Estuaries and Coasts (2022) 45:2675–2689 https://doi.org/10.1007/s12237-022-01094-6 Use of Biodegradable Coir for Subtidal Oyster Habitat Restoration: Testing Two Reef Designs in Northwest Florida Becca Hatchell1 · Katie Konchar1 · Maria Merrill1 · Colin Shea2 · Kent Smith1 Received: 23 June 2021 / Revised: 26 May 2022 / Accepted: 27 May 2022 / Published online: 27 July 2022 © Coastal and Estuarine Research Federation 2022 Abstract Oyster reefs are among the most threatened habitats in the world having suffered cosmopolitan decline, and studies evaluating reef construction materials and designs are critical to their successful restoration and management. Current restoration practice commonly employs the use of high-density polyethylene (HDPE) plastic materials to contain oyster shell (cultch); however, as scientists begin to understand more about the problematic ecological and health effects of microplastics in marine environments, testing alternatives to these materials has become increasingly important. In this study, we used biodegradable coconut fiber (coir) materials to construct a network of subtidal oyster reefs and evaluate two reef designs in West Bay, St. Andrew Bay, Florida. These designs differed in the quantity of cultch used and therefore overall reef height. Through an analysis of changes in reef area and reef height, as well as mollusc coverage, density, and size-frequency distribution over a 5-year period, we compare the performance of low- and high-profile reef construction designs and assess the suitability of coir for subtidal oyster reef restoration. Results indicate that a high-profile reef design involving a perimeter wall of coir oyster bags and a loose cultch interior is suitable for creating oyster reef habitat in the low-wave energy, subtidal conditions of St. Andrew Bay. Coir adequately contained cultch until live oysters could colonize the surface, indicating a viable alternative to using HDPE plastic materials in subtidal oyster reef restoration. Results also show the importance of reef height to sustaining oyster habitat at restoration sites subject to mobile sediments. Keywords Coconut fiber · Cultch · Oyster reef · Plastic · Reef height · St. Andrew Bay Introduction In the USA, Eastern oysters (Crassostrea virginica) naturally inhabit Atlantic and Gulf of Mexico (GOM) nearshore estuarine systems where freshwater input moderates water salinity (zu Ermgassen et al. 2012; VanderKooy 2012). Known as ecosystem engineers, oysters build their own three-dimensional habitat and provide habitat for a variety of invertebrate and fish species. They also provide vital ecosystem services such as Communicated by Eric N. Powell * Becca Hatchell Becca.Hatchell@MyFWC.com 1 Division of Habitat and Species Conservation, Florida Fish and Wildlife Conservation Commission, 620 South Meridian Street, Tallahassee, FL 32399, USA 2 Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, 100 8th Avenue Southeast, St. Petersburg, FL 33701, USA water filtration, wave energy reduction, and shoreline protection (Coen et al. 2007). Recent estimates quantify that 85% of the world’s oyster reefs have been lost due to pollution, overfishing, and habitat loss (Beck et al. 2011). Despite significant declines, oyster habitat in the GOM remains in fair condition and is considered the best opportunity for oyster conservation and sustainable fishery management on a large-scale (Beck et al. 2011). Restoration and management efforts to improve oyster populations have increased since 1999, with over 260 subtidal reef sites constructed or restored in the GOM alone (La Peyre et al. 2014). Following the 2010 Deepwater Horizon oil spill, $8.1 billion was allocated to replenish and protect coastal and marine resources including oyster and seagrass habitat within the GOM and surrounding coastal systems (DWH Trustees 2016). Efforts to understand the effectiveness of various reef materials and designs have become important topics for the future of oyster reef restoration. The main building materials for oyster reef restoration efforts replicate or mimic natural oyster shell (cultch). 13 Vol.:(0123456789) 2676 Composed largely of calcium carbonate, cultch provides the appropriate chemical signature for the settlement and attachment of oyster larvae (Crisp 1967). Access to cultch is limited however, as oyster populations and harvests have declined globally (Beck et al. 2011). As oyster habitat and fishery restoration efforts have increased, what cultch is available has become increasingly scarce and expensive (Goelz et al. 2020; Levine et al. 2017). As a result, new construction techniques using less cultch and/or alternate materials are growing in popularity (e.g., Goelz et al. 2020). If cost-effective designs are successful, managers can restore, enhance, or create larger areas of oyster reef and increase ecosystem services for significantly less cost. In addition, oyster habitat construction using cultch often requires the use of materials or structures that will contain and secure the cultch until living oysters are able to stabilize the substrate. For the State of Florida, the Department of Environmental Protection (FDEP) regulates the restoration, establishment, and enhancement of low profile oyster habitat via several rules adopted under Florida Administrative Code. Specifically, Rule 62–330.632 (2018) requires that oyster reef materials “be firmly fixed on the substrate, bagged, or otherwise contained in such a way to prevent movement away from the [project] footprint.” The most commonly used material to contain cultch is ultraviolet-resistant, aquaculture-grade, high-density polyethylene (HDPE) plastic mesh matting or bags. The widespread use of HDPE plastic is of ecological concern as it can break down into microplastic particles that persist in the marine environment (Hidalgo-Ruz et al. 2012). As filter feeders, bivalves such as oysters are especially susceptible to microplastic exposure. In an investigation on the level of microplastic exposure in wild Eastern oysters on Florida’s Atlantic coast, Waite et al. (2018) found an average of 16.5 pieces per adult individual, more than the amount found in other bivalves studied. When exposing Pacific oysters (Crassostrea gigas) to a cocktail of