Effects of the Black River (Cape, South Africa) on the distribution and survival of marine psammofauna

Volume16/Number2/February1985 0025-326X/85 S3.00"H).O0 C 1985PergamonPressLtd.

Maline Pollution Bulletin, Vol. 16,No. 2, pp. 69-75, 1985.

Printedin GreatBritain

Effects of the Black River (Cape, South Africa) on the Distribution and Survival of Marine Psammofauna A. DAVID SCARFE*, ALAN B. THUM~ and CINDY L. WILKE

Zoology Department, Universityof Cape Town, Republic of South Africa and Sterling C. Evans Library, TexasAddVl University, USA *Present address: Department of Veterinary Anatomy, Texas A & M University, College Station, TX 77843, U S A tPresent address: Lockheed Ocean Science Laboratory, 6350 A. Yarrow Drive, Carlsbad, C A 92008, U S A

The physicochemistry of the Black River and adjacent marine beach, in terms of tidal, daily and seasonal fluctuations of temperature and salinity, presents a highly stressed environment exacerbated by canalization of the river mouth. Riverine pollution further stresses this environment, producing low densities and diversities of marine infauna adjacent to the river. No fauna were found in the river mouth where typical estuarine conditions exist only during high tide. Tolerance of the dominant marine species, Cerebratulusfuscus, to temperature, salinity and river water combinations showed that mortality was more rapid after exposure to river water dilutions than after exposure to distilled water dilutions. Computer-generated models suggested that this species should survive conditions closer to the river than were observed. This confirmed additional stress due to pollution carried in the river from urban and industrial drainage. Lower river water temperatures during winter appear to ameliorate pollution stress, enhancing survivability of the infauna.

(1972), on the other hand, found that this latter canalized zone of the river had an extremely depressed biotic community suggesting an extremely harsh environment due to the effects of canalization, its proximity to the marine environment and/or a toxic pollution load. Because of the difficulty in detecting periodic discharges of poilutants, no specific assessment of pollution loads has been attempted. Frequently in such situations perturbations in faunal distributions have been used to assess the effects of organic or toxic pollution (Filice, 1954, 1958, 1959; Hynes, 1960; Alderdice, 1966; Brinkhurst & Jamieson, 1971; Warren, 1971; Swartz et al., 1976, 1982; Pearson & Rosenberg, 1978; Bellan-Santini, 1980).

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The Black River entering Table Bay about 3.5 km north of Cape Town (Fig. 1) is a narrow river with a relatively constant flow 0?=68 750 m 3 day-l) (Banks et al. 1971). The river is periodically polluted by effluents from industrial and residential areas in and around Cape Town. Pollutants have included hexane and associated compounds, oils and paraffins from industrial sources and algicides and herbicides used in vegetation control along the river (Basset & Prentice, Cape Town City Engineer's Department, pers. comm.). The major portion of the river is characterized by low species diversity with high densities of tubificid oligochaetes (Campbell 1972) indicating organic pollution (Brinkhurst & Jamieson, 1971). At the mouths of many tributaries entering the Black River, and at outlets of many industrial effluent pipes, both the species diversity and tubificid numbers are reduced or absent. Chemical engineers concluded, on the basis of dissolved oxygen (DO), chemical oxygen demand (COD) and NH 3 concentrations, that the river was polluted down to about 2000 m from the mouth (Banks et al., 1971) but thereafter 'recovered'. Campbell

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PAARDENEILAND (IndustrialArea)

~

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Fig. 1 Location and drainage of the Black River and its tributaries (inset) and the location of transects in the river mouth and adjacent beach of Table Bay.

69

Marine Pollution Bulletin

oxygen was determined by the Winkler method and oxygen saturation levels were calculated from the temperature-salinity measurements at each transect (Gilbert et al., 1968).

25

Tolerance experiments _2 ~5 .a

10

TRANSECT NUMBER

(Relative Position)

Fig. 2 Salinity means (+S.D.) and ranges (n=7) at transects in the Black River mouth (1-3) and north along the beach of Table Bay (4-9).

