Tiina Nõges

The effect of extreme water level decrease on hydrochemistry and phytoplankton in a shallow eutrophic lake

A drought-induced drastic decrease in mean depth and volume brought about several effects in the ecosystem of L. Vortsjarv (270 km2). Differences in alkalinity, Si and inorganic N in two consecutive winters can be attributed to the concentration effect resulting from different ice/water volume ratios in the lake. Smaller initial amount of oxygen that remained under the ice in winter 1995/96 accounted largely for the observed anoxia. Increase in phytoplankton biovolume represented another concentration effect, as a similar areal primary production in 1995 and 1996 resulted in about twice a higher density of algal crop in the summer of 1996, when the average amount of water in the lake was 1.7 times smaller. Due to a better mixing of shallow water, integral oxygen concentration during the ice-free period in 1996 was systematically higher than in 1995. Stronger sediment resuspension in 1996 enriched the water with suspended solids and nutrients, resulting in a slight increase in the trophic state. Lower water level in 1996 improved light conditions by ‘cutting off’ less illuminated and less productive deep layers. This was probably one of the main factors which caused the collapse of the common dominants Limnothrix redekei and L. planctonica and initiated mass development (max. 36 g m−3)of Cyanonephron styloides. The observed changes in the phytoplankton composition in 1996 could be caused also by the selective grazing of larger zooplankton.

First Experiences in Mapping Lake Water Quality Parameters with Sentinel-2 MSI Imagery

The importance of lakes and reservoirs leads to the high need for monitoring lake water quality both at local and global scales. The aim of the study was to test suitability of Sentinel-2 Multispectral Imager’s (MSI) data for mapping different lake water quality parameters. In situ data of chlorophyll a (Chl a), water color, colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) from nine small and two large lakes were compared with band ratio algorithms derived from Sentinel-2 Level-1C and atmospherically corrected (Sen2cor) Level-2A images. The height of the 705 nm peak was used for estimating Chl a. The suitability of the commonly used green to red band ratio was tested for estimating the CDOM, DOC and water color. Concurrent reflectance measurements were not available. Therefore, we were not able to validate the performance of Sen2cor atmospheric correction available in the Sentinel-2 Toolbox. The shape and magnitude of water reflectance were consistent with our field measurements from previous years. However, the atmospheric correction reduced the correlation between the band ratio algorithms and water quality parameters indicating the need in better atmospheric correction. We were able to show that there is good correlation between band ratio algorithms calculated from Sentinel-2 MSI data and lake water parameters like Chl a (R2 = 0.83), CDOM (R2 = 0.72) and DOC (R2 = 0.92) concentrations as well as water color (R2 = 0.52). The in situ dataset was limited in number, but covered a reasonably wide range of optical water properties. These preliminary results allow us to assume that Sentinel-2 will be a valuable tool for lake monitoring and research, especially taking into account that the data will be available routinely for many years, the imagery will be frequent, and free of charge.

The budgets of nitrogen and phosphorus in shallow eutrophic Lake Vortsjarv (Estonia)

The nutrient budget, phytoplankton primary productionand sedimentation rate were studied weekly in the large(270 km2) and shallow (mean depth 2.8 m)eutrophic Lake Vortsjarv in 1995. The annualexternal loading was 7.7 g m™2 y™1 of totalnitrogen (TN) and 0.2 g m™2 y™1 of totalphosphorus (TP), including 7% of both inputs asdirect atmospheric precipitation. The external budgetrevealed a retention of 53% of TN and 28% of TPannual input. About 80% of the total loss of nitrogenwas accounted for by denitrification (3.3 g m™2 y™1) and only 20% was buried into the sediment.Wind-induced sediment resuspension played the majorrole in the upward nutrient flux formation and, thus,in the formation of the temporal pattern of nutrientconcentration during the ice-free season. Other fluxesas the external loading or new sedimentation ofautochthonous production were overcome and masked bythe powerful resuspension – sedimentation cycle,exceeding the former by one or two orders ofmagnitude. The intensity of upward flux of nutrients(mainly caused by resuspension) increased inaccordance to decreasing water level in autumn andcorrelated with the weekly average wind speed. Thesummer population of filamentous blue-greens dominatedby Limnothrix redekei was light-limited untilthe minima of TN:TP ratio (<10) and DIN:TN ratio(∼ 0) in July initiated a clear peak of N_2-fixingalgae (Aphanizomenon gracile, Anabaena spp.)which lasted until September.

Nitrogen fixation in a large shallow lake: rates and initiation conditions

The fixation of molecular nitrogen (N2fix) by cyanobacteria in situ and in PO4-P enrichment experiments was investigated in large shallow Lake Võrtsjärv in 1998–2000. In this lake, N2fix started when TN/TP mass ratio was about 20, which is much higher than Redfield mass ratio 7. The rate of N2fix varied between 0.81 and 2.61 μgN l−1 d−1 and maximum rate (2.61 μgN l−1 d−1) was measured in 15.08.2000. In L. Võrtsjärv a lag period of a couple of weaks occurred between the set-up of favourable conditions for N2fix as the appearance of N2-fixing species and depletion of mineral nitrogen, and the real N2fix itself. However, if the favorable conditions for N2fix occurred in the lake, N2fix started after enrichment with PO4-P in mesocosms even then when no N2fix was detected in the lake. N2fix in mesocosms was also more intensive than in lake water. In our experiments PO4-P concentrations higher than 100 μgP l−1started to inhibit N2fix.

