Summer Research Fellowship Programme of India's Science Academies

Impact of groundwater intrusion/submarine groundwater discharge on coastal phytoplankton

Aditi Sharma

MSc. Environmental Sciences, Jawaharlal Nehru University, New Delhi

Dr. VVSS Sarma

Senior Principal Scientist, NIO-RC, Visakhapatnam


The groundwater has high magnitude of nutrient concentration due to various natural as well as anthropogenic activities. This nutrient-rich groundwater gets exchanged with the sea water at the ocean-land interface, and it is called Submarine Groundwater Discharge, resulting in the increased nutrient level in the coastal water. For the marine phytoplankton, major limiting nutrients are nitrogen and phosphorus. Due to the Submarine Groundwater Discharge, the phytoplankton biomass increased due to input of abundant amount of nutrients causing them to flourish. The phytoplankton are the base of the marine food webs, so any change in the phytoplankton composition or abundance will have huge impacts on all the other dependent living organisms of coastal ecosystem. To understand the effect of groundwater intrusion on marine phytoplankton, several ground water samples were collected in and around Visakhapatnam city region to evaluate the spatial variability in nutrients levels in the ground water. The coastal waters samples were also collected, and some proportion of ground water was added from different regions and incubated for 5 days to examine the change in phytoplankton composition. The incubated samples for phytoplankton are being analyzed to examine the response of ground water addition to the coastal water. Nutrients (nitrate and phosphate) are measured following colorimetric technique using autoanalyzer.

Keywords: nitrogen, phosphorus, phytoplankton abundance, nutrient enrichment in coastal water, algal blooms


SGD Submarine Groundwater Discharge
HABs Harmful Algal Blooms
GW Groundwater
SW Seawater



The primary objective of the work was to understand the influence of nutrients input through submarine groundwater discharge on marine phytoplankton.

Submarine Groundwater Discharge

Submarine groundwater discharge (SGD) has been defined as “direct groundwater outflow across the ocean–land interface into the ocean” (Church, 1996). It is place at the ocean-land interface where the groundwater mixes with the seawater. The physical as well as chemical properties of sea water changes because of the discharge, which delivers the groundwater nutrients to the sea water. Since the groundwater is highly dependable natural resource, it is extremely polluted due to various anthropogenic activities. The concentrations of nutrients in the sea water and ground water are different; much higher in groundwater. The exchange of ground water with coastal water increases nutrients level in the coastal water and have significant impact on coastal phytoplankton abundance and composition. The excessive nutrient addition to coastal water may result in the formation of red tides or Harmful Algal Blooms (HABs), which have significant impact on the ecosystem dynamics. Nitrogen and phosphorus are the major limiting nutrients for marine organisms. For the balanced growth, marine phytoplankton requires nitrogen and phosphorus in a ratio of 16N:1P. The submarine groundwater discharge delivers plenty of the limiting nutrient to the marine organisms. This lessens the stress of limiting nutrient and results in proliferation of phytoplankton in the marine ecosystem. Thus, resulting the change in abundance and composition of the phytoplankton.

The phytoplankton provides the base of the food chain in the marine ecosystem. Any change in the phytoplankton abundance or composition will therefore affect the whole marine biodiversity. The SGD causes enormous growth of phytoplankton, sometimes resulting in eutrophication, which in turn causes anoxic condition and hence death of the marine organisms. The coastal population living along the coastline that highly depends on seafood as a source of their sustenance and income might be affected due to these changes. SGD may result in the formation of HABs which will have toxic effect on marine organisms and on the human beings consuming it.


Glassware/ plasticware

DO bottles (200ml), 20-liter Nalgene bottles, 10-liter Groundwater collection bottles, 1-liter storage bottle, funnel, zooplankton mesh (200 micron), phytoplankton mesh (10 micron), measuring cylinder (1 liter), suction tube, 1 ml pipette, Sedgwick-Rafter cell counting chamber (microscopic slide).

