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Inter-annual variations in the hydrochemistry of Alappuzha mud banks, southwest coast of India
Continental Shelf Research 177 (2019) 42–49
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Inter-annual variations in the hydrochemistry of Alappuzha mud banks, southwest coast of India
T
Dayana Mathewa, T.R. Gireeshkumara,∗, P.B. Udayakrishnanb, K.R. Muraleedharana, N.V. Madhua, M. Naira, C.Revichandrana, K.K. Balachandrana a b
CSIR - National Institute of Oceanography, Regional Centre, Kochi, 682 018, India CSIR - National Institute of Oceanography, Regional Centre, Mumbai, 400 053, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Upwelling Mud bank Hypoxia Nutrients Fish availability
The changes in the hydro-chemical properties of a near shore waters of Alappuzha, southwest coast of India, where the mud banks and coastal upwelling are co-existent, are presented based on two time-series measurements taken during 2014 and 2016. The meteorological data show that the wind speed was significantly high during 2014 as compared to that during 2016 (p < 0.01). As a result, the shoreward propagation and persistence of hypoxic upwelling waters were more prominent in the mud banks during 2014, even though the coastal upwelling was generally more intense during 2016. This indicates that irrespective of the intensity of upwelling, the incursion of hypoxic waters towards the shallow coastal environment is dependent on the local meteorological conditions such as winds. The coastal upwelling generally increased the concentrations of nutrients and chlorophyll a but marked increase in the nitrate levels during 2014 favoring a massive diatom bloom (Fragelaria oceanica) is noteworthy, as it is an indicator of the abundance of Indian oil sardines (Sardinella longiceps), the popular mud bank fishery.
1. Introduction There is a growing concern over the intensification of hypoxia in coastal systems globally since the 1960s (Diaz and Rosenberg, 2008; Vaquer-Sunyer and Duarte, 2008) leading to the large-scale expansion of dead zones in the sea (Schmidtko et al., 2017). These regions are found to be severely affected in their biogeochemical interactions leading to increased emission of greenhouse gases (Breitburg et al., 2018). Upwelling induced coastal hypoxia are found to intensify due to very strong winds in the near shore areas (Feely et al., 2008; Sydeman et al., 2014). Upwelling along the western continental shelf of India and its implications on the primary, secondary and tertiary trophic levels are widely studied (Banse, 1968, Banse et al., 1996; Reghunathan et al., 1984; Shetye et al., 1990; Sudheesh et al., 2016; Jyothibabu et al., 2018). The development of an acute oxygen deficient zone in the coastal region subsequent to the upwelling, has been attributed to the combined effect of the sub surface upwelling and increased oxygen demand for the oxidation of autochthonous organic matter (Naqvi et al., 2000; Sudheesh et al., 2016). Another concurrent process that occurs along the southwest coast of India during peak summer monsoon period is the development of mud
∗
banks in certain shallow regions such as the coastal waters of Alappuzha (Tatavarti and Narayana, 2006; Narayana et al., 2008; Hareeshkumar and Anand, 2016). Mud banks are semi-circular patches of calm coastal waters (10 m) with a fluid muddy layer in the bottom that continuously attenuate the incident waves to create a calm environment conducive for fishing, when the rest of the coastal environment is hostile due to the high monsoonal waves (Jyothibabu et al., 2018). A recent time-series measurement has reported that there is an incursion of hypoxic waters into the mud bank regions of Alappuzha during peak southwest monsoon (Gireeshkumar et al., 2017). This time series measurement is the first report from such a shallow depth (5 m–13 m depth) from an Indian coast which was possible only due to the existence of calm mud banks. The other time-series measurements so far reported from Indian coastal environments are from much deeper depths such as that off Goa (20 m) and Kochi (50 m) and at much longer frequency of 30–45 days (Banse, 1959; Shenoy et al., 2002; Sudheesh et al., 2016). The development of hypoxic zones in shallow (3 m) coastal environment should be treated with caution and the causative factors should be identified. Therefore, the time-series measurements conducted during 2014 at Alappuzha were repeated during 2016 to understand the inter-annual variability in the hydrochemistry of the
Corresponding author. E-mail address: [email protected] (T.R. Gireeshkumar).
https://doi.org/10.1016/j.csr.2019.03.008 Received 27 July 2018; Received in revised form 23 February 2019; Accepted 20 March 2019 Available online 23 March 2019 0278-4343/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. The location of the sampling sites (M1, M2 and M3) of the Alappuzha mud bank, in the south west coast of India. The white dashed line shows the approximate boundary of the persistent mud bank location. Red star marks represent the major mud bank locations along the south west-coast of India. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
water samples were collected using a 5 L Niskin sampler and sub-sampled for dissolved gases, inorganic nutrients and chlorophyll a. Samples for dissolved oxygen (DO) were collected in 125 ml glass bottles without entering air bubbles and immediately fixed using Winkler's reagent. The samples for nutrient analysis were kept in ice and brought to the laboratory.
