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Browsing by Subject "Brownification"

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  • Rajala, Salla (2022)
    Tiivistelmä * Referat * Abstract Brownification is a serious environmental problem, which means a substantial increase in water color. Suggested reasons for brownification are soil recovery from acidification, land-use change, especially ditching, climate change. Brownification changes the light conditions in the lakes, increases thermal stratification, and sedimentation. Despite the considerable effects of brownification on lake ecosystem, currently in the EU Water Framework Directive, there is no sufficient biological indicator for lake browning. Biological indicators of the Water Framework Directive are mainly targeted to eutrophication, and thus are not sufficient for measuring the effects of brownification. Macrophyte chlorophyll a:b ratio has been shown to decrease with decreasing light conditions, thus making it a potential indicator for browning. The seasonal variation of macrophyte chlorophyll content is not well studied. This master’s thesis aims to find out, how the chlorophyll content of yellow water lily (Nuphar lutea) changes during the growing season in lakes with variable water quality. N. lutea was chosen as a study species, as it is very common in different lakes in Finland. It is important to study the seasonal variation of the macrophyte chlorophyll content, because if the chlorophyll content reacts rapidly for example changes in the weather, it might be too sensitive to be used as a long-term indicator for lake browning. It is also needed to study, if the seasonal variation is different in lakes with variable water quality, so that in the future, the possible sampling can be timed right in different lakes. The study lakes were clear water lakes, humic and eutrophic lake, and their water quality and the chlorophyll content of N. lutea were examined with two weeks intervals from June to September. The concentration of chlorophyll a and chlorophyll b were measured from the floating leaf and from the petiole at 10 cm intervals. Based on these, the total chlorophyll concentration (CHL a+b) and CHL a:b ratio was calculated. In addition to macrophyte samples, water samples were taken from each lake from both epilimnion and hypolimnion. From the water samples, dissolved organic carbon, water color, iron concentration and specific ultraviolet absorbance at 254 nm were measured. In addition, water temperature, light intensity, oxygen concentration and water turbidity were measured at each sampling time. The results of this thesis showed, that the CHL a:b ratio, and total CHL concentration (a+b) were significantly lower in the humic than in the clear water lake. In the eutrophic lake, CHL a:b ratio and CHL a+b concentration was higher than in the humic lake but lower than in the clear water lake. In each lake, the CHL a:b ratio increased during the growing season, together with the increasing water clarity. The change in the CHL a:b ratio was detected best in the 10-30 cm petiole depth. In the floating leaf, CHL a:b ratio and CHL a+b concentration varied a lot, meaning that the floating leaf is not a sufficient indicator for brownification. The change in CHL a:b ratio took approximately 5 to 7 weeks, and the seasonal variation was similar in each lake. Based on the results of this thesis, the CHL a:b ratio of the N. lutea petiole seems to be a very promising long-term indicator for lake browning. In the future, it would be, however, important to study, how the carbon sequestration changes as a result of brownification, if the macrophyte chlorophyll concentration decreases, since this might have considerable effects on lake carbon sequestration. Also, the eutrophic study lake of this thesis had a water color over 100 mg Pt l-1, so it would be important to study, what is the macrophyte chlorophyll content in clear, but eutrophic lakes.