Broad-Scale Patterns in CDOM and Total Organic Matter Concentrations of Inland Waters – Insights from Remote Sensing and GIS

The rise in CDOM (coloured dissloved organic matter) is likely to be relatively more pronounced in remote northern regions. However, there is a lack of monitoring to confirm this. For this reason, there is a strong incentive to develop remote sensing-based methods to map CDOM in lakes across broader geographical scales and to include geograghic contex in such analysis. There is a lack of understanding of the mechanisms behind changes in water colour (i.e. CDOM) at large scales. The CDOM variations could be due to varying drivers, such as climate and landscape patterns or catchment features. This means that currently, we do not know the extent to which aquatic ecosystems need conservation efforts, such as management of the surrounding vegetation, to prevent CDOM leakage. Thus, there is need to better understand the drivers behind CDOM changes in inland waters.Over the last few decades, remote sensing technologies and methods have developed dramatically for terrestrial ecosystems. Coupled with the broader availability of remote sensing data, free access to different data sources and the increased resolution of satellite platforms, remote sensing technology now has a significant impact on land monitoring. Due to the increasing demand for high-quality remote sensing data, the technology continues to improve, which makes remote sensing critical for reducing time and funding costs. Similar to these advances in terrestrial remote sensing, there is an increasing potential to provide information about inland waters by using remote sensing. For instance, recent advancements in designing remote sensors, such as the Landsat 8 operational land imager (OLI) and Sentinel-2 multispectral instrument (MSI), have solved past radiometric sensitivity issues and provide high spatial resolution. This thesis explored CDOM patterns on spatial and temporal scales. The overall aim was to investigate the capabilities of remote sensing (RS) and geographic information systems (GIS) to extend CDOM patterns from a regional to a broad scale. Different study sites in Europe, mainly Northern Scandinavia, including large numbers of lakes and rivers, were tested on different scales.The results shows how climate changes (from wet to dry) can result in a combination of changes in hydrology, vegetation type and productivity, which can lead to intra-annual variations in the CDOM of recipient waters. It is also shown that drought can temporarily decrease values of CDOM in boreal lakes. In addition, it is demonstrated that combining remote sensing and GIS tools is an effective way to reveal the impact of different catchment parameters and morphometry on lake CDOM concentration. Moreover, the thesis shows that utlizing long-term remote sensing records of CDOM from the last few decades is a successful approach to fill the gaps of the missing lake data from in situ assessments. Finally, the results helped to explore links between water browning and the organic matter degradation rates in temperate European rivers at a continental scale. In conclusion, this thesis demonstrates the pogential use of remote sensing for mapping CDOM in a wide range of inland waters that are situated in complex, inaccessible regions that are not well- monitored

Evaluation of Sentinel-2A imagery for regional lake water quality assessments by observing colored dissolve organic matter (CDOM) in freshwater lakes in Västerbotten and Jämtland in northern Sweden

his study is intended to demonstrate the significant shifts in DOM concentration with space and time within and across inland waters (lakes), which can be acquired by the analysis of DOM concentration using Sentinel-2A imagery. Using a remote sensing method (based on Sentinel-2A data) to evaluate DOM quantity or concentration in freshwater (lakes), optical properties shifts of CDOM of spatial and temporal observations at sites in Västerbotten and Jämtland in Northern Sweden can be understood in terms of the landscape features and hydrological events at those locations. Using Sentinel-2A as a data source for these remote sensing methods can provide an effective approach for estimating CDOM concentrations in water to obtain water quality information regularly, and also could help to accurately estimate the CDOM absorption coefficient (aCDOM ) via Sentinel-2A data of the study area. The estimation of (aCDOM ) by using Sentinel-2A could help to study DOM in inland waters at global scale by using following equation aCDOM(420)=5.13(B2/B3)-2.67 Key words: Dissolved Organic Matter component (DOM), water quality, sentinel -2

Decreasing organic carbon bioreactivity in European rivers

European rivers experience increased loading of total organic carbon (TOC) from terrestrial sources due to factors involving changes in land use, climate and soil acidity. However, little is known about how increased TOC is linked to changes in the bioreactivity of organic matter in these rivers on a continental scale. We compiled paired measurements of TOC and biological oxygen demand in 5‐day 20°C dark incubations from 3,486 EU monitoring rivers. Assuming first‐order decay and a fixed respiratory quotient, annual average TOC and biological oxygen demand values were used to calculate 11,060 values of the decay coefficient k . The k decreased by two orders of magnitude as a power function of increasing TOC. This relationship could partly be explained by carbon quality, as the C:N ratio of the organic matter was the lowest in high‐reactivity low‐TOC rivers, and vice versa. A trend analysis showed that TOC increased by 18% from 1996 to 2012, while k decreased by as much as 50%. As a consequence, the biological oxygen demand in the water decreased over time in spite of the water browning trend (increased TOC). Together, these results suggest that reactivity is low near terrestrial hot spots for TOC export, and low during years with high terrestrial TOC loading, but increases in rivers with low TOC concentrations where internal processes in the water have high relative influence on bulk TOC quality. Thus, browning of European freshwaters is linked to strong decreases in TOC reactivity on a continental scale, suggesting that the impacts of browning on microbial water deoxygenation and greenhouse gas production are less severe than previously thought

Mapping conservation priorities in alpine and subartctic Swedish lakes affected by rapid climate change

Swedish alpine and subarctic areas undergo rapid transitions due to climate change, especially in relatively humid areas where the forest vegetation is expanding. It has been suggested that lakes in these areas are in transition from clearwater into brownwater state, because of the humus layer building up in surrounding soils, with negative consequences, e.g., for the biomass production of zooplankton and fish. However, this idea remains a hypothesis to be tested at a large scale. We used the new high-resolution satellite Sentinel-2 to evaluate the relationship between lake color and forest succession in 250 widely distributed lakes across Sweden. Preliminary results show that lakes remain surprisingly clear in areas of recent forest expansion, suggesting that there is lag time before brownification occurs. Our study could be used to map regions that are sensitive to future water brownification

Large-Scale Retrieval of Coloured Dissolved Organic Matter in Northern Lakes Using Sentinel-2 Data

Owing to the significant societal value of inland water resources, there is a need for cost-effective monitoring of water quality on large scales. We tested the suitability of the recently launched Sentinel-2A to monitor a key water quality parameter, coloured dissolved organic matter (CDOM), in various types of lakes in northern Sweden. Values of a(420)CDOM (CDOM absorption at 420 nm wavelength) were obtained by analyzing water samples from 46 lakes in five districts across Sweden within an area of approximately 800 km2. We evaluated the relationships between a(420)CDOM and band ratios derived from Sentinel-2A Level-1C and Level-2A products. The band ratios B2/B3 (460 nm/560 nm) and B3/B5 (560 nm/705 nm) showed poor relationships with a(420)CDOM in Level-1C and 2A data both before and after the removal of outliers. However, there was a slightly stronger power relationship between the atmospherically-corrected B3/B4 ratio and a(420)CDOM (R2 = 0.28, n = 46), and this relationship was further improved (R2 = 0.65, n = 41) by removing observations affected by light haze and cirrus clouds. This study covered a wide range of lakes in different landscape settings and demonstrates the broad applicability of a(420)CDOM retrieval algorithms based on the B3/B4 ratio derived from Sentinel-2A. View Full-Tex