Water chemistry and nutrients
The physico-chemical characteristics of the reservoir water play vital role in the productivity of the system. The important physico-chemical features ofKanhirapuzha reservoir are presented in Significant variations in these parameters were not observed between months or seasons and they largely depicted the oligotrophic nature of the reservoir.
Temperature of the reservoir water is of primary importance since it regulates biotic growth rates and life stages and defines fishery habitat. It leads to speeding up of the chemical reactions in water. The average surface water temperature of the reservoir varied widely from 25°C during the monsoon season (rainy) to 30.5°C in the summer season (dry). But in certain months, September 2008 at Sl and S4 and February and March 2009 at SI, the maximum temperature difference of 3.5- 4.0°C was observed between the epilirnnetic and hypolirnnetic layer.
The intensity of the light penetrating through water media is known as transparency. Turbidity is a measure of the cloudiness or murkiness of water due to suspended particles and can be caused by organic particles, such as decomposed plant and animal matter, or living biological organisms (algae), inorganic particles (silt, clay and natural chemical compounds like calcium carbonate). Surface turbidity occurs due to heavy rain, flooding and monsoon runoff landslides. The transparency of Kanhirapuzha reservoir recorded a low of 135.3cm during summer (May) and was higher 251.3 em (October) following the monsoon season during the entire study period. The transparency level explains the heavy monsoon influx has not lowered the secchi depth because the water content is less turbid and clear.
Dissolved oxygen (DO) content in the water reflecting the physical and biological process prevailing in the reservoir was high and conducive (5.1 to 9.0 mgl,”) at surface throughout the study due to the wind activated turbulence. A state of anoxic condition was also observed during the study period as the oxygen level went to a low of 0.6 to 3.2 mgl.i’in the middle layers of site S1, S2and S5 during May and June 2009 and nil to 1.6 mgl,” during the other months. But no signs of fish mortality were observed at that time. Biological oxygen demand was also measured for 5 days and it has not shown any significant difference indicating the scant presence of organic substances.
pH, the measure of the intensity of acidity or alkalinity rising by photosynthesis and declining by respiration is an essential indicator of reservoir productivity. Generally, peninsular reservoirs record alkaline property by the presence of carbonates. But Kanhirapuzha probably receives humic acid from the rainfed forest, registered acidic to neutral pH (6.2 -7.8) which is the common characteristic of reservoirs of Kerala. A low pH of 5.7 was also recorded in the late South-West monsoon season, in September 2007.
Free carbon dioxide is essential for the synthesis of carbohydrate through photosynthetic activities and it increased as the depth increased. During the period of study, the free carbon-dioxide values ranged from 0.4 to 5.1 mgl,”.
Alkalinity ofthe water is its capacity to neutralize a strong acid and is characterized by the presence of all hydroxyl ions capable of combining with hydrogen ion. It also determines the productivity of the reservoir. The total alkalinity in the range of> 90.0, 40-90 and < 40 mgl.i’are estimated to be high, medium and low productive, respectively (Sugunan, 1995). Accordingly, the epilimnetic total alkalinity values registered in this reservoir (17.3 to 35.6 mgl.”) indicate that the reservoir is less productive.
Conductivity is a measurement of the ability of an aqueous solution to carry an electrical current. The reservoir water exhibited an average electrical conductivity of 45.1rnhos cm’ and seasonal fluctuations are absent probably due to the absence of charged ions in allochthonous input.
Hardness is the property of water which prevents the lather formation with soap and increases the boiling point of water. The hardness of the reservoir on an average was low
Calcium and Magnesium
Calcium is one of the most abundant substances of the natural waters whereas the concentration of magnesium is lower than the calcium. Both are leached from the rocks. Magnesium is required by the flora to build its chlorophyll and for the growth of algae. The depletion of magnesium acts as a limiting factor for the growth of phytoplankton and it rauged from 2.45 to 107.0 mgC (Goel et ai.,1986; Khatavkar et al., 1990). The concentration of both calcium (2.1 to 5.8 mgl,”) and magnesium (1.7 to 9.3 mgl,”) in the water samples registered lower values indicating the low productive status of the reservoir and contributing poor supplement for phytoplankton growth.
Trophic State Index
Trophic state is defined as the total weight of living biological material in a water body at a specific location and time. It is used to classify the nature of the waterbodies as oligotrophic, mesotrophic and eutrophic. The Carlsson’s Trophic State Index (TSI) value based on secchi depth (27.28) and phosphorous (44.67) reiterated that Kanhirapuzha reservoir is a classic oligotrophy with clear water, saturated with oxygen round the year in the hypolirnnetic zone. Oligotrophic water bodies are characterised by low productivity, low nutrient content, low algal production and clear water
The rocks in which most of the phosphorus is bound are generally insoluble in water,and hence the phosphorus content of natural freshwaters is low and biological growth is limited due to this fact. Phosphorus is considered an important element for the growth of plankton and mostly it is in inorganic forms in natural waters. Since, Kanhirapuzha reservoir is devoid of any pollution the phosphorus content is trivial (0 to 0.03 mgl,”) thus indicating that the inflow to the reservoir is from non-polluting sources.
