The study region experiences distinct physico-chemical conditions based on the seasons. To emphasize the seasonal variations, the results have been clubbed into seasons covering the months of February to May (pre-monsoon), June to September (southwest monsoon; SWM) and October to January (post-monsoon) and arithmetic means of the observations have been calculated for each season. For each season, data is presented by calculating the arithmetic means of the observations. Also, the four sampling depths have been averaged to get a clear picture of all the observations except copepods samples (a single collection of the entire water column).
Physicochemical properties of the water column
Characteristically, the highest water temperatures were recorded during the pre-monsoon season (Fig. 2). The surface temperature range (28.6–29.4 ⁰C) showed little inter-annual variability. The pigment concentrations were low, chlorophyll a and phaeopigment ranged between 1.61 and 1.84 µg/L and 0.25 and 0.91 µg/L, respectively. While mean chlorophyll concentrations were similar in both years (2012 and 2013), the measured phaeopigment levels were twice as high in 2012 (0.91 µg/L) as compared to 2013 (0.43 µg/L). The highest chlorophyll a concentration (3.81 µg/L) during this season was observed in March 2013.
During the southwest monsoon, the coastal region along the west of India receives heavy rainfall also runoff from the numerous mountainous rivers draining into a coastal region (Maya et al. 2011). Coastal upwelling events also introduce colder, more saline waters in this region. A combination of all these factors resulted in water temperatures being lower by 3–6 ⁰C from pre-monsoon values (Fig. 2). Mean temperature in 2012 was nearly 4⁰C lower than that in 2013. Both chlorophyll a (1.80–4.29µg/L) and phaeopigment (2.20–2.31 µg/L) concentrations were the highest during this season. Inter-annual variability was also observed; while mean chlorophyll a in 2012 was higher than that in 2013, the reverse trend was observed in case of phaeopigments (see Fig. 2).
As the monsoon recedes, the water temperature increased again in post-monsoon and ranged from 26.50–28.30 ⁰C (Fig. 2). The pigment concentrations were moderate, chlorophyll a in the range 1.56–2.36 µg/L and phaeopigment 0.37–1.36 µg/L. The lowest chlorophyll a of this season was measured in 2012.
Seasonal variability of copepods
Seasonal as well as inter-annual variations were observed in the percent abundance of Subeucalanus spp., Acartia spp., Temora spp., Calanoida and Poecilostomatoida (Fig. 3). Oncaeidae followed by Paracalanidae contributed the most to the total copepod abundance. The seasonal variations of in the abundance of four copepod orders in the study region have been described by D’souza and Gauns (2018). Briefly, copepods dominated the mesozooplankton abundance by > 80% and seasonally, low and high copepod abundance was observed during monsoon and post-monsoon, respectively.
During pre-monsoon, the highest percent contribution by Subeucalanus spp. and Oncaeidae was noticed. Precisely, highest abundance of Subeucalanus spp. (33280 individual/100m3), Oncaeidae (178347 individual/100m3) and Paracalanidae (52907 individual/100m3) were observed during March 2013 that corresponds to the sudden increase in the chlorophyll concentration in the ambient water. Although, highest abundance of Paracalanidae was noticed in pre-monsoon, the highest percent contribution was observed during monsoon 2013. Conspicuously, the highest abundance of Acartia spp. was observed during August 2012 (4267 individual/100m3). The herbivore, Temora spp., showed its highest percent abundance during post-monsoon. Precisely, the highest abundance was observed during December 2012 (12800 individual/100m3). On the other hand, the copepod, Acartia spp., showed highest percent abundance during the post-monsoon 2013.
