Fluctuation of water level: A new abiotic index for studying insect biodiversity variations, in a large waterbody.

Dr.Asma Ali (Ecologist)

Many researchers have shown the importance of water chemistry and benthic habitat characteristics for the conservation of the freshwater macroinvertebrate biodiversity. However very few authors have examined the physical effects of extreme water level fluctuations in lakes. 

The present study set out to determine, through a comparative study between four sampling stations of Upper Lake, how water level fluctuation affect the structure of the shoreline macroinvertebrate communities. The present paper investigates the influence of water level fluctuations on the community indices of shoreline insect’s assemblage and discusses the role of disturbance, physical parameters and biological interactions in structuring communities of shoreline insects in Upper Lake.

INTRODUCTION: 

Numerous studies have been conducted on the impact of the stream regulation on the various biotic communities (Hoffman and Kilambi 1970,Lehmkuhl 1972, Trotosky and Gregorry 1974) of temperate water bodies. Some investigations were carried out on the distribution and long-term changes of benthic invertibrate communities in relations to flow characteristics (Armitage & Pardo 1995, Bickerton 1995, Fjellhein 1996, Hamill 2009).

In lake habitate, water level fluctuations plays an important role in structuring macroinvertebrate communities (Resh et al. 1998, Richter et al., 2003, Voldovinos et al.,2007) . Particularly in large lakes, shore zones is affected by water flow variations and drying up, both of which affect community and population structure of shoreline communities. Shorelines of large lakes offer a wide range from periodically to permanently flooded areas. In such a gradient, ecological theory predicts that maximum species diversity is reached in sites with medium frequency and intensity of disturbance ( Connel 1978, Hutson 1979).

MATERIALS AND METHODS:

                                      The present study was conducted on the four sampling sites of shorelines of Upper Lake Bhopal. The site Upper Lake of Bhopal, is a vast man-made lake of the capital city of Madhya Pradesh , a centrally located state of India. The essential features of the Upper Lake are given in table 1. Four sampling stations of Upper Lake were taken to study the effect of water level fluctuation i.e. Bhadbhada (S1), Van-vihar (SII), Pump-house (SIII) and Bairagarh (SIV).

    Table-1: Essential statistics of the Upper Lake of Bhopal ( India).

Period of construction	11th Century A.D.
Type of Dam	Earthen
Location	Latitude Longitude	23º12’-23º16’N 77º 18,-77º 23’E
Periodicity of water level fluctuation	Seasonal
Trophic status	Oligotrophic
Catchment area (Sq.km. )	361
Submergence area at FTL (Sq. km. )	36.54
Full tank level(MSL)m.	508.65
Dead storage level (MSL) m.	503.53
Storage capacity (million Cu.m)	117.05
Maximum depth(m)	11.7
Designed flood discharge(Cu.m /sec. )	2208
Source of water	Rain water
Main use of water	Potable water supply
Inflow points (Nos.)	31
Sewage water inflow (MLD)	50.47

              Sampling was carried out at monthly intervals regularly for a period of 24 months. Water samples for chemical analysis were collected from four sampling stations in the morning hours between 10 AM to 12 PM. Excepting water level fluctuation, temperature, transparency and pH all parameters such as Chloride, Calcium hardness, Total hardness, Alkalinity, Dissolved oxygen were analyzed in the laboratory by APHA(1998) methods. 

The insect fauna was collected by transect method using insect net ( Knapp,1998; Taki and Kevan 2007 ) in field based upon the different preservation methods for variable insect orders. Shoreline insects of each sampling station were identified to major taxonomic groups and counted.

Data analysis:

                    Sampling sites were characterized through an index that quantifies the extent of drying up over two years. For each sampling point the area under the water level curve when the actual water level was below the sampling point was calculated as distance to actual water level in meters.

RESULTS:

                  The climate of Bhopal is relatively moderate and dry except in the monsoon season, indicating a seasonal rhythm of weather. During the two years of study period the maximum ambient temperature was recorded 46 degree.

