Daugavpils University
Faculty of Natural Sciences and Mathematics
DĀVIS GRUBERTS
THE FLOOD PULSE CONCEPT
IN THE ECOLOGY OF FLOODPLAIN LAKES
OF THE MIDDLE DAUGAVA
Summary of the thesis for applying for the Doctoral degree in Biology
(specialty Ecology)
Daugavpils 2006
INTRODUCTION
Importance of the study
Floodplains of the lowland rivers are among the most productive and threatened
ecosystems on Earth. Today, 90 % of all floodplains in Europe and North
America are artificially regulated (Tockner, Stanford 2002). Floodplain lake
ecosystems of the Daugava River are threatened by plans of Latvian and Belarus
politicians to construct several new flood protection dams and hydroelectric
power stations along the Middle and Upper Daugava during the next 10 years
(Корсак 2003; Bruņenieks 2005, nepubl.). Establishment of the Daugava-Dnepr
waterway has been proposed also (Балиев 2003). These projects could destroy
natural hydrological regime of the whole Daugava and, therefore, could have an
adverse impact on the complex structure, biodiversity and ecological role of its
floodplain lakes (Грубертс 2003). Thus, it is necessary to perform ecological
studies of the Daugava's floodplain lakes now.
Scientific novelty
Until recently, ecological studies in Latvia have been focused on the lakes, whose
hydrology is influenced mostly by their drainage area characteristics. Exception
is the lagoon-type lakes of Piejūras lowland, whose water level and aquatic
chemistry is significantly influenced also by an intrusion of the salt-water masses
from the Riga Bay during storms. Continuous studies have been performed also in
Rīgas, Ķeguma and Pļaviņu HEPS reservoirs along the Lower Daugava and
characterised by a hydrology of deep, slowly flowing and artificially regulated
water bodies (Spriņģe et al. 1999). Ecological studies of Latvian floodplain lakes
have not been performed before. Thus, this study is the first such study in Latvia.
This study expands also "geography" of ecological studies of East European
floodplain lakes. Until now, such studies in Eastern Europe have been performed
only in the Prypjat floodplain in Belarus (Nagorskaya et al. 2002) and in the
Danube Delta in Hungary. In addition, during the last century, hydrological
regime has been artificially modified in almost all European river floodplains,
which have been studied before (Tockner, Stanford 2002). In the Middle Daugava,
it remains more or less natural. Therefore, the study of these floodplain lakes is a
scientific novelty not only for Latvia, but also for Europe.
By this study, the flood pulse concept (Junk et ah 1989; Junk, Wantzen 2003) has
been applied in Latvia for the first time. This concept explains temporal and
lateral unity of river floodplain ecosystems and is among of the most significant
concepts in river ecology today (Ward et ah 2002). It also serves as the main
theoretical background for restoration projects of natural hydrological regime in
river floodplains (Tockner et al. 1998; Middleton 2002). Though, it has not been
applied in hydroecological studies in Latvia before.
During this study, flooding characteristics of the Daugava's floodplain lakes have
been explored, hydrological classification of these lakes was created, the role of
floods in development of phytoplankton was stated, and main factors, which
accounts for the variability in limnological parameters during the summer
isolation, were identified for the first time. This study consists also Latvian
translation and explanation of several new terms, which are closely related to the
flood pulse concept and floodplain lake morphometry, hydrology and ecology.
Therefore, it is possible to use it for further studies in this field in Latvia.
Object of the study
Spring floods of the Daugava River usually start early, during the snowmelt and ice
break, when macrophytes and other biotic components have not been developed
yet. On the other hand, development of phytoplankton communities in lakes of
Eastern Latvia stars shortly after the ice cower degradation and improvement of
underwater lighting conditions (Trifonova 1993). Thus, compositional and
structural changes, observed in phytoplankton communities, are the most
significant indicators, which show the role of spring floods in functioning of
floodplain lake ecosystems of the Middle Daugava. Besides, due to their short life
span, communities of phytoplankton algae are very usable for testing of different
ecological hypotheses (Huszar, Reynolds 1997). Because of that, phytoplankton of
the Middle Daugava floodplain lakes has been selected as the main object of this
study. In order to gain better picture of the ecological role of the Daugava's floods,
characteristics of zooplankton and macro-zoobenthos communities and macrophyta
vegetation during the summer low water period have been also analysed.
Main hypotheses
1. Spring floods have no disruptive impact on seasonal development of phytoplankton in floodplain lakes of the Daugava, whereas the flash floods have.
2. Interaction between the flood pulse and ice cover has a stimulating effect on
floodplain lake ecosystems of the Daugava.
3. Spring floods have a long lasting impact on floodplain lake ecosystems of the
Daugava during the summer isolation. This impact is closely related to the
flooding frequency and position of these lakes within the floodplain.
Goal of the study
To adjust the flood pulse concept for the Baltic region according to the results of
ecological studies in floodplain lakes of the Middle Daugava
Main tasks
1. To determine location, origin and hydrographical and morphometrical
parameters of floodplain lakes of the Middle Daugava.
2. To determine hydrological connectivity between these lakes and the Daugava
according to the differences in the long-term annual flooding frequency.
3. To find out an impact of interaction between the wave of spring floods and the
ice cover on these floodplain lake ecosystems.
4. To clear out, whether, and under what conditions, the floods could be
regarded as a disturbance in seasonal development of algae communities in
these lakes.
5. To determine, whether the flooding frequency has an impact on limnological
parameters of these lakes during the summer low water period.
6. To determine suitability of the flood pulse concept for the Middle Daugava.
Approbation
Main results of this study were presented in the following international
conferences:
- Partnerships for Sustainable Life in Lake Ecosystems. Shiga, Japan, 2001;
- Ecohydrological Processes in Northern Wetlands. Tallinn, Estonia, 2003;
- International Environmental Experience: Applications for Belarus. Vitebsk,
Belarus, 2003;
- Research and Conservation of Biological Diversity in Baltic Region.
Daugavpils, Latvija, 2005;
- Shallow Lakes in a Changing World. Dalfsen, the Netherlands, 2005;
- 4th Symposium for European Freshwater Sciences. Krakow, Poland, 2005;
- Management of Lake Basins for Their Sustainable Use: Global Experiences
and African Issues. 11th World Lake Conference. Nairobi, Kenya, 2005;
- Global Challenges Facing Oceanography and Limnology. ASLO Summer
Meeting. Victoria, Canada, 2006;
- Use of Algae for Monitoring Rivers. 6th International Symposium. Hungary,
Baltonfured, 2006.
International publications
The main results of the study are described in 10 scientific publications. 7 of them
are published, the rest are submitted for reviewing (see the list on page 79).
Role of the author
Selection and hydrological classification of lakes; field explorations in 2004-2005;
management of the DU research project "Seasonal dynamics of hydrobiological
parameters of the Daugava's floodplain lakes" (2005); summarisation and
statistical analysis of obtained data series; identification of principal components
(factors); the first author in all 7 papers published already.
