The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community

Abstract

Lay AbstractAquatic macrophytes (such as macroalgae and sea grasses) are able to take up nutrients directly from the surrounding water in which they inhabit. How fast nutrients are transported into their structures (such as leaves or fronds) can be influenced by water flow. However, macrophytes tend to form canopies made up of many individual structures, and the presence of these canopies has large effects on local water flow. This implies that in shallow regions the architecture of different canopies may well be a key factor in setting rates of nutrient transport to the sea floor. Predicting these effects is complicated because in many natural systems macrophyte communities often form patchy distributions consisting of bare and vegetated areas, as well as patches of different species with different canopy types. Here we use a new application of stable isotope labeling to directly measure the in situ uptake of ammonium at the transition between short, dense Caulerpa prolifera (a macroalga) and tall, sparse Cymodocea nodosa (a sea grass) canopies. We found that as water flowed over the dense macroalgae into the sparse sea grass canopy, it accelerated, resulting in horizontal and vertical variations in water velocity and mixing that appeared to explain variations in the ammonium uptake of the different types of organisms living on and within the canopies. These results highlight how the interplay between the physical structure of organisms and flow may determine the nutrient transport of individual species within macrophyte communities, creating nutrient transport microhabitats that may influence biodiversity and ecosystem functioning.