7-9Mean rates of secondary production in the Mandovi, Zuari and Cumbarjua canal, Goa, India were 16.9, 35.9 and 32.4 mg C/m2/day respectively. The general hydrographic conditions of the Zuari were responsible for the higher secondary production. In general, the saline period was more productive (12.9 mg dry wt/m3/day) compared to the low saline period (11 mg dry wt/m3/day). The average secondary production in the estuarine system was 21.4 mg dry wt/m3/day or 1078 tonnes carbon/yr. The coefficient of energy transfer from primary to secondary level was 6.6%. Theoretical estimate of fish biomass in this estuarine system was 1007 tonnes/yr

Madhupratap, M

Nair, Vijayalakshmi R

Selvakumar, R A

English

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Indian Journal of Marine SciencesVol. 9, March 1980, pp. 7-9Seasonal Variations in Secondary Production of the Mandovi-Zuari EstuarineSystem of GoaR A SELVAKUMAR, VIJAYALAKSHMI R NAIR & M MADHlJPRATAPNational Institute of Oceanography, Dona Paula, Goa 403 ()()4Received 16 April 1979: revised received 25 JUlie 1979Mean rates of secondary production in the Mandovi, Zuari and Cumharjua canal were 16.9. 35.Q and 32.4 mg C/m2/dayrespectively. The general hydrographic conditions of the Zuari were responsible for the higher secondary production. Ingeneral, the saline period was more productive (24.9 mg dry wt/m ' /day) compared to the low saline period (II mg dry wt/m3/day). The average secondary production in the estuarine system was 21.4 mg dry wl/m3jday or 1078 tonnes carbon/yr. Thecoefficient of energy transfer from primary to secondary level was 6.6%. Theoretical estimate of fish biomass in this estuarinesystem was 1007 tonnes/yr.The Mandovi and Zuari rivers and the interconnectingCumbarjua canal form the major estuarine system ofGoa. Earlier reports indicate that the primary! andbenthic? productions in the estuary are quite high.Studies on zooplankton from this area 3 - 7 are limitedto the abundance and diversity of different groups. Inthis communication, an estimate of secondaryproduction of the Mandovi-Zuari system is presented.From the yield ratio of carbon production in the area, atheoretical estimate of fish biomass is also made.The Mandovi and Zuari rivers open into theArabian Sea and the Cumbarjua canal connects themat a distance of about 15 and 12 km respectively fromthe mouths. The width of the Mandovi and Zuari riversat the mouth region is 3.? and 4.9 km respectively. Theaverage depth at the midstream of Mandovi is 7 m andthat ofZuari 8.5 m. The average rainfall of Go a is about3000 mm, the major part being contributed by thesouthwest monsoon during June-September. A largenumber of tributaries are connected to the Mandoviand consequently the estuary receives a lot more freshwater than the Zuari, The tides in the estuaries are ofsemi diurnal type with a tidal range of about 2.5 m.Compared to the drastic variations in salinity,fluctuations in temperature and dissolved oxygen areconsidered unimportant to affect the secondaryproduction of this estuarine system.Materials and MethodsFortnightly observations were made from June 1971to May 1972 at 12 stations in the estuarine system(Fig. 1). The distance from st 1 to st 12 is about 43 km.Most of the collections were made during the high tide.Zooplankton was collected in surface tows lasting5 min using a HT net (mouth area 0.25 m? and meshsize 0.3 mm) with a flow meter attached. Surface watersamples were collected for determining salinity. Dryweight of samples was determined after removinglarger organisms. A factor of 33.1~{, of dry weightestimated as the organic carbon content of estuarinezooplankton" was used to compute the secondaryproduction in terms of carbon. Although collectionswere made fortnightly it was assumed that thisrepresented the secondary production for the period toconvert it to daily production".Depending on the salinity regime in this estuarinehabitat, mid-June to mid-September was considered asthe low saline