A Principal Component Analysis ( PCA ) Approach to Seasonal and Zooplankton Diversity Relationships in Fishing Grounds of Mannar Gulf , India

Principal component analysis (PCA) is a technique used to emphasize variation and bring out strong patterns in a dataset. It is often used to make data easy to explore and visualize. The primary objective of the present study was to record information of zooplankton diversity in a systematic way and to study the variability and relationships among seasons prevailed in Gulf of Mannar. The PCA for the zooplankton seasonal diversity was investigated using the four seasonal datasets to understand the statistical significance among the four seasons. Two different principal components (PC) were segregated in all the seasons homogeneously. PCA analyses revealed that Temora turbinata is an opportunistic species and zooplankton diversity was significantly different from season to season and principally, the zooplankton abundance and its dynamics in Gulf of Mannar is structured by seasonal current patterns. The factor loadings of zooplankton for different seasons in Tiruchendur coastal water (GOM) is different compared with the Southwest coast of India; particularly, routine and opportunistic species were found within the positive and negative factors. The copepods Acrocalanus gracilis and Acartia erythrea were dominant in summer and Southwest monsoon due to the rainfall and freshwater discharge during the summer season; however, these species were replaced by Temora turbinata during Northeast monsoon season.


Introduction
Zooplankton inhabits oceans at all the depths and occupies every ecological niche that is considered as the chief index of utilizing aquatic biotope at the secondary trophic level (Pitchaikani and Lipton, 2015).Sea surface temperature, salinity and inorganic nutrients are some of the important factors that are reported to cause spatial changes among zooplankton population (Lawrence et al., 2004).Among the zooplankton groups, copepods are the important grazers of phytoplankton and micro zooplankton, and hence they form a major trophic link to many predatory invertebrates and fish (Atkinson, 1996).Studies related to seasonal variation and zooplankton diversity in offshore waters, particularly in fishing grounds of Gulf of Mannar, is meager; hence, the present study was established to investigate the relationship between seasons and zooplankton diversity.
Pelagic copepods are a key trophic group in the marine plankton, where they play important roles in both the transfer of energy from primary producers to higher trophic levels and biogeochemical cycles (Roemmich et al., 1995;Beaugrand et al., 2009).Principal component analysis (PCA) is a technique used to emphasize variation and bring out strong patterns in a dataset.It is often used to make data easy to explore and visualize.The primary objective of the present study was to record information of zooplankton diversity in a systematic way and to study the variability and relationships among seasons prevailed, which in turn should be useful for an understanding of highly variable aquatic environments in general, besides Gulf of Mannar.

Materials and Methods
The present investigations of principal component analyses (PCA) on zooplankton diversity was carried out and in the coastal waters off Tiruchendur, Southeast coast of Tamil Nadu (Gulf of Mannar), India.The study was carried out at three different fishing grounds (Fig. 1): Station-1 (Lat: 8°.27'.28.48"N,Long: 78°.8'.18.48"E), .06"E), , for a period of two years, extending from January 2009 to December 2010.The area located between the Southeast coast of India and West coast of Sri Lanka is a unique The correlation matrix loadings of the significant principal components for the four seasons are given in Tables 1 to 4.
For a few coastal areas, it has been suggested that protists occasionally constitute the main food source for calanoid and cyclopoid copepods (Kleppel et al., 1991;Pierce et al., 1992;Levinsen et al., 2000), which would have important implications for food web dynamics.In order to provide an insight into this idea, the present study was aimed to attempt statistically the feeding of small copepods on the full food size-spectrum (phytoplankton) and predation on juvenile appendicularians.

Principal component analyses (PCA)
In the post monsoon season, two different components of the principal component analyses explained 100% of total variance with eigene values exceeding 1 as could be boted from Table 1.The total variance observed in the study was directly linked with zooplankton forage predicting the routine (PC1) and opportunistic grazers (PC2).The extracted principal component 1 (PC1) explained 74.9% of the total variability and this could be used to differentiate the zooplankton groups present within the extracted fishing grounds.Earlier studies also emphasized similar findings in Gulf of Manine (Pershing et al., 2005).marine environment, and rich in biodiversity (Pitchaikani and Lipton, 2015).
The zooplankton diversity data used in the present study was obtained from the published data of the first author of this paper (Pitchaikani and Lipton, 2015).To examine the relationship among the four seasonal data sets, Principal Component Analyses (PCA) was performed on the zooplankton species diversity using program PAST ver.2.01 (Hammer, 2001).This program makes PCA of the correlation matrix of two sets of data (environmental data and zooplankton counts) and also calculates the correlations between the components of the two sets.