Size and structure of the thesis
The size of the thesis is 127 pages. It consists of 40 figures and 14 tables. 141
references, 12 unpublished papers and 5 Internet sources were cited. The structure
of the thesis consists of introduction, review of scientific literature, description of
study area, materials and methods, results, discussion, conclusions,
acknowledgements, list of references and 3 appendixes. Materials and methods,
main results, their analysis and conclusions are summarised in this volume.
MATERIALS AND METHODS
In order to clarify, whether the flooding frequency has an impact on limnological
parameters of floodplain lakes of the Middle Daugava during the summer low water
period, 24 floodplain lakes were selected at first. They are located mainly in
Daugavpils region and have different relative height above the Daugava riverbed
(Fig. 1).
Figure 1. Study area - floodplain of the Daugava River at Daugavpils
During July 18-28, 2004, the origin, morphometrical and physicochemical
parameters, vegetation cover and the amplitude of water level fluctuation of these
lakes was determined. Simultaneously, the samples of water, phytoplankton,
zooplankton and macrozoobenthos were collected and macrophyta vegetation
composition explored (Gruberts et al. 2005b; 2006, unpubl.). In order to clear out,
whether and under what conditions vernal floods and flash floods can be regarded
as a disturbance factors in seasonal development of phytoplankton communities,
the lakes Skuķu, Dvietes, Koša and Ļubasta as well as the Daugava River
upstream and downstream from Daugavpils City were selected for further
observations (Fig. 1). In 2005, these sites were monitored for 11 times, from
March till October. At every field trip, physicochemical parameters of these study
sites were determined and samples of water, phytoplankton and zooplankton
collected. In addition, hydrological and meteorological observations were
performed.
In order to select the lakes for this study, topographic maps of the former USSR
Army and the long-term observation data of the Daugava River water level at
Krāslava, Daugavpils, Vaikuļāni, Buivīši, Jersika and Jēkabpils hydrological
posts (Государственный водный кадастр 1987) were applied. By using these
data, the long-term annual flooding frequency of the lakes was determined. It was
also used to characterise hydrological connectivity between the lakes and the
river (Gruberts et al. 2006, unpubl.).
Maximum depth of the lakes was measured in the field by the acoustics sounding.
Surface area was calculated from the available topographic maps. Seasonal
amplitude of water level fluctuation was determined by instrumental leveling of
the highest position of the spring flood debris to the water level in each lake in
July 2004.
Seasonal water level observations in 2005 were performed from the bridges
across the Dviete River at Bebrene and Dviete and across the Lacesa in Grīva
(Daugavpils) (Fig. 1). Main meteorological elements were registered by the
weather station Vantage Pro2 Plus located in Putnu Sala (Bebrene's municipality)
(Fig. 1). All data series were saved in the memory of station every hour
(Gruberts, Volka 2006).
Water temperature, pH, conductivity, total diluted solids, concentration and
saturation of dissolved O2, ox-red potential and water turbidity was measured at
0,5 m depth and down to bottom at 1 m interval in the deepest parts of the lakes
by Hydrolab Surveyor 4 miniprobe. Measurements in the Daugava were
performed in the shoreline at 0,5 m depth. Water transparency was measured by
Secchi disc, 30 cm in diameter.
Water samples were collected in central part of the lakes and in the shoreline of
the Daugava at the 0,5 m depth by the Ruttner type sampler. Chemical analyses of
water samples were performed at Ecological Laboratory of Daugavpils Regional
Environment Board and at Laboratory of Environmental Chemistry of Daugavpils
University. Samples were analysed for total nitrogen and phosphorus,
ammonia, nitrates, nitrites, phosphates and total silica according to standard
methods accredited in Latvia.
Phytoplankton samples were collected in central part and littoral zone of the lakes
as well as in the shoreline of the Daugava at 0,5 m depth by the Ruttner type
sampler and fixed with lugol's solution. Systematic analysis of phytoplankton
samples was performed at Laboratory of Hydrobiology, Faculty of Biology,
University of Latvia, according to the method of Utermőhl (1958) by inverted
microscope and taxonomic literature of the Middle Europe freshwater flora (Ettl
1983; 1988; Krammer, Lange-Bertalot 1986; 1988; 1991; Starmach 1985).
Biological diversity of phytoplankton communities was calculated according to the
Shannon's equation, in which data about the biomass of all taxa was used.
Concentrated zooplankton samples (100 1 in volume) were collected in epilimnion
of central parts and littoral zones of the lakes and in the shoreline of the Daugava by
65 µm plankton mesh, 25 cm in diameter, and fixed in the field by 4 %
formaldehyde solution. The light microscope was used for the zooplankton
species identification and enumeration.
Macrozoobenthos samples were collected from the deepest part and littoral zone of
the lakes by the Ekman-type grab in July 2004. A sieve with a mesh size 0,5 mm
and 4% formaldehyde solution was used for the zoobenthos samples concentration
and fixation.
Composition and distribution of aquatic vegetation of the lakes was observed in the
field in July 2004.
In order to determine correlation between limnological parameters of the lakes,
explored in July 2004, Spearman's rank correlation method was used (Krebs 1989).
Rank correlation coefficients were calculated between 57 series of morphometrical,
hydrological, physicochemical and biological data. In order to extract main factors,
which explain the observed variance in limnological parameters during summer
low water period, method of Principal Component (Factor) Analysis was applied to
20 previously selected data series. After the initial extraction, principal components
were rotated in order to increase their scores in relation to the variables according to
the Varimax method. The rank correlation and Factor Analysis was performed by
SPSS 11.5 for Windows statistic software package.
To determine if the selected water bodies can be classified together or need to be
separated, calculations of Renkonen's coefficients (the percentage similarity) were
performed by using phyto- and zooplankton community data and after their
standardization in terms of percentages. They were used further for average
linkage clustering of selected lakes according to the unweighted pair-group
method using arithmetic averages (UPGMA) (Krebs 1989).
RESULTS
Hydrography, morphometry and origin of the lakes
Today, there are at least 23 individual lakes and several groups of small oxbows
and quarry pits, which are located along the Middle Daugava between Naujene and
Jēkabpils and inundated at the highest possible flood water level. Largest number of
such lakes is located in Daugavpils region and belongs to the Berezovka (Dviete)
and Līksna catchments (Fig. 1). The four largest floodplain lakes in Latvia and the
whole Daugava floodplain (Skuķu, Dvietes, Koša, Ļubasta) are also located in this
area (Gruberts 2002; 2003). Floodplain lakes of the Middle Daugava, which where
explored in summer 2004, are located at different heights above the sea level, but
their difference is less than 10 m. Differences in relative heights of the lakes are the
same. Most of them are located 5-7 m above the mean summer water level in the
Daugava. Usually, they are small and shallow (Tab. 1). Correlation between the
size and depth of the lakes is not significant. Due to their melioration, the largest
lakes are also the shallowest ones.
Most of them have been originated during geological development of valley and
floodplain of the Daugava in Holocene. According to their genesis these lakes are
classified as former side arms, the flood scours, depressions between natural
levees and oxbows. In addition, there are also numerous lakes of glacial and
artificial origin located on floodplain of the Middle Daugava (Gruberts et al.
2006, unpubl.).
