2:45 PM - 3:00 PM
[AHW22-14] Integrated multi-isotope model to assess nutrient balances in watershed ecosystems
Keywords:In-stream nutrient metabolism, Nitrate triple oxygen isotope anomaly, Nutrient balance, Nutrient spiral metrics, Phosphate oxygen isotope ratio, Reoligotrophication
1. Introduction Since the Anthropocene, mass production and consumption have led to disturbance of natural biogeochemical cycles of macronutrients, such as phosphorus (P) and nitrogen (N), in watersheds of the world. Nutrient loading has caused serious cultural eutrophication. In order to reduce the nutrient loading, developed countries have installed WWTPs, leading to mitigation of the eutrophication. At present, however, the developed countries are facing with a new social-environmental issue. Some researchers pointed out that recent reduction of fishery production can be attributed to the reoligotrophication, which decreases the ecosystem productivity. In Seto Inland Sea, for instance, Hyogo Prefecture recently implemented a legislation to discharge untreated sewage wastes in order to enhance the ecosystem productivity. However, this manipulation has not yet resulted in the recovery of fishery production, while it poses a risk that the coastal ecosystem may move back to eutrophic state.
In general, P:N ratio is a primary factor to determine ecosystem processes because of its scarcity relative to other macronutrients. According to the theory of ecological stoichiometry, there exists an optimal nutrient balance to maximize the ecosystem processes. However, no one knows what is the optimum for the whole ecosystem though some studies demonstrated that organismal growth rate is maximized around the Redfield ratio (N:P=16:1) under microcosm experiments. If we can diagnose the optimal nutrient balance for a focal ecosystem, the best way to maximize the ecosystem processes is addition of limiting nutrient or removal of excess nutrient relative to the optimum. Here we aim to develop an integrated isotope model to assess nutrient balances in watershed ecosystems based on in-situ measurements of their N and P metabolism.
2. Materials & Methods Prior to application for the whole watershed ecosystem, we conducted field research in 3 small streams of Yasu River, the largest tributary of the Lake Biwa Watershed, in 4 seasons to validate the utility of isotope model using nutrient spiral metrics. We adopted a pulse injection method to measure in-stream N and P turnover rate. In parallel, we collected stream water samples for Δ17ONO3-δ18ONO3 and δ18OPO4 analyses. The stream water Δ17ONO3-δ18ONO3 was used to assess in-stream N metabolism. Tsunogai et al. (2016) hypothesized that upward deviation (σNO3) of stream water δ18ONO3 from a Δ17ONO3-δ18ONO3 isotope mixing line of atmospheric and remineralised NO3 would reflect in-stream NO3 assimilation.
By contrast, the stream water δ18OPO4 will reach an isotope exchange equilibrium (δ18OPO4-IEE) as the proportion of biologically recycled P increases in a stream P pool. Therefore, we hypothesize that absolute deviation (σPO4) of δ18OPO4-stream from δ18OPO4-IEE will be smaller when in-stream PO4 recycling is higher. In order to examine if the σNO3 and σPO4 can be indicators for in-stream N and P metabolism, respectively, we tried to find any correlations between these isotope variables and N or P spiral metrics.
3. Results & Discussion The study streams often had lower ratios of TN:TP than the Redfield ratio due to P loading from sedimentary rocks. In-stream PO4 metabolism indicated by spiral metrics was affected by nutrient balances: it increased with increasing TN and decreasing TP. By contrast, N metabolism was less sensitive to the nutrient balances. Contrary to our expectation, interestingly, σNO3 was negatively correlated with in-stream NO3 uptake indicated by the spiral metrics, suggesting that streams with the higher σ which had a higher NO3 metabolic rate have the lower potential for NO3 uptake in response to the NO3 addition. A similar pattern was also observed for PO4 metabolism: σPO4 decreased with the decreasing potential for PO4 metabolic rate indicated by the spiral metrics. In conclusion, these isotope variables may be promising indicators for in-stream nutrient metabolism.
In general, P:N ratio is a primary factor to determine ecosystem processes because of its scarcity relative to other macronutrients. According to the theory of ecological stoichiometry, there exists an optimal nutrient balance to maximize the ecosystem processes. However, no one knows what is the optimum for the whole ecosystem though some studies demonstrated that organismal growth rate is maximized around the Redfield ratio (N:P=16:1) under microcosm experiments. If we can diagnose the optimal nutrient balance for a focal ecosystem, the best way to maximize the ecosystem processes is addition of limiting nutrient or removal of excess nutrient relative to the optimum. Here we aim to develop an integrated isotope model to assess nutrient balances in watershed ecosystems based on in-situ measurements of their N and P metabolism.
2. Materials & Methods Prior to application for the whole watershed ecosystem, we conducted field research in 3 small streams of Yasu River, the largest tributary of the Lake Biwa Watershed, in 4 seasons to validate the utility of isotope model using nutrient spiral metrics. We adopted a pulse injection method to measure in-stream N and P turnover rate. In parallel, we collected stream water samples for Δ17ONO3-δ18ONO3 and δ18OPO4 analyses. The stream water Δ17ONO3-δ18ONO3 was used to assess in-stream N metabolism. Tsunogai et al. (2016) hypothesized that upward deviation (σNO3) of stream water δ18ONO3 from a Δ17ONO3-δ18ONO3 isotope mixing line of atmospheric and remineralised NO3 would reflect in-stream NO3 assimilation.
By contrast, the stream water δ18OPO4 will reach an isotope exchange equilibrium (δ18OPO4-IEE) as the proportion of biologically recycled P increases in a stream P pool. Therefore, we hypothesize that absolute deviation (σPO4) of δ18OPO4-stream from δ18OPO4-IEE will be smaller when in-stream PO4 recycling is higher. In order to examine if the σNO3 and σPO4 can be indicators for in-stream N and P metabolism, respectively, we tried to find any correlations between these isotope variables and N or P spiral metrics.
3. Results & Discussion The study streams often had lower ratios of TN:TP than the Redfield ratio due to P loading from sedimentary rocks. In-stream PO4 metabolism indicated by spiral metrics was affected by nutrient balances: it increased with increasing TN and decreasing TP. By contrast, N metabolism was less sensitive to the nutrient balances. Contrary to our expectation, interestingly, σNO3 was negatively correlated with in-stream NO3 uptake indicated by the spiral metrics, suggesting that streams with the higher σ which had a higher NO3 metabolic rate have the lower potential for NO3 uptake in response to the NO3 addition. A similar pattern was also observed for PO4 metabolism: σPO4 decreased with the decreasing potential for PO4 metabolic rate indicated by the spiral metrics. In conclusion, these isotope variables may be promising indicators for in-stream nutrient metabolism.