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Questions: Coastal vegetated systems are known to play a fundamental role in climate change mitigation as a result of their efficiency sequestering and storing atmospheric CO2. While most of the work evaluating carbon sequestration capacity has focused on global change factors that can affect carbon release from plant litter decomposition through changes in (large‐scale) environmental conditions, less is known about the possible effects of the loss (or replacement) of dominant species. We hypothesized that dominant marsh plants can influence decomposition not only through changes in litter quality but also through changes in (microscale) soil environmental conditions such as humidity, soil temperature or solar radiation. Location: We performed a field manipulative experiment in a southwestern (SW) Atlantic salt marsh in Argentina. Methods: We simulate a selective disturbance (i.e., removal of the dominant grass species Spartina densiflora) thus allowing removal plots to develop an alternative plant community. To evaluate the effect of the dominant grass species on litter decomposition, in an experiment we performed a litterbag approach three years after the establishment of the removal plots. Results: Results showed that the presence of S. densiflora significantly decreased litter decomposition directly by producing less labile litter, but also by effects that seem to be related to its structure as standing dominant vegetation. The experimental removal of S. densiflora led to an alternative plant community, formed by otherwise subordinate species, which is less densely packed, allowing higher radiation incidence on the soil and elevated midday soil temperature. Conclusions: Our results suggest that salt marsh litter decomposition, and thus C sequestration, is determined in part by the identity of the dominant plant, not only because of the quality of the produced litter but also as a consequence of the vegetation structure. Changes in species diversity, above all changes in the dominant species in these coastal systems, could have large impacts on the carbon turnover and mitigation capacity of these ecosystems. [ABSTRACT FROM AUTHOR]

Question: Does the effect of droppings and herbivory by wild guinea pigs on salt‐marsh vegetation vary across microhabitats? Location: Upper salt marsh in the Mar Chiquita coastal lagoon (37°44′52″ S, 57°26′6″ W, Argentina). Methods: During autumn 2014, we performed a micro‐scale experiment manipulating droppings of wild guinea pigs (Cavia aperea). After 15 days, we compared the content of nutrients (i.e., NH4+, NO3−, and PO43−) in soil. Then, we started two factorial experiments in the high marsh, specifically one in marsh matrix (i.e., tall vegetation) and the other in open patches (i.e., short vegetation). Units were randomly assigned to different treatments (with and without herbivores and droppings). In early summer, we compared total plant cover, maximum height, above‐ground biomass, plant species richness and the composition of plant assemblages between treatments. Results: Droppings increased the content of PO43− in soil by five times, but decreased the content of NO3−. Herbivory reduced total cover, plant height, and above‐ground biomass in the marsh matrix and the open patches. In the marsh matrix, droppings increased the total cover, whereas herbivory reduced the abundance of Spartina densiflora (cover and above‐ground biomass) and subordinate species (cover). Additionally, the cover of subordinate species was negatively associated with the cover of Spartina densiflora. Therefore, in the marsh matrix, by increasing light availability at the ground level, herbivory increased plant species richness, but with low cover of subordinate species. In contrast, in open patches, herbivory and droppings interacted to reduce plant species richness. Conclusion: Herbivory by small mammals can control the above‐ground biomass, structure, and plant community composition of the high marsh, but non‐trophic mechanisms (droppings) can also affect community composition. Nonetheless, ecological impacts of small mammals depend on the context, because the magnitude and direction of trophic (herbivory) and non‐trophic (droppings) effects change across microhabitats. Our understanding of trophic and non‐trophic effects of small herbivores remains relatively poor compared to that of large herbivores. Our experiment, simultaneously manipulating wild guinea pig herbivory and droppings in a salt marsh, shows that these factors can interact, and the magnitude and the direction of these trophic and non‐trophic ecological effects depend on the environmental context. [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]

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. [ABSTRACT FROM AUTHOR]

Theory predicts that ecosystem engineers should have their most dramatic effects when they enable species, through habitat amelioration, to live in zones where physical and biological conditions would otherwise suppress or limit them. Mutualisms between mycorrhizal fungi and plants are key determinants of productivity and biodiversity in most terrestrial systems, but are thought to be unimportant in wetlands because anoxic sediments exclude fungal symbionts. Our field surveys revealed arbuscular mycorrhizal associations on salt marsh plant roots, but only in the presence of crabs that oxygenate soils as a by-product of burrowing. Field experiments demonstrate that fungal colonization is dependent on crab burrowing and responsible for nearly 35% of plant growth. These results highlight ecosystem engineers as ecological linchpins that can activate and maintain key mutualisms between species. Our findings align salt marshes with other important biogenic habitats whose productivity is reliant on mutualisms between the primary foundation species and micro-organisms. [ABSTRACT FROM AUTHOR]

