Isotopic composition of nitrate (
Nitrogen is an essential macronutrient wherein the availability often limits
primary production in aquatic ecosystems. It is a polyvalent element that
undergoes redox transformation between the terminal oxidation states of
In the eutrophic Lake Lugano, the highly depleted
There are a large number of natural freshwater lakes as well as man-made reservoirs in India. In fact, India has the third-highest number of dams (around 4300) in the world, after China and USA. However, these systems have not been well investigated for biogeochemical cycling. In the very first study of its kind, Narvenkar et al. (2013) sampled eight dam reservoirs spread across India and observed strong thermal stratification during summer in all reservoirs. Six of these reservoirs were found to experience varying degrees of oxygen depletion in the hypolimnia, ranging from hypoxia to complete anoxia, in spring–summer. Anoxia has been found to greatly affect the distribution of nitrogen species in these systems. In order to gain insights into biogeochemical cycling in these poorly investigated water bodies, we selected the Tillari Reservoir for detailed studies. These included measurements of natural abundance of nitrogen and oxygen isotopes in nitrate, and nitrogen and carbon isotopes in POM. These data, first of their kind generated from any Indian freshwater body, facilitate an understanding of biogeochemical processes (especially involving nitrogen) that should be typical of any relatively pristine, tropical, monsoon-affected freshwater body.
Map of the sampling location (Tillari Reservoir). T1 shows the main sampling location at the deepest point of the reservoir.
The Tillari Reservoir is situated in the Dodamarg taluka in the
Sindhudurg district of Maharashtra (15
The Tillari Reservoir is a dimictic water body. Relatively low air temperatures and cool winds descending from the Western Ghats, located immediately to the east of the reservoir, result in convective mixing and well-oxygenated conditions in winter. The water column gets thermally stratified in spring and remains this way until the strong southwest monsoon (SWM) winds and supply of relatively cold water homogenize the water column again. The water column gets stratified after the SWM. Stratification during spring–summer leads to an anoxic condition that is most intense (sulfidic in most years) just before the onset of mixing in June–July. A previous study (Kurian et al., 2012) showed that the occurrence of sulfidic conditions within the euphotic zone supports anoxygenic photosynthesis with brown sulfur bacteria in this reservoir. Methane has been found to accumulate in high concentrations below the thermocline during this period; however, its emissions into the atmosphere are not very high (Narvenkar et al., 2013). Direct human impacts on nutrient inventory of the reservoir are relatively minor, as the basin is located amidst thick forests with low human population density and minimum agricultural activities.
Sampling was conducted at one station located at the deepest part of the
reservoir. Water samples from pre-fixed depths were collected with 5 L
Niskin samplers attached to nylon ropes and equipped with reversing
thermometers to measure temperature. Subsamples for dissolved oxygen (DO) and
hydrogen sulfide (H
Dissolved O
Sampling for isotopic analyses of POM commenced in March 2010 and continued
on a monthly basis till 2012. From 2012 to 2015 samples were collected on a
seasonal basis. Samples for nitrate isotopic measurements were collected from
2011. The facility for nitrate isotope analysis was created in 2014 and
samples from 2014 and 2015 were analysed immediately for natural abundance of
N and O isotopes. Samples from 2011 and 2012 were also analysed on a
selective basis. Samples (up to 3 L) for isotopic analyses of POM and DIN
(dissolved inorganic nitrogen; i.e. NO
Samples for isotopic analysis of nitrate were preserved in two ways. While
samples collected in 2011 and 2012 were acidified with HCl to pH 2.5, those
taken in 2014 and 2015 were frozen immediately and analysed within a week.
Prior to the isotopic analyses, nitrate and nitrite concentrations were
measured colorimetrically. Isotopic analyses of nitrogen and oxygen in
NO
Nitrite concentration was insignificant in most of the samples; sulfamic
acid was added in a few samples that contained nitrite in concentrations
exceeding 0.1
Sodium azide (2M solution) and 20 % acetic acid were mixed in 1 : 1
proportion (by volume) to yield the azide–acetic acid buffer (A–AA buffer)
solution. In 20 mL crimp vials, samples and standards were diluted with LNSW
for a final concentration of 20 nanomoles and a final volume of 15 mL. Two
international nitrite standards (N23 and N20) were added in this step to
check the efficiency of N
The “chemical” method yielded a very low blank (
Samples for measurements of
The analyses of
Surface sediment collected from the reservoir during the May 2012 field trip was analysed on only one occasion to investigate its role as an ammonium source. The freeze-dried, homogenized sample was analysed following similar protocol.
Based on the vertical temperature distribution it appears that the reservoir
gets vertically mixed through convective overturning in winter (December to
February, with the exact duration of mixing depending upon meteorological
conditions prevailing in a given year). In spring, stratification sets in and
is the most intense from April to June/July (with a surface-to-bottom
temperature difference of 7–8
The epilimnion was always oxic. During the stratification periods, the DO
concentrations dropped rapidly within the thermocline. The water column
became well-oxygenated following the onset of the SWM. H
A thorough analysis of nutrient dynamics in the Tillari Reservoir is provided by
Naik et al. (2017). Here we provide a brief
description of nitrate profiles during the study period. Surface-water
nitrate concentrations were typically low throughout the year ranging from
below detection limit to 0.7
Large variations in the isotopic composition of nitrate and ammonium were
observed in space and time. Isotopic composition of nitrate in the epilimnion
could not be measured on several occasions due to low concentrations.
