Trophic models in sandy beaches with contrasting morphodynamics: Comparing ecosystem structure and biomass flow

Abstract

We model and compare for the first time the ecosystem structure and trophic networks of two sandy beaches with contrasting morphodynamics (i.e. dissipative and reflective). To this end, an Ecopath model was implemented to represent the macroscopic food web on each sandy beach ecosystem. The dissipative beach model comprised 20 compartments and the reflective nine, including detritus, phytoplankton, zooplankton, benthic invertebrates, fishes and seabirds. Input data mainly came from direct surveys, whereas additional information was gathered from published and unpublished sources. Results revealed a major number of top predators and higher trophic levels (TLs) in the dissipative beach (seabirds, fishes, gastropods and the polychaete Hemipodus olivieri) than in the reflective one (fishes and Hemipodus olivieri). Detritivorous and filter feeding benthic invertebrates constituted intermediate trophic levels on both beaches. Exportation of most primary production and detritus was a common feature, with a detritivory:herbivory ratio of 0.42 in the dissipative beach and 0.51 in the reflective, indicating a higher utilization of the primary production in the former. The aggregation analysis showed five TLs in the dissipative beach and four in the reflective. Lower transfer efficiencies in the reflective beach could be attributed to a lower diversity of predators when compared to the dissipative system. Comparison of global ecosystem properties showed that the dissipative system had higher values for total system throughput, total biomass, net production, Ascendency and total number of pathways. We concluded that the greater diversity and biomass in the dissipative beach are reflected in a higher dimension and ecosystem organization than in the reflective beach, thus providing new evidences at the ecosystem level about well-established differences in biological descriptors between beach types. Finally, we stressed the need for acquiring experimentally-based information on benthic invertebrate's consumption rates and the inclusion of the surf zone microbial loop and the interstitial compartment to test for differences in structure and functioning of these coastal ecosystems.