Many of these lakes are hydrologically open systems or they have an outlet. The
Lake Gaišais and reservoirs of the Elerne's gravel quarry are closed systems.
Hydrology of the lake Skuķu and Dvietes is especially complicated because the river
Dviete, which is among the largest tributaries of the Middle Daugava, flows through
them (Fig. 1).
Flooding frequency of the lakes
According to the height of the Daugava's water level, at which flooding occurs, it is
possible to divide these lakes in 6 hydrological groups (Tab. 2). Each group of the
lakes is characterised by different mean annual flooding frequency, which is
measured in points (0-5) (Gruberts et al. 2006, unpubl.). More than half of them are
located between the mean annual flood level and the mean highest summer (autumn)
water level (Fig. 2). Therefore, they can be classified as regularly or repeatedly
flooded lakes (Tab. 3).
Table 1 (shortened). Main limnological parameters of floodplain lakes of the Middle
Daugava, July 2004 (Gruberts et al. 2006, unpubl.)
* - transparency down to bottom
Seasonal fluctuation of water level
Seasonal amplitude of water level fluctuation varied among the lakes depending
on their relative height above the mean summer water level in the river. The
largest seasonal amplitude of water level fluctuation (6-7 m) was stated for the
water bodies, characterised by low position within the floodplain (Lake Ruģeļu,
Berezovka bay etc.), but in most cases it varied between 2 and 4 m per year (Tab.
1). In addition, very close linear correlation between the amplitude of water level
fluctuation in 2004 and the long-term annual flooding frequency was stated (r =
0,966; p < 0,01; N = 24) (Gruberts et al. 2006, unpubl.), indicating that the longterm data of hydrological observations, which are used in this study, are adequate
and usable for the hydrological classification of these lakes and also for the
further statistical analysis.
Table 2. Hydrological groups of the Daugava's floodplain lakes (Gruberts et al.
2006, unpubl.)
Group
I
II
Ш
IV
V
VI
Flooding frequency of the lakes
Not flooding at all even at the highest floods because of
artificial isolation
Flooding rarely (once or twice in a century), only at the
highest floods
Flooding regularly (once or twice in every 10 years), at the
mean flood level
Flooding repeatedly (once or twice per year), not only in
spring but also at the most highest summer (autumn) water
level
Flooding frequently (several times per year), not only in
spring but also at the mean highest summer (autumn) water
level
Flooding very frequently (many times per year), even at the
mean summer water level
Points
0
1
2
3
4
5
Seasonal observations in 2005 demonstrated, that the largest floodplain lakes of
the Daugava can indeed be flooded twice per year, and that their flooding
frequency depends on their position within the floodplain, as predicted by their
hydrological classification described above. At the beginning of April 2005, the
medium size wave of spring floods was observed in the Daugava floodplain near
Dvietes village (Fig. 3). It inundated the lakes Skuķu, Dvietes and Ļubasta, whose
water level increased for several meters, whereas the lake Koša was not inundated
because of its higher position. At the beginning of May, it was followed by
unusually high wave of flash floods, caused by heavy local rainstorms (Gruberts,
Volka 2006). This wave reached its maximum in mid-May and raised water level
in all four lakes.
Figure 2. Position of floodplain lakes of the Middle Daugava in relation to typical
water levels in the river (names of the lakes see in Table 3)
Table 3. Hydrological grouping of floodplain lakes of the Middle Daugava
explored in summer. 2004
Nr.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Name
Little Daugava
Lake Dubaks I
Lake Skuķu
Lake Dvietes
Berezovka oxbow II
Lake Dubaks II
Berezovka oxbow I
Berezovka bay
Lake Paukštes
Lake Koša
Lake Gaišais
Lake Pjatačoks
Lake Apaļais
Reservoir Linmārka
Lake Ļubasts
Lake Piskuņicas
Reservoir Pļintovkas
Lake Bezdonka
Group
V
IV
IV
IV
V
V
VI
VI
III
III
I
II
II
Ш
IV
III
IV
III
Flooding frequency
several times per year
once or twice per year
once or twice per year
once or twice per year
several times per year
several times per year
many times per year
many times per year
once or twice in every 10 years
once or twice in every 10 years
not flooding because of isolation
once or twice in a century
once or twice in a century
once or twice in every 10 years
once or twice per year
once or twice in every 10 years
once or twice per year
once or twice in every 10 years
Table 3 (continued)
Nr.
Name
Group
Flooding frequency
19
Pond at DU
I
20
Lake Ruģeļu
VI
many times per year
21
Reservoir Elernes I
IV
once or twice per year
22
Reservoir Elernes IV
IV
once or twice per year
23
Reservoir II
IV
once or twice per year
24
Reservoir III
IV
once or twice per year
not flooding because of isolation
Figure 3. Water level fluctuation and rainfall intensity in the Daugava floodplain
near Dvietes village in 2005
Physicochemical parameters and their seasonal variation
Despite of their common hydrology and origin, there were very large differences in
physicochemical parameters of floodplain lakes of the Middle Daugava explored
in summer, 2004 (Tab. 1) (Gruberts et al. 2006, unpubl.). Besides, large differences
were stated even for the morphologically and hydrologically similar lakes like the
lake Dvietes and Skuķu, which are located close to each other, are glacial by their
origin and are drained by the same river Dviete. Distinct thermal and chemical
stratification was found also in deepest floodplain lakes of the Middle Daugava in
summer, 2004. In most cases the thermocline was located at 1-3 m depth and was
characterised by a sharp decrease in water temperature and dissolved oxygen
concentrations (Gruberts et al. 2006, unpubl). However, its
scale was different, indicating significant impact of the lakes' morphometry on the
water column stability.
Seasonal observations in the lakes Skuķu, Dvietes, Koša and Ļubasta and in the
Daugava upstream and downstream from Daugavpils in 2005 indicated similar
seasonal patterns of physicochemical parameters and nutrients (Fig. 4, 5), which
were influenced by the weather, water level fluctuations and seasonal development
of phytoplankton communities and macrophyta vegetation. At the beginning of
spring floods, there was a sharp increase in water temperature, pH, dissolved O2
and a decrease in conductivity, turbidity and concentrations of nutrients, which was
observed in floodplain lakes in the Daugava, when compared to the ice cover
period (Fig. 4, 5). At the same time, turbidity in the Daugava increased due to an
increased runoff and stream velocity (Gruberts 2006, unpubl.). At the end of the
spring floods, short period of stabilisation in physicochemical parameters
established, which was interrupted by the wave of flash floods at the beginning of
May.
During the flash floods, water temperature and concentration of dissolved O2
decreased whereas the amount of nutrients and turbidity increased again. At the end
of the flash floods, the opposite trend was observed and the previous tendencies in
seasonal variation of physicochemical parameters resumed. In summer,
concentration of nutrients in these lakes was low, when compared to spring, due to
their accumulation in biomass of phytoplankton and macrophyta vegetation.
Composition and structure of phytoplankton communities in summer
In total, 125 taxa of phytoplankton from 9 systematic groups were stated in
floodplain lakes of the Middle Daugava in summer, 2004 (Gruberts et at. 2006,
unpubl.).
