Publications

Ecological engineering with oysters for coastal resilience : Habitat suitability, bioenergetics, and ecosystem services

Chowdhury, Mohammed Shah Nawaz

Summary

Ecosystem based coastal management has gained in interest over the last decades. Development was achieved by incorporating different ecosystems services into coastal protection that can deal with threats related to climate change, such as accelerating sea level rise and increased storminess. The ecosystem-based approach not only tries to minimize anthropogenic impacts of coastal protection infrastructures on ecosystems but also aims at offering possibilities to enhance ecosystem functioning and resilience. Natural coastal ecosystems, such as biogenic reefs, dunes, beaches and tidal wetlands have potential value in protecting the coast from erosion and waves, with the benefit that these systems have some ability to self-repair and adapt in changing climate. The use of sustainable ecosystems that integrate human society with its natural environment for the benefit of both is called ecological engineering. It attempts to combine engineering principles with ecological processes to reduce environmental impacts from built infrastructure. Certain key species inhabiting those coastal habitats are known as ecosystem engineers. A number of ecosystem engineers such as coral reefs, reef forming bivalves, vegetation of kelps and seagrasses, marshes and mangroves are known to play engineering roles in shallow estuarine and coastal areas.

Reef forming bivalves that occur in coastal waters can attenuate erosive wave energies, stabilize sediments and reduce marsh retreat. Oysters are commonly said to be ecosystem engineers as they form structures that influence the environment around them in ways that are beneficial to other species. There is a positive feedback of oyster reefs on the settlement of new recruits which makes the reefs self-sustaining. They provide a variety of ecologically and economically valuable goods and services. Oyster reefs serve as natural coastal buffers, absorbing wave energy directed at shorelines and reducing erosion from boat wakes, sea level rise, and storms. Given adequate recruitment and survival, oyster reefs could be self-sustaining elements of coastal protection that enhance other habitats. More than fifty studies were conducted throughout the world since 1995 to evaluate the different ecosystem services provided by oyster reefs including coastal defence. Several studies showed that created oyster reefs can reduce the coastal erosion rate in comparison to control sites with no reefs. This PhD study utilized this concept of oysters as ecosystem engineers and studied the rock oyster, Saccostrea cucullata, in a subtropical, monsoon dominated environment in Bangladesh. This particular environment imposes dynamic conditions for oysters to grow and act as ecosystem engineers. This study investigated the critical factors that determine oyster (S. cucullata) growth and development in a dynamic, monsoon dominated coastal ecosystem of Bangladesh. This study performed experiments by using oyster breakwater reefs to evaluate their eco-engineering effect on: (1) erosion control; and (2) biodiversity of benthic macroinvertebrates and fishes. It was aimed that the application of oyster breakwater reefs can be beneficial to mitigate erosion of tidal flats, promote sediment accretion and facilitate habitats for increasing saltmarsh growth and faunal abundance.

At first, the question was where oysters can settle and grow out, so the focus is on boundary conditions in terms of habitat quality (Chapter 2). To answer this, a habitat suitability index (HSI) model was developed to identify potential suitable sites around the south-eastern Bangladesh coast, where oysters can establish. Seven habitat factors were used as input variables for the HSI model: water temperature, salinity, dissolved oxygen, particulate inorganic matter (PIM), pH, Chlorophyll-a, and water flow velocity. Comprehensive field surveys were conducted at 80 locations to collect geo-spatial environmental data, which were used to determine HSI scores using habitat suitability functions. The model results clearly showed that sites from the mouth of Sangu River to the tip of Teknaf, including the offshore islands (Kutubdia and Maheshkhali), are found suitable (HSI >0.50) habitats for oysters, except a few areas near small river mouths which become dynamic with freshwater flashes during monsoon months. These areas showed relative high salinity, Chlorophyll-a, dissolved oxygen and pH. In contrast, freshwater dominated estuaries and nearby coastal areas (i.e. northern part of the study area coving Sandwip, Feni, Mirsarai, Chittagong) with high suspended sediment concentrations from river discharges were found less suitable (HSI <0.50) for oysters. Salinity, Chlorophyll-a, dissolved oxygen and pH were identified as driving factors that determine the habitat quality for oyster in Bangladesh coast. The HSI model results match the current distribution of oysters throughout the investigated area. The good correspondence with the field data enhances the reliability of the presented HSI model as an interactive and quantitative tool for planning and managing oyster resources along the south-eastern coast of Bangladesh.

