This experimental study was conducted to get baseline information about SRP flux between the bottom sediments and the overlying water of KR during monsoon and summer seasons, and to find out the spatial variation of potential release rate of phoshorus, besides the efficiency of oxygen in the control of phosphorus release from the bottom sediments was studied.
3.1 Seasonal Variation of SRP Flux and Potential Release Rate of Phosphorus from Krishnagiri Reservoir Sediments
SRP flux under the anaerobic condition of sediments collected during the monsoon season is illustrated in Fig. 2. After 30 days of the incubation period, final SRP concentrations were highly variable under anaerobic conditions. Lacustrine zone sediments exhibited a wider range of P flux than riverine and transition zone sediments. The highest value (1.56 mg l− 1) of SRP was recorded on the eighth day of incubation in the columns with lacustrine zone sediments. Sediments at the Transition-D showed the maximum SRP concentration (1.3 mg l− 1) on the eighth day of incubation; whereas sediments collected from the riverine (1.21 mg l− 1) and Transition-S (0.81 mg l− 1) zones exhibited the maximum concentration on the ninth and eleventh day respectively.
The SRP flux of the sediments collected during the summer season under anaerobic conditions. SRP values increased linearly during the first few days of incubation and the highest value (1.62 mg l− 1) was recorded in Lacustrine zone sediments on the eleventh day. The columns containing Riverine and Transition-S zone sediments exhibited the maximum SRP values of 1.24 mg l− 1 and 0.94 mg l− 1 on the ninth day of anaerobic incubation. Hence, the linear release rate of SRP from the KR sediments during anaerobic incubation might be caused by the reason of the decomposition of organic matter accumulated at the bottom sediments and the consequent release of Organic-P. Kleeberg and Schlungbaum (1993) found that P fractions other than the reductant SRP were released from the sediments. The decomposition of Organic matter resulted in the degradation of organic phosphorus due to oxidation and mobilization of organic phosphorus is also possible during an anoxic environment via sulphate reduction (Kleeberg 1997).
Table 1 elucidates the potential release rate of phosphorus in KR sediments collected from four different zones during the monsoon and summer seasons. Under anaerobic conditions, the SRP concentrations were increased linearly with time in all the sediments collected during monsoon seasons.
Table 1
Potential Release Rate of Phosphorus from KR sediments collected during monsoon season
Sl. No. | Sediment Zone | Potential Release Rate of Phosphorus in mg m− 2 d− 1 during monsoon season | Potential Release Rate of Phosphorus in mg m− 2 d− 1 during summer season |
Min. | Max. | Mean | Min. | Max. | Mean |
1 | Riverine | 6.0 | 107.6 | 53 | 35.8 | 95.6 | 49 |
2 | Transition (Shallow) | 6.0 | 35.9 | 17.3 | 12.0 | 87.5 | 36.6 |
3 | Transition (Deep) | 12 | 77.7 | 53.8 | 17.9 | 113.6 | 55.7 |
4 | Lacustrine | 53.8 | 102 | 65 | 23.9 | 125.5 | 69.9 |
Especially higher phosphorus flux was observed with the lacustrine zone sediments during anaerobic incubation than riverine and transition zone sediments. The maximum of 102.0 mg m− 2 d− 1 potential release rate of phosphorus was observed on the eighth day of incubation in the lacustrine zone sediments collected during monsoon season with the highest mean concentration (65.0 mg m− 2 d− 1). The highest potential release rate of phosphorus (107.6 mg m− 2 d− 1) in the riverine zone sediments was noticed on the ninth day of anaerobic incubation revealed that there was a considerable release of phosphorus, which could be from the allochthonous input. The Transition-D zone exhibited the highest potential release rate of phosphorus on the eighth day and the Transition-S zone sediments showed the maximum release rate of phosphorus on the eleventh day of anaerobic incubation. There was a great difference between the two different locations of shallow and deep regions in the transition zone.
The lacustrine zone exhibited a maximum potential release rate of phosphorus (125.5 mg m− 2 d− 1) during the summer season under anaerobic incubation with a mean concentration of 69.9 mg m− 2 d− 1. The Transition (Deep) zone showed the next highest (113.6 mg m− 2 d− 1) potential release rate of phosphorus with the mean value of 55.7 mg m− 2 d− 1 during the summer season. The Transition (Shallow) zone showed the minimum potential release rate of phosphorus (12 mg m− 2 d− 1) with the lowest mean value of 36.6 mg m− 2 d− 1.
