Improving knowledge on food webs in Latvian and Estonian lakes

During the last research season (April – June) Katrit Karus, the post-doctorate of the Institute for Environmental Solutions (IES) finished data analysis in laboratory for the samples gathered in 2019 fieldwork campaigns. The analysis will give an insight on the components of the overall food web – from bacteria to phytoplankton-zooplankton in Latvian and Estonian lakes.

“I think that we are among the few researchers who are trying to study food webs in comprehensive manner. This will allow us to understand how food webs function, so we can describe the mechanisms that determine the mortality of larval fish in water bodies which is crucial for fish stock management (fish farming, aquaculture)” post-doctorate Katrit Karus describes the importance of the research.

In this article you will find the results of analysis of the samples gathered during 2019 spring, summer and autumn seasons.

Methods used during sample analysis

For the analysis of the gathered samples researchers used standard methods to assess abundance and biomass for all the samples as well as community structure of some of the samples (metazooplankton, phytoplankton, ciliates). For heterotrophic nanoflagellates and bacterioplankton sample analysis researchers used DAPI direct count method using epifluorescence microscope. For phytoplankton and ciliate sample analysis researchers used “Utermöhl” technique using inverted microscope. And for metazooplankton sample analysis they used “Bogorov’s” counting chambers with stereomicroscope.

Metazooplankton sample analysis

Overall, total abundance and biomass of metazooplankton differed greatly lake by lake, as well as seasonally and between studied biotopes – coastal and open-water areas of the lakes.

Metazooplankton biomass and seasonal fluctuations were highest in Lake Akste, Lake Vārzgūnes, Lake Riebinu and Lake Nohipalo Valgjärv. Total biomass and abundance of metazooplankton was lowest in Lake Laukezers and in Lake Prossa (Image 1). Metazooplankton biomass peaks occurred mostly in autumn or spring except for Lake Vārzgūne where the biomass was several times higher in summer (Image 1).

Overall, the species richness was higher in coastal areas of the lakes compared to open-water sites. The number of species was highest in Lake Vārzgūnes. The biomass of cladoceran and copepod groups was generally lower in summer during the active feeding period of young fish compared to spring and autumn (Image 2).

In summer, the total biomass of metazooplankton was always higher in open-water areas (except for Vārzgūnes) (Image 1) presumably indicating lower feeding activity of young fish in this area compared to coastal territories.

Phytoplankton sample analysis

Researchers analysed taxonomic composition, abundance and biomass of phytoplankton. Results shows that taxonomic composition and biomass of phytoplankton differs between lakes and variates seasonally but are similar between studied biotopes – coastal and open-water areas of the lakes.

Ciliate sample analysis

From all ciliate sample analysis, researchers determined total abundance and biomass (Image 3). They also determined ciliate species as far as possible and divided the found species into two broad categories: picovorous (ciliate species whose main food is bacterioplankton) and nanovorous (ciliate species whose main food is nanoplankton or even larger objects).

Researchers found that ciliate abundance and biomass varied considerably between studied lakes and showed also great variability between different biotopes (Image 3 & Image 4). Highest biomass values in open-water areas were found in Lake Prossa and in coastal areas of Lake Vārzgūnes. Corresponding lowest biomass values were found in Lake Akste (open-water area) and in Lake Nohipalu Valgjärv (in coastal area) and lowest abundances in Lake Auciema (in open-water area) and in Lake Nohipalu Valgjärv (in coastal area). Highest values were generally found in autumn or in summer. Overall, the average biomass in open-water areas was higher than in coastal areas.

Also, the proportion of pico- and nanovorous ciliates varied greatly (Image 4). Highest share of picovores was found in Lake Laukezers (average 71.2% from biomass) and lowest in Lake Auciema (average 18.6% from biomass). In coastal picovorous species showed slightly higher biomass than nanovores. In open-water areas nanovores were slightly more abundant.

Heterotrophic nanoflagellate sample analysis

Researchers analysed heterotrophic nanoflagellate community (abundance and biomass) from water samples collected during 2019 fieldwork campaigns.

Total abundance of heterotrophic nanoflagellates was highest in Lake Akste where they occured in extremely high abundances (up to 70 770 ind/ml), especially in spring and summer in coastal area of the lake (Image 5). That makes the lake quite extraordinary because in our knowledge there are no literature data about such exceptionally abundant heterotrophic nanoflagellate communities in lakes. Heterotrophic nanoflagellate abundance was lowest in Lake Prossa during whole vegetation period. Researchers concluded that the total abundances of heterotrophic nanoflagellates were lowest during the peak of metazooplankton biomass. 

