Biophysical feedbacks between seagrasses and hydrodynamics in relation to grazing, water quality and spatial heterogeneity: consequences for sediment stability and seston trapping (material costs)

The overall aim of the seagrass projects within the research cluster is to explore and analyse whether nonlinear feedback relations within seagrass ecosystems explain sudden and irreversible shifts between vegetated seagrass meadow and a bare state. Testing this alternative stable state concept for seagrasses does require mechanistic insight in various feedback loops. In this project we focus on the biophysical feedback loop between hydrodynamics and seagrasses, which has an overriding importance for seagrass functioning. This biophysical feedback loop is known to importantly influence both sediment trapping and sediment stability within a meadow, and, for bare sediments, affect seston (here defined as phytodetritus and other particulate organic matter) exchange between column water and pore water (only 1 study for seagrasses available). In the present study, we focus on three factors that affect the biophysical feedback loop in Indonesian seagrass meadows: i) grazing by e.g. turtles, ii) sediment loading of the water column due to e.g. riverine inputs, and iii) spatial heterogeneity. We hypothesize that: - grazing will affect a broad range of hydrodynamically driven feedback process in seagrass meadows (e.g. seston exchange plus trapping and stabilization of sediment) due to modifying the meadow characteristics (shoot length & density). - seagrass meadows may buffer sediment loading in coastal systems, but that this uffering capacity strongly depends on the vegetation characteristics shoot length and shoot density. - that spatial heterogeneity will amplifies hydrodynamic energy and turbulence within seagrass meadows, and thereby enhance seston exchange between pore water and column water, but reduce trapping and stabilization of sediment. These hypotheses will be tested by quantifying, in a combination of flume and field experiments, the effect of: 1. canopy characteristics (height and density as affected by grazing) on the within meadow i) hydrodynamic properties and ii) sediment dynamics; 2. canopy characteristics in combination with sediment porosity on the within meadow seston exchange to the pore water; 3. spatial heterogeneity of seagrass meadows on the afore mentioned processes. The research is innovative in that until now, very few studies have provided insight in: - the consequences of grazing for seagrass functioning from an hydrodynamic perspective - seston exchange within seagrass meadows (and certainly not in combination with shoot properties as affected by grazing) - effects of spatial heterogeneity for biophysical interaction between seagrass and hydrodynamics The proposed project, will address these items in an integrated manner.