Polychaetes and burrowing crabs are widely distributed in intertidal soft bottom environments, playing an important role in structuring infaunal assemblages through trophic and non-trophic interactions such as bioturbation. In southwestern Atlantic (SWA; 37°40′S, 57°23′W) intertidal mudflats, the polychaetes Laeonereis acuta and Neanthes succinea coexist with the burrowing crab Neohelice (Chasmagnathus) granulata. N. granulata and L. acuta strongly modify the sediment characteristics at different spatial scales, changing the infaunal assemblages and probably affecting feeding mode of N. succinea , which could prey upon L. acuta . Here, we experimentally evaluated the effects of constant densities of L. acuta and N. succinea on the benthic species assemblages and sediment organic matter (OM) content in two contrasting scenarios of crab bioturbation: inside and outside crab beds. We found that (1) both polychaetes did not affect the mean density of other macrofaunal organisms neither the meiofauna in general, but L. acuta modifies the abundance of specific groups such as foraminiferans; (2) polychaetes produced changes in meiofaunal spatial distribution probably by adding habitat heterogeneity; and (3) no evidence of predation of N. succinea on L. acuta were observed. Additionally, the variable effects of polychaetes on chlorophyll a nd OM content showed that they were species-specific and also modified by crab bioturbation. Moreover, effects of crab bioturbation on primary producers, quality and content of OM, and on some macro and meiofaunal organisms were found. Our results suggest that the effects of L. acuta and N. succinea on benthic species and OM content are mostly species-specific and, with regard to food sources (OM content and microphytobenthic biomass), strongly modified by larger scale crab bioturbation. [ABSTRACT FROM AUTHOR]

Abstract: In this work we evaluate the potential grazing impact of the invasive reef-forming polychaete Ficopomatus enigmaticus in a SW Atlantic coastal lagoon (Mar Chiquita coastal lagoon; 37° 40'' S, 57° 23'' W; Argentina). This gregarious species feeds on suspended detritus and phytoplankton. Given the large area dominated by reefs, suspension feeding by this species might reduce the lagoon phytoplankton concentration and even hinder local eutrophication. To evaluate this hypothesis in situ replicated mesocosm experiments were performed in spring 2005 and, summer and winter 2006. Mesocosms enclosing reefs and without reefs were installed and grazing intensity was measured as the difference in chlorophyll a concentration and turbidity between the reef-treatment and the treatment without reefs. Reefs of F. enigmaticus decreased the mean chlorophyll a concentration, more during summer (56% decrease) than spring (25% decrease) and winter (19% decrease). Reefs also decreased water turbidity during summer (54% decrease) and spring (23% decrease). While previous studies indicate that the physical structure of these reefs alters water flow increasing water turbidity, our evidences show that their suspension-feeding activity can counteract this effect. Water turbidity was positively correlated with chlorophyll a concentration, which suggests that phytoplankton grazing by F. enigmaticus decreased light attenuation through the water column, with a potential for enhancement of benthic primary productivity. Therefore, our results suggest that grazing by this polychaete affects overall estuarine primary production as well as the relative importance of planktonic and benthic carbon sources to higher trophic levels. [Copyright &y& Elsevier]

We present a comparative analysis of the biomass dynamics of the two dominant Spartina plant species in South West Atlantic saltmarshes: Spartina densiflora Brong. (high intertidal zone) and Spartina alterniflora Loesel. (mid-low intertidal zone). We assessed aboveground biomass, live: dead ratios, net production, mortality and turnover rates, and then used this information to understand if saltmarshes dominated by one or the other Spartina species have different ecosystemic roles in the recycling of organic matter. Through field sampling we found that S. densiflora had larger live, dead and total biomass than S. alterniflora . When comparing within each species, S. alterniflora had larger live than dead biomass for most of the year, while S. densiflora had more dead than live biomass. Through the Weigert and Evans (1964) net aerial primary production (NAPP) estimating method we found that S. densiflora had larger annual NAPP, annual mortality and live turnover rates than S. alterniflora . In both species the amount of NAPP was similar to the amount of annual mortality. These results suggest that S. densiflora aboveground biomass is a carbon accumulation zone, while S. alterniflora is a carbon recycling one. [ABSTRACT FROM AUTHOR]

