Phytoplankton, Nethravati estuary, salinity, chlorophyll, plankton indices.

Estuaries are transition zones linking freshwater and marine systems, and are therefore characterized by gradients of chemical, physical and biological components in the water column. These gradients strongly influence the spatial and temporal distribution and abundance of phytoplankton in estuaries. Estuaries in India are influenced by monsoonal rainfall, hence they are called monsoonal estuaries (Sarma et al., 2014; Ranjith et al., 2017), where clarity of the water plays a major role than nutrients in deciding plankton productivity. They receive land driven nutrients through rivers and these nutrients support rich phytoplankton production in estuaries (Neill, 2005; Chalapathi et al., 2018). The magnitude of river discharge controls the water column stability, residence time, salinity and nutrient composition. Due to these changes, river discharge not only affect phytoplankton biomass but also functional groups of phytoplankton community. The species composition, biomass, relative abundance and their spatial and temporal distribution of this aquatic biota are an expression of the environmental health or biological integrity of a particular water body (Ekwu and Sikoki 2006; Dattatreya et al., 2018). Phytoplankton biomass and community composition are highly dynamic at the land-sea interface where diverse human actions and climate variability intersect to drive complex patterns of change over time (Cloern and Jassby 2008), thereby serve as a key element in assessing the ecological quality status in these transitional waters.

The present investigation was carried out in Nethravati limb of Nethravathi-Gurupur estuary, which is formed by the confluence of the Netravathi and Gurupur river, located in coastal urbanized city Mangalore, India.

Composite samples were collected from surface waters at monthly intervals for a period of 16 months from the upper stretch of Nethravati estuary at Adyar, covering pre-monsoon, monsoon and post-monsoon seasons to analyze salinity and chlorophyll-a content of water. Salinity of water was estimated in the laboratory by following Mohr’s method (Strickland and Parsons 1972) and the results are expressed in psu.

Water samples collected for the estimation of chlorophyll-a were filtered through 198 μm nylon bolting silk net to remove the grazers. Then a known volume (1000 mL) was filtered immediately through a Millipore membrane filter of 47 mm diameter, having a pore size of 0.45μm by adding two drops of magnesium carbonate suspension during filtration. Particulate matter on the filter paper was extracted with 10 mL of 90% v/v acetone under dark, at low temperatures by keeping over night with periodic shaking. Then the extract was centrifuged for 20 minutes at 2000 rpm. The supernatant was decanted into 1cm path length cuvette, to measure the extinction at different wave lengths i.e., 630, 647, 664 and 750 nm against an acetone blank. Chlorophyll-a concentration was then calculated by using the equation, recommended by Parsons et al. (1989) and the values are expressed in terms of μg/L. The absorbance was measured colorimetrically using Spectrophotometer (Systronics UV-VIS Spectrophotometer 119). 

Standard Plankton net was used to collect plankton samples. In the laboratory, the plankton samples were again filtered through a 198 μm nylon bolting silk cloth to remove the zooplankton trapped, if any. The filtrate along with the phytoplankton was made up to a known volume (100 mL) and was preserved in Lugol’s solution. The ‘net phytoplankton’ (includes phytoplankton retained after filtration i.e., in the size range of 60 μm - 198 μm) present in quadruple aliquots of 1mL from a subsample (25% of total sample) was analyzed both qualitatively, based on morphology following standard keys (Davis, 1955; Bellinger and Sigee, 2010) and quantitatively using Sedgwick Rafter cell and plankton abundance was expressed in number/m3. OLYMPUS - CKX41 (Inverted microscope) and OLYMPUS - CX 21 microscopes were used in the qualitative and quantitative analysis of phytoplankton. Alpha diversity indices of plankton were estimated using Primer Software. 

Salinity: In estuaries where salinity often variable due to tides and river runoff, salinity is one of the key parameters along with nutrients for regulation of phytoplankton abundance and species assemblages. 

