Meso-scale barrier estuary disturbance, response and recovery behaviour: Evidence of system equilibrium and resilience from high-resolution particle size analysis

Abstract

Establishing the future trajectories of coastal wetlands, especially the nature of their disturbance, response and recovery regimes, is of critical importance for a wide range of stakeholders and environmental managers. Reconstructing meso-scale behaviour in coastal environments can serve to attune coastal resource management strategies to natural scales of system operations, thus fostering genuine sustainability. Sediments from Pescadero Marsh, a backbarrier coastal wetland in California, were analysed down-core for their particle size distribution in consecutive 2-mm sections. The particle size data reflect changing hydrodynamics in the back-barrier area driven by past variations in barrier coherence. When considered together, the down-core mean particle size trend and particle size distribution curve styles provide considerable insight into meso-scale system behaviour, revealing barrier/back-barrier disturbance-response-recovery regimes, regime shifts and the role of aperiodic high-energy events in disturbing these regimes. Over sub-annual and multi-annual time periods, the behaviour of the Pescadero system was consistently characterised by both dynamic response to disturbance and recovery through negative feedback. Furthermore, over the duration of the analysed core section, that is, 1200-2300 years, the system was determined to have adopted a series of static equilibrium states. The barrier estuary behaviour reconstructed from the Pescadero sediment record is indicative of innate environmental resilience.

Particle‐size evidence of barrier estuary regime as a new proxy for ENSO climate variability

“…At carefully selected locations, back‐barrier sediments from Pescadero Marsh provide a high temporal resolution record of late Holocene system behaviour (Clarke et al ., , b). A time series of maps and aerial photographs was used to first identify areas of Pescadero Marsh where sedimentation had consistently been influenced by seasonal changes in barrier openness, i.e.…”

“…the duration and extent of annual barrier inlet opening, and identifiable regime shifts in relation to disturbance and recovery from extreme events. The mean particle‐size trend can therefore be used in combination with a robust chronology (based on multiple chronological markers and radiometric dating) to evidence changes in depositional energy at sub‐annual resolution – if sampled at sufficient resolution – over a period of >150 years (in the specific case of PM08, Clarke et al ., , b).…”

“…relative sea level, coastal hydrodynamics, accommodation space, accumulation rate, resuspension, estuary mixing, changing sediment provenance, and the ecohydrological role of vegetation (Dyer, ; Boyd et al ., ; Dalrymple et al ., ; Christiansen et al ., ; Dalrymple and Choi, ; Edwards, ; Fitzgerald et al ., ; Shi, ). These shortcomings aside, particle‐size analysis is an established tool for distinguishing depositional environments (Folk and Ward, ; Visher, ; Tanner ), which can be used effectively to reconstruct past environments, and environmental change, when and where observational records are unavailable (Lario et al ., ; Clarke et al ., , b; Rahman and Plater, ).…”

“…The basis for this connection between ENSO climate variability and barrier estuary particle size derives from previous work on Pescadero Marsh (Figure ) (as well as understanding of other bar‐built estuaries – see previous section) in which the system undergoes a seasonal switch from a back‐barrier lagoon to an open (or partly restricted) barrier estuary. Each year is, in principle, characterized by a depositional couplet of fine grained lagoonal muds and coarser barrier estuary sandy‐silts laid down on a salt marsh and tidal channel margin; over time the changing balance between these two components is conceptualized as barrier ‘openness’ – with episodic storms being preserved as sandy layers of variable thickness and extent (Clarke et al ., , b). Here we don't attempt to distinguish the specific driver of barrier ‘openness’ due to the interaction of high rainfall and coastal erosion during El Niño years, nor can we resolve specific overwash events due to the temporal resolution of the sediment record.…”

“…Extended periods of barrier closure will increase the relative proportion of the lagoonal muds, while longer periods of open barrier estuary conditions will limit their deposition. Even though extensive exploration of particle size distribution shape (Clarke et al ., ) and statistical analysis of grain size components (Clarke et al ., ) has been undertaken, mean particle size has proved to be a very effective, integrative proxy for variability in barrier ‘openness’, i.e. the duration and extent of annual barrier inlet opening, and identifiable regime shifts in relation to disturbance and recovery from extreme events.…”