Shell Hash Research Articles

Phascolion kamchatikus sp. nov., the first interstitial sipunculan (Sipuncula, Phascolionidae) from the Pacific Ocean

A new species of meiobenthic sipunculan, Phascolion kamchatikus sp. nov. collected from coarse sand and shell hash in shallow waters in the northwestern Pacific, is described and illustrated using light and electron microscopy. This new species is well distinguished from all other congeners by a unique combination of main taxonomic characters. This is the only sipunculan having only two primary tentacles in both juvenile and sexually mature worms. There are no hooks and holdfast papillae in the adult stages. Because of the presence of three nearly equal in size retractor muscles forming a retractor column divided for most of its length, this species is assigned to the subgenus Isomya of the genus Phascolion. Unlike most other species of the genus Phascolion, P. kamchatikus sp. nov. has an interstitial mode of life moving among the coarse sand grains and shell hash accumulated between rocks and stones in shallow water areas alongside the Pacific coast of Kamchatka, Aleutian Islands and Alaska Peninsula. Phascolion kamchatikus sp. nov. constitutes the only third interstitial species of the phylum Sipuncula. This is also the ninth species of Phascolion in the northwestern Pacific and the first interstitial representative of this genus in the Pacific Ocean.

Effects of Experimental Addition of Algae and Shell Hash on an Infaunal Mudflat Community

Effects of Experimental Addition of Algae and Shell Hash on an Infaunal Mudflat Community

Relating side scan sonar backscatter data to geotechnical properties for the investigation of surficial seabed sediments

Side scan sonar is a common tool for seafloor imaging and surveying due its efficiency and high resolution. The backscatter information from side scan sonar enables to identify sediment types and seabed roughness, often used to study sediment dynamics. Theory suggests that side scan sonar backscatter can be correlated to the geotechnical properties of the seabed surface. This could enhance the prediction of erodibility and efficiency of seabed sediment characterization, considering that side scan sonar can offer large spatial coverage in a short time. In this study, high-frequency (1000 kHz) side scan sonar backscatter data, sediment samples, and in-situ seabed strength profiles were collected of the seabed surface at ten locations. Statistical analysis of the backscatter data compared with geotechnical data showed trends between mean backscatter, soil strength, and textural sediment properties. Generally, mean backscatter increased when sediment strength and mean grain size increased and when water content and fines content decreased. However, roughness from bedforms, the presence of oysters, shell hash as well as variations in water content (i.e., porosity) of the seafloor heavily influenced the backscatter and sometimes masked any relationships with the strength properties directly.

Impacts of infauna, worm tubes, and shell hash on sediment acoustic variability and deviation from the viscous grain shearing model.

Infauna influence geoacoustic parameters in surficial marine sediments. To investigate these effects, an experiment was conducted in natural sand-silt sediment in the northern Gulf of Mexico. In situ acoustic measurements of sediment sound speed, attenuation, and shear speed were performed, and sediment cores were collected from the upper 20 cm of the seabed. Laboratory measurements of sound speed and attenuation in the cores were conducted, after which the core contents were analyzed for biological and physical properties. Since no model currently accounts for the effects of infauna, a deviation from model predictions is expected. To assess the extent of this, acoustic measurements were compared with the viscous grain shearing model from Buckingham [J. Acoust. Soc. Am. 122, 1486 (2007); J. Acoust. Soc. Am. 148, 962 (2020)], for which depth-dependent profiles of sediment porosity and mean grain size measured from the cores were used as input parameters. Comparison of acoustic results with distributions of infauna, worm tubes, and shell hash suggests biogenic impacts on acoustic variability and model accuracy are important in surficial marine sediments. The presence of infauna and worm tubes were correlated with higher variability in both sound speed and attenuation and greater deviation from the model near the sediment-water interface.

