the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Feb 2024
Assessment of nutrient cycling in an intensive mariculture system
Abstract. Rapid expansion of mariculture during past decades has raised substantial concerns about impacts on the coastal environment, notably eutrophication. This study focuses on one of the world’s highest density mariculture sites, Sansha Bay, Fujian Province, China, featuring integrated multi-trophic aquaculture practices involving croaker, kelp and oyster, based on examination of nutrient distributions and releases. A two-endmembers-mixing model showed significant addition of dissolved inorganic nitrogen (DIN; 6.9 ± 4.1 μmol L-1) and phosphorus (DIP; 0.45 ± 0.29 μmol L-1) associated with mariculture activities in spring 2020. A mass balance model estimated an annual release of N and P from cage fish farming systems fed with mixed trash fish feed and formulated feed of (2.42 ± 0.15)×104 tons and (5.33 ± 0.37)×103 tons, respectively. Of the total feed input, 52.8 ± 4.7 % of DIN and 33.0 ± 3.7 % of DIP were released into seawater, values much higher than the riverine input and exchange with offshore coastal waters. A co-culture strategy involving kelp and oyster production in 2020 removed (1.08 ± 0.01)×103 tons of N and (1.56 ± 0.08) ×102 tons of P, respectively. Therefore, adjusting feed strategies and improving feed conversion rates could alleviate eutrophication caused by mariculture expansion in this ecosystem.
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RC1: 'Comment on egusphere-2023-3155', Anonymous Referee #1, 13 Mar 2024
This manuscript studies the spatial variations in dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in Sansha Bay, an intensive mariculture system, and investigates the contribution of mariculture activities to nutrient variations. A cruise survey was conducted over the Bay in May 2020, sampling and analyzing water temperature, salinity, chlorophyll a, inorganic nutrient concentrations (N, P, and Si), and total alkalinity. Some samples of the cultivated fish, kelp, and oysters from the Bay were also collected to measure their N and P content. A two-endmember mixing model and a mass balance model were subsequently applied to 1) explain the observed spatial variations in DIN and DIP within the Bay; and 2) estimate how much the fish feed from the cage farming systems contribute to the nutrient budget as well as how much the kelp and oyster production remove the nutrient from the system.
I think this manuscript addresses an important topic and falls within the scope of the journal. However, the manuscript is not well structured and has major shortcomings in the mass balance model estimation and interpretation. There are also some language issues, partially making it hard to follow the story. Please see my detailed comments below.
Major comments
1. An important assumption adopted by this manuscript is that the N and P in feed input (I_N,P) are equivalent to fish production (P_f) multiplying the feed conversion ratio (FCR) (Eq. 6). The FCR is not defined in the manuscript, so I have to speculate it based on other literature. The ratio is commonly defined as the weight of feed intake divided by the weight gained by the animal. It follows that the P_f in Eq. 6 should be the weight increase of the cultivated fish within the interested time period of the budget calculation (the time period is not specified in the manuscript). Therefore, directly assigning the total fish production in 2020 to P_f in Equation 6 will overestimate the actual feed input.
2. Descriptions of the mass balance model are confusing and missing important information to understand the estimated N and P budgets.
- What’s the relationship between feed loss ratio LR, feed input (I), and the total waste discharge (L)?
- What’s the relationship between I_f (in Equation 7) and I (Equation 5)? Some statements in the manuscript suggest I = I_f * number of individual fish, while some suggest that I is the feed input.
- My common 1 also applies to the estimation of G_f in Equation 7.
- The same parameter C_N,P is adopted for the N and P content of feed as well as the fish, which is misleading.
- Missing important information to evaluate the mass balance model. I don’t have knowledge about the aquaculture species here, including fish (L. crocea), oysters (C. angulate), and kelp (L. japonica). What are their common growing seasons and practices? Do their N and P content as well as assimilation efficiency change at different life stages? Are the ratios of soft tissue and shell of oysters considered constant? Are these variations in parameters accounted for in the budget calculation? If not, what are the introduced uncertainties in the budget calculations?
3. Regarding the N and P budget estimation, it is unclear what source/sink terms are measured, what terms are hypothesized or derived (based on what assumptions), how the measured DIN and DIP concentrations from the cruise survey were related to the calculation, and what contribute to the presented uncertainties in the budget. The presented budgets in Figure 8 are thus very confusing.