However, when the measurements could be made it was observed that the
The water column remains weakly stratified for a large part of the year,
usually from October to March. A trend of increasing concentrations of
isotopically light (
Elevated nitrate concentrations occur throughout the water column during the
SWM. The
The suspended particulate organic matter in the Tillari Reservoir showed
distinct seasonal and depth-wise variations in its isotopic and elemental
compositions (Fig. 2). Primary productivity in the epilimnion led to higher
Mean annual variations of
Nitrate concentrations in surface waters of the Tillari Reservoir varied from
below detection limit during the pre-monsoon period to 10.7
Time series of nitrate concentrations
The isotopic composition of the DIN source exerts the key control on the
In March, when nitrate was close to the detection limit, surface
Stratification in the Tillari Reservoir sets in soon after the decline of the
monsoon-fed inflow following which nitrate concentrations increased in
oxygenated bottom waters with a concomitant decrease in ammonium
concentrations, indicating the occurrence of nitrification. The nitrate
concentrations ranged from below detection limit in the upper 10 m to nearly
10
Nitrogen and oxygen isotopic composition of dissolved nitrate during three different periods in 2014. February represents the early or weak stratification period with two distinct clusters of epilimnetic (0–10 m) and hypolimnetic (15–48 m) samples. April is a period of intense water-column stratification and a denitrification signal is observed in the bottom waters. July is a period of monsoon holomixis when the water column has uniformly high nitrate values.
Ammonium, oxygen and carbon dioxide are the major substrates needed for
nitrification (Christofi et al., 1981). While ammonium largely comes from the
sediments, oxygen is supplied from aerated surface waters. During the early
stratification period, conducive conditions exist for nitrifiers to grow
within the hypolimnion. However, as the bottom waters turn increasingly more
oxygen-depleted with the intensification of stratification, the
“ammonium-oxygen chemocline” (Christofi et al., 1981) moves upward in the
water column and the metalimnion becomes more suitable for the occurrence of
nitrification. In April 2014,
The
During the period of strong stratification, the water column loses oxygen
below the thermocline, which apparently results in N loss. Along with a
decrease in nitrate, there also occurs an increase in NH
The values of nitrogen (
Our data from the Tillari Reservoir indicates the occurrence of
denitrification in the suboxic hypolimnion under stratified conditions.
However, this process is restricted to a narrow depth range of 10–20 m,
which limits the number of data points. There may be several factors
responsible for the low (
Assuming the N loss was largely through denitrification, an attempt was made to compute the fractionation factor using a Rayleigh “closed-system” model (Lehmann et al., 2003). Although there have been several attempts to compute the nitrogen isotope enrichment factors in marine systems, groundwater and laboratory cultures (Table 1), similar information is relatively scarce from freshwater lakes and reservoirs.
The available information on oxygen isotope fractionation is even scarcer.
The values of
Denitrification strongly discriminates among the two N isotopes, leaving
behind
The isotopic composition of ammonium should reflect that of the sedimentary
organic matter being degraded. In Lake Kinneret, Israel,
A negative linear relationship between
Schematic diagram depicting major biogeochemical processes taking place in the Tillari Reservoir over an annual cycle. This information is based on monthly sampling in the reservoir for several years (Shenoy et al., 2017).
As the summer intensified and oxidized nitrogen was fully utilized,
facultative bacteria apparently began to utilize sulfate as an electron
acceptor as indicated by the accumulation of H
Vertical profiles of NO
The reservoir gets vertically mixed during the months of July, August and
September due to a combination of lower atmospheric temperature, strong winds
and an inflow of relatively cold water during the SWM. Nitrate
concentrations are moderately high throughout the water column, although
variable from one year to another. The mean water-column nitrate
concentrations were 7.26
Thus, looking solely at the high nitrate concentrations in the water column, atmospheric wet deposition may be a major nitrate source to the water column during the monsoon season. However, this inference is based on a single measurement where the isotopic composition is also different. Moreover, the river water is also rainfed and it is not clear why its isotopic composition is much lower at the most upstream station. At the same time, the isotopic composition of POM indicates an influence of the upstream waters. Variable inputs from the atmosphere and by river runoff to the DIN pool probably account for the inter-annual variability, but more studies are needed to identify and quantify these contributions in detail.
Using stable isotopes of nitrate, ammonium and particulate organic matter,
we have been able to identify distinct water-column conditions and
transformation processes of reactive nitrogen in the Tillari Reservoir. The
reservoir gets vertically mixed during the SWM season as well
as in winter; the water column remained stratified during other parts of the
year. The most intense stratification occurs during summer just before the
monsoon onset. The relative importance of microbial processes, such as
nitrification, denitrification, ammonification and sulfate reduction in the
water column, varied depending on intensity of stratification and associated
DO levels in the hypolimnion. These processes produced unique isotopic
signatures in the dissolved and particulate matter. Our results suggest the
occurrence of microbial chemosynthesis using methane and ammonium as primary
C and N sources, producing organic matter in the anoxic bottom waters that
is highly depleted in
The authors declare that they have no conflict of interest.
We thank the Director, CSIR-NIO for providing necessary support for this work and the management body of the Tillari Reservoir for permission to carry out this study. This research was carried out as a part of INDIAS IDEA project funded by the Council of Scientific & Industrial Research (CSIR). The authors wish to thank Mark Altabet and Laura Bristow for sharing their expertise. We thank Sugata Hazra and the School of Oceanographic Studies, Jadavpur University for their support and encouragement. Puja Satardekar is acknowledged for analyzing the nutrient samples. Sujal Bandodkar (DTP section, CSIR-NIO) is thanked for her creative inputs. P. Bardhan thanks CSIR for the award of Senior Research Fellowship. The authors are also grateful to M. V. Maya for her initial assistance in isotopic analyses and to H. Dalvi, A. Methar, J. Lobo and S. Yanamandra for their help during field work. This is NIO Contribution no. 5978. Edited by: B. A. Pellerin Reviewed by: two anonymous referees