Chemicals: Lugol’s iodine solution

S.no. Components Formula Amount used
1. Potassium iodide KI 50 g
2. Glacial acetic acid CH3COOH 50 ml
3. Iodine I2 25 g
4. Distilled water H2O 500 ml

The procedure followed for Lugol’s iodine solution preparation is:

  • 500 ml distilled water was measured in a dark brown bottle.
  • 100 g KI was dissolved.
  • 50 g I2 was added and mixed thoroughly.
  • 100 l glacial acetic acid was added.
  • The bottle was stoppered and stored in dark.


Microscope (Olympus IX71) Phosphate (SYSTEA µMAC-1000) and Nitrate (EcoLAB Insitu) nutrient autoanalyzer pH analyzer (Titrando titrator)


  • Sample collection
  • Mixing and Incubation
  • 5-days collection
  • Sample analysis – phytoplankton abundance & nutrient analysis
  • Interpretation

1) Sample collection

i) Groundwater sample: The groundwater samples were collected from two different sites in the Visakhapatnam city.

Site Amount of sample collected Latitude Longitude
Gandhinagar street, Peddawaltair   10 liters   17.732° N   83.336° E
Palikiwar Street (near fish market)   10 liters 17.734° N 83.336° E

ii) Sea water sample: The sea water sample was collected from the Ramakrishna Beach situated along the East coast of Bay of Bengal. The sample was filtered through the zooplankton mesh (200 micron) and collected in the 20-liter Nalgene bottles. Total six bottles were collected in the same way from the location with coordinates 17.717589° N and 83.330416° E.

2) Mixing and incubation

To understand the effect of groundwater intrusion on sea water phytoplankton, a specific amount of groundwater sample is mixed with the sea water sample and incubated at room temperature in a semi-shady place. With the groundwater collected from two different locations, two sets of experiment were set up. In each set there were three samples:

  • Sea water sample (control)
  • Mixing sample in a ratio 19L: 1L and
  • Mixing sample in a ratio of 18L:2L

3) 5-days collection

The samples from each set were collected after every 24-hours of incubation. For phytoplankton abundance, 1 litre of each sample was collected and 2-3 drops of Lugol’s iodine solution were added to it to preserve it until examination. For nutrient analysis, 200 ml of each sample was collected and stored in refrigerator at 4° C until examination. The samples were collected every 24-hour for 5 days.

4) Sample analysis – nutrient analysis & phytoplankton abundance

The samples collected for 5 days were analyzed for nutrient concentration (nitrogen & phosphorus) and the phytoplankton abundance. The nutrient concentrations were measure by the colorimetric technique using the autoanalyzer. The nitrate concentrations in the samples were measured with the EcoLAB Insitu nutrient autoanalyzer. The phosphate concentrations were measured using SYSTEA µMAC-1000 autoanalyzer. For the phytoplankton abundance analysis, the following steps were followed: a) 1 litre sample that was collected was siphoned using a siphoning pipe and phytoplankton mesh of size 10 micron. b) The volume of the sample was reduced to 10 ml and was stored in polypropylene tubes. c) 1 ml of the siphoned sample was taken and spread over the gridded microscopic slide and observed under the microscope. d) The number of phytoplankton was counted in 10 squares from each side and the centre of the slide under the microscope at 40X resolution. e) The total number of phytoplankton for each sample were added up and used for further calculations. f) The phytoplankton count from each square corresponds to the number in 1 microliter of sample, since the 1 ml sample is distributed in 1000 squares on the gridded slide. So, counting 10 squares from each corner and the center of the slide will give the number of phytoplankton in 50 microliter of sample which can be extrapolated for the total amount of sample.


The data for nutrient concentration and phytoplankton abundance were compared for all set of samples and the results were interpreted accordingly.


The pH of the groundwater samples collected were analyzed using the Titrando titrator. The pH of the two samples collected from different locations are:

Site pH
Gandhinagar street, Peddawaltair 6.108
Palikiwar Street (near fish market) 6.334

The pH data indicate that the groundwater sample from both the locations are slightly acidic. There is no health-based guideline value for pH according to WHO, but pH lower than 6.5 can have corrosive effect and can leach metals.


All the samples in both sets (set A and set B) were analyzed for nutrient concentration using the autoanalyzer. The detection is based on the spectrophotometry. The absorbance of standards and the samples are measured based on which the concentrations of the samples are calculated.

