role of hydrology, nutrients and fish in interaction with global climate

ROLE OF HYDROLOGY, NUTRIENTS AND FISH
IN INTERACTION WITH GLOBAL CLIMATE CHANGE
IN EFFECTING ECOLOGY OF SHALLOW LAKES IN TURKEY
Meryem Beklioğlu
Middle East Technical University, Biology Department 06531, Ankara
[email protected]
ABSTRACT
By 95% of the worl lakes are shallow. In effecting the primary production and
associated species diversity, availability of nutrients [Nitrogen and Phosphorus] are
critical. Extensive eesearch on eutrophicated the north temperate shallow lakes has
shown deterioration of water clarity through increased phytoplankton biomass and
suspended matter amount, and loss of ecological and conservation values through
disappearance of submerged plants, predatory fish and waterfowl. Results of a longterm monitoring of Lakes Eymir and Mogan have shown that nutrient enrichment
induced high phytoplankton biomass and plankti-bentivorous fish feeding induced
turbid state resulted in loss of submerged plants. Furthermore, In Turkish shallow
lakes located in a semi-arid to arid Mediterranean climate, where the hydrological
changes are common phenomenones, are presumably very sensitive to water level
fluctuations (WLF). Investigation on the relationships between WLF and submerged
macrophyte development in five Turkish shallow lakes revaled that in all lakes, WLF
emerged as a major factor determining submerged plant development. High submerged plant coverage was observed when the water level was low. Through global
warming that lake levels are expected to go down. However, warmer water temperature, low dissolved oxygen availability associated with longer hydraulic residence
stimulated salinity, internal phosphorus loading and suppression of denitrification.
These processes are likely to increase nitrogen and phosphorus availability and in
turn, increased turbid water condition through global warming. Unders such turbid
conditions submerged plant development were supressed. Furthermore, warmer
conditions stimulated anoxic water that led to major fish kill. On the otherhand,
increased salinity is expected to lead to loss of freshwater species.
Key words: carp, nitrogen, phoshorus, submerged plants, water level fluctutation
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INTRODUCTİON
Freshwaters are crucial to the establishment of any human community and to
the conservation of all land-based wild life. Most of the world’s freshwaters area are
shallow and made up of small individual lakes dominated by littoral [shallow plantdominated] communities that are more productive per unit area of water than deep
ones and are more versatile in the uses to which they have been put [Moss, 1998].
These plant-dominated shallow waters are collectively called wetlands and include a
huge range water courses from swamps to forests. Dominance by submerged plants
is often closely related to increased ecological and conservation values owing to
recovery in water quality and, with it, increased diversity and abundance of species,
especially waterfowl [Hargeby et al. 1994, Noordhuıs et al. 2002, Van Gest et al.
2005]. Consequently, the impact of water level fluctuations on the establishment of
extensive and high diversity littoral zones in shallow lakes needs further investigation aimed to develop potential restoration tools [Coops & Hosper 2002]. The value
of wetlands on a world scale is immense as shown by budgets made by Costanza et
al. [1997] for the value of natural goods and services provided by them. The value of
wetlands proved to be greater than that of the drier systems, including forests and
grasslands and amounted to trillions of dollars per year. The total value of the
World’s natural systems amounted to over thirty trillion US dollars per year, or three
times the combined gross domestic products [GDP] of all the World’s economies
combined. The contribution of freshwater wetlands was over 10% and of wetlands as
a whole, over a quarter.
During the past century, increased urbanization and sewage disposal, regulation
of wetlands and streams and more intensive crop and animal farming practices have
increased the nutrient especially nitrogen and phosphorus [N and P, respectively]
loading to many shallow lakes causing a world-wide problem of eutrophication
[Jeppesen 1998]. Eutrophication of shallow lakes may result in deterioration of water
clarity and loss of ecological and conservation values through disappearance of submerged plants, predatory fish and waterfowl [Scheffer et al. 1993, Moss et al. 1996,
Jeppesen et al. 2003]. The whole system and its integrity are very important in maintaining values for nature conservation and human exploitation. The results of eutrophication are high algal biomass often accompanied by massive summer blooming of
cyanobacteria or green algae, few submerged macrophytes, dominance of planktibenthivorous fish and low water clarity [Moss, 1998]. Moreover, feeding of benthivorus fish like carp and bream deteriorate the ecological value and water quality of
shallow lakes. During the past 10-30 years, several countries have made a great effort
to improve the ecological quality of lakes by combating external loading [Sas, 1989],
sometimes in combination with additional restoration measures such as biomanipulation [Hansson et al., 1998; Meijer et al., 1999; Beklioglu et al., 2003]. To overcome
biological resilience, plankti-benthivorous fish removal has been widely applied in
BASIN WATER MANAGEMENT
585
north temperate lakes with some success [Hansson et al. 1998, Mehner et al. 2002,
Gulati & Van Donk 2002], whereas success of fish removal has been debatable in
warm temperate and subtropical lakes [Scasso et al. 2001, Beklioglu et al. 2003, Jeppesen et al. 2005]. However, trophic structure varies among warm lakes depending on
whether they are located in dry or wet and low or high altitudes. Such as whole lake
fish removal study was carried out in a semi-dry high altitude shallow Mediterranean lake and successfully initiated a clear-water state [Beklioglu et al. 2003].