Abstract: Interaction strength is a key component in food-web dynamics being highly variable over time and space, depending on biotic and abiotic conditions. Intertidal soft bottom ecosystems are essential nursery and feeding sites for migratory shorebirds and marine fishes, which in turn affect infaunal assemblage and the food web dynamic. In these systems, organisms that modify the resource availability to other species (i.e. burrowing crabs) are expected to have significant consequences on infauna distribution and the interactions among organisms. Here we simultaneously evaluated the interaction strength of birds and fishes on infaunal prey in areas with and without crab bioturbation. Field experiments showed weak predator–prey interactions of both predators; however, birds exerted high impact on meiofauna inside bioturbated areas during the warm season. Moreover, the effect of fishes differed depending on bioturbation and another factor such as climatic events (e.g. El Niño Southern Oscillation (ENSO)). Thus, our results suggest that interaction strength of birds and fishes on prey are context dependent, varying between different areas, tidal time exposition, ENSO episode and seasons. Also, our results exemplify how the outcome of predator–prey interaction strength can depend on other factors such as the activity of other species that modify the environment. [Copyright &y& Elsevier]

The effects of mound building ants on soil and vegetation have been described worldwide; however, few studies have explored their effects in salt marsh communities. Here, we studied the effects of the black fire ant Solenopsis richteri on the cordgrass Spartina densiflora in a southwestern Atlantic salt marsh. We found that S. richteri nests are more abundant in the high marsh than in the low marsh and in the terrestrial environment. Sediment characteristics were examined in the above-ground and below-ground portions of nests, and in the surrounding soil apart from them. Nests' sediment had lower organic matter content, lower bulk density and higher pH values than the non-nest soil. These differences were in general more marked in the above-ground portion of nests. Grain size distribution was biased towards coarser grain particles in nests, with the smallest particles being more abundant in the non-nest soil. Ammonium content was higher in the nests than in the non-nest soil, despite nitrate and dissolved inorganic nitrogen were not different. Phosphate content was higher in the below-ground portion than in the above-ground portion of nests. Samplings and transplant experiments showed that plants growing in contact with mounds grew longer, had wider stems and higher root biomass than plants apart from mounds. These results indicate that by affecting sediment characteristics, ants indirectly and positively affect plants condition, with potential cascading up effects on community structure and dynamics. Because mounds are relocated every ~3.5 months, bioturbation impacts may extend over time through the whole high marsh, highlighting the role that black fire ants may have in the ecology of southwestern Atlantic marshes. • Black fire ants inhabit southwestern Atlantic salt marshes. • Ant nests are more abundant in the high than low marsh and terrestrial environment. • Ants modify the physico‐chemical characteristics of the marsh sediment. • Plants in contact with ant mounds perform better than plants apart from mounds. • Bioturbation effects may extend over time through the marsh due to mound relocation. [ABSTRACT FROM AUTHOR]

Nutrient input drive macroalgal blooms and increases in photosynthetic activity in coastal ecosystems. An intense macroalgal photosynthetic activity can increase the surrounding pH and it could prevent the acidification that often follows an eutrophication process. We tested this hypothesis with field sampling and experiments in a macrotidal (up to 9 m in amplitude) coastal system within a semi-desert region with contrasting eutrophic conditions and Ulva lactuca blooms in the northern Argentinean Patagonia (San Antonio Bay). Our results indicate that daily pH variability during low tide could be controlled by the photosynthetic activity of Ulva lactuca under eutrophic conditions. At seasonal scale, the pH variations were related to environmental features, particularly seawater temperature. Both environmental (i.e. high solar radiation, negligible freshwater inputs and large tidal action) and anthropogenic nutrient inputs into the studied area promote the Ulva lactuca blooms, which in turn increases the surrounding pH in well oxygenated seawater through the intense photosynthetic activity. Our study shows that eutrophication instead of being a driver of acidification, could contribute to its prevention in well oxygenated marine coastal systems located within semi-desert regions. • Macroalgal photosynthesis (MP) controls daily pH variability during low tide. • Environmental factors control pH variability at seasonal scale. • Ulva lactuca photosynthetic activity increased the pH of seawater. • Macrotidal action and MP prevent coastal acidification in an eutrophic system. [ABSTRACT FROM AUTHOR]