plastics commonly used in aquaculture, including HDPE, polypropylene, and polyvinyl chloride, Bringer et al. (2021) demonstrated reductions in settlement success and spat growth. In a similar laboratory study, Sussarellu et al. (2016) demonstrated reductions in Pacific oyster egg production, sperm motility, larval survival, growth rates, and overall development after exposure to polystyrene microspheres. While these authors are unaware of direct evidence demonstrating that HDPE plastic mesh bags used in oyster restoration provide a measurable source of microplastics to the environment directly surrounding oyster habitat they are used to create, there are many other sources of HDPE plastic pollution. Thus, the use of alternative, biodegradable materials for oyster habitat restoration presents an opportunity for practitioners to reduce the overall input of HDPE plastic to the marine environment. Coconut fiber or (coir) is a renewable, biodegradable natural fiber commonly used 13 Estuaries and Coasts (2022) 45:2675–2689 for erosion control and living shorelines (Partnership for the Delaware Estuary 2013; Barry et al. 2015). For example, Orange County Coastkeeper placed 20 tons of Pacific oyster shell in hand-sewn coir fiber bags to create a living shoreline in Upper Newport Bay, California (Herr 2021). Coir also has applications for seagrass restoration (Beth Fugate, 01/25/2022, FDEP, personal communication). With a higher lignin and lower cellulose content than other natural fibers, coir offers desirable properties such as strength and resistance to weathering (Thyavihalli Girijappa et al. 2019). In a comparative study evaluating several biodegradable alternatives to aquaculture grade plastic, coir showed minimal wear and was deemed a recommended replacement for plastic in living dock applications on the east coast of Florida (Soucy 2020). However, there is limited research demonstrating the success and limitations of coir for oyster habitat restoration in subtidal conditions. Study Aims As part of a larger effort to restore lost seagrass habitat in West Bay, St. Andrew Bay, the Florida Fish and Wildlife Conservation Commission (FWC) created a network of subtidal oyster reefs using two different oyster reef construction designs involving biodegradable coir with varied quantities of cultch. In doing so, this study tested (1) the suitability of coir for subtidal oyster reef construction and (2) the effectiveness of a low versus high-profile reef design. We hypothesized that coir materials would persist in the low-energy system of our project site long enough (e.g., 6–12 months) to effectively contain loose cultch until live oysters could settle, accrete, and successfully stabilize the reef substrate. We also hypothesized that, if the low-profile reef construction design was successful at establishing reef structural and functional ecological services, managers could restore, enhance, or create oyster reef habitats using significantly less cultch. Methods Site Selection Located in Northwest Florida, West Bay is the western arm of the St. Andrew Bay estuary and has experienced a variety of anthropogenic impacts ranging from the construction of the Gulf Intracoastal Waterway in 1938, commercial shrimp farming in the 1970s, and point source discharges of wastewater effluent from 1971 to 2011 (Brim and Handley 2007; NWFWMD 2017). These effects likely contributed to the degradation of estuarine habitats, with especially large losses of seagrass (approximately 7.5 ­km2 since 1953) documented in West Bay (Brim and Handley 2007). Seagrass habitat is often considered an indicator for the health of a Estuaries and Coasts (2022) 45:2675–2689 system (Madden et al. 2009). Thus, documented increases in West Bay seagrass coverage from 1992 to 2010 followed by steady coverage from 2015 to 2017 (Carlson et al. 2020) provided impetus for the FWC to construct subtidal oyster reef habitat for the purposes of reducing stressors preventing further seagrass recovery (i.e., wave energy; Carlson et al. 2020) and fostering continued water quality improvements. The western shoreline of West Bay (see Fig. 1) was selected due to the known presence of large amounts of oyster spat and limited amount of hard substrate for oyster recruitment. Site suitability was confirmed via a 15-month oyster recruitment study (June 2011–Sept 2012) which indicated a ready supply of oyster spat from elsewhere in the system, identified peak oyster recruitment periods (spring and fall), and guided the selection of suitable depths (0.6 to 1.2 m MLW) for oyster survival. The FWC worked in concert with the Bay County Oystermen’s Association to identify all existing commercially harvestable oyster reefs and avoid impacts to those areas during the placement of new oyster habitat. With the support of local oyster harvesters, FWC then restricted oyster harvesting from the restoration project area (per Florida Administrative Code Rule 68B27.0175) with the intent that the newly constructed reefs would contribute spat to commercially harvestable oyster habitat elsewhere in the system in the long term. Reef Design and Installation Mimicking the morphology and orientation of string reefs (long reefs positioned perpendicular to tidal flow) that occur naturally throughout the eastern oyster’s range (Colden et al. 2016), fourteen curvilinear, subtidal oyster reefs were constructed in September 2015 using two oyster reef construction designs: oyster bag reefs and oyster mattress reefs. Both reef types were composed of biodegradable coir and recycled, cured cultch (~ 7.6–15.2 cm in shell height) obtained from shuck houses on adjacent Apalachicola Bay. To avoid historic seagrass habitat extent, all reefs were installed along the 1.2 to 1.8 m (MLW) contour, a depth slightly greater than the range tested during the feasibility study (0.6 to 1.2 m MLW) but one that followed the historic seagrass habitat boundary. Reef placement did not overlay or otherwise affect any existing oyster or seagrass habitat in the project area. Reef units were separated by gaps averaging 43 m north to south and 24 m east to west to allow for adequate water flow and wildlife passage. Seven oyster bag reefs (approximately 35 m length by 6 m width) were constructed using a high-profile, loose