Secondly, seasonal dynamics in oyster performances are analysed by measurements of the physiological performance of the oysters as a function of environmental conditions (Chapter 3 and 4). Chapter 3 provides physiological information of S. cucullata related to different ecological parameters, which were synthesized from large number of eco-physiological experiments and the outcomes were further used to estimate the DEB model parameters. It is concluded that the hydrometeorological aspects, i.e. a monsoon regime and high turbidity levels, are quite different from temperate regions and drives the physiological traits of shellfish organisms in Bangladesh coastal waters. The estimated DEB parameters for Saccostrea cucullata and their related univariate data provided opportunities (see chapter 4) to simulate the oyster growth in a monsoon dominated hydrodynamic environment. Chapter 4 utilizes the dynamic energy budget (DEB) theory, which allows to establish links between the physiology of an organism and its environment by capturing the metabolic dynamics of an individual organism through its entire life cycle. Developed DEB model was validated by simulating S. cucullata growth under varying hydro-biological conditions. The model results are compared with independent field observations on the growth (length and weight) of S. cucullata at three different sites (Sonadia, Kutubdia and Inani) located in the south-eastern coast of Bangladesh, covering a distinct environmental gradient. The sites vary spatially and temporally in environmental conditions such as salinity, total particulate matter (TPM) and Chlorophyll-a concentrations due to the monsoonal river discharges. At the three sites, field observations of oyster growth, temperature and food availability (Chlorophyll-a and Particulate Organic Matter-POM) have been carried out in the period between September 2014 - August 2017. The DEB model reproduced temporal as well as spatial variation in oyster growth as a function of the prevailing environmental conditions. Growth rates of oysters were highest (shell length: 3cm yr-1) in Sonadia Island due to better food conditions. Whereas, the growth rates were relatively low (1.94 cm yr-1) in Kutubdia and none of oysters survived in Inani during the monsoon event due to high suspended load (889 ± 101 mg l-1) and low Chlorophyll-a (1.86 ± 0.16 µg l-1) conditions. Temporal variation is largely monsoon driven: the period between November to May was the main growing season for oysters along the Bangladesh coast, while growth slowed down in the monsoon months (June-September). DEB model simulations for S. cucullata showed good fit (>8.54 score out of 10) with measured growth data under the different in situ conditions throughout the seasons. It means that the DEB model for S. cucullata demonstrated accuracy for simulating growth in its natural environment along the Bay of Bengal. Therefore, the model can be used to evaluate potential sites for oyster culture development or restoration to enhance coastal resilience.

Thirdly, in Chapters 5 and 6, it was tested if the application of oyster breakwater reefs contribute to reducing coastal erosion in the context of monsoon-dominated subtropical coast and at the same time be beneficial in facilitating other habitats (i.e. mudflat, saltmarsh) and species (macro-invertebrates, fishes). Therefore a suitable site was chosen based on model outputs and observations, namely an eroding mudflat on Kutubdia Island. Here, concrete rings with oysters overgrown for 2 years were placed as oyster breakwater reefs in the lower intertidal zone of the mudflat. The oyster breakwater reefs were tested to see whether it reduced sediment erosion, promoted mudflat stability and enhanced seaward salt marsh expansion and growth, in comparison with areas without such reefs. The results demonstrated that oyster breakwater reefs are particularly useful to reduce erosion at lower intertidal areas as the reefs successfully trapped sediments by dissipating waves. Oyster breakwater reefs modified the mudflat morphology up to 35 m distance at the lee side with accretion of 29 cm clayey sediments and erosion rate was two times lower during the monsoon period compared to control sites. By doing so, it enhanced the growth of new salt marsh vegetation and expanded their seaward edge effectively, thereby further stabilizing the unconsolidated sediments. Therefore, along the coast of Bangladesh, where oyster larval supply is abundant, the eco-engineered breakwater structures have the potential to contribute to a more sustainable shoreline protection against erosion.

