Increase of organic matter transport between marshes and tidal flats by the burrowing crab Neohelice (Chasmagnathus) granulata Dana in SW Atlantic salt marshes

Abstract

Marshes are considered exporters of organic material, with tides being one of the most studied transport mechanisms. However, animal movements can enhance this energy transport and even, in some cases, import energy to marshes by fecal deposition. In this work, we analyze if Neohelice (Chasmagnathus) granulata, an abundant intertidal burrowing crab that changes its diet between marsh (herbivorous) and tidal flat (deposit feeder) zones and moves between zones during the tidal cycle, enhance energy transfer between marshes and tidal flats by fecal deposition. Given that higher tidal levels increase these movements, two intertidal habitats with different tidal amplitudes were compared (Mar Chiquita Coastal lagoon 37° 40' S -MCCL, max 2. m and San Antonio Bay 40° 46' S -SAB, max 9. m). The seasonal and tidal movements between zones were studied using movement traps. Crabs moving towards each direction were kept in containers to calculate fecal production and the total organic matter (OM) release and OM% of their feces. With these data, a resampling technique was performed to compare with a 3 way ANOVA the net transport of feces seasonally, between zones, in both intertidals. The results showed that the movement of N. granulata enhances the exchange of OM among habitats by fecal deposition between marsh and tidal flats with seasonal variations in its importance. Fecal transport showed differences between locations being 10 times higher in winter and decreasing towards summer in MCCL, while the opposite trend was found in SAB. There were no differences in the OM deposited in each zone in SAB but in MCCL there was 45% more feces deposited in the marsh than in the tidal flat. The fecal OM% is higher in SAB than in MCCL suggesting that biodeposition in SAB, where it can reach 40%OM (surrounding sediment is lower than 2%) could be more important. The fecal production in MCCL is about 80% of the rate of detritus production in the marsh. In conclusion, fecal deposition is a very important input of energy in marsh zones, being crabs an important mechanism in transporting OM mainly towards the marsh zones.