Seasonal variability in elemental composition (POC, PON and C/N) of SPOM and copepods
During pre-monsoon, large inter-annual variability was observed in POC and PON contents of SPOM with values in 2013 (POC = 44.23 µM, PON = 6.08 µM) more than twice of those in 2012 (POC = 16.75 µM, PON = 2.73 µM). In this season, copepods revealed species-specific variability in the carbon and nitrogen contents that apprehended for Paracalanidae (C = 5.47–6.04 µM; N = 1.04–1.32 µM), Oncaeidae (C = 2.40–2.64 µM; N = 0.49–0.53 µM), Subeucalanus spp. (C = 9.05–12.72 µM; N = 1.65–2.94 µM), Acartia spp. (C = 1.55–6.13 µM; N = 0.34–1.44 µM) and Temora spp. (C = 2.99 µM; N = 0.71 µM). The mean SPOM–C/N ratio in 2012 and 2013 were 6.05 and 7.27, respectively (Fig. 4). Similarly, Copepod-C/N ratios varied on a narrow range between 4.56 and 5.28 during 2012 and from 4.23 to 5.65 in 2013.
The POC and PON values of SPOM increased and C/N ratio decreased during SWM. Overall, the highest carbon (20.78 µM) and nitrogen (4.06 µM) concentrations were attained in SWM by Subeucalanus spp. and the lowest in Poecilostomatoida (C = 2.82 µM; N = 0.51 µM). The C/N ratio for copepod species fluctuated from 4.12 (in Acartia spp. in 2013) to 5.78 (in Calanoida in 2012).
The POC (2.62 − 9.88 µM) and PON (0.50–1.80 µM) of SPOM loading were considerably reduced during post-monsoon and exhibited identical trends. Low values were observed in Poecilostomatoida (2012) and high values in Subeucalanus spp. (2013). The C/N ratio of SPOM varied from 7.23 to 8.00 and copepods–C/N ratio fluctuated from 4.5 to 6.2 (Fig. 4).
Overall, C/N ratios of copepods were consistent in pre-monsoon, monsoon and post-monsoon unlike C/N ratios of SPOM that showed seasonal variability. However, inter-annual variability in the enrichment of C and N contents for different copepods was noticed. Moreover, it is noteworthy that Poecilostomatoida attained least nitrogen content in all seasons.
Seasonal variability of isotopic composition (δ13C and δ15N) of SPOM and copepods
As expected the copepods were always isotopically more enriched than SPOM throughout the study period along with notable seasonal variations. During pre-monsoon, δ13C–SPOM ranged between − 21.23 and − 19.48‰ where as δ15N–SPOM between 1.55 and 4.28‰ (Figs. 5A and 5D). The δ13C–copepods varied between − 20.35 and − 18.10‰ with the more depleted values observed in Oncaeidae (2013) and more enriched in Temora spp. (2013). Concurrently, the δ15N–copepods varied from 6.94 to 9.91‰; the extreme values (low & high) were respectively perceived in Acartia spp. (2011) and Subeucalanus spp. (2013). A poor correlation (r2 = 0.29) was observed between δ13C and δ15N of copepods (Fig. 6A).
While δ13C of SPOM was more enriched during monsoon as compared to the preceding pre-monsoon, the δ15N values further declined during this time (Figs. 5B and 5E). Likewise, the δ13C of copepods during this period (− 20.24 to − 17.72‰) was also slightly enriched when compared to pre-monsoon, the minimum value observed in Poecilostomatoida (2011) and maximum in Calanoida (2012) (Fig. 5B). The distribution of δ15N in copepods showed species-specific variations with the low and high values observed in Poecilostomatoida (6.12‰ in 2011) and Acartia spp. (9.1‰ in 2012), respectively (Fig. 5E). statistically, a poor correlation (r2 = 0.06) was detected between δ13C and δ15N of copepods (Fig. 6B).
During post-monsoon, the δ13C-SPOM showed minor depletion (− 22.38 to − 21.24‰) while the δ15N–SPOM was comparatively more enriched (4.17–6.07‰) (Fig. 5C and 5F). The δ13C–copepods (− 20.60 to − 16.59‰) showed more enriched values as compared to other seasons, the minimum value observed in Poecilostomatoida (2012) and maximum in Temora spp. (2011; Fig. 5C). The δ15N-copepods varied from 3.76 to 9.78‰ and the low and high values were observed in Poecilostomatoida and Subeucalanus spp. during November and December of 2012, respectively (Fig. 5F). Statistically, in copepods a significant correlation (r2 = 0.49) was witnessed in δ13C and δ15N (Fig. 6C).