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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. [ABSTRACT FROM AUTHOR]

Organic matter (OM) remineralization plays a key role in controlling the biogeochemistry of marine sediments. Through their burrowing activities, bioturbating macrofauna not only induces physical, chemical and biological modifications, which can affect microbial communities responsible for organic matter remineralization, but it could also directly affect the distribution and bioavailability of sedimentary organic matter. Through in situ experiments manipulating crab and burrow density in intertidal soft-bottoms, we assessed if crab-bioturbation affects benthic metabolism, and the amount, distribution, and bioavailability of sedimentary OM. Crab-bioturbation enhanced overall benthic metabolism and benthic flux of dissolved OM toward the water column at both mudflat and saltmarsh zones. Moreover, our results revealed that bioturbation also changes the quality, bioavailability and distribution of sedimentary OM in mudflats and saltmarshes. Overall, bioturbation enhanced the proportion of labile organic carbon of bioturbated sediments and homogenized the sediment column in terms of their proportion of labile organic carbon. However, crabs also generated biogenic structures (e.g., mounds) that could promote spatial heterogeneity of high nutritional-value OM. Bioturbation-induced changes on benthic metabolism and on OM availability would result in a reduction of the storage capacity of carbon in our intertidal systems. Previous works indicated that crab-burrows trap detritus and OM-rich sediments. Our results suggest that detritus are efficiently remineralized at bioturbated sediment, and finally they are quickly exported to the water column as CO 2 and DOC. Thus, crabs are modifying the OM processing at intertidal soft bottoms, and the ways in which carbon is exported to coastal waters. [ABSTRACT FROM AUTHOR]

Abstract: Biogenic bottom features, animal burrows and biological activities interact with the hydrodynamics of the sediment–water interface to produce altered patterns of sediment erosion, transport and deposition which have consequences for large-scale geomorphologic features. It has been suggested that depending on the hydrodynamic status of the habitat, the biological activity on the bottom may have a variety of effects. In some cases, different bioturbation activities by the same organism can result in different consequences. The burrowing crab Neohelice granulata is the most important bioturbator at SW Atlantic saltmarshes and tidal plains. Because of the great variety of habitats that this species may inhabit, it is possible to compare its bioturbation effects between zones dominated by different hydrodynamic conditions. Internal marsh microhabitats, tidal creeks bottoms and basins, and open mudflats were selected as contrasting zones for the comparison on a large saltmarsh at Bahía Blanca Estuary (Argentina). Crab burrows act as passive traps of sediment in all zones, because their entrances remain open during inundation periods at high tide. Mounds are generated when crabs remove sediments from the burrows to the surface and become distinctive features in all the zones. Two different mechanisms of sediment transport utilizing mounds as sediment sources were registered. In the first one, parts of fresh mound sediments were transported when exposed to water flow during flooding and ebbing tide, with higher mound erosion where currents were higher as compared to internal marsh habitats and open mudflats. In the second mechanism, mounds exposed to atmospheric influence during low tide became desiccated and cracked forming ellipsoidal blocks, which were then transported by currents in zones of intense water flow in the saltmarsh edge. Sedimentary dynamics varied between zones; crabs were promoting trapping of sediments in the internal saltmarsh (380gm−2 day−1) and open mudflats (1.2kgm−2 day−1), but were enhancing sediment removal in the saltmarsh edge (between 10 and 500gm−2 day−1 in summer). The implication is that biologically mediated sedimentological changes could be different among microhabitats, potentially leading to contrasting geomorphologic effects within a particular ecosystem. [Copyright &y& Elsevier]

Abstract: The role of positive interactions is often crucial in communities with intense abiotic stress such as intertidal environments. Grasses acting as ecosystem engineers, for example, may ameliorate intertidal harsh physical conditions and modify the community structure. The mud snails Heleobia australis d''Orbigny frequently inhabit the SW Atlantic marshes, mainly associated to intertidal marsh plants (mainly the smooth cordgrass Spartina alterniflora Loisel) probably due to the plant indirect effects. The purpose of this work was to investigate the magnitude of these association and the processes that generate the pattern. Samples of the snail abundance in six SW Atlantic coastal marshes show that H. australis is associated to coastal areas of low energy and low or none freshwater input. This result is important because this species is being used as bioindicator of coastal estuarine systems during the Holocene. Thus the paleontological interpretation based on this species should be revised. Within the studied areas, snails are associated to intertidal marsh plants. However, stable isotope analysis shows that neither plant nor their epiphytes are their main food sources. Field experiments show that snails actively select areas with plants, although tethering experiments show that plants do not provide shelter from predators. However, plants do buffer physical stress factors such as temperature, which generate important mortality outside plants covered areas. These positive interactions have large effects on H. australis distributions in marsh communities; increasing the habitats available for colonization and affecting their local distribution. [Copyright &y& Elsevier]

