Spatio-Seasonal Hypoxia/Anoxia Dynamics and Sill Circulation Patterns Linked to Natural Ventilation Drivers, in a Mediterranean Landlocked Embayment: Amvrakikos Gulf, Greece

Amvrakikos Gulf is a Mediterranean landlocked, fjord-like embayment and marine protected area suffering from natural, human-induced hypoxia/anoxia and massive fish mortality events. Seasonal marine geophysical and oceanographic surveys were conducted focusing on the water-circulation patterns at the sill and the spatial-seasonal distribution of dissolved oxygen (DO) in the gulf. Detailed surveys at the sill, the only communication route between the gulf and the open sea, revealed a two-layer water circulation pattern (top brackish outflow–bottom seawater inflow) and the role of the tide in the daily water exchange. Statistical analysis of the known natural drivers of DO distribution (density difference between the Ionian Sea and Amvrakikos, river inflow, wind) revealed that horizontal density gradients strongly affect anoxia reduction and seafloor oxygenation, while river inflow and wind mainly oxygenate volume/areas located above or within the pycnocline range, with DO concentrations > 2 mg/L. Complex geomorphology with well-formed internal basins contributes to the development and preservation of low DO conditions below the pycnocline. Finally, 43% of the seafloor and 36% of the gulf’s total water volume are permanently hypoxic, and reach a maximum of 70% and 62%, respectively, in September and July. This work is tailored to future ecosystem management plans, decisions, and future research on coastal ecosystems.

Water Balance and Influence Mechanism Analysis: A case study of Hongjiannao Lake, China

Hongjiannao groundwater exchange was the largest desert freshwater lake in China (57.25 km2 in 1986). However, it shrank sharply over the past 34a (1986–2019), with the smallest lake area 31.41 km2 in 2015. The objective of this study was to use the Landsat images, ASTER GDEM V2 data, and meteorology and statistics data, in combination with the water balance model to calculate the dynamics of water balance elements, quantify and characterize the interannual variations in lake-groundwater exchanges, and analyzed its influencing factors by using the geographical detector. The results showed that in the stable stage (1986–1997), the average rate of the lake area, water level, and lake volume change was − 0.26 km2/a, -0.0483 m/a, and − 0.0009 km3/a, respectively. Precipitation, river inflow, and groundwater were 0.0203 km3, 0.0485 km3, and 0.0098 km3, which accounts for the whole input were 25.83%, 61.70%, and 12.47%, respectively, evaporation was 0.0786 km3. In the reduction stage (1998–2015), the average rate of the lake area, water level, and lake volume change was − 1.21 km2/a, -0.2422 m/a, and − 0.0101 km3/a, respectively. Before 2006, precipitation, river inflow, and groundwater were 0.0154 km3, 0.0475 km3, and − 0.0025 km3, respectively; from 2006 to 2009, precipitation, river inflow, and groundwater were 0.0143 km3, 0.0334 km3, and 0.0058 km3, respectively; after 2009, precipitation, river inflow, and groundwater were 0.0139 km3, 0.0199 km3, and 0.0085 km3, respectively. Evaporation decreased from 0.0714 km3 to 0.0480 km3 from 1998 to 2015. In the growth stage (2016–2019), the average rate of the lake area, water level, and lake volume change were 1.38 km2/a, 0.27 m/a, and 0.0088 km3/a, respectively. Precipitation, river inflow, and groundwater were 0.0209 km3, 0.0005 km3, and 0.0373 km3, which accounts for the whole input were 46.63 %, 52.12 %, and 1.25 %, respectively, evaporation was 0.0187 km3. Compared with the stable stage, groundwater in the growth stage reduced from 12.47 % (0.0098 km3) to only 1.25 % (0.0005 km3). From 1998 to 2004, Hongjiannao Lake experienced continuous losing conditions (discharge from the lake to groundwater), with a variable exchange volume of up to -0.01582 km3 in 1999. Through geographical detector analysis, it was found that temperature was the dominant factor from 1988 to 1997, while human factors were the dominant factors from1998 to 2015.

Dependence structure analysis of multisite river inflow data using vine copula-CEEMDAN based hybrid model

Several data-driven and hybrid models are univariate and not considered the dependance structure of multivariate random variables, especially the multi-site river inflow data, which requires the joint distribution of the same river basin system. In this paper, we proposed a Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN) Vine copula-based approach to address this issue. The proposed hybrid model comprised on two stages: In the first stage, the CEEMDAN is used to extract the high dimensional multi-scale features. Further, the multiple models are used to predict multi-scale components and residuals. In the second stage, the residuals obtained from the first stage are used to model the joint uncertainty of multi-site river inflow data by using Canonical Vine. For the application of the proposed two-step architecture, daily river inflow data of the Indus River Basin is used. The proposed two-stage methodology is compared with only the first stage proposed model, Vector Autoregressive and copula-based Autoregressive Integrated Moving Average models. The four evaluation measures, that is, Mean Absolute Relative Error (MARE), Mean Absolute Deviation (MAD), Nash-Sutcliffe Efficiency (NSE) and Mean Square Error (MSE), are used to observe the prediction performance. The results demonstrated that the proposed model outperforms significantly with minimum MARE, MAD, NSE, and MSE for two case studies having significant joint dependance. Therefore, it is concluded that the prediction can be improved by appropriately modeling the dependance structure of the multi-site river inflow data.

Study of Internal Waves in the Persian Gulf

The Persian Gulf (PG), as a semi-enclosed water basin extends in [47-57] E, [24-30] N, geographic domain. Particularly, northern part of the PG shows more baroclinicity and turbulence because of the river inflow from the Arvand, bottom and costal stresses. Furthermore, wind stress has many effects rather than in mid deep domain of the PG. Thermocline development in the PG is observed because of studying the data measured in the Mt. Mitchell cruise in 1992 by different models from winter to summer. The studied turbulence in the northern part of the PG is navigated from winter to summer due to the internal wave’s activity and stability intensified through water column.

High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian hydroelectric reservoir

Reservoir sediments sequester significant amounts of organic carbon (OC), but at the same time, high amounts of methane (CH4) can be produced and emitted during the degradation of sediment OC. While the greenhouse gas emission of reservoirs has received a lot of attention, there is a lack of studies focusing on OC burial. In particular, there are no studies on reservoir OC burial in the Amazon, even though hydropower is expanding in the basin. Here we present results from the first investigation of OC burial and CH4 concentrations in the sediments of an Amazonian hydroelectric reservoir. We performed sub-bottom profiling, sediment coring and sediment pore water analysis in the Curuá Una (CUN) reservoir (Amazon, Brazil) during rising- and falling-water periods. The spatially resolved average sediment accumulation rate was 0.6 cm yr−1, and the average OC burial rate was 91 g C m−2 yr−1. This is the highest OC burial rate on record for low-latitude hydroelectric reservoirs. Such a high rate probably results from a high OC deposition onto the sediment, which compensates the high OC mineralization at a 28–30 ∘C water temperature. Elevated OC burial was found near the dam and close to major river inflow areas. C:N ratios between 10.3 and 17 (average ± SD: 12.9±2.1) suggest that both land-derived and aquatic OC accumulate in CUN sediments. About 23 % of the sediment pore water samples had dissolved CH4 above the saturation concentration. This represents a higher share than in other hydroelectric reservoirs, indicating a high potential for CH4 ebullition, particularly in river inflow areas.