In semi-arid to arid regions, in addition to nutrient availability and associated
trophic state, the functioning of lake ecosystems is controlled by the quantity and
periodicity of the water. Shallow lakes are particularly sensitive to any rapid change
in water level and input. Water level fluctuations may have an overriding effect on
the water chemistry [pH, salinity, nutrients] and in turn, the ecology of shallow
lakes, especially on the submerged plant development [Talling 2001, Coops et al.
2003, Beklioglu et al. 2006, Tan and Beklioglu 2006]. Understanding the role of water
level change in ecosystem functioning is important, given the current concerns about
global climate change. However, most research into climate change impacts has focused on high latitude or temperate lakes [Carvalho & Moss 1999, Jeppesen et al.
2006], while less is known about low latitude lakes, especially in regions exhibiting
an arid climate, such as the Mediterranean climatic region, which is particularly
sensitive to global warming due to the predictions of a drier and hotter climate
[Sánchez et al. 2004].
Turkey is a rich country in freshwater resources with 200 natural lakes, 75 dams,
and 700 ponds [Kazanci et al, 1995]. The total surface area covered by these lakes is
equal to 10,000 km2. Also, most of the lakes of Turkey are shallow and have large
surface areas [Lakes Beysehir, Marmara, Işıklı, Uluabat etc.]. These shallow lake
wetlands that have rich aquatic vegetation community are also rich in biodiversity.
Lakes in Turkey are subjected to nutrient [N & P] enrichment namely eutrophication
and strong water level fluctuations induced through drought/wet conditions. The
impact of the latter is predicted to be different according to trophic status or availability of nutrients. In addition to this feeding of benthivorus carp effect on ecology
of shallow lakes. Here I aimed to evaluate the effects of hydrology, eutrophication,
feeding of benthivorous fish, and biomanipulation as a restoration tool on the ecology of Turkish shallow lake wetlands. Further, the impacts of all these effects were
discussed in the reflection of global climate change.
Material and Methods
Lakes, which are included in this study, are Lakes Beyşehir, Marmara, Uluabat, Mogan
and Eymir vary in size and depth and are located in a zone ranging from southern to northern Anatolia, Turkey. Data related with water level fluctutations, fish, waterfowl and submerged plant development in Lakes Beyşehir, Marmara, Uluabat and Mogan were taken from
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INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
Beklioglu et al. [2006]. Furthermore, the data on hydrology, major ions, nutrients and ecology
of Lakes Eymir and Mogan were taken from Beklioglu et al. [2003], Tan & Beklioglu [2005],
Karapınar [2005], Ozen [2006], Beklioglu & Tan [submitted]. Methodology on data collection
and evaluations are thoroughly discussed in the literature cited above.
Results and Discussion
In shallow lake wetlands, dominance by submerged plants is often closely related to increased ecological and conservation values owing to recovery in water
quality and, with it, increased diversity and abundance of species, especially waterfowl.
