Spatial and Temporal Variability in the Carbon and Oxygen Systems in an Indigenous Aquaculture System: Heʻeia Fishpond, Oʻahu, Hawaiʻi
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2022
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University of Hawaii at Manoa
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Abstract
Carbon and oxygen are integral components in a range of organic and inorganic coastal processes. Variability in carbon and oxygen systems can be driven by a range of temporal and spatial factors. The following report is composed from the results of a 12-month study assessing the magnitude of spatial and temporal variability in the carbon and oxygen systems within an Indigenous Hawaiian aquaculture system at Heʻeia Fishpond, located on the western edge of Kāneʻohe Bay. We observe patterns in the spatial variability of both DIC and % O2 saturation between north and south mixing regions within the pond. An analysis of the ΔTA/ΔDIC relationship of the observed spatial variability revealed that aerobic respiration was the dominate metabolic pathway, particularly in the south of the pond. The respiration signal was identified across the 12-month field campaign, in addition to tidal and diel temporal cycles. During neap tide sampling the respiration signal was typically measured as a drawdown of oxygen and an increase in dissolved inorganic carbon originating in the benthos. However, both diel field measurements and carbon parameter analysis suggest that the spatial signal in DIC and % O2 includes both respiration and production in comparable measure. Given the observation of in-situ regimes of significant respiration and production, the typically observed respiration signal must be driven by the benthic metabolism of autochthonous organic matter. Likewise, while the spatial respiration signal in carbon and oxygen was observed throughout the field campaign, it was observed to weaken during the spring tide. The variability in the strength of the respiration signal caused by tidal flux, particularly from Heʻeia Stream, suggests that the physical dynamics of flow direction and residence time within the pond are sensitive to both tidal flushing and elevated streamflow. Taken together, the spatial and temporal variability observed in this project demonstrate the potential for sheltered estuarine ecosystems to develop microenvironments of biogeochemical change. At Heʻeia we observed that such microenvironments seemed to be connected to both the flow of water from Heʻeia stream and the location of invasive mangroves and algae. Both the enhancement of streamflow and invasive species removal are targets of the ongoing restoration work at Heʻeia. Our data suggests that continued progress towards these goals is likely to reduce the significance of low % O2 saturation and elevated DIC associated with the respiration signal we observed by equalizing residence time across the fishpond and removing the benthic algae that may be one of the drivers of the signal.
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Fish pond ecology, Seawater--Calcium carbonate content, Water--Dissolved oxygen, Biogeochemical cycles
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Hawaii--Oahu
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