The structure of salt marsh soil mesofauna food webs – The prevalence of disturbance

Mesofauna taxa fill key trophic positions in soil food webs, even in terrestrial–marine boundary habitats characterized by frequent natural disturbances. Salt marshes represent such boundary habitats, characterized by frequent inundations increasing from the terrestrial upper to the marine pioneer zone. Despite the high abundance of soil mesofauna in salt marshes and their important function by facilitating energy and carbon flows, the structure, trophic ecology and habitat-related diet shifts of mesofauna species in natural salt marsh habitats is virtually unknown. Therefore, we investigated the effects of natural disturbance (inundation frequency) on community structure, food web complexity and resource use of soil mesofauna using stable isotope analysis (15N, 13C) in three salt marsh zones. In this intertidal habitat, the pioneer zone is exposed to inundations twice a day, but lower and upper salt marshes are less frequently inundated based on shore height. The mesofauna comprised 86 species / taxa dominated by Collembola, Oribatida and Mesostigmata. Shifts in environmental disturbances influenced the structure of food webs, diversity and density declined strongly from the land to the sea pointing to the importance of increasing levels of inundation frequency. Accordingly, the reduced diversity and density was associated by a simplification of the food web in the pioneer zone as compared to the less inundated lower and upper salt marsh with a higher number of trophic levels. Strong variations in δ15N signatures demonstrated that mesofauna species are feeding at multiple trophic levels. Primary decomposers were low and most mesofauna species functioned as secondary decomposers or predators including second order predators or scavengers. The results document that major decomposer taxa, such as Collembola and Oribatida, are more diverse than previously assumed and predominantly dwell on autochthonous resources of the respective salt marsh zone. The results further suggest that Mesostigmata mostly adopt an intraguild predation lifestyle. The high trophic position of a large number of predators suggests that intraguild predation is of significant importance in salt marsh food webs. Presumably, intraguild predation contributes to stabilizing the salt marsh food web against disturbances

The structure of salt marsh soil mesofauna food webs – The prevalence of disturbance - Fig 4

<p>(A) δ¹⁵N signatures of all soil mesofauna species in the upper salt marsh (USM), lower salt marsh (LSM) and pioneer zone (PZ). Boxplots represent mean (red line) and median (black line) of δ¹⁵N signatures; different letters represent significant differences between zones (Tukey’s HSD; p < 0.05). (B) δ¹³C and δ¹⁵N stable isotope values (means with standard deviation) of dominant mesofauna species: Collembola (orange squares), Mesostigmata (blue squares) and Oribatida (green squares). Black dashed horizontal lines represent estimated trophic level boundaries with each trophic level spanning 3.4‰ δ¹⁵N: I = primary decomposers, II = secondary decomposers, III = first order predators, and IV = second order predators. Number of replicates are included in parentheses; see Table C in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0189645#pone.0189645.s001" target="_blank">S1 File</a> for full species names.</p

Effect of water hardness/alkalinity and humic substances on the toxicity of peracetic acid to zebrafish embryos and pathogenic isolates

The prophylactic use of peracetic acid (PAA)-based disinfectants is becoming more popular in aquaculture due to rising concerns regarding sustainability, fish welfare and food safety. However, specific and effective PAA dosing protocols have not been developed to guide the aquaculture industry under diverse production conditions. In the present study, the effect of water hardness/alkalinity and humic substances (HS) on the toxicity of PAA to zebrafish Danio rerio embryos and the efficacy of PAA against the in vitro growth of Yersinia ruckeri and Saprolegnia parasitica was investigated. PAA concentrations that were safe to fish embryos demonstrated strong bactericidal, but limited fungistatic properties. In higher hardness/alkalinity water, or when HS was added, the same concentration of PAA resulted in a smaller pH decrease accompanied by a smaller increase of oxidation-reduction potential (ORP), and showed lower toxicity and weaker antimicrobial effects than in lower hardness/alkalinity waters. We suggest the determining factor of PAA toxicity and its antimicrobial capacity was likely ORP. At low hardness/alkalinity conditions, strong pH reduction (resulting in pH&lt;5) was the dominant role in PAA toxicity to D. rerio embryos. In aquaculture settings, lower PAA doses should be used under lower hardness/alkalinity conditions. Addition of HS under lower hardness/alkalinity conditions can assist with reducing toxicity and the risk to fish. Finally, we determined that repeated PAA disinfection is necessary to achieve a sustained prophylaxis, and we caution that the instant formation of aggregates by HS at high hardness conditions and the subsequent attachment of bacteria may reduce their susceptibility to PAA disinfection