A Model for Understanding the Effects of Sediment Dynamics on Oyster Reef Development

Abstract

Previous field studies have demonstrated that sedimentation is an important factor that can limit oyster reef growth and restoration success. High relief reefs are more productive and resilient than low relief reefs, in part, because increasing reef height reduces sedimentation and enhances oyster growth. In this study, we investigated the relationship between initial reef height and reef development using a simple model. The model contains three coupled differential equations that describe changes in oyster volume, shell volume, and sediment volume per unit area of reef with time. The model was used to investigate how parameters such as flow speed, sediment grain size, and food concentration affect reef survival and final reef height. Whether or not a reef survives depends primarily on the shape of the sediment concentration profile relative to the initial reef height. Over a long time period, three different steady-state reef heights are possible, depending on the environmental parameters and initial reef height: (1) If growth outpaces sedimentation, the reef achieves the maximum possible height, which is independent of sedimentation parameters; (2) if deposition outpaces growth and the shear stress does not exceed the critical shear stress, the reef is buried in sediment and dies; and (3) if deposition outpaces growth and the shear stress exceeds the critical shear stress, a reduced steady-state height is achieved that depends on both growth and sedimentation parameters. The model can be used to assess the ways in which measurable environmental parameters affect reef restoration success.

Reef height drives threshold dynamics of restored oyster reefs

“…If flow rates are sufficient to promote sediment erosion from the reef and break down feeding-inducing concentration gradients, this could result in increased recruitment, growth, and survival of the oyster population (Lenihan 1999, Byers et al 2015. Jordan-Cooley et al (2011) suggest a bifurcation point for reef populations above which reefs and live oyster populations increased to a non-zero equilibrium and persisted over time and below which reefs were overwhelmed by sediment and degraded to extinction in less than 20 yr. Other models demonstrate similar dynamics to those described by Jordan-Cooley et al (2011) in response to different initial reef height conditions (Wil berg et al 2013, Housego & Rosman 2016.…”

“…Population models suggest that sediment deposition mediated by initial reef height is capable of producing alternative stable equi libria similar to reef outcomes observed in the field (JordanCooley et al 2011, Wilberg et al 2013, Housego & Rosman 2016. In these models, a stable, non-zero equilibrium is achieved when live oyster and shell volume growth outpaces sediment de position, which varies with reef height and reaches a maximum at the seafloor (Jordan-Cooley et al 2011, Housego & Rosman 2016.…”

“…Modeling studies suggest that these outcomes represent 2 alternative stable states of oyster reef ecosystems (Jordan-Cooley et al 2011, Wilberg et al 2013, Housego & Rosman 2016) that result from biophysical feedbacks re sponding to initial reef height. Elevation of the reef above the seafloor restricts the height of the water column as it passes over the reef, resulting in faster flow rates relative to non-constricted flow (Lenihan 1999).…”

Restoration of eastern oyster populations with positive density dependence

“…, Nystrom et al. , Housego and Rosman ). In systems with alternative stable states, restoration becomes particularly challenging as transitions between desired and undesired states can occur through sudden, often unpredictable, phase shifts, and successful restoration often requires the conditions of the system be returned to levels more extreme than those immediately prior to the phase shift (Scheffer et al.…”

Restoration of Eastern oyster populations with positive density dependence

“…Demographic modeling has shown that positive feedbacks between living oysters and shell substrate can lead to thresholds between population persistence and extinction, as well as possible alternative stable states (Jordan-Cooley et al 2011, Nystrom et al 2012, Housego and Rosman 2016. In systems with alternative stable states, restoration becomes particularly challenging as transitions between desired and undesired states can occur through sudden, often unpredictable, phase shifts, and successful restoration often requires the conditions of the system be returned to levels more extreme than those immediately prior to the phase shift (Scheffer et al 2001, Beisner et al 2003, Scheffer and Carpenter 2003, Hastings and Wysham 2010.…”