3.2 Spatial Variability and Predominant Zones of Potential Release Rate of phosphorus
The riverine and Transition-S zone sediments exhibited a linear release rate on the ninth day of incubation. Transition-D zone and lacustrine zone sediments showed the maximum release rate under the anaerobic condition on the tenth and eleventh day respectively. At the end of the incubation period, zone-specific differences in the release rate of phosphorus were still evident and highly significant (Table 2) with the p values of 0.035.
The high release rate of phosphorus may also be possible due to the high concentration of loosely bound phosphorus (NH4Cl-P) in the sediments as well as due to high sedimentation rates of phytoplankton as declared in the Lake Arresø, Denmark by the authors Søndergaard et al. (1992). In KR, NHCl-P constituted about 16.9–25.7% NH4Cl-P% of total phosphorus during the Northeast monsoon season (Sudha and Ambujam 2012b). Hence the more accumulation of NH4Cl-P in the bottom sediments of KR collected during the Northeast monsoon season might be the reason for the high potential release rate of phosphorus during the study.
Table 2
Two way ANOVA for P release rate at four zones in KR sediments
Sl. No. | Sediment collection Zones | Mean Square | SD | F Value | p-Value |
1 | Riverine | 27.92 | 32.52 | 4.315 | 0.035 |
2 | Transition (Shallow) | 13.15 | 17.78 |
3 | Transition (Deep) | 29.20 | 29.52 |
4 | Lacustrine | 36.23 | 36.11 |
According to the statement of Mortimer (1941 and 1942), in anaerobic environment manganese and iron undergo a reduction process and this results in high P dynamics from the bottom sediments. In the earlier study, the authors Sudha and Ambujam (2012b) were observed the high accumulation of total iron (52 gm kg-1 DW) in the lacustrine zone sediments of KR. Hence, this could explain the reason for the high potential release rate of phosphorus during the experimental study using KR sediments. Therefore, from the present experimental study, Lacustrine and Transition (Deep) zones were identified as the zones where the high sediment P release rate is taking place. The results showed that the release rate was moderate to high.
3.3 Efficiency of Oxygen on the Control of SRP Flux and Potential Release Rate of Phosphorus from Krishnagiri Reservoir sediments
3.3.1 Efficiency of Oxygen on the control of P release from sediments collected during monsoon season
To find out the efficacy of oxygen on sediment phosphorus release, the concentration of SRP in all 30 days was taken to estimate the highest rate of positive retention of phosphorus in the sediments during aerobic incubation. Excellent control over the phosphorus release in riverine and transition zone sediments was observed using oxygen gas (Fig. 3). A remarkable reduction in the SRP concentration was observed on the 30th day at the Transition-S zone sediments with the lowest concentration (0.241 mg l− 1) during the aerobic incubation period. The riverine zone sediments showed a minimum of 0.931 mg l− 1 on the 30th day of aerobic incubation.
A reduction in the SRP values in the Riverine and Transition-S zone sediments revealed that P undergoing sedimentation when they were exposed to oxygen. SRP concentration in Riverine and Transition-S zone sediments remain the same till the 30th day of the aerobic incubation supporting the study of Zhang et al. (2011), as they reported in their experimental study that the concentration of SRP in the overlying water attained a steady concentration between 15 ~ 20 days of incubation. Whereas a very slow release of SRP was observed in the columns packed with Lacustrine and Transition-D sediments. The maximum concentration of SRP (1.43 mg l− 1) in the overlying water of the Lacustrine zone sediments and 1.03 mg l− 1 of SRP in the Transition-D zone sediments were recorded on the 21st day of the aerobic incubation.
The efficiency of oxygen on the control phosphorus release sediments of different zone sediments collected during the summer season showed linear increases in the SRP values (Fig. 3). The concentration of SRP kept on increased in all the water columns except Transition-S zone sediments. The highest SRP values of 1.058 mg l− 1 (Riverine) and 1.742 mg l− 1 (Lacustrine zone) were recorded on the eleventh day of incubation. Although various processes are concerned with sediment phosphorus release, two major influencing processes are Ironbound phosphorus release under redox conditions and the involvement of microbes in phosphorus releasing mechanisms (Søndergaard et al. 2003). Redshaw et al. (1990) revealed that the equilibrium SRP concentration during aerobic incubations is the sign of a high potential for phosphorus dynamics. This present experimental study using the sediments of KR exhibited that the phosphorus flux among water and sediment was high when incubated under anaerobic conditions compared to aerobic incubation and the same condition was revealed by few scientists Sen et al. (2007); Haggard et al. (2005); and Holdren and Armstrong (1980).