mud banks. 2. Materials and methods 2.1. Study area and sampling In view of the above objective, the time series measurement carried out in the Alappuzha during 2014 was repeated during 2016. The observations are discussed in detail in Gireeshkumar et al. (2017). The observations in 2014 were for 18 weeks (22nd April to 20th September) at 3 locations of M1, M2 & M3. Station M2 (7 m) represented the Alappuzha mud bank, while M1 (7 m) is a non-mud bank region and a reference point located 7 km north of M2. Station M3 is in the offshore located much deeper (13 m) and westward of M2 (Fig. 1). The measurements during 2016 were from 06th April to 14th October. In all observations, a CTD profiler (SBE 19 plus) was used to record the salinity (accuracy of ± 0.0005 S/m in terms of conductivity) and temperature (accuracy of ± 0.0005 °C) from each location. Wind data for 2014 and 2016 (ERA-Interim data sets) was obtained from European Centre for Medium-Range Weather Forecasts (ECMWF) at a closest grid to the Alappuzha mud bank region. Water samples were collected at all stations at every 2 m interval in 2014 and 3 m interval in 2016. The
2.2. Laboratory analysis DO samples were analyzed using Winkler's method using an automatic titration system (Metrohm 865 Dosimat plus) following standard methods with a detection limit of 2 μM (Grasshoff et al., 1983). The Water samples for nutrients were filtered using 0.7 μm GF/F paper and analyzed for ammonium (NH4+), nitrite (NO2−), nitrate (NO3−), phosphate (PO43−) and silicate (SiO4). The nutrient analysis during 2014 was carried out using a UV-VIS spectrophotometer (Shimadzu UV 1800), while a Skalar San++ Auto-analyzer was used for estimation in 2016. Both instruments were standardized by calibration to minimize analytical errors. The detection limits of NH4+, NO2−, NO3−, PO43− and SiO4 are 0.07, 0.07, 0.07, 0.05 and 0.05 μM respectively. Chlorophyll a from each sampling location was analyzed by passing 1 L of sample through 0.7 μM GF/F filter paper followed by extraction in 90% 43
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August 2014, which were in accordance with the upwelling period (Fig. 4a). During this period, the bottom waters also experienced a steady decline in the dissolved oxygen levels to as low as 6 μM. The persistence of hypoxic waters in the offshore station (M3) was more pronounced and durable in 2016, though it was not properly reflected on the oxygen profiles of the nearshore stations (M2, M1). The offshore waters (M3 station) becomes oxygen deficient by 14th June that was further expanding to reach almost the surface layer on 24th June. The intrusion of low oxygenated waters into the nearshore areas (M1 and M2) were only for short periods from 24th June to 10th August and from 7th September to 23rd September. Whereas the surface layer was more oxygenated (> 160 μM), the subsurface waters at M3 continued to remain hypoxic (7–19 μM) till the end of September. 3.3. Inorganic nutrients The nutrient concentrations showed almost similar trend during both years. There was a progressive increase in the concentrations of NO3−, NO2−, PO43− and Si(OH)4 in the bottom in response to the upwelling. Detailed explanation of nutrient distribution in 2014 was published in Gireeshkumar et al. (2017). However, the enrichment of NO3− (≈15 μM) at M3 during 2014 was exceptional (Fig. 5a) compared to that during 2016 (7.5 μM∼). There was an increase in the NO2− concentration (> 2 μM) between 13 June to 2nd August 2014 and from 10th July to 23rd September 2016 well in accordance with the lowest DO values during both years. However, the enrichment was not consistent, as it varied whenever the water column changed to oxic. NH4+ concentrations (Fig. 5c) remained very low during 2014, but showed high concentrations during 2016. In general, the concentrations varied widely during the upwelling periods and was relatively high (2–24 μM) during 2016 compared to that (0.5–5.2 μM) during 2014. The PO43− concentration (Fig. 6a) was generally low (< 0.5 μM) up to the second week of June 2016, compared to the PO43− values during the same period in 2014 with the progression of upwelling. However, the subsurface and bottom waters in July (2014) and September (2016) displayed significant high values of ≥2 μM PO43−, with sharp spike in 2016. Considering the Si(OH)4 values, the accumulation of Si(OH)4 up to 20 June (> 10 μM), were decreased to < 5 μM by September 2014 (Fig. 6b). However, by the end of July to October 2016, mud bank water column witnesses replete Si(OH)4 values.
Fig. 2. Monthly variation of wind speed (m/s) in 2014 (blue line) and 2016 (black line) of the Alappuzha mud bank. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
acetone for 24 h in the dark at 4 °C and estimation using Fluorometer (Turner designs, 7200) with a detection limit of 0.02 mg/m3. 3. Results 3.1. General hydrography The water column generally remained warm in the surface (32 °C) and well mixed till the end of May in 2014 and 2016. The strengthening of NNW winds was associated with the upwelling of relatively cold (26 °C) bottom waters at M3 on 22nd May 2014, which progressed to the coastal locations. The onset of monsoon on 6th June 2014 was indicated by strong winds (Fig. 2) along with a sudden cooling by 3 °C at M3 bottom (Fig. 3a). This water was found to move shoreward (M2 and M1) through bottom within a week. The water column at all stations experienced uniform cooling from 6th June to 20th June, and was further cooled by the incursion of cold water (< 24 °C) in the bottom from 10th July to 16th August. The shoreward progress of these cold waters was in close correspondence with the increasing wind speed, especially during 2014. There was a slight change in the onset (2nd June) of upwelling to surface in 2016, but it was more intense, as the cold water (< 24 °C) was persistent in the deeper location (M3) for longer duration. In contrast to the wide variations in the water temperature, the salinity (35) remained consistent with minor variations in the surface layers on rainy days (Fig. 3b). There was an occasion when the salinity was lowered to 34 in accordance with thermal conditions caused by rains. Based on the physical oceanographic properties of the study region, strong winds are found to favor homogeneous water column in the Alappuzha mud banks (Muraleedharan et al., 2017). There was occasional freshening of surface layers followed by stratification of water column during 2016 forming an upper low salinity lens.