Most of them belonged to Chlorophyta, Bacillariophyta and
Cyanophyta groups (Gruberts et al. 2005b; 2006; 2006, unpubl.). Only 5 algae
taxa (Апаbаепа sp., Euglena sp., Cryptomonas sp., Gymnodinium sp., Synedra
acus Kütz.) were considered as common whereas 40 taxa were rare. The largest
number of phytoplankton taxa (48) was found in the lake Ļubasts. Biological
diversity (the Shannon's index) varied between 0,03 and 3,55 (Tab. 2). Its highest
value was stated for the small reservoir of the Elerne's gravel quarry, the lowest - for
the lake Koša, related to the mass development of Ocillatoria (Planktothirx) sp.
during the summer low water period (Gruberts et al. 2006). In these lakes,
Cyanophyta, Chlorophyta and Cryptophyta formed total biomass of phytoplankton
communities, though there were large differences between the lakes in its amount
and composition (Fig. 6).
Figure 4. Seasonal variation in physicochemical parameters of the Daugava and
its largest floodplain lakes in 2005
Figure 5. Seasonal variation of nutrient concentrations in the Daugava and its largest
floodplain lakes in 2005
Figure 6. Composition and amount of total phytoplankton biomass in floodplain
lakes of the Middle Daugava in summer, 2004
Significant differences between the lakes was also found, when they were
compared by the Renkonen's percentage similarity method. The most similar
phytoplankton communities were found in the lake Ļubasts, reservoir Pļintovkas and
the Little Daugava (Renkonen's index > 85 %). The most different was the lake
Koša, which has very low similarity with the other lakes (Renkonen's index > 1 %).
Such result can be explained by the fact, that in many lakes there were only one or
two dominant groups of phytoplankton, which formed the largest fraction of total
biomass (Fig. 6). In some lakes, the single species dominated. For example,
Ocillatoria (Planktothirx) sp., which usually forms large biomasses in shallow,
eutrophic lakes and reservoirs in summer (Druvietis 1998; 2003), dominated in the
lake Koša, whereas Chloromonadophyta Gonyostomum semen (Ehr.) Dies., which
is commonly found in the peat bog lakes (Fott 1971; Rudzroga 1995), formed the
largest fraction of total biomass in the lake Pjatačoks.
Figure 7. Clustering of the Daugava floodplain lakes, based on the phytoplankton
community data (Gruberts et al. 2006, unpubl.)
These differences are well seen in cluster tree diagram (Fig. 7), which results from
the comparison of relative weights of all phytoplankton taxa in total biomass of
the lakes in summer, 2004. Position of the lakes within this tree indicates, that their
similarity is determined at first by the relative weight of Cryptomonas sp. biomass.
In this case the most similar are the lakes Little Daugava, Ļubasts, Pļintovkas and
Ruģeļu, which have the highest percentage of these taxa (Fig. 6). When compared
to other lakes, higher percentage of Cryptophyta was stated also in the lakes
Dvietes and Skuķu as well as in the reservoirs Elernes I and Ш (Gruberts et al. 2006;
2006, unpubl.).
Seasonal development of phytoplankton communities in 2005
149 taxa from 8 phytoplankton groups were stated in the Daugava and its largest
floodplain lakes in 2005. Most of them are Bacillariophyta and Chlorophyta
species. The largest number of taxa was found in the lake Ļubasts and in the
Daugava downstream from Daugavpils (> 90 taxa) (Gruberts 2006, unpubl.;
Gruberts, Druvietis 2006).
Some taxa were observed constantly, like Cryptomonas sp. in the lakes Skuķu and
Dvietes or Melosira varians Ag. in the Daugava upstream from Daugavpils.
Several other taxa have been found in all sampling sites. The largest number of
such taxa was observed in May 15, during the flash floods, whereas during the
winter and summer low water periods there were no such taxa at all. Biological
diversity, the number of taxa and total biomass of phytoplankton communities in
the Daugava and its largest floodplain lakes was influenced by the seasonal
variation of water level. Biological diversity of phytoplankton had the tendency to
decrease during the spring floods and increase during the flash floods and the
second half of summer whereas the number of taxa and total biomass increased
during the spring floods and decreased during the flash floods (Gruberts 2006,
unpubl.; Gruberts, Druvietis 2006; Paidere et al. 2006, unpubl.) (Fig. 8). Relative
weight of different algae groups and dominant taxa in total biomass of the
phytoplankton was also influenced by hydrological conditions (Fig. 9; Tab. 4).
Peculiarities of Zooplankton communities in summer
There were 50 taxa of Zooplankton found in floodplain lakes of the Middle
Daugava in summer, 2004, which belong mostly to Rotatoria group (Gruberts et al.
2005b; 2006; 2006, unpubl.). The most common taxa were Synchaeta sp.,
Polyarthra sp., Keratella cochlearis Gosse, Keratella quadrata Müll., Bosmina
longirostris Müll., Cyclops strennus Fischer (Gruberts et al. 2005b), whereas 9 taxa
can be considered as rare, found in the single sample only. The largest number of
taxa (20) has been found in the lake Ļubasts as well as in the pond at Daugavpils
University; the largest number of Zooplankton individuals per 1 m3 -in the lake
Paukštes (13,8 × 106 ind. m-3 ). In contrast to phytoplankton, the summer
communities of Zooplankton of these lakes were much more similar to each other.
Their similarity was determined mostly by the differences in relative weight of
Copepoda taxa and individuals (Gruberts et al. 2006, unpubl.).
Macrozoobenthos zoocenosis during the summer low water period
In total, 77 taxa of macrozoobenthos were found in floodplain lakes of the Middle
Daugava in summer, 2004. The four taxa were found most frequently: Limnodrilus
hoffmeisteri Claparede, Chaoborus flavicans Meigen, Chironomidae, Culicoidae
Gen. sp. (Gruberts et al. 2005b; 2006; 2006, unpubl; Poppeis et al. 2005), whereas
37 taxa were found rarely. When compared to other systematic groups, the
percentage of rare taxa was the largest within the macrozoobenthos zoocenosis. The
largest number of taxa (27) was found in the lake Dvietes, whereas the reservoir
Piintovkas had the largest number of individuals (47 × 102 ind. m-2). In general, the
most abundant in these lakes were Oligochaeta and Chironomidae organisms, but
molluscs formed the largest part of total biomass.
Table 4 (shortened). Dominant taxa in phytoplankton communities of the largest
floodplain lakes of the Daugava in 2005
Hydrological
conditions
The end of the
ice cover period
Lake Skuķu
Ceratium
hirudinella,
Cryptomonas sp.
Lake Dvietes
Nitzschia sp.,
Gymnodinium
sp., Cryptomonas
sp.
Pandorina
morum
Lake Koša
Closterium sp.
Maximum of
the spring
floods
Coelastrum
microporum,
Pandorina
morum,
Anabaena
lemmermannii
Beginning of
drainage phase of
the spring floods
The end of
drainage phase
of the spring
floods
Cryptomonas sp.,
Pandorina
morum
Cryptomonas sp.
Cryptomonas sp.,
Pandorina
morum
Cryptomonas sp.