A. Hydrology
i. Water Level Fluctuations - Submerged Plant Development and
Waterfowl
Shallow lakes located in different geographic regions ranging from the north
temperate zone to the Mediterranean, and in the southern hemisphere as well, respond to water level fluctuations, and their submerged plant coverage and density
may thus vary widely regardless of top-down mechanisms [Blindow et al. 1997,
Coops et al. 2003, Havens et al. 2004, Van Der Valk 2005]. Large water level fluctuation results in a large amplitude change, which is a common phenomenon in semidry to dry Mediterranean climates due to high evaporative loss that is not balanced
by an adequate water income, which occurs primarily during the rainy season in
winter [Naselli-Flores 2003]. In all of the study lakes, water level fluctuations
emerged as the major factor affecting submerged plant coverage. Lakes Beyşehir,
Uluabat, Marmara and Mogan had significantly higher vegetation cover during the
low water level [LWL] period in comparison to the high water level [HWL] period
(Figure 1). However, the duration and magnitude of the LWL period varied greatly
from lake to lake. For example, in Lakes Beyşehir and Uluabat, throughout a year
significant monthly approximately about 2 m water level differences occurred between LWL and HWL periods and vegetation coverage expanded 30 % and 55 %,
respectively. In Lake Marmara, however, the vegetation cover was high, while significant monthly differences (over 2 m) occurred between LWL and HWL periods
except for winter months (Oct.-Dec.). Similarly, in subtropical Lake Okeechobee
submerged plants recovered when water level dropped by 2 m alleviating stress of
multiple years of high water [Havens et al. 2004]. Also, an approximate 40-cm drop
in water level occurring only in May in Lake Mogan was sufficient to expand the
submerged plant coverage. This is similar to the results of Blindow et al. [1997], who
demonstrated that a 10-15 cm lower/higher water level than the spring average led to
a catastrophic shift in the state of Swedish Lake Krankesjön. Consequently, LWL
occurring only during the growing season, especially in spring and early summer
BASIN WATER MANAGEMENT
587
together with water clarity, can be critical for vegetation development and the magnitude of the water level decrease to allow submerged plants to recover can vary
among lakes [Van Nes et al. 2002]. With an increase in submerged plants, waterfowl
abundance, especially that of herbivorous coot, increased in all of the study lakes.
Coot dominated the waterfowl community (75%), followed by diving ducks e.g.
pochard and tufted duck. The increased waterfowl densities during the vegetated
periods of Lake Beyşehir caused it to become an A-class wetland and Important Bird
Area (IBA). Similarly, Lake Mogan acquired the status of a Specially Protected Area
as well as an IBA during the high vegetation period. Finally, in Lake Uluabat, nearly
half a million waterfowl were recorded during the vegetated state in 1996, which is
the highest waterfowl density recorded in a Turkish wetland since 1970. Based on
this waterfowl census, the lake was designated a Ramsar site in 1997 [Yarar & Magnin 1997].
ii. Water Level Fluctutations and Water Residence Time and Major Ions
Shallow lakes located in arid to semi-arid regions are particularly sensitive to
water stock. As a result, sensitivity to hydrological conditions also has significant
consequences for the in-lake concentration of major ions and nutrients, especially for
determining the salinity and conductivity [Wetzel 1983, Talling 2001]. The hydraulic
residence time, which is estimated by dividing the lake volume (Vlake) by the volume
of water flowing into the lake (Vin) per unit of time, is prone to hydrological alterations. Long term data from Lakes Eymir and Mogan revealed a significant increase in
hydraulic residence time from around 1 yr-1 to nearly 7 yr-1 coinciding with the low
water level periods (Figure 2a). The consequence of increased residence time was a
several fold increase in the salinity. Lakes Eymir and Mogan with a salinity slightly
above 1‰ can be classified as hard water lakes [Wetzel 1983]. The exceptionally dry
conditions in 2001, 2004 and 2005 resulting in the study’s lowest lake level and highest retention time, caused an increase in salinity and conductivity to the highest values recorded during the study period [Figure 2b]. Hydrological extremes [floods and
dry periods] are predicted to follow the global climatic changes. Semi-arid or arid
Mediterranean lakes are predicted to receive lower input of water due to shorter
precipitation seasons coupled with higher incidence of summer dry periods [Sánchez
et al. 2004]. Extrapolation of these results suggests that if such conditions occur,
many freshwater lakes located in the Mediterranean region would turn saline in the
future. Evidently, in Turkish lakes changes in salinity level in the past have frequently triggered shifts between saline and freshwater conditions [Karabiyikoglu et
al. 1999]. A modelling study based on the worse climate change scenario, which
assumed no reduction in the carbon dioxide emission to the atmosphere, with increases in air temperature (2 0C) and evaporation (10%), and decreases in precipitation (5%) and runoff water (30%) [Arnell, 2003], predicted more than two fold in-
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INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
crease in the salinity levels [Klepper, & Beklioglu, unpublished data]. Such an increase would likely to result in the loss of freshwater species diversity.