4. It’s unclear how the nutrient removal by oyster production is calculated. The C and N content of the dry tissue and shell of oysters were measured but the data was not presented in the manuscript. In the calculation of nutrient removal, the production of oyster soft tissue and shell were applied but we can’t tell whether the dry weight or wet weight of the production was adopted. If it’s the wet weight, the calculated removal will be a substantial overestimation.
5. The manuscript also compares different nutrient fluxes, including nutrient input from fish farming, nutrient removal by kelp and oyster aquaculture, nutrient discharge from rivers, and nutrient exchange between the bay and the shelf. It’s unclear to me what time scale this comparison focuses on. It seems to be over a year. If so, it’s problematic to use the observations of nutrient concentration in May 2020 to represent the entire year when calculating the riverine nutrient input and the nutrient exchange flux. Plus, the flow rates also change in different months.
6. The presentation quality of the manuscript needs substantial improvement. Please check my specific comments below.
Specific comments
L13-14. No direct evidence to attribute the addition of DIN and DIP to “mariculture activities in Spring 2020”. Other sources are not excluded.
L16-18. What’s the time scale for this comparison? Please make it clearer.
L19-21. The logic is weak here. The preceding sentence describes how promising kelp and oyster production can remove nutrients, while here advocates adjusting feed strategies and feed conversion rates to mitigate eutrophication. Can’t see how these two sentences can be connected with a “therefore”.
L40. Please explain how feed conversion rate (FCR) is defined, otherwise, it’s difficult to interpret the listed range.
L41-42. These percentages are higher than those listed in the abstract. Does it suggest that Sansha Bay has a higher feed conversion rate?
L47. Suggest replacing “Thus” with “For example”
L77. What does “semi-quantitatively” mean?
L95. What is tidal prism referred to?
L105. What does seawater half-exchange time mean?
L115. The unit m3 should be m2.
L116-117. Are these fish of different wet weights co-cultured? Are densities part of the estimation for the budget?
L127. oyster production didn’t show a linear increase in Figure 2b.
L129. DIN didn’t show a steady increase. It started to decline in 2006.
L113-132. Should provide more information about the common growing seasons and practices of cultivated fish, kelp, and oysters in Sansha Bay. The starting and lasting time of aquaculture will affect the nutrient dynamics. Such knowledge is needed to help readers understand the nutrient budget analyses.
L144-145. This statement suggests that DIN in April 2006 was higher than that in May 2006. Any supporting evidence?
L148. What does “measured continuously” mean? Continuously in time?
L165-166. Is it necessary to provide the detail of where the reference material is obtained, in terms of reproducing these analyses?
L169-170. These are very important parameters. More information regarding the biological samples is needed, e.g., weights of these biological samples (considering that individual organism’s N and P contents might vary depending on the weight), how many individual fish and oysters were sampled, how much kelp and fish feed were collected for the measurement.
L182. Average over time or water depths?
L188. percent -> percentage
L215-216. Does this apply to Sansha Bay only or generally to all mariculture? Please make it clearer and provide a reference.
L261. Insert “at different depths” after “nutrient concentrations”
L432, L436, L467. “consumption” is a bit misleading. Suggest replacing it with “demand”.
L444. Delete “however”.
There are many long sentences, making it difficult to follow the story. Each of them can be broken into two or even more shorter sentences to improve the clarity. Below I provide line numbers for some of them:
L9-12; L54-57; L60-63; L66-68; L134-136; L174-178; L298-302; L451-454.
Examples of poor or difficult language:
L37-38. incomplete sentence
L38-41. poor logic
L47-51. “therefore” and “thus” in a row
L63-65. poor logic
L68. Is this the goal of this study or that of Song et al. (2023)?
L75-77. Redundant sentence. Can delete “proposed to” and “science-based”.
L206-208. Confusing.
L439-431. Poor logic
L466. The preceding sentence only describes the removal efficiency of kelp but not oysters. It can’t derive the conclusion following the “therefore”.
Citation: https://doi.org/10.5194/egusphere-2023-3155-RC1 -
RC2: 'Comment on egusphere-2023-3155', Anonymous Referee #2, 08 Apr 2024
This manuscript reports spring nutrient variabilities determined in Sansha Bay, which is heavily influenced by mariculture activities. Then they used a two end-member mixing model and a mass balance model (LOICZ) to construct a nutrient budget for the bay, which is a key selling point of the paper.
