Faunal engineering stimulates landscape-scale accretion in southeastern US salt marshes

Abstract

The fate of coastal ecosystems depends on their ability to keep pace with sea-level rise—yet projections of accretion widely ignore effects of engineering fauna. Here, we quantify effects of the mussel, Geukensia demissa, on southeastern US saltmarsh accretion. Multi-season and -tidal stage surveys, in combination with field experiments, reveal that deposition is 2.8-10.7-times greater on mussel aggregations than any other marsh location. Our Delft-3D-BIVALVES model further predicts that mussels drive substantial changes to both the magnitude (±<0.1 cm·yr−1) and spatial patterning of accretion at marsh domain scales. We explore the validity of model predictions with a multi-year creekshed mussel manipulation of >200,000 mussels and find that this faunal engineer drives far greater changes to relative marsh accretion rates than predicted (±>0.4 cm·yr−1). Thus, we highlight an urgent need for empirical, experimental, and modeling work to resolve the importance of faunal engineers in directly and indirectly modifying the persistence of coastal ecosystems globally.

The Role of a Faunal Engineer, Geukensia demissa, in Modifying Carbon and Nitrogen Regulation Services in Salt Marshes

“…Mean sediment accumulation rates measured by (Letzsch & Frey, 1980) on Sapelo Island were highest in the low marsh zone (0.9 cm yr −1 ), which experiences the longest inundation period and supports higher density mussel populations, relative to transitional (0.45 cm yr −1 ) and high marsh (0.1 cm yr −1 ) zones. Amplified sedimentation via mussel suspension feeding and biodeposition, as seen in our Dean Creek study alongside others (Crotty et al., 2023), is likely an important mechanism underpinning the allochthonous shift we measured in accumulated OM, suggesting that the mussel effects we observed at the sediment‐water interface (i.e., increased benthic‐pelagic coupling and sedimentation) persist into the sediment layer.…”

“…(2017) observed that organic C burial was positively correlated with within‐reef live oyster ( Crassostrea virginica ) density, attributing this relationship to increasing suspension feeding and baffling by reef structure (i.e., sedimentation) as well as shell production. Thus, we expect that density‐dependent sedimentation, which is also positively correlated within mussel aggregations (Crotty et al., 2023), is likely driving the continuous increase in accumulated C and N. Although previous modeling efforts suggest that mussel populations drive sediment deposition and vertical accretion at both creekshed (∼2,500 m 2 ) and landscape (∼10,000 m 2 ) scales as their biodeposits are dispersed across the salt marsh platform (Crotty et al., 2023), our study revealed that density‐dependent effects on C and N accumulation was not upheld in adjacent, non‐mussel areas. One mechanistic explanation for this may be that organic fractions of biodeposits are more readily remineralized locally while mineralogical fractions more effectively redistribute across salt marsh platforms.…”

“…Elevated POM and chlorophyll a concentrations are observed in spring and summer months and during spring, as opposed to neap, tides (Berg & Newell, 1986;Cadee, 1982;Toro et al, 1999). These temporal patterns in seston availability are reflected in mussel biodeposition rates, which peak during summer spring tides in our study region (Crotty et al, 2023). To assess these more active periods of mussel biodeposition, we collected samples during Summer 2019 and 2020 and, when applicable, during spring tides.…”

“…For all models related to bulk geochemistry described here and below, model formulas are summarized (i.e., response distributions and link functions) in Table S2 in Supporting Information S1. We multiplied dry weight by %TOC and %TN to calculate the amount of organic C and N (g) captured by the benthic filters and then extrapolated these values to estimate sediment deposition per square meter of mussel aggregation and cordgrass only areal coverage (g m −2 ) across the marsh platform during a single, spring high tide (see Crotty et al (2023) for similar methods of extrapolation). Pelagic filter duplicates were analyzed for either ẟ 13 TOC, after acidification, or ẟ 15 N. These values were later incorporated into isotope mixing models described below.…”

“…Mussels are thus important biogeochemical couplers of the water column and the marsh bottom, facilitating the exchange of C and N in these ecosystems (Bilkovic et al., 2017; Dame et al., 2002; Kuenzler, 1961). Further, mussel density, both within individual aggregations at the patch scale and across populations of aggregations at the landscape scale, likely modulates the effect of these organisms on C and N processes (Bertness, 1984; Crotty et al., 2023; Moody & Kreeger, 2020; Smith & Frey, 1985). Despite the key, multifaceted importance of these abundant and productive bivalves, the fate of mussel biodepostion as it relates to source‐specific sedimentation, accumulation, and storage of C and N—and how mussel density affects these pathways—in salt marshes has not been explored.…”