Eugenia Fanjul

Abiotic stress mediates top-down and bottom-up control in a Southwestern Atlantic salt marsh

Increasing evidence has shown that nutrients and consumers interact to control primary productivity in natural systems, but how abiotic stress affects this interaction is unclear. Moreover, while herbivores can strongly impact zonation patterns in a variety of systems, there are few examples of this in salt marshes. We evaluated the effect of nutrients and herbivores on the productivity and distribution of the cordgrass Spartinadensiflora along an intertidal stress gradient, in a Southwestern Atlantic salt marsh. We characterized abiotic stresses (salinity, ammonium concentration, and anoxia) and manipulated nutrients and the presence of the herbivorous crab Neohelice (Chasmagnathus) granulata, at different tidal heights with a factorial experiment. Abiotic stress increased at both ends of the tidal gradient. Salinity and anoxia were highest at the upper and lower edge of the intertidal, respectively. Nutrients and herbivory interacted to control cordgrass biomass, but their relative importance varied with environmental context. Herbivory increased at lower tidal heights to the point that cordgrass transplants onto bare mud substrate were entirely consumed unless crabs were excluded, while nutrients were most important where abiotic stress was reduced. Our results show how the impact of herbivores and nutrients on plant productivity can be dependent on environmental conditions and that the lower intertidal limits of marsh plants can be controlled by herbivory.

Grazing effects of the periwinkle Echinolittorina peruviana at a central Peruvian high rocky intertidal

Echinolittorina peruviana is the most common gastropod in the high intertidal zone of Peru, representing more than 80% of the individuals present at that zone. Experimental removal of snails was used to evaluate their effects on (a) abundance of epilithic biofilm, (b) barnacle recruitment, and (c) abundance of macroalgae under “normal” conditions of the El Niño Southern Oscillation (ENSO). Experiments were carried out from October 2005 to April 2007 at two intertidal levels of a semi-protected rocky shore of central Peru. Results demonstrated that E. peruviana is able to control biofilm abundance and barnacle recruitment at both heights investigated, with marked effects in the lower zone. Erect macroalgae (Ulva spp. and Gelidium spp.) were less affected by grazing; but negative effects were observed on macroalgal crusts. Season and physical stress seem to play a more important role in the abundance of macroalgae in the high intertidal. Our results are similar to those reported elsewhere for high shore littorinids and represent baseline data to understand how the role of intertidal consumers will vary under the cold (La Niña) and warm (El Niño) phases of ENSO on these shores.

Can a Single Species Challenge Paradigms of Salt Marsh Functioning

Over the history of ecology, well-established generalizations were refined or even changed after the appearance or consideration of new evidence. Here, we review results obtained in Southwestern Atlantic salt marshes (between southern Brazil −32° 1′ S- and the Argentinean Patagonia −53° 48′ S-). Most of these salt marshes are inhabited by the intertidal burrowing crab Neohelice granulata, a species that influences many ecological processes through bioturbation and herbivory. The experimental evaluation of these processes shows that in some cases, the results were not consistent with generalizations and models of salt marsh ecological functioning. However, this does not imply that the generalizations grounded mainly on the results from North American sites are not valid. In turn, we suggest that these apparently conflicting results emerged because two major processes, herbivory and bioturbation, have been overlooked until recently. Thus, their relative contribution has not been included in the models of salt marsh functioning. In conclusion, we believe that there is a need for performing parallel and simultaneous experiments comparing distant sites with varying environmental (i.e., abiotic and biotic) conditions to be able to uncover common processes and causes of contingencies. Particularly, Southwestern Atlantic salt marshes could lead the way in providing information to better incorporate herbivory and bioturbation into current models or paradigms about how salt marshes work.