Rainfall was observed between .0003 to 21.67 mm.Whereas the water level in Upper Lake is regulated by an earthen dam towards the east, near the Kamla park from where, there is a regular seepage, which forms the main source of water for the lower lake and a Bhadbhada weir towards the south having a sluice gate for releasing excess water into Kaliasot dam. Thus fluctuation of water level of Upper Lake is seasonal and temporary.

The total 83 species of different orders were recorded at all four sampling stations of shorelines of Upper Lake during the study period. The total number of taxa was recorded slightly higher at stations II and IV then stations I and III. Low water period in summer season due to drying up, station I showed maximum water level fluctuation from actual sampling point (minus m.) and the most capable species of order strepsiptera became dominant, whilst some odonatans, dipterans and coleopterans become rare in the same season. 

On the other hand adults of orthopterans, hemipterans and lepidopterans abundance was not found to be associated with high intensity of water level fluctuations, due to the migratory nature of insects belonging to these orders.

Table-2: Mean abundance of shoreline macroinvertebrates per m² at all sampling stations of Upper Lake during two years).

Orders	SI	SII	SIII	SIV
Hemiptera	2.24	3.42	2.45	2.86
Hymenoptera	-	4.8	1.36	1.45
Strepsiptera	2.18	1.95	-	1.32
Dictyoptera	0.73	1.16	1.11	1.16
Odonata	3.7	3.08	4.96	4.93
Lepidoptera	2.17	3.33	2.20	3.32
Diptera	6.6	4.75	4.18	3.26
Orthoptera	3.2	3.91	3.14	4.2
Coleoptera	2.48	3.5	2.96	3.43

At station III water level fluctuations were a function of changes in the amount of water in lakes. These fluctuations, generally short in duration, were due to wind or barometric pressure and seasonal, because the Upper Lake was generally at their lowest levels in the summer months at all sampling stations.

In the summer, when the air above the lake was warm and moist and the lake was relatively warm, evaporation from the lake was great. With more water leaves the lake then entered, the water level decline to their seasonal lows, similarly evaporation from the lakes was least in the early winter when the air above the lakes was cold and moist and the lake was cold, condensation on the lake surface replaced evaporation. In the rainy season, more water entered in to lake, than left, this process caused water level rise (fig.1).

                

Table-3: a List of several shoreline insects/20m² collected at all four sampling stations of Upper Lake of Bhopal during two years of study period.

Taxa	Order	SI	SII	SIII	SIV
N. maculata	Hemiptera	506	220	396	624
H.stagnorum	-,,-	522	425	724	672
B.indicum	-,,-	396	400	518	396
G.lacustris	-,,-	542	578	470	672
C. varicunda	-,,-	356	424	258	362
R.varipes	-,,-	440	588	404	828
N.viridula	-,,-	196	236	316	278
P.auriculacea	Coleoptera	658	612	466	762
P.albuguttata	-,,-	626	622	612	622
C.septempunctata	-,,-	692	644	318	422
D. indicus	-,,-	499	414	554	606
D. unidentatus	-,,-	488	366	268	144
Boyeria	Odonata	404	532	374	206
Brachythemis	-,,-	286	422	336	314
S. striolatum	-,,-	220	273	258	268
L.quadrimaculata	-,,-	120	274	356	226
I. elegans	-,,-	240	256	292	308
P. numphulla	-,,-	-	82	-	-
A. crenulata	Ortoptera	398	382	402	330
G.africana	-,,-	-	382	338	206
P. spumarius	-,,-	-	344	130	-
P. griseoaptera	-,,-	-	126	-	104
S. gregaria	-,,-	4014	288	550	322
C. trachypterus	-,,-	282	330	-	294
C. spenariodes	-,,-	-	430	-	332
T. tendipes	Diptera	446	530	503	535
T. kiffrulus	-,,-	520	620	538	304
D. baumbauri	-,,-	262	432	381	355
D. irrorata	-,,-	220	280	349	205
C. longiarolata	-,,-	428	544	260	-
B. wallaci	Strepsiptera	225	202	-	104
X. adusta	-,,-	320	224	-	94

