Geographic variation in salt marsh structure and function

Hydrodynamic and anthropogenic disturbances co-shape microbiota rhythmicity and community assembly within intertidal groundwater-surface water continuum

Tidal hydrodynamics drive the groundwater-seawater exchange and shifts in microbiota structure in the coastal zone. However, how the coastal water microbiota structure and assembly patterns respond to periodic tidal fluctuations and anthropogenic disturbance remains unexplored in the intertidal groundwater-surface water (GW-SW) continuum, although it affects biogeochemical cycles and coastal water quality therein. Here, through hourly time-series sampling in the saltmarsh tidal creek, rhythmic patterns of microbiota structure in response to daily and monthly tidal fluctuations in intertidal surface water are disentangled for the first time. The similarity in archaeal community structures between groundwater and ebb-tide surface water (R2=0.06, p = 0.2) demonstrated archaeal transport through groundwater discharge, whereas multi-source transport mechanisms led to unique bacterial biota in ebb-tide water. Homogeneous selection (58.6%-69.3%) dominated microbiota assembly in the natural intertidal GW-SW continuum and the presence of 157 rhythmic ASVs identified at ebb tide and 141 at flood tide could be attributed to the difference in environmental selection between groundwater and seawater. For intertidal groundwater in the tidal creek affected by anthropogenically contaminated riverine inputs, higher microbial diversity and shift in community structure were primarily controlled by increased co-contribution of dispersal limitation and drift (jointly 57.8%) and enhanced microbial interactions. Overall, this study fills the knowledge gaps in the tide-driven water microbial dynamics in coastal transition zone and the response of intertidal groundwater microbiota to anthropogenic pollution of overlying waters. It also highlights the potential of microbiome analysis in enhancing coastal water quality monitoring and identifying anthropogenic pollution sources (e.g., pathogenic Vibrio in aquaculture) through the detection of rhythmic microbial variances associated with intertidal groundwater discharge and seawater intrusion.

Dramatic change of bacterial assembly process and co-occurrence pattern in Spartina alterniflora salt marsh along an inundation frequency gradient

Exotic Spartina alterniflora has become widely distributed along most of the coastlines in China in a wide range of inundation frequencies. However, the assembly processes and co-occurrence patterns of the bacterial community in S. alterniflora wetlands under different inundation frequencies remain elusive. In this study, an in-situ mesocosm was established to investigate the changes in soil bacterial community. We found that soil water content was the most decisive factor in influencing the bacterial community. Balanced variation, rather than abundance gradients, accounted for the major shifts in bacterial communities and was significantly and positively correlated with the changes in water content, suggesting that species substitution was facilitated by the increased water content. Deterministic processes were dominant in community assembly, and a large degree of change in water content increased variable selection. Co-occurrence network revealed that increasing water content significantly decreased the average degree and the relative abundance of keystone species, resulting in a network with less complexity. Structural equation modelling suggests that increasing inundation frequency has strong impacts on bacterial community, primarily by altering water content, network degree, and the relative abundance of keystone species. Overall, our results illustrate that increasing inundation frequency significantly influences the bacterial community assembly processes and co-occurrence patterns.

Distinct Community Assembly Processes of Abundant and Rare Soil Bacteria in Coastal Wetlands along an Inundation Gradient

Coastal wetlands are one of the important ecosystems that play a crucial role in the regulation of climate change. Rare taxa (RT) exist in one habitat along with abundant taxa (AT).

Microbial communities commonly consist of a large number of rare taxa (RT) and few abundant taxa (AT), and it is important to identify the differences of the community assembly processes between RT and AT in response to environmental changes. However, the community assembly processes governing AT and RT in coastal wetland soils along an inundation gradient remain elusive. Here, an in situ mesocosm, with continuous inundation gradients and native mangrove Kandelia obovata or exotic cordgrass Spartina alterniflora, was established to determine the patterns and driving factors of community turnover and assembly processes of AT and RT. We found that RT exhibited a remarkably lower turnover rate than AT, and the niche breadth of RT was significantly narrower than that of AT. In comparison with AT, RT presented stronger phylogenetic signals for ecological preferences across environmental gradients. Null model analyses revealed that RT were more phylogenetically clustered and primarily governed by homogeneous selection, while AT were more overdispersed and dominated by dispersal limitation. Soil water content was the most decisive factor for community turnover and assembly processes of both AT and RT. Structural equation modeling analysis showed that RT were strongly associated with K. obovata biomass rather than S. alterniflora biomass, suggesting a strong relationship between RT and the growth of mangrove K. obovata. Overall, our study revealed distinct assembly processes of soil AT and RT communities in coastal wetlands, which is crucial for mechanistic understanding of the establishment and maintenance of soil microbial diversity in coastal wetlands under conditions of global environmental changes.