Zooplankton—phytoplankton interactions in lakes Võrtsjärv, Peipsi (Estonia) and Yaskhan (Turkmenia)

Zooplankton—phytoplankton interactions have been studied in three shallow lakes of different trophic state. In strongly eutrophic, large and very shallow Lake Vortsjarv the grazing does not play a leading role in controlling phytoplankton production and its standing stock. Small-size zooplankton can not eat filamentous blue-greens. The nutrient regeneration by zooplankton has a weak impact on phytoplankton, the latter being limited rather by underwater light than by nutrient availability. In large moderately eutrophic Lake Peipsi the presence of concentrated zooplankton in the experimental vessel mostly stimulated steady-state phytoplankton growth and negative grazing values were measured. Most probably the nutrients (N, P), excreted by zooplankton in grazing chamber stimulated the growth of larger phytoplankton which dominated because of heavy grazing pressure on edible forms. In Lake Peipsi phytoplankton seems to be nutrient-limited and heavily controlled by zooplankton community. In eutrophic, macrophyte-dominated Lake Yaskhan zooplankton in grazing chamber also mainly stimulated the growth of phytoplankton community which seemed to be nutrient-limited because of strong competition with macrophytes.

Nitrogen dynamics in the steeply stratified, temperate Lake Verevi, Estonia

The dynamics of different nitrogen compounds and nitrification in diverse habitats of a stratified Lake Verevi (Estonia) was investigated in 2000–2001. Also planktonic N2-fixation (N2fix) was measured in August of the observed years. The nitrogen that accumulated in the hypolimnion was trapped in the nonmixed layer during most of the vegetation period causing a concentration of an order of magnitude higher than in the epilimnion. The ammonium level remained low in the epilimnion (maximum 577 mgN m−3, average 115 mgN m−3) in spite of high concentrations in the hypolimnion (maximum 12223 mgN m−3, average 4807 mgN m−3). The concentrations of NO{sk2/−} and NO{sk3/−} remained on a low level both in the epilimnion (average 0.94 and 9.09 mgN m−3, respectively) and hypolimnion (average 0.47 and 5.05 mgN m−3, respectively). N2fix and nitrification ranged from 0.30 to 2.80 mgN m−3 day −1 and 6.0 to 107 mgN m−3 day −1, respectively; the most intensive processes occurred in 07.08.00 at depths of 2 and 5 m, accordingly. The role of N2fix in the total nitrogen budget of Lake Verevi (in August 2000 and 2001) was negligible while episodically in the nitrogen-depleted epilimnion the N2fix could substantially contribute to the pool of mineral nitrogen. Nitrification was unable to influence nitrogen dynamics in the epilimnion while some temporary coupling with ammonium dynamics in the hypolimnion was documented.

Diel dynamics of bacterioplankton activity in eutrophic shallow Lake Võrtsjärv, Estonia

The diel dynamics of bacterio- and phytoplankton as main compartments in the pelagic foodweb were followed in order to assess the coupling between algal photosynthesis and bacterial growth during a diel cycle in Lake Võrstjärv, Estonia. Three diurnal studies were carried out, on July 12th–13th, 1994; on June 25th–26th, 1995 and on July 17th–18th, 1995 with a sampling interval of 3–4 hours. Diel variations in bacterial number, biomass and productivity, in phytoplankton primary production and extracellular release of photosynthetic products, in ciliate number and biomass were followed. Phytoplankton was dominated by filamentous species: Limnothrix redekei, Oscillatoria sp., Aulacoseira (Melosira) ambigua and Planktolyngbya limnetica. The abundance of bacteria ranged from 4.1 to 14.6 · 1012 cells m-2 (median 9.88). The production of heterotrophic bacteria varied from 0.6 to 11 mgC m-2 h-1 (median 3.65), the variation during diel cycle was high. Depth integrated values of particulate (PPpart) and extracellular primary production (PPdiss) ranged from 6 to 55 and from 17 to 90 mgC m- 2 h-1, respectively. About 50 ciliate taxa were identified among them more abundant were bacterivores, bacterivores- herbivores and omnivores. Biomass of bacterivorous ciliates (TCbact) varied from 8 to 427 mgC m-2. Bacterioplankton production constituted not more than 20% of total primary production (particulate + released), dynamics of bacterial production was related to the primary production, the correlation was negative with PPpart and positive with PPdiss. Different types of potential controlling factors of bacterioplankton (N and P nutrient control, bottom-up control by food and top-down control) are discussed.