In the present investigation, majority of the period during monsoon and early post-monsoon seasons, study area witnessed strictly limnetic (<0.5 psu) conditions, indicating almost total flushing of the estuarine water during this period, thereby making the estuary, nearly a freshwater system.  Salinity showed a wide variation (25.58 psu) ranging from 0.04 psu (in the monsoon) to 25.62 psu (in the pre-monsoon). Similar type of observations was made by Tripathi (2002) in the waters of Nethravati-Gurupur estuary (1.75 to 32.25 psu), Kumary et al. (2007) in Adimalathura estuarine waters (0.007 to 36.27 psu), Martin et al. (2008) from Cochin estuary (‘0’ to 30 psu), Kumar et al. (2009) in Tapi estuarine waters (0.11 to 32 psu). In the present investigation, salinity acted as a significant explanatory variable for the observed temporal variability in plankton abundance.

Temporal variations in the salinity of water are presented in Fig. 1.

Observations 1 to 4 represents post-monsoon, 5 to 8 represents pre-monsoon, 9 to 12 represents monsoon and 13 to 16 represents post-monsoon seasons respectively.

Fig. 1.  Temporal variations in the salinity of water.

Chlorophyll-a: Chlorophyll-a is an important biochemical component in the molecular apparatus of microalgae that is responsible for photosynthesis. It serves a dual role in oxygenic photosynthesis, i.e., in light harvesting as well as in converting energy of absorbed photons to chemical energy. This photosynthetic pigment is present in all species of phytoplankton, including eukaryothic (algae) and prokaryotic organisms (cyanobacteria) and thus it is a reliable and commonly used proxy for total phytoplankton biomass, thereby gives a clue about the trophic status of the water body concerned. 

Temporal variations in the Chlorophyll-a content were presented in Fig. 2. In the present study it fluctuated between 2.95µg/L and 6.86µg/L, thereby indicated mesotrophic nature, in accordance with Mineeva (2000). Some earlier works that are in line with the present study includes, Chlorophyll-a content of Sunderban mangrove estuary, Bangladesh, is 3.09µg/L (Rahaman et al., 2013); and that of Godavari mangrove estuary, India, is 12.49µg/L (Tripathy et al., 2005). These observed spatio-temporal variations could be conveniently ascribed to a combination of varying light availability and grazing pressure.

Observations 1 to 4 represents post-monsoon, 5 to 8 represents pre-monsoon, 9 to 12 represents monsoon and 13 to 16 represents post-monsoon seasons respectively.

Fig. 2.  Temporal variations in chlo-a content of water.

The temporal variations in Chlorophyll-a content, coincided by and large, with the respective trend in plankton abundance. The relationship between chlorophyll-a and total plankton count is represented through Fig. 3.

Fig. 3. Linear regression between chlorophyll a and total plankton count (as No.*104).

Phytoplankton community structure. Communities are recognized as recurrent organized systems of organisms responding to changes in the environment and any disturbance in the environment produces changes in many growth variables of an individual, which ultimately leads the community to become reorganized (Smayda, 1963). Phytoplankton community composition is important in establishing ecosystem structure and function. Of the many abiotic factors that influence the growth of phytoplankton community of an estuarine ecosystem, the monsoonal runoff and tidal activity being the major contributors (Cloern, 1996; Sin et al., 1999). Higher phytoplankton diversity was observed in the south Indian estuaries than north Indian estuaries due to the long water residence time in the former (Bharathi and Sarma 2019). In the present study, 65 phytoplankton genera were contributed to community structure, and the generic diversity was in the order of Chrysophyta (24) > Chlorophyta (22) > Cyanophyta (18) > Rhodophyta (1) Pyrophyta (0). Phytoplankton biomass in estuaries is controlled by complex biological and chemical processes that control growth and mortality, and physical processes that control transport and dilution. Phytoplankton abundance was in the order of Cyanophyta > Chrysophyta > Chlorophta > Rhodophyta, with complete absence of Pyrophyta. 

Top ten phytoplankton genera (based on regularity & dominance) found at this station are Merismopedia, Hydrodictyon, Aphanizomenon, Coscinodiscus, Desmidium, Spirogyra, Melosira, Pediastrum, Tabellaria and Gomphosphaeria spp. Among the observed plankton during the present investigation, Coscinodiscus, Tabellaria spp. were present continuously throughout the study period. 