   At station II and Iv the high growth of aquatic plants such as Eichhornia crassipes, Potamogeton pectinatus, P.nousus, Lemna minor, Leersia hexendra and Vallisnaria spiralis affected the flow of water. Plant growth is part depended on the weather, and vary from month to month and year to year. In the summer, aquatic plant growth reduced the flow of water on average by 2 percent, therefore due to dense population abundance of shoreline macrophytes  at stations II and IV helped to decrease fluctuation in water level and also decreased possibility of habitat loss for marginal insects, which is a major threat to biodiversity and ecosystem function. 

Interestingly fluctuated water, picked up materials, aquatic plants from shoreline areas and deposited them wherever the water is slowed down and picked up again when the velocity of the water increased. This process  the equilibrium of aquatic ecosystem, with regard to littoral vegetation and insect abundance. It was also observed that fluctuated water picked up nutrients and deposited them at shoreline zone which increased nutrient level of lake margins and promote dense abundance of flora and fauna.

Table-4: List of several shoreline macrophytes and their population abundance/20m² at all four sampling stations of Upper Lake.

Shoreline vegetation	Bhadbhada(SI)	Van-vihar(SII)	PumpHouse(SIII)	Bairagarh(SIV)
I.aquatica	24	152	43	116
I.fistulosa	26	148	34	107
C. demersum	34	88	34	64
H. Verticillata	36	55	13	40
A. pinnata	16	38	-	34
Najas minor	54	142	53	112
P. pectinatus	44	170	36	166
P.crispus	54	152	52	140
E. crassipus	78	182	40	140
L. hexeandra	34	120	25	118
A.philoxroides	48	80	18	86
J. repens	54	90	46	98
P.glabrum	40	52	34	84
V. spiralis	56	92	38	154
M.spathulatum	80	34	30	88
P.hysterophorus	-	76	-	106
C.forskalli	36	40	22	46
L.minor	70	66	65	96

        In the present study it was observed that several species of order Odonata (,Brachythemis Sympetrum steriolatum, Libellula quadrimaculata, Ishnaru elegans ,Pyrrhosoma numphulla and nainads), Orthoptera (Atrectomorpha crenulata, Gastrimargus africana, Philaenus spumarius and Pholidoptera griseoaptera ) and Diptera (Dioctria baumbauri and Dolicocephala irrorata) were seriously affected at station I through decline in water level in summer . 

On the other hand migratory nature of certain adults of shoreline insects belonging to orders, Hemiptera  (,Notonecta maculata, Hydrometra stagnorum Gerris lacustris,Corixa varicunda and Nazara viridula) Coleoptera (Protaetia, Aurichalacea, Dineutus indicus, Dineutus unidentatus) and Strepsiptera (Batocera wallacei, Xylorhiza adusta)  were not found to be affected by water level fluctuation during the study period; because the subtle changes in water level of Upper Lake gave time to adult shoreline insect to maintain their population abundance by migration towards the favorable habitat (Table-3).

                      (A)

     (B)

    (C)

                          (D)

Fig-1 (A-D): Monthly water level fluctuations at different sampling stations of Upper Lake of Bhopal (India)

DISCUSSION:

Many researchers have shown the importance of water chemistry and benthic habit characteristics of lakes, for the conservation of freshwater macroinvertebrate biodiversity (Brodersen et al. 1998, Allan and Flecker 1993). In contrast, few authors have given attention to the physical effect of extreme water level fluctuation of lakes on the conservation of benthic macroinvertebrate assemblages. This situation contrasts with the case of the rivers, in which abundant information exists (Galat & Lipkin 2000, Richter et al. 2003). The impact of water level fluctuation on shoreline insects as observed in the present study has been documented for the first time in tropical waterbody, particularly in freshwater lake.