3.4. Chlorophyll a Chlorophyll a concentrations (Fig. 7) were moderate (≤4 mg/m3) at M1 and M2, followed by the significant increase (> 12 mg/m3) from the end of June to 2 September in 2014, whereas from 26 September to 7 October in 2016. The relatively low Chlorophyll a biomass in M3 as well attained a maxima (> 10 mg/m3) during this period respectively. The exceptionally high Chlorophyll a values obtained from M2, during 2016 (> 16 mg/m3) with no marked difference between surface and subsurface layer, were compatible with the DO profiles. 4. Discussion According to Indian Meteorological Department (IMD) Reports, there were episodes of heavy rains during the peaking of southwest monsoon 2014 (Muraleedharan et al., 2017). It is evident from the Fig. 2 that average wind speed was greater during 2014 compared to that during 2016. The wind intensity is an important factor influencing the mixed layer dynamics of a coastal environment (Feng et al., 2012; Li et al., 2016). The water column was in general, warm (32 °C) and vertically mixed at all stations till the onset of monsoon (Fig. 3). Towards the end of May 2014 and May 16, a relative cooling in the bottom was coinciding with the strengthening of the NW winds (Muraleedharan et al., 2017). The onset of monsoon made the water column stratified during both periods. Assuming the temperature of the
3.2. Dissolved oxygen (DO) The dissolved oxygen levels were generally saturated in the study region up to May, till the onset of monsoon during both years. The presence of low oxygenated (< 125 μM) water in the bottom was first measured on 22nd May 2014, which intensified and spread to shallow depths (3 m) during the next few weeks (Gireeshkumar et al., 2017). Thus, the bottom waters remained hypoxic (≤65 μM) for two spells spanning from 13th June to 4th July and again from 19th July to 2nd 44
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Fig. 3. Distribution of (a) temperature (b) salinity in 2014 and 2016 from M1, M2 and M3.
Fig. 4. Distribution of dissolved oxygen in time series stations M1, M2 and M3 during 2014 and 2016 in the Alappuzha mud bank region.
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Fig. 5. Distribution of NO3− (a), NO2− (b) and NH4+ (c) in 2014 and 2016 from mud bank stations M1, M2 and M3.
The consumption of oxygen is generally much higher in coastal regions as compared to that in open ocean (Rabalais and Nixon, 2002; Diaz and Rosenberg, 2008). Therefore, development of oxygen deficient zones in coastal areas are normally associated with major upwelling systems (Gilbert et al., 2010; Carstensen et al., 2014). In the case of Alappuzha mud banks, the incursion of hypoxic waters (Fig. 4) showed
upwelled water as 26 °C, the cooling at M3 bottom started on 13 June 2014 and 14 June 2016 respectively, and this cool water has advected to the coastal region (M2 and M1) within a week's time (20/24thJune 2014/16) under the influence of winds. During 2014, the meteorological evens (winds, upwelling and precipitation) were intermittently varying compared to a sustained upwelling during 2016. 46
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Fig. 6. Distribution of PO43− (a) and Si(OH)4(b) in 2014 and 2016 from mud bank stations M1, M2 and M3.
Fig. 7. The seasonal variations of Chlorophyll a (mg/m3) in 2014 and 2016 from M1, M2 and M3. 47
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entire period could also be due to its contribution from the reductive dissolution of Fe and Mn (oxy) hydroxides in sediments. Chlorophyll a concentrations during 2014 increased with the intensity of upwelling (Fig. 7) and peaked in the surface (> 16 mg/m3) during July (Jagadeesan et al., 2017). It was relatively low (< 3 mg/m3) during 2016 but increased abruptly to > 20 mg/m3 for a short duration during the upwelling period (Fig. 6b). In 2014, silicate concentrations were moderate (2–10 μM), but was relatively high (> 25 μM) during 2016 due to re-suspension (9200 mg/l) in the bottom (Shynu et al., 2017). The plankton biomass in the near shore waters are found to increase substantially due to upwelling (Jyothibabu et al., 2008). The phytoplankton community in the coastal waters is usually dominated by diatoms while incidents of plankton blooms (Noctiluca) are also reported (Subrahmanyan et al., 1975; Nair et al., 1984). The substantial increase in the nitrate levels in the Alappuzha mud banks during 2014 has led to the massive growth of chain forming diatoms known as Flagilaria (Karnan et al., 2017). Noctiluca, one of the major predators of diatoms, was also abundant along with the diatom blooms (Arunpandi et al., 2017). The low Si(OH)4 concentrations during 2014 monsoon could be due to its uptake by this diatom blooms. The increased grazing of Noctiluca alternately removed a significant portion of diatoms. The NO3− enrichment in 2016 was not perceptible, and hence, there was no intense diatom blooms to consume the silicate. Turbidity can also increase the concentration of Si(OH)4 in the coastal environment (Muraleedharan et al., 2017). It is known that seasonal hypoxia can alternately transform a coastal environment into a state of disturbed system (Conley et al., 2009). The present study observed that although the mud bank was prevalent for a longer period during 2016, the presence of nutrient-rich hypoxic waters in the mud bank was for a longer period during 2014. This is because the relatively stronger winds during 2014 were able to carry the hypoxic waters more towards shallow depths as compared to the weak winds during 2016. The high abundances of archaea, bacteria and phytoplankton and their cascading effects in the tertiary stocks were quite evident during 2014 (Gireeshkumar et al., 2017; Anas et al., 2018; Jyothibabu et al., 2018). Fragilaria blooms were dominant during peak monsoon in 2014 (Jyothibabu et al., 2018), which is the preferred food to fishes such as sardines (Nair and Subrahmanyan, 1955). Further, the persistence of hypoxia in the bottom is a physiological stress to fishes, as they tend to aggregate in the mud bank surface waters Gireeshkumar et al. (2017).
