J. KAU: Mar. Sci., vol. 5, pp. 25-34 (1414 A.H./1994 A.D.
Humic Substances in the Sediments of Lake Edku, Egypt:
I -Occurrence and Distribution
MOHAMEDA. ELSAYED,WAFIKAABOULNAGA,YOUSEFHALIM* and ALII. BELTAGY
National Institute of Oceanography and Fisheries, Alexandria, Egypt, and
* Faculty of Science, Dept. of Oceanography, Alexandria University, Egypt
ABSTRAcr. Humic substance (HS) as humic and fulvic acids were deter- mined in the surface sediments of Lake Edku (Egypt). Their sources, dis- tribution and behavior were assessed. The lake is geographically subdivided into eastern, central and western sub-basins; subdivision is based on the presence of natural barriers and proximity to the drains or to the sea-lake connection. The central basin showed the highest concentrations of HS, which is attributed to inte,nsive development of macrophytes and to en- hanced phytoplankton primary production. In the eastern basin, high tur-
bidity and water turbulence downstream from the drains limit primary pro- duction. In the western basin, however, water turbulence, sandy bottom, elevated salinity and decrease in nutrients due to mixing with seawater limit the growth of macrophytes as well as the phytoplankton productivity.
Introduction
Humic substances (HS) are the largest reservoir of organic carbon in the aquatic and terrestrial environments (Gjessing 1976; Mantoura et al., 1978). The bulk of the or- ganic matter in most soils, waters and sediments consists of humic substances
(Schnitzer and Khan 1972), which may constitute as high as 70% of the organic mat- ter in the lake sediments (Kemp 1971). This fairly stable and extremely complex copolymers have received a great deal of interest due to their active participation in the biogeochemical processes in the aquatic environment (Duursma 1963; Nissen- baum & Kaplan 1972; Prakash et al., 1973; Stumm & Brauner 1975; Frimmel &
Christman 1988).
The origin of the HS is of great importance since it will determine their chemical compo~itionand structural characteristics and both will define their fate and reactiv-
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29 Humic Substances in the Sediments (Part I).
TABLE 1. Percentage of total organic matter (TOM) and mud in sediment samples.
Stations % TOM % Mud
I II III IV V
2.2 4.4 8.8 2.5 2.7
9.8 32.8 79.4 50.9 49.2
4.1
2.5
44.4 22.9 meanS.D.
waters and mainly due to the proliferation of the macrophytes which reduce the water velocity and favors deposition of the fine sediments.
In the eastern sub-basin (stations IV, V), mud gave a large proportion of the sedi- ments (about 50%). Evidently, most of the coarse fraction carried by the drain, sedi- ments in this basin which is still under the influence of the water current generated by the inflowing water. Calcareous shell fragments contributed also to the coarse frac- tion.
TOM varied approximately in the same way, peaking in the central basin (8.8% at Station III) and decreasing in both the eastern and western basins. Evidently, or- ganic carbon co-varied with the mud content, decrease in the mean particle diame- ter, due to the increase of the mud content, results in the increase of the exposed sur- face, which favors surface reactions like sorption (Bordovskii 1965).
However, plot of TOM against the mud content (Fig. 2) reveals .that the eastern basin was relatively deficient in TOM. It is worthy noting that TOM in the sediments from the eastern basin (2.5%) lies in the range found for most of the coastal lagoons, which eliminates the possibility of a rapid mineralization of the land derived organic matter. Evidently, the in situ primary productivity, particularly macrophytes, contri- buted largely to the TOM in the central basin, the western basin was consequently af-
fected.Distribution oltoml humic substance
Concentrations of total humic substances (THS), as well asTHSrrOM FA/fHS 19tios in the different sediment samples are given in Table 2. The average of THS
concentration in the lake sediments was 25.27 :!: 31.14mg g -1, ranging between 4.37 and 85.54 mg g -1. Obviously, a considerable variation existed between the sampling sites.
