Feedback between sediment and light for seagrass: Where is it important?

Abstract

A feedback between seagrass presence, suspended sediment and benthic light can induce bistability between two ecosystem states: one where the presence of seagrass reduces suspended sediment concentrations to increase benthic light availability thereby favoring growth, and another where seagrass absence increases turbidity thereby reducing growth. This literature review identifies (1) how the environmental and seagrass meadow characteristics influence the strength and direction (stabilizing or destabilizing) of the seagrass-sediment-light feedback, and (2) how this feedback has been incorporated in ecosystem models proposed to support environmental decision making. Large, dense seagrass meadows in shallow subtidal, noneutrophic systems, growing in sediments of mixed grain size and subject to higher velocity flows, have the greatest potential to generate bistability via the seagrass-sediment-light feedback. Conversely, seagrass meadows of low density, area and height can enhance turbulent flows that interact with the seabed, causing water clarity to decline. Using a published field experiment as a case study, we show that the seagrass-sedimentlight feedback can induce bistability only if the suspended sediment has sufficient light attenuation properties. The seagrass-sediment-light feedback has been considered in very few ecosystem models. These models have the potential to identify areas where bistability occurs, which is information that can assist in spatial prioritization of conservation and restoration efforts. In areas where seagrass is present and bistability is predicted, recovery may be difficult once this seagrass is lost. Conversely, bare areas where seagrass presence is predicted (without bistability) may be better targets for seagrass restoration than bare areas where bistability is predicted.

Peer Review #2 of "Comparison of shallow-water seston among biogenic habitats on tidal flats (v0.1)"

“…This habitat-specificity in initial values was also apparent in Chl: 6.4 (0.8 SE), 4.7 (0.6 SE), and 3.7 (0.3 SE) µg L -1 . The finding that biogenic habitats did not further influence seston differentially with respect to bare suggests that the mosaic of intertidal water properties is established at patch borders, for instance because erosional processes tend to be enhanced at leading edges of structured habitats (Adams et al 2016). Factors expected to result in build-up of effects during drifts, such as suspension feeder activity, may not differ among habitats as expected from oysters alone, given that we did not sample for infaunal suspension-feeders (i.e.…”

Water residence time controls the feedback between seagrass, sediment and light: Implications for restoration

“…On the other hand, hydrodynamic simulations can more accurately predict T residence , without explicitly requiring an ecological process model to be run simultaneously. This is convenient because large-scale ecosystem model suites are often structurally designed so that the hydrodynamic model runs separately and prior to the ecological model, which means that it is difficult to explicitly simulate ecological feedbacks on the local hydrodynamics in these model suites (Adams et al, 2016a). For ecosystem model suites that allow feedbacks of ecological processes back onto the hydrodynamics (e.g.…”

“…There is quantitative evidence that the SSL feedback has the potential to produce alternative stable states of seagrass presence and absence in the same location, which is an ecosystem characteristic called "bistability" (Adams et al, 2016a). Several theoretical models have also predicted that the SSL feedback can cause bistability (van der Heide et al, 2007;Carr et al, 2010Carr et al, , 2012aCarr et al, ,b, 2016.…”

Biophysical Interactions in Fragmented Marine Canopies: Fundamental Processes, Consequences, and Upscaling

“…Enhanced sediment deposition creates new substrate, which provides nutrients and anchoring, encouraging enhanced canopy growth, and this positive feedback maintains spatial correlation between canopy and substrate distributions (Baattrup-Pedersen and Riis, 1999). Sediment resuspension reduces light levels, which reduce canopy growth rates, leading to sparser canopies, enabling further resuspension (Adams et al, 2016). Resuspension can occur because of enhanced turbulence or enhanced mean flow -thus sediment may be resuspended within patches even if mean flow speeds are below the threshold of sediment motion, due to stem wake turbulence (Lefebvre et al, 2010).…”

“…Further studies are needed into the mechanics of interactions between multiple canopy patches and gaps at all scales -from interactions between two patches, through studies of patch mosaics (Schoelynck et al, 2018) and fragmented canopies with more complex spatial distributions, to whole-landscape scales. These need to take into account the roles of a wide range of different variables, including those related to hydrodynamics (waves, currents, turbulent mixing), sediment (erosion, resuspension, transport, deposition), and other physical variables such as light levels (e.g., Koch, 2001;Adams et al, 2016) and water temperature. From a chemical perspective, they need to include concentrations of nutrients, dissolved gasses, pollutants and a wide range of biogenic chemicals, as well as their flux rates, both in terms of physical movement between the substrate, water column, biota and atmosphere, and in terms of chemical changes, for example from dissolved to particulate form, or organic to inorganic form.…”