Habitat deterioration despite protection: long-term declines of littoral area of fishponds in Czech nature reserves

Fishponds play a key role in current pondscapes in many developed countries. Their littoral areas, supporting multiple ecosystem functions including the maintenance of aquatic and riparian biodiversity, have been adversely affected by the move shift towards more intensive aquaculture and widespread eutrophication in the middle 20th century. To counteract these changes, many fishponds received some protection, but its long-term efficiency has not been studied. Here we focus on the role of conservation status in protecting the area of littoral areas of fishponds in Czechia between the years 1950 and 2019. We found that the conservation status of these fishponds did not prevent habitat deterioration in most of the fishponds, especially during the second half of the 20th century. Moreover, we detected no significant effects of the reserve establishment year, fishpond area and conservation target on the littoral areas. This suggests that the conservation measures are insufficient across fishpond reserve types. We attribute the negative trends to persisting high fish stocks, especially of common carp, and eutrophication resulting from additional feeding, pond manuring, and ongoing nutrient inputs from the pond catchments. Sediment dredging and high grazing pressure by waterfowl in some reserves can further aggravate the situation. We conclude that effective protection of the littoral areas requires a paradigm shift towards less intensive fish stock management, more frequent summer drainage, and effective reduction of all nutrient inputs to increase the water quality. Such measures can help recover the littoral areas and the associated biota.

Dianchi Shallow Lake Management

Lake Dianchi in the Chinese province Yunnan is a shallow lake suffering from algae blooms for years due to high pollution. We conducted a thorough survey of the water quality of the northern part of the lake called Caohai. This study was intended as the basis for the system understanding of the shallow lake of Caohai. The study consisted of two steps. First we collected available environmental, hydrological and pollution data from Kunming authorities and other sources. It was possible to parameterise a lake model model based on the preliminary data set. It supported first estimations of management scenarios. But the first and quick answers came with a relevant vagueness. Relevant monitoring data was still missing like P release from lake-internal sediment.Because data uncertainty causes model uncertainty and model uncertainty causes planning and management uncertainties, we recommended and conducted a thorough sediment and river pollution monitoring campaign in 2017. Examination of the sediment phosphorus release and additional measurements of N and P was crucial for the improvement of the shallow lake model of Caohai. In May 2018 we presented and discussed the results of StoLaM shallow lake model of Caohai and the outcomes of a set of management scenarios.The StoLaM shallow lake model for Caohai used in SINOWATER indicates that sediment dredging could contribute to the control of algae by limitation of phosphorus, but sediment management can only produce sustainable effects when the overall nutrient input and especially the phosphorus input from the inflows will be reduced significantly.

The Long-Term Effects of Dredging on Chromium Pollution in the Sediment of Meiliang Bay, Lake Taihu, China

Sediment dredging is a common remediation tool for polluted water bodies. However, the long-term effects of dredging on chromium (Cr) contamination remain unclear. This study was conducted to evaluate the long-term effects of sediment dredging on Cr contamination in Lake Taihu, six years after dredging was performed. In this study, high-resolution equilibrium dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) sampling techniques were used for sampling total dissolved Cr and DGT-labile Cr(VI) at the sediment water interface. The results show that the vertical averaged concentrations of total dissolved Cr in summer (112.6 ± 28.8 μg/L) and winter (115.3 ± 29.9 μg/L) in the non-dredged site were above the fisheries water quality standard (AEPC, 2002). They were 38% lower in overlying water and 20% lower in sediment pore water in the dredged site in winter, while in summer the reduction was not evident. The concentration of total dissolved Cr in the dredged site was significantly higher in spring and autumn than those in the non-dredged site, which was probably caused by the large rainfall and river discharge during the two seasons. The vertically averaged concentrations of DGT-labile Cr(VI) in both the non-dredged and dredged sites did not exceed the drinking water quality standard requirements (WHO, 1993). Modeling of DGT-induced fluxes from sediment into overlying water showed a higher response time (Tc) and lower adsorption rate (k1) and desorption rate (k−1) in the dredged site except in summer, indicating that sediment dredging decreased Cr mobility in sediments. Overall, these results confirm that sediment dredging decreased the risk of Cr contamination in winter in Lake Taihu.

