We documented the frequency and effect on live biomass of five different types of disturbance over 14 years in creekbank and mid‐marsh zones of eight salt marshes dominated bySpartina alterniflorain Georgia,USA. Wrack (floating debris) and creekbank slumping were the most common disturbances at the creekbank, and snails were the most common disturbance agent in the mid‐marsh. Disturbance frequency varied among sites due to differences in plot elevation and landscape position. Wrack disturbance at the creekbank was positively correlated with plot elevation, and both initial slumping and terminal slumping of creekbank plots were negatively correlated with plot elevation. Wrack disturbance at the creekbank and snail disturbance in the mid‐marsh were also most common at barrier island vs. interior marshes. Disturbance varied up to 14‐fold among years. Wrack disturbance at the creekbank was negatively correlated with river discharge and sea level, and initial slumping of creekbank plots was also negatively correlated with sea level. The different disturbance types varied in their effects on end‐of‐year standing plant biomass. At the creekbank, wrack disturbance reduced biomass in affected plots by ~46%, but slumping did not affect biomass until the plot was totally lost. In the mid‐marsh, slumping and wrack were not important disturbances, but snail disturbance reduced biomass in affected plots by ~70%. In addition, abiotic conditions (river discharge, maximum monthly temperature, sea level, and precipitation) strongly affected year‐to‐year variation in biomass. Across the entire landscape, fewer than a quarter of the plots on average were disturbed, and disturbance reduced overall standing biomass by ~18% in the creekbank zone and ~3% in the mid‐marsh zone. Our results indicate that wrack has fairly strong effects on end‐of‐year biomass at the creekbank. Overall, however, variation in abiotic conditions among years had stronger effects on end‐of‐year standing biomass in both marsh zones than did disturbance.

In salt marshes, disturbance by wrack (floating mats of dead vegetation) is common and affects plant productivity and species composition, but little is known about how the timing of disturbance mediates these effects, nor how it interacts with herbivory. Using a field experiment on the Georgia coast, we simulated the effects of wrack disturbance at different times of the year on the marsh grass Spartina alterniflora and its stem‐boring herbivorous insects. The timing of disturbance throughout the growing season strongly affected fall biomass, stem height, the proportion of stems flowering, and the proportion of stems colonized by stem‐boring herbivorous insects. End‐of‐season biomass in plots disturbed in March did not differ from undisturbed controls, but biomass was reduced by 50% in plots disturbed in May, and by over 90% in plots disturbed in September. Disturbance in March and May stimulated flowering, but disturbance later in the growing season suppressed it. Plots disturbed late in the growing season had a low frequency of stem‐boring herbivores. Stems containing stem borers rarely flowered. These results indicate that the timing of disturbance matters in coastal salt marshes. Late‐season disturbances had the strongest effects on S. alterniflora and its herbivores. Disturbances early in the growing season did not affect end‐of‐season biomass, and stimulated flowering, suggesting parallels between fire disturbance in grasslands and wrack disturbance in salt marshes. Late‐season disturbance did reduce herbivory by stem‐boring insects, but not enough to compensate for the direct effects of disturbance on the plants. Future studies of disturbance in salt marshes should consider how the timing of experimental disturbance treatments relates to the timing of natural disturbances.

In order to understand the spatial distribution and ecological risk of heavy metal pollution in the sediments of the Three Gorges Reservoir, surface sediment samples in six typical tributaries and estuaries were analyzed. The heavy metal content in sediment was detected by ICP-MS (PE Elan DRC Ⅱ, PerkinElmer, Waltham, MA, USA) after digestion by mixed acid. The data were evaluated and analyzed using multiple methods, including the potential ecological risk index, the geoaccumulation index (I_geo), and cluster analysis. The results show that the tributary surface sediments were polluted by heavy metals to a certain extent. Cu, Zn, Pb, and Cr have been enriched in sediments in recent years. The results of I_geo show that the pollution degree is as follows: Pb > Cu > Zn > Cr > Cd > Ni > As > Hg. According to the potential ecological risk index, the grades of Yunyang, Xiaojiang, and Xinjin are all medium risk, and Cd has the highest contribution rate. Combined with field investigation data, cluster analysis, and correlation analysis, we conclude that Cu, Zn, and Cr are likely to come from agricultural chemical fertilizers and industrial sewage, while Pb mainly comes from the discharge of ship fuel.