In the present study, Centrales were represented by regular/dominant forms like Campylodiscus (0 to 48000 cells/m3), Coscinodiscus (6000 to 1980000 cells/m3), Melosira (0 to 1430000 cells/m3), and by rare forms like Chaetoceros, Cyclotella, Triceratium spp. While, Pennales were represented by the regular /dominant forms like Bacillaria (0 to 180000 cells/m3), Gyrosigma (0-44000 cells/m3), Nitzschia (0-12000 cells/m3), Pleurosigma (0 to 18000 cells/m3), Tabellaria (4000 to 290000 cells/m3), and rare forms like Asterionella, Fragilaria, Gomphonema, Navicula, Pinnularia, Surirella, Synedra, Thalassionema and Thalassiothrix spp. Cyanophyta was represented by the regular/dominant forms like Aphanizomenon (0 to 1280000 cells/m3), Gomphosphaeria (0 to 576000 cells/m3), Lyngbya (0 to 24000 cells/m3), Merismopedia (0 to 7680000 cells/m3), Oscillatoria (0 to 14000 cells/m3), Phormidium (0 to 62000 cells/m3), Spirulina (0 to 16000 cells/m3), Stigonema (0 to 96000 cells/m3) and rare forms like Anabaena, Aphanocapsa, Aphanotheca, Coelosphaerium, Gleocapsa, Heterohormogonium, Marssoniella, Microcystis, Nostoc, Trichodesmium spp. Chlorophyta was represented by the regular/dominant forms like Basicladia (0 to 36000 cells/m3), Closterium (0 to 40000 cells/m3), Desmidium (0 to 1000000 cells/m3), Dichotomosiphon (0 to 24000 cells/m3), Hydrodictyon (0 to 2000000 cells/m3), Mougoetia (0 to 110000 cells/m3), Pediastrum (0 to 448000 cells/m3), Spirogyra (0 to 864000 cells/m3) and rare forms like Actinastrum, Cladophora, Kirchneriella, Microthamnion, Pandorina, Pleurotaenium, Prasinocladus, Scenedesmus, Sirogonium, Sphaerocystis, Spitotaenia, Stigeoclonium, Triploceros, Ulothrix and Zygnema spp. Rhodophyta was represented by single genera Lemanea spp (0 to 12000 cells/m3). 

Seasonal variations in the plankton community structure based on abundance was shown in Fig. 4. and temporal variations in abundance of plankton taxonomic groups were represented through Table 1, while, Indices worked out on the basis of plankton dynamics were presented in Table 2. 

With respect to relative abundance, present investigation revealed that, Cyanophytes overwhelmingly dominated (74.23%) its competent counterparts, i.e., Chlorophytes (19.93%) in organizing the plankton community structure during pre-monsoon. Predominance of blue–green algae during this period might be due to the existence of brackish water and stagnant water conditions due to less discharge as opined by Bharathi and Sarma (2019). With change in season from pre-monsoon to monsoon, with respect to cell density, a remarkable decrease in case of cyanophytes (from 74.23% to 56.62%) and increase in case of chlorophytes (from 19.93% to 37.38%) was noticed. Post-monsoon season witnessed a notable increase in chrysophyte abundance (18.58%), whose abundance was only less than 6% during the rest of the seasons. Rhodophytes contribution to total standing crop of plankton was very meagre, i.e., not even contributed to 0.5%, across the seasons. 

A stressed environment typically has low diversity and greater representatives from the existing taxa (Gao and Song 2005). As the study area witnessed strictly limnetic conditions (<0.5 ppt salinity) during monsoon and early post-monsoon seasons, thereby transition to ensuing pre-monsoon has created a stressful environment, hence early pre-monsoon witnessed a bloom of Coscinodiscus spp., while alone had contributed to 78.26% of total phytoplankton.

Fig. 4.  Seasonal variations in the plankton community structure.

Table 1: Temporal variability in the phytoplankton dynamics (Cells/m3) of water.

Observations 1 to 4 represents post-monsoon, 5 to 8 represents pre-monsoon, 9 to 12 represents monsoon and 13 to 16 represents post-monsoon seasons respectively.

Table 2: Temporal variations observed in the plankton indices of water (upper stretch of Nethravati estuary).

Observations 1 to 4 represents post-monsoon, 5 to 8 represents pre-monsoon, 9 to 12 represents monsoon and 13 to 16 represents post-monsoon seasons respectively.

K. Madhavi, A. Padmanabha,  N. Jesintha and  Adnan Amin  (2024). Phytoplankton Community Assemblage of Upper Stretches of Nethravati Estuary Dakshina Kannada, Karnataka. Biological Forum – An International Journal, 16(3): 276-280.