Nutrients Distribution Patterns in Tudor Estuary (Mombasa, Kenya) During Rainy Season J.M

Kenya Journal of Sciences Series B (1989) 10(1-2): 47 61 Nutrients distribution patterns in Tudor estuary (Mombasa, Kenya) during rainy season J.M. Kazungu Kenya Marine and Fisheries Research Institute P.O. Box 81651, Mombasa, Kenya

Frank Dehairs and Leo Goeyens Lab. of Analytical Chemistry Free University of Brussels Pleinlaan 2 1050 Brussels, Belgium

SUMMARY

Nutrients distribution pattern for the Tudor Creek, covering the period April, May, June and July 1986 was investigated. At the beginning of the rainy season (April), a salinity gradient with low values upstream and high values towards the open sea was evident -- giving the creek estuarine characteristics. After the rainy season, salinity values were more-or-less uniform throughout the creek. The low salinity waters near the river mouths had high nutrient concentrations which decreased gradually towards the open sea. The highest nutrient concentrations were recorded in May which also corresponded with the rainfall peak. Though river runoff is shown to be the main source of nutrients in Tudor estuary, the Coast General Hospital's sewage system (CGHSS) is also identified as a possible local source of nutrient supply into the estuary. However, its effect is not noticed during the peak of the rainy season due to the rich nutrient waters from rivers upstream. Conservative mixing of phosphates and nitrates is shown to persist in May. However, after the heavy rains, non-conservative mixing is noticed mainly due to the reduced freshwater nutrient input and nutrient loading by the CGHSS.

INTRODUCTION phytoplankton growth. Availability of these elements therefore plays an A nutrient element is defined as one essential role in controlling primary which is functionally involved in the productivity. processes of living organism. Though Tudor Creek (fig. 1), which is the term has been applied almost approximately 20 km long forms the exclusively to silicon, phosphorus and north-eastern boundary of the inorganic nitrogen, strictly speaking, Mombasa island. The characteristics other major constituents of sea water, of the creek are such that during the together with many essential trace rainy season, a salinity gradient metals are also nutrient elements. develops due to river drainage system Silicon, phosphorous and inorganic upstream. The creek then acquires nitrogen occur in sea water in low estuarine characteristics. However, concentrations and at times are known immediately after the rainy season, to act as limiting factors for salinities become more or less uniform

© The Kenya National Academy of Sciences 48 J.M. Kazungu, Frank Dehairs and Leo Goeyens throughout the creek due to reduced project on the identification of various freshwater input from the rivers. water types within the Tudor and Because of these seasonal changes, Kilindini estuaries. different water types are likely to exist within the estuary. Revis and MATERIALS AND METHODS Okemwa (1988); report that certain copepod species are common in certain Figure 1 shows sampling stations of areas of the estuary and not in others. the Tudor estuary. Station AÍ This could be evidence of the existence represents the open sea water, while of different water types within the station A6 is the farthest station estuary. Though a lot of work has upstream-near the river mouth. been done on Zooplankton distribution Water samples were chemically in the creek (Kimaro 1987; Revis and fixed immediately after collection Okemwa (1988); Reay and Kimaro and stored in ice boxes at temperature 1984) very little is known about below 5°C. On return to the nutrients distribution therein. laboratory, phosphate and silicate Norconsult (1975) is the only samples were analysed immediately, available report on nutrients while nitrate samples were deep distribution in Tudor Creek. However, frozen and analysed the following the report is not comprehensive since day. Nitrate samples were collected all their nutrient sampling stations in one liter bottles and fixed with 1 ml were close to the open sea and did not mercuric chloride (50g/1000 ml) before extend deeper into the creek. The being put into the ice box. Samples for land surrounding the rivers and the phosphate and silicate tributaries that feed Tudor creek are determinations were collected in one intensively cultivated for liter plastic bottles and fixed with 5 agricultural production and m l chloroform. Salinity substantial amount of fertilizer is determinations were done using the used in this area during the long rainy Knudsen titration method where the season. In 1986 about 2850 kg of a water sample is titrated against fertilizer commonly known as N:P:K:, standardised silver nitrate solution 20:20:0: (containing 20% nitrogen, 20% (Strickland and Parsons 1968). phosphorus, 0% potassium) was used. Quantitative detrmination of Also used was about 710 kg of triple nitrates, phosphates and silicates super phosphates (TSP) which was done spectrophotometrically as contains approximately 46% described by Parsons, Maita and Lalli phosphates (Mr O. Adede, Kilifi (1984). A Shimadzu UV-150-02 District Agricultural Officer, Double-Beam spectrophotometer was personal communication). Some of the used for the absorption measurements. fertilizer used is washed into the All surface samples were collected rivers during the rainy season and at about 30 cm below the water surface ultimately find their way into the during low tides on board a small creeks. Effects of such inputs on the pelagic communities of the receiving shallow boat of about 7 m long. creek waters are little known and Samples at 5 m depth and below were represent one of our long-term research collected using a Nansen water goals. sampler. However, station A4 and A5 The main objective of the present were so shallow (less than 5 m depth) paper is to report on the influence of and so only surface samples were river drainange on the nutrients distribution within Tudor Creek. This analysed for these two stations. is part of an ongoing Kenya/Belgium Nutrients distribution patterns in Tudor Creek 49

