The importance of allochthonous organic matter quality when investigating pulse disturbance events in freshwater lakes: a mesocosm experiment

Extreme precipitation is occurring with greater frequency and intensity as a result of climate change. Such events boost the transport of allochthonous organic matter (allo-OM) to freshwater ecosystems, yet little is known about the impacts on dissolved organic matter (DOM) quality and seston elemental stoichiometry, especially for lakes in warm climates. A mesocosm experiment located in a Turkish freshwater lake was designed to simulate a pulse event leading to increased inputs of allo-OM by examining the individual effects of increasing water colour (HuminFeed®, HF), the direct effects of the extra energetic inputs (alder tree leaf leachate, L), and the interactions of the single treatment effects (combination of both sources, HFL), along with a comparison with unmanipulated controls. Changes in the DOM quality and nutrient stoichiometry of the allo-OM treatment additions was examined over the course of the experiments. Results indicated that there was an increase of high recalcitrant DOM components in the HF treatment, in contrast to an increase in less aromatic microbially derived molecules for the L treatment. Unexpectedly, seston C:P ratios remained below a severe P-limiting threshold for plankton growth and showed the same temporal pattern in all mesocosms. In contrast, seston N:P ratios differed significantly between treatments, with the L treatment reducing P-limiting conditions, whilst the HF treatment increased them. The effects of the combined HFL treatment indicated an additive type of interaction and chlorophyll-a was highest in the HFL treatment. Our results demonstrate that accounting for the optical and stoichiometric properties of experimental allo-OM treatments is crucial to improve the capacity to explain extrapolated conclusions regarding the effects of climate driven flooding on freshwater ecosystems in response to global climate change. Graphical abstract

STRUCTURAL AND FUNCTIONAL CHARACTERISTICS OF PHYTOPLANKTON OF LOWER DNIPRO WATERFLOWS

The study of waterflowsin the lower currentsof the Dniproin 2016–2020 showed that groups of plankton algae are quite rich in species composition and characterized by polydominant structure, and Chlorophyta, Bacillariophyta and Cyanophyta (in the amount of 82%)are the main structure-forming divisions. 145–261 WWs from 9 divisions were found in rivers and tributaries. The floristic spectrum of planktonic algaethereis similar and formed mainly by green (32%), diatoms (30%) and blue-green (20%) algae; The river Viryovchyna is an exception, in which the third place in terms of the number of WWsis occupied by the Euglenophyta division (14%), and the indicators of phytoplankton developmentare 2.3 and 5.7 times higher than the following indicators in the rest of the studied water bodies: 2.7 million. cells / dm3and 0.967 g / m3against 6.2 million cells / dm3and 5.537 g / m3due to the inflow of allochthonous organic matter with wastewater from urban treatment facilities. The dominant phytoplankton complex of waterflows is formed mainly by diatoms.The primary production of algae,during the research period,averaged 3.55 g O2/ m3× day and in different waterflowswas 1.6–2.0 times higher than the destruction. Destructive processes, similarly to the productive ones, took place most intensively in the river Viryovchyna, where in all seasons the p / p was not less than 6.19 g O2/ m3× day. Gross p / p in the lower Dnipro waterflows averaged 3.55 g O2/ m3× day during the study period and was 1.6–2.0 times higher than destruction.The destruction of organic matter (OM)in the studied water bodies averaged 2.06 g O2/ m3× day, net production -1.49 g O2/ m3× day, which indicates the accumulation of organic matter in the studied waterflows of the lower Dniproduring 2016-2020.In the river Viryovchyna,during all the seasons,the gross p / p was not less than 6.19 g O2/ m3× day.Key words:lower Dnipro, waterflows, structure of phytoplankton, primary products, destruction of organic matter. Антропогенне навантаження на річки, особливо ті, які протікають через міста, безумовно відбивається на видовому складі фітопланктону. Проведено дослідження річкової води біля міста Херсон. Особлива увага була приділена річці Вірьовчина, до якої відбувається скид води з очисних споруд міста. Дослідження водотоків нижньої течії річки Дніпра 2016–2020 роківпоказало, що угруповання водоростей планктону досить багаті за видовим складом і характеризуються полідомінантною структурою, а основними структуроутворюючими відділами є Chlorophyta, Bacillariophytaі Cyanophyta(усумі 82% флористичного спектру).Урічках і рукавах річки Дніпро виявлено 145–261 ввт з 9 відділів. Флористичний спектр планктонних водоростей в них схожий і формується переважно зеленими (32%), діатомовими (30%) і синьозеленими (20%) водоростями; виключенням є річка Вірьовчина, в якій третє місце за кількістю ввт займає відділ Euglenophyta(14%), а показники розвитку фітопланктону в 2,3 і 5,7 рази перевищують такі показники в решті досліджених водних об’єктів: 2,7 млн. кл/дм3 і 0,967 г/м3проти 6,2 млн. кл/дм3 і 5,537 г/м3за рахунок надходження алохтонної органіки зістічними водами міських очисних споруд.Зазначене явище являє собою підтвердження антропогенного забруднення стічними водами притоки річки Дніпро.Домінуючий комплекс фітопланктону водотоків формується переважно діатомовимиводоростями.Первинна продукція водоростей уперіод польових досліджень в середньому складала 3,55 г О2/м3×добу і в різних водотоках в 1,6–2,0 рази перевищувала деструкцію. Деструкційні процеси, аналогічно продукційним, найбільш інтенсивно проходили урічціВірьовчина, де в усі сезони п/п була не менше 6,19 г О2/ м3×добу. Валова п/п уводотоках нижнього Дніпра становила в середньому за досліджуванийперіод 3,55 г О2/ м3×добу і в 1,6–2,0 рази перевищувала деструкцію. Деструкція органічної речовиниудосліджених водних об’єктах в середньому становила 2,06 г О2/ м3×добу, чиста продукція –1,49 г О2/ м3×добу, що свідчить про накопичення органічної речовини в досліджених водотоках нижнього Дніпра протягом 2016–2020 рр. УрічціВірьовчина в усі сезони валова п/п була не менше 6,19 г О2/ м3×добу.Ключові слова: нижній Дніпро, водотоки, структура фітопланктону, первинна продукція, деструкція органічної речовини.