B. Nutrients (N & P) & Fish.
i. Eutrophication and Planti-benthivorous Fish
Worldwide, lake eutrophication from excessive inputs of phosphorus and nitrogen, leads to water quality deterioration with significant losses of biodiversity, goods
and services [Kristensen & Hansen, 1994; Dodson et al., 2000]. However, the earliest
study from the pre-eutrophicated period on Lake Eymir revealed that the lake water
was clear, with a summer Secchi disc transparency averaging >4 m, maximum value
being 6 m [Geldiay 1949]. However, over 25 years of raw sewage effluent discharge
shifted Lake Eymir from a state dominated by submerged plants to a turbid water
state with high concentrations of TP, DIN, chlorophyll-a, and suspended solids, in
addition to low Secchi depth [Beklioglu et al., 2003]. Further, eutrophic conditions in
the lake shifted the fish stock to a conditions of being being dominated by planktivorous carp and tench and the share of piscivorous pike became insignificant [Beklioglu et al., 2003]. Several studies illustrate that, in shallow lakes, Benthivorous fish
such as carp can be potentially destructive for aquatic plants, either directly through
grazing and up-rooting or indirectly through increasing turbidity and nutrient release [Crivelli 1983, Breukelaar et al. 1994]. Beklioglu et al. [2006] showed that in
Lakes Marmara and Uluabat, sparse submerged plant development coincided with
high carp biomass. Carp biomass and the binary index of submerged plant coverage
also displayed a strong inverse correlation for these lakes (r:-0.85 & r:-0.72, respectively).
For restoration of Lake Eymir, removal of about half of the tench and carp
stocks, namely biomanipulation led to a 2.5-fold increase in annual Secchi disc transparency (to 262±15 cm from 101±43 cm), largely because of a 4.5-fold decrease in the
inorganic suspended solids concentrations and, to some extent, decreased chlorophyll-a concentrations (9.4±6 mgl-1 and 11.4±2.6 µg l-1, respectively) [Beklioglu et al.
2003] (Figure 3). Fish removal triggered clear-water conditions resulting in a severalfold increase in especially spring Secchi depth during a five-year period. Rapid recolonisation of submerged plants, Potamogeton pectinatus L. and Ceratophyllum. demersum L., occurred, and the growth remained high for a consecutive period of four
years, the cover ranging from 45 to 90% of the lake surface area. Biomanipulation of
Lake Eymir is the only case from warm-temperate or semi-dry Mediterranean climatic region. The case showed that biomanipulation can be an effective restoration
measure in arid climates for shifting a lake to the clear-water state with redevelopment of denser vegetation
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a
3
1125.0
2
1
1124.0
0
1123.0
-1
WL
2001
1999
1997
z -s c ore
1999
z-sc
Lake Marmara
81.0
3
1
77.0
0
75.0
z -s c ore
2
79.0
-1
WL
d
1999
1997
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
1973
-2
1971
73.0
1969
z-sc
974.0
3
2
1
0
-1
-2
-3
973.0
972.0
WL
07. 02
01. 02
07. 01
07. 00
01. 00
07. 99
01. 99
07. 98
01. 98
07. 97
01. 97
971.0
z -s c ore
Lake Mogan
01. 01
Water Level (m . a. s . l . )
c
1997
1995
-2
1993
-1
4.0
1991
0
5.0
1989
1
6.0
1987
2
7.0
1985
3
8.0
1983
9.0
WL
Water Lev el (m . a. s . l . )
1995
z-sc
Lake Uluabat
1981
Water Level (m . a. s . l . )
b
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
-3
1973
-2
1121.0
1971
1122.0
z -s c ore
1126.0
1969
Water Lev el (m . a. s . l . )
Lake Beysehir
z-sc
Figure 1. Changes in water level [m.a.s.l] [WL] and the cumulative z-scores of water level for a]
Lake Beyşehir, b] Lake Uluabat, c] Lake Marmara, d] and Lake Mogan, Lines on the second X-axis
indicate the state of submerged plant development: thick dark line [■] = high vegetation cover, thin
dark line [─] = low vegetation cover, and broken line [---] = lack of data.
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-1
)
INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
hydraulic residence time (t yr
14
12
a)
=
LakeEymir
10
Lake Mogan
8
6
4
2
0
1997
2.2
1998
1999
2001
2002
2003
2004
2005
b)
2
Salinity (ppt)
2000
Lake Eymir
Lake Mogan
1.8
1.6
1.4
1.2
1
98
99
2000
2001
2002
2003
2004
2005
Figure 2. a) Hydraulic residence time and b) salinity values measured
in Lakes Eymir & Mogan from 1998 to 2005.
ii. Eutrophication and climate change
Low water level years in Lakes Eymir and Mogan, which are associated with
longer hydraulic residence times, probably provide great opportunities for sedimentwater contact [Figure 2a] [Saunders & Kalff, 2001].
Chl a
70
SS
Secchi-depth
450
Chl-a (µg/l) SS (mg/l)
350
50
300
40
250
30
200
150
20
100
10
Secchi deptjh (cm)
400
60
50
0
0
Bef.