NITRATE: The nitrate concentration was measured using the EcoLAB Insitu Nutrient Auto analyser. The alternative days samples were chosen to be examined i.e., day 1, day 3 and day 5 sample. The concentration of nitrate in the samples of both sets A and B, calculated from the absorbance values are listed in the table below:

Nitrate concentration
Sample Day1 Conc. (µM) Day 3 Conc. (µM) Day 5 Conc. (µM)
GW 1031.3 1154.6 1124.3
SW 7.2 6.6 0.35
19:1 SW:GW 85.8 66.9 34.6
18:2 SW:GW 125.6 121.2 105.8

Graphical comparison of nitrate concentration in samples of set A:

    The concentrations of nitrate in the samples of set B:

    Nitrate concentration
    SET B
    Sample Day1 Conc. (µM) Day 3 Conc. (µM) Day 5 Conc. (µM)
    GW 491.0 483.9 495.4
    SW 8.0 3.6 1.29
    19:1 SW:GW 1021.1 716.0 370.2
    18:2 SW:GW 68.0 55.3 43.8

    Graphical comparison of nitrate concentration in samples of set B:

      PHOSPHATE: The phosphate concentration was measured using SYSTEA µMAC-1000 autoanalyzer. Similar to nitrate, the alternative days concentration was calculated for phosphate in the samples of both sets A and B, listed as in the table below:

      Phosphate concentration
      SET A
      Sample Day1 Conc. (µM) Day 3 Conc. (µM) Day 5 Conc. (µM)
      GW 1.37 1.32 1.42
      SW 1.12 0.84 0.35
      19:1 SW:GW 1.32 0.35 0.99
      18:2 SW:GW 1.74 1.24 0.97

      Graphical comparison of the phosphate concentration in samples of set A:

        The concentration of phosphate in samples of set B:

        Phosphate concentration
        SET B
        Sample Day1 Conc. (µM) Day 3 Conc. (µM) Day 5 Conc. (µM)
        GW 0.99 0.42 0.82
        SW 0.74 1.18 0.09
        19:1 SW:GW 1.42 1.32 1.38
        18:2 SW:GW 1.30 1.18 0.37

        Graphical comparison of the phosphate concentration in samples of set B:

          Phytoplankton Abundance

          The number of phytoplankton were enumerated in the sample by counting in 10 squares from each corner and the center of the slide. This will give the number of phytoplankton in 50 microliters of sample. The count was extrapolated to calculate the number in the total volume of the sample.

          The phytoplankton abundance in sample of set A:

          Phytoplankton abundance
              SET A  
            Day 1 Abundance Day 3 Abundance Day 5 Abundance
          Sample in 50µl 20 litres in 50µl 20 litres in 50µl 20 litres
          SW 2615 104.6*107 1588 63.5*107 2309 92.3*107
          19:1 SW:GW 3217 128.6*107 3529 141.1*107 3402 136.0*107
          18:2 SW:GW 2076 83.0*107 1367 54.6*107 2602 104.0*107

           The abundance of various samples was compared graphically:

            The phytoplankton abundance in samples of set B:

            Phytoplankton abundance
                SET B  
              Day 1 Abundance Day 3 Abundance Day 5 Abundance
            Sample in 50µl 20 litres in 50µl 20 litres in 50µl 20 litres
            SW 2753 110.1*107 3385 135.4*107 2454 98.1*107
            19:1 SW:GW 3413 136.5*107 2333 93.3*107 2841 113.6*107
            18:2 SW:GW 2276 91.0*107 2729 109.1*107 >10,000 >400*107

              The overall data of Set-A and Set-B can be compared from the tables given below:

