Persistent effects of physical disturbance on meiobenthos in mangrove sediments

Disturbance is an important factor in structuring ecological communities, exerting its influence through changes to the physical environment and to the trajectories of successional processes. Marine environments are subject to a wide range of disturbances and while much is known about the effects of disturbance on macrobenthos in unconsolidated sediments, little is known about the responses of meiobenthos to disturbance in consolidated sediments, such as mangroves. Trampling was used to study the response of meiobenthos to disturbance in mangrove sediments. Even light trampling appeared to break up the mangrove root mat and increased the proportion of fine sediment. Densities of meiobenthos increased 2-3-fold in disturbed sediments, but there was no evidence of disproportionate abundance. Temporal variability was similar in all treatments, but spatial variability increased 4-5-fold in disturbed sediments. Effects persisted for at least 24 months, with little evidence of convergence of treatments. Meiobenthos may have exploited the increase in habitat resulting from loss of the root mat and possibly benefited from increased food from the decomposition of root material. These effects are likely to persist for several years because of the minimal recovery of the root mat.     

Benthic Disturbance and Impact Experiments in the Central Indian Ocean Basin

As a part of the Environmental Impact Assessment studies for nodule mining, a long-term program has been initiated in the Central Indian Basin. Multidisciplinary studies on geological, biological, physical, and chemical parameters were carried out in an area selected on the basis of baseline data collected in the first phase of the program. A benthic disturbance was simulated with a hydraulic device also used in the previous experiments in the Pacific Ocean. A site of 3,000 ×200 m was repeatedly disturbed by a combination of fluidizing pump and suction pump to dislodge and discharge sediment from the seafloor into the water column 5 m above the seafloor. During 9 days of operation, 26 tows were carried out for 47 h of disturbance, resuspending about 6,000 m 3 of sediment along an 88-km line. Data for postdisturbance impact assessment were collected with sediment traps, deep-towed cameras, seafloor samples, and conductivity-temperature depth sensor (CTD)-rosette observations. Seafloor data, sediment samples, and water column studies were aimed at evaluating the impact of benthic disturbance, on the basis of pre- and postdisturbance data collected during the experiment. Observations show that vertical mixing of sediment as well as its lateral movement and resedimentation because of plume migration alters various parameters and leads to changes in the environment around the area.     

Microscale Response of Sediment Variables to Benthic Disturbance in the Central Indian Ocean Basin

The response of sediment bacteria and biochemical variables to benthic disturbance was investigated in the Central Indian Ocean Basin from cores at nine stations. Generally, bacterial density and biochemical variables declined vertically with depth in the upper layers of the cores. Mechanical disturbance caused by a hydraulic disturber brought about a substantial decrease in total bacterial numbers. However, the numbers of retrievable bacteria increased by two orders of magnitude. The biochemical parameters decreased in quantity with a shift in the linear relationship. The data suggest that abiotic artifacts will directly affect the biomass and biochemistry of the sediment.     

Geotechnical Properties of Surface Sediments in the INDEX Area

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0-10 cm) have a shear strength of 0-1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312-577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm 3 . Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.     

ICP-OES法测定岩石矿物中镍和钴的方法改进

对ICP-OES法测定镍、钴的适用条件进行探讨和改进。检出限分别为0.018μg/mL和0.0042μg/mL,相对标准偏差为0.62%~1.55%,结果令人满意。     

Response of the zooplankton community to extreme hydrological variations in a temporary lake in a neotropical floodplain system

Extreme events can cause species substitution or biodiversity losses. In floodplains, flood and drought phases are considered disturbances when their amplitude or intensity are atypical. These unpredictable events can affect the structure of aquatic communities. In this context, we evaluated the response of the zooplankton community to two extreme droughts that occurred from 2001−2002 and 2014−2016. We assessed species richness and composition on three occasions: before extreme drought (phase 1); during the phase after extreme drought (phase 2) characterized by floods, and lastly, after flood (phase 3), when the fluctuations in water levels in the lowlands stabilized. We hypothesized that (i) zooplankton composition in the lake is distinct in the phases before extreme drought and flood, as the extreme drought event causes changes in the community structure and (ii) after the flood (phase 3), the composition of species in the lake would be more similar to the phase 1, with a large number of species shared in these. Contrary to what we expected, similarity in the species composition of the lake was greater between the phase 1 and phase 2, which may indicate the resilience capacity of the community, through specialized adaptations capable of tolerating extreme. The lowest similarity was observed between the phase 1 and phase 3 and this difference may be a consequence of the dilution effect of the community in the flood (phase 2). Our results suggest that the community structure in the lake can be considered more resilient to extreme drought. We also highlight the importance of the hydrological dynamics of a floodplain for the structure of communities, considering atypical droughts in periods of different climatic events.     