Bioman.
Dur.
Bioman.
2000
2001
2002
2003
Figure 3. Mean concentrations of suspended solids, chlorophyll-a, and Secchi disk
transperancy recorded before and during biomanipulation, and the consequent years in Lake Eymir.
Thus, during drought periods the in-lake phosphorus concentrations in Lakes
Eymir and Mogan increase several-fold and thus became more dependent on internal
processes rather than external loading (Figure 4). Despite the fact that the external TP
load was low, the in lake total phosphorus (TP) concentrations remained high (Figure 4). Further, in Lake Eymir, there has been a late increase in the ammonium concentration coinciding with decreased availability of dissolved oxygen (Figure 5).
Anoxic conditions, which probably resulted from the increase in phytoplankton
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BASIN WATER MANAGEMENT
production, interrupt the process of nitrification, leading to accumulation of ammonium and, in turn, reduced or completely stopped the process of denitrification. This
is in aggrement with findings from the Parana wetland, Argentina, where ammonium and phosphorus increased along with an increase in anoxic conditions
[O’Farrell et al. 2003, Izagurre et al. 2002]. However, the results of an investigation
comprising of 100 European lakes demonstrated the occurrence of nitrogen depletion
in these lakes within the past 15 years, which is partly attributed to enhanced temperatures [Weyhenmeier et al. submitted]. Thus, increased temperatures may result
in different responses in nutrient concentrations, especially nitrogen, between cold
and warm lakes due to variations in the denitrification processes in response to parameters such as the oxygen amount. Furthermore, it has recently been suggested
that nitrogen may play a far more important role for the stability of submerged
plants upon recovery, especially at relatively high phosphorus levels [Moss 2001].
The late increase in ammonium concentrations may have contributed to the instability of clear-water conditions in Lake Eymir. Thus, in shallow warm lakes, the effects
of global warming may require establishment of even lower thresholds not only for P
but also for nitrogen loadings than those so far suggested to obtain a stable submerged plant dominated clearwater state [Romo et al. 2004].
In Lake Eymir nutrient enrichment led to a several fold increase in the concentrations of chlorophyll-a associated with a significant decrease in the dissolved oxygen availability that result in occasional fish-kills in the summer occurred in 2004 and
2005 [Ozen & Beklioglu, unpublished data]. This may induce temporary unstable
clear water conditions since the nutrient enrichment is high and the stability of induced clearwater conditions is expected to be short-lasting.
TPload
3000
7000
2500
6000
5000
2000
4000
1500
3000
1000
2000
500
TPload (kg)
TPlake (kg)
TPlake
1000
0
0
1997 1998 1999 2000 2001 2002 2003 2004
Figure 4. Total phosphorus (TP) load from the catchments and in- lake TP amounts
recorded between 1997 and 2004 in Lake Eymir.
592
Dissolved oxygen concentration (mg -1
l)
INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
mean oxygen
5
4.5
hypolimnetic oxygen
4
3.5
3
2.5
2
1.5
1
0.5
0
Bef.
Bioman.
Dur.
Bioman.
2000
2001
2002
2003
Figure 5. Mean and hypolimnetic dissolved oxygen concentration recorded
in Lake Eymir during before and during biomanipulation, and the consequent years in Lake Eymir.
In sum, ecology of Turkish shallow lakes and their associated species diversity
are very sensitive to hydrological alterations, namely water level fluctuations and
hydraulic residence time. In nutrient poor lakes, low water levels may enhance submerged plant development with obvious advancement of the littoral zone with associated species diversity. Eutrophication in Turkish shallow lakes may also lead to the
loss of submerged plants with increased turbidity resulting from benthiplanktivorous fish feeding and increased phytoplankton production. Biomanipulation as a restoration measure can offer a help as removal of half of tench and carp
populations led to the re-development of submerged plants with clearwater conditions. However, global climate warming appeared to complicate the lake processes
through increasing salinity and in turn reducing the freshwater species diversity.
Further the increase in salinity appears to be inevitable as shown by the predictive
models. Furthermore, global warming may lead to further, enhancement of eutrophication through increasing bottom-up processes as a provider of nutrients. Therefore,
it appears to be very critical to take control measures to prevent eutrophication of
lakes and implement restoration methods along with better hydrological management. At present time, these are the biggest challenges of lake scientists, lake managers and state authorities.
Acknowledgement. Several MSc and PhD students worked towards producing
data that have been presented here, I am grateful to Arda Özen, Can Ozan Tan and
Burcu Karapinar. Sara Banu Akkaş for linguistic help.
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BASIN WATER MANAGEMENT
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