This paper describes and relates infauna distribution and physicochemical conditions in the mouth of the Black River and along its adjacent beach of Table Bay. In addition, multilevel tolerance studies were undertaken with the dominant species, Cerebratulusfuscus, to determine the effect of the temperature, salinity and additional stress due to river pollution on their survival and distribution.

Methods and Materials Sampling Three transects were located across the mouth of the river where estuarine conditions prevailed: immediately across the mouth and 150 and 300 m upstream (Fig. 1). Six beach transects located north of the river (at 6, 65, 170, 500, 1250 and 4000 m north) were chosen to include similar beach slopes and obvious contact with the discoloured river water. Sampling stations were selected within each transect at 16 m intervals, and each transect was divided into two zones with respect to mean sea level 0VISL): zone A, MSL to - 0 . 5 m; zone B, - 0 . 5 to --1.0 m. Sparse fauna was encountered above MSL at all transects. Fauna were sampled at each station during June, July and August, to a depth of 30 cm, with five cores, each 15.5 cm in diameter similar to that described by Wells (1971) and the average density of each species per zone was calculated for each transect. Preliminary sampling showed core size and number was large enough to overcome patchiness (Wells, 1971). Sediment cores (3 cm diam., 30 cm deep) were taken adjacent to the faunal cores and analysed to determine grain size (Folk, 1966). Preliminary measurements of interstitial conditions showed porewater to be within 2-3% of surface water at a falling tide at each transect. Salinity, temperature and D O were therefore sampled at each transect site just above the sediment surface at high and low, spring and neap tides. Salinity was determined by the Knudson method (Strickland & Parsons, 1968) and calculated to 0.1%o according to Weyl's (1970) formula. Dissolved 70

Tolerance of the dominant species, Cerebratulus fuscus, a nemertean worm, to combinations of temperatures (10", 15" and 20"C) and salinity (0, 10, 20, 30 and 3 5 ° o ) was determined experimentally under controlled laboratory conditions to relate to its distribution and tolerance to the major physicochemical features in the field. To identify additional pollution-induced stress tolerance experiments using seawater diluted with distilled water were compared to those using seawater diluted with river water. A total of 315 animals was collected outside the study area. They were maintained unfed in clean aerated seawater for 52 h prior to experimentation and allowed to void their gut. Groups of 15 animals were placed in aerated normal seawater (35%o) in 500 ml beakers, placed in water baths, and brought to and maintained at the appropriate temperature (+0.2"C) and salinity over a 2 h period. All animals were examined for detrimental effects every 60-90 rain. Dead and moribund animals were removed and tested for sensitivity to a drop of 0.1% formaldehyde. Lack of response indicated death, and no animals so classified recovered after being placed in clean seawater. A control group of animals, subjected to the formaldehyde sensitivity test each time an experimental animal was tested, showed no adverse effects to the treatment during the whole experimental period. Tolerance data were analysed in two ways. Linear regressions were fitted to each mortality curve for each combination after the first death in each combination. These regressions were then compared with all others (within, and between distilled water and river water dilutions) for similarity in intercepts and slope (Ostle & Mensing, 1975) in order to compare ratios of mortality in each condition. The tolerance data were also analysed using a stepwise multiple regression model (Box &

19 18

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17

I 4

TRANSECT NUMBER

1 6

I 7

t 8

i 9

(Relative Position)

Fig. 3 Water temperature means (+S.D.) and ranges (n=7) at transects in the Black River mouth (1-3) and north along the beach of Table Bay (4-9).

Volume 16/Number 2/February 1985

Youle, 1955) to allow estimation and partitioning of both the linear and quadratic effects of temperature, salinity, and the interactions of temperature and salinity, over time. This model was then used with computer-generated coefficients, produced from the survival data, to model mortality contours for experimental temperaturesalinity combinations. Tests on the level of significance of the regression coefficient were carried out on the largest contributions to mortality at the 0.05 and 0.01 levels of significance. The experimental design permitted statistical comparisons between the significant contributors to mortality and the total contribution of all bterms for both distilled water and river water dilutions, thus separating the effects of river water constituents from the effects of temperature and salinity per se.

