SW Atlantic Salt Marsh Research Articles

South West Atlantic salt marshes as model systems for community and ecosystem ecology

Just as some species are used as model systems in organismal biology (e.g., physiology, genetics), many ecosystems are commonly used as model systems in ecology. Salt marshes, for instance, are great models to perform manipulative field experiments, and thus, were historically used to understand the drivers of community and ecosystem function. Decades of experimental work, indeed, made a strong contribution to community ecology as a discipline, but most of the emerged hypotheses and models were grounded in a few sites. When studies from new sites came onboard, looking to enlarge generalities, their results challenged the prevailing ideas. Here, we review more than 25 years of intense experimentation in South West Atlantic salt marshes, which helped not only to increase the knowledge about salt marsh functioning, but also to expand this knowledge beyond salt marshes helping to refine community and ecosystem function theory. We show that results coming from SW Atlantic marshes significantly contribute to understand 1) the separate and interactive effect of biotic and abiotic stress for species distribution and even for ecosystem stability, 2) the integrated role of species that can function as ecosystem engineers and as consumers, 3) the balance between stochastic and deterministic forces as drivers of community structure and 4) the regulation of cross-ecosystem fluxes. Nevertheless, we believe SW Atlantic salt marshes still have a lot more to offer, not only as conceptual models that help satisfy our intellectual curiosity, but also as key ecosystems that provide valuable benefits to our societies.

Driving forces behind latitudinal variations in plant-herbivore interactions in SW Atlantic salt marshes

Fil: Canepuccia, Alejandro Daniel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina

Population structure of the burrowing crab Neohelice granulata (Brachyura, Varunidae) in a SW Atlantic salt marsh

Neohelice granulata inhabits estuarine and protected coastal areas in temperate regions, and is the most dominant decapod crustacean in the Bahía Blanca estuary. The population structure was studied during a year in a SW Atlantic salt marsh located in the Bahía Blanca estuary, Argentina. Crabs were sampled monthly from August 2010 to July 2011. The maximum observed density was 39 crabs/m2 in February and 70 burrows/m2 in May. The maximum carapace width (CW) in males was 32 mm and 27.5 mm in females. Medium size crabs were between 16 and 20 mm CW. Significantly smaller sized crabs were observed at the lower intertidal regions (p

Biological and physical factors affecting the colonization of vegetation-free patches in a SW Atlantic salt marsh

Biological and physical factors affecting the colonization of vegetation-free patches in a SW Atlantic salt marsh

Biogeomorphically driven salt pan formation in Sarcocornia-dominated salt-marshes

Salt-marshes are under increasing threat, particularly from sea-level rise and increased wave action associated with climate change. The development and stability of these valuable habitats largely depend on complex interactions between biotic and abiotic processes operating at different scales. Also, interactions between biotic and abiotic processes drive internal morphological change in salt-marshes. In this paper we used a biogeomorphological approach to assess the impact of biological activities and interactions on salt pan formation in Sarcocornia-dominated salt marshes. Salt pans represent a key physiographic feature of salt-marshes and recent studies hypothesized that biogeomorphic processes could be related to salt pan formation in SW Atlantic salt-marshes. The glasswort Sarcocornia perennis is one of the dominant plants in the salt-marshes of the Bahía Blanca Estuary (Argentina) where they form patches up to 8m in diameter. These salt-marshes are also inhabited in great densities by the burrowing crab Neohelice (Chasmagnathus) granulata whose bioturbation rates are among the highest reported for salt-marshes worldwide. A set of biological interactions between N. granulata and S. perennis appears to be responsible for salt pan development in these areas which has not been described elsewhere. The main objective of this work was to determine the ecological interactions occurring between plants and crabs that lead to salt pan formation by using field-based sampling and manipulative experiments. Our results showed that S. perennis facilitated crab colonization of the salt-marsh by buffering otherwise stressful physical conditions (e.g., temperature, desiccation). Crabs preferred to construct burrows underneath plants and, once they reach high densities (up to 40 burrowsm−2), the sediment reworking caused plant die-off in the central area of patches. At this state, the patches lose elevation and become depressed due to the continuous bioturbation by crabs. Thus, salt pans are generated in this case by a set of biogeomorphic processes that include pure ecological interactions such as plant facilitation of crab settlement and also indirect negative effects of crabs on plant survival. Furthermore, crab bioturbation affects sediment structure due to concentration of burrowing activity under plant canopies promoting elevation loss and leading, after a few years, to salt pan formation in a previously vegetated substrate.

Nutrients and Abiotic Stress Interact to Control Ergot Plant Disease in a SW Atlantic Salt Marsh

Fil: Daleo, Pedro. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina

Effects of nutrient enrichment and crab herbivory on a SW Atlantic salt marsh productivity

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.

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

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.

Relationship between anthropogenic sewage discharge, marsh structure and bird assemblages in an SW Atlantic saltmarsh

One of the main effects of urbanization on coastal areas is through the discharge of sewage, which increases nutrient concentrations in the receiving environment. Salt marshes, like other coastal marine environments, are limited by nutrients, mainly nitrogen, and thus increasing nutrient loadings to a marsh may have consequences on marsh characteristics. We evaluated how the effects of nutrient enrichment in the form of sewage input, affected the vegetation structure and bird assemblages in a Spartina alterniflora salt marsh system near Bahía Blanca, Argentina (39° 01′ S – 56° 25′ W). Surveys of nutrient concentration, vegetation and birds were made at three different distances from the sewage discharge source. The concentration of ammonium, phosphate, and nitrate and the percent organic matter was higher in marshes nearest to the sewage discharge source. Bird composition and abundance, and vegetation physiognomy changed along a gradient of nutrient concentration. The increased habitat complexity found near the areas of higher nutrient concentration was exploited by birds that use neighboring interior and coastal habitats, including Spartina densiflora marshes, freshwater marshes and upland shrubby habitats. Our results show that local increases of nutrient inputs directly changed the vegetation physiognomy, and indirectly the composition and abundance of bird assemblages.

