Limits to oyster development on breakwater structures

Objectives

The objectives of this study are to:

1. Compare rates of (a) wild oyster spat recruitment and (b) seeded oyster mortality across the intertidal gradient at multiple coastal restoration sites featuring breakwaters.
2. Identify ecological factors that limit (a) oyster spat recruitment and (b) oyster post-settlement survival to guide the design of Living Shoreline breakwaters that maximize oyster habitat.

Methodology

This study will occur in Portersville Bay, near Bayou La Batre, Alabama. Portersville Bay has a long history of coastal restoration with several high-profile living shoreline restoration projects completed over the past couple of decades and others in progress or in planning. A desired outcome of most of these projects is the establishment of oyster populations on the breakwater structures, however, our research has shown highly variable success in this regard (Bland et al., 2024). Portersville Bay is known to be an important destination for oyster larvae (Gancel et al., 2021), and is expected to have variable environmental conditions including freshwater discharge and different regimes of oyster predators (Belgrad et al., 2023).

We will contract the Auburn Shellfish Laboratory to seed ceramic tiles with live oyster spat. The seeded tiles will be fixed to deployment poles designed to remain in the field at fixed intertidal elevations; the tiles on each pole will range from being near-continuously inundated to being near-continuously exposed. The height of each tile in meters NAVD88 will be measured using a Real Time Kinematic (RTK) GPS receiver. The seeded tiles will be frequently monitored after field deployment to look for new oyster recruits and quantify any oyster mortality. Some of the poles will include predator exclusion cages to estimate rates of mortality attributable to predation versus non-predation sources. At least one pole at each site will include a cage control (i.e. a cage that permits predator access) to assess the effect of caging.

Each site will include 2-3 additional poles with bare tiles. These tiles will be visited and replaced every three weeks to monitor rates of wild oyster larval settlement. In addition, each site will have one deployed water level logger to determine the inundation patterns for each tile, and a temperature logger mounted near the top tiles on one caged and one uncaged tiles to monitor aerial thermal stress on the most exposed tiles. Finally, we will deploy water quality sensors when possible to continuously monitor water temperature, salinity, and dissolved oxygen. These data will inform how water quality contributed to rates of oyster recruitment and
survival.

In summary, the proposed methodology will provide data on two key determinants of oyster population levels, i.e. rates of spat recruitment and post-settlement survival, across the intertidal gradient at existing and potential restoration sites. Therefore, we will be able to identify the optimal substrate elevation for oyster restoration at target restoration sites; this will inform the design of future oyster reefs and breakwaters that best accommodate the ecological requirements of oysters and are more likely to develop into successful oyster reefs. Additionally, we will collect concurrent data on key ecological stressors of oyster settlement and survival, i.e. water quality, aerial temperatures and predation. Understanding the locally important stressors will allow restoration managers to better predict the likelihood of successful oyster restoration, identify sites conducive for oyster restoration, and take targeted actions to release oysters from stressors.

Rationale

Coastal communities are increasingly threatened by rising sea levels, worsening storm impacts and the degradation of valuable coastal habitats. Coastal managers attempt to mitigate these threats with a variety of approaches; ecosystem-based coastal defense is one category of management practices for sustainable coastal protection coupled with ecosystem restoration (Temmerman et al., 2013). For example, intertidal oyster reefs can buffer wave energy and provide ecosystem services including fish habitat (Scyphers et al., 2011) while self-maintaining under scenarios of sea level rise (Rodriguez et al., 2014). While many coastal restoration activities provide substrate for oyster settlement, oyster reef development is highly variable across projects (Morris et al., 2021; Wellman et al., 2021).

There are numerous potential barriers to oyster reef development in coastal restoration contexts. Would-be reefs may be spat limited (Gancel et al., 2021), or oysters may be subject to post-settlement stressors such as unfavorable water quality (Baker and Mann, 1992; Shumway, 1996) or predation (Brown and Stickle, 2002; Grabowski, 2004). Critically, intertidal oysters experience a gradient of stressors across their vertical tidal range (Johnson and Smee, 2014); therefore, oysters may settle or survive at different rates depending on their inundation frequency (Morris et al., 2021; Notz et al., 2023). Together, these ecological factors represent the local oyster ecology of marine systems, which are recognized as critical considerations for effective oyster restoration (Morris et al., 2021; Powers et al., 2009; Wellman et al., 2021). For example, the optimal oyster reef breakwater material and breakwater crest height both depend on local site conditions (Morris et al., 2021; Wellman et al., 2021). Prior knowledge of these and other ecological factors could inform breakwater siting and design for more successful oyster restoration, but unfortunately, restoration is typically performed with little understanding of local oyster ecology.

