﻿The variations in physical and chemical 	 

parameters of the Alleppey mudbank waters are 	 

discussed in this paper. The fluctuations in tem	 

perature, salinity, oxygen and the comparatively 	 

high nutrient contents of the mudbank waters 	 

are highlighted

Gopinathan, C P

Mathew, K J

Murty, A V S

Rao, D S

Regunathan, A

CMFRI Digital Repository

SIX ECOLOGY OF MUDBANKS-D. S. RAO, K. J. MATHEW, C. P. GOPINATHAN, - H Y D R O G R A P H Y A. REGUNATHAN and A. V. S. MURTY ABSTRACT The variations in physical and chemical parameters of the Alleppey mudbank waters are discussed in this paper. The fluctuations in tem-perature, salinity, oxygen and the comparatively high nutrient contents of the mudbank waters are highlighted. INTRODUCTION Observations on the physical and chemical properties of the mudbank are very meagre. Damodaran and Hridayanathan (1966) and Damodaran (1973) have studied some of the hydrographical properties of the Narakkal mudbank and recently Kurup (1977) reported the temperature and sal inity distr ibut ion at the mudbank at Purakkad. However, a detai led study on the physico-chemical aspects of the mudbank area is s t i l l lacking. The fo l l ow ing account deals w i t h the physical and chemical aspects of Al leppey mudbank, based on observations made during 1971-72, wh ich covered two mudbank seasons. In the year 1971-72, mudbank was formed be', ween Kakkazham in the north aTid Karoor (near Purakkad) in the south w i th Ambalapuzha at the centre, and it extended about 3 km along the coast and about 5 km of fshore. The average depth was about 4 m. MATERIAL AND METHODS A country-craft was used for carrying out investigations in the mudbank. For regular moni tor ing of the b io logical and ecological parameters, 4 stations were f ixed in the mudbank, (F ig . 1, Chapter 5). The observations were made at surface and bot tom for tempe-rature, sal ini ty, d issolved oxygen, reactive phosphate, reactive si l icate, nitr i te and nitrate. Observations on temperature was made using a bucket thermometer for surface waters and reversing thermometer for bo t tom. Seawater samples for chemical analysis were col lected by Nansen bott le. Sal in i ty was determined by the t i t ra t ion method, d issolved oxygen by Winkler's method and the nutrients by the method of Strickland and Parsons (1968). CMFRI BULLETIN 31 25 RESULTS Temperature: The variations of the mean temperature at surface and bottom are pre-sented in f i g . 1 . Before the start o f the mudbank season, the temperature was about ^S.S'C bo th at the surface and bo t tom. During the mudbani< season (June-August ) , the tem-perature of the waters was between 26 and 27*0 at the surface and between 25and 26°C at the bot tom. During November-December 1971 the temperature of the surface was around 290c and at the bot tom it was 28 'C. During o 30r E 4 . 0 z UJ X o 2.0 -I 1—1_ _i l_J_ J J A S O N O J F M A M J 1971 1972 J A S Fig- 1 Monthly mean variations of liydrographic para-meters: a- temperature (surface and bottom); b. Salinity (surface and bottom); c. dissolved Oxygen (Surface and bottom). January-IVIarch 1972, the temperature for surface water was 28°C, and at the bot tom it was \°C less. Salinity: The sal ini ty values at both surface and bot tom were relat ively high during the pre-mudbank and post-mudbank seasons. During the 1 972 mudbank season the average sal inity values reached as low as 28.5%o. It is interesting to note that the sal ini ty values at the bot tom were s l ight ly lower than at the surface during the mudbank seasons ( f i g . 1 b ) . Dissolved oxygen: The f i g . 1 c represent the mean values of dissolved oxygen at the surface and at the bo t tom. The value rose to around 4 ml/I during the pre-mudbank season. The dissolved-oxygen values in the mudbank season were the same as the pre-monsoon values at surface of 1971. However, during the 1972 mudbank season the dissolved oxygen of both surface and bot tom waters was reduced to about 3.2 m l / 1 . Reactive phosphate: The variations of the values of reactive phosphate of surface and bot tom waters are given in f i g . 2 a. The values during winter were generally lower at the two levels and less than 2 (xg at P/l The o- o Surface — — — Sotlom O - o ' — I — I — » • - 6r z o 4 f-v'-V • J — I I—I 1 I I 1 I ' • • • • ' I Fig.2 Monthly mean variations of hydrographic parame-ters: a, phosphate (surface and bottom); b, nitrite (surface and bottom; c, nitrate (surface and bot-tom); d, silicate (surface and bottom). 26 MUDBANKS seasonal value of phosphate of the surface water, as we l l as the bot tom water, indicated the maximum during the mudbank season of the years 1971 and 1972. However, a sharp rise in the phosphate values was noted in March 1972. As is usually the case, the phosphate values were more at the bot tom than at the surface. Nitrite and nitrate: Fig. 2 b shows the var i -ations in the values of ni t r i te content. Generally speaking, the values during 1972 mudbank season were much less than those corresponding to the 1971 season. The nitrate nitrogen ( f i g . 2 c) was low during the pre-mudbank season and appareciably high during the mudbank season. The bot tom and surface values did not show much difference in the case of nitrates. The nitrate ni t rogen values showed maximum during post-mudbank season at both surface and bot tom. Reactive silicate: The f igure 2 d gives the variations in reactive si l icate values at surface and bot tom. The si l icate values at the t w o levels were at their ebb, being less than 10 (A g at S1O3/I during the pre-mudbank and post-mudbank seasons. During the mud -bank season, the si l icate values were high. In general, the bottom sil icate values were higher than there at the surface during the post-mud-bank season. DISCUSSION The reduced temperature observed during the mudbank season could be due to the r e d -uced incoming solar radiation on account of the thick monsoon clouds. The observed reduction in sal ini ty values f rom the p re -mud-bank to the itiudbank season may be attr ibuted to precip i tat ion, run of f and also due to the freshwater discharge f rom the nearby spi l lway The sl ight decrease of sal inity at the bot tom of the water column during the mudbank season may be attr ibuted to the seepage of water-borne mud from the subterranean porous stratum of the coastal str ip, an obseivat ion supporting Crawford's v iews (see Chapter 3 on mudbank format ion) . Heavy blooming of organism Noctiluca can cause a rapid reduction in the d isso lved-oxygen content of the water, especially when it is calm. Thus during the post-mudbank season (September-November), the minimum values of d issolved oxygen can be attr ibuted to the observed blooms of Noctiluca miliaris. The nitrogenous compounds and the phos-phate contents of the water column showed an approximate inverse relat ionship ( f i g . 2), wh ich indicates that the planktonic algae uti l ises one of the nutrient compounds at a faster rate during a particular season. The high sil icate values during the mud-bank season may be due to the f ine sediments present in suspension in the water. This i n -crease may also indicate the heterogenous character in space and time of the silica recycling mechanism by the diatoms during its g rowth and mul t ip l icat ion. CMFRI BULLETIN 31 27 

Ecology of mudbanks - Hydrography

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Ecology of mudbanks - Hydrography

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