Survey of sediment oxygenation in rhizospheres of the saltmarsh grass - Spartina anglica.[Pubmed: 28262356]

Although transport of oxygen via the aerenchyma tissue and subsequent oxygen loss across root surfaces is well-documented for salt marsh grasses, only few studies have measured the oxygenation of sediment surrounding roots and rhizomes. In this study, sediment oxygenation was assessed in situ in rhizospheres of the intertidal salt marsh grass, Spartina anglica - an invading species, vigorously spreading in many wetlands around the world. The rhizospheres of two populations of S. anglica with differing plant morphology growing in different sediment types were investigated in situ using a novel multifiber optode system with 100 oxygen probes. No oxygen was detected inside the rhizospheres at any depth in either location, indicating a limited impact of plant-mediated sediment oxygenation on the bulk anoxic sediment. Subsequent planar optode studies imaging the oxygen content around the roots substantiated these findings showing that sediment oxygenation was present in both locations, but it was confined only to the immediate vicinity of the root tips. The size of the oxic zones surrounding the root tips differed between sediment-types: in S. anglica growing in permeable sandy sediment, oxic root zones extended 1.5mm away from the roots surface compared to only 0.4mm in muddy tidal flat deposit, which had a substantially higher oxygen demand. The oxygen concentration inside the oxic root zones remained stable during continuous light and air-exposure of the aboveground biomass. In comparison, sediment oxygenation generated by burrowing infauna (Hediste diversicolor) showed to be markedly more temporally variability, reaching anoxic conditions multiple times during a 5-h period.

Toxicity of an herbicide and adjuvant to saltmarsh invertebrates in the management of invasive grass; Comparative laboratory and field tests.[Pubmed: 27262496]

Coastal weeds are often treated with herbicides without knowledge of non-target impacts, and toxicity data from standardized test species can have limited applicability. We evaluated toxicity to invertebrates from Fusilade Forte® and the adjuvant Hasten® in the control of invasive salt marsh grass, Spartina anglica. For 3 of 4 local invertebrates, Fusilade Forte® was moderately toxic (96h LC50 5.4-144mgL(-1)), whereas Hasten® was less toxic (14.2-450mgL(-1)). For most species, the mixture was more toxic than the herbicide alone, with 96h LC50 reduced 23-45%. However, a field experiment applying typical concentrations (1000×the lowest 96h LC50) showed low concentrations of herbicide residues and no detrimental impacts on invertebrates over 6months. The results reveal the importance of testing locally relevant species for potential toxicity, and of comparison tests with field exposures to determine the realised toxicity in nature.