Ecological thresholds of phytoplankton community across environmental gradients in the harmful algal blooms-frequently-occurring,subtropical coastal waters,East China Sea

Phytoplankton communities can response immediately and directly to environmental changes,and thus have been applied as reliable biotic indicators in aquatic systems.This study provided insights into the relationships concerning ecological thresholds of phytoplankton communities and individual taxon in response to environmental changes in coastal waters of northern Zhejiang Province,East China Sea.Results demonstrated that there existed seasonal variations of phytoplankton community ecological thresholds of which spring being higher than those in summer.As for individual species,Prorocentrum donghaiense and Noctiluca scintillans were identified as the most tolerant and sensitive indicator species in spring and summer,respectively.They exhibited strong indications in response to environmental changes.These findings highlighted that phytoplankton community structure in this region was stable when environmental gradients were below the thresholds of sensitive species,whereas potential harmful algal blooms may occur when environmental gradients exceeded the thresholds of tolerant species.

Soil characteristics and ecological thresholds of Suaeda salsa wetlands

Suaeda salsa is an annual euhalophyte in estuarine wetlands.Soil properties of wetlands have an important influence on S.salsa growth.Therefore,the soil ecological thresholds is valuable for the restoration of degraded S.salsa wetlands.The objectives of this present study were to analyze the soil physicochemical properties and evaluate the soil ecological thresholds in the typical degraded areas for S.salsa growth.Soil text components became coarser with increased sand contents and less clay contents,as the higher degree of wetland degradation.Meanwhile,the salt contents in different soil depths gradually increased with the increased degree of degradation of wetlands.Evident changes in soil water content,organic matter content,and cations concentrations were not observed,while the concentrations of these factors were higher in the soil layer of 0-10 cm than those in the 20-30 cm.The soil pH in the 0-10 cm soil layer was lower than that in the 20-30 cm.The content of the three available nutrients did not change evidently with the increasing degree of degradation.The optimum thresholds of soil salinity and water content were 7.073-16.613 g/kg and 31.8-63.2%,respectively.

Advances in Climate Change Research 12(2021)48-65 Review

In boreal and arctic regions,forest fires exert great influences on biogeochemical processes,hydrothermal dynamics of the active layer and near-surface permafrost,and subsequent nutrient cycles.In this article,the studies on impacts of forest fires on the permafrost environment are reviewed.These studies indicate that forest fires could result in an irreversible degradation of permafrost,successions of boreal forests,rapid losses of soil carbon stock,and increased hazardous periglacial landforms.After forest fires,soil temperatures rise;active layer thickens;the release of soil carbon and nitrogen enhances,and;vegetation changes from coniferous forests to broad-leaved forests,shrublands or grasslands.It may take decades or even centuries for the fire-disturbed ecosystems and permafrost environment to return to pre-fire conditions,if ever possible.In boreal forest,the thickness of organic layer has a key influence on changes in permafrost and vegetation.In addition,climate warming,change of vegetation,shortening of fire return intervals,and extent of fire range and increasing of fire severity may all modify the change trajectory of the fire-impacted permafrost environment.However,the observations and research on the relationships and interactive mechanisms among the forest fires,vegetation,carbon cycle and permafrost under a changing climate are still inadequate for a systematic impact evaluation.Using the chronosequence approach of evaluating the temporal changes by measuring changes in the permafrost environment at different stages at various sites(possibly representing varied stages of permafrost degradation and modes),multi-source data assimilation and model predictions and simulations should be integrated with the results from long-and short-term field investigations,geophysical investigations and airborne surveys,laboratory testing and remote sensing.Future studies may enable quantitatively assess and predict the feed-back relationship and influence mechanism among organic layer,permafrost and active layer processes,vegetation and soil carbon under a warming climate at desired spatial and temporal scales.The irreversible changes in the boreal and artic forest ecosystem and their ecological and hydrothermal thresholds,such as those induced by forest fires,should be better and systematically studied.