The influence of allochthonous macroalgae on the fish communities of tropical sandy beaches

Abstract

This study evaluates the hypothesis that detached macroalgae (drift algae) influence the structure of fish communities in the surf-zones of two tropical sandy beaches from the central coast of Brazil. Surf-zone seine hauls were conducted weekly during three consecutive 6-week periods according to the occurrence of drift algae – in pre-drift, drift and post-drift periods. Drift algae comprised 33 macroalgae species and likely came from offshore rhodolith beds. Fish biomass, density and species richness increased from pre-drift to drift periods, and species composition differed significantly between periods, particularly in the more sheltered of the sampled beaches. The density of young-of-the-year (YOY) fishes also increased significantly from the pre-drift to drift periods, suggesting the importance of drift algae to early fish stages. The gut contents of the two most abundant species during the drift algae period, Trachinotus falcatus and Trachinotus goodei, demonstrated the importance of macrophyte-associated amphipods in their diets, suggesting that the drift algae may be used as feeding habitats for these species. Our results showed that the influx of allochthonous drift algae may structure fish communities of tropical sandy beaches and that it may also represent an important alternative resource for YOY fish by providing shelter and food during autumn and winter.

Introduction

Surf zones of sandy beaches are important nursery grounds for many fish species worldwide, particularly during their early life (McLachlan and Brown, 2006). They are also used as transient habitats for fish migrating between inner shelf zones and coastal ecosystems such as estuaries and mangroves (Cowley et al., 2001, Able, 2005). The high wave-energy and shallow waters usually observed in sandy beaches provide shelter from predators and support elevated densities of the mobile invertebrates that provide food resources for juvenile fish (BatisticTutman et al., 2005, Niang et al., 2010). Given their importance as nursery grounds, natural and anthropogenic disturbances in these habitats can profoundly affect fish communities, leading to ecological responses that may vary in time and space (Syms and Jones, 2000). For example, fish from sandy beaches may be subject to anthropogenic impacts such as fishing, beach nourishment and oil spills (Schlacher et al., 2007, Defeo et al., 2009). However, natural disturbances such as freshwater runoff, storms and seasonal winds may occasionally trigger the input of allochthonous organic and inorganic materials into sandy beaches (Colombini and Chelazzi, 2003, Gomes et al., 2003).

Coastal ecosystems worldwide usually receive large amounts of allochthonous macroalgae and seagrasses, also referred to as macrophytes, which may provide food and habitat for marine fauna in the surf zones or when it is stranded as beach-cast wrack (Rossi and Underwood, 2002, Crawley and Hyndes, 2007). Allochthonous macrophytes can be detached from their substrate by storms or strong currents (Colombini and Chelazzi, 2003). Upon detachment, algal tissues may be called drift algae or simply detached macrophytes. When algal drifts approach surf zones, they are mostly consumed by benthic crustaceans (Colombini and Chelazzi, 2003, Crawley et al., 2009) and may be finally deposited ashore. These benthic crustaceans in surf zones may in turn be fed on by juvenile fish (Helmer et al., 1995). Fish abundance and biomass have been positively correlated with the mass of detached macrophytes in temperate sandy beaches (Robertson, 1984, Crawley et al., 2006). Therefore, detached and non-detached macrophytes in coastal areas may provide better nursery grounds for juvenile fish before they recruit to adult habitats and may support higher fish diversity than bare sand habitats (Ornellas and Coutinho, 1998, Gullström et al., 2008). The number of studies focusing on the effects of detached macrophytes in coastal environments has recently increased (e.g., Duong and Fairweather, 2011, Gonçalves and Marques, 2011). However, only a few studies have attempted to investigate their influence on the fish communities in surf zones of sandy beaches, particularly in temperate regions (Robertson, 1984, Lenanton and Caputi, 1989, Crawley et al., 2006). While abiotic factors (e.g. climate variability) are preponderant in shaping fish assemblages in temperate regions, biotic factors as competition and predation play a major role in tropical regions (Schemske et al., 2009). Therefore, drift algae may be even more important in structuring fish assemblages in tropical regions than they are for temperate ones. To our knowledge, there is no available information on the effect of drift algae on fish communities from tropical beaches, nor on sandy beaches from the western Atlantic.

In sandy beaches from the central coast of Brazil, drift algae occur a couple of times a year, mostly during early autumn (April), as a result of strong south winds following the first seasonal cold fronts after summer. Once arriving in the surf zones, these dense accumulations are deposited on the beach face, where they are colonized by macrofaunal organisms. In tropical beaches, this whole cycle lasts for approximately 8–20 weeks depending on factors such as wind speed, tide strength and human removal (Colombini and Chelazzi, 2003). In the study region, local fishermen believe that drift algae in nearshore areas are nursery habitats for crustaceans and fish, which reinforces the idea that macroalgae potentially affects fish populations. However, neither the direct effect of algal drifts on local fish assemblages nor the processes underlying those effects are clearly understood in tropical sandy beaches.

This study aimed to examine the effects of drift algae on fish communities from two tropical sandy beaches in the central coast of Brazil. More specifically, we evaluate how the local fish assemblages respond to intrusions of drift algae in terms of species composition and the proportions of life stages. In addition, we analyzed the diet of two locally dominant fish species as indicators of the importance of drift algae-associated organisms to the fish diet.