shell fill design (Fig. 2a). Woven oyster bags (60 cm length by 25 cm width) with no less than 58% (600 g/m2) and no more than 65% (460 g/m2) weave openness were filled with approximately 19 L of cultch and tied shut using coir string. To construct oyster bag reefs, two oyster bags were stacked along 2677 the entire reef perimeter to create an approximately 25 cm high and 25 cm wide outer wall. Self-contained underwater breathing apparatus (SCUBA) divers then pushed 0.9 m rebar stakes through each stack of two bags (one approximately every 0.6 m) and into the substrate to secure their position. A long reach excavator was used to deposit loose cultch to a desired height of 25–35 cm inside each oyster bag reef perimeter wall. Seven oyster mattress reefs (approximately 45 m length by 4.5 m width) were constructed using a low-profile design (Fig. 2b). Woven oyster mattresses were assembled from two coir layers (4.5 m length by 1.5 m width) with no less than 58% (600 g/m2) and no more than 65% (460 g/m2) weave openness and filled with a thin layer of cultch at a density of 130–190 shells per square meter. The woven coir layers were tied together to enclose cultch material by weaving coir string along each mattress seem edge. To construct oyster mattress reefs, individual mattresses were placed in the water by a long-reach excavator, unrolled, positioned parallel and adjacent to the previously placed mattress, and then secured to the substrate with 1.2 m rebar stakes by SCUBA divers. Once installed, oyster mattress reefs were no greater than 25 cm in height and did not overlap more than 15 cm in any direction. Monitoring Coir material was assessed qualitatively during monitoring events by manually grasping individual fiber strands and hand pulling gently to test tensile strength and document the degree of decomposition in situ. Degree of decomposition was observationally described and noted as fiber strands present and mesh intact; present but mesh able to be pulled apart; present but no longer in mesh formation; or no longer present. Oyster reef habitat monitoring closely followed the methods outlined by Baggett et al. (2014) and was completed annually for a minimum of three and, for a few metrics, up to 5 years following construction. Metrics assessed include reef areal dimensions, reef crest height, and mollusc percent coverage, density, and size-frequency. (Following the completion of this study and during later phases of the project, it was discovered that common slipper shells (Crepidula fornicata), slipper snails (Crepidula depressa), and common jingle shells (Anomia simplex) were being grouped with Eastern oyster (Crassostrea virginica) density, size-frequency, and percent coverage measurements. Due to uncertainty about how often this occurred during this study, we use the more generic term “mollusc” instead of “oyster” for the data reported herein. Although Eastern oysters were the primary species observed throughout the project, similar physical characteristics and geographic distributions among these molluscs prevented data separation by species.) 13 2678 13 Estuaries and Coasts (2022) 45:2675–2689 Estuaries and Coasts (2022) 45:2675–2689 ◂Fig. 1 Fourteen project reefs were installed along the western shore- line of West Bay, St. Andrew Bay, in Northwest Florida. Seven reefs were constructed using the oyster bag design and seven using the oyster mattress design Reef areal dimensions were measured annually for three years post construction (2016–2018) by running a vessel equipped with side-scan sonar (Model: Humminbird Helix 12 CHIRP GPS SI) parallel with each reef (n = 14) while a sub-meter accuracy Global Positioning System (GPS) unit collected continuous location readings. Reef crest height was measured annually for 3 years post construction by recording measurements every 5 m along the length of each reef at the approximate crest (n = 14). Mollusc percent coverage was simultaneously measured by tossing three (0.25 by 0.25 m) quadrats haphazardly at the same 5-m intervals and visually estimating live coverage within each quadrat. Additional reef crest height and mollusc percent coverage data were collected at 5 years post construction (2020) from a subset of mattress and bag reefs (n = 8). Low visibility water conditions made individual mollusc density and size measurements difficult to monitor in situ. Instead, a removable monitoring unit was constructed and deployed directly adjacent to the southern tip of each reef at the time of construction (2016; n = 14). Each monitoring unit consisted of a 0.25 by 0.25 m polyvinyl chloride (PVC) frame. A stiff panel of plastic poultry/garden fencing was attached across the span of each frame using cable ties. Four PVC stakes were attached to each frame corner to secure the unit to the sandy substrate. Each removable monitoring unit was designed to mimic the reef design on which it was placed and involved either the attachment of an individual oyster bag or a 0.25 by 0.25 m oyster mattress to the mesh panel using cable ties. Monitoring molluscs within the removable units was destructive, and they could not be redeployed for subsequent monitoring. As such, following the first year of post construction monitoring (2016), newly designed units were deployed on six randomly selected oyster bag reefs. These new, re-deployable monitoring units were constructed using plasticized 12.5-gauge 2.5 cm steel mesh panels placed on the bottom and four sides (approximately 13 cm in height) of each individual unit. Each monitoring unit was then filled with approximately 38 L of cultch from the previously undisturbed and adjacent reef to mimic the loose cultch centers of oyster bag reefs. Monitoring for mollusc density and size-frequency continued on three bag reefs at 2 years post construction (2017) and a separate set of three reefs at 3 years post construction (2018). New monitoring units were not redeployed adjacent to oyster mattress reefs due to difficulties in replicating the mattress reef design. Mollusc density was measured by counting the total number of live molluscs from a standardized volume of material 2679 contained within each monitoring unit. These data were then used to estimate the total number of live molluscs per square meter. To