Results Physical conditions Variations in salinity within transects and with distance from the mouth of the river are given in Fig. 2. Lowest salinities were found in the river mouth (0.8%0 at LWS), and the highest at transects 7, 8 and 9 (34.5%o), the latter being similar to that found offshore in Table Bay (32.0-34.5%0) (van Ieperen, 1971). The largest ranges were found at the mouth and the immediately adjacent beach (transects 3 and 4) where the greatest mixing of river and marine waters occurred. The average salinity increased with distance from the river mouth north along the beach to transect 7, thereafter remaining constant. Temperature decreased with distance from the river (Fig. 3). Water temperatures at transects 8 and 9 were the least variable, irrespective of time and tide, and were similar to temperatures of offshore water (van Ieperen, 1971). The three stations in the river mouth showed reduced oxygen concentrations during low tide which increased with wave action during high tide (Fig. 4). Along the open beach DO was supersaturated at all tides. Sediment in the area was characterized by fine sand ()2 particle size range---2.68o [0.156 ram] to 2.21o [0.216 mm]), with the largest sediment near the river, the smallest in the river mouth and at transect 9 and no statistical difference between transects. Two kilometres upstream from the mouth, where the river is not canalized, sediments were characterized by soft black (anoxic) mud.

11

10

i L

1

2

i

i

i

i

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i

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3

4

5

6

7

8

9

TRANSECT NUMBER (Relative Position)

Fig. 4 Measured dissolved oxygen (open symbols) and calculated oxygen saturation levels (solid symbols) in the Black River mouth (1-3) and north along the beach of Table Bay at LWS (triangles) and HWS (circles).

relatively low numbers of animals (apparently due to sediment erosion in this local area during the study), these data were omitted from further analysis. Species diversity (Margalef, 1958)t at each transect, differed (Fig. 5). The diversity in Zone A increased as a linear function of distance (p=0.0001, R2--0.997) and in Zone B as a quadratic function (p--0.03, R2--0.969), indicating a consistently lower diversity at higher tidal elevations. The density of the dominent species, C. fuscus, at each transect was bimodally distributed along the beach, peaking at transects 6 and 9 (Table 1). Tolerance experiments The linear regressions (Fig. 7), fitted from the time of first mortality, approximate the rate of mortality at each dilution and temperature. All regressions were significant

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Faunal distribution The distribution of the dominant macrofauna found along the beach of Table Bay is summarized in Table 1. No organisms were found in any of the samples taken from the river mouth (Transects 1, 2 and 3) and despite numerous random sampling only one dead tubificid (presumably washed downstream) was discovered. Except for transect 9, the number of species and population densities generally increased with distance from the river mouth. In addition, within each transect there were generally greater numbers of animals in the lower tidal elevations (Zone B). Since transect 9 exhibited

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Fig. 5 Relationship of species diversity and distance from the river mouth for each transect (ll A), for Zone A ( I . . . . I) and Zone B ( 0 . . . . 0). Data from 4 km are not included in regressions, see text for explanation.

_ S-I tSpecies diversity (0~)--i**~N, where S=number of species and N=number of individuals (per m-').

71

Marine Pollution Bulletin

TABLE 1 Species and number of organisms/m: (calculated from five cores of 15.5 cm diameter sunk to a depth of 30 cm), in two zones (Zone A, MSL to - 0 . 5 m and Zone B, - 0 . 5 to - 1 . 0 m) along six beach transects north of the Black River m o u t h ( M S L W is 0.808 m below MSL) Transect No. Zone Species

4 A

Distance from river (m)

Polychaeta Sigalion capense Glycera bengeullana Nephtys capensis Lumbrineris tetraura Scolelepis squamata Dispiomagna Nemertea Cerebratulusfuscus Gastropoda Bullia digitalis Mysidacea Gastrosuccus psammodytes Isopoda Eurydice longicornis

5 B

A

6

6 B

A

65

A

170

8 B

9

A

500

B

A

1250

B 4000

0 0 0 0 0 0

0 0 23 0 0 0

0 0 0 0 11 0

0 0 11 0 0 0

0 0 6 0 23 0

0 0 0 0 11 0

0 0 0 23 0 0

0 6 6 28 0 28

0 0 23 0 23 17

11 0 17 6 0 177

0 11 0 0 0 0

0 0 6 0 0 11

11

57

11

187

238

0

34

23

40

68

170

124

11

23

0

11

0

6

6

17

0

62

0

11

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0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