Cryptomonas sp.,
Closterium
aciculare
Oscillatoria
(Planktothirx)
sp., Synedra
acus, Closterium
sp., C. aciculare
no data
Maximum of
the flash floods
Cryptomonas sp.,
Nitzschia
acicularis
Cryptomonas sp.,
Nitzschia
acicularis
Oscillatoria
(Planktothirx)
sp., Closterium
aciculare,
Asterionella
formosa, Synedra
ulna
Pandorina
morum,
Cryptomonas sp.,
Melosira varians
Beginning of
drainage phase
of the flash
floods
The end of
drainage phase
of the flash
floods
Summer low
water period
(first part)
Cryptomonas sp.,
Dynobrion
sertularia,
Synedra ulna
Pandorina
morum.
Cryptomonas sp.
Cryptomonas sp.,
Dynobrion
sertularia,
Synedra acus
Cryptomonas sp.,
Nitzschia
acicularis,
Navicula sp.
Euglena sp.,
Euglena acus
Oscillatoria
(Planktothirx)
sp., Nitzschia
acicularis
Oscillatoria
(Planktothirx)
sp., Cryptomonas
sp.
Oscillatoria
(Planktothirx) sp.
Cryptomonas sp.,
Pandorina
morum
Euglena sp.,
Euglena acus
Cryptomonas sp.
Lake Lubasts
Navicula sp.,
Melosira
varians, Euglena
sp.
Pandorina
morum, Synedra
ulna, Melosira
varians, Euglena
sp.
Cryptomonas sp.,
Aulacoseira
italica, Nitzschia
acicularis,
Peridinium
cinctum
Pandorina
morum,
Staurastrum sp.,
Melosira varians
Gonyostomum
semen,
Pandorina morum,
Staurastrum sp.,
Botryococcus
braunii,
Gomphosphaeria
lacustris,
Cryptomonas sp.
Figure 8. The biomass, number of taxa and biological diversity of phytoplankton
communities of the Daugava and its largest floodplain lakes in 2005
Figure 9. Relative weight of different algae groups in total phytoplankton biomass of
the Daugava and its largest floodplain lakes in 2005
Composition and distribution of macrophyta vegetation
85 taxa of macrophyta were found in floodplain lakes of the Middle Daugava in
summer, 2004. The most common were Agrostis stolonifera L., Alisma plantagoaquatica L., Carex acuta L., Elodea canadensis Michx., Equisetum fluviatile L.,
Hydrocharis morsus-ranae L., Lemna minor L., Lemna trisulca L., Nuphar lutea
(L.) Sm., Potamogeton sp., Rorripa amphibia (L.), Besser Sagittaria sagittifolia
L., Sparganium erectum L. and Spirodela polyrhiza (L.) Schieid. (Gruberts et al.
2005b; 2006; 2006, unpubl). 26 taxa were found rarely. In the lake Skuķu, the
largest number of macrophyta taxa (31) was stated. The most vegetated were the
shallow lakes Dvietes and Apaļais (vegetation cover 95-100% of the water
surface) due to reduction of their summer water level after melioration.
Correlation between limnological parameters
In total, 1596 correlation coefficients were calculated after the rank correlation
analysis of 57 limnological parameters, which were obtained in summer, 2004. In
90 cases, correlation was significant at the level a = 0,01, whereas in 108 cases -at
the level a =0,05.
The long term annual flooding frequency and seasonal amplitude of water level
fluctuation significantly correlated with the main physicochemical parameters
(water temperature, transparency, conductivity) and also with some biological
parameters, such as the biomass of Cryptomonas sp., the number of Zooplankton
taxa and individuals as well as the number of Rotatoria, Copepoda, Cyclopus sp.
and Oligochaeta individuals (Gruberts et al. 2006, unpubl.). These biological
parameters correlated also to each other and to other limnological parameters. For
example, biomass of Cryptomonas sp. correlated to the number of Cyclopus sp.
individuals (r = 0,-67; p < 0,05; N = 22) as well as to oxidation-reduction
potential (r = 0,60; p < 0,01; N = 24).
Therefore, the rank correlation analysis alone was unsuitable for clear
identification of the main factors, which determine the differences in limnological
parameters of these lakes during the summer low water period.
Results of Factor Analysis
By initial application of the Factor (Principal Component) Analysis to these data,
6 components (factors) were extracted. They explained 81 % of total variance
(Tab. 5). The first factor better explains the variation in flooding frequency, water
level amplitude, conductivity and number of Zooplankton taxa in these lakes.
Factor 2 explained variation in the area/depth ratio, temperature, dissolved O2 and
number of Cladocera individuals; factor 3 - in pH, red-ox potential, total P,
Anabaena sp. biomass and number of Copepoda individuals; factor 4 - in total N
and Cryptomonas sp. biomass; factor 5 - in number of Rotatoria and Cyclopus sp.
individuals; factor 6 - in Euglena sp. biomass. After some additional analyses, the
number of Oligochaeta individuals was most closely related to the first factor,
vegetation cover - to the second one (Gruberts et al. 2006, unpubl.).
Table 5. Rotated component matrix (Gruberts et al. 2006, unpubl.)
DISCUSSION
Suitability of the flood pulse concept for the Middle Daugava
Initially, the flood pulse concept was based on studies in floodplains of the
Amazon and Mississippi rivers, characterised by natural hydrological regime and
warm, wet weather (Junk, Wantzen 2003). Later on, this concept was extended
according to the studies in some lowland and alpine floodplains of Central Europe
(Tockner et al. 2000). In order adjust this concept for Baltic region it is necessary at
first to concretise, how the nature of the Middle Daugava floodplain differs from that
of the other sites explored before.
From geomorphologic point of view, there is an obvious similarity between the
floodplain of the Middle Daugava and other large lowland rivers (Danube,
Volga, Orinoco etc.), which are characterised by straight, slowly flow, an
overload by the suspended matter and branching of riverbed. The Daugava in its
middle reach crosses East Latvian lowland, its valley is relatively straight,
shallow and simple shaped and its bed is not meandering (Эберхард 1972). In
contrast to the floodplains of tropical rivers, the surface of the Middle Daugava
floodplain is formed by erosion channels and depressions, which have been
created during ice jams, and which are typical for northern river floodplains
(Smith, Pearce 2002). In addition, the origin of many floodplain lakes of the
Middle Daugava is also related to the ice jams during the spring floods. Besides,
the floodplain of the Middle Daugava is relatively new: it started to form only at the
end of the last glaciation (Эберхард 1972).
From climatologic point of view, extended version of the flood pulse concept would
be more suitable for Baltic region, because it is based on the studies in temperate
climatic zone. The most similar climatic conditions could be found in the Middle
Danube floodplain, which has been explored in order to extent the concept (Tockner
et al. 2000). The Daugava as well as the Danube is located in transition zone
between maritime and continental temperate climate. Drainage basins of these two
rivers have very similar characteristics of mean annual precipitation and air
temperatures in winter and summer. Though, the Daugava basin has the maximum
of precipitation in summer, whereas in the Danube basin it is more or less constant
through the year.