assess size-frequency distribution, a caliper or ruler was used to measure the shell height of up to 50 molluscs from each monitoring unit sample (per Baggett et al. 2014). Statistical Analysis To assess among-year differences in reef area and height, we fit linear regression models, and for percent coverage, we used a beta regression with a logit link function for oyster bag and mattress reefs. We fit three candidate regression models for each of three response variables. For each response variable, the first model included reef type, year, and their interaction; the second included reef type and year, excluding their interaction; and the third model included only reef type. We note that for the percent coverage analysis, we transformed the percent coverage response vari[y×(N−1)+0.5] following Smithson and Verkuilen able as y� = N (2006) due to a single observation of 100% coverage, which was not permitted under a beta distribution. Here, y was the observed mean percent coverage and N was the number of observations. All predicted probabilities from the beta regression models� were back-transformed to the original [y ×(N)−0.5] scale (i.e., y = ). To assess differences in the N−1 number of live molluscs between oyster bag and mattress reefs in 2016 (the only year in which both reef types were sampled), and among-year differences in the number of live molluscs in bag reefs from 2016 to 2018, we fit two sets of negative binomial regression models. In all negative binomial regression models, we included survey area as an offset such that parameter estimates were expressed as the mean number of live molluscs per square meter; further, we fit an additional model that included reef type (model set 1: oyster bag vs. mattress reefs in 2016) and year (model set 2: oyster bag reefs from 2016 to 2020) in the dispersion portion of the negative binomial regression model to account for between-reef and among-year differences, respectively, in overdispersion. We used Akaike’s information criterion (AIC; Akaike 1973) with a small-sample bias adjustment (AICc; Hurvich and Tsai 1989) to assess the relative support of candidate models, with lower AICc values indicating a better supported model (Burnham and Anderson 2002), and we based all inferences on the best-supported models (i.e., lowest AICc). Where applicable, we assessed among-year differences by examining 95% confidence intervals associated with post hoc, pairwise contrasts based on marginal means as implemented in the R package “emmeans” (Lenth 2019). We fit all linear, beta, and negative binomial regression models in R v3.6.1 (R Core Team 2019) using the R package “glmmTMB” (Brooks 13 2680 13 Estuaries and Coasts (2022) 45:2675–2689 Estuaries and Coasts (2022) 45:2675–2689 ◂Fig. 2 a Oyster bag reefs were constructed using coir oyster bags filled with cultch to build a containment wall for centralized loose cultch fill material. b Oyster mattress reefs were constructed using two coir layers filled with a layer of cultch et al. 2017). Lastly, for all models, we assessed goodness-offit using a simulation-based residual assessment approach, implemented in the R package “DHARMa” (Hartig 2020). We used a two-sample, non-parametric Kolmogorov–Smirnov (KS) analysis to compare the size frequency distributions of oysters collected on oyster bag versus oyster mattress reefs in 2016. Similarly, we conducted three additional KS tests to compare the size frequency distributions of molluscs in bag reefs among years (2016, 2017, and 2018). Results Evaluation of Coir Fiber Coir fibers maintained their structural integrity and ability to contain cultch (i.e., fiber strands present and mesh intact) for an average of 9 months post construction on both reef types evaluated. Following that period, coir remained present but appeared brittle and was easily torn when pulled by hand to test tensile strength. Other physical disturbances to coir material were also observed, for example tears in oyster bags and burrows through oyster mattress layers. Five years following reef installation, coir fiber was mostly absent from oyster bag reefs but remained persistent underneath the sediment overlaying oyster mattress reefs. At that time, macroalgae was also observed throughout the mattress reefs utilizing the coir fiber as an attachment point. Goodness of Fit For all linear, negative binomial, and beta regression models described below, the simulation-based assessments of residuals based on the best-approximating models indicated that all models provided an adequate fit to the data, with no evidence of unexplained patterns in model residuals owing to non-normality, heteroscedasticity, or overdispersion (where relevant). Reef Areal Dimensions and Reef Crest Height The best-approximating linear regression model for reef area included reef type, year, and their interaction. Parameter estimates and post-hoc contrasts indicated that although reef area remained relatively constant for mattress reefs from 2016 to 2018 (i.e., no significant differences among years), reef area 2681 increased steadily for bag reefs over the same time period, with significant differences among all 3 years (Fig. 3a). From 2016 to 2018, mean bag reef area increased by 27%, whereas mattress reefs did not show a significant increase. The best-approximating linear regression model for reef crest height included reef type and year, with no interaction. Parameter estimates and post-hoc contrasts indicated indeterminant reef crest height adjustment between 2016 and 2017 for both bag and mattress reefs, although both reef types trended toward an increase in height. Both reef types decreased in height significantly by 2018 and again in 2020 (Fig. 3b). Model-based estimates indicated that bag reefs, measured at an average height of 22 cm in 2016, declined by an average of 14% (to 19 cm) by 2018 and 27% (to 16 cm) by 2020. Mattress reefs, measured at an average height of 8 cm in 2016, declined by an average of 28% (to 5.7 cm) by 2018 and 65% (to 3 cm) by 2020. During field monitoring, cultch was observed outside the oyster bag perimeter wall and underneath accumulated sediment in lower elevation portions of the interior of bag reefs. Sediment had accumulated across the entire surface of mattress reefs by 