Effects of physical features on production of three macrofaunal species in different sandy beach zones in South America

Abstract

Habitat Harshness Hypothesis (HHH) and Habitat Safety Hypothesis (HSH) successfully predict trends in diversity, abundance, individual growth, survival rate, and reproductive success of intertidal and supralittoral macrofauna species according to beach morphodynamic states. However, the effects of beach physical features on secondary production are still poorly known, especially concerning the response of species that inhabit different beach zones. Here we tested the hypothesis that production and P/B ratio of common species of lower and middle intertidal, the wedge clam Donax hanleyanus and the isopod Excirolana armata, respectively, follow HHH, increasing towards less severe dissipative/intermediate conditions. Conversely, the turnover rate (production-to-biomass ratio: P/B) of both species is expected to increase towards harsher reflective conditions according to Allen's relationship (instantaneous mortality rate Z = P/B). On the other hand, the production and P/B of the supralittoral isopod Excirolana braziliensis are expected to follow HSH, i.e. an opposite pattern to that of intertidal species, with higher production on reflective beaches and higher P/B on dissipative ones. To test these predictions, relationships of production and P/B with physical characteristics of beaches (grain size, beach slope, and width) were assessed using 34 estimates gathered from literature, obtained from 14 microtidal beaches located in the Southwestern Atlantic coasts. The results showed that D. hanleyanus production increased towards dissipative conditions following HHH, although P/B ratio followed the production trend. These results, however, agree with those previously observed in the mole crab Emerita brasiliensis, reinforcing these trends for inhabitants of the swash zone, the harshest beach zone. Similarly, E. armata production increased sharply towards dissipative conditions, finer sand and gentler slopes, an expected pattern for a middle intertidal, highly-substrate specific species. Production and P/B of the supralittoral species E. braziliensis followed HHH rather than HSH, with higher production in intermediate/dissipative conditions, and inversely, higher P/B and Z on reflective beaches. This study shows that the production of intertidal species both in the lower and in the middle intertidal increases from reflective to intermediate/dissipative conditions.