Effects of Shell Hash on Friction Angles of Surficial Seafloor Sediments near Oysters

Effects of Shell Hash on Friction Angles of Surficial Seafloor Sediments near Oysters

A 4000-year paleoenvironmental reconstruction and extreme event record from Laguna Nuxco, Guerrero, Mexico

Reconstructing paleoenvironments on Mexico's Pacific coast is crucial to determine past climate conditions and natural hazard periodicities, vital for predicting future activities. Two sediment cores extracted from Laguna Nuxco, a coastal lagoon in Guerrero, were analyzed to provide a 4000-year paleoenvironmental record. Sediments dominated by shelly clay with low water and organic content, high carbonate content, low Ti/Ca ratio and the presence of Anomalocardia subrugosa suggest that a sheltered bay existed from ~4000–~1300 cal yr BP. During this stage, intermittent beach ridge gaps facilitated the deposition of storm-induced shell deposits, likely from tropical cyclones. Frequent thick shell hash deposition toward the top of the lithologic unit indicates heightened storminess likely from an increase in ENSO activity. An intense tropical cyclone, or perhaps multiple storms during an active period deposited a thick shell hash layer at ~1500 cal yr BP, which helped strengthen the beach ridge system and lead to the modern lagoon phase (~1300 cal yr BP to present), as suggested by the presence of Mytella sp. A more isolated lagoon with significant terrestrial sediment input was reflected by the predominance of lagoonal clay with decreased shell and carbonate contents, increased water and organic contents, and increased Ti/Ca ratio. A quiet period with lower tropical cyclone activity occurred from 1300 to 800 cal yr BP and from 300 cal yr BP to the present, as inferred from no or relatively few shell hash deposits, respectively. A period of hyperactivity occurred from 800 to 300 cal yr BP, as evidenced by seven shell hash layers. During the modern lagoon phase, shell hash deposition was not caused by marine inundation processes, but rather ‘blowout’ events driven by intense precipitation during heightened ENSO activity. Findings shed additional light on Late Holocene environmental changes in a dynamic coastal zone related to regional controlling factors such as tropical cyclone activity, ENSO, and beach ridge development. • A 4000-yr reconstruction at Laguna Nuxco uncovers a transition from bay to lagoon. • Thick shell hash units indicate marine deposition via tropical cyclones. • Thin shell hash layers indicate lagoon “blowouts” from heavy precipitation events. • Activity regimes are largely tied to periods of heightened ENSO. • Notable activity periods occurred from 1500 to 1300 and 800–300 cal yr BP.

An evaluation of the efficacy of shell hash for the mitigation of intertidal sediment acidification

AbstractOur objectives were twofold: (1) to determine whether the addition of shell hash to intertidal sediments would mitigate porewater acidification and (2) whether its effectiveness was dependent on the type of sediment as described by organic matter (OM) and particle grain size (PGS). Field experiments were conducted at two sites within Burrard Inlet, British Columbia; Maplewood Mudflats (MM), high in OM and silt and Whey‐ah‐Wichen/Cates Park (WAW), low in OM and an equal PGS among very coarse, coarse, fine sand, and silt. Shell hash was added to triplicate treatment plots matched with triplicate controls at each site and porewater pH measured at flood and ebb tide over eight tidal cycles. Sampling occurred during June and July when tidal cycles were at their maximum inundation and exposure. Porewater pH was significantly greater for ebb versus flood tide and also between sites with MM significantly lower (7.59) as compared to WAW (8.03). Although pH was not mitigated by the shell hash, for WAW, variation in pH was reduced as compared to MM, as indicated by coefficients of variation over the 6‐week sampling period. We suggest that the application of shell hash to reduce the impact of ocean acidification (OA) on intertidal sediments will be site dependent. The combined processes of eutrophication in sediments with high OM and respiration of infauna, especially at high densities, could act in concert with OA to create an intertidal region unsuitable for bivalve larvae settlement and development.