period and mid-September to mid-Juneas the saline period. Rates of secondary productionwere estimated separately for saline and low salineperiods and also for different regions in the system.Fig. 1-Location of stations [Secondary production was estimatedfor the hatched area)7INDIAN J. MAR. SCI., VOL. 9, MARCH 1980Results and DiscussionSalinity- The estuary becomes seawater dominatedduring the summer months (April/May) resulting inhigh salinity (> 35%0 except at sts 4 and 5) whereas itbecomes freshwater dominated during the monsoon(Fig. 2). A salinity gradient along the longitudinal axisdecreasing from mouth to the head of the estuariesoccurred even during the high saline period. With theonset of southwest monsoon in mid-June, salinity getsreduced drastically (<0.35" /00) in the interior stations(3-8). At stations located towards the mouths salinityvalues were slightly higher during the monsoon periodowing to their proximity to the sea, the lowest valuesobserved in July from Mandovi and Zuari were 0.93%0and 4.27%0 respectively. In general, the Zuari estuaryshowed relatively higher salinity than the Mandoviestuary, probably because of the greater tidal influence,larger opening at the mouth and lesser riverinedischarge. It was reported 10 that due to the influence ofZuari estuary, salinity value in the Cumbarjua canalwas higher than the interior parts of Mandovi duringhigh saline period and the present study also confirmsthis observation.Zooplankton production- The changes in thehydrographical conditions of this estuarine systemdirectly influenced the zooplankton abundance.Zooplankton biomass was low in the estuarine systemduring the low saline period. High salinity periodsustained relatively higher amount of zooplankton.Seasonal variations in the secondary production of theMandovi-Zuari-Cumbarjua estuarine system are givenin Tables 1 and 2. Mean production in the Mandoviwas estimated as 12.8 mg dry wt/m3/day. Stationslocated near the mouth of Mandovi had higherzooplankton biomass during the low saline periodthan the interior stations (Table 1). However, duringthe saline period the interior stations sustained higherproduction probably because these areas provide astabler environment compared to turbulent conditionsprevailing near the mouth.The Cumbarjua canal had a production ratecomparable to that of the Mandovi during the lowsaline period. During the saline season, the averageproduction from this area was 29.4 mg dry wt/m3/day,the highest for the estuarine system. Meanzooplankton production in the canal was 23.3 mg drywt/m3/day.The Zuari estuary sustained a higher yearly meanproduction rate compared to the Mandovi andCumbarjua canal (Table 2; Fig. 3). Average zooplank-ton biomass at different stations indicated that highervalues were confined to sts 9-12 due to generalconditions of higher salinity prevailing in this estuary.Thus, the Zuari showed highest production (Table 1)for the low saline period compared to the other regions.8JUN ~!§§~~;;::~r---------- -- IJUL ~~ .•.----',-5 J- I -AUG ----.---.,NOVC/):I:DEeI-Zo~ JANFEBMARAPRMAY2 3 4 567STATIONS8 9 10 II 12Fig. 2-Distribution of salinity (%0) in the surface waters ofMandovi-Zuari estuarine system during June 1971 to May 1972Table I-Secondary Production (mg dry wt/m3/day) atDifferent Locations during Low Saline and Saline PeriodsRegion Low saline SalineMouth of Mandovi 5.8 13.5(sts I & 2)Interior part of Mandovi 3.3 24.2(sts 3.4 & 5)Mouth of Zuari 14.6 29.7(sts 11 & 12)Interior part of Zuari 11.3 24.5(sts 9 & 10)Table 2-Secondary Production (mg dry wt/m3/day) in theMandovi-Zuari Estuarine System during Low Saline andSaline periodsPeriod RegionMandovi Cumbarjua Zuari Entirecanal system5.3 5.3 14 II15.3 29.4 , 28.8 24.912.8 23.3 25.1 21.4Low salineSalineWhole yearSELVAKUMAR et al.: SEASONAL VARIATIONS IN SECONDARY PRODUCTION••.•. HOOYI C. CANAL lUARI1 ~."j ... . I .-:::==,STATIONSFig. 