Crab bioturbation and herbivory reduce pre- and post-germination success of Sarcocornia perennis in bare patches of SW Atlantic salt marshes

Colonization of bare patches is a key process during community development given that pioneers usually have positive and negative effects on the forthcoming species. A variety of biotic and abiotic factors influence the process of colonization of bare patches. In salt marshes, however, the emphasis has been on abiotic factors and plant-plant interactions, while comparatively little attention has been paid to the role of plant-animal interactions in the successful colonization by pioneer plants. Thus, the goal of the present study was to evaluate whether bioturbation and herbivory by the bur- rowing crab Neohelice (Chasmagnathus) granulata affected the number of seedlings of the pioneer plant Sarcocornia perennis in SW Atlantic salt marshes. To evaluate this, we conducted an experiment using exclosures deployed at different times (post-dispersal and post-germination). The results showed that post-germination exclosures had 62% fewer seedlings than post-dispersal exclosures but 8 times more than plots always accessible to crabs. We also used glass beads to experimentally eval- uate the potential effect of crab bioturbation on seed availability, and we used 1 yr old transplants to evaluate whether herbivory could explain post-germination mortality. Crab bioturbation reduced the number of glass beads on the surface by 56%, and transplants were highly consumed when crabs were present. These results suggest that seed burial by bioturbation exerts a pre-germination control while herbivory exerts a post-germination control. The results also highlight the importance of con- sidering biotic factors when analyzing the success of marsh plants colonizing bare surfaces.

The burrowing crab Neohelice granulata affects the root strategies of the cordgrass Spartina densiflora in SW Atlantic salt marshes

Salt marshes are among the most productive systems of the world, with plant primary production limited by soil oxygen deficiency and nutrient availability. Nevertheless, root adaptations to anoxia and nutrient acquisition are different and often incompatible. The SW Atlantic salt marshes are characterized by high densities of the deep (up to 1 m) burrowing crab Neohelice granulata (Dana, 1851) that may change soil physical and chemical characteristics by burrow construction. In this work, we experimentally evaluated the hypothesis that crab burrowing can enhance soil oxygenation, causing changes in Spartina densiflora Brongniart below ground tissues from structures adapted to anoxia to systems adapted for nutrient acquisition. This response, in turn, would enhance plant productivity. Results from field observations show that oxygen availability is higher in zones with high burrow densities. As burrow densities increased, the plant root distribution changed from shallow (associated to low oxygen availability) to deeper and vertically homogeneous, with a positive correlation between burrow density and plant aboveground biomass. Experimental exclusion of crabs shows that they induce changes of root strategies from anoxia toleration to nutrient efficient acquisition, with increasing plant productivity. The invasive success that this plant shows in other parts of the world is likely to be due to their ability to tolerate harsh environmental conditions. Our results suggest that the morphological plasticity of S. densiflora is also important in their native zone given the characteristics of their specific habitat.

Sediment dynamics modulated by burrowing crab activities in contrasting SW Atlantic intertidal habitats

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 (380 g m −2 day −1) and open mudflats (1.2 kg m −2 day −1), but were enhancing sediment removal in the saltmarsh edge (between 10 and 500 g m −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.

Changes in rainfall pattern affect crab herbivory rates in a SW Atlantic salt marsh

Climatic fluctuations usually change the intensity of existing interactions. Thus, in the context of the global climate change, it is important to consider new potential interactions or changes that may appear. Heavy rainy periods (one of the consequences of global climate change in eastern-central Argentina) can promote flooding in some estuaries (mainly on coastal lagoons), and thus, affect interactions between species. In this work we investigate if climatic fluctuations can affect Spartina densiflora Brong. (dominant marsh plant) survival through a chain of biotic and abiotic interactions in a SW Atlantic costal lagoon (37° 40′S, 57° 23′W; Mar Chiquita, Argentina). To achieve this, the long-term rainfall behavior of this region, and the effect of rainy periods on submergence of estuarine marsh areas (using satellite images) were analyzed. Then, the effect of flooding on the activity of the dominant herbivore of this system, the burrowing crab Neohelice granulata (= Chasmagnathus granulatus), was studied using pitfall traps. Finally, the effect of flooding on crab herbivory rates and plant survival were analyzed using transplants, stem-marking and flooding experiments. Long-term rainfall behavior showed that mean annual rainfall has increased during the last century, with the occurrence of more rainy years, and increases in cumulative monthly rainfall increased the submerged area of the S. densiflora marsh. Also, crab activity in the marsh largely increased during periods of flooding, associated with more than 100% increments in herbivory rates and stem mortality. These results reveal that increments in rainfall regime can trigger a cascade of abiotic and biotic interactions leading to increased marsh mortality, and stresses the importance of considering both, biotic and abiotic factors, together to predict changes in community organization.