Also, from hydrologic point of view, floodplain of the Middle Daugava differs from
that of the Danube. Although regular floods in both rivers are caused by intense
melt of seasonal snow cover in their drainage basins, peak of the flood pulse in the
Danube floodplain is observed in early summer (Tockner et al. 2000) whereas in the
Daugava floodplain it already occurs in spring (Пасторc 19876; Нежиховский
1988; Briede et al. 2001). Besides, annual floods of the Daugava is characterised by
the ice flow, which is frequently associated with the ice jams in the shallow sites of
the riverbed (Пасторc 1987a; Briede et al. 2004). Interaction between flood wave
and ice cover of the Daugava is very complicated and related to many interacting
factors. Because of that, beginning, duration and scale of spring floods in the
Daugava are very difficult to predict (Нежиховский 1988).
In contrast to the Danube floodplain, where floodplain lakes are isolated from the
river by dams, hydrological connectivity of floodplain lakes along the Middle
Daugava is determined by their relative height and the water level in the river, as
stated during this study and as predicted by the flood pulse concept (Junk 1997). It
means, that hydrologic regime of the Middle Daugava is preserved as more or less
natural and typical for climatic conditions of Baltic region.
Ecological aspects of interaction between the flood pulse and ice cover
According to Junk and Wantzen (2003), prolonged ice cover and low
temperatures strongly affect the biota in high latitude river-floodplain systems
and it might require as many adaptations to these events as to the flood pulse.
Hoverer, the interaction of the flood pulse with these environmental factors is not
sufficiently understood (Prowse, Culp 2003). On the other hand, such interaction is
observed frequently in the Middle Daugava floodplain (Пасторе 1987а). It is
possible to evaluate some of its ecological aspects by taking into account seasonal
studies in floodplain lakes of the Daugava in 2005. Process of complete ice cover
degradation in the Middle Daugava at Vaikuļāni and Daugavpils takes about 5 days
(Государственный водный кадастр 1987). During the spring floods, it occurs
even faster; if the ice melts gradually, it needs much more time. Therefore, speed of
ice cover degradation in floodplain lakes of the Middle Daugava depends on timing
of the floods as well as on location of the lakes. If the flood wave arrives at the end
of March, ice cover of the lower floodplain lakes disintegrates very fast, in few
days. If it is arrives later, period of ice degradation extends.
When floodwaters of the Daugava enter its floodplain lakes, their ice cover is lifted
up along with the stands of macrophyta, frozen in it. By the acting of wind or
streams, ice cover quickly breaks up and might be transported out of the lake along
with its load. It slows down overgrowing of these lakes by the reeds and other
macrophytes. Such "self-cleaning" during the spring floods is typical phenomenon
for the lake Skuķu. Thus, it is still functioning as a refugee for fish and
macroinvertebrate populations during summer low water period in spite of its
melioration and extended macrophyta cover (Gruberts 2000, unpubl.). According to
this study, the wave of the spring floods causes also a significant decreasing of water
turbidity (Fig. 4) and improvement of underwater lighting, which acts as the
limiting factor for phytoplankton development in winter (Wetzel 2001). It
promotes fast development of vernal diatoms and greens, such as Aulacoseira
italica, Cyclotella sp. and Pandorina morum (Gruberts, Druvietis 2001; Gruberts et
al. 2005a). For lakes in temperate climate, this phenomenon usually occurs during
spring circulation (Reynolds 1984; Wetzel 2001). In Latvia, spring maximum of
these groups is usually observed in mesotrophic and eutrophic lakes (Druvietis
1995).
Because of the faster ice break-up, seasonal development of macrophyta
vegetation might also start earlier in the most frequently flooded lakes of the
Daugava floodplain. It promotes also mixing and aeration of their water columns,
as observed in the lakes Skuķu, Dvietes and Ļubasta at the beginning of April
2005. Due to this, atmospheric oxygen could reach also the deepest layers of
water columns and therefore improve existence of fish and macroinvertebrate
populations, which survived under the prolonged ice cover in winter. On the
other hand, it provides food resources for large populations of spawning fish and
migrating water birds, for which the Daugava's floodplain in the Dvietes ancient
valley is very important resting site during their annual migration (Gruberts 2000,
unpubl.; Račinskis 2004).
Thus, interaction between the flood pulse and ice cover can be regarded as a
stimulating factor, which promotes faster circulation of matter and energy in the
most frequently flooded lakes of the Middle Daugava floodplain. It also slows
down their overgrowing and effectively restricts development of reed stands in their
basins. Therefore, it maintains the necessary conditions for existence of different
aquatic communities during the summer low water period.
Impact of floods on seasonal development of phytoplankton
If the floods act as a disturbance factor in seasonal development of phytoplankton
communities of these lakes, an increase in biological diversity and decrease in
biomass should be observed (Reynolds 1993a; Sommer et al. 1993). Though,
according to this study, the beginning of the spring floods in the Middle Daugava
could not be regarded as a disturbance, rather vice versa. At the end of March and
beginning of April 2005, an interaction between the flood pulse and the ice cover in
the largest floodplain lakes of the Daugava resulted in fast development of their
phytoplankton communities, characterised by simultaneous increase in total
biomass, the number of taxa and, in some cases, biological diversity (Fig. 8). Such
changes immediately after the ice cover degradation have been observed in other
East Latvian lakes also (Trifonova 1993), and are regarded as the initial stage of
phytoplankton development. Further development of phytoplankton communities in
the Daugava downstream from Daugavpils as well as in its most frequently flooded
lakes was not interrupted by the water level stabilisation and gradual decrease at the
end of the spring floods. In contrast, it was characterised by further increase in total
biomass and decrease in biological diversity (Fig. 8), which indicates the more or
less undisturbed successional development (Reynolds 1993a).
On the contrary, quick decease in total biomass and increase in biological
diversity was observed in these lakes and also in the Daugava at the beginning of the
flash floods (Fig. 8) (Gruberts 2006, unpubl.; Gruberts, Druvietis 2006; Paidere et
al. 2006, unpubl.). Such characteristics according to current understanding could
be regarded as indicators of intermediate disturbance (Sommer et al. 1993).
Though, even it was influenced by location of the lakes within the Daugava's
floodplain. Insignificant decrease in the number of taxa and biological diversity was
observed at this time in the lakes Koša and Ļubasta, which are located higher
above the Daugava riverbed. Total phytoplankton biomass in these two lakes even
slightly increased (Fig. 8), which shows, that the flash floods had insignificant impact
on their phytoplankton communities.
Main influencing factors during the summer low water period
By taking into account results of the Factor Analysis, it is possible to identify main
factors, which determines variation in physicochemical parameters as well as
peculiarities of phytoplankton, Zooplankton, macrozoobenthos communities
and macrophyta vegetation in floodplain lakes of the Middle Daugava during the
summer low water period (Gruberts et al. 2006, unpubl.).
Both hydrological parameters were most closely related to the same factor (Table 5).