2020, however, making them difficult to locate with only a minimal number of live oyster clusters present. Mollusc Density Model Set 1: Bag and Mattress Reefs in 2016 The best-approximating negative binomial regression model relating reef type to the total number of live molluscs included reef type and a single overdispersion term (i.e., overdispersion was similar between bag and mattress reefs in 2016). Parameter estimates from the best-approximating model indicated that the mean number of live molluscs was significantly higher in mattress reefs (mean 6459 per ­m2, 95% CI 5548–7519; Fig. 3c) compared to bag reefs (mean 4633, 95% CI 3972–5403; Fig. 3c) in 2016. Model Set 2: Bag Reefs in 2016–2018 The best-approximating negative binomial model relating year to the total number of live molluscs in bag reefs included the additive effects of reef type and year, along with a year term in the dispersion model (i.e., overdispersion in bag reefs varied among years). Parameter estimates and post-hoc contrasts based on the best-approximating model indicated that the mean number of live molluscs in bag reefs declined significantly from 2016 (mean 4633, 95% CI 3942–5444) to 2017 (mean 2879, 95% CI 2196–3776) and further in 2018 (mean 1381, 95% CI 830–2300; Fig. 3c). 13 2682 Estuaries and Coasts (2022) 45:2675–2689 Fig. 3 For all figures pictured, dark gray coloration represents oyster bag reefs and light gray represents oyster mattress reefs, error bars represent 95% confidence intervals and lettering in parenthesis indicates among-year and among-reef-type differences based on post-hoc contrasts. a Mean reef areal dimensions in bag and mattress reefs in 2016, 2017, and 2018. b Mean reef crest height in bag and mattress reefs in 2016, 2017, 2018, and 2020. c Mean number of live molluscs per square meter in bag and mattress reefs in 2016 and in bag reefs in 2016, 2017, and 2018. d Mean percent coverage of live oysters estimated within 0.25 by 0.25 m quadrats in bag and mattress reefs in 2016, 2017, 2018, and 2020 Mollusc Percent Coverage Mollusc Size‑Frequency Distribution The best-approximating beta regression model included reef type, year, and their interaction. Parameter estimates and post-hoc contrasts indicated that both bag and mattress reefs experienced substantial annual declines in mean live mollusc percent coverage from 2016 to 2020. In bag reefs, backtransformed model-based predictions indicated that mean percent coverage declined from 93.4% in 2016 to 5.5% in 2020, whereas mattress reefs declined from 36.5 to 1.8% live mollusc coverage over the same 5-year study period (Fig. 3d). The KS analysis used to test for a difference in the size distribution of live molluscs between bag and mattress reefs indicated that the two reef types did not differ significantly in 2016 (D = 0.12, P = 0.1608) (Fig. 4a, c, and e). The KS analyses used to test for among-year differences in the size distribution of molluscs for bag reefs only indicated that the size distribution in 2016 was significantly different from 2017 (D = 0.61, P < 0.001) and 2018 (D = 0.67, P < 0.001), and 2017 was significantly different from 2018 (D = 0.21, P < 0.001) (Fig. 4b, d, and f). Over the 3-year survey period, 13 Estuaries and Coasts (2022) 45:2675–2689 2683 Fig. 4 Size-frequency histogram for molluscs collected from a oyster bag reefs (dark gray) and oyster mattress (light gray) reefs in 2016, and b bag reefs in 2016 (dark gray), 2017 (medium gray), and 2018 (light gray). Shell height box plot for molluscs collected from c oyster bag reefs (dark gray) and oyster mattress (light gray) reefs in 2016, and d bag reefs in 2016 (dark gray), 2017 (medium gray), and 2018 (light gray). Cumulative size-frequency plot for molluscs collected from e oyster bag reefs (dark gray) and oyster mattress (light gray) reefs in 2016, and f bag reefs in 2016 (dark gray), 2017 (medium gray), and 2018 (light gray) the mean height of molluscs on bag reefs declined from 47 to 14 mm (Fig. 4d). coir bag persistence 2 years post-installation (Brianna Group, 02/04/2022, TNC, personal communication). Even though neither project directly compared coir fiber to HDPE plastic in situ, these results provide evidence that coir may be a suitable replacement for HDPE plastic oyster bags in low-energy, subtidal environments. Shorter periods of performance may be expected in intertidal conditions, however. Restoration projects along Florida’s intertidal zones indicate coir oyster bags persist up to 6 months along the high energy Atlantic coast (Annie Roddenberry, 11/29/2018, FWC, personal communication) and up to 8 months along the lower energy Gulf Coast (Corey Anderson, 11/29/2018, FWC, personal communication). These studies illustrate that coir fiber functionality for cultch containment is dependent upon site conditions. The successful use of coir is also subject to overall design elements and environmental dynamics. Stressors that place strain on fiber strands such as those characteristic of the Discussion Suitability of Coir Fiber for Reef Restoration This study qualitatively evaluated the use of coir as a temporary cultch containment material for oyster reef restoration in the low-energy, subtidal conditions of West Bay. Results show that coir maintained its structural integrity and the ability to contain cultch for 9 months post construction, a period sufficient for oysters to recruit to and stabilize underlying cultch material. In similar low-energy, subtidal conditions of Great Bay, New Hampshire, The Nature Conservancy (TNC) deployed coir oyster bags in 2019 and has observed ongoing 13 2684 intertidal zone (i.e., direct exposure to ultra-violet rays, the wet/dry regime of tides, and erosive wave forces) should be considered when evaluating suitable applications. To address this limitation, the authors recommend that coir fiber be used in cases where desired organismal recruitment is anticipated prior to the expected timeframe of material degradation. For example, coir fiber oyster bags should be deployed concurrent with the peak oyster settlement period in the system to ensure organismal recruitment and stabilization of contained cultch. Reef Areal Dimensions and Reef Crest Height Reef area and reef height metrics provide valuable insight into the persistence