11

0

0

79

0

0

11

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1°°c

(p <0.001). In control groups maintained at 35%0 for 4 days no mortality occurred at any temperature. All animals held in pure river water (RW) or distilled water (DW) died within 30-60 min., irrespective of temperature. The results of all possible comparisons for these mortality curves (comparisons of slopes and intercepts) indicate that all but four rates of mortality between RW and DW dilutions differ significantly (p <0.05). While mortality rate does not differ between DW 10*/10%o and RW 10°/10%o, DW 15*/10%o and RW 10"/10°/o0, DW 10*/30%o and RW 10*/30% o, and DW 15*/30%o and RW 20°/30%0, mortality rates are generally higher in river water dilutions than in distilled water dilutions (Figs 6 and 7). Within the RW dilutions and DW dilutions only the mortality rates of, respectively, RW 20*/10%o and RW 15°/10%o, and DW 20*/10%o and DW 15"/10%o were similar. The differences in mortality rates within each of these data sets are indicative of increasing survivability at lower temperatures and higher salinities. In DW dilutions the range of temperature-salinity conditions for total survival decreases with increased exposure and after an extended period (74 h) survival is limited to the lower temperatures and higher salinities (Fig. 8). A similar effect is seen with the RW dilutions but the progression of reduced survivability with exposure time is more advanced than in DW dilutions. The relative contribution of linear, quadratic and interaction effects of temperature and salinity to the mortality of C. fuscus was assessed by fitting the data presented in Figs 6 and 7 at 6, 10, 24, 46, 58 and 74 h to the model of Box & Youle (1955) by multiple regression analysis (Table 3). Up to 24 h, the linear and quadratic effects of salinity contributed significantly more to mortality in DW and RW than temperature or the interaction of temperature and salinity. At 58 and 74 h, temperature contributed, albeit not significantly, to the mortality of animals in DW. A similar response to temperature began 12 h earlier with the animals in RW. In addition, the total contribution of all variables in the model (total R Evalues; 72

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Fig. 6 Mortality rates of C. fuscus in combinations of temperature and salinity using distilled water dilutions.

Table 3) decreases with time in the RW dilutions while in DW dilutions the total contribution of these variables remained near 0.9. This suggests that additional variables, not present in the distilled water dilutions, contributed to mortality of animals exposed to fiver water. Discussion

A harsh environment exists in the mouth of the Black River with tidally induced salinity and temperature fluctuations unlike those found in normal estuaries (Caspers, 1967; Pilchard, 1967; Day, 1981). Canalization of the fiver mouth has prevented expansion into a more typical estuary in which gentle mixing of marine and river water could occur. The river flow and currents of Table Bay have therefore created estuarine-like conditions along the open beach north of the fiver. After about 1 km north the waters are typically more marine and similar to those

Volume 16/Number 2/February 1985

found in Table Bay (van Ieperen, 1971) and other areas of the Cape. Sediment characteristics were similar at all transects. Although DO was below saturation in the fiver mouth, wave action supersaturated the water immediately it left the confines of the mouth. The density and diversity of the infauna were most closely related to the stability of temperature and salinity conditions that increased with distance from the river. Although the assemblage of infauna in the study area was similar to other areas in the Cape (Day, 1970; Brown, 1971; Scarfe & Thum, unpublished data) their densities were substantially lower. In addition, no benthic or hard substrate fauna was found in the sediments or on canal walls and rocks in the river mouth and for several hundred metres upstream, despite additional random sampling. It therefore appeared that organic or toxic wastes periodically carried in the river (Banks et al., 1971; Campbell, 1972; Basset & Prentice, pers. comm.) further limited or depressed the biotic communities in the river and adjacent marine beach. The greater density of C. fuscus, particularly near the river indicated that the species is the most tolerant to the local conditions. Although C fuscus is known to be a predator of polychaetes (Wheeler, 1940; Day, 1969), its distribution was very similar to the distribution of indicator species (Hynes, 1960; Brinkhurst & Jamieson, 1971; Warren, 1971; Filice, 1954, 1958, 1959) found in other areas of the world. In such areas, many of which have been reviewed by Pearson & Rosenberg (1978), the distributional abundance of opportunistic species increase rapidly to peak maximally near a source of organic input, and thereafter progressively decreases with distance from the source. Tolerance studies of C. fuscus to temperature/salinity/ river water combinations provided insight not only into survival in local conditions but also into additional