Fig. 8. The seasonal variability of bottom water dissolved oxygen (stations M1 and M2) and wind speed during the study period during 2014 and 2016.
a close correspondence with the local winds. It can be seen that the incursion of hypoxic waters in the mud banks for a longer period during June and July 2014 were mainly associated with swift winds (> 4.5 m/ s) that sustained for several days (Fig. 8). However, the winds during the same period were relatively weak (< 4.5 m/s) and intermittent during 2016, which considerably reduced the period of occurrence of hypoxic waters in the mud banks. To sum up, though the intensity of upwelling was much strong during 2016, the presence of hypoxic waters in the Alappuzha mud banks was for a longer period during 2014. The coastal environments are normally expected to receive high amounts of nutrients through surface runoff (Diaz and Rosenberg, 2008; Rabalais and Turner, 2001). The present study region is located away from any rivers and hence, the contribution from surface runoff was insignificant, except for some weeks towards the late monsoon period. The nutrients in the mud bank region generally remained moderate in the subsurface as a result of upwelling, in accordance with earlier studies (Rao et al., 1984, Muraleedharan et al., 2017; Balachandran, 2004). Thus, the NO3− concentration increased up to 15 μM during 2014 as compared to 9 μM during 2016 (Fig. 5a). NO2− levels were insignificant initially due to oxygen saturation, but as the upwelling intensified, there was an accumulation of NO2− up to 3 μM corresponding to the development of hypoxia (Fig. 5b). This indicates that at times, there may be events of denitrification in the sediment (Sudheesh et al., 2016) or even transient in the fluid mud layer caused by hypoxia. PO43− distribution in the study region needs a special mention as it has been already established that the mud banks are the storehouse of phosphorus (Sheshappa, 1953). PO43− concentration was low (< 0.75 μM) prior to the southwest monsoon, but increased to > 1.5 μM during the upwelling period (Fig. 6a). It was concurrent with peak upwelling period in 2014, but it was delayed further in 2016 and the increased was noted towards the end. The substantial increase in PO43− in the shallow regions (M1 and M2) during 2016 could be due to re-suspension (Kraal et al., 2012; Hietanen and Lukkari, 2007). The consistently high PO43−(> 3 μM) in the mud banks (M2) during the
5. Conclusion The present study makes a comparison in the hydrochemistry of Alappuzha mud banks during two time-series observations conducted in 2014 and 2016. The coastal upwelling was found to have varied influence on the mud banks during each monsoon. The coastal upwelling was generally more intense during 2016, though the shoreward propagation and persistence of hypoxic waters were more prominent in the Alappuzha mud banks during 2014 due to strong winds. This indicates that irrespective of the intensity of upwelling, the incursion of hypoxic waters towards the shallow coastal environment is dependent on the local meteorological conditions such as winds. Future studies should have more focus on the impact of hypoxia on the outbreak of specific blooms, their preference to tertiary fauna and their adaptations to various environmental stressors. Acknowledgements The authors thank the facilities and the support provided by the Director, National Institute of Oceanography and the Scientist in Charge, Regional Centre-Kochi. We are thankful to all our colleagues in CSIR-NIO who helped to carry out the filed sampling successfully. The critical comments given by reviewers have considerably helped to improve the clarity of the manuscript. This is NIO contribution No. 6374. 48
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.csr.2019.03.008. References Anas, A., Sheeba, V.K., Jasmin, C., Gireeshkumar, T.R., Mathew, D., Krishna, K., Nair, S., Muraleedharan, K.R., Jayalakshmy, K.V., 2018. Upwelling induced changes in the abundance and community structure of archaea and bacteria in a recurring mud bank along the southwest coast of India. Reg. Stud. Mar. Sci. 18, 113–121. Arunpandi, N., Jyothibabu, R., Jagadeesan, L., Gireeshkumar, T.R., Karnan, C., Naqvi, S.W.A., 2017. Noctiluca and copepods grazing on the phytoplankton community in a nutrient-enriched coastal environment along the southwest coast of India. Environ. Monit. Assess. 189, 351. Balachandran, K.K., 2004. Does subterranean flow initiate mud banks off the southwest coast of India? Estuarine. Coast Shelf Sci. 59, 589–598. Banse, K., 1959. On upwelling and bottom trawling off the south-west coast of India. J. Mar. Biol. Assoc. India 1, 43–49. Banse, K., 1968. Hydrography of the Arabian Sea shelf of India and Pakistan and effects on demersal fishes. Deep-Sea Res. I 15, 45–79. Banse, K., Vijayaraghavan, Sumitra, Madhupratap, M., 1996. On the possible causes of the seasonal phytoplankton blooms along the southwest coast of India. Indian J. Mar. Sci. 25, 283–289. Breitburg, D., Oschlies, A., Grégoire, M., Chavez, F.P., Conley, D.J., Garçon, V., Gilbert, D., Gutiérrez, D., Isensee, C., Jacinto, G.S., Limburg, K.E., Montes, I., Naqvi, S.W.A., Pitcher, G.C., Rabalais, N.N., Roman, M.R., Rose, K.A., Seibel, B.A., Telszewski, M., Yasuhara, M., Zhang, J., 2018. Declining oxygen in the global ocean and coastal waters. Science 359 (6371). Carstensen, J., Andersen, J.H., Gustafsson, B.G., Conley, D.J., 2014. Deoxygenation of the baltic sea during the last century. Proc. Natl. Acad. Sci. U.S.A. 111 (15), 5628–5633. Conley, D.J., Paerl, H.W., Howarth, R.W., Boesch, D.F., Seitzinger, S.P., Havens, K.E., Lancelot, C., Likens, G.E., 2009. Ecology controlling eutrophication: nitrogen and phosphorus. Science 323, 1014–1015. Diaz, J.R., Rosenberg, R., 2008. Spreading dead zones and consequences for marine ecosystems. Science 321 (5891), 626–629. Diaz, R.J., Rosenberg, R., 2008. Spreading dead zones and consequences for marine ecosystems. Science 321, 926–929. Feely, R.A., Sabine, L.C., Hernandez-Ayon, M.J., Ianson, D., Hales, B., 2008. Evidence for upwelling of corrosive “Acidified” water onto the continental shelf. Science 320, 1490–1492. Feng, Y., DiMarco, S.F., Jackson, G.A., 2012. Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico. Geophys. Res. Lett. 39. Gilbert, D., Rabalais, N.N., Diaz, R.J., Zhang, J., 2010. Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean. Biogeosciences 7, 2283–2296. Gireeshkumar, T.R., Mathew, D., Pratihary, A.K., Naik, H., Narvekar, K.U., Araujo, J., Naqvi, S.W.A., 2017. Influence of upwelling induced near shore hypoxia on the Alappuzha mud banks, South West Coast of India. Cont. Shelf Res. 139, 1–8. Grasshoff, K., Ehrhardt, M., Kremling, K., 1983. Methods of Sea Water Analysis, second ed. VerlagChemie, Weinhein. Hareeshkumar, P.V., Anand, P., 2016. Coastal upwelling off the southwest coast of India: observations and simulations. Int. J. Digital Earth 9 (12), 1256–1274. Hietanen, S., Lukkari, K., 2007. Effects of short-term anoxia on benthic denitrification, nutrient fluxes and phosphorus forms in coastal Baltic sediment. Aquat. Microb. Ecol. 49, 293–302. Jagadeesan, L., Jyothibabu, R., Arunpandi, N., Karnan, C., Balachandran, K.K., 2017. Dominance of coastal upwelling over Mud Bank in shaping the mesozooplankton along the southwest coast of India during the Southwest Monsoon. Prog. Oceanogr. 156, 252–275. Jyothibabu, R., Balachandran, K.K., Jagadeesan, L., Karnan, C., Arunpandi, N., Naqvi, S.W.A., Pandiyarajan, R.S., 2018. Mud Banks along the southwest coast of India are not too muddy for plankton. Sci. Rep. 8 (1), 1–13.
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Inter-annual variations in the hydrochemistry of Alappuzha mud banks, southwest coast of India
T
Dayana Mathewa, T.R. Gireeshkumara,∗, P.B. Udayakrishnanb, K.R. Muraleedharana, N.V. Madhua, M. Naira, C.Revichandrana, K.K. Balachandrana a b
CSIR - National Institute of Oceanography, Regional Centre, Kochi, 682 018, India CSIR - National Institute of Oceanography, Regional Centre, Mumbai, 400 053, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Upwelling Mud bank Hypoxia Nutrients Fish availability
The changes in the hydro-chemical properties of a near shore waters of Alappuzha, southwest coast of India, where the mud banks and coastal upwelling are co-existent, are presented based on two time-series measurements taken during 2014 and 2016. The meteorological data show that the wind speed was significantly high during 2014 as compared to that during 2016 (p < 0.01). As a result, the shoreward propagation and persistence of hypoxic upwelling waters were more prominent in the mud banks during 2014, even though the coastal upwelling was generally more intense during 2016. This indicates that irrespective of the intensity of upwelling, the incursion of hypoxic waters towards the shallow coastal environment is dependent on the local meteorological conditions such as winds. The coastal upwelling generally increased the concentrations of nutrients and chlorophyll a but marked increase in the nitrate levels during 2014 favoring a massive diatom bloom (Fragelaria oceanica) is noteworthy, as it is an indicator of the abundance of Indian oil sardines (Sardinella longiceps), the popular mud bank fishery.
1. Introduction There is a growing concern over the intensification of hypoxia in coastal systems globally since the 1960s (Diaz and Rosenberg, 2008; Vaquer-Sunyer and Duarte, 2008) leading to the large-scale expansion of dead zones in the sea (Schmidtko et al., 2017). These regions are found to be severely affected in their biogeochemical interactions leading to increased emission of greenhouse gases (Breitburg et al., 2018). Upwelling induced coastal hypoxia are found to intensify due to very strong winds in the near shore areas (Feely et al., 2008; Sydeman et al., 2014). Upwelling along the western continental shelf of India and its implications on the primary, secondary and tertiary trophic levels are widely studied (Banse, 1968, Banse et al., 1996; Reghunathan et al., 1984; Shetye et al., 1990; Sudheesh et al., 2016; Jyothibabu et al., 2018). The development of an acute oxygen deficient zone in the coastal region subsequent to the upwelling, has been attributed to the combined effect of the sub surface upwelling and increased oxygen demand for the oxidation of autochthonous organic matter (Naqvi et al., 2000; Sudheesh et al., 2016). Another concurrent process that occurs along the southwest coast of India during peak summer monsoon period is the development of mud
∗
banks in certain shallow regions such as the coastal waters of Alappuzha (Tatavarti and Narayana, 2006; Narayana et al., 2008; Hareeshkumar and Anand, 2016). Mud banks are semi-circular patches of calm coastal waters (10 m) with a fluid muddy layer in the bottom that continuously attenuate the incident waves to create a calm environment conducive for fishing, when the rest of the coastal environment is hostile due to the high monsoonal waves (Jyothibabu et al., 2018). A recent time-series measurement has reported that there is an incursion of hypoxic waters into the mud bank regions of Alappuzha during peak southwest monsoon (Gireeshkumar et al., 2017). This time series measurement is the first report from such a shallow depth (5 m–13 m depth) from an Indian coast which was possible only due to the existence of calm mud banks. The other time-series measurements so far reported from Indian coastal environments are from much deeper depths such as that off Goa (20 m) and Kochi (50 m) and at much longer frequency of 30–45 days (Banse, 1959; Shenoy et al., 2002; Sudheesh et al., 2016). The development of hypoxic zones in shallow (3 m) coastal environment should be treated with caution and the causative factors should be identified. Therefore, the time-series measurements conducted during 2014 at Alappuzha were repeated during 2016 to understand the inter-annual variability in the hydrochemistry of the
Corresponding author. E-mail address: [email protected] (T.R. Gireeshkumar).
