The Effects of Benthic Organisms on Intertidal Sediment Erodibility
On intertidal sandflats, the dispersal of juvenile organisms can rely on bedload transport, a critical process during small-scale disturbance recovery. The erosion of surface sediments is also vital to benthic-pelagic coupling, where resuspended microbes and organic material become available to suspension feeders and/or transported to adjacent habitats. On sandflats, the organisms living within sediments have been proven to be key drivers of ecological function, such as benthic-pelagic coupling and biogeochemical cycling. Although benthic organisms have been recognized as important to ecological function, the role of multiple species and their interactions on sediment movement is rarely described in non-cohesive sandflats. Furthermore, the role of benthic organisms in sediment movement can vary with environmental conditions, making general sediment-benthos relationships difficult to establish. In this thesis, I examined subtle variations in the biotic and abiotic factors that influence sediment movement on intertidal sandflats. The thesis comprises three research chapters, each evaluating the relationships between benthic community structure and sediment properties. I used a backdrop of environmental stressors common to intertidal sandflats in order to incorporate the natural variations in sediment-benthos relationships that occur under environmental stress. Throughout this thesis, erosion potential was characterized by: erosion thresholds (Ʈc; N m⁻²), erosion rates (ER; g m⁻² s⁻¹), and change in erosion rate with increasing bed shear stress (me; g N⁻¹ s⁻¹). When describing erosion, a decrease in (-) Ʈc signifies an increase in initial bed erosion and an increase in (+) ER represents an increase in surface erosion. Therefore -Ʈc and +ER demonstrate surface erosion. In contrast, me was recorded during a higher bed shear stress, so that an increase in (+) me denotes more rapid changes in subsurface erosion. In chapter 2, I used a spatial gradient of increasing sediment mud (≤ 63 μm) content (0-56 %) to represent temporal increases in fine sediments that can occur as a result of terrestrial inputs. Samples were collected from intertidal flats in three estuaries (n = 45), and data pooled across estuaries to achieve the increasing mud gradient. Distance-based linear modeling (DistLM) was employed to account for the variation in erodibility using biotic and abiotic sediment characteristics. Small bioturbating macrofauna, predominantly freely motile species < 5 mm in size, destabilized surface sediments (-Ʈc and +ER), whereas microbes and organic matter were resuspended during surface erosion. In contrast, increasing mud and mean grain size stabilized subsurface sediments (+me) explaining 61 % of the variation. This study highlights the importance of abundant small bioturbating macrofauna to surface erosion, and describes the natural variation in erosion measures that occur with changes in biotic and abiotic sediment properties. In chapter 3, I manipulated macrofaunal deposit feeding grazing pressure (i.e., Macomona liliana density, hereafter Macomona) and added shade to stress the microphytobenthic community on an exposed sandflat. Biotic and abiotic properties of sediments were measured, and DistLM used to account for the variation in erosion measures with increasing grazing pressure. In this study, the density of abundant shallow-dwelling bioturbators was linked to initial bed erosion (-Ʈc), whereas density of larger deep-dwelling adult Macomona was linked to stabilization (+Ʈc). This study also confirmed several positive feedbacks between abundant shallow-dwelling macrofauna and microbial biomass. However, despite positive feedbacks, net results demonstrate the importance of bioturbating macrofauna to initiating sediment transport in a tidally driven wave dominated system. In chapter 4, I used a disturbance-recovery experiment to measure changes in benthic macrofaunal community structure and sediment erosion after exposure to decomposing macroalgae (Ulva spp). Since a small-scale disturbance response can be influenced by pre-disturbance benthic community structure, I considered the effects of decomposing macroalgae at two sites: mixed (deposit feeding, suspension feeding, and predators) and Macomona (grazing deposit feeder) dominated. After 30 d, decomposing Ulva was removed, and multivariate measures of sediment properties, microbial biomass, and macrofaunal functional groups (based on feeding mode) compared to changes in erosion measures. Despite similarities in sediment properties and microbial biomass, erosion was greater (-Ʈc, +ER, and +me) at the Macomona dominated site than in control plots. After Ulva exposure, I measured a difference in surface erosion by site, with an increase in surface erosion (-Ʈc and +ER) at the mixed site (1 d post-Ulva removal) and more stable surface sediments (+Ʈc and -ER) at the Macomona site (up to 2 weeks post-Ulva removal). This study highlights the importance of benthic macrofaunal community structure to surface erosion and suggests that the small-scale variations may aid in larger-scale disturbance recovery. Throughout this thesis, I describe the role of abundant bioturbating macrofauna in enhancing surface erosion (-Ʈc and +ER) on intertidal sandflats. I also demonstrate positive feedbacks between macrofauna and microbial biomass, which typically resulted in surface erosion (-Ʈc and +ER). Subsurface erosion (+me) was distinct, and more accurately predicted by sediment grain size characteristics. When considering anthropogenic inputs (e.g., terrestrial fine sediments or excess macroalgae), these results suggest that benthic infaunal organisms play a key role in regulating their sedimentary environment, and that areas with a healthy abundance of small bioturbating macrofauna may exhibit greater resilience.
macrofauna, microphytobenthos, sand, erosion, resuspension, benthic microbes, environmental stress
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