The highest THS concentration was associated with the sediments having
maximum mud and TOM content. Fig. 2 and Tables 1 and 2 show that THS is well
correlated with TOM and mud content which agrees with the observations of
Bojanowski & Pempkowiak (1980). It seems that the macrophytes represent an im-
TABLE 2. Total humic substances (THS), humid acid (HA), fulvic acid (FA), THSrrOM and F A/rHS in sediment samples.
THS.
HA*Stations
FA. THSrrOM F~S
I II III IV V
6.4 26.8 85.5 4.7 44
1.0 14.9 38.4 J..o
5.5
11.9 47.2 3.8 3.3
0.3 0.6 1.0 0.2 0.2
85.3 44.4 55.2 78.9 75.5
MeanS.D. 25.6 31.1
11.2 14.6
14.3 16.7
0.5 0.3
68.1 15.6 Oms, HA and FA in mg g-1 sediment (
portant source for sedimentary humic substances. Mayer (1985) stated that mac- rophyte debris may serve as an important site of alteration/humification process.
Humification is also enhanced in muddy sediments (Cauwet 1985).
It is surprising that the sediments at station I contained almost the same content of THS as those at stations IV and V, despite the fact that the mud content of the later stations was about 5 times higher than that in the sediments at station I. This might result from the flocculation/precipitation of dissolved humic substances due to the in- crease in salinity in the area of fresh water/seawater mixing (Sholkovitz 1976). It has also been demonstrated that increased salinity enhanced adsorption of humic sub- stances on clay minerals (Rashid et al., 1972).
Whatever is the total organic carbon in the sediments, the extractable organic mat- ter is an important parameter; it may reflect the degree of 'association between the organic matter and mineral support, as well as the nature of the organic matter (Cauwet 1985). THSrrOM ratio increased regularly from 0,3 at station I to almost 1.0 at station III, then decreased to almost a constant level of 0.17 and 0.19 in the most eastern part of the lake (Table 2).
The plot of THSrrOM ratio against the mud content reveals the distinct character of the eastern basin and the rest of the lake (Fig. 3). The central and western basins showed an increasing extractability with the increase in the fine fraction. Despite the high mud ratio in the eastern basin, the degree of extractability was very low (6- 7.5%). It has been stated (Cauwet 1985) that, the nature of the humic substances particularly the degree of polymerization may be one of the factors controlling the binding of the humic substances to clay part~cles (Cauwet 1985).
Humic substances and relative abundance of humic and fulvic acids
The concentrations of humic and fulvic acids, as well as F A/fHS ratio in the humic
extracts are given in Table 2. Humic substances in Lake Edku sediments were
cJtaracterized by dominance of the fulvic acid fraction whatever the sampling site and
consequently the origin of the organic matter. This seems to be a general feature of
Humic Substances in the Sediments (Part I).
Brown, M. (1975) High molecular weight material in Baltic Seawater. Mar. Chem., 3:253-261.
Cauwet, G.C. (1985) Dynamique de la matiere organique dans les mileux marins et polyhalins; son role dans les processus geochimiques. These d'Etat, Universite de Perpignan, 178 p.
Duce, R.A. and Duursma, E.K. (1977) Input of organic matter in the ocean. Mar. Chem. 5: 319-341.
Duursma, E.K. (1963) The production of the dissolved organic matter in the sea, as related to the primary gross production of organic matter. Netherland Journal of Sea Research, 3: 391-405.
EI-Samra, M.A. (1973) Chemical and physical properties of the water of Lake Edku and the mixed waters between the lake and the sea. M.Sc. Thesis. Faculty of Science, Alexandria University, Egypt, 110
p.
EI-Sarraf, W.M. (1976) Some limnological studies on the submerged macrophytes in Lake Edku, with spe- cial reference to their value as food offish. M.Sc. Thesis. Faculty of Science., Alexandria Univer- sity, Alexandria, Egypt, 117 p.
