-frequency data for metabolism calculations but then show only a value of each season/event plus the statistical p-value. Also, I think you should expand the caption of the table to explain all variables again as otherwise the reader has to go back to the text every time.

Response: Thank you for your valuable suggestions. Yes, the values in the table are the medians of 2018 seasons for ExLF and medians of the 2014-2017 seasons. We will mention this explicitly in the table caption. The reviewer mentioned standard deviations or quantiles of the datasets. This is sensible. We will include them in Table 1 in the revised version. We will expand the captions of the table to explain all variables in it to increase readability.

• L312: “increased”, do you mean “suggested that increased phytoplankton growth rates led to higher […]”? But isn’t that related as higher growth rates cause higher conc.?

Response: Thank you for the reviewer to mention this. Yes, the sentence should be “suggested that increased phytoplankton growth rates led to higher…”

• L347: Wouldn’t areal ER always indicate oxygen consumption and should be always negative?

Response: Yes, you are absolutely correct that ER indicates oxygen consumption and should be always negative. We will explicitly clarify this in the Methods section about our sign convention: "Following standard aquatic metabolism conventions, positive GPP values indicate oxygen production while negative ER values represent oxygen consumption."

• L350: But can your analysis for sure determine if conditions were “less heterotrophic” as the p-value is not significant?

Response: Thank you for the comment and observation. Yes, indeed the NEP values between ExLF and LF were not significantly different according to the p-value. The saying of “less heterotrophic” came from the comparison of the median values of the NEP. We will modify the text as follows: "Though not significant (p = 0.45), the observed median NEP values suggested a potential trend toward reduced heterotrophy during ExLF conditions (Table 1)."

• L353 and onwards: I am missing a discussion of atmospheric exchange rates here in affecting overall NEP rates. Are they negligible and indeed all O2 changes can be attributed to GPP and ER?

Response: We appreciate the reviewer’s important question regarding the role of atmospheric exchange in our NEP calculations. We would like to clarify and expand on how reaeration was incorporated into our analysis and its potential impacts during ExLF.

In our methodology (Section 2.4), we explicitly accounted for atmospheric exchange by estimating the reaeration rate using the O’Connor-Dobbins formula, which is particularly suitable for slow-flowing streams (Chapra, 2008). This approach incorporates key hydraulic variables including water depth and flow velocity, obtained from both gauging station measurements and hydrodynamic modeling results (Huang et al., 2022). Additionally, dissolved oxygen saturation levels were carefully determined based on water temperature, salinity, and barometric pressure measurements following standard methods (APHA, 1998).

During ExLF conditions, we recognize that atmospheric exchange becomes particularly influential on NEP calculations due to two primary factors: First, the reduced flow velocity decreases turbulence, leading to lower reaeration coefficients (k₂). This results in slower oxygen exchange with the atmosphere, making dissolved oxygen dynamics more sensitive to biological processes (GPP and ER). Second, the diminished gas exchange can amplify observed DO swings - potentially exaggerating daytime peaks from GPP and nighttime declines from ER.

While our method does account for changes in velocity and depth during ExLF when estimating reaeration rates, we acknowledge that NEP calculations become more sensitive to reaeration estimates under these extreme conditions. Any inaccuracies in reaeration estimation could introduce bias in our NEP results, potentially explaining why some of our NEP comparisons between ExLF and LF conditions showed non-significant differences. For instance, an overestimated reaeration rate could make the system appear more autotrophic, while an underestimation could bias results toward more heterotrophy. The above-mentioned discussion will be supplemented in the revised manuscript.

• L356: Phytoplankton growth and GPP_P are inherently linked in eq. 1, so isn’t this expected? Would growth rates benefit only because biomass is elevated due to lower flushing rates?

Response: We appreciate the reviewer's observation regarding the relationship between phytoplankton growth and GPP_P in our model. The reviewer is correct that phytoplankton growth rate (units of d⁻¹) and GPP_P are inherently linked through Eq. 1. However, we would like to clarify that while the growth rate itself was not directly enhanced by lower flushing rates, the longer residence time during extreme low-flow (ExLF) conditions did lead to greater phytoplankton biomass accumulation along the study reach. This increased biomass (C_PHY in Eq. 1) subsequently resulted in higher GPP_P values during ExLF periods. To improve accuracy, we will revise the term "phytoplankton growth" to "phytoplankton biomass accumulation" throughout the manuscript to better reflect this distinction.

• L364: Could you add GPP_B to Table 1 please.

Response: Yes, Ok. We will include it in Table 1 in the revised version.

• L433: Would solar exposure be really higher or just longer?

Response: We thank the reviewer for raising this important question regarding solar exposure during ExLF. In this study, we analyzed long-term observational data of hourly shortwave global radiation (J/cm²) obtained from the German Weather Service. These radiation measurements were converted to Langley per day (Ly/d) to enable standardized comparison between LF and ExFL, as shown in Table 1.

Our results demonstrate that the total solar energy, represented by the hourly sum of shortwave global radiation, was significantly higher during ExLF conditions compared to LF periods. This metric reflects the integrated product of both solar irradiance intensity and duration. Here, solar exposure means total solar energy received over time, which could be represented by this metric. So, we can draw conclusions on the solar exposure are really higher. However, we acknowledge that our current analysis do not determine whether this increased solar exposure resulted from higher irradiance intensity, longer sunshine duration, or some combination of both factors. But we could incorporate the duration comparison as the German Weather Service also provide this parameter.

To improve the clarity in the manuscript, we will make the following revisions: We will explicitly specify in Table 1 that the solar radiation values represent cumulative solar energy, clearly indicate the data source (DWD hourly radiation measurements), and include both original (J/cm²) and converted (Ly/d) units and add the statistics of sunshine duration. Additionally, we will expand the Methods section to provide more detail about the radiation data processing and the distinction between total solar energy and sunshine duration.

• L447: Could nutrient limitation or photosensitivity also play a role here that could be discussed?

Response: We appreciate the reviewer's suggestion regarding additional factors that could influence our results. In response to the insightful comment:

Regarding nutrient limitation, our WASP model simulations do indeed provide information about nutrient constraints during both LF and ExLF periods. We will incorporate this analysis into the revised discussion to examine how nutrient availability may have interacted with the observed temperature and light effects.

Concerning photosensitivity, while our current modeling framework includes light limitation through the Half-Saturation equation, we acknowledge that alternative formulations (such as Smith or Steele equations) could potentially yield different results. However, as we did not conduct sensitivity analyses comparing these different light limitation approaches in this study, we are unable to rigorously evaluate the potential role of photosensitivity effects at this time.

References

Huang, J., Borchardt, D., and Rode, M.: How do inorganic nitrogen processing pathways change quantitatively at daily, seasonal, and multiannual scales in a large agricultural stream?, Hydrol. Earth Syst. Sci., 26, 5817–5833, https://doi.org/10.5194/hess-26-5817-2022, 2022.