Plant species invasion effects on litter dynamics in subtropical streams

Aim We evaluated the effect of the presence of Hovenia dulcis Thunb. (Rhamnaceae) in riparian zones on the organic matter dynamics of small subtropical streams. Methods We conducted this study in three subtropical Atlantic Forest streams with different densities of H. dulcis in riparian vegetation located in southern Brazil. In each stream, we quantified the input of allochthonous organic matter for one year using buckets (area: 0.04 m2/bucket) suspended about 1 m from the streambed in three different sections (15 buckets/stretch = 45 buckets/stream). Monthly, the plant material retained in the buckets was collected individually, dried (40±5 °C/72 h), identified (native litter together and H. dulcis litter alone) and weighed. Results The largest input of native organic matter occurred during the winter months (~55 g.m-2), ranging from ~31 g.m-2 (summer) to ~46 g.m-2 (spring) over the year. The input of H. dulcis organic matter was concentrated in the autumn (~56 g.m-2) and summer (~28 g.m-2), being scarce in the other seasons (~3 g.m-2 in the spring and winter). Only the contribution of native organic matter was associated with precipitation. Contrary to that observed with native vegetation (input of organic matter related with rainfall), H. dulcis input was related to the phenology of the species, which is deciduous, with leaf fall strongly marked, occurring especially during the autumn. Conclusions When present at high densities (dominant), the presence of H. dulcis in riparian stream vegetation makes the supply of allochthonous plant resources scarce at some periods of the year, altering the energy availability in these ecosystems and, potentially, the functioning of subtropical streams.

Changes in the hydrochemical regime of Onegо Lake since the early 1990s

The supply of allochthonous organic matter with river water to the lakes rises in the new climatic conditions of Karelia (mild winters, an increase in the amount of liquid precipitation, less freezing of the soil). In connection with the geochemical peculiarities of Fennoscandia, more quantity of humic substances in a complex with iron and phosphorus enter the water bodies. These processes can lead to a change in the hydrochemical regime, water quality and habitat of the biota. For the first time for lakes of Karelia, long-term changes (1963–2017) of parameters, which are markers of allochthonous organic matter, were estimated on the example of Petrozavodsk Bay of Onego Lake. It was found that since the 1990s, the following characteristics significantly increase in Petrozavodsk Bay water: the color of water (from 56 to 73 degrees), the content of suspended matter (from 1.6 to 3 mg/l), iron (from 0.12 to 0.42 mg/l), phosphorus (from 12 to 22 μg/l). This leads to changes in the carbonate system of the bay water. The concentration of carbon dioxide increases significantly (from 1.2 to 3.0 mg/l), the pH value drops (from 7.22 to 7.12) and the oxygen content diminishes (from 101 to 92% of saturation). The Spearman correlation coefficients between the chemical characteristics and the year of study were the highest for the spring period, when the bay is separated from the open part of the lake by thermal bar and is strongly influenced by river water. Simultaneously with the change in the hydrochemical regime, there is an increase in the amount of iron in the upper layer of silts (from 0.65 to 4.8% of the air-dry sample). This led to a decrease in the number of macrozoobenthos 6–7 times.