S J U D O R CR&EK

COAST GENERAL / HOSP

MOMBASA PORT REITZ ISLAND

Figure 1. Sampling stations (A1-A6) of the Tudor cstuar) 50 J.M. Kazungu, Frank Dehairs and Leo Goeyens

RESULTS AND DISCUSSION rivers. A third likely source of nitrate supply into Tudor Creek is the Results of the nutrients (nitrate, bacterial remineralisation activity at silicate and phosphate) and salinity the surface of the bottom sediment. In many estuaries, bacterial distribution analyses for the Tudor activity decompose organic litter of estuary are presented in tables 1 to 4 bottom sediments (Selmer 1988; and their corresponding profiles are Caperon et al 1979) releasing presented in figures 2 to 5. River runoff inorganic nutrients into the water during the rainy season has a column. At the beginning of the rainy profound effect on the nutrients season (April), a lot of organic matter from the mangrove trees surrounding distribution pattern in Tudor estuary Tudor Creek are washed into the (figures 2-5). creek. Some of this organic litter sinks to the bottom and possibility of re Nitrate distribution mineralisation by bacterial activity is high. Though no conclusive Figure 2 shows the nitrate evidence is presented here, tables 3 distribution within the estuary. At and 4 show that nutrients the start of the rainy season in April concentration of the bottom samples of (figure 8) high nitrate values were station A5 were higher than those of observed at station A5 and A6 than at the surface indicating the possibility any other station within the estuary. of nutrients supply from the bottom. Nitrate concentrations reduced However, more nutrients regeneration gradually downstream and the lowest experiments need to be conducted to concentrations were recorded at confirm this source. Stations A4 and station AÍ. The highest nitrate A6 were very shallow (< 3 meters concentrations for this period was depth) and so no bottom samples were about 9.4 pg-at N /l recorded at taken. station A6, while the lowest was Though river runoff has been about 0.2 pg-at N /l recorded at shown to supply high nutrient waters station AÍ. into the estuary during the rainy During the peak of the long rains in season, Tudor estuary should still be May (figure 8) nitrate concentrations referred to as "low nutrient estuary". were higher than for the previous Low nutrient estuaries are categorised month. Again the distribution trend as those in which nitrate values in was similar to that of April with the the freshwater end range from about highest value (22.6 pg-at N/l) 10 to 40 pg-at N /l and decrease recorded at stations A6 and the lowest linearly to about zero in the full salt (2.75 pg-at N/l) recorded at station end of the estuary (Sharp 1983). AÍ. In June and July, after the rains Other examples of low nutrient had stopped, nitrate concentrations tropical estuaries are the Zaire River fell so low (<_ 1.0 p g - a t N /l) estuary (Van Bennekom et al 1978) throughout the estuary. The two and the Magdelena River in South small peaks recorded at station A2 America (Fanning and Maynard 1978). during these two months are most High nutrient estuaries usually have probably due to the sewage system very high nitrate values at the from the Coast General Hospital. freshwater end. A typical example of During the months of April and May, this is the Tamar estuary in England the sewage effect is overshadowed by whose nitrate values at the the high nutrient waters from the freshwater end ranges between 100 Nutrients distribution patterns in Tudor