Effects of Spartina Wrack on Surface-Active Arthropod Assemblage Under Different Environmental Contexts in Southwest Atlantic Salt Marshes

Abstract

Large amounts of tidally accumulated detritus (i.e., wrack) are an important source of disturbance affecting different abiotic and biotic characteristics in salt marshes, which could in turn affect the macrofauna assemblage. The purpose of this study was to evaluate the importance of wrack disturbance in Southwest Atlantic (SWA) salt marshes and its effects on the surface-active arthropod assemblage under different environmental contexts. By sampling the most important SWA salt marshes (from 36° 19′ S to 41° 01′ S), we found that wrack is a widespread disturbance in this region, present in all the salt marshes and periods sampled. However, the biomass and type of wrack (Spartina alterniflora vs. S. densiflora) vary according to the species that dominates each salt marsh. At two of these sites (Bahia Blanca (BB), 38° 59′ S and San Clemente (SC), 36° 19′ S), chosen because they represent the two salt marsh types in the SWA region (dominated by Spartina alterniflora or S. densiflora), we performed a field experiment by manipulating the presence and absence of wrack and conducting field samplings of sediment organic matter content and water content. We found that wrack affects surface arthropod assemblage but that this effect was not consistent for the different salt marshes: in BB, it changed the surface-active arthropod assemblage (shifted towards more detritivorous taxa) and increased the number of total individuals but had no effect on the number of species or diversity. At SC, wrack had no effect on any of the parameters evaluated. We suggest that the type of wrack in each salt marsh modulates the amount of organic matter content in the sediment: BB had wrack of better nutritional quality (dominated by S. alterniflora) and in turn had greater organic matter content in the sediment of wrack zones than in no-wrack zones, while in SC (dominated by S. densiflora), there is no differences between the two zones. We also suggest that depending on the original surface-active arthropod assemblage, those modifications will either favor (BB) or not favor (SC) wrack colonization by the surface-active arthropod assemblage. Moreover, considering that SWA wrack has different compositions and that the biomass differs among the different salt marshes, we expect wrack effects in the SWA, and probably in other regions, to be site-specific.