Increasing in its value resulted in higher frequency of flooding and larger amplitude
of water level fluctuation. Obviously, it could be identified with hydrological
connectivity of the lakes to the river. According to the flood pulse concept,
hydrological connectivity via surface waters is the key factor for ecological
functioning of the large river floodplain ecosystems in tropics (Junk 1997; Lewis et
al. 2000). Its significant role has been stressed also in ecological studies of
floodplain lakes along the other European rivers (Van den Brink et al. 1994;
Bornette et al, 1998; Tockner et al. 1998; Hein et al. 2001). The second factor could
be identified by the morphology of the lakes, because it explains variation in surface
area/depth ratio, water temperature and dissolved O2 (Table 5). In this case, the most
significant parameter is the maximum depth of the lakes: larger depth results in
lower area/depth ratio, higher temperature and larger concentration of dissolved O2
in the surface water layer. It is well illustrated by the differences in thermal
stratification, observed in these lakes in summer, 2004. Stratification was especially
pronounced in the small and deep lake Pjakačoks whereas in the much larger lake
Koša it was relatively weak (Gruberts et al. 2006, unpubl.).
Factors 3 and 4 explain variations in total N and total P between the lakes. On the
other hand, they are not related to hydrological conditions, because they are not
correlating to them. Probably, these factors could be identified by an internal
loading of N and P from sediments after their depletion from the water column,
which could occur in shallow lakes in summer caused by the mass development of
phytoplankton and macrophyta vegetation (Van den Brink et al. 1994; Scheffer
2004). In addition, indirect evidence shows that the main source of nutrients in
sediments of these lakes is not the floodwater of the Daugava, but the local runoff.
For instance, according to hydrochemical observations of the river Dviete in May
1999, its water had much higher nutrient concentrations than that of the lake Skuķu
at the same time (Gruberts 2000, unpubl.). Besides, mass development of common
reed Phragmites australis in this lake at the inlet of the river Dviete also indicates
that its eutrophication is caused mostly by local runoff.
Characteristics of the factor 5 show, that variation in Cyclopus sp. individuals in
these lakes is determined mostly by throphic interactions between
Zooplankton groups (Tab. 5). Acording to Stemberger and Evans (1984),
predation by Cyclopus is among the most important factors affecting seasonal
development of Rotatoria in the Lake Michigan (USA).
Finally, the sixth factor, which explains variance in Euglena sp. biomass, could be
identified with local source of dissolved organic matter. Increasing in its value
resulted in higher biomass of Euglena sp. (Tab. 5). According to Reynolds
(1993b), euglenoids are feeding by assimilation of dissolved organic matter
instead of nitrogen (Reynolds 1993).
Hydrological connectivity and biological diversity
The role of hydrological connectivity in maintenance of high biological diversity is
among the central themes in the ecology of floodplain lakes today (Junk, Wantzen
2003; Roozen 2005). The highest diversity is expected in the lakes with intermediate
level of hydrological connectivity. This prediction is confirmed by the recent
studies in floodplain of the Danube River, in Austria (Tockner et al. 1998).
According to this study, the number of Zooplankton taxa is the only variable, which
significantly correlates to the long-term annual flooding frequency and seasonal
amplitude of water level fluctuation in floodplain lakes of the Middle Daugava (Fig.
10). In spite of all expectations, species diversity of phytoplankton,
macrozoobenthos and macrophyta communities are not correlating with these
parameters (Gruberts et al. 2005b; Gruberts et al. 2006, unpubl.). On the other hand,
it does not mean, that the Daugava's floods have no long-term impact on these
groups in summer, as stated in recent studies of semi-isolated floodplain lakes of the
Waal River in the Netherlands (Roozen 2005). In this study, the highest biological
diversity of phytoplankton communities has been observed in the lakes with
intermediate amplitude of water level fluctuation, which completely agrees with the
latest statements of the flood pulse concept (Junk, Wantzen 2003) and shows that
hydrological connectivity has the long-term impact on phytoplankton communities
of the Daugava's floodplain lakes, although it is not possible to prove it by the
applied statistic methods. Importance of hydrological connectivity in the ecology
of the Daugava's floodplain lakes is confirmed also by the highest number of
macrozoobenthos taxa in the lakes with intermediate amplitude of water level
fluctuation (Fig. 12.). It coincides with results of recent studies in floodplain of the
Danube River, in Austria (Tockner et al. 1998), where the same phenomenon has
been observed. Although the species diversity of macrophyta vegetation in these
floodplain lakes was related mostly to their morphology and origin, mean number of
all taxa in all systematic groups was observed in the lakes with intermediate
flooding frequency (Fig. 12). This result coincides very well with the current
understanding of regular floods as disturbance factor in the ecology of river
floodplains (Ward et al. 2002; Junk, Wantzen 2003) and with the similar results
obtained by Tockner and his colleagues in the study of the Danube's floodplain
(Tockner et al. 1998). It also shows the importance of hydrological connectivity in
the ecology of floodplain lakes in natural hydrological conditions.
Figure 10. Correlation between the amplitude of water level fluctuation and number
of Zooplankton taxa in floodplain lakes of the Middle Daugava in summer, 2004
Figure 11. Correlation between water level fluctuation and biological diversity of
phytoplankton in floodplain lakes of the Middle Daugava in summer, 2004
Figure 12. Mean number of taxa of different groups in 15 floodplain lakes of the
Middle Daugava in summer, 2004
Nutrient and energy cycling on floodplain of the Daugava River
According to the flood pulse concept, regular flooding stimulates the exchange of
nutrients and energy between terrestrial and aquatic communities and explains
the high productivity of most floodplain ecosystems (Junk 1997). By the analogy
to other floodplains, nutrient and energy cycling on the Daugava's floodplain can
be divided into two phases: aquatic phase (the spring flood period) and
terrestrial phase - the low water period, which includes summer, autumn and
winter seasons (Fig. 13). Though, phytoplankton communities, whose growth is
promoted by interaction between the flood pulse and the ice cover, play central role
in nutrient and energy cycling on this floodplain during the aquatic phase and not
the macrophytes, which dominate in floodplain lakes of large tropic rivers (Junk
1997).
At the end of the spring floods, growth of terrestrial and aquatic vegetation on
floodplain of the Daugava River quickly starts. It determines nutrient and energy
cycling in terrestrial and aquatic habitats during the summer low water period.
According to this study, most floodplain lakes of the Middle Daugava are very
shallow and can be classified as the macrophyte type lakes, in which the growth of
aquatic plants controls the nutrient cycling (Scheffer et al. 1993; Scheffer 2004).
Figure 13. Nutrient and energy cycling on floodplain the Daugava River
(modified after Junk 1997)
The winter low water period can be regarded as the period of slow cycling of
nutrients and energy on floodplain of the Daugava, which is characterised by the
decay of organic matter, its mineralization and transport out of the system. Human
activity is the most significant factor, which determines this process, because
floodplain forests are used for firewood collection during the winter. In addition,
fishing in the floodplain's lakes and rivers also proceeds until the beginning of the
spring floods.
At the beginning of the floods, seasonal cycling of nutrients and energy in
floodplain of the Daugava starts again. Though, it can be interrupted by the flash
floods at any season, caused, for example, by the heavy rainfalls or unexpected thaw
of the snow cover. Flash floods significantly disturb seasonal development of
terrestrial and aquatic vegetation and phytoplankton communities and worsen the
existence of those animal species, which have been adapted to specific hydrological
conditions. They also result in large economic losses because their timing and scale
is unpredictable.
