Seasonal Beach-Profile Variation and Macroalgal Subsidies Shape Wrack-associated Macrofauna on Tropical Sandy Beaches of the Southwestern Atlantic

Abstract

Sandy beaches are dynamic ecotones where beach-profile morphology, sediment properties, and allochthonous organic subsidies interact with intertidal benthic communities. We assessed seasonal variation in beach morphology, sediment granulometry, beach-cast macroalgal wrack, and associated macrofaunal assemblages at two tropical sandy beaches on the central Brazilian coast: Putiri Beach and Fifteen Beach. Both beaches were classified within the intermediate morphodynamic range according to Dean’s dimensionless fall-velocity parameter, with Ω = 3.29 at Fifteen Beach and Ω = 4.38 at Putiri. Breaking wave conditions were comparable between beaches, with Hb ≈ 119.4 cm and T ≈ 7.57 s at Fifteen Beach and Hb ≈ 116.6 cm and T ≈ 7.83 s at Putiri. Sediment mean grain size remained within the fine-sand class at both beaches and in both seasons, but lower phi values in summer indicated relative coarsening within this class: Putiri shifted from phi 2.67 to phi 2.24, whereas 15 Beach shifted from phi 2.69 to phi 2.25. Sediment organic matter was also higher in summer than in winter, increasing from 2.81 ± 0.68% to 3.80 ± 0.49% at Putiri and from 2.87 ± 0.39% to 3.78 ± 0.80% at Fifteen Beach. Wrack biomass remained low and relatively stable at Putiri between winter and summer (53.81 ± 7.35 and 54.55 ± 1.05 g m⁻², respectively) and at Fifteen Beach in winter (56.93 ± 12.55 g m⁻²), but increased sharply at Fifteen Beach in summer (219.52 ± 83.31 g m⁻²), mainly due to brown algae and terrestrial plant material. Macrofaunal assemblages differed significantly among beach–season combinations, with Talitridae, Oligochaeta, and Cirolanidae together accounting for 98.4% of total abundance. Assemblage differences were supported by Bray–Curtis PERMANOVA, while relative-abundance patterns showed strong dominance of amphipods at Fifteen Beach in winter and increased contributions of oligochaetes and insects in summer assemblages. Together, these separate but complementary analyses indicate that beach–season combinations differed simultaneously in beach-profile morphology, sediment texture, wrack subsidies, and macrofaunal assemblage composition, supporting an association between physical habitat variation, trophic subsidies, and benthic assemblage structure.

Bibliographic Information