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Fig. 7 Mortality rates of C. fuscus in combinations of temperature and salinity using river water dilutions.

effects of river pollution loads. These results (Fig. 6, Table 2) demonstrated salinity to be the predominant factor controlling mortality, while temperature contributed to mortality only after 46 h of exposure. Lower temperature enhanced survivability at all salinities, indicating a greater tolerance to these conditions during winter than summer. Mortality of C. fuscus was significantly greater in fiver water dilutions of seawater at all temperature/salinity combinations than in distilled water dilutions. In addition, the contribution of linear, quadratic and interaction effects of temperature and salinity (R e values; Table 2) were progressively less over time in fiver water dilutions than in distilled water dilutions, indicating that some factor contained in river water, in addition to temperature and salinity, influenced survival (see Alderdice,

TABLE 2 Predominant contribution of temperature (T) and salinity (S) to the mortality of Cerebratulusfuscuscalculated using the Box & Youle (1955) model (see text). Proportional contribution (R 2) is cumulative Time (h) 6

10

24

46

58

74

Variable

Distilled water dilutions R-"

F

Variable

River water dilutions R2

S S2 Total S S2 Total

0.6483 0.9198 0.9550 0.6743 0.9566 0.9669

18.44* 30.46*

S S2 Total S S2 Total

0.6659 0.9447 0.9513 0.6430 0.9123 0.9527

19.93* 45.34*

S S2 Total S S2 Total S T Total Sz T Total

0.6995 0.9552 0.9623 0.7177 0.8288 0.8822 0.8338 0.8751 0.8978 0.8370 0.8778 0.9005

23.27* 51.39"

S S2 Total S T Total S~ T Total S2 T Total

0.7127 0.8550 0.8736 0.7389 0.8235 0.8502 0.6385 0.7525 0.7761 0.6055 0.6325 0.7210

24.81" 16.85"

20.70* 58.50*

25.43* 5.84(ns) 50.19" 2.97(ns) 28.02* 1.65(ns)

F

18.01" 27.62*

28.30* 4.32(ns) 46.65* 4.16(ns) 27.46* 3.04(ns)

*p "< 0.001; ns=not significant.

73

Marine Pollution Bulletin RW6h

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Fig, 8 Estimation of the mortality of Cerebratulus fuscus exposed to constant temperature and salinity combinations for 6, 24 and 74 h. Isopleths represent percentage mortality of individuals in distilled (DW) or river water (RW) dilutions and are calculated using a two-factor second-order response surface model and survival data for 16 temperature-salinity combinations.

1966; Alderdice & Forrester, 1968, for discussion of unexplained variance in response regression models). Mortality curves (Fig. 8) of C. fuscus predicted that the animals could survive field conditions 65 m from the fiver within each 6 h tide cycle, providing river water was not present. C. fuscus densities initially peak at 170 m (Table 2) and the highest species diversity was encountered 1200 m from the fiver. River water therefore further stresses the animals preventing their survival closer to the river. Low temperatures encountered closer than 65 m ameliorate the conditions to allow the survival of the relatively low densities of C. fuscus in this region. Failure to survive sustained conditions (i.e. 74 h or longer) in the area indicates that the tide assists survival by providing better conditions every 6 h. Fluctuating conditions which simulate field conditions more closely are not usually considered in tolerance studies (see Kinne, 1970; Alderdice, 1972), but in order to predict the real tolerance to existing field conditions they could be more closely considered with methods similar to those