Therefore, flash floods also should be regarded as very important factor, which has
a significant impact on this river-floodplain system. Local authorities for further
development planning in the Daugava's floodplain also should consider it. It is
even more important in the near future, when significant increase in frequency of
extreme meteorological events including the flash floods in rivers of the North-East
Europe is expected (Impacts of... 2004).
CONCLUSIONS
1. The largest group of floodplain lakes of the Middle Daugava is located in
Daugavpils region and belongs to the Berezovka and Līksna drainage basins.
They are located 5-7 meters above the mean summer water level in the
Daugava, and most of them are classified as glacial lakes, the flood scours and
oxbows. Most of them are small and shallow and could be flooded repeatedly,
not only during the mean annual spring floods, but also at the highest water
level in summer-autumn season. Seasonal amplitude of water level fluctuation in
these lakes varies between 0,1 and 7 meters depending on their relative height
and highest water level in the river in a particular year.
2. Differences in physicochemical parameters between the floodplain lakes of the
Middle Daugava in summer are related to their location, thermal stratification,
development of aquatic vegetation and phytoplankton communities and impact
of local runoff. Seasonal variation of physicochemical parameters is also related
to water level fluctuation in the Daugava River. Significant impact of the
Daugava's floods on water transparency, conductivity, temperature, oxidationreduction potential and concentration of nutrients, dissolved O2 and other
physicochemical parameters of these lakes has been found. Though, this impact
is short-term and concentrations of nutrients in summer are determined by local
factors.
3. During summer, Chlorophyta, Bacillariophyta and Cyanophyta species
dominate phytoplankton communities of floodplain lakes of the Middle
Daugava. However, there are large differences between these lakes in
phytoplankton's composition, structure and diversity. Their similarity in summer
is determined mostly by the relative weight of Cryptomonas sp. in total biomass.
Largest biological diversity has been found in the lakes with intermediate
amplitude of water level fluctuation.
4. There are large seasonal differences in composition, structure and diversity of
phytoplankton communities in the Daugava and its largest floodplain lakes,
which are closely related to their hydrology. Biological diversity decreases
during the spring floods and increases during the flash floods and at the end of
summer whereas the number of taxa and total biomass has the opposite
tendencies.
5. Biomass and dominant taxa of different algae groups also changes in relation to
water level within the floodplain. At the beginning of the spring floods,
percentage of Chlorophyta Pandorina morum and Dinophyta Gymnodinium sp.
in total biomass of the lower floodplain lakes significantly increases. During
the drainage phase of the spring floods, the percentage of Crypomonas sp.
increases whereas during the flash floods higher percentage of Bacillariophyta is
observed.
6. Zooplankton communities of floodplain lakes of the Middle Daugava are more
similar to each other than phytoplankton communities. During summer,
Rotatoria taxa dominate. Total number of Zooplankton taxa correlates to
hydrological connectivity of the lakes. When connectivity increases, the number
of Zooplankton taxa decreases. Its variation is also influenced by the
morphology of the lakes, macrophyta vegetation and thropic interrelationships.
7. Macrozoobenthos zoocenosis of these lakes are characterised by high
percentage of rare species found in the single sample each. The largest number
of individuals was stated in Oligochaeta and Chironomidae groups whereas
molluscs formed the highest percentage in total biomass. Repeatedly flooded
lakes are characterised by the largest number of macrozoobenthos taxa and
Oligochaeta individuals, which correlates also with hydrological connectivity.
8. Most of the lakes are characterised by rich and diverse macrophyta vegetation,
which is related to their morphology, and by an impact of melioration and local
sources of water pollution. Seasonal development of floating macrophyta in the
lower floodplain lakes starts shortly after the break-up of the ice cover.
9. Significant rank correlations between many limnological parameters of these
lakes were found in summer. Flooding frequency and seasonal amplitude of
water level fluctuation of the lakes correlated with their relative height, water
temperature, transparency, conductivity, Cryptomonas sp. biomass, number of
Zooplankton taxa and individuals and other parameters. Though, rank correlation
does not reveal the main influencing factors, because these parameters correlate
also to each other and to other limnological parameters.
10. The Factor Analysis extracted 6 main factors, which explain variation in selected
limnological parameters of these lakes during the summer low water period. They
are identified by hydrological connectivity, the morphology of the lakes, internal
loading of N and P from sediments, trophic interactions between the Zooplankton
groups as well as by local source of organic matter.
11. Geomorphologic, climatic and hydrologic characteristics of the Daugava's
floodplain are explained by the flood pulse concept and its extended version only
partly. In contrast to other river floodplains studied before, the floodplain of
the Daugava is quite new. Its largest floodplain lakes are created by the glacial
processes at the end of last glaciation. It also has natural hydrological regime,
which has not been modified by the large-scale hydrotechnical engineering.
Besides, the floodplain of the Daugava River is characterised by colder and more
continental climate, seasonal snow and ice cover and ice jams during the spring
floods.
12. Regular spring foods should not be regarded as a disturbance factor in seasonal
development of phytoplankton communities in the largest floodplain lakes of
the Daugava River. In the most frequently flooded lakes,
spring floods promote faster degradation of ice cover, improvement of
underwater lighting conditions and mixing of the water column. Therefore, better
conditions for the rapid growth of Bacillariophyta and Chlorophyta are created.
13. Interaction between the pulse of spring floods and ice cover, which is typical for
the Daugava River in winter, could be regarded as a stimulating factor, which
fastens nutrient and energy cycling in these floodplain lakes. On the other hand,
it slows down their overgrowing by reeds and other macrophytes. Therefore,
appropriate conditions for the existence of different aquatic communities during
the summer low water period are maintained.
14.
In contrast, the flash floods caused by heavy rainstorms in summer act as an
intermediate disturbance, which decreases phytoplankton biomass and
increases biological diversity in more frequently flooded lakes, as predicted by
the hypothesis of this study. However, this impact depends on location of the
lakes within the floodplain and it diminishes, if relative height of the lakes
increases.
15.
Although the largest species diversity of different systematic groups was
observed at different levels of hydrological connectivity, the largest total
number of all taxa was found in the lakes, characterised by intermediate
flooding frequency. It conforms to the hypothesis, according to which flooding
is an essential factor for maintaining high biological diversity in river
floodplain ecosystems. It also shows the importance of hydrological connectivity
in the ecology of floodplain lakes of the Middle Daugava.
16.
In contrast to tropical river floodplains, nutrient and energy cycling on
floodplain of the Daugava River interacts with seasonal cycles of solar
radiation, day length, water temperature and snow and ice cover.
Phytoplankton communities, whose growth is promoted by the interaction
between the flood pulse and the ice cover, play the central role in nutrient and
energy cycling on this floodplain during the aquatic phase (the spring floods). It
is significantly influenced also by the flash floods, which can be caused by heavy
rainfalls or thaw of the snow cover. They disturb cycling of nutrients and energy
and have an adverse impact on terrestrial and aquatic communities of the whole
floodplain of the Daugava River.