A critical analysis of the direct effects of dredging on fish

Dredging can have significant impacts on aquatic environments, but the direct effects on fish have not been critically evaluated. Here, a meta‐analysis following a conservative approach is used to understand how dredging‐related stressors, including suspended sediment, contaminated sediment, hydraulic entrainment and underwater noise, directly influence the effect size and the response elicited in fish across all aquatic ecosystems and all life‐history stages. This is followed by an in‐depth review summarizing the effects of each dredging‐related stressor on fish. Across all dredging‐related stressors, studies that reported fish mortality had significantly higher effect sizes than those that describe physiological responses, although indicators of dredge impacts should endeavour to detect effects before excessive mortality occurs. Studies examining the effects of contaminated sediment also had significantly higher effect sizes than studies on clean sediment alone or noise, suggesting additive or synergistic impacts from dredging‐related stressors. The early life stages such as eggs and larvae were most likely to suffer lethal impacts, while behavioural effects were more likely to occur in adult catadromous fishes. Both suspended sediment concentration and duration of exposure greatly influenced the type of fish response observed, with both higher concentrations and longer exposure durations associated with fish mortality. The review highlights the need for in situ studies on the effects of dredging on fish which consider the interactive effects of multiple dredging‐related stressors and their impact on sensitive species of ecological and fisheries value. This information will improve the management of dredging projects and ultimately minimize their impacts on fish.     

Short-term temperature dependence of heterotrophic soil respiration after one-month of pre-incubation at different temperatures

Quantification of microbial activities involved in soil organic carbon (SOC) decomposition is critical for the prediction of the long-term impact of climate change on soil respiration (SR) and SOC stock. Although the temperature sensitivity of SR is especially critical in semi-arid regions, such as North West Tunisia, where the SOC stock is low, little research has been carried out in these environments. More needs to be known about factors, such as SOC availability that influence temperature sensitivity. In this study, soil samples were incubated with and without glucose addition for 28 days after a 28-day pre-incubation period. Pre-incubation and incubation was carried out at 20 °C, 30 °C, 40 °C and 50 °C. Respiration measurements were taken with temperature, glucose addition and incubation time as independent variables. The highest pre-incubation temperature reduced the temperature sensitivity of SR during the subsequent incubation period, both with and without glucose addition. Soil samples pre-incubated at 50 °C had the lowest SR at all subsequent incubation temperatures and the lowest temperature sensitivity of SR, even after glucose addition. However, after glucose addition, the effect of a high pre-incubation temperature on soil respiration lasted only two days. Measuring the water-soluble carbon (WSC) in soil samples suggested that the high pre-incubation temperature may have killed part of the microbial biomass, modified microbial communities or solubilized SOC. For quantifying the possible effect of global warming, in particular heat waves, on soil respiration in the soil studied, the results indicate a moderate response of soil respiration to temperature at high temperatures, as shown by Q₁₀ close to 1.7, even in the range 40-50 °C.     

Habitat connectivity and complexity underpin fish community structure across a seascape of tropical macroalgae meadows

Context

In heterogeneous landscapes, local patterns of community structure are a product of the habitat size and condition within a patch interacting with adjacent habitat patches of varying composition and quantity. While evidence for local versus landscape factors have been found in terrestrial biomes, support for such multi-scale effects shaping marine ecological communities is equivocal.

Objectives

We investigated whether within-patch habitat condition can override seascape context to explain the community structure of macroalgae-associated reef fishes across a tropical seascape.

Methods

We mapped the distribution and abundance of a diverse family of reef fishes (Labridae) occupying macroalgae meadows within a tropical reef ecosystem, and using best-subsets model selection, investigated the potential for habitat structural connectivity and/or local habitat quality for predicting variations in fish community structure across the seascape.

Results

Local habitat quality (canopy structure, hard habitat complexity) and area of coral-dominated habitat within 500 m of a macroalgal meadow provided the best predictors of fish community structure. However, the specific importance of a given predictor varied with fish life history stage and functional trophic group. Interestingly, macroalgae meadow area was among the least important predictors.

Conclusions

Given the complex interplay between local habitat quality and spatial context effects on fish biodiversity, our study reveals the multi-scale predictors that should be used in spatial conservation and management approaches for tropical fish diversity. Moreover, our findings question the ubiquity of habitat area effects in patchy landscapes, and cautions against a sole reliance on habitat quantity in spatial management.

     

Impacts of land use and cropland management on soil organic matter and greenhouse gas emissions in the Brazilian Cerrado

The Brazilian Cerrado is a large and expanding agricultural frontier, representing a hotspot of land-use change (LUC) from natural vegetation to farmland. It is known that this type of LUC impacts soil organic matter (SOM) dynamics, particularly labile carbon (C) pools (living and non-living), decreasing soil health and agricultural sustainability, as well as increasing soil greenhouse gas (GHG) emissions, and accelerating global climate change. In this study, we quantified the changes in the quantity and quality of SOM and GHG fluxes due to changes in land use and cropland management in the Brazilian Cerrado. The land uses studied were native vegetation (NV), pasture (PA) and four croplands, including the following management types: conventional tillage with a single soybean crop (CT), and three no-tillage systems with two crops cultivated in the same year (i.e., soybean/sorghum (NT_SSo), soybean/millet (NT_SMi) and maize/sorghum (NT_MSo)). Soil and gases were sampled in the rainy season (November, December and January) and dry season (May, July and September). The highest soil C and nitrogen (N) stocks (6.7 kg C m⁻² and 0.5 kg N m⁻², 0–0.3-m layer) were found under NV. LUC reduced C stocks by 25% in the CT and by 10% in the PA and NT. Soil N stocks were 30% lower in the PA and NT_MSo and 15% lower in the croplands with soybean compared to NV. δ¹³C values clearly distinguished between the C-origin from NV (−25‰) and that from other land uses (−16‰). Soil (0–0.1 m) under NV also presented higher labile-C (625 g C m⁻²), microbial-C (70 g C m⁻²) and microbial-N (5.5 g N m⁻²), whereas other land uses presented values three times lower. GHG emissions (expressed as C-equivalent) were highest in the NV (1.2 kg m⁻² year⁻¹), PA (1.3 kg m⁻² year⁻¹) and NT_MSo (0.9 kg m⁻² year⁻¹) and were positively related to the higher SOM turnover in these systems. Our results suggest that in order to maintain SOM, it is necessary to adopt “best” management practices, that provide large plant residue inputs (above- and belowground). This can be seen as a pathway to achieving high food production with low GHG emissions.