The marine environment is an intricate and interdependent ecosystem affected by a variety of physical, chemical, and biological processes. It boasts of an extensive array of flora and fauna, making it a crucial resource for supporting life [1,2]. In particular, the coastal region is highly dynamic owing to its relationship with the terrestrial zone, leading to variations in physicochemical parameters [3]. Sandy beaches, which serve as popular sites for tourism and recreation, provide crucial ecosystem functions including nutrient recycling, water quality regulation and maintenance of ecological sustainability. Water quality is a vital determinant of the resources available for sustaining life in marine ecosystems [4].

One significant player in the dynamics of the marine ecosystem is zooplankton, which acts as predators of primary producers, prey for higher trophic levels, and a link between the classical food web and the microbial food web [5,6]. The various physicochemical and biological processes influence the zooplankton productivity in marine environments. Extensive research has concentrated on the impact of temperature [7]. Salinity on the spatiotemporal variability of zooplankton [8-11]. However, limited research has been conducted on zooplankton population dynamics in Nagapattinam coastal waters [12]. The Nagapattinam coast is dominated by fishing activity. This area has been subjected to diverse anthropogenic pressures, including land discharge, habitat degradation with the subsequent decline of plant and animal populations, and the final diminishing of fish harvests. Many zooplankton population have broad distributions but show preferences for particular environmental conditions within their range. There is also a growing body of evidence that the production rate and distribution pattern of zooplankton may be significantly affected by ozone depletion and the consequent increase in ultraviolet (UV) radiation [13-17]. The merger of environmental sensitivity, short life cycles, and limited mobility makes zooplankton an excellent indicator of environmental change. Zooplankton are also an important mid-level component in the food web, serving as a tropical connection between small organic particles (e.g., detritus and microalgae) and fish that feed on plankton [18,19]. Understanding of the spatiotemporal variability of estuarine zooplankton composition and abundance essential for grasping ecosystem dynamics, as temporal changes in environmental conditions significantly affect the distribution of zooplankton species [20]. Even though the Nagapattinam coastal environment is ecologically, and socially important, only limited information is available about its hydrobiological characteristics. The present investigation pertains to the influence of physicochemical features on the distribution and abundance of zooplanktonic copepods along the Nagapattinam coast.

Description of the Study Area

The present study was carried out in the Bay of Bengal and adjacent Kaduvaiyar estuary on the Nagapattinam coast of Tamil Nadu, Southeast India. The freshwater influence due to tidal fluctuations and the influx of freshwater during monsoons because the Kaduvaiyar Estuary is debouching in the Bay of Bengal. The sampling sites are illustrated in Fig.1. Three sampling stations viz; ST-1 (10°45'53.98"N; 79°52'20.29"E), ST-2 (10°45'54.63"N; 79°51'42.32"E) and ST-3 (10°45'52.80"N; 79°51'7.49") were fixed and monthly samplings were carried out from February 2019 to January 2020.

Fig 1: Map Showing the Study Area.

Sampling and Analysis

In the current study, we conducted monthly sampling of seawater and zooplankton to monitor various meteorological and chemical factors. Meteorological factors include rainfall, atmospheric and surface-water temperatures, dissolved oxygen, pH, and inorganic nutrients present in the water. Standard mercury-filled centigrade thermometers were used to measure atmospheric and surface water temperatures. To estimate salinity, a Hand Refractometer (ERMA, Japan) was used, and pH was measured using a pH meter (Elico Grip). Dissolved oxygen was estimated using the modified Winkler method [21]. For nutrient analysis, surface water samples were collected in clean polyethene bottles, transported in an ice box to the laboratory, filtered using a Millipore filtering system (MFS), and analyzed for dissolved inorganic phosphate, nitrate, nitrite, reactive silicate, and ammonia, using the standard methods described by Strickland and Parsons (1972). The collected copepod (zooplankton) samples from the surface water by using the Indian Ocean Standard Plankton net with a mouth diameter of 0.35 m and mesh size of 158 μm. Regular monthly sampling was carried out by horizontal towing of the plankton net for 30 min, and the collected samples were preserved in 5% neutralized formalin for identification. The copepods were identified by referring to the standard keys of Kasturirangan (1963) and Perumal et al. (1998), and a quantitative estimate of copepods was carried out by filtering 500 l of water through a bag net of the same mesh size. The numerical analysis was performed using an inverted microscope (Micros, Austria). Finally, biodiversity indices were calculated using the following standard formulae: species diversity: H1 = -S Pi log Pi; I = 1; richness: D 132=1-C; C=SPi2; Pi=ni/N and evenness: J’=H’/log2S [22,23].

