Oyster breakwater reefs promote adjacent mudflat stability and salt marsh growth in a monsoon dominated subtropical coast

Abstract

Oyster reefs have the potential as eco-engineers to improve coastal protection. A field experiment was undertaken to assess the benefit of oyster breakwater reefs to mitigate shoreline erosion in a monsoon-dominated subtropical system. Three breakwater reefs with recruited oysters were deployed on an eroding intertidal mudflat at Kutubdia Island, the southeast Bangladesh coast. Data were collected on wave dissipation by the reef structures, changes in shoreline profile, erosion-accretion patterns, and lateral saltmarsh movement and related growth. This was done over four seasons, including the rainy monsoon period. The observed wave heights in the study area ranged 0.1-0.5 m. The reefs were able to dissipate wave energy and act as breakwaters for tidal water levels between 0.5-1.0 m. Waves were totally blocked by the vertical relief of the reefs at water levels <0.5 m. On the lee side of the reefs, there was accretion of 29 cm clayey sediments with erosion reduction of 54% as compared to control sites. The changes caused by the deployed reefs also facilitated seaward expansion of the salt marsh. This study showed that breakwater oyster reefs can reduce erosion, trap suspended sediment, and support seaward saltmarsh expansion demonstrating the potential as a nature-based solution for protecting the subtropical coastlines.

Figure 1

Percentage of wave height dissemination by the reefs, as measured at five water levels related to tidal height (25, 50, 75, 100 and 125 cm) of wave measurement points, classified in five different wave heights (WH). Lines show the linear regressions for each wave height. They indicate the reduction of wave height in relation to increase in tidal (water) levels.

Figure 2

(a) Elevation profiles, based on seasonal measurements from March 2016 to February 2017, along three transects crossing the constructed reefs (right) and two transects crossing control sites (left) without any reefs, during the pre-monsoon, monsoon, post-monsoon and winter; (b) comparison in changing elevation at seaward mudflat (0–10 m), landward mudflat (10–25 m), salt marsh (25–80 m) and mangrove (>80 m) areas of reef and control sites by the end of pre-monsoon (May 2016), monsoon (September 2016), post-monsoon (November 2016) and winter (February 2017). Elevation changes were the difference between two consecutive seasons, whereas changes in post-monsoon were measured by the differences between the initial transect profile surveyed in March 2016 and the transect profile of May, 2016. Star on the bar showing significant difference in reef and control sites.

Figure 3

Changes observed for mudflat elevation areas around the oyster breakwater reefs. (A) Showing the differences in elevation between October 2015 and October 2016; the position of reefs (RS, grey bars) and control sites (CS). (B) Zoom (40 m × 40 m) in view on RS1, RS2 and RS3 to observe the sediment accreted areas around the breakwater oyster reefs (grey bars).

Figure 4

(A) Mean elevation change across the transect lines over one year period, and (B) net sediment accretion or erosion rates by distance classes along the reef (right) and control (left) sites.

Figure 5

Seasonal dynamics in the movement of salt marsh edge at control (CS) and reef (RS) sites.

Figure 6

Photographs showing the difference in salt marsh growth at (a) reef and (b) control sites in December 2017, and (c) seaward salt marsh expansion (photograph was taken in February, 2019).

Figure 7

(a) Study area in the Kutubdia Island, Bangladesh; (b) Google earth satellite image (2017) showing the experimental reef sites (RS = reef site; solid white rectangles), and control sites (CS = control site; dotted white rectangle) with ecological settings (mudflat, salt marsh, mangroves) of the area. Thick dotted white lines were transect lines for measuring monthly and seasonal changes of shore profiles; Black dots were sediment sampling stations; Black stars were wave gauges; White arrows were marsh retreat/seaward expansion monitoring points, measuring the distance between a benchmark stick at the reef edge and the salt marsh edge. Black square: quadrates for salt marsh density measurement; (b) Reef dimension. A 20 m long reefs was constructed by placing 41 concrete rings in two rows next to each other.

Figure 8

Abundance of: (A) oysters; and (B) reef forming polychaetes (Sabellaria sp.) with anemones in the upper (top 50 cm) and lower (bottom 30 cm) parts of the oyster breakwater reefs, respectively. Photographs were taken in October 2017, twenty months after deployment of the concrete rings.