3-Average values of zooplankton displacement volume (mIl100 m ') obtained from different stations in the Mandovi-Zuariestuarine system during June 1971 to May 1972Further, the difference in production between lowerand upper reaches during the saline as well as lowsaline periods was not very pronounced.Biomass obtained from the entire system over theseasons ranged from 0.3 to 158 mg dry wt/m '. Theaverage production in the estuarine system (Fig. 1)withan approximate area of 92 km2 was 21.4 mg dry wt/m3/day which is less than the figure (31.8 mg dry wt/m3/day) reported? for Cochin backwaters. The meancolumn production of zooplankton, computed fromthe column depths of stations, for Mandovi, Zuari andCumbarjua canal were 16.9, 35.9 and 32.4 mg C/m2/day, respectively. The average rate uf secondaryproduction for the entire system was 29.6 mg C/m2/day. These values fell within the ranges estimatedearlier! for zooplankton production in the system.Production at different trophic levels-The estimatedproduction at different trophic levels for the entireestuarine system is presented in Table 3. The totalprimary production in the system was estimated as16388 tonnes carbon from a mean net production rateof 485.97 mg C/m2/dayll. The yearly secondaryproduction was estimated to be 1078 tonnes carbon.The tertiary production of plankton eating fishes wasaveraged from 1% of prima.y production and 10% ofsecondary production and raising the figures by afactor of7.41 given by Vinogradov'? to obtain the wetweight offish. The theoretical estimate offish yield was1007 tonnes per year.The coefficient for energy transfer from primary tosecondary level in the system was 6.6% (Table 3). Thetransfer coefficient of 2.7% for Cochin backwaters?would become 7.4% if the carbon content forzooplankton was raised to 33.1% of dry weight used inthe present study and this fairly agrees with thecoefficient obtained. This also compares well with theaverage consumption rate of phytoplankton byTable 3-Production at DitTerent Trophic Levels in theMandovi-Zuari Estuarine SystemArea (km")Primary production(tonnes carbon/yr)Secondary production(tonnes dry wt/yr)Secondary production(tonnes carbonjyr)Transfer coefficient (%)Tertiary production(based on primary production)Tertiary production(based on secondary production)Mean tertiary production92.1416388325710786.612157991007zooplankton (7%), given by Qasirn ':' for the Mandovi-Zuari estuarine complex.AcknowledgementThe authors are thankful to Dr S.Z. Qasirn, Directorand Dr T.S.S. Rao, Head of the BiologicalOceanography Division, for their suggestions.References1 Bhattathiri P M A, Devassy V P & Bhargava R M S,lndian) marSci, 5 (1976) 83.2 Parulekar A H &-Dwivedi S N, in Recent researches in estuarinebiology, edited by R. Natarajan (Hindustan Publ Cor, Delhi)1973, 2l.3 Goswarni S C & Singbal S Y S, I ndia/l J mar Sci. 3 (1974) 51.4 Pant A, Bhargava R M S & Goswami S C, Indian) mar Sci, 5(1976) 103.5 Goswami S C & Selvakumar R A, in Proceedings of thesymposium on warm warer zooplankton (Special PublicationUNESCOfNIO) 1977, 226.6 Achuthankutty C T, George M ] & Goswami S C, in Proceedingsof the symposium on warlll warer zooplankcon (SpecialPublication UNESCOfNIO) 1977,412.7 Nair V R & Selvakumar R A, Mahasagar=Bult nacn Ins/Oceanogr, 12 (1979) 17.8 Madhupratap M. Venugopal P & Haridas P, Indian) lIlar Sci, 8(1979) 155.9 Madhupratap M. Rao T S S & Haridas P, in Proceedings of/hesymposium 011 warm WlIler zooplankton (Special PublicationUNESCOfNIO) 1977,515.10 Das P K. Murty C S & Varadachari V V R./llelian J mar Sci, 1(1972) 95.II Bhattathiri P M A, Personal communication.12 Vinogradov A P, Mem Sears Fdn mar Res, 2 (1953) 647pp.13 Qasim S Z, in Proceedings of the symposium On warm W{l/eYzooplankton (Special Publication UNESCOfNIO) 1977,700.9

Seasonal variations in Secondary Production of the Mandovi-Zuari Estuarine System of Goa

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Seasonal variations in Secondary Production of the Mandovi-Zuari Estuarine System of Goa

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