and condition of the restored area over time. They provide information about substrate movement such as subsidence or spreading as well as the quality of ecosystem services and habitat provided for associated resident and transient species (Baggett et al. 2014). Decreases in reef height post construction may be caused by spreading of cultch, sedimentation, subsidence (e.g., Powell et al. 2006) or a combination of these factors (Coen et al. 2011). The simultaneous increase in area and decrease in height of reefs constructed using the oyster bag design suggests that cultch material spread over the course of this study. Following an initial pulse in oyster recruitment and growth, oyster density and percent coverage declined. Without continuous live oyster growth, exposed cultch is susceptible to movement via wave action, bioerosion from boring organisms (e.g., clionid sponges), and sedimentation, all factors which can drive reef degradation (Pace et al. 2020; Powell et al. 2006). In contrast with bag reefs, mattress reefs did not show a significant change in overall areal dimensions but declined in reef crest height to 3 cm relative to the surrounding substrate. While coir fiber effectively contained cultch deployed on mattress reefs, the lower profile of the mattress reef design likely resulted in a higher rate of sediment deposition. This is supported by field observations made at the conclusion of the study. While bag reefs were easily located and visually recognizable by the presence of live oyster clusters and an exposed foundation of cultch, mattress reefs were difficult to locate without GPS assistance and recognizable only by the presence of macroalgae attached to coir matting. Mattress reefs had very few live oyster clusters and no exposed foundation of cultch. Once buried by sediments, cultch contained within oyster mattresses and placed at or near the substrate likely succumbed to degradation through natural taphonomic processes (Powell et al. 2006). This study supports others that have shown reef height to be a driver of degradation or persistence by influencing sedimentation buildup and the settlement and survivorship of oyster spat (e.g., Colden et al. 2016; Lenihan 1999). Reef profile influences flow speed with higher reefs providing greater 13 Estuaries and Coasts (2022) 45:2675–2689 vertical mixing, lessening sediment deposition, and increasing food resources relative to lower profile reefs, thus increasing oyster density (Lenihan 1999; Housego and Rosman 2016). Oyster reefs in Chesapeake Bay built to heights greater than 30 cm showed oyster densities almost four times higher than those under 30 cm (Colden et al. 2017). Similar results have been obtained for higher profile reefs in Virginia whose oyster densities were five times larger than for lower reef profiles and were less affected by disturbances (Housego and Rosman 2016). Greater reef height also increases the potential for habitat complexity (e.g., Henderson and O’Neil 2003) and specifically, interstitial space within the reef structure, a key factor for oyster recruitment and survival (Lavan 2019). Even though various site-specific biotic or abiotic variables can affect the success or failure of oyster reefs, this study further supports the role that height plays in reef sustainability. The higher profile oyster bag reef design exhibited greater resiliency to the stress of sedimentation in the West Bay system. However, adaptive modifications to the design should test the deployment of cultch at great heights, for instance above the 30 cm threshold recommended by Colden et al. (2017). Mollusc Density Live oyster density relays vital information about oyster recruitment and survivorship and therefore overall population size. To allow for the easy removal and measurement of reef materials deployed in low visibility waters, removable monitoring units were built to mimic bag and mattress reefs to the closest extent possible. Due to challenges described above with replicating the oyster mattress design in a removable monitoring unit, live mollusc density could only be quantified on both reef types at one year post construction (2016). Those data indicate that live mollusc density was significantly greater on mattress reefs than on bag reefs at that time. However, the difference in density between the two reef types as quantified from monitoring units was not observed on the reefs themselves. Rather, estimates of percent live mollusc coverage indicate lower values on mattress reefs than bag reefs in all years. The lack of consistency between these two metrics raises further concerns regarding the placement and design of monitoring units. Firstly, the placement of monitoring units adjacent to each reef atop a removable 1 inch PVC frame may have resulted in a slightly higher elevation for oyster mattress units than the low-profile oyster mattress reefs themselves. Due to the influence of reef height discussed above, this may have contributed to artificially high live density values within oyster mattress monitoring units (e.g., 140% higher than bag reefs). Secondly, to construct oyster bag monitoring units, cultch was added to an individual oyster bag, which was then tied shut and lifted onto the mesh panel of each monitoring unit. During this process, the weight of the cultch settled to the bottom Estuaries and Coasts (2022) 45:2675–2689 and left an excess of loose coir fiber bunched at the top of each bag. Once placed in the water, this excess coir fiber was observed by staff to cover a large portion of each oyster bag monitoring unit’s upper surface. This may have resulted in less exposed cultch per monitoring unit area than the adjacent oyster bag reef and artificially reduced oyster density counts from oyster bag monitoring units. This issue was not observed on constructed oyster bag reefs, as the cultch and fiber of oyster bags placed along each reef perimeter were spread out evenly to create a contiguous barrier wall. Neighboring St. Andrew Bay to the east, Apalachicola Bay is one of the most studied bays with respect to oyster populations in Northwest Florida (Parker et al. 2013). Despite concerns regarding the accuracy of density counts derived from monitoring units, 2016 live mollusc densities on bag (4633/m2) and mattress (6459/m2) monitoring units were comparable to oyster densities recorded in Apalachicola Bay (nearly 6000 oysters/m2) following the placement of loose fossilized shell on existing commercially harvested reefs (Parker et al. 2019). The declines in density values observed over time on bag reef monitoring units in this study, which fell to 1381/m2 at 3 years post construction (2018), were also comparable to those seen on Apalachicola Bay reefs at 1 year (approximately 1500/m2) and 2 years (approximately 500/m2) post cultch placement (Parker et al. 2019). Following an initial pulse of settlement in small size classes, such declines in total live density are expected as fewer larger oysters survive to effectively colonize and stabilize the substrate. Mollusc Percent Coverage Collection of percent coverage estimates is the only measure of live mollusc settlement available from both reef types across multiple years (2016–2020). While visual estimates of percent coverage are less quantitatively accurate than direct measures of density, percent coverage estimates likely provided a better picture of the overall performance of each reef type due to the challenges with density measurements taken from removable monitoring units discussed above. Although percent coverage declined significantly from initial settlement observations on both reef types, percent coverage remained an average of 2.63 times greater on bag reefs than on mattress reefs over the course of this study. The bag reef design included both a greater volume of shell per unit area and more exposed cultch, thus providing both more interstitial space and surface area available for settlement than the mattress design. Bag reefs were also a higher profile, likely driving higher percent coverage values on bag reefs. These results are consistent with an evaluation of oyster reefs in noharvest sanctuaries; Powers et al. (2009) found oyster densities nearly twice as high at the crest of reefs than the base. 2685 As our mattress reefs were designed in a low-profile (Fig. 2), it is reasonable to describe the mattress reefs with little to no reef crest and thus, having less observed molluscs. While initial live oyster coverage is a direct indicator of how each construction design and material type performed with respect to settlement and early recruitment (Baggett et al. 2014), continued annual assessments provided an overall picture of how each reef design fostered oyster growth as oyster habitat over time. Declines in live mollusc coverage on both reef types occurred concurrent with accumulating sediments observed across the project footprint throughout all years of the study. Project staff also observed the movement and deposition of a large pulse of sediment over the project site following Hurricane Michael in October 2018. These observations were made concurrent with FDEP observations of sediment on oyster reefs throughout the St. Andrew Bay system following Hurricane Michael, but most notably in West Bay, where cultch material was either displaced or buried (FDEP 2020). Hatchell et al. (2022) recorded a median value of 5.65 cm in sediment accumulation along the western shoreline during a 2019–2020 seagrass transplantation study. The stressors of sedimentation in the bay likely contributed to the negligible 1.8% live mollusc coverage values observed on mattress reefs by 2020, leaving little to no substrate exposed for future recruitment events. Shifting sediments have been shown to limit oyster reef development in intertidal systems as well (Taylor and Bushek 2008). Although bag reefs maintained only slightly higher average live coverage values by 2020 (5.5%), the remaining foundation of substrate (16.5 cm on average) indicates that these reefs have the potential to sustain reef growth contingent upon naturally recurring oyster settlement and recruitment events and subsequent oyster survival and persistence within the West Bay system. Without those natural recurring inputs of juvenile oysters however, we recommend oyster reefs contain material with greater resistance to storm energy (e.g., limestone) to ensure persistence during naturally variable oyster spat periods, thus increasing the resiliency of this reef construction design. Mollusc Size‑Frequency Distribution Oyster size-frequency distribution relays information about oyster recruitment as well as survivorship and mortality of individual cohorts over time. Despite a slightly lower mean mollusc shell height on mattress reefs (43 mm) than on bag reefs (47 mm), there was no statistically significant difference in size-frequency distributions between the two reef types at 1 year post construction. These results are comparable to a mean oyster shell height previously recorded for St. Andrew Bay (44 mm; Baggett et al. 2014) and within seven other Florida estuaries (40–45 mm; Parker et al. 2013). Although these results indicate similar patterns of 13 2686 recruitment across both reef designs initially, this was followed by a statistically significant decline in mean shell height on bag reefs over the 3-year study. Decreases in mean shell height can be attributed to low recruitment of molluscs from small to larger size classes, and from juvenile to adult life stages, throughout the course of the study. Oyster populations are naturally temporally variable. Nonetheless, the few numbers of individuals observed in larger size-classes in this study reflects a concerning trend of adult oyster mortality observed elsewhere in the region (e.g., in Apalachicola Bay: Parker et al. 2013; Wang et al. 2008). This is likely attributable to a suite of abiotic and biotic stressors acting in concert within these estuaries. Water salinity and water temperatures in West Bay are typically within the optimal range for adult oysters (see Burrell 1986). However, low recruitment into larger size classes can be caused by extreme salinity fluxes such as those possible following tropical storm events (e.g., Edmiston et al. 2008). Salinity extremes can be further exacerbated by high water temperatures (La Peyre et al. 2013; Rybovich et al. 2016) or associated increases in predator abundance or disease (Menzel et al. 1966). For example, Perkinsus marinus (Dermo), which is abundant across the GOM