Table 3 illustrates the efficiency of oxygen on the potential release rate of phosphorus from four different zones of KR. The maximum potential release rate of phosphorus (32.3 mg m− 2 d− 1) was recorded with the lacustrine zone sediments on the fourth day of aerobic incubation with the highest mean value of 4.8 mg m− 2 d− 1. All four different zone sediments showed the positive retention of P and the highest sedimentation rate of -4.8 mg m− 2 d− 1 was observed in the Transition-S sediments.
Table 3
Efficiency of Oxygen on potential release rate of phosphorus from KR sediments collected during monsoon season
Sl. No. | Sediment Zone | Potential Release Rate of Phosphorus mg m− 2 d− 1 during monsoon season | Potential Release Rate of Phosphorus mg m− 2 d− 1 during summer season |
Min. | Max. | Mean | Min. | Max. | Mean |
1 | Riverine | -3.0 | 3.6 | -1.2 | -1.8 | 5.4 | 0.9 |
2 | Transition (Shallow) | -4.8 | 6.0 | -1.5 | -3.0 | 3.0 | 0.2 |
3 | Transition (Deep) | -1.2 | 21.5 | 2.5 | -2.4 | 36.0 | 4.8 |
4 | Lacustrine | -3.0 | 32.3 | 4.8 | -1.8 | 41.8 | 5.2 |
Sediments from four different zones showed positive retention of phosphorus under aerobic incubation during summer season. But, also the release of phosphorus was observed in all the four zones with the highest value of the potential release rate of phosphorus (41.8 in mg m− 2 d− 1) in the Lacustrine zone sediments. The mean values of potential release rate of phosphorus ranged between 0.2–5.2 in mg m− 2 d− 1.
Table 4 exhibits the significant differences between the incubation condition and the release of phosphorus with the p values of 0.00. This indicates that the sediment phosphorus release rate was effectively controlled by the supply of oxygen.
Table 4
Two way ANOVA for phosphorus release rate under different incubation conditions with different seasons in KR sediments
Incubation Conditions with Different Seasons | Mean Square | SD | F Value | p-Value |
Monsoon Anaerobic | 47.25 | 20.72 | 37.73 | 0.00 |
Monsoon Aerobic | 1.15 | 3.03 |
Summer Anaerobic | 55.30 | 13.86 |
Summer Aerobic | 2.77 | 2.59 |
The consequences of phosphorus release from the bottom sediments in Krishnagiri Reservoir, including the major impact on essential ecosystem services such as the supply of water for drinking, domestic, irrigation, industries; fish production; livestock rearing, and recreation. As the soil erosion in the catchment area of KR was controlled by soil conservation programs in the past few years there is a necessity for the effective control of the internal loading of phosphorus to hasten the recovery process once the external input of phosphorus from the catchment has been reduced. The main aim of the efforts to restore the Krishnagiri Reservoir from hyper-eutrophication condition is the assumption that a reduction in the concentration of SRP in the water column will subsequently reduce the hyper-eutrophication condition and thereby the improvement of water quality and ecosystem services would be achieved effectively. From this site-specific experimental study, it was discovered that the sediments from four different zones in KR exhibited different potential release rate of phosphorus and are acting as a source, especially the Transition (D) and Lacustrine zones exhibited high potential release rate of phosphorus. The effective control of phosphorus release was identified in this study in the sediments of all the four different zones by using oxygen. Especially in the sediments collected during the summer season when the external input of P was almost nil, the supply of oxygen has control over phosphorus release and enhance the sedimentation rate of -2.4 mg m− 2 d− 1 ( i.e positive retention of phosphorus) at the Transition-D zone and − 1.8 mg m− 2 d− 1 at the Lacustrine zone. Transition-S zone sediments collected during monsoon season exhibited the maximum sedimentation rate of P (-4.8 mg m− 2 d− 1). The riverine sediments exhibited the highest sedimentation rate of -3.0 mg m− 2 d− 1 when collected during the monsoon season. The sediments at the riverine and Transition-(S) (Northern part of KR) may tend to control over P release whenever the natural mixing is taking place under the influence of the wind as they are at the shallow depth. The main outcome of this experimental study is that the Transition (D) and Lacustrine zones were identified as the zones which are releasing more P from the bottom sediments. Thus, these zones can be considered for the restoration processes to control sediment P releases. Besides, this study also proved that oxygen effectively controls sediment P release in all four different zones of KR.