1:45 PM - 3:15 PM
[ACG41-P04] Evaluation of nutrient absorption and release characteristics of seagrass beds in subtropical waters.
Global warming and various anthropogenic impacts have been reported to cause the decline of coral communities in tropical and subtropical areas. On the other hand, seagrass beds and mangrove forests near the coast of coral reefs are expected to reduce the pollution load and provide favorable conditions to coral communities spreading offshore by absorbing and trapping high concentration of nutrient and suspended solids and, in turn, releasing decomposed organic matter as nutrients at an adequate concentration. However, there need to be more quantitative data on nutrient fluxes and the factors causing their variability. This study aimed to evaluate nutrient fluxes at the water column-sediment interface based on the hypothesis that differences in groundwater flow, weather conditions, and benthic vegetation cause spatiotemporal changes in nutrient concentrations.
Field survey was carried out at Bise coral reef, located in the northern part of Okinawa Island, Japan, from September 13 to 25, 2022. The coastal areas were mainly categorized into two distinct vegetation: a seagrass zone inhabited mainly by Thalassia hemprichii and a mixed zone inhabited by seagrasses, corals, and seaweeds. We periodically collected water samples using flow-tracking techniques with a submerged drogue, and monitored concentration changes of salinity, DO, and nutrients in the water mass. The nutrient fluxes were calculated from temporal changes in nutrient concentration and interpreted with characteristic benthic vegetation and changes in DO concentration.
Nutrient fluxes at the water column-sediment interface showed significant variability with positive values (release) and negative values (uptake). However, there was no clear relationship between nutrient fluxes or their magnitude and changes in DO concentration and benthic classification. It was inferred that various error factors such as 1) time lag between photosynthetic rates and nutrient uptake rates, 2) the temperature dependence of heterotrophic respiration activity, and 3) spatiotemporal changes in groundwater nutrient concentrations may have influenced the flux calculations. In our presentation, we will introduce the results in detail with other parameters, including the unanalyzed carbonate data and several isotope data sets.
Field survey was carried out at Bise coral reef, located in the northern part of Okinawa Island, Japan, from September 13 to 25, 2022. The coastal areas were mainly categorized into two distinct vegetation: a seagrass zone inhabited mainly by Thalassia hemprichii and a mixed zone inhabited by seagrasses, corals, and seaweeds. We periodically collected water samples using flow-tracking techniques with a submerged drogue, and monitored concentration changes of salinity, DO, and nutrients in the water mass. The nutrient fluxes were calculated from temporal changes in nutrient concentration and interpreted with characteristic benthic vegetation and changes in DO concentration.
Nutrient fluxes at the water column-sediment interface showed significant variability with positive values (release) and negative values (uptake). However, there was no clear relationship between nutrient fluxes or their magnitude and changes in DO concentration and benthic classification. It was inferred that various error factors such as 1) time lag between photosynthetic rates and nutrient uptake rates, 2) the temperature dependence of heterotrophic respiration activity, and 3) spatiotemporal changes in groundwater nutrient concentrations may have influenced the flux calculations. In our presentation, we will introduce the results in detail with other parameters, including the unanalyzed carbonate data and several isotope data sets.