The effects of storms and a transient sandy veneer on the interannual planform evolution of a low-relief coastal cliff and shore platform at Sargent Beach, Texas, USA

Abstract. Coastal cliff erosion is alongshore-variable and episodic, with retreat rates that depend upon sediment as either tools of abrasion or protective cover. However, the feedbacks between coastal cliff planform morphology, retreat rate, and sediment cover are poorly quantified. This study investigates Sargent Beach, Texas, USA, at the annual to interannual scale to explore (1) the relationship between temporal and spatial variability in cliff retreat rate, roughness, and sinuosity and (2) the response of retreat rate and roughness to changes in sand and shell hash cover of the underlying mud substrate as well as the impact of major storms using field measurements of sediment cover, erosion, and aerial images to measure shore platform morphology and retreat. A storm event in 2009 increased the planform roughness and sinuosity of the coastal cliff at Sargent Beach. Following the storm, aerial-image-derived shorelines with annual resolution show a decrease in average alongshore erosion rates from 12 to 4 m yr−1, coincident with a decrease in shoreline roughness and sinuosity (smoothing). Like the previous storm, a storm event in 2017 increased the planform roughness and sinuosity of the cliff. Over shorter timescales, monthly retreat of the sea cliff occurred only when the platform was sparsely covered with sediment cover on the shore platform, indicating that the tools and cover effects can significantly affect short-term erosion rates. The timescale to return to a smooth shoreline following a storm or roughening event, given a steady-state erosion rate, is approximately 24 years, with the long-term rate suggesting a maximum of ∼107 years until Sargent Beach breaches, compromising the Gulf Intracoastal Waterway (GIWW) under current conditions and assuming no future storms or intervention. The observed retreat rate varies, both spatially and temporally, with cliff face morphology, demonstrating the importance of multi-scale measurements and analysis for interpretation of coastal processes and patterns of cliff retreat.

Beyond the single index: Investigating ecological mechanisms underpinning ecosystem multifunctionality with network analysis.

Ecosystems simultaneously deliver multiple functions that relate to both the activities of resident species and environmental conditions. One of the biggest challenges in multifunctionality assessment is balancing analytical simplicity with ecosystem complexity. As an alternative to index‐based approaches, we introduce a multivariate network analysis that uses network theory to assess multifunctionality in terms of the relationships between species' functional traits, environmental characteristics, and functions. We tested our approach in a complex and heterogeneous ecosystem, marine intertidal sandflats. We considered eight ecosystem function, five macrofaunal functional trait groups derived from 36 species, and four environmental characteristics. The indicators of ecosystem functions included the standing stock of primary producers, oxygen production, benthic oxygen consumption, DIN (ammonium and NOx efflux) and phosphate release from the sediments, denitrification, and organic matter degradation at the sediment surface. Trait clusters included functional groups of species that shared combinations of biological traits that affect ecosystem function: small mobile top 2 cm dwellers, suspension feeders, deep‐dwelling worms, hard‐bodied surface dwellers, and tube‐forming worms. Environmental characteristics included sediment organic matter, %mud, %shell hash, and %sediment water content. Our results visualize and quantify how multiple ecosystem elements are connected and contribute to the provision of functions. Small mobile top 2 cm dwellers (among trait clusters) and %mud (among environmental characteristics) were the best predictor for multiple functions. Detailed knowledge of multifunctionality relationships can significantly increase our understanding of the real‐world complexity of natural ecosystems. Multivariate network analysis, as a standalone method or applied alongside already existing single index multifunctionality methods, provides means to advance our understanding of how environmental change and biodiversity loss can influence ecosystem performance across multiple dimensions of functionality. Embedding such a detailed yet holistic multifunctionality assessment in environmental decision‐making will support the assessment of multiple ecosystem services and social‐ecological values.