Abstract: The Bahía Blanca Estuary (38° 50′ S, and 62° 30′ W) presents salt marshes where interactions between the local flora (Sarcocornia perennis) and fauna (Chasmagnathus granulatus) generate some kind of salt pans that alter the normal water circulation and condition its flow and course towards tidal creeks. The crab–vegetation dynamics in the salt marsh presents variations that cannot be quantified in a reasonable period of time. The interaction between S. perennis plant and C. granulatus crab is based on simple laws, but its result is a complex biological mechanism that causes an erosive process on the salt marsh and favors the formation of tidal creeks. To study it, a Cellular Automata model is proposed, based on the laws deduced from the observation of these phenomena in the field, and then verified with measurable data within macroscale time units. Therefore, the objective of this article is to model how the interaction between C. granulatus and S. perennis modifies the landscape of the salt marsh and influences the path of tidal creeks. The model copies the basic laws that rule the problem based on purely biological factors. The Cellular Automata model proved capable of reproducing the effects of the interaction between plants and crabs in the salt marsh. A study of the water drainage of the basins showed that this interaction does indeed modify the development of tidal creeks. Model dynamics would likewise follow different laws, which would provide a different formula for the probability of patch dilation. The patch shape can be obtained changing the pattern that dilates. [Copyright &y& Elsevier]

Abstract: After intense debate it is now accepted that nutrients (a bottom-up process) and herbivores (a top-down process) are both important controls of plant productivity in many systems. Besides their direct effects, herbivores may also have profound positive or negative indirect effects that can be modulated by nutrients and time. The interactive relationships between time, nutrient availability and herbivore impacts (direct and indirect) on plant growth dynamics are an emerging research topic that merits further effort. Here we did several experiments in a SW Atlantic marsh to contribute towards that gap by focusing on the dominant plant, Spartina densiflora, and one of the dominant herbivores, the crab Neohelice (Chasmagnathus) granulata, in the marsh. Herbivory by the crab was highly seasonal, with most of the consumption occurring in fall. Even though crabs preferred nutrient enriched leaves, nitrogen content was not the driver of these seasonal variations. Crab herbivory had markedly indirect negative impacts on S. densiflora leaves, reducing their growth rates and increasing their senescence. These deleterious impacts may partially explain the seasonal decline in leaf growth and a net loss in leaf biomass observed in the fall. Fertilization did not seem to alter these processes. Adding nutrients increased leaf growth in the spring, where ambient herbivory was low, but it also increased herbivory in the fall, resulting in similar patterns as the ones observed under non-fertilized conditions. Herbivory by the crab also greatly affected the dynamics of S. densiflora stems. Increases in stem density in relation to initial conditions were larger in non-grazed than in grazed plots regardless of whether nutrients were added or not. Together, these results indicate that, in Southwestern marshes populated by S. densiflora and N. granulata, herbivory by the crab represents an important direct and indirect control of plant growth. Our results also emphasize the importance of considering impacts on growth rates and not only on biomass because not considering reduced growth after herbivory may lead to improper calculations of nutrient cycling or detritus production. [Copyright &y& Elsevier]

Abstract: Two cordgrass species of the genus Spartina cohabit in SW Atlantic (southern Brazil 31º48′ S to Argentinean Patagonia, 43º20′ S) salt marshes. Some salt marshes are dominated by the dense-flowered cordgrass Spartina densiflora (which inhabits the upper intertidal level) and others by the smooth cordgrass Spartina alterniflora (which inhabits the lower intertidal level). We investigated how the different species dominance affects the detritus dynamics in the Bahia Blanca estuary (38º47′ S, 62º20′ W Argentina). Field measurements of annual detritus production using destructive methods show that both plants are similar. However, detritus of S. alterniflora shows higher decomposition rates than that of S. densiflora. This difference may be due to a larger N content, lower lignocellulose content and lower C/N ratio of S. alternifora when compared with S. densiflora. Moreover, field sampling shows that S. alterniflora has a larger amount of trapped litter that, according to the litterbag method, has higher decomposition rates. Therefore it is highly likely that S. alterniflora salt marshes contribute towards more profitable detritus for estuarine food webs than marshes dominated by S. densiflora. These results illustrate that the composition of the coastal plant community can determine the quality and profitability of the detritus that support estuarine food webs. They also illustrate that salt marshes belonging to a same biogeographic group and even coexisting in great proximity can have very different ecosystemic roles. [Copyright &y& Elsevier]