IMPORTANCE Coastal wetlands are one of the important ecosystems that play a crucial role in the regulation of climate change. Rare taxa (RT) exist in one habitat along with abundant taxa (AT). In this study, we found that RT exhibited narrower niche breadth and stronger phylogenetic signals than AT. Null model analyses showed that RT were more phylogenetically clustered and primarily governed by homogeneous selection, while AT were more overdispersed and dominated by dispersal limitation. Revealing the differences in the community assembly processes between AT and RT in coastal wetlands is critical to understand the establishment and maintenance of soil microbial diversity in coastal wetlands with regard to environmental changes.

Biogeographic context dependence of trophic cascade strength in bromeliad food webs

Ecosystem functions and the biomass of lower trophic levels are frequently controlled by predators. The strength of top-down control in these trophic cascades can be affected by the identity and diversity of predators, prey, and resources, as well as environmental conditions such as temperature, moisture, and nutrient loading, which can all impact interaction strength between trophic levels. Few studies have been able to replicate a complete community over a large geographic area to compare the full trophic cascade in a manipulative experiment. Here, we identify geographic dependency in trophic cascade strength, and the driving factors and specific mechanisms behind it, by combining geographically replicated experiments with a novel approach of community analogues of common garden and transplant experiments. We studied a predator-detritivore-detritus food web in bromeliads in Puerto Rico, Costa Rica, and Brazil. We found that interaction strengths between resources, consumers, and predators were strongly site-specific, but the exact mechanism differed between trophic levels. Large bodied predators created strong interaction strengths between predator and consumer trophic levels, reducing consumer abundance regardless of the geographic location, whereas small-bodied predators created weak interactions with no impact on consumer abundances in any site. In contrast, the interaction strength between consumers and resources varied among sites, depending on the dominant species of leaf detritus. More labile leaf species in Costa Rica created a strong consumer-resource interaction and therefore strong trophic cascade, whereas tougher leaf species in Brazil created a weak consumer-resource interaction, and an overall weaker trophic cascade. Our study highlights the importance of replicating experiments over geographic scales to understand general patterns of ecological processes.

Mosquito distribution in a saltmarsh: determinants of eggs in a variable environment

Two saltmarsh mosquitoes dominate the transmission of Ross River virus (RRV, Togoviridae: Alphavirus), one of Australia's most prominent mosquito-borne diseases. Ecologically, saltmarshes vary in their structure, including habitat types, hydrological regimes, and diversity of aquatic fauna, all of which drive mosquito oviposition behavior. Understanding the distribution of vector mosquitoes within saltmarshes can inform early warning systems, surveillance, and management of vector populations. The aim of this study was to identify the distribution of Ae. camptorhynchus, a known vector for RRV, across a saltmarsh and investigate the influence that other invertebrate assemblage might have on Ae. camptorhynchus egg dispersal. We demonstrate that vegetation is a strong indicator for Ae. camptorhynchus egg distribution, and this was not correlated with elevation or other invertebrates located at this saltmarsh. Also, habitats within this marsh are less frequently inundated, resulting in dryer conditions. We conclude that this information can be applied in vector surveillance and monitoring of temperate saltmarsh environments and also provides a baseline for future investigations into understanding mosquito vector habitat requirements.

Nutrient presses and pulses differentially impact plants, herbivores, detritivores and their natural enemies

Anthropogenic nutrient inputs into native ecosystems cause fluctuations in resources that normally limit plant growth, which has important consequences for associated food webs. Such inputs from agricultural and urban habitats into nearby natural systems are increasing globally and can be highly variable, spanning the range from sporadic to continuous. Despite the global increase in anthropogenically-derived nutrient inputs into native ecosystems, the consequences of variation in subsidy duration on native plants and their associated food webs are poorly known. Specifically, while some studies have examined the effects of nutrient subsidies on native ecosystems for a single year (a nutrient pulse), repeated introductions of nutrients across multiple years (a nutrient press) better reflect the persistent nature of anthropogenic nutrient enrichment. We therefore contrasted the effects of a one-year nutrient pulse with a four-year nutrient press on arthropod consumers in two salt marshes. Salt marshes represent an ideal system to address the differential impacts of nutrient pulses and presses on ecosystem and community dynamics because human development and other anthropogenic activities lead to recurrent introductions of nutrients into these natural systems. We found that plant biomass and %N as well as arthropod density fell after the nutrient pulse ended but remained elevated throughout the nutrient press. Notably, higher trophic levels responded more strongly than lower trophic levels to fertilization, and the predator/prey ratio increased each year of the nutrient press, demonstrating that food web responses to anthropogenic nutrient enrichment can take years to fully manifest themselves. Vegetation at the two marshes also exhibited an apparent tradeoff between increasing %N and biomass in response to fertilization. Our research emphasizes the need for long-term, spatially diverse studies of nutrient enrichment in order to understand how variation in the duration of anthropogenic nutrient subsidies affects native ecosystems.