Chapter 6 aims to analyze the effects of these breakwater reefs on abundance and composition of macrobenthic soft-bottom assemblages together with transient and resident mobile fauna (fish, shrimp, crabs and other macro-invertebrates) in comparison with adjacent control sites without reefs. Seasonal influences were also considered to understand whether the effects of reefs depend on seasons. This study clearly indicates that oyster breakwater reefs had a positive effect on mudflat fauna communities. It shows higher abundances and biomass of fish and macroinvertebrates relative to the adjacent control sites. Seasonal variation was obvious, but didn’t overrule the reef impact. Multivariate analyses also demonstrated that the reef sites held distinct faunal communities, which differed from the control sites. Changes in macrobenthic community composition were associated with the variations in sediment load and characteristics, which were influenced by the breakwater reefs. Oyster breakwater reefs help to stabilize find sediments locally in lee side (landward) of the reefs, which is found as key reason to observe higher rates of macrobenthic colonization. Higher abundance of transient fish and mobile macro-invertebrates in reef sites indicated that breakwater oyster reefs attract mobile species as the reefs offer food and shelter. In fact, the study suggested that three-dimensional oyster breakwater reefs not only provide the shelter functions for mobile resident fauna, but also extend the ecosystem services related to nursing, breeding and foraging for numerous transient species by augmenting different prey resources for them. Though the ecological impact of oyster breakwater reefs was limited to a local area surrounding the reefs, this study provided hands-on evidence of ecological benefits using these reef configurations in estuarine and coastal habitats.

This PhD study demonstrates that the use of the oyster breakwater reefs has multiple benefits. It can locally protect tidal flats against erosion and promote saltmarsh growth at the lee side of the reefs. These reefs act as breakwater and dissipate wave energy that accelerate the soft sediment deposition behind the structure and increase the bed level. This type of morphological changes may provide opportunities for mangrove planting. The study also showed that eco-engineered oyster reefs can support a high density of macro-benthos in reef areas, sessile macrofauna (oysters, barnacles, sea anemones etc.) on surface of reef substrates, large number of motile macro-invertebrates in reef system that attract transient nektons. The oyster breakwater reefs clearly has the potential to improve fishery production by providing high quality habitat and prey to a variety of commercially and ecologically important fishes, shrimps and crabs. Despite of having these benefits and opportunities, oyster breakwater reefs also have some limitations. Oysters need to settle, survive and grow at the designated place i.e. substrates in order to achieve long-term, persistent structures and self-sustainable reefs. This depends on the habitat characteristics of the site in the first place. Not all sites are equally suitable for oyster settlement survival and growth. Selection of the right site for creating oyster reefs is crucial. Therefore, we developed a HSI model that showed to be helpful in identifying potential sites (Chapter 2). The site can be further critically evaluated by a DEB model to understand seasonal dynamics in predicting oyster growth and reproduction (Chapter 4). Particularly, burial by sediment can cause significant loss of reef habitat. It can be avoided by increasing the heights of reef substrates based on the characteristics of the site. Additional constraints are the vulnerability of oysters for diseases and predation. Oyster drills (Urosalpinx spp.) and stone crabs (Myomenippe spp.) were found as meso-predators in the investigated sites, but their effects on oyster population need to be investigated.

The intertidal rock oyster, S. cucullata can be ecologically engineered by providing hard substrates to settle on, that offers a kick-start for reef formation at places where they were lost or are desirable for coastal protection. Reef formation and development is however strongly dependent on the local environmental conditions governing oyster recruitment, survival and growth dynamics. These conditions can be highly dynamic, for example during the monsoon season. S. cucullata shows abilities to adapt to these conditions by regulating their physiological activities. The study shows that the S. cucullata populations are able to sustain in many estuarine areas along the southeast coast of Bangladesh as they can cope with the monsoonal climate. This makes them suitable for the role as eco-engineers for coastal protection. The study showed that artificial substrates can be used to develop self-sustaining oyster populations that contribute to coastal protection. Furthermore, oyster breakwater reefs dissipate the wave energy that reduces the hydrodynamic pressure on the foreshore of the primary dike and thus reduce the dike maintenance cost. Integration of oyster reefs with other ecosystems can add more benefits. Even it can enhance the possibility of doing oyster culture by enhancing larval supply in the area. Moreover, coexisting with other ecosystems viz., salt marsh and mangrove along with oyster breakwater reefs in the intertidal zone can act as bio-shield to prevent erosion and reduce the effect of cyclonic storm surges in the region. Therefore, oysters provide a great chance for Bangladesh to utilize them for the benefit of coastal people and environment.