used in multifactorial adaptation studies (e.g. Alderdice, 1972). 74

We thank Dr J. H. Day (Zoology Dept, UCT) for assistance in polychaete identification; Ms Patricia Preston (Civil Engineering Dept, UCT) for some chemical analysis; Mr I. MacDonald and Mr van der Smit (Computer Sciences Center, UCT) for computer software assistance; Dr A. O. Fuller (Geology Dept, UCT) for discussion on particle size analysis and G. Basson and J. Booysen for assistance in the field. Drs Jim Matis (Inst. of Statistics, Texas A&M Univ.) and Jim Heltshe (Dept of Experimental Statistics, Univ. of Rhode Island) assisted with statistical designs and Drs Herman Kleerekoper and Merrill Sweet (Biology Dept, Texas A&M Univ.) commented on early drafts of this paper.

Alderdice, D. F. (1966). The detection and measurement of water pollution. Biological assays. In Pollution and Our Environment. A conference held in Montreal from 31 October to 4 November 1966. Canadian Council of Resource Ministers, Ottawa. Vol. 3 (Paper D251-3) (also in Can. Fish. Rep., 9, 33-39, 1967). Alderdice, D. F. (1972). Environmental factors: Factor combinations-Responses of marine poikilotherms to environmental factors acting in concert. In Marine Ecology, (O. Kinne, ed.) Vol. 1, pp. 1659-1722. Alderdice, D. F. & Forrester, C. IL (1968). Some effects of salinity and temperature on early development and survival of the English sole (Parophrys vetulus). J. Ftsh. Res. Bd Can., 25, 495-521. Ballan-Santini, D.. (1980). Relationship between populations of amphipods and pollution. Mar. Pollut. Bull., 11,224-227. Banks, D. G., Farrugia, R. A. & McNamara, R. (1971). Pollution survey of the Black River Complex. Technical Report, Department of Civil Engineering, University of Cape Town.

Volume 16/Number 2/February 1985 Box, G. E. P. & Youle, P. V. (1955). The exploration and exploitation of response surface; an example of the link between the field surface and the basic mechanism of the system. Biometrica, 11,297-323. Brinkhurst, R. O. & Jamieson, B. G. M. (1971 ). Aquatic Oligochaeta of the World. Oliver & Boyd. Edinburgh. Brown, A. C. (1971). The ecology of the sandy beaches of the Cape Peninsula, South Africa. Part 1:Introduction. Trans. R. Soc. S. Afr., 39, 247-279. Campbell, B. (1972). An ecological survey of the Black and Vygieskraal river complex with special reference to the effects of pollution. Technical Report, Zoology Department, University of Cape Town. Caspers, H. (1967). Estuaries: Analysis of definition and biological considerations. In Estuaries, (G. H. Lauff, ed.), pp. 6-8. Amer. Ass. for the Adv. of Sci., Washington, Pub. 83. Day, J. H. (1969). A Guide to Marine Life on South African Shores. A. A. Balkema, Cape Town. Day, J. H. (1970). The biology of False Bay, South Africa. Trans. R. Soc. S. Afr., 30, 211-221. Day, J. H. (ed.) (1981). Estuarine Ecology--with Particular Reference to Southern Africa. A. A. Balkema, Rotterdam. Filice, E P. (1954). A study of some factors affecting the bottom fauna of a portion of the San Francisco Bay estuary. Wasmann J. Biol., 12,257292. Filice, E P. (1958). Invertebrates from the estuarine portion of San Francisco Bay and some factors influencing their distribution. WasrnannJ. Biol., 16, 159-211. Filice, E P. (1959). The effects of wastes on the distribution of bottom invertebrates in the San Francisco Bay estuary. Wasmann J. Biol., 17, 1-17. Folk, R. L. (1966). A review of grain size parameters. Sedirnentology, 6, 73. Gilbert, W., Pawley, W. & Park, K. (1968). Carpenter's oxygen solubility tables and nomograph for seawater as a funtion of temperature and salinity. Technical Data Report, No. 29, Dept. Oceanography, Oregon State University, Corvallis, Oregon.

