Patterns in size and species diversity of benthic macroinvertebrates in mediterranean salt marshes

Abstract

Mediterranean salt marsh macroinvertebrate assemblages are abiotically filtered due to harsh environmental conditions. We hypothesized that in these environmentally fluctuating systems, biotic interactions would have more relevance than abiotic ones and that size-based interactions would dominate. The manuscript focuses on the response to biotic and abiotic factors of 2 diversity metrics: (1) one that relies on species identity (Shannon-Wiener diversity), and (2) another based on the shape of biomass size spectra (size diversity). Benthic macroinvertebrates (organisms >1 mm) were the target group of this study and were sampled monthly in 6 basins during 2 flooding cycles (from October 1997 to July 1999). The high inter-annual variability characteristic of Mediterranean systems revealed the robustness of the observed patterns. Abiotic and biotic interactions appeared to be significant for both diversity metrics, with species diversity more abiotically controlled than size diversity. The relationships between size diversity and environmental variables were more interannually stable than those for species diversity. Only water total organic carbon had a robust relationship with species diversity, but dissolved inorganic nitrogen and predatory pressure had a robust relationship with size diversity. Additionally, size diversity had a significant succession pattern related to days after flooding (time), whereas species diversity did not. Our results suggest that in abiotically filtered environments, macroinvertebrate size-based interactions become especially relevant.