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

Abstract

Aquatic 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. In low‐nutrient, macrophyte‐dominated coastal zones, benthic ammonium (NH 4 + ) uptake may be influenced by the structural properties of plant canopies via their effect on near‐bed hydrodynamics. Using a dual‐tracer (uranine and 15 NH 4 + ) method that does not require enclosures, we examined how this process affects nutrient uptake rates within a tidally dominated, patchy Caulerpa prolifera – Cymodocea nodosa landscape. NH 4 + uptake was determined by calculating tissue 15 N excesses and correcting for 15 N enrichment as derived from uranine concentration. Vertical hydrodynamic profiles were measured in the downstream flow direction from outside to inside of the C. nodosa bed by using an array of acoustic Doppler velocimeters. The transition from a C. prolifera to a C. nodosa bed included a change in both benthic canopy properties (short and dense to tall and sparse) and sediment topography (0.2‐m increase in water column depth) that resulted in an increase in longitudinal advection and turbulent diffusivity within the C. nodosa canopy between 0.5 and 1.5 m from the leading edge. Vertical differences in canopy water exchange appeared to explain variations in uptake between biotic functional groups; however, no clear differences in longitudinal uptake were found. Using in situ labeling, this study demonstrated for the first time the role of hydrodynamics in structuring NH 4 + uptake within an undisturbed, patchy macrophyte landscape.