Holocene climate–fire–vegetation interactions at a subalpine watershed in southeastern British Columbia,Canada

Vegetation assemblages and associated disturbance regimes are spatially heterogeneous in mountain ecosystems throughout the world due to the complex terrain and strong environmental gradients. Given this complexity, numerous sites describing postglacial vegetation and fire histories are needed to adequately understand forest development and ecosystem responses to varying climate and disturbance regimes. To gain insight into long-term historical climate–fire–vegetation interactions in southeastern British Columbia, Canada, sedimentological and paleoecological analyses were performed on a sediment core recovered from a small subalpine lake. The pollen assemblages, stomata, and macroremains indicate that from 9500 to 7500 cal yr BP, Pinus-dominated forests occurred within the catchment and Alnus was also present. Climate was an important control of fire and fire frequency was highest at this time, peaking at 8 fires 1000 yr^− 1, yet charcoal accumulation rates were low, indicative of low terrestrial biomass abundance. From 7500 to 4600 cal yr BP, Pinus decreased as Picea, Abies and Larix increased and fire frequencies decreased to 3–6 fires 1000 yr^− 1. Since 7500 cal yr BP the fire regime varied at a millennial scale, driven by forest biomass abundance and fuel accumulation changes. Local scale (bottom-up) controls of fire increased in relative importance since at least 6000 cal yr BP.     

Floods,drought, and seed mass of riparian plant species

Plant communities in dryland riparian ecosystems are influenced by flood disturbance and water availability. As global climate change alters stream flow regimes, there is increasing need to understand which traits allow plant species to persist under increased stress and disturbance. Small seed mass is part of a strategy that adapts for disturbance, but the role of seed mass as an adaptation for drought is less well documented. For dryland riparian plant communities, we asked, does seed mass vary with water availability and flood frequency? We compared community seed mass between sites that vary in flow permanence (longitudinal water gradient) and between hydrogeomorphic surfaces within sites (lateral gradients of moisture and disturbance). Using data from four rivers in Arizona, we contrasted seed mass between plant groups. We found community seed mass to be greater at sites with ephemeral than perennial flow, and to increase laterally from wet, frequently-flooded channel edges to dry, less disturbed terraces. Seed mass varied by moisture group (smallest for hydroriparian species) and by disturbance group (smallest for disturbance species), and showed a trend for being greater in introduced species. We conclude that small seed mass is independently associated with wet and disturbed conditions in dryland riparian ecosystems.     

Using long-term datasets to study exotic plant invasions on rangelands in the western United States

Invasions by exotic species are generally described using a logistic growth curve divided into three phases: introduction, expansion and saturation. This model is constructed primarily from regional studies of plant invasions based on historical records and herbarium samples. The goal of this study is to compare invasion curves at the local scale to the logistic growth curve using long-term datasets. Five datasets ranging 41–86 years in length were recovered from five sites in four western states. Data for the following seven exotic species were analyzed using regression analysis to evaluate fit to a non-linear sigmoidal logistic curve: crested wheatgrass (Agropyron cristatum), dwarf alyssum (Alyssum desertorum), cheatgrass (Bromus tectorum), Lehmann lovegrass (Eragrostis lehmanniana), halogeton (Halogeton glomeratus), Russian thistle (Salsola tragus) and tumble mustard (Sisymbrium altissimum). A greater variety of curve shapes was documented by long-term datasets than those published based on herbaria sampling. Only two species from three different sites and with three different data types met the criteria for fitting a logistic curve. Many of the other species/location combinations were characterized by sporadic spikes and crashes. The general lack of fit with the model may be the results of the complex interactions that drive vegetation change in rangeland environments.     

Hydrological shifts and tree growth responses to river modification along the Apalachicola River,Florida

Shifts in the hydrologic regime of Florida’s Apalachicola River have been attributed to anthropogenic changes throughout its watershed, including local dam construction. To assess impacts of those shifts on floodplain forests, we reconstructed tree growth using dendrochronology and compared these trends with hydrological and climatic variables. Comparisons of stream-gage data before and after dam construction on the Apalachicola River revealed statistically significant mean declines in annual average stage. Mean minimum annual stages, rise rates, and fall rates also decreased, while hydrograph reversals increased. Growth in four tree species correlated strongly with site-specific inundation parameters. A wetter climate in the two decades following dam construction and fine-scale fluctuation of the hydrograph may have set the stage for positive growth releases. Logging and hurricane wind throw events may have also contributed. However, drier conditions in the last two decades are now exacerbated by stage-discharge declines that had been masked previously. Tree growth rates and recruitment have decreased and, in the absence of a major disturbance, the forest canopy is composed of an older cohort of individuals. Our findings highlight how hydrograph variability, climate change, and vegetation disturbance are all relevant for gaging and anticipating the range of impacts of river modification on floodplain forests.