In the present study significant difference occurred in the abundance of shoreline insects community at all sampling stations The amplitude and patterns of water - level fluctuations exhibited by different unregulated natural lakes are highly variable. In Upper lake, water level remains constant over a long period and showed considerable short term variations. Fluctuation in water level increased in summer, when there were high ambient temperature (35.5-41.2ºC), high wind velocity and low air humidity (12.8-44.7%) which facilitated desiccation of the first few meters of the substrate.

Temperature extremes and desiccation directly affect the marginal insects within the exposed shore zone (Palomaki 1994, Hamill 2009). However, water - level fluctuations also influence the benthic communities of chironomidae and offsprings of odonata, diptera, coleoptera and hemiptera, indirectly by reducing or eliminating aquatic plants, modifying chemical conditions, and increasing erosion, thereby altering the benthic habitat (Batzer & Wissinger 1996, Friday 1987, Keddy & Reznicek 1986, Palomaki 1994 and Voldovinos et al. 2007). Marginal surface inhabiting insects were not found to be much affected by water level fluctuations in the present study at stations II, III and IV because of the high population abundance of shoreline vegetations, which caused decline of water level fluctuations.

    The present investigation revealed that no significant differences occurred in the abundance of shoreline insect communities at highly dense vegetation sampling stations such as SII and SIV of Upper Lake. Maximum diversity of insect fauna was observed at station II that had intermediate disturbance levels of water fluctuation due to dense population abundance of marginal vegetations of Upper Lake. Because shoreline macrophytes of any waterbody helps to reduce water fluctuation and provides holistic environment to marginal fauna, including insects and mollusks.Similar results with respect to invertebrate species diversity have been reported in several other rivers by Ward & Stanford 1983 ,Robinson & Minshall 1986 and Reckendorfer et.al. 1996.

           Low water level in summer, showed maximum number of shoreline insect, ranged between 3.9 to 14.6 sps/m2. Shallow nature of lake margin and peak abundance of marginal free floating macrophytes in summer, provides hiding place and prey to shoreline insects whereas decomposed rooted macrophytes helped to increase their population abundance. The effect of drying up in sustaining high species diversity has been demonstrated in seasonal oxbow lakes and wetlands and it has been proposed that the removal of organism during low-water periods may account for high marginal insect species richness (Outridge 1987, Growns et al. 1992, Balla & Davies 1995).

    Diversity and abundance of marginal insects depend on the evenness of water level of any waterbody. Species richness showed a curvilinear relationship with disturbance, but the initial increase was less steep than in diversity. Evenness of water level was positively related to insect diversity where a close relationship between insect abundance and water level fluctuation implies that a habitat is species-saturated (Mackey 1977, Drade 1982, Tockner 1993). In such a situation, deterministic processes which gain considerable importance and the most capable species of order strepsiptera such as Xylorhiza adusta (320 species/20m²) and Batocera wallacii (225 species/20m²) becomes more dominant in high fluctuation of water level than other insect species of different order of station I of shoreline of Upper Lake in summer.

    Our results suggested that in Upper lake margins, water level fluctuation effect more on benthic macroinvertebrates such as Chironomids than the other shoreline insects, because changes of water level at Upper lake margins were found to be seasonal and subtle, which effect only benthic or sessile macroinvertebrate animals (Tockner 1993, Reckendorfer 1996). Although marginal insects of order coleoptera, Hemiptera, Strepsiptera and Odonata showed migratory nature, which supported them to survive on the water body where water level decline and rise, seasonally.

    Dense growth of aquatic weeds such Eichhornia Apenogeton, Leersia, Potemogeton, Vallisnaria, Verbascum, Polygonum and Ipomoea at station II and IV found to be responsible to decrease the flow of water and maintain evenness of water level. Thus low fluctuation values at these stations don’t leave any large impact on shoreline insects.

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