Distribution and abundance of dinoflagellates from the coastal waters of Karachi , Pakistan , northern part of the Arabian Sea

This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract The present study reports on seasonal and spatial variations in diversity, distribution and abundance of dinoflegellates and indicates the presence of HAB species in Pakistan waters. A total of 179 taxa, recorded in this study from offshore and near-shore waters, belong to 41 genera in 26 families and 10 orders. The high species count (149 species) was recorded from Manora Island offshore station (MI-1) and 105 spp, 109 spp and 115 spp were encountered from the Mubarak village offshore station (MV-1), Manora near shore station (MI-2) and Mubarak Village near-shore station (MV-2) respectively. Tripos furca was the dominant and frequently occurring species (> 1 x103 to > 25 x103 cells L-1 from coastal and >1x 105 cells L-l from near-shore stations) in addition to less abundant Alexandrium catenella, Alexandrium sp., Alexandrium minutum, and Prorocentrum micans (>103 to 25x 103cells/L). Another 44 species occurred in relatively low numbers (<103 cell L-l). Seventy species were found throughout the study period at all four stations. High number of species in three genera (Tripos (38), Protoperidinium (34) and Prorocentrum (20) was recorded. Potently toxic (16 genera 43 species) and HAB related (19 genera and 30 species) dinoflagellate taxa were also recorded. The percent contribution of dinoflagellates in total phytoplankton population generally remained below 20% except for a few instances. Manora Island stations had comparatively higher Shannon index and equitability and slightly lower dominance index. The PCA plot showed strong positive correlation among chlorophyll-a concentration, dissolved oxygen, total number of phytoplankton and dinoflagellates.


Distribution and abundance of dinoflagellates from the coastal waters of Karachi, Pakistan, northern part of the Arabian Sea
Distribuição e abundância de dinoflagelados nas águas costeiras de Karachi, Paquistão, parte norte do Mar da Arábia

Results
Occurrence and distribution of dinoflagellate species in the coastal and near-shore waters of Pakistan is shown in Table 1. A total 179 taxa and 154 species belonging to 41 genera were recorded. Offshore waters had 105 (MV-1) to 149 (MI-1) species in 27 (MV-1) and 36 (MI-1) genera, respectively. On the other hand, near-shore waters had representation of 109 species at MI-2 to 115 species at MV-2 representing 30 (MI-2) and 29 (MV-2) genera, respectively. Variable numbers of species were recorded in 41 genera of dinoflagellates (Table 2). Three genera (Tripos (38 species), Protoperidinium (34 species) and Prorocentrum (20 species)) had high species counts. In addition, 5-9 spp. were recorded in 4 genera; 3-4 species in six genera; 2 spp. in 10 genera and only one species in 18 genera. About
A number of potential blooms forming species have been reported from coastal waters of Pakistan in the northern Arabian Sea (Saifullah and Chaghtai, 1990;Baig et al., 2006). In general, reports on phytoplankton from Pakistan are scarce and only a few studies are (Latif et al., 2013;Khokhar et al., 2018Khokhar et al., , 2020 and the remaining contributions are restricted to taxonomy and coastal waters (Munir et al., 2011(Munir et al., , 2013(Munir et al., , 2015Mansoor and Saifullah, 1995). The few reports on dinoflagellates were available on growth rate and seasonal abundance (Munir et al., 2016;Khokhar et al., 2018).
The present study reports on seasonal and spatial variations in diversity, distribution and abundance of dinoflegellates occurring in the coastal and near-shore off Karachi. The presence of HAB species in coastal and near-shore waters, where most of the artisanal fishery operations are located, is alarming for fisheries industry as well as human and environmental health.

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Dinoflagellates distribution and diversity along Karachi coast 39% of the total species (70 species) were distributed at all stations (Table 2). Diversity indices (Table 1) showed that Manora Island stations (MI-1 and MI-2) had slightly higher Shannon index and equitability, and slightly lower dominance thereby depicting high dinoflagellate diversity compared to Mubarak Village stations (MV-1 and MV-2). Variability in occurrence and abundance of dinoflegellate species was recorded at different stations (Table 2, Figure 3). Only one species, Tripos furca, was dominantly abundant at all stations (> 1 x10 3 cells L -1 at MV-1 and MI-2, > 25 x10 3 cells L -1 at MV-2 and >100x 10 3 cells L -l from MI-1).Twenty species were commonly present at all stations (between 1x10 3 and 25x10 3 cells L -1 ). Some of the species were termed as frequent (51 spp.; present at three stations; <1x10 3 cells L -1 ), and the rest (107 spp.; <1x10 3 cells L -1 ) were classified as occasional (present at two stations) and rare (present at only station). The highest total cell density of dinoflagellates was generally observed during NE monsoon i.e., October (149x10 3 cellsL -l at MI-1) and November (25x10 3 cells L -l at MV-1) ( Figure 2).

