Unravelling the role of local and large-scale factors in structuring sandy beach populations: the wedge clam Donax hanleyanus

Abstract

Changes in the structure and dynamics of sandy beach macrofaunal populations result from the simultaneous action of local and regional factors acting synergistically. This is particularly noticeable in transitional interfaces between freshwater and marine ecosystems, where large-scale gradients affect local beach morphodynamics and resident fauna. The wedge clam Donax hanleyanus occurs on beaches with contrasting morphodynamics along the salinity gradient generated by the Rio de la Plata estuary. We conducted sampling surveys to determine a fine-grained comprehensive coverage of the species’ distribution in Uruguay to assess the concurrent effects of large-scale variations in salinity and local beach morphodynamics on wedge clam populations along 400 km (16 sandy beaches) over 2 yr. The main factor controlling wedge clam occurrence was salinity, overriding the effects of local habitat features. On beaches where the species was present, total abundance was higher on oceanic dissipative shores, which also harbored a higher abundance and relative representation of recruits. Recruitment was almost lacking in reflective beaches. However, biomass of adults with larger body sizes prevailed on reflective beaches, suggesting a scaling of abundance to body size and potential density-dependent effects. In this metapopulation, estuarine beaches are sinks, whereas oceanic beaches act as source habitats. We concluded that large-scale gradients generated by salinity variations translated into local population patterns (distribution, abundance and population structure) and processes (recruitment), which were secondarily driven by local morphodynamics. The relative importance of pre- and post-settlement processes across the morphodynamic spectrum from reflective to dissipative beaches has yet to be elucidated.

Testing macroecological hypotheses in sandy beach populations: the wedge clam Donax hanleyanus in South America

“…interspecific competition, top-down and/or bottom-up processes, or density-dependence at population level) could play a role . In agreement with our findings, Celentano et al (2022) found a higher abundance of wedge clams in dissipative beaches (see Fig. 3e) and an increased relative representation of older adults with higher body size on reflective beaches (see Fig.…”

“…Salinity, followed by chl a and SST, were the most relevant factors that may trigger large-scale differences in wedge clam population attributes, whereas local habitat features (Dean's para meter, grain size, beach slope) played a secondary role. Our observations reinforce regional-scale findings that highlighted salinity as the main factor controlling wedge clam populations in sandy beaches (Celentano et al 2022, Risoli et al 2022.…”

“…In turn, salinity may affect abundance, growth, survival, and recruitment success . Low abundances of this species were documented in beaches under the influence of the RdlP in both Argentina and Uruguay (Celentano et al 2022, Risoli et al 2022, as well as in other Donax populations around the world (Ansell 1983, Schoeman & Richardson 2002.…”

“…The chl a data used are from July 2002 to June 2021. Surface salinity data were used because the wedge clam inhabits beaches affected by continental freshwater discharge (Celentano et al 2022, Risoli et al 2022. Salinity data were obtained from the World Ocean Database (Boyer et al 2018; https://www.ncei.noaa.gov/products/world-oceandatabase), and from the 'Base Regional de Datos Oceanográficos (BaRDO) -INIDEP' oceanographic database (https://www.argentina.gob.ar/inidep).…”

“…Latitudinal patterns in life-history traits have been observed in a broad range of intertidal organisms (Dugan et al 1994, Defeo & McLachlan 2005, Mc -Lachlan & Defeo 2018, including bivalves that dominate sandy-beach biota (Cardoso & Veloso 2003, Fiori & Defeo 2006, Moss et al 2016). These species also exhibit higher individual growth rates and lower longevity with increasing SSTs (Marquardt et al 2022), and a reduced abundance, survival, growth, and fecundity in beaches affected by large-scale salinity gradients (Schoeman & Richardson 2002, Risoli et al 2020, Celentano et al 2022 or low food availability (Risoli et al 2022). Concerning the role of local morphodynamics, a negative impact on life-history traits is expected in populations inhabiting reflective beaches compared to dissipative ones, according to the habitat harshness hypothesis (HHH).…”