An Optical Assessment of Weathervane Scallop Density and Abundance Off Kodiak Island, Alaska

Since 2000, information to support the sustainable management of the weathervane scallop Patinopecten caurinus in the Gulf of Alaska is largely derived from fishery-dependent observations from a small fleet (≤8 vessels total). As a supplement to this information, the Alaska Department of Fish and Game collected more than one million high-resolution underwater photographs of the sea floor in five previously unsurveyed scallop beds off Kodiak Island during the spring of 2014. Weathervane scallop distribution, abundance, and size composition in these beds were assessed by analyzing more than 150,000 images subsampled in 50-image strips (stations) using a 100-m grid. In addition, the influence of local abiotic habitat features, including depth, substrate, and shell remains, on weathervane scallop distributions was explored. An estimated 26.8 [95% confidence interval (CI) = 23.5–30.7] million scallops in the five beds, with densities ranging from 0.08 (95% CI = 0.067–0.084) to 0.26 (95% CI = 0.221–0.297) ind.•m–2 among the beds. Shell size compositions differed significantly despite the relative proximity of the beds. Weathervane density was positively related (in order of influence) to gravel, shell debris, shell hash, mud, and cobble. Optical assessment of these five previously unsurveyed scallop beds in the Gulf of Alaska demonstrates the efficacy of this approach for weathervanes and suggests that the expansion of optical survey coverage to include other known beds as a viable path to overcoming the limitations of fishery-dependent information for scallop assessment and management in Alaskan waters.

Small invertebrates in bivalve-cultivated and unmodified habitats of nearshore ecosystems

Many nearshore ecosystems are modified by aquaculture, including bivalve culture to produce food and restore extirpated populations. Small invertebrates in nearshore ecosystems support fundamental ecological processes, but the effects of bivalve culture on invertebrates are incompletely understood. Here, we compared invertebrate assemblages from multiple studies of bivalve-cultivated and unmodified nearshore habitats along the US west coast. In general, unmodified eelgrass and nearby off-bottom culture habitats with eelgrass present were inhabited by a greater abundance, richness, and diversity of epibenthic invertebrates than bottom culture and bare (mud, sand) habitats that both lacked eelgrass. Findings of individual studies suggested: minor differences in epibenthic invertebrate assemblages associated with various aquaculture practices; restoring native oysters to mudflats did not detectably alter epibenthic invertebrate abundances; epibenthic invertebrates were more abundant on shell hash introduced to mudflats than unmodified mudflats; and benthic invertebrates were less abundant, rich, and diverse in habitats cultured on bottom by Manila clams. Considering the range of these patterns, there appears to be potential for coastal communities to restore extirpated bivalve populations or develop bivalve culture practices that meet objectives to grow food while maintaining nearshore ecosystems’ fundamental processes supported by robust invertebrate assemblages.

Lake-stream transition zones support hotspots of freshwater ecosystem services: Evidence from a 35-year study on unionid mussels

Securing adequate supply of high-quality water is of increasing global importance and relies in large part on ecosystem services provided by freshwater biota. Unionid mussels are important keystone species and habitat engineers that shape freshwater ecosystems through water filtration, nutrient cycling and provision of habitats; their rapid global declines result in dramatic losses of ecosystem functions. Maintenance and enhancement of the services they provide depend on the identification of their crucial habitats. Following theoretical assumptions, this study analyses the importance of lake-stream transition zones for unionid mussels, based on data collected in 1984 and 2019 from an undisturbed stream flowing through five consecutive lakes. Mussel distribution matched the distribution of host fish and was strongly influenced by lakes: densities were highest near lake outlets, reaching 290 ind. m−2 (14.7 kg m−2) in 2019, and declined with downstream distance following a negative power function. This pattern was spatially consistent and sustained over time. All six unionid species native to north-central Europe were present, but common species (Anodonta anatina, Unio pictorum, U. tumidus) contributed about 80% of individuals and were responsible for most of the ecosystem services provided by unionid mussels. Estimated 1.9 × 106 mussel individuals inhabiting 3.2 km of stream length filtered a water volume equivalent to the total stream discharge approximately 2.5 times daily. Aggregations of spent shells, up to 17 kg m−2, accumulated downstream of lakes, forming extensive shell and mussel beds, providing habitats and contributing shell hash that improved stream-bed conditions. Globally invasive Dreissena polymorpha was present at low densities and did not spread or increase in abundance, indicating a long-term biotic resistance of the natural native community. Our study underscores the importance of undisturbed lake outlets, longitudinal connectivity of riverine ecosystems, and of common mussel species in maintaining freshwater ecosystem functionality and provision of vital services.