Results and Discussion
A total of 49 species of zooplankton have been recorded during the study.The PCA for the zooplankton seasonal diversity was investigated using the four seasonal datasets to understand the statistical significance among the four seasons viz.post monsoon (January-March), summer season (April-June), southwest monsoon (July-September) and northeast monsoon (October-December).Two different principal components (PC) were segregated in all the seasons homogeneously.The component matrix which is exceeding 0.6 could be taken for interpretation in the present study (Udayakumar et al., 2009;Jose et al., 2012).In the PC1 of the post monsoon, species such as Acartia erythrea, A. spinicauda, Paracalanus parvus, P. simplex, Calanopia elliptica, Temora turbinata, Oncaea venusta, T. stylifera, Calanopia minor, Euterpina acutiferons, Oithona rigida, O. spinirostris, Favella brevis, Tintinnopsis cynensis, Gastropod larvae, Gastropod veliger, Euphasid zoea, Porcellidium spp., Dotilla fenestrate and Nauplius of balanus were showed with positive significant correlation, which indicated their preference for sharing the same feeding habitat devoid of antagonism as suggested by Jose et al. (2012).However, the positive loadings in the PC1, most of the copepods species were herbivores, except that of Oithona rigida and O. spinirostris as similarly reported by Madupradap et al. (2003).Carnivore species also congregated in the same ground along with herbivores though they limit their feeding habit.This result therefore suggests that different feeding habituated zooplankton species gathered in the fishing grounds during the post monsoon season.
However, the PC1 components showed negative correlation with Acrocalanus gibber, A. gracilis, Eucalanus elongatus, E. subcrassus, Oncaea conifer, Clausocalanus arcuicornis, Metacalanus aurivillii, Oithona similis, O. nana, Tintinnopsis cylindrical, Sagita enflata, Bivalve veliger, Portunidae spp.and Globigerina spp.It was thus clearly noticed that, the significant positive loading species of copepods, which were also dominant exerted grazing pressure, which in turn depressed the remaining copepods by the way of squeezing them away from their feeding grounds (Jose et al., 2012).However, during the post monsoon season, most of the copepods remain there and only a few opportunistic species invade the fishing grounds with respect to the seasonal current pattern, as well as grazing pressure.In the Arabian Sea, upwelling and down welling are the prime factors that control the invading species along with grazing pressure.But in the case of Gulf of Mannar, seasonal current patterns are playing a major role on zooplankton species abundance (Jagadeesan et al., 2013).
The results of the present study clearly demonstrated the following species as opportunistic invaders: Acartia erythrea, Paracalanus simplex, Temora turbinate, Clausocalanus arcuicornis, Metacalanus aurivillii, Calanopia minor, Euchaeta marina, Euterpina acutiferons, Oithona nana, O. spinirostris, Crab zoea, Globigerina spp.These opportunistic invaders could have possibly entered from northern part of the Gulf of Mannar and Palk Bay due to the monsoon induced water circulation pattern.From the PCA analyses, results reinforced the idea that the zooplanktonic community of Gulf of Mannar could be strongly influenced by hydrological circulation patterns.Similar inference could be noted from earlier studies in Gulf of Mannar and in Gulf of Maine (Pershing et al., 2005).The results of the present study thus agreed with findings in other regions and explained that copepods constituted the major taxa in Catalan Sea (Jyothibabu et al., 2013), in Arabian Sea (Madhupratap et al., 2001;Jose et al., 2012), and in Mondego estuary (Marques et al., 2007).
PC2 represented the following species viz.Eucalanus elongatus, Temora turbinata, Metacalanus sp., Temora discaudata, Corycaeus sp., Corycaeus sp., Lucifer sp., Portunidae spp., with positive significant correlation.Most of the species were herbivores and preferred to grazing phytoplankton.Thus the PCA analyses tend to conclude that Portunidae spp. was the only opportunistic species available during the post monsoon season.This may be due to the coastal water circulation pattern in Gulf of Mannar.Increased abundance of both herbivores and carnivores in the fishing grounds could thus lead to sequential community development, subsequent to the start of nutrient supply either by current pattern or upwelling as recorded by Madhupratap and Haridas (1990).
Similar to the post monsoon season, the herbivores and carnivores zooplankton species were abundantly observed in PC1 during summer season.Among the recorded zooplankton, the herbivores dominated in terms of biomass, which suggested that the richest areas of zooplankton production in turn are associated with areas of high primary productivity (Padmakumar et al., 2010).In PC1, the following species identified as opportunistic species, invaded the fishing grounds based on the seasonal current pattern and grazing pressure: Acartia erythrea, Paracalanus simplex, Oncaea venusta, Temora turbinata, T. Euterpina acutiferons, Crab zoea, Portunidae spp., Globigerina spp., Mysis larvae and Cypris of balanus.While comparing post monsoon, more opportunistic species have invaded the fishing area in the summer season, due to the favorable environmental conditions such as salinity, sea surface temperature and coastal current pattern along the western boundary of Gulf of Mannar.Most of the studies on zooplankton in the Indian coastal waters with respect to the physical oceanographic factors mostly limited to the upwelling and eddies (Madhupratap, 1987;Jagadeesan et al., 2013).
In summer season, PC2 represented the following species viz.Acrocalanus Acrocalanus gracilis, Eucalanus elongatus, E. subcrassus, Temora turbinate, Clausocalanus arcuicornis, Metacalanus aurivillii, Temora discaudata, Oithona nana, Corycaeus sp., Sagita enflata, Lucifer sp., Gastropod Bivalve veliger, Dotilla fenestrata and Globigerina spp., with significant positive correlation.Among these positive groups, the following species identified as opportunistic species: Temora turbinata, Clausocalanus arcuicornis, Metacalanus aurivillii, Oithona nana, Globigerina spp., while the rest of the species could be grouped as routine ones, being uniformly abundant throughout the year.From the factor analyses, it was derived that the grazing power of zooplankton on phytoplankton was in balanced condition (Verity and Smetacek, 1996) and development of algal bloom could be controlled by zooplankton.Since the grazing power of zooplankton over phytoplankton was in a balanced state, there was no algal bloom in the study area.
The second component of PC2 represented the routine and opportunistic groups.The routine group included species such as Calanopia minor, Oithona brevicornis, O. rigida, Euphasid zoea and Cypris of balanus.The grazing zooplanktons of opportunistic invaded species including herbivores and carnivores viz.Tintinnopsis cynensis, Labidocera acuta, Portunidae spp.and Mysis larvae.
In the southwest monsoon also, two different principal components PC1 and PC2 (Table 3) were extracted consisting 74.3% and 25.6% variability with eigene values exceeding 1.The first component showed strong positive loading on all Acartia erythrea, A. spinicauda, Paracalanus parvus, P. simplex, Copepodite labidoceropavo, Oncaea venusta, Temora stylifera, Calanopia minor, Euterpina acutiferons, Oithona nana, O. spinirostris, Favella brevis, Tintinnopsis cylindrical, Gastropod larvae, Gastropod veliger, had large proportional changes in their seasonal cycle.A similar trend was explained to occur along the Gulf of Maine (Pershing et al., 2005).During the southwest monsoon season, phytoplankton population density had decreased gradually evidently due to the least availability of a few phytoplankton species: many of the zooplankton species which foraged in the food-rich surface layers would have possibly migrated to deeper waters form a direct source of flux of organic matter into the ocean interiors (Madhupratap and Parulekar, 1993).The PC2 showed significant positive correlation among copepods assemblage such as A.