https://doi.org/10.1016/j.csr.2019.03.008 Received 27 July 2018; Received in revised form 23 February 2019; Accepted 20 March 2019 Available online 23 March 2019 0278-4343/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. The location of the sampling sites (M1, M2 and M3) of the Alappuzha mud bank, in the south west coast of India. The white dashed line shows the approximate boundary of the persistent mud bank location. Red star marks represent the major mud bank locations along the south west-coast of India. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
water samples were collected using a 5 L Niskin sampler and sub-sampled for dissolved gases, inorganic nutrients and chlorophyll a. Samples for dissolved oxygen (DO) were collected in 125 ml glass bottles without entering air bubbles and immediately fixed using Winkler's reagent. The samples for nutrient analysis were kept in ice and brought to the laboratory.
mud banks. 2. Materials and methods 2.1. Study area and sampling In view of the above objective, the time series measurement carried out in the Alappuzha during 2014 was repeated during 2016. The observations are discussed in detail in Gireeshkumar et al. (2017). The observations in 2014 were for 18 weeks (22nd April to 20th September) at 3 locations of M1, M2 & M3. Station M2 (7 m) represented the Alappuzha mud bank, while M1 (7 m) is a non-mud bank region and a reference point located 7 km north of M2. Station M3 is in the offshore located much deeper (13 m) and westward of M2 (Fig. 1). The measurements during 2016 were from 06th April to 14th October. In all observations, a CTD profiler (SBE 19 plus) was used to record the salinity (accuracy of ± 0.0005 S/m in terms of conductivity) and temperature (accuracy of ± 0.0005 °C) from each location. Wind data for 2014 and 2016 (ERA-Interim data sets) was obtained from European Centre for Medium-Range Weather Forecasts (ECMWF) at a closest grid to the Alappuzha mud bank region. Water samples were collected at all stations at every 2 m interval in 2014 and 3 m interval in 2016. The
2.2. Laboratory analysis DO samples were analyzed using Winkler's method using an automatic titration system (Metrohm 865 Dosimat plus) following standard methods with a detection limit of 2 μM (Grasshoff et al., 1983). The Water samples for nutrients were filtered using 0.7 μm GF/F paper and analyzed for ammonium (NH4+), nitrite (NO2−), nitrate (NO3−), phosphate (PO43−) and silicate (SiO4). The nutrient analysis during 2014 was carried out using a UV-VIS spectrophotometer (Shimadzu UV 1800), while a Skalar San++ Auto-analyzer was used for estimation in 2016. Both instruments were standardized by calibration to minimize analytical errors. The detection limits of NH4+, NO2−, NO3−, PO43− and SiO4 are 0.07, 0.07, 0.07, 0.05 and 0.05 μM respectively. Chlorophyll a from each sampling location was analyzed by passing 1 L of sample through 0.7 μM GF/F filter paper followed by extraction in 90% 43
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August 2014, which were in accordance with the upwelling period (Fig. 4a). During this period, the bottom waters also experienced a steady decline in the dissolved oxygen levels to as low as 6 μM. The persistence of hypoxic waters in the offshore station (M3) was more pronounced and durable in 2016, though it was not properly reflected on the oxygen profiles of the nearshore stations (M2, M1). The offshore waters (M3 station) becomes oxygen deficient by 14th June that was further expanding to reach almost the surface layer on 24th June. The intrusion of low oxygenated waters into the nearshore areas (M1 and M2) were only for short periods from 24th June to 10th August and from 7th September to 23rd September. Whereas the surface layer was more oxygenated (> 160 μM), the subsurface waters at M3 continued to remain hypoxic (7–19 μM) till the end of September. 3.3. Inorganic nutrients The nutrient concentrations showed almost similar trend during both years. There was a progressive increase in the concentrations of NO3−, NO2−, PO43− and Si(OH)4 in the bottom in response to the upwelling. Detailed explanation of nutrient distribution in 2014 was published in Gireeshkumar et al. (2017). However, the enrichment of NO3− (≈15 μM) at M3 during 2014 was exceptional (Fig. 5a) compared to that during 2016 (7.5 μM∼). There was an increase in the NO2− concentration (> 2 μM) between 13 June to 2nd August 2014 and from 10th July to 23rd September 2016 well in accordance with the lowest DO values during both years. However, the enrichment was not consistent, as it varied whenever the water column changed to oxic. NH4+ concentrations (Fig. 5c) remained very low during 2014, but showed high concentrations during 2016. In general, the concentrations varied widely during the upwelling periods and was relatively high (2–24 μM) during 2016 compared to that (0.5–5.2 μM) during 2014. The PO43− concentration (Fig. 6a) was generally low (< 0.5 μM) up to the second week of June 2016, compared to the PO43− values during the same period in 2014 with the progression of upwelling. However, the subsurface and bottom waters in July (2014) and September (2016) displayed significant high values of ≥2 μM PO43−, with sharp spike in 2016. Considering the Si(OH)4 values, the accumulation of Si(OH)4 up to 20 June (> 10 μM), were decreased to < 5 μM by September 2014 (Fig. 6b). However, by the end of July to October 2016, mud bank water column witnesses replete Si(OH)4 values.