Effects of Sediment Dredging on Nutrient Release and Eutrophication in the Gate-Controlled Estuary of Northern Taihu Lake

Estuarine zones are regarded as the ecotones connecting the rivers and lakes. Sediment dredging is a conventional treatment technology that is widely used to remove the internal loading in estuarine zones worldwide. However, what is the characteristic of nutrient release in the gate-controlled estuary and how long this practice is effective are still unclear. Hence, sediment and water samples were collected from dredged and undredged regions around the gate-controlled estuary of northern Taihu Lake for laboratory experiments, in which they were subjected to different temperatures, depths, and disturbance levels. The total nitrogen (TN) and total phosphorus (TP) concentrations of the dredged region were lower than those from the undredged region under stable hydrodynamic conditions. A high dynamic release rate (R) of nutrients in the dredged sediments (RTN = 164.75 mg/m2·d and RTP = 5.83 mg/m2·d) existed under dynamic release conditions (stirring speeds: 90 and 120 r/min). The effect of disturbance and temperature on release rate and nutrient form was completely different for the static and dynamic release cases. The nutrient loads from dynamic release were 4–17 times greater than those from static release. For unstable hydrodynamic conditions, the release rate from the bottom sediment exceeded that from the surface sediment in the undredged region. These results indicated that, under stable hydrodynamic conditions, dredging improves long-term water quality. However, dredging alone in unstable hydrodynamic conditions may not remove the potential risk of internal release in the long term. Specific ecological and engineering measures should be combined with dredging practice to restore estuary habitats and minimize the release of internal pollutants.

GIS in environmental monitoring and assessment of dredging activites in a perturbed estuarine Lagoon

The study aimed at mapping and characterizing the benthic communities of Lagos Lagoon based on how the sediment define the distribution of organisms in response to dredging activity of the area. The hydrographic survey of was carried out with the aid of an echo-sounder and side-scan sonar and plotted in ArcGIS 10.3.1. The benthic macroinvertebrate ecology in relation to human activities was defined and studied in ten selected dredged stations. Habitat suitability maps of dominant species were plotted using spatial analyst tool. The removal of organisms with the sediment resulted into increased turbidity, suspended solids and modified sediment characteristics in the study area. The known venus, pachymelania, mangrove, estuarine-amphioplus and estuarine-rock macroinvertebrate communities were intact. However, there were changes in the composition of the communities with Donax acutangulus going into extinction. Multivariate analysis showed that the observed benthic communities and sediment characteristics were sandy in nature. The observed assemblage of organism was mostly dominated by bivalve - Atactodea striata of the family Mesodesmatidae at Ogun river station which was characterised by sand. The results revealed a patchy distribution of organisms having abundant macroinvertebrates with less diversity which may be as a result of the on-going dredging activities in the area. Keywords: Macrobenthos, bathymetry, sediment, dredging, GIS

Mitigating a global expansion of toxic cyanobacterial blooms: confounding effects and challenges posed by climate change

Managing and mitigating the global expansion of toxic cyanobacterial harmful algal blooms (CyanoHABs) is a major challenge facing researchers and water resource managers. Various approaches, including nutrient load reduction, artificial mixing and flushing, omnivorous fish removal, algaecide applications and sediment dredging, have been used to reduce bloom occurrences. However, managers now face the additional challenge of having to address the effects of climate change on watershed hydrological and nutrient load dynamics, water temperature, mixing regime and internal nutrient cycling. Rising temperatures and increasing frequencies and magnitudes of extreme weather events, including tropical cyclones, extratropical storms, floods and droughts, all promote CyanoHABs and affect the efficacy of ecosystem remediation measures. These climatic changes will likely require setting stricter nutrient (including both nitrogen and phosphorus) reduction targets for bloom control in affected waters. In addition, the efficacy of currently used methods to reduce CyanoHABs will need to be re-evaluated in light of the synergistic effects of climate change with nutrient enrichment.

Urban sediment in infiltration wells: A lesson from the northern area of greater Yogyakarta city

The development of housing in Yogyakarta eventually led to increased levels of runoff. To mitigate runoff increases, infiltration wells were constructed in a specific network. The northern part of Yogyakarta is now facing urbanization, and there are many housing blocks that are being constructed complete with the runoff drainage system. This study aims to reveal the role of the infiltration wells network about runoff and its effectiveness concerning the sediment load in the urban area. The drainage networks in the research area function as a watershed, but not with a single outlet. The runoff flow was spreading through the drainage system and did not accumulate on the single outlet. It can be seen that some infiltration wells supposed as a well, which is a place of accumulation of runoff and sediment. Most of the infiltration wells have been filled with sediment up to over a quarter, or even more than half, of their carrying volume. Granulometry measurement shows that the percentage of clay, loam, sand, and gravel in each well tend to differ for every sample. The differences between filled wells and their sediment grain depend on the location of those wells. Maintenance of the wells is required, such as sediment dredging, to rejuvenate them and improve their efficiency.