Table 1 .
Eigene analyses of the correlation matrix loadings of the significant principal components in post-monsoon season

Table 3 .
Eigene analyses of the correlation matrix loadings of the significant principal components in SW monsoon Crab zoea, Porcellidium spp.and Dotilla fenestrate.Most of the species were herbivores except Oithona nana, O. spinirostris and Porcellidium spp.Similar to the post Porcellidium spp.and Dotilla fenestrate.The opportunistic group included Paracalanus simplex, Calanopia minor, O. nana and O. spinirostris.The opportunistic zooplankton observed during the southwest monsoon was similar with post monsoon species.Oithona was ubiquitous in the Gulf of Mannar, however, their large biological differences, in the community shift (eg.O. nana) Tintinnopsis cylindrical, Lucifer sp., Bivalve veliger and Nauplius of Balanus.Similar to the post monsoon season and summer season, herbivores and carnivores species were recorded in southwest monsoon as well.However, there was a sharp declining in Leucosiidae zoea, Porcellidium spp., Portunidae spp., Calanopia elliptica, Oithona nana, Calanopia elliptica and Sagita enflata in the fishing grounds.Particularly, temporal and spatial variation in Lucifer sp.abundance was noticed during the study.During the first year of study, density of Lucifer sp.showed slight declining trend in southwest monsoon, and Lucifer sp. was not observed at station 3, in the second year of the study.Jacob et al. (1981) reported sharp declining trend of Lucifer sp.during southwest monsoon season in Vizhinjam coastal waters.In the PC2, the following strong negative correlation were noticed viz.Metacalanus sp., Calanopia minor, Euchaeta marina, Oithona brevicornis, Corycaeus danae, Favella brevis, Tintinnopsis cynensis, Labidocera acuta, Leucosiidae zoea, Portunidae spp., Dotilla fenestrate, Mysis larvae and Cypris of Balanus.

Table 4 .
Eigene analyses of the correlation matrix loadings of the significant principal components in NE monsoon