Observed Dispersal of Invasive Yellow Flag Iris (Iris pseudacorus) through a Saline Marine Environment and Growth in a Novel Substrate, Shell Hash

Invasive in North America, Iris pseudacorus is commonly considered a freshwater species; however, I. pseudacorus can tolerate some saltwater, and is observed in brackish coastal habitats such as estuaries and marshes. Despite this tolerance, saltwater depresses I. pseudacorus reproduction, growth, survival, and dispersal ability. This raises the question of whether I. pseudacorus can spread through a marine environment and establish away from sources of freshwater. This study reports the observation of 104 established juvenile I. pseudacorus individuals in coastal habitat on two islands in southern British Columbia, Canada. Two islands that are devoid of aboveground freshwater sources such as streams, rivers, or lakes that connect to marine habitats. Individuals were also observed growing in a novel substrate, shell hash (fragments of bivalve and gastropod shells that accumulate in areas of low hydrological energy). To the best of our knowledge, this is the first confirmed case of I. pseudacorus growing in a high salinity (30 PSU) shell hash environment. Despite the need for more study, this observation suggests that a broader range of coastal habitat may be susceptible for invasion by I. pseudacorus than previously thought.

Interactive effects of shell hash and predator exclusion on 0-year class recruits of two infaunal intertidal bivalve species in Maine, USA

Biotic and abiotic factors affecting survival and growth of recently settled infaunal bivalves not only determine the strength of 0-year class populations, but the structure and function of benthic soft-bottom communities. At mudflats across north-central Casco Bay, a 517 km2 embayment in the western Gulf of Maine, USA, intertidal sediments are generally acidic (pore water pH range = 7.09–7.85), which can negatively affect settlement and subsequent recruitment success of infaunal bivalves due directly or indirectly to shell dissolution. In addition, predation on bivalves by invasive green crabs, Carcinus maenas, and native consumers is intense in this region. Two commercially important infaunal bivalve species occupy these sediments (softshell clams, Mya arenaria; hard clams, Mercenaria mercenaria). Fisheries managers, legislators, and others have suggested that adding crushed shells of M. arenaria to the surface of mudflats can ameliorate negative effects of acidic sediment pore water through chemical buffering. We investigated the interactive effects of modifying surface sediments using crushed and weathered shells of Mya and predator exclusion on abundance and size of 0-year class individuals of these two bivalve species in large-scale plots (9.3 m2) and small-scale experimental units (EU; 182.4 cm2). Field experiments were conducted over three years (170–204 days yr−1; each initiated prior to spawning and continuing well after settlement had ceased). Shell hash in large-scale plots (mean particle size = 19.3 mm) varied across three levels (0, 0.63, and 1.27 kg m−2), and between 0 and 1.27 kg m−2 in EU where shell size varied from 1.9–19.3 mm. Small-scale experiments also used granite chips that, like crushed shells, increased habitat heterogeneity but did not buffer sediments. Density and size of both bivalve species at the end of most field trials were significantly greater in predator-exclusion treatments vs. controls independent of shell treatment. In all trials, neither Mya nor Mercenaria responded positively to the presence of shell additions. Fisheries managers should focus attention on mitigating effects due to predators instead of spreading shell hash to buffer intertidal sediments.