The most important source of the nitrate is biological oxidation of organic nitrogenous substances. Run-off from agricultural fields is also high in nitrate. Kanhirapuzha reservoir located at the foothills of Western Ghats would naturally record low nitrate values (traces to 0.2 mgL-1 ), the source of water being pure and not much contaminated.
The next most abundant element in the earth after oxygen is silicon but it occurs in meagre quantities in Kanhirapuzha water. The solubility of silica has been found to be more at high pH or high temperature. Silicate is the core component of bacillariophyceae. The value of silica in this reservoir ranged from traces to 37.0 mgl,” .
Thermal and oxygen stratification
The thermal and oxygen stratification of a waterbody refers to a change in the temperature or oxygen at different depths in it. The sudden decline in values refers to the productivity status occurring mostly in the middle layer of water bodies. They are referred as oxycline and thermocline which were absent during study period in either of the transitional or lacustrine zone. During April 2008 anoxic conditions (0.08 ppm) were observed at the bottom of station 2 where the depth was 16 m. Thermal or oxygen stratification could not be observed in 2008 but a distinct oxycline was observed in Station Sl, (the Lentic zone near the sluice) and station S5 (the protected zones in between the two small hillocks) during April, May, June 2009. It was probably due to the non-mixing of water as there was no water flux during this period, hence the oxycline formed was undisturbed. During these three months, the water was anoxic below 7.0 m depth.
The basin of this reservoir is largely covered by laterite stone beds with intermittent alluvial soil. Soil pH was acidic ranging from 5.01 to 5.84. Electrical conductivity registered from the low of 32.2 umhos/cm in February 2009 to the maximum of 70.4 umhos/cm in June 2009. Electrical conductivity ofthe soil was comparatively lower in summer season than that of rainy season (June – November, 2007) where the maximum value recorded was 158.8 umhos/cm. This could be attributed to the marginal input of allochthonous sources during rainy season. The average available phosphorus values ranged from 0.12 (S3) to 1.08 mg/100 g soil in S5. The vast variation in the phosphorus content in the different sites may be due to the nutrient flow affected by hydrodynamics at the reservoir. The percentage of organic carbon content in the soil recorded more than 2.3 during dry season which may be due to pooling of alluvial soil scattered at the bottom basin of the reservoir.
Chlorophyll a is a well-accepted index for phytoplankton abundance and population of primary producers in an aquatic environment. It is a reliable and commonly used proxy for total phytoplankton biomass. In reservoirs chlorophyll a concentration generally ranges between 3.49 and 47.34 IlgL-‘ (Gregor and Marsalek, 2004). The average Chlorophyll a was quantified in 2007 -08 for this reservoir wasl6.6 IlgL-‘which reduced to 11.6 IlgL-‘ in the succeeding year 2008-2009, corresponding to the decline in plankton count. The other pigments chlorophyll band c, carotenoids are also estimated. The
verage chlorophyll b also showed similar trend measuring 23.2 IlgL-‘ and 18.1 IlgL-‘ in 2007-08 and 2008-2009, respectively, Chlorophyll c pigments of annual average recorded very high values 159.3IlgC’ in 2007-08 and plummeted to 26.1 IlgL-‘ in the following year. Carotenoids are the other pigments estimated which are responsible for bright colors in various biological organisms and accumulated in chloroplasts in all green plants.
Primary production or carbon synthesis is a variable complex and function of available nutrients and intensity of light. The primary production of the reservoir was estimated every month. The quarterly average gross carbon synthesis in Kanhirapuzha reservoir fluctuated from 16.66 to 64.23 mgC m’ hrduring the study period and the net primary production from 5.20 to 48.35 mgC m-3hf’.The community respiration recorded in this reservoir ranged from a low13.12 to moderate 42.04 mgC m-3hf’. The Net.Gross ratio recorded higher value (1.56) during peak monsoon season July-September 2007, indicating corresponding maximum net photosynthetic activity and so also respiration. During Oct – Dee 2007 late monsoon, the P: R ratio was high (2.49), indicating that the community respiration is lower while net production had not significantly subsided
Energy flow of the reservoir
The process of energy transformation by chlorophyll bearing photosynthetic organisms from solar electromagnetic radiation to chemical energy (primary production) and to fish production is given below. The percentage conversion of primary production to fish yield of Kanhirapuzha reservoir (0.013) is lower compared to some of the Indian reservoirs (Bhavanisagar: 0.182, Nagarjunasagar: 0.0456, Rihand: 0.0311, Govindsagar: 0.123).
|1. Light energy to Primary production||0.35 %|
|2. Light energy to fish production||0.00047%|
|3. Primary production to fish yield||0.013 %|
- central Inland Fisheries Research Institute, Barrackpore