              Overall data of set A
              Set- A
              Sample Nitrate Conc.(µM) Phosphate Conc.(µM) Abundance (in 50µl)
              Day1 Day 3 Day 5 Day1 Day 3 Day 5 Day1 Day 3 Day 5
              GW 1031.3 1154.6 1124.3 1.37 1.32 1.42 - - -
              SW 7.2 6.6 0.35 1.12 0.84 0.35 2615 1588 2309
              19:1 SW:GW 85.8 66.9 34.6 1.32 0.35 0.99 3217 3529 3402
              18:2 SW:GW 125.6 121.2 105.8 1.74 1.24 0.97 2076 1367 2602
              Overall data of set B
              Set- B
              Sample Nitrate Conc. (µM) Phosphate Conc.(µM) Abundance (in 50µl)
              Day1 Day 3 Day5 Day1 Day3 Day 5 Day1 Day3 Day 5
              GW 491.0 483.9 495.4 0.99 0.42 0.82 - - -
              SW 8.0 3.6 1.29 0.74 1.18 0.09 2753 3385 2454
              19:1 SW:GW 1021.1 716.0 370.2 1.42 1.32 1.38 3413 2333 2841
              18:2 SW:GW 68.0 55.3 43.8 1.30 1.18 0.37 2276 2729 >10,000


              To understand the effect of groundwater intrusion on the marine phytoplankton, we need to study the relationship between the nutrient concentration (NO3 & PO4) and the phytoplankton abundance. In both the experiments, significant increase in seawater nitrate concentrations were observed after addition of groundwater by an order of magnitude, which suggests that nitrate would not limit the phytoplankton abundance. In contrast, the phosphate was decreased in Set-A (19:1) experiment in mixed water sample and seems to be limiting the phytoplankton growth, whereas it was moderately increased in Set-A (18:2) and Set-B (both mixings). As a result, increase in phytoplankton abundance of Set-B was 10% in 19:1 mixing proportion and 25% in 18:2 mixing proportion, whereas 40% increase was observed in Set-A (18:2) and no significant increase was observed in the Set-A (19:1). The variations in response of phytoplankton abundance may also be contributed by abundance of grazers in each set. The variation may be caused by micro zooplankton grazing; therefore, the net impact should be considered. The mesozooplankton do not have any significant effect on phytoplankton abundance in this experiment as they were already removed from the sample water before the incubation.


              The groundwater contains high concentration of nutrients, especially nitrate. The nutrient concentration increases even more when the groundwater gets polluted due to the anthropogenic activities. When this nutrient-rich groundwater is exchanged with the coastal water, it delivers the nutrients. Our results suggest that mixing of ground water with coastal seawater results in significant increase in nitrate, however the increase in phosphate was rather minimal. Therefore, the mixing experiments conducted in this study were mainly controlled by availability of phosphate rather than nitrate. Increase in the phytoplankton abundance was observed in 3 out of 4 experiments by 10–400%. This change must be considered as a net increase, as some of the phytoplankton might have been removed by the micro zooplankton grazing. These experimental results can be further used for comprehensive studies by measuring the phytoplankton composition, growth rate, grazing rates of micro zooplankton to examine the gross impact on phytoplankton composition and the possible changes in diversity of phytoplankton due to mixing of ground water with coastal seawater.


              I would like to thank the Indian Academy of Science (IAS), Bangalore for selecting me as a summer research fellow for the SRFP-2019 and providing me the wonderful opportunity. I take immense pride to have worked under the supervision of Dr. VVSS Sarma, Senior Principal Scientist, National Institute of Oceanography-Regional Centre (NIO-RC), Visakhapatnam. He has been extremely kind in accepting me to work under him and giving me the chance to get a great learning experience and exposure. He has given me his valuable time in guiding me throughout the project. He has been patient in teaching me and correcting my mistakes whenever and wherever needed. I am thankful to Dr. Jagadeesan Loganathan, Scientist at NIO-RC for his support and guidance while handling the microscope. I am also thankful to Mr. Arindam Basu, intern at NIO-RC for providing the nutrient concentration data of the groundwater samples and for all the support. Lastly, I would like to thank my parents and brothers for always being very supportive and encouraging towards my studies.



              2. Boesch, Donald F. et al. 1996. Harmful Algal Blooms in Coastal Waters: Options for Prevention, Control and Mitigation. NOAA Coastal Ocean Program Decision Analysis Series No.10. NOAA Coastal Ocean Office, Silver Spring, MD. 46 pp. + appendix.

              Written, reviewed, revised, proofed and published with