Effects of environmental gradients on sandy beach macrofauna of a semi‐enclosed bay

Abstract

Macrofauna of sheltered and pocket beaches located in embayments, estuaries or landward of protective reefs has not been adequately assessed in beach ecology. In this setting, the concurrent role of large‐scale morphodynamics and wave‐exposure gradients in shaping biological patterns is still uncertain. To examine variations in community descriptors and the influence of physical factors on macrofauna, 12 sandy beaches on five islands within Sepetiba Bay (SE Brazil) were characterized in terms of beach morphodynamics and dimensions (length and width), such as geographical position relative to the bay mouth. A total of 80 species were collected and identified. Community descriptors of macrofauna responded to morphodynamics and exposure gradients. Increases in species richness, and abundance, from harsh reflective (coarse sands, steep slopes) to benign dissipative (fine sands, gentle slopes) beaches, and from the bay mouth (exposed beaches) towards the inner bay (sheltered beaches) were observed. Mollusks were the most abundant macrofaunal group, followed by crustaceans and polychaetes. Crustaceans dominated the beaches near the bay mouth, whereas mollusks were more abundant on the beaches farther from the bay mouth. Canonical correspondence analysis showed that the degree of exposure, distance from the bay mouth, beach index, and beach length and width significantly affected the macrofauna distribution and abundance, creating an environmental gradient along Sepetiba Bay. Differences in macrofauna composition among the beaches studied were associated with beach length and width. In this sense, spatial variation of macrofauna among beaches can be a function of intertidal area. These results indicate that community characteristics in the sandy beaches studied are affected by several physical characteristics, but also by other factors that are affected by coastal processes.

Population dynamics of two suspension-feeding bivalves on a sheltered beach in southeastern Brazil

“…Because of the species' high abundance, the production of A. brasiliana accounts for a high percentage of the overall production of the local macrofaunal community. Among mollusks, A. brasiliana and the gastropod Cerithium atratum comprise more than 80% of all individuals in the macrofaunal community (Cardoso et al 2011). On the other hand, the P/B ratio calculated for D. punctata was higher than for A. brasiliana, a difference which possibly relates to the former species' lower individual body mass (Brey and Clarke 1993;Tumbiolo and Downing 1994) and longevity (Waters 1977;Robertson 1979).…”

“…Several studies have focused on community ecology and population dynamics of macrofauna from exposed beaches McLachlan 2005, 2011), while sheltered beaches have been a historically neglected area of sandy-beach ecology (Cardoso et al 2011). Surprisingly, few studies have analyzed population parameters of sheltered beach macrofauna (Denadai et al 2004;Abrahão et al 2010), even though these environments support a high diversity of species (Dexter 1992).…”

“…Many species of bivalves coexist on sheltered beaches, including the suspension-feeding bivalves Anomalocardia brasiliana (Veneridae) and Diplodonta punctata (Ungulinidae) (Cardoso et al 2011). A. brasiliana has a wide geographical distribution, from the West Indies to Uruguay (Rios 1994).…”

Variability of macrofauna distribution along a dissipative log-spiral sandy beach in Rio de Janeiro, Southeastern Brazil

“…At its western end, the Sepetiba Bay is connected to Ilha Grande Bay by a narrow channel in which the main current direction is from west to east (Signorini 1980, Fragoso 1999. Cardoso et al (2012), studying 12 sandy beaches on five islands within Sepetiba Bay, showed that the species richness and total abundance markedly increase in the inner bay, where the wave action is minimal. They also verified that wave exposure variability, sediment variables (e.g.…”

“…Based on the environmental characteristics, the bay can be divided into three sectors: the inner sector (influenced by freshwater discharges from several small rivers), the outer sector (influenced by oceanic waters and in contact with the Atlantic Ocean by a wide mouth located at its western end) and the middle sector (a mixed zone with intermediate environmental conditions influenced by freshwater discharges and by oceanic waters). The outer sector is marked by the presence of islands with several sheltered and exposed sandy beaches (Cardoso et al 2012). The mean tidal range of the region is 1.50 m and currents, with a maximum speed of approximately 1 m/s, mainly regulate the hydrodynamic conditions (Fragoso 1999).…”