First Pliocene fish otolith assemblage from the Gulf Coastal Plain, Dauphin Island, Mobile County, Alabama, USA

ABSTRACT The first Pliocene teleostean otoliths from the Gulf Coastal Plain are described from Dauphin Island, Mobile County, Alabama, USA. The 250 otolith specimens, mainly sagittae, were obtained from shell hash dredged from an offshore Pliocene unit deposited onshore as part of recent beach nourishment. Analysis of the otoliths revealed 22 species from 11 teleostean families. Sciaenid fishes dominate the assemblage and account for 79.6% of the total specimens, while the other 10 families (including indeterminates) comprise approximately 20% of the total. Comparison to modern biological and ecological requirements for the fishes represented by the otoliths suggest a shallow (0–20 m) neritic environment with some brackish and rare deep-water influence. Several of the most abundant taxa indicate shallow, soft bottom estuarine habitat with upstream oligohaline creeks (0.5 to 5.0 ppt salinity) serving as primary nursery area. However, the percentage of marine-only species may indicate more interaction with open estuary and shallow coastal areas and subtropical waters. The lack of invertebrate settlement on the otoliths likely indicates limited surface residence-time. All 22 taxa recovered from the Dauphin Island site are highly significant since they represent the first Pliocene occurrences of these taxa within the Gulf Coastal Plain of North America.

After the Fall: Legacy Effects of Biogenic Structure on Wind-Generated Ecosystem Processes Following Mussel Bed Collapse

Blue mussels (Mytilus edulis) are ecosystem engineers with strong effects on species diversity and abundances. Mussel beds appear to be declining in the Gulf of Maine, apparently due to climate change and predation by the invasive green crab, Carcinus maenas. As mussels die, they create a legacy of large expanses of shell biogenic structure. In Maine, USA, we used bottom traps to examine effects of four bottom cover types (i.e., live mussels, whole shells, fragmented shells, bare sediment) and wind condition (i.e., days with high, intermediate, and low values) on flow-related ecosystem processes. Significant differences in transport of sediment, meiofauna, and macrofauna were found among cover types and days, with no significant interaction between the two factors. Wind condition had positive effects on transport. Shell hash, especially fragmented shells, had negative effects, possibly because it acted as bed armor to reduce wind-generated erosion and resuspension. Copepods had the greatest mobility and shortest turnover times (0.15 d), followed by nematodes (1.96 d) and the macrofauna dominant, Tubificoides benedeni (2.35 d). Shell legacy effects may play an important role in soft-bottom system responses to wind-generated ecosystem processes, particularly in collapsed mussel beds, with implications for recolonization, connectivity, and the creation and maintenance of spatial pattern.

High‐energy inundation events versus long‐term coastal processes – room for misinterpretation

AbstractCoastal boulder deposits and chevrons are two features whose origin have triggered controversial discussions. Boulders are often used as indicators of past tsunamis and storms, with the former interpretation in many cases preferred due to the clast size. Chevrons, defined as large parabolic sand bodies, were previously attributed to (mega‐)tsunami, potentially caused by oceanic impacts, because of their dimensions, height above sea level and alignment of the central axis. This study documents that chevrons along the Quobba coast in Western Australia are parabolic dunes and not related to tsunami inundation; their age is consistent with an arid period at about 3·9 to 2·3 ka when the sea level was 1 to 2 m higher than today. The internal age distribution proves an inland migration. Weakly developed soil horizons represent phases of intermittent dune stabilization and later reactivation. The calculated velocities required for wind transport and the prevailing wind directions are consistent with on‐site meteorological parameters. The boulders at Quobba are most likely to be remnants of in situ platform denudation that produces shell hash, coral clasts and boulders. An unknown portion of the boulders was certainly moved by tropical cyclones. A previously proposed tsunami origin is unsustainable because the observed features can be explained by processes other than tsunamis. Boulders were tilted during gravitative platform collapse, standing water caused dissolution of the boulder bottoms, creating ‘pseudo‐rockpools’, consequently not applicable as upside‐down criteria, and ages of attached encrusting organisms document their colonization at higher sea level and (sub)recent frequent inundation by wave splash during rough seas.