Hynes, H. B. N. (1960). The Biology of Polluted Waters. Liverpool Press. Liverpool. Kinne, O. (1970). Environmental factors: Temperature. In Marine Ecology, (O. Kinne, ed.), Vol. 1, pp. 321-616. Margalef, R. (1958). Information theory in ecology. Gen. Syst., 3, 36-74. Ostle, B. & Mensing, R. W. (1975). Statistics in Research. Iowa State University Press, Ames. Pearson, T. H. & Rosenberg, R. (1978). Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanogr. Mar. Biol. A. Re~., 16,229-311. Pritchard, D. W. (1967). What is an estuary: physical viewpoint, in Estuaries (G. H. Lauff, ed.) pp. 3-5. Amer. Assoc. for the Adv. of Sci., Washington, Pub. 83. Strickland, J. D. H. & Parsons, T. R. (1968). A practical handbook of seawater analysis. Bull. Fish. Res. Bd Can., 167, 311. Swartz, R. C., Walker, J. D., DeBen, W. A. & Cole, E A. (1976). Structural analysis of stressed marine communities. In Water Quality Research of the U.S. Environmental Protection Agency, USEPA Ecological Research Series EPA-600/3-76-079. Swartz, R. C., Deben, W. A., Sercu, K. A. & Lamberson, J. O. (1982). Sediment toxicity and distribution of amphipods in Commencement Bay, Washington, USA. Mar. Pollut. Bull., 13, 359-364. van leperen, M. P. (1971). Hydrology of Table Bay. Ph.D. thesis. Department of Oceanography, University of Cape Town. Warren, C. E. (1971). Biology and Water Pollution Control. W. B. Saunders, Philadelphia. Wells, J. B. J. (1971). A brief review of methods of sampling the meiobenthos. In Proceedings of the First International Conference of Meiofauna (Hulings, N. C., ed.), pp. 183-186. Smithsonian contributions of Zoology. U.S. Gov. Printing Office, Washington. Weyl, P. R. (1970). Oceanography: An Introduction to the Marine Environment. John Wiley, New York. Wheeler, J. F. (1940). Some nemerteans from South Africa and a note on Lineus corrugatus Mclntosh. J. Linn. Soc. Lond. (ZooL ), 41, 20-49.

(X|25-326X/85 $3.00-t41.00 O 1985 Pergamon Press Ltd.

Marine Pollution Bulletin, Vol. 16, No. 2, pp. 75-78, 1985. Printed in Great Britain

Bottom Layer Anoxia in the Central Part of the Gulf of Trieste in the Late Summer of 1983 J. FAGANELI, A. AVCIN, N. FANUKO, A. M A L E J, V. TURK, P. TU~NIK, B. VRI~ER and A. VUKOVIC Marine Research Centre, 66 330 Piran, Yugoslavia Anoxie conditions in the near-bottom layer below the pycnocline were observed in September 1983 causing mass mortality of benthic macrofauna in the central part of the Gulf of Trieste. The vertical transport of particulate organic matter and decomposition of abundant pelagic and benthic organic matter during the summer produced a low oxygen level in the near-bottom layer below the pycnocline when this layer was sufficiently close to the bottom. A high sea water temperature and vertical stability contributed to the development of anoxic conditions in September 1983 in the near-bottom layer.

In September 1983 anoxic conditions in the near-bottom (approx. 2 m above the sediment) layer below the pycnocline were observed, causing mass mortality of benthic macrofauna in the central part of the Gulf of Trieste (Northern Adriatic). Similar events were also observed in

the late summer of 1974 and 1980, although the affected area was smaller. The purpose of this report is to elucidate the causes and to discuss the consequences of anoxia. M a t e r i a l s and M e t h o d s

Sea-water samples for chemical and phytoplankton analyses were collected approximately weekly during the summer of 1983 from four depths (0.5, 5, 10, 20 m) at the fixed sampling point (maximal depth of 22 m) about 1 nautical mile offshore of Piran (Fig. 1). In September 1983 chemical, phyto-, bacterio- and zooplankton samples were collected below and above the pycnocline. For chemical and phyto- and zooplankton analyses the standard methods (Grasshoff, 1976; UNESCO, 1966, 1976) were used; for bacteriological analyses the pourplate method on marine agar 2216 E (Oppenheimer & 75