Fig. 2. Monthly variation of wind speed (m/s) in 2014 (blue line) and 2016 (black line) of the Alappuzha mud bank. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
acetone for 24 h in the dark at 4 °C and estimation using Fluorometer (Turner designs, 7200) with a detection limit of 0.02 mg/m3. 3. Results 3.1. General hydrography The water column generally remained warm in the surface (32 °C) and well mixed till the end of May in 2014 and 2016. The strengthening of NNW winds was associated with the upwelling of relatively cold (26 °C) bottom waters at M3 on 22nd May 2014, which progressed to the coastal locations. The onset of monsoon on 6th June 2014 was indicated by strong winds (Fig. 2) along with a sudden cooling by 3 °C at M3 bottom (Fig. 3a). This water was found to move shoreward (M2 and M1) through bottom within a week. The water column at all stations experienced uniform cooling from 6th June to 20th June, and was further cooled by the incursion of cold water (< 24 °C) in the bottom from 10th July to 16th August. The shoreward progress of these cold waters was in close correspondence with the increasing wind speed, especially during 2014. There was a slight change in the onset (2nd June) of upwelling to surface in 2016, but it was more intense, as the cold water (< 24 °C) was persistent in the deeper location (M3) for longer duration. In contrast to the wide variations in the water temperature, the salinity (35) remained consistent with minor variations in the surface layers on rainy days (Fig. 3b). There was an occasion when the salinity was lowered to 34 in accordance with thermal conditions caused by rains. Based on the physical oceanographic properties of the study region, strong winds are found to favor homogeneous water column in the Alappuzha mud banks (Muraleedharan et al., 2017). There was occasional freshening of surface layers followed by stratification of water column during 2016 forming an upper low salinity lens.
3.4. Chlorophyll a Chlorophyll a concentrations (Fig. 7) were moderate (≤4 mg/m3) at M1 and M2, followed by the significant increase (> 12 mg/m3) from the end of June to 2 September in 2014, whereas from 26 September to 7 October in 2016. The relatively low Chlorophyll a biomass in M3 as well attained a maxima (> 10 mg/m3) during this period respectively. The exceptionally high Chlorophyll a values obtained from M2, during 2016 (> 16 mg/m3) with no marked difference between surface and subsurface layer, were compatible with the DO profiles. 4. Discussion According to Indian Meteorological Department (IMD) Reports, there were episodes of heavy rains during the peaking of southwest monsoon 2014 (Muraleedharan et al., 2017). It is evident from the Fig. 2 that average wind speed was greater during 2014 compared to that during 2016. The wind intensity is an important factor influencing the mixed layer dynamics of a coastal environment (Feng et al., 2012; Li et al., 2016). The water column was in general, warm (32 °C) and vertically mixed at all stations till the onset of monsoon (Fig. 3). Towards the end of May 2014 and May 16, a relative cooling in the bottom was coinciding with the strengthening of the NW winds (Muraleedharan et al., 2017). The onset of monsoon made the water column stratified during both periods. Assuming the temperature of the
3.2. Dissolved oxygen (DO) The dissolved oxygen levels were generally saturated in the study region up to May, till the onset of monsoon during both years. The presence of low oxygenated (< 125 μM) water in the bottom was first measured on 22nd May 2014, which intensified and spread to shallow depths (3 m) during the next few weeks (Gireeshkumar et al., 2017). Thus, the bottom waters remained hypoxic (≤65 μM) for two spells spanning from 13th June to 4th July and again from 19th July to 2nd 44
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Fig. 3. Distribution of (a) temperature (b) salinity in 2014 and 2016 from M1, M2 and M3.
Fig. 4. Distribution of dissolved oxygen in time series stations M1, M2 and M3 during 2014 and 2016 in the Alappuzha mud bank region.
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Fig. 5. Distribution of NO3− (a), NO2− (b) and NH4+ (c) in 2014 and 2016 from mud bank stations M1, M2 and M3.
The consumption of oxygen is generally much higher in coastal regions as compared to that in open ocean (Rabalais and Nixon, 2002; Diaz and Rosenberg, 2008). Therefore, development of oxygen deficient zones in coastal areas are normally associated with major upwelling systems (Gilbert et al., 2010; Carstensen et al., 2014). In the case of Alappuzha mud banks, the incursion of hypoxic waters (Fig. 4) showed
upwelled water as 26 °C, the cooling at M3 bottom started on 13 June 2014 and 14 June 2016 respectively, and this cool water has advected to the coastal region (M2 and M1) within a week's time (20/24thJune 2014/16) under the influence of winds. During 2014, the meteorological evens (winds, upwelling and precipitation) were intermittently varying compared to a sustained upwelling during 2016. 46
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Fig. 6. Distribution of PO43− (a) and Si(OH)4(b) in 2014 and 2016 from mud bank stations M1, M2 and M3.