Statistical Analyses

Correlation coefficient analysis was performed to interpret the relationship between the copepod diversity and physicochemical factors.

The recorded rainfall ranged between 2.7 and 365.4 mm, with the lowest value of 2.7 mm in April and the highest of 365.4 mm in November (Fig.2). At the atmospheric temperature, the highest value was 33^oC °C. A maximum atmospheric temperature of 33^oC °C was recorded in March and June (at stations 1 and 2) and a minimum of 25^oC °C in December and January at station 3 (Fig. 2). A surface water maximum temperature of 29^oc was recorded in June and July at station 1, and a minimum of 24 ^oC was recorded in December and January at stations 2 and 1, respectively (Fig. 2). In the present investigation, a wide range of salinity variations was recorded among the three stations. At station 1, a maximum of 34 PSU was observed in May. At station 3, a minimum of 18 PSU was observed in October (Fig. 2). At all three stations, the hydrogen ion concentration (pH) remained alkaline throughout the year. The highest pH value was noticed in stations 1 and 8.24 during February, and the lowest of 7.9 in September at station 3 (Fig. 2). The dissolved oxygen concentration was maximum (5.48 mg/l) during May at station 1, and a minimum of 3.71 mg/l was recorded during October at station 2 (Fig. 2).

In the present study, inorganic nutrient concentrations were found to vary from 11.11 to 21.82 µmolL-1 at station1, the maximum nitrate (21.82) was recorded in November, and the minimum (11.11) in February, at station 1 (Fig. 2). The nitrite content was ranged between 0.54 and 2.61 µmol L-1. At station 2, the maximum value of 2.61 µmol L-1 was recorded in May and the minimum value (0.54 µmol L-1) in November at station 1 (Fig. 3). The phosphate content was noticed between 5.65 and 13.14 µmol L-1. At station 1, the maximum value (13.14 µmol L-1) was observed during September and the minimum phosphate value (5.65 µmol L-1) in April (Fig. 3). The silicate concentration varied from 29.49 to 35.10 µmol L-1. At Station 2, the maximum (35.10 µmol L-1) silicate content was found in June, and a minimum of 24.94 µmolL-1 was obtained in January at Station 3 (Fig. 3). The ammonia values varied from 0.60 to 1.53 µmol/L at station 1, with the maximum (1.53 µmol L-1) in August and the lowest value of ammonia 0.60 µmol L-1 was noticed in June at station 3 (Fig. 3). The chlorophyll ’a’ values were found to range between 0.18 and 0.59 mg L-1 (Fig. 3) at station 1, the maximum value (0.59 mg L-1) was recorded in May and the minimum value (0.18 mg L-1) was observed in October at station 3. During the one-year study period, a total of 38 species of copepods were recorded from Nagapattinam coastal waters, with calanoids being the most dominant order (in all stations), followed by cyclopoida and harpacticoida forms. The dominant copepod species identified were Pontella sp., Labidocera pavo, Centropages furcatus, Nannocalanus minor, Pseudodiaptomus aurivilli, Rhincalanus sp., Lucicutia flavicornis, Calanopia sp., Nitocra affinis, Thermocyclops inversus and Dioithona rigida.

The copepod population density ranged from 1492 to 14763 individuals. L-1. The maximum population density (14763 ind. L-1) (Fig.3) was observed in June at station 3, and the lowest population density (1492 ind. L-1.) was recorded in February at Station 1. Species diversity varied from 1.46 to 2.99 (Fig. 4). The maximum diversity (2.99) was observed in May at station 1, and the minimum (1.46) in January at station 3. Copepod species richness varied from 0.51 0.98 (Fig. 3). The maximum richness was observed in the month of 0.98 in May at station 3. The lowest species richness (0.51) was observed in January at Station 1. Species evenness varied from 0.82 to 0.98 respectively. Maximum evenness was observed during May, June, August, and September at station 3, but minimum evenness (0.82) was observed during November at station 1 (Fig 3).

For full article please go through this link: https://www.pubtexto.com/pdf/?spatiotemporal-changes-of-the-copepods-population-concerning-physicochemical-characteristics-at-nagapattinam-coast-southeast-india