and most likely to infect adult oysters (Ford and Tripp 1996), is most prevalent during high temperature, high salinity conditions (Soniat 1996). Even though the GOM has seen decreasing trends in Dermo pathogen infection rates from 1995 to 2009 (Apeti et al. 2014), Dermo should not be overlooked as a potential weakening factor (Radabaugh et al. 2019) among many possible causes for the low adult oyster recruitment observed in this study. Despite a low number of individual molluscs recruiting into adult size classes, juvenile molluscs continued to recruit in high numbers to bag reefs through the second and third year of this study. With continued recruitment, even a low abundance of live adult oysters can provide valuable ecosystem services (Powers et al. 2009). The oyster reefs created through this study are anticipated to continue serving as breakwater structures that reduce suspended sediments (see also Lenihan 1999) and improve water quality and in turn, provide ecosystem services directly applicable to the larger effort to restore seagrass habitat in West Bay. Conclusion This study has identified a viable new approach using biodegradable coir for creating or restoring oyster reef habitat in the low-energy, subtidal conditions of St. Andrew Bay. While coir fiber successfully contained cultch material and Eastern oysters successfully recruited to both reef designs initially, the most successful strategy involved installing coir oyster bags around centralized loose cultch built to an 13 Estuaries and Coasts (2022) 45:2675–2689 average reef height of 22 cm. This higher profile oyster bag reef design maintained its structural integrity, continued to recruit oysters, and increased in reef area over the course of this study. It also exhibited greater resiliency to the stress of sedimentation in the West Bay system. Even though this study demonstrates the viable use of coir in a low-energy area, its application should be tested in other subtidal areas subject to a range of environmental conditions. In addition, coir fiber may not be suitable in spat limited systems where live oysters are unable to naturally recruit to and stabilize cultch prior to coir degradation. Finally, as alternative materials such as coir gain popularity in oyster restoration and enhancement projects, we recommend future studies assessing any potential effects of coir microfibers on filter feeders. The higher profile bag reefs continued to provide substrate for oyster settlement and recruitment 5 years postconstruction and in turn, continued to provide ecosystem services beneficial to seagrass recovery in the West Bay system. However, the continuous decline in reef height coinciding with declines in oyster density and percent coverage indicates a need for further research to elucidate causes driving reef degradation over time. Without live oyster growth, the half-life of Eastern oyster shells is known to be 2–10 years across its range (Pace et al. 2020). Mass mortality events caused by stressors such as low periods of dissolved oxygen can lead to shell degradation via a cascading sequence of bioerosion, shell fragmentation, and shell dissolution (Pace et al. 2020). Stacking additional coir fiber oyster bags, placing taller oyster mattresses, or simply adding more loose shell in the interior of the oyster bag reefs would have resulted in a higher structure. Results indicate, however, that reefs constructed even in this way would not likely persist over the long-term with the current conditions in West Bay. We acknowledge several interacting abiotic and biotic stressors likely affecting oyster reef persistence in the St. Andrew Bay estuary. Hence, a larger analysis of those stressors and the metapopulation dynamics of oysters within the St. Andrew Bay estuary is warranted. The FWC has incorporated other lessons learned during this study into the design and monitoring of future project phases. Design adjustments have included the use of limestone as more stable oyster reef substrate coupled with building reefs above 30 cm, the height threshold recommended by Colden et al. (2017). Limestone provides the appropriate calcium content and chemical suitability for oyster settlement, is not readily affected by storm energy, and retains its structural integrity (Goelz et al. 2020) when used to create reefs. These changes to both the base material and constructed reef height are anticipated to support oyster growth and survival under pulsed recruitment regimes and further recruitment success in a system with high sediment deposition. Monitoring adjustments include the use of SCUBA divers to remove live oyster material from constructed reefs Estuaries and Coasts (2022) 45:2675–2689 for a more accurate assessment of oyster density and sizefrequency metrics in situ. Additionally, the use of sonar technology to estimate the height of higher profile reefs has been used to reduce the time required for manual in-water measurements and allowed for repeat assessments of reef crest height during low visibility and/or cold-water conditions. Finally, additional hands-on training in species identification has been provided to project observers who assist in oyster density, size-frequency, and percent coverage surveys. Acknowledgements This project was funded by the National Fish and Wildlife Foundation (Award #8006.14.040897) and the State of Florida’s Marine Resources and Conservation Trust Fund. Special thanks to all volunteers who tirelessly contributed over 1400 h to fill coir oyster bags and build oyster mattresses. We gratefully acknowledge Jacob Berninger for his assistance with monitoring and data management from 2016 to 2018. We also thank Matthew Davis and Annie Roddenberry who contributed valuable time and effort to review and edit previous versions of this manuscript. We also would like to extend our appreciation to Mike Hunter (FWC’s Office of Community Relations) for his artistic assistance on Figs. 1 and 2 and to Chris Anderson (FWC’s Center for Spatial Analysis) for cartographic assistance with Fig. 1. Data Availability Data for this study are available in the FWC Digital Library repository, https://f50006a.eos-intl.net/F50006A/OPAC/Detai ls/Record.aspx?BibCode=5598669. 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