If a leaf fell in a mangrove and no-one was there to hear it, would it make a noise? OK, it’s a silly question, but the amount of organic material – plant, animal, whatever – which ends up in our coastal areas is huge. Most people know estuaries, the places where rivers meet the sea, from fishing trips or muddy school excursions. What’s not immediately apparent is that estuaries are highly dynamic and play an important role in many important natural systems, from carbon cycling to fisheries.
The processes driving estuaries are diverse in type and scale; the mixing of a river into the ocean can operate on large scales, but a single leaf falling from a mangrove plant onto the sediment can trigger a whole cascade of effects which, when added up, are very important.
In sediments (like soils, but under a water body), organic material isn’t always broken down in a way we’re used to. We all respire using oxygen; that is, the cellular machinery in our bodies ‘burn’ sugars in a controlled reaction with oxygen, which releases carbon dioxide, water and energy. However, some kinds of bacteria don’t need oxygen for their respiration, and are capable of using substituting other chemicals, including iron oxides (rust) and sulfate (SO4). These chemicals perform the same function for bacteria that oxygen does for people.
Sulfate and iron are particularly important in estuaries, because they are present at much higher concentrations than oxygen and can be responsible for facilitating up to 90% of organic matter breakdown. If this didn’t happen, lots of important nutrients would be lost from natural systems, locked away under the ground.
During my Honours year, I worked to develop and expand a gel-based sampling method for studying the patterns of these chemicals at high resolution in two dimensions. As sediments are extremely complex, varying in chemical composition in three dimensions and over time, our understanding of how they operate hinges on how well we can get information about what’s going on. Under the guidance of my supervisors, I produced the first ever simultaneous, high-resolution images of both sulfur and iron in local sediments.
As you can see, the iron and sulfide maps mirror each other, with high concentrations of one found where the other is at lower concentration. So far, we’ve published field data from several Gold Coast locations, as well as the results of laboratory studies looking at the effect of burrowing animals on sediment dynamics. By interpreting these results, and refining our methods, we hope to improve our understanding of the small-scale processes which drive major environmental systems in estuaries.
Similar research in lakes has revealed that up to 30% of organic matter breakdown can occur in tiny hotspots comprising about 1% of the actual sediment volume – in the image above, you can see some small points which have higher concentrations than the areas around them. Such hotspots are completely destroyed by conventional sampling techniques, so our understanding of their role carbon, sulfur and metal cycling in the rich coastal sediments was negligible until the past few years. It’s only by making finer-scale measurements that we can detect, measure and understand such processes. Similar hotspots are rife in estuarine and marine sediment, but their relative importance hasn’t been studied yet – estuaries are even more complicated than most lakes, so it’s a challenge!
There’s still a lot of room for more study and improvement – nature is so complex and detailed that we’ll only ever reach an approximation of the full picture – and the project is being carried on at Griffith by more students (poor suckers – sulfide is, literally, rotten egg gas! Surprisingly, you get used to it…), as well as other researchers around the world.
So there you have it – some of my contribution, so far, to science!