No. of Taxa
The PCA plot of hydro-biological variables (Figure 4) showed two significant components (PC-1 and PC-2) which represent 99.9% of the total variability in water quality. PC-1 accounted for 94.86% of the total variance, which was  The PCA plot showed that axis I was highly and positively correlated with DO, chlorophyll a, cell counts of phytoplankton and dinoflagellates, whereas axis II was strongly correlated with water temperature, pH, and salinity. The transparency of water appeared to be negatively correlated with axis I. The difference in the relative size of axis I and axis II was small with eigen values of 6.37 and 3.44, respectively ( Figure 4).

Discussion
This study reports on the seasonal variability in population structure and composition of dinoflagellates in the offshore and nearshore waters of Karachi (Northern Arabian Sea). The dinoflagellates are one of the major components in marine ecosystem and play significant role in energy transfer between different trophic levels (Nagata et al., 1996;Ganjian and Makhlogh, 1998;Nuuk, 1999;Ganjian and Makhlogh, 2003;Carter et al., 2005) and their blooms can cause eutrophication (Micheli, 1999) and change water quality (Richardson and Schoeman, 2004;Leterme et al., 2006). Dinoflagellate maintained low proportion (<20%) in the total phytoplankton population with a few exceptions, which is in conformity with other studies reported previously (Azov, 1986;Zingone et al., 1995) and has been attributed to surface temperature and wind conditions (Hinder et al., 2012). It may be noted (Figure 2) that stations towards the west (MV-1 and MV-2) showed peaks of phytoplankton abundance slightly earlier compared to stations towards the east (MI-1 and MI-2). This may be due to the eastwards circulation of water, bringing up-sloped nutrient rich water that has been upwelled off the coast of Oman during the SW monsoon (Swallow, 1984;Elliott and Savidge, 1990). The phytoplankton abundance during the NE monsoon along the Pakistan coast is also the effect of delayed transmission of nutrient rich upwelled water in this area. Higher dinoflagellate abundance in the coastal waters at MI-1 and MV-1 compared to the corresponding near-shore stations may be generally attributed to the coastal effect which may be noted much enhanced in the Manora Island stations where the coastal and near-shore waters appeared to have inputs of sewage and industrial wastes and agricultural runoff through Malir and Layari rivers (Nergis et al., 2012;Chan et al., 2021). The regional effect of temperature and wind conditions and upwelled nutrient-rich water is also evident on a seasonal scale. For the same reason discussed above, the diversity of dinoflagellates is higher at Manora Island stations compared to other two stations off of Mubarak Village. Again the diversity and the species richness were higher in the coastal waters as oppose to the corresponding near-shore stations.
Only six genera (Tripos, Protoperidinium, Prorocentrum, Alexandrium, Diniphysis and Gonyaulax) that represent high species numbers (seven or more) are recorded in this study. Similar data has been obtained from the Arabian Gulf and Sea of Oman (Polikarpov et al., 2016) where all these genera had higher diversity except for Alexandrium. Tripos (38 spp.) had highest number of species including five new records, whereas 53 species were reported previously (Shameel and Tanaka, 1992;Munir et al., 2016;Latif et al., 2013) that give rise to a total of 58 species of Tripos from coastal and shelf area of Pakistan. Thirty four taxa and 32 spp of Protoperidinium, including 12 new records, together with previously reported 27 species (Munir et al., 2016;Latif et al., 2013;Gul and Nawaz, 2014) make a total of 59 species in this genus. A total of 24 species of Prorocentrum are now known from Pakistan, including 20 taxa (18 species) and 5 new records (current study) and 15 previously recorded species (Shameel and Tanaka, 1992;Munir et al., 2013). Four new records of Alexandrium (current study; 9 taxa, 8 species) improved previously reported 5 species (Munir et al., 2016) to 9 species in this genus. Eight taxa and 7 species of genus Dinophysis (including one new record) and 7 species of genus Gonyaulax are reported here. This report together with previously recorded species makes a total of 16 species of Dinophysis and 8 species of Gonyaulax (Saeed et al., 1995;Baig et al., 2006;Saifullah, 2010, 2012;Yaqoob et al., 2013;Latif et al., 2013).