Habitat complexity and benthic predator-prey interactions in Chesapeake Bay

In Chesapeake Bay, the soft-shell clam Mya arenaria (thin-shelled, deep-burrowing) exhibits population declines when predators are active, and it persists at low densities. In contrast, the hard clam Mercenaria mercenaria (thick-shelled, shallow-burrowing) has a stable population and age distribution. We examined the potential for habitat and predators to control densities and distributions of bivalves in a field caging experiment (Mya only) and laboratory mesocosm experiments (both species). In the field, clams exposed to predators experienced 76.3% greater mortality as compared to caged individuals, and blue crabs were likely responsible for most of the mortality of juvenile Mya. In mesocosm experiments, Mya had lower survival in sand and seagrass than in shell hash or oyster shell habitats. However, crabs often missed one or more prey items in seagrass, shell, and oyster shell habitats. Predator search times and encounter rates declined when prey were at low densities, likely due to the added cost of inefficient foraging; however, this effect was more pronounced for Mya than for Mercenaria. Mercenaria had higher survival than Mya in mesocosm experiments, likely because predators feeding on Mercenaria spent less time foraging than those feeding on Mya. Mya may retain a low-density refuge from predation even with the loss of structurally complex habitats, though a loss of habitat refuge may result in clam densities that are not sustainable. A better understanding of density-dependent predator-prey interactions is necessary to prevent loss of food-web integrity and to conserve marine resources.

Habitat effects of macrophytes and shell on carbonate chemistry and juvenile clam recruitment, survival, and growth

Adverse habitat conditions associated with reduced seawater pH often, but not always, negatively affect bivalves in early life history phases. Improving our understanding of how habitat-specific parameters affect clam recruitment, survival, and growth could assist natural resource managers and researchers in developing appropriate adaptation strategies for increasingly acidified nearshore ecosystems. Two proposed adaptation strategies, the presence of macrophytes and addition of shell hash, have the potential to raise local seawater pH and aragonite saturation state and, therefore, to improve conditions for shell-forming organisms. This field study examined the effects of these two substrate treatments on biological and geochemical response variables. Specifically, we measured (1) recruitment, survival, and growth of juvenile clams (Ruditapes philippinarum) and (2) local water chemistry at Fidalgo Bay and Skokomish Delta, Washington, USA, in response to experimental manipulations. Results showed no effect of macrophyte or shell hash treatment on recruitment or survival of R. philippinarum. Contrary to expectations, clam growth was significantly greater in the absence of macrophytes, regardless of the presence or absence of shell hash. Water column pH was higher outside the macrophyte bed than inside at Skokomish Delta and higher during the day than at night at Fidalgo Bay. Additionally, pore-water pH and aragonite saturation state were higher in the absence of macrophytes and the presence of shell. Based on these results, we propose that with increasingly corrosive conditions shell hash may help provide chemical refugia under future ocean conditions. Thus, we suggest adaptation strategies target the use of shell hash and avoidance of macrophytes to improve carbonate chemistry conditions and promote clam recruitment, survival, and growth.

Effects of burrow and tube construction by infauna on sound propagation through marine sediments

Infauna, animals living in marine sediments, modify sediment structure by burrowing, constructing burrows and tubes, and irrigating burrows. These activities can change the bulk porosity and density as well as create heterogeneity in sediment structure. We test the hypothesis that these activities alter sound speed and attenuation in sediments by manipulating homogenized sediments to mimic animal activities. Specifically, we examine the effects of burrow excavation, burrow wall compaction, burrow irrigation, and construction of tubes from shell hash on sound speed and attenuation at 100, 200, and 400 kHz. Wavelengths corresponding to these frequencies span the size of the burrows constructed, and measurements were conducted at several depths within the upper 10 cm of sediment in which infauna are commonly found. Each of these activities or functions is performed by multiple species of animals that comprise a functional group. Our results will help identify functional groups that have important impacts on sediment acoustics and will be used to interpret field data in which deviations from predicted sound speed and attenuation are correlated with different and diverse communities of infauna.