Fig. 7. The seasonal variations of Chlorophyll a (mg/m3) in 2014 and 2016 from M1, M2 and M3. 47
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entire period could also be due to its contribution from the reductive dissolution of Fe and Mn (oxy) hydroxides in sediments. Chlorophyll a concentrations during 2014 increased with the intensity of upwelling (Fig. 7) and peaked in the surface (> 16 mg/m3) during July (Jagadeesan et al., 2017). It was relatively low (< 3 mg/m3) during 2016 but increased abruptly to > 20 mg/m3 for a short duration during the upwelling period (Fig. 6b). In 2014, silicate concentrations were moderate (2–10 μM), but was relatively high (> 25 μM) during 2016 due to re-suspension (9200 mg/l) in the bottom (Shynu et al., 2017). The plankton biomass in the near shore waters are found to increase substantially due to upwelling (Jyothibabu et al., 2008). The phytoplankton community in the coastal waters is usually dominated by diatoms while incidents of plankton blooms (Noctiluca) are also reported (Subrahmanyan et al., 1975; Nair et al., 1984). The substantial increase in the nitrate levels in the Alappuzha mud banks during 2014 has led to the massive growth of chain forming diatoms known as Flagilaria (Karnan et al., 2017). Noctiluca, one of the major predators of diatoms, was also abundant along with the diatom blooms (Arunpandi et al., 2017). The low Si(OH)4 concentrations during 2014 monsoon could be due to its uptake by this diatom blooms. The increased grazing of Noctiluca alternately removed a significant portion of diatoms. The NO3− enrichment in 2016 was not perceptible, and hence, there was no intense diatom blooms to consume the silicate. Turbidity can also increase the concentration of Si(OH)4 in the coastal environment (Muraleedharan et al., 2017). It is known that seasonal hypoxia can alternately transform a coastal environment into a state of disturbed system (Conley et al., 2009). The present study observed that although the mud bank was prevalent for a longer period during 2016, the presence of nutrient-rich hypoxic waters in the mud bank was for a longer period during 2014. This is because the relatively stronger winds during 2014 were able to carry the hypoxic waters more towards shallow depths as compared to the weak winds during 2016. The high abundances of archaea, bacteria and phytoplankton and their cascading effects in the tertiary stocks were quite evident during 2014 (Gireeshkumar et al., 2017; Anas et al., 2018; Jyothibabu et al., 2018). Fragilaria blooms were dominant during peak monsoon in 2014 (Jyothibabu et al., 2018), which is the preferred food to fishes such as sardines (Nair and Subrahmanyan, 1955). Further, the persistence of hypoxia in the bottom is a physiological stress to fishes, as they tend to aggregate in the mud bank surface waters Gireeshkumar et al. (2017).
Fig. 8. The seasonal variability of bottom water dissolved oxygen (stations M1 and M2) and wind speed during the study period during 2014 and 2016.
a close correspondence with the local winds. It can be seen that the incursion of hypoxic waters in the mud banks for a longer period during June and July 2014 were mainly associated with swift winds (> 4.5 m/ s) that sustained for several days (Fig. 8). However, the winds during the same period were relatively weak (< 4.5 m/s) and intermittent during 2016, which considerably reduced the period of occurrence of hypoxic waters in the mud banks. To sum up, though the intensity of upwelling was much strong during 2016, the presence of hypoxic waters in the Alappuzha mud banks was for a longer period during 2014. The coastal environments are normally expected to receive high amounts of nutrients through surface runoff (Diaz and Rosenberg, 2008; Rabalais and Turner, 2001). The present study region is located away from any rivers and hence, the contribution from surface runoff was insignificant, except for some weeks towards the late monsoon period. The nutrients in the mud bank region generally remained moderate in the subsurface as a result of upwelling, in accordance with earlier studies (Rao et al., 1984, Muraleedharan et al., 2017; Balachandran, 2004). Thus, the NO3− concentration increased up to 15 μM during 2014 as compared to 9 μM during 2016 (Fig. 5a). NO2− levels were insignificant initially due to oxygen saturation, but as the upwelling intensified, there was an accumulation of NO2− up to 3 μM corresponding to the development of hypoxia (Fig. 5b). This indicates that at times, there may be events of denitrification in the sediment (Sudheesh et al., 2016) or even transient in the fluid mud layer caused by hypoxia. PO43− distribution in the study region needs a special mention as it has been already established that the mud banks are the storehouse of phosphorus (Sheshappa, 1953). PO43− concentration was low (< 0.75 μM) prior to the southwest monsoon, but increased to > 1.5 μM during the upwelling period (Fig. 6a). It was concurrent with peak upwelling period in 2014, but it was delayed further in 2016 and the increased was noted towards the end. The substantial increase in PO43− in the shallow regions (M1 and M2) during 2016 could be due to re-suspension (Kraal et al., 2012; Hietanen and Lukkari, 2007). The consistently high PO43−(> 3 μM) in the mud banks (M2) during the
5. Conclusion The present study makes a comparison in the hydrochemistry of Alappuzha mud banks during two time-series observations conducted in 2014 and 2016. The coastal upwelling was found to have varied influence on the mud banks during each monsoon. The coastal upwelling was generally more intense during 2016, though the shoreward propagation and persistence of hypoxic waters were more prominent in the Alappuzha mud banks during 2014 due to strong winds. This indicates that irrespective of the intensity of upwelling, the incursion of hypoxic waters towards the shallow coastal environment is dependent on the local meteorological conditions such as winds. Future studies should have more focus on the impact of hypoxia on the outbreak of specific blooms, their preference to tertiary fauna and their adaptations to various environmental stressors. Acknowledgements The authors thank the facilities and the support provided by the Director, National Institute of Oceanography and the Scientist in Charge, Regional Centre-Kochi. We are thankful to all our colleagues in CSIR-NIO who helped to carry out the filed sampling successfully. The critical comments given by reviewers have considerably helped to improve the clarity of the manuscript. This is NIO contribution No. 6374. 48
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Appendix A. Supplementary data
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