Coastal Alabama features several coastal restoration sites utilizing the hybrid Living Shoreline design, i.e., a vegetated salt-marsh shoreline is protected by offshore breakwater structures. In addition to attenuating waves, breakwaters are assumed to provide suitable substrate for oysters and other encrusting organisms. Our preliminary data suggests that live oyster densities has been spatially and temporally variable on Alabama breakwaters (Bland et al., 2024), but maximum oyster densities in Alabama are dwarfed by densities found on oyster reef breakwaters elsewhere, including breakwaters constructed with similar materials in comparable estuarine environments (Morris et al., 2021; Wellman et al., 2021). There are multiple hypotheses as to why Alabama oyster reef breakwaters have to date underperformed compared to other systems; our goal is to identify the specific limiting factors in our local ecological context and locate optimal elevations for oyster settlement and survival at local restoration sites.

This study would build upon a preliminary study that assessed patterns of oyster settlement and post-settlement survival across an intertidal gradient at two Alabama living shoreline sites (Notz et al., 2023). The previous study used a novel oyster tile design that deployed tiles across an intertidal gradient and permitted retrieval for sampling while maintaining each tile’s intertidal treatment. We found very high rates of oyster mortality, but oyster survival was generally greatest at intertidal locations. We identified a mismatch at the Coffee Island Living Shoreline site; Coffee Island featured smaller, subtidal breakwaters that did not provide substrate at the intertidal elevation we identified as optimal for oysters. Taller emergent breakwaters, such as those being implemented in the upcoming Coffee Island restoration, may improve oyster survival and oyster reef development. In terms of the potential drivers, most of the mortality was likely attributable to predation, especially by oyster drills on more inundated tiles, and thermal and exposure stress, especially for oysters on the more exposed tiles. This suggests that oysters face a variety of environmental stressors and that may need to be ameliorated for successful oyster restoration.

This study would build upon the previous work in multiple important ways: (1) Unlike the previous study, we plan to incorporate predator exclusion cages in our study design. These cages would be incorporated into the existing intertidal tile pole design and help distinguish between mortality from predation versus from other sources like thermal stress. (2) We plan to include two new sites across Portersville Bay plus one repeat site. We have identified multiple locations within a short geographic distance with differing salinity regimes and predator regimes. We hypothesize that salinity dictates the distribution of common oyster predators such that we will observe differing patterns of oyster settlement and survival at the different sites.

Depending on the local limiting factors we identify, restoration managers will be able to make specific decisions to improve the likelihood of successful oyster restoration. For example, if predation is a limiting factor, restoration managers should consider remote setting oyster variants that better withstand predators (Belgrad et al., 2023). If we identify a favorable inundation range and elevation for oysters, restoration managers can design breakwaters with a crest height at or greater than that elevation to provide favorable substrate for oysters. If we identify unsuitable water quality or predator regimes, restoration managers may locate sites with favorable local conditions and prioritize these sites for oyster restoration. Importantly, if it is not feasible to support oysters in a particular environment due to unsuitable local conditions, restoration managers may select breakwater designs that do not prioritize oyster settlement or depend on oyster recruitment. For example, we are monitoring multiple restoration sites that include degraded breakwaters that have lost their structural integrity due to a lack of oyster recruitment (e.g. Alabama Port, Coffee Island, Point aux Pines); these sites are now more exposed to wave attack. Such sites would be better protected by alternative breakwater designs such as pre-cast concrete that can function regardless of oyster reef development. Finally, this framework of relating oyster ecology to specific restoration decisions is a significant asset for oyster restoration causes. We envision future work assaying oyster recruitment and survival across the northern Gulf of America to (1) inform oyster habitat suitability models and (2) develop an oyster restoration decision tree, providing restoration managers with the tools needed for the most effective restoration outcomes.

The proposed work would leverage and complement ongoing funded research. The Baker Lab is presently monitoring Alabama Living Shoreline sites as part of NOAA RESTORE Living Shorelines Comprehensive Monitoring Project and the NOAA IIJA Coffee Island Restoration Project. These projects involve monitoring the densities of oysters on Living Shoreline breakwaters to compare their efficacy in developing oyster reefs. However, the current scope of work would not be able to determine the ecological drivers underpinning the variable oyster densities. Furthermore, monitoring oysters on existing breakwaters would not provide information on potential oyster substrates that are not currently available onsite, i.e. at elevations above the breakwater heights. The proposed research would provide information on specific environmental drivers of oyster densities on Living Shoreline breakwaters, including at substrates not currently available on existing breakwaters. The NOAA RESTORE and IIJA grants will provide additional personnel, materials, and boat days to conduct the proposed research. Additionally, Co-PI Bland is fully funded off of the NOAA RESTORE project and is therefore able to conduct and present the proposed research.