Only one spring data is not rich enough. And the logic of writing is weak. It lacks detail and explanation of many statements and calculations, and it is full of speculations and weak discussion. Thus, it is quite difficult for the reader to follow the calculation. Most structures, data analysis, and discussion appear too similar to Han et al., 2021 (JGR) but without citation. Quite limited new knowledge is given from this manuscript. Furthermore, it also needs some major editing of grammar, sentence structures, and too many textual detail flaws. Biogeosciences is a high-quality journal, but the manuscript is far from the requirements of this journal (including the dataset, innovation, and writing (calculation)). Some improvements can be made to increase the readability and quality of the paper. My detailed suggestions are below.
Major comments:
1 The Introduction is not well structured, and the logic is confused. The author may rewrite the first paragraph. The ideas mentioned in the Introduction should be relevant to the highlighted points in your following sections. For example, “ Norwegian salmon farming industry....”, ”upwelling”, ”58%-62% carbon”, are they mentioned/important in your Discussion?
2 Alkalinity-salinity relationship is a useful tool to analyze (two) endmember mixing processes in general coastal areas. For intensive mariculture, is it possible that the base value of alkalinity itself is too large and unable to reflect the variations of three end-member mixing? Can the authors analyze the properties of end-member mixing via T-S diagram to see if there are other endmembers, which may be due to longer residence time and/or locations (for example, the authors defined mariculture zone/stations around Sandu Island (S1, S2, S6, S26, etc. In Figure 6). How is the uncertainty derived?
3 The interpretation and the estimation for the “mass balance of N and P in fish farming systems” in section 2.4 is not clear. It is difficult to follow the calculation processes. I can not see the values of the parameters according to Table S1. In addition, is the “total waste discharge (L)” the same as “feed loss (L)”? The author adopted some parameters from reference, it would be better if the authors could assess if is it feasible for this study first.
4 Section 2.5 kelp and oyster removal. The authors considered kelp and oyster and emphasized that the bay is an IMTA system, are there any other key/primary mariculture species/production in the bay? How to assess the individual species production related to the growth period and the sampling period? On the other hand, the oyster zone is located in Yantian Harbor. Does the oyster production/removal will influence the nutrient flux/budget in the bay/stations? Can the much more detailed functional areas of various aquaculture (fish, kelp, oyster, etc.) be shown on the map?
5 Section 3.3, it is quite difficult to follow the nutrient release estimation. How is the nutrient flux derived? What the data 21±1% and 10±1% come from? Is there any single cage for trash fish feed and formulated feed fish farming in the bay? How did they obtain the numbers in the manuscript?
Minor comments:
Line 100, it is suggested to quote more classical hydrology and biogeochemical literature.
Line 113, 69%, what is the data source, reference?
Line 159, It is a nutrient manuscript. The measurement methods of nutrients are important but the authors did not cite any published paper.
Line 226, “Nitrogen” should be “N”
Line 26, “Fao” should be capitalized “FAO”
Line 253, Chl a in the manuscript is too low with regards to the mariculture bay. Is there any in situ field measured Chl a data? Compare to historic references? The author displays Chl a in Figure 5e but does not introduce clearly, the value for Chl a is as high as 0.8 μg L-1 ?
Line 315, Jia et al., 2003 is omitted in the References.
Line 395, how did the authors obtain 32.8% and 34.8%?
Figures 6,7, the color should be constant.
Figure 7, most of the stations in the “mariculture zone” are actually of salinity >25, meanwhile, the relationship between DIN and DIP is ~23.38. The DIN/DIP ratio for the mariculture zone seems to be dominated by only two dots in Fig. 7a.
Map 1, have you compared the satellite map with Google Earth map and/or other images? The mariculture zone is probably underestimated.
Table 1. Four stations (LJ21, LJ22, ND41, and ND42) in offshore coastal waters should be introduced in the MS, and be shown on the map. What is the formulation for u? It seems the uncertainty u is too large for Vex, FDIN, and FDIP? Is it feasible to say outflow for DIN and DIP?
Citation: https://doi.org/10.5194/egusphere-2023-3155-RC2
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