Heterotrophic bacterioplakton sample analysis

To characterize heterotrophic bacterioplakton communities, researchers analysed abundances and biomasses of collected water samples. Bacterial total abundance results differed between lakes, seasonally and between studied biotopes (Image 7 & Image 8). In general, total abundance of heterotrophic bacteria shows food web state of the water body.

Total number of bacteria was lowest in Lake Nohipalo Valgjärv and Lake Laukezers (both oligotrophic lakes and characterized by poor macrophyte and fish communities) and highest in Lake Auciema, Lake Kaiavere, Lake Riebinu and Lake Prossa (all considered as eutrophic lakes with rich fish communities and macrophyte rich communities (except Lake Kaiavere which is macrophyte-poor lake)) (Image 7).

Total abundance and biomass of bacterioplankton peaked mostly in summer and/or spring, in some lakes the highest values occurred also during autumn (Lake Riebinu, Laukezers). In case of two studied biotopes (coastal and open-water areas of the lakes) total number of bacteria was strongly higher in open-water areas of Lake Riebinu and Lake Auciema compared to coastal zones of the lakes, in all the other lakes total abundances of bacteria was quite similar in open-water and coastal areas.

Macrophytes sample analysis

Researchers were looking at different parameters during macrophyte community studies, such as coverage, frequency etc. Among these parameters total number of macrophyte species during growing season and species frequency in sampling areas were chosen as best characteristics to describe lake macrovegetation. In general, total number of macrophyte species expresses taxonomic diversity of studied lakes and frequency at the same time indicates domination of certain species.

Average number of emergent, floating & floating-leaved, and submerged plant species in Latvian lakes during whole studied vegetation period (2019) was 4 species in emergent plant zone, 3 in floating & floating-leaved plant zone, and 5 species in submerged plant zone. Apparently, species diversity was highest in submerged plant zone and mainly in summer or autumn (Image 9).

In case of Estonian lakes, during whole vegetation period the average number of species was 5 species in emergent plant zone, 2 species in floating & floating-leaved plant zone and 4 species in submerged plant zone (Image 10).

Fish community sample analysis

During the last research period researchers analysed fish communities only in Estonian lakes. In Latvian lakes they used formerly collected data (summer, 2018). The number of species caught by gillnets were highest in Latvian lakes: 12 species in Lake Riebinu, 9 species in Lake Auciema, 8 species in Vārzgūnes ezers and 5 species in Laukezers. In Estonia, species diversity was highest in Lake Kaiavere (8 species) and Lake Prossa (7 species), from Lake Akste and Nohipalo Valgjärv only 2 species per lake were caught. Total fish catches WPUEs (weight per unit effort, in grams) were also highest in Latvian lakes, especially in Lake Auciema and Lake Riebinu (Image 11).

Larval fish sample analysis

Larval fish samples were collected once a year (in spring) from coastal and open-water areas of the lakes. Researchers successfully caught fish larvae in spring from all lakes except Laukezers – in that lake the sampling was probably carried out too early and fish larvae were not yet hatched (we found a quantity of perch eggs in coastal zone). Researchers detected fish larvae from following species: perch, roach, sunbleak, rudd, bleak and pike. Most common species caught was perch. Researchers also analysed fish diet using gut content method under epifluorescence microscope.

Researchers also estimated the abundance of larval fish in lakes Auciema, Varzgune, Kaiavere, Akste and Prossa. Highest larval abundances were found in Lake Auciema and lowest in Lake Varzgune (Image 12).

Young fish (0+ fish) community analysis

For young fish (0+ fish) community samples studies and collection researchers used beach-seine method in all the 8 study lakes both in summer and autumn. In summer and autumn, they estimated the 0+ fish abundance and feeding from the coastal zones. Highest numbers of 0+ fish were found in summer from Lake Riebinu and lowest in Lake Nohipalo Valgjärv (Image 13). Highest number of species was caught in summer from Lake Varzgune (7 species) and lowest in lakes Akste, Nohipalo Valgjärv and Laukezers (1 species).

After sample analyses researchers will evaluate collected data and results in order to understand how food webs function in different lake types and how it impacts fish stock. To get the overall understanding IES researchers will conduct more sample gathering campaigns to retrieve reference information. The main objective of the research is to improve knowledge on how lake food webs function, thereby enabling the development of scientific and data-based plans and decision making for the management of lakes that combine economic and ecological criteria.

The research is a part of the project „Fish feeding conditions in lakes with different planktonic food web structure and microvegetation “(MICROFISH), No.1.1.1.2/VIAA/1/18/301. Agreement with State Education Development Agency of the Republic of Latvia No. Programme number 1.1.1.2/16/I/001. The project is financed by the European Regional Development Fund, the State budget of the Republic of Latvia and the foundation „Institute for Environmental Solutions”.

Find out more about the project here.