Seasonal dynamics of zooplankton in lakes of different trophic status

In the article, we present the data about the dominant complexes of zooplankton species in the pelagic zone of lakes Naroch, Myastro, Batorino (2014 –2019), Malye and Bolshie Shvakshty (2014 –2015), Svir (2018). The seasonal dynamics of zooplankton is analysed. The contribution of taxonomic groups and individual species in the abundance and biomass of zooplankton in lakes with different trophic status is studied. We noted two peaks in the seasonal dynamics of zooplankton. Rotifers prevailed in the spring, while Crustacea dominated in the summer-autumn period. The relative biomass of cladocerans increased and the proportion of copepods decreased with the increasing of trophic status of the lakes.

All day-long: Sticklebacks effectively forage on whitefish eggs during all light conditions

The three-spined stickleback Gasterosteus aculeatus invaded Lake Contance in the 1940s and expanded in large numbers from an exclusively shoreline habitat into the pelagic zone in 2012. Stickleback abundance is very high in the pelagic zone in winter near the spawning time of pelagic whitefish Coregonus wartmanni, and it is hypothesized that this is triggered by the opportunity to consume whitefish eggs. Field sampling has qualitatively confirmed predation of whitefish eggs by stickleback, but quantification has proven difficult due to stormy conditions that limit sampling. One fundamental unknown is if freshwater stickleback, known as visual feeders, can successfully find and eat whitefish eggs during twilight and night when whitefish spawn. It is also unknown how long eggs can be identified in stomachs following ingestion, which could limit efforts to quantify egg predation through stomach content analysis. To answer these questions, 144 individuals were given the opportunity to feed on whitefish roe under daylight, twilight, and darkness in controlled conditions. The results showed that stickleback can ingest as many as 100 whitefish eggs under any light conditions, and some individuals even consumed maximum numbers in complete darkness. Furthermore, eggs could be unambiguously identified in the stomach 24 hours after consumption. Whitefish eggs have 28% more energy content than the main diet of sticklebacks (zooplankton) based on bomb-calorimetric measurements, underlining the potential benefits of consuming eggs. Based on experimental results and estimates of stickleback abundance and total egg production, stickleback could potentially consume substantial proportions of the total eggs produced even if relatively few sticklebacks consume eggs. Given the evidence that stickleback can feed on eggs during nighttime spawning and may thereby hamper recruitment, future studies aimed at quantifying actual egg predation and resulting effects on the whitefish population are urgently needed.

Feeding Ecology of Invasive Three-Spined Stickleback (Gasterosteus aculeatus) in Relation to Native Juvenile Eurasian Perch (Perca fluviatilis) in the Pelagic Zone of Upper Lake Constance

The fish community of Lake Constance, a large, deep, oligotrophic lake has undergone drastic changes in recent years, with the sudden rise to dominance of invasive three-spined stickleback (Gasterosteus aculeatus) in the pelagic zone, a rather atypical habitat for this species in Central Europe. The core objective of this study was to compare the feeding ecology of stickleback and young Eurasian perch (Perca fluviatilis) in this unique situation to identify reasons for this unexpected dominance, a possible food niche and feeding time overlap, and to discuss consequences for the reshaped pelagic fish community. The diel feeding patterns and prey compositions of pelagic sticklebacks and juvenile (0+) perch sampled in October 2018 and March 2019 were studied analyzing stomach contents. The diets of both species mostly comprised zooplankton, with copepods appearing in the greatest numbers. Benthic and airborne insects were consumed occasionally, mostly by sticklebacks. Both species exhibited peaks of feeding activity early in the morning, afternoon and dusk, and in both species, stomachs were fullest at dusk. Stickleback stomachs contained about 20% more prey at night than perch, and mean estimated nocturnal stomach fullness values were almost 50% greater. Night feeding in sticklebacks was confirmed by digestive states, pointing to a possible competitive advantage over perch. Dietary composition varied over a 24-h cycle and the pattern of consumption of different prey varied between the species. Perch consumed more comparatively small cladocerans (Bosmina spp.), while larger Daphnia appeared more often in stickleback stomachs. In both species, seasonal variation in diet mirrored food availability, indicating some degree of opportunism. A Morisita-Horn index value of 0.95 confirmed dietary niche overlap between species, suggesting the large population of sticklebacks may exert a competitive effect on juvenile perch when resources are limited. Both the longer feeding periods and greater intake of nutritive high quality prey like daphnids can contribute to the rapid success of stickleback in dominating the pelagic zone of Lake Constance.

Observations of a Desmodesmus (Chodat) S.S. Ann, Friedl & E. Hegewald (Sphaeropleales, Chlorophyta) and two Didymocystis Korshikov (Chlorellales, Chlorophyta) in Lake Tanganyika

The occurrence of three species belonging to the small coenobia forming green algae Desmodesmus and Didymocystis is reported from the northernmost part of Lake Tanganyika, East Africa. During a monitoring between September 2011 and October 2013 the highest number of cells of Desmodesmus bicellularis and Didymocystis comasii were observed in August and September, at the end of the dry season. On the other hand, the third taxon, Didymocystis cf. inermis, was only sporadically observed between September 2011 and March 2012. According to existing databases, it should be the first records from Africa of these three species. However, D. bicellularis (as Didymocystis bicellularis) was already reported in 2003 in phytoplankton samples of the pelagic zone from more southern located places in Lake Tanganyika.

Abundance, habitat use and food consumption of seabirds in the high-Arctic fjord ecosystem

To monitor the rapid changes occurring in Arctic ecosystems and predict their direction, basic information about the current number and structure of the main components of these systems is necessary. Using boat-based surveys, we studied the numbers and distribution of seabirds foraging in Hornsund (SW Spitsbergen) during three summer seasons. The average number of seabirds foraging concurrently in the whole fjord was estimated at 28,000. Little Auks Alle alle were the most numerous, followed by Northern Fulmars Fulmarus glacialis, Brünnich’s Guillemots Uria lomvia and Black-legged Kittiwakes Rissa tridactyla. The pelagic zone was exploited by some 75% of the birds. Their density was the highest (> 400 ind. km−2) in the tidewater glacier bays, where kittiwakes were predominant, and the lowest in the coastal glacier bays. The seabirds in Hornsund daily consumed c. 12.7 tons of food, i.e. c. 0.2% of the summer mesozooplankton and fish standing stocks available in the fjord. This food consisted primarily of copepods, amphipods and molluscs (c. 70%), whereas fish made up < 15%. More than 50% of this biomass was ingested by pursuit divers, while surface feeders took c. 29% and benthophages c. 13%. About three-quarters of the food biomass was taken from the pelagic zone. This paper describes, for the first time in quantitative terms, the structure and function of a seabird community foraging in an Arctic fjord. It also provides a baseline for future studies on climate-induced changes in the importance of seabirds in the Arctic food web.

Vertical distribution, seasonal differentiation and trajectory of movement of zooplankton and pelagic fish of Lake Baikal

In Lake Baikal, the majority (87%) of fish biomass is generated in the pelagic zone. Two species of Comephorus, three species of Cottocomephorus and Coregonus migratorius inhabit in the pelagic zone of the lake. Currently, only C. migratorius is considered a resource species, the harvest of which accounts for about 50% of the total fish harvest. Food items for pelagic fish are copepods Epischura baikalensis and pelagic amphipods Macrohectopus branickii. The seasonal distribution and formation of zooplankton concentrations determine the food migrations of fish. The patterns of the seasonal distribution of zooplankton and pelagic fish were described by M.M.Kozhov (1954, 1964). Comparative analysis of the abundance of zooplankton in South Baikal over time, showed that during the period of maximum development of E. baikalensis in the months of July, August, and October, zooplankton concentration in the decade 1997 to 2007 was twice more than in 1951 (Kozhov 1954; Kiprushina 2010). The distribution of E. baikalensis is directly dependent on the water temperature. Yet there is an inverse relationship in the surface layer 0–25 meters. During warming up of the surface water layers, which had been observed in 2002 year E. baikalensis left the upper zone and migrated deeper, to a layer of 50–100 meters. In other months of the year, no changes in the average values of the E. baikalensis abundance were revealed. At present, when the ecosystem of Lake Baikal is changing, there is a need to restart such complex studies, and to use the results obtained by M.M. Kozhov for comparative analysis.