Frimmel, F .H. and Christman, R.F. (1988) Humic Substances and Their Role in the Environment. John Wiley & Sons, New York, 271 p.
Gagosian, R.B. and Lee, C. (1981) Processes controlling the distribution of biogenic organic compounds in seawater. In: Duursma, E.K. and Dawson, R. (eds.), Marine Organic Chemistry, Elsevier Scien- tific Publication Company, Amsterdam, 521 p.
Kemp, A.L. W. (1971) Organic carbon and nitrogen in the surface sediments of lakes Ontario, Erie and Huron. Journal of Sedimentology and Petrology, 41: 537-548.
King, L.H. (1967) Isolation and characterization of organic matter from glacial marine sediments on the Scottian Shelf. Bedford Institute of Oceanography. Report No. 67(4), p. 18.
Majumdar, S.K. and Rao, C. V.N. (1978) Physicochemical studies on enzyme-degraded fulvic acid. Jour- nal of Soil, 29: 489-497.
Mantoura, R.F .C., Dickson, A. and Riley, J.P. (1978) The complexation of metals with humic materials in natural waters. Estuar. Coast. Mar. Sci., 6: 387-408.
Mayer, L.M. (1985) Geochemistry of humic substances in estuarine environments. In: Aiken, G.R., McKnigilt, D.M., Wershaw, R.L. and MacCarthy, P. (eds), Humic Substances in Soil, Sediments and Water. John Wiley & Sons. New York, pp. 211-233.
Mook, D.H. and Hoskin, C.M. (1982) Organic determination by ignition: Caution advised. Estuar. Coast.
ShelfSci., 15: 697-699.
Mopper, K. and Degens, E. T. (1979) Organic carbon in the ocean: nature and cycling. In: Bolin, B., De- gens, E. T. and Kemp, S. (eds.), The Global Carbon Cycle, Wiley, New York, pp. 293-316.
Nissenbaum, A. and Kaplan, I.R. (1972) Chemical and isotopic evidence for the in situ origin of marine humic substances. Limnol. Oceanogr., 17: 570-582.
Prakash, A., Rashid, M.A. and Rao, D. V,S. (1973) Influence of humic substances on the growth of marine phytoplankton: Diatoms. Limnol. and Oceanogr., 18: 516-524.
Rashid, M.A. and King, L.H. (1969) Molecular weight distribution measurements from marine clays on the Scottian Shelf. Geochim. Cosmochim. Act., 33: 147-151.
Rashid, M.A., Buckly, D.E. and Robertson, K.R. (1972) Interaction of humic acids with different clay minerals and natural sediments in ionic waters similar to waters of marine estuaries. Mezdumar Geokhim. Kongr., 4 (pt. 3.): 125-133.
Riley, G.A. (1963) Organic aggregates in sea water and the dynamics of their formation and utilization.
Limnol. Oceanogr., 8: 72-381.
Samman, A.A. (1974) Primary production of Lake Edku. Bull. Inst. Oceanogr. & Fish., A.R.E., 4: 259- 317.
Schnitzer, M. and Khan, S.U. (1972) Humic Substances in the Environment, Marcel Dekker, New York, 327p.
Sholkovitz, E.R. (1976) Flocculation of dissolved organic matter during the mixing of river water and sea water. Geochim. Cosmochim. Acta. 40: 831-845.
Stevens41n, F .J. (1982) Humus Chemistry. John Wiley & Sons, New York, 443 p.
Stumm,W. and Brauner, D.A. (1975) Chemical speciation. In: Riley, J.P. and Chester, R. (eds.), Chemi- calOceanography, 2nd ed., Academic Press, pp. 173-234.
Wangersky. P.J. (1978) Production of Dissolved Organic Matter. In: Kinne, O. (ed.), Marine Ecology, IV, Dynamics. Wiley, New York, pp. 115-220.
