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Effective monitoring and management of inland waterbodies depend on reliable assessments of water quality through remote sensing technologies. Match-up analysis plays a significant role in investigating the comparability between in situ and remote sensing data of physical and biogeochemical variables. By exploring different spatial aggregations and temporal windows, we aimed to identify which configurations are most effective and which are less effective for the assessment of remotely sensed water quality data within the context of governmental monitoring programs. Therefore, in this study, remote sensing data products, including the variables of Secchi depth, chlorophyll-a, and turbidity, derived from the Copernicus satellites Sentinel-2 and Sentinel-3, were compared with in situ laboratory data from >100 waterbodies (lakes and reservoirs) in Germany, covering a period of 5 years (2016–2020). Processing was carried out using two different processing schemes, CyanoAlert from Brockmann Consult GmbH and eoapp AQUA from EOMAP GmbH & Co. KG, in order to analyze the influence of different processors on the results. To investigate appropriate spatial aggregations and time windows for validation (the match-up approach), we performed a statistical comparison of different spatial aggregations (1 pixel; 3 × 3, 5 × 5, and 15 × 15 macropixels; and averaging over the whole waterbody) and time windows (same day, ±1 day, and ±5 days). The results show that waterbody-wide values achieved similar accuracies and biases compared with the macropixel variants, despite the large differences in spatial aggregation and spatial variability. An expansion of the temporal window to up to ±5 days did not impair the agreement between the in situ and remote sensing data for most target variables and sensor–processor combinations, while resulting in a marked rise in the number of matches.

Remote sensing for water quality evaluation has advanced, with more satellites providing longer data series. Validations of remote sensing-derived data for water quality characteristics, such as chlorophyll-a, Secchi depth, and turbidity, have often remained restricted to small numbers of water bodies and have included local calibration. Here, we present an evaluation of > 100 water bodies in Germany covering different sizes, maximum depths, and trophic states. Data from Sentinel-2 MSI and Sentinel-3 OLCI were analyzed by two processing chains. Our work focuses on analysis of the accuracy of remote sensing products by comparing them to a large in situ data set from governmental monitoring from 13 federal states in Germany and, hence, achieves a national scale assessment. We quantified the fit between the remote sensing data and in situ data among processing chains, satellite instruments, and our three target water quality variables. In general, overall regressions between in situ data and remote sensing data followed the 1:1 regression. Remote sensing may, thus, be regarded as a valuable tool for complementing in situ monitoring by useful information on higher spatial and temporal scales in order to support water management, e.g., for the European Water Framework Directive (WFD) and the Bathing Water Directive (BWD).

Lake Sevan is the largest freshwater lake in the Caucasus region and one of Eurasia's largest alpine lakes. The lake has been experiencing growing cyanobacteria blooms; however, the relevance of external and internal phosphorus (P) loading to its eutrophication is still not known. We carried out a sediment investigation of Lake Sevan to estimate the potentially mobile P, which could be a source of internal total phosphorus (TP)-loading; we also estimated external TP-loading and retention. The study was carried out at four sampling points of different depths to determine the spatial variability of P in 2018. The sediments had elevated TP contents at the sediment surface; potentially mobile P ranged from 20 to 60% in the top 2 cm. The upper 4 cm sediment had an elevated content of TN (8 to 16.1 mg N g-1 dw) and TP (1.2 to 1.7 mg P g-1 dw). Spatial variability of most of the measured parameters was more prominent in the upper 3 cm. External TP loading was estimated to be 110 tons annually, and the TP retention was 85%. The estimated TP stored in the top cm of the sediment is 1,500 tons. The potential for P release is high; the short-term exchange between oxic and anoxic overlying water could release 0.01 to 0.02 mg P L-1 from the top cm of the sediment, and long-term diagenesis and burial could release about 0.12 mg P cm-2. Internal P-loading in Lake Sevan may play an essential role in eutrophication, especially given the long flushing time of Lake Sevan.

High-frequency in-situ monitoring of nutrients in lakes is increasingly becoming relevant to understand event-driven nutrient pulses better. These pulses are essential for phytoplankton development but are usually not captured during conventional sampling programs. In-situ monitoring of nutrients in lakes has been lagging despite the role of nutrients in eutrophication. We evaluated an in-situ wet chemistry soluble reactive phosphorus (SRP) analyzer to establish a methodological basis for high-frequency measurements of SRP. The device (based on the molybdate blue method) was evaluated in laboratory and field tests by comparing it with conventional laboratory analysis. The results of the in-situ analyser during laboratory were similar to the conventional analysis (R2 > 0.995, NSE = 0.989, PBIAS = 8.4 %). Repeated measurements by the in-situ analyzer in a 0.1 mg L-1 test solution had a mean and standard deviation of 0.1005 ± 0.0047 and a MAPE of 3.39, indicating the stability of the measurements. During field tests in a hypertrophic lake with anoxic hypolimnion, the in-situ analyzer SRP concentrations were lower and divergent from the laboratory analysis; due to potential interference of the in-situ analyzer's reagents by sulfides. Under aerobic conditions during field tests, the in-situ P results of a eutrophic, mixed shallow lake were similar to those from water samples analyzed in the laboratory (R2 = 0.562 – 0.812, NSE = 0.397 – 0.707, PBIAS = -8.5 – 24.6 %). There was no difference in SRP concentration among the unfiltered, 0.2 µm, and 10 µm pore size filtered samples; filter pore size did not play a role in the measured SRP concentrations. The in-situ SRP analyzer revealed previously unobserved phenomena, e.g., multiple strongly discontinuous SRP peaks reaching almost 0.1 mg L-1; it therefore, offers a solid methodological basis for high-frequency monitoring of SRP. Incorporating high-frequency in-situ nutrient analysis in lake monitoring provides insight

Water quality degradation of urban rivers has become a serious constraint to the sustainable development of big cities in sub-Saharan Africa, and few systematic quantitative studies have been conducted on this issue. Here, we studied three main urban rivers: Mzinga River, Kizinga River, and Msimbazi River in Dar es Salaam, the largest port city on the west coast of the Indian Ocean. The spatial and temporal changes of the physicochemical parameters including DO, pH, oxidation–reduction potential (ORP), electrical conductivity (EC), total dissolved solids (TDS), turbidity, total nitrogen (TN), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), chemical oxygen demand (CODMn), total phosphorus (TP), total dissolved phosphorus (TDP), soluble reactive phosphate (SRP), and water quality index (WQI) were investigated. The results showed that the middle and lower reaches of the three rivers were severely polluted with N (nitrogen) and P (phosphorus) with pollution increasing from the upstream to the downstream. WQI results showed that the water quality of Msimbazi River was in the “poor” category and fluctuated temporally and spatially. Principal component analyses (PCA) implied that redox status and N were the main factors affecting the water quality of the rivers. Unregulated discharges of untreated municipal and industrial wastewaters were the main drivers of water quality degradation in the rivers. Rapid urbanization characterized by population explosion and the small handicraft industry aggravated the situation. Source control and end treatment are urgently needed to prevent the water quality of the urban rivers in Dar es Salaam from deteriorating further.

Lake Sevan (Armenia) is one of the large freshwater high-mountain lakes of Eurasia. Detailed information about the extent and fate of trace elements on lake sediment and water quality has not been published yet. For this reason, surface sediment and water samples were collected from the southern and northern basins of Lake Sevan to determine trace element concentrations and assess the trace element behaviour. Geo-accumulation index, potential ecological risk index, and hazard index were calculated to estimate the environmental risk potential. In comparison to reference values, the investigated sediment samples contained elevated concentrations frequently for V, Cr, Co, Ni, Mo, Cd, Be, Ti, Rb, Sr, Se, Hf, and Th and occasionally for Cu, As, Li, B, Ag, Sb, Tl, Bi, U, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm. An overall potential ecological risk posed by all the investigated trace elements in the sediments was assessed to be moderate-considerable, however, Mo, Hg, and Cd were the elements with the highest ecological risk potential. The two basins of the lake showed significantly different behaviour according to the investigated trace element contents in the sediments to be higher in the bigger basin compared to the smaller basin. In comparison to reference concentrations in water samples according to the use of the adapted geo-accumulation index, elevated values for Ti, Cr, Cu, Cd, and Pb were observed. Elevated concentration was also observed in the case of B in nearly all water samples in comparison with literature values. Nevertheless, several water samples can be seen as not strongly anthropogenic influenced by Co, Ni, Sn, Sb, Ag, Hg, and Bi. The concentrations of trace elements in the lake water caused health risks to humans particularly children in the case of lake water used for drinking purposes, moreover, As was the main element posing health hazards. The results point out further attention to the sources of elevated trace elements in Lake Sevan, inc

Temperature and dissolved oxygen concentration are critical factors affecting the exchange of solutes between sediment and water; both factors will be affected by warming of lakes and thereby influence water quality. Temperature and oxygen responses of single solute fluxes are well known, however not much is known about interaction of temperature and oxygen in regulating the balance of different fluxes in the benthic environment. We analyzed benthic flux (mobilization and immobilization) data of various solutes (DOC, CH4, NO3--N, NH4+-N, SRP, SO4-, Fe, Mn, and O2) collected from laboratory incubations of 142 sediment cores from 5 different reservoirs incubated under varying in-situ temperature and oxygen conditions. Oxygen was the primary driver of benthic fluxes while temperature and total organic content were secondary. Temperature effects on benthic fluxes were stronger under anoxic conditions which imply that warming will substantially increase the benthic fluxes if the sediment surface becomes anoxic. The varying temperature response of processes underlying the studied fluxes will result in a shift of their relative importance in the benthic environment especially in shallow lakes that are more vulnerable to warming. For example more anoxic conditions will shift the equilibrium between net sulfate reduction and methane release towards the latter. We also predict that physical effects of warming leading to hypolimnetic oxygen depletion i.e. stronger stratification and longer hypolimnetic confinement will increase the benthic mobilization of phosphorus, dissolved organic carbon, and methane into water and immobilization of sulfate by the sediments even in deep lakes.