Groundwater-surface water exchange in a mangrove tidal creek: evidence from natural geochemical tracers and implications for nutrient budgets

Assessing submarine groundwater discharge (SGD) in mangroves can be challenging due to the complex, heterogeneous nature of sediments and temporally dynamic water flows. Here, we use natural tracers (i.e., radon and radium isotopes) to quantify groundwater–surface water exchange in a mangrove tidal creek at the ecosystem scale, and to assess whether SGD was a source of dissolved inorganic and organic nutrients to the nearby estuary. Hourly sampling for over 27 h in the winter and 30 h in the summer revealed clear tidal trends for 222Rn, 224Ra, 223Ra, 226Ra, dissolved oxygen, ammonium, and dissolved organic nitrogen (DON) in a mangrove tidal creek with no upstream freshwater inputs (Kangaroo Island, Southern Moreton Bay, Australia). Nitrate, phosphate and dissolved organic phosphorus (DOP) did not show a clear tidal trend. A mass balance revealed that 5–12% of the tidal prism volume infiltrated the sediments and drained back to the creek at low tide. Groundwater exchange accounted for nearly all the ammonium and about 30% of the DON exported by the creek. Crab burrows were a major factor influencing groundwater exchange in this tidal creek. A detectable 226Ra (half-life of 1600 years) enrichment in the creek at low tide could only be explained by the construction of new crab burrows during each tidal cycle exposing old sediments. Overall, the concentration of groundwater tracers and nutrients was higher in the summer most likely due to increased crab activity and organic matter degradation in the warmer months. Respiration of belowground organic matter apparently released nitrogen to groundwater seeping out from the creek bank at low tide and increased N:P ratios of surface water draining from mangroves. Therefore, we hypothesize that outwelling from mangroves with high SGD may influence the composition of downstream biological communities by altering estuarine N or P limitation conditions

Importance of budgets for estimating the input of groundwater-derived nutrients to an eutrophic tidal river and estuary

Groundwater is often overlooked as a source of nutrients to estuaries and most previous groundwater-surface water exchange studies did not consider the input of dissolved organic nutrients. Here, we hypothesize that groundwater is contributing to high dissolved inorganic and organic nutrient concentrations in an eutrophic subtropical tidal river and estuary (Caboolture River, Queensland, Australia). Several spatial radon (222Rn, a natural groundwater tracer) surveys indicated that the majority of groundwater discharge occurred in the tidal river just upstream of the estuary, and that the radon hotspot did not necessarily coincide with the nutrient hotspot. A radon mass balance revealed that groundwater discharge into the tidal river was equivalent to about 50% of the gauged river flow in February 2012. Groundwater discharge apparently contributed 85% of ammonium and 35% of phosphate entering the estuary. In spite of significant correlations between radon and nitrate and dissolved organic nitrogen (DON) during spatial surveys, groundwater could account for only 7% of nitrate and 9% of DON inputs due to low groundwater concentrations and other sources (i.e., apparently a sewage treatment plant for nitrate and floodplain tributaries for DON). Because total dissolved nitrogen (TDN) was dominated by DON (69%) and nitrate (23%), the groundwater ammonium inputs were a minor source to the TDN pool within the tidal river and estuary. This study demonstrated that correlations between a groundwater tracer and nutrient concentrations do not necessarily illustrate causation. To assess how groundwater drives nutrient dynamics in estuaries, it may be important to include the tidal river (not only the estuarine salinity gradient) in field investigations, consider DON (not only ammonium and nitrate), and perform detailed budgets that include minor tributaries

Groundwater-derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: the missing mangrove carbon sink?

A majority of the global net primary production of mangroves is unaccounted for by current carbon budgets. It has been hypothesized that this ‘‘missing carbon’’ is exported as dissolved inorganic carbon (DIC) from subsurface respiration and groundwater (or pore-water) exchange driven by tidal pumping. We tested this hypothesis by measuring concentrations and d13C values of DIC, dissolved organic carbon (DOC), and particulate organic carbon (POC), along with radon (222Rn, a natural submarine groundwater discharge tracer), in a tidal creek in Moreton Bay, Australia. Concentrations and d13C values displayed consistent tidal variations, and mirrored the trend in 222Rn in summer and winter. DIC and DOC were exported from, and POC was imported to, the mangroves during all tidal cycles. The exported DOC had a similar d13C value in summer and winter (, 230%). The exported d13C-DIC showed no difference between summer and winter and had a d13C value slightly more enriched (, 222.5%) than the exported DOC. The imported POC had differing values in summer (, 216%) and winter (, 222%), reflecting a combination of seagrass and estuarine particulate organic matter (POM) in summer and most likely a dominance of estuarine POM in winter. A coupled 222Rn and carbon model showed that 93–99% of the DIC and 89–92% of the DOC exports were driven by groundwater advection. DIC export averaged 3 g C m22 d21 and was an order of magnitude higher than DOC export, and similar toglobal estimates of the mangrove missing carbon (i.e., , 1.9–2.7 g C m22 d21)

Stable isotopes reduce parameter uncertainty of an estuarine carbon cycling model

Quantifying estuarine carbon cycling is complex due to the highly-variable environmental conditions associated with the interaction between tides, riverine inflows, meteorological forcing and internal biogeochemical processes. A Markov-Chain Monte Carlo algorithm was utilized to perform unbiased calibration of parameters used by a 1-D isotope-enabled carbon model applied to stable isotope data collected in Caboolture River Estuary, Australia. The parameter posteriors were ported into a 3-D finitevolume isotope-enabled carbon model and run over a range of hydro-meteorological conditions that occurred during a 1.5-year simulation period. The model highlighted the spatio-temporal variations and uncertainties associated with carbon cycling within the estuary, including the shift from being strongly heterotrophic in the upper estuary with a higher water-atmosphere flux of CO2, to a more balanced trophic state in the lower estuary. The approach demonstrates the usefulness of isotope data to constrain model uncertainty and advances our ability to undertake carbon budgeting in coastal environments

Decolonising Design Education through Playful Learning in a Tertiary Communication Design Programme in South Africa

This article introduces playful learning as part of the decolonising project at institutes of higher learning in South Africa with specific reference to the discipline of communication design. Not only does the article interrogate the content of design education, specifically design for development, but more specifically the way that design for social innovation is taught. The article begins with a contextualisation of the decolonisation debate both locally and internationally, which is followed by a brief historical overview of formal design education. Design education’s trajectory informs the contemporary conception of design as a form of rhetoric. Design and play are then interfaced theoretically, and pragmatically through a case study to explore how deeper learning was made possible by play in a curriculum‐based project. Ultimately, the article aims to highlight the value of playful learning in design higher education to nurture alternate modes of design thinking that favour localised practice, intersubjective relationships between designers and their stakeholders and the enabling of students’ self‐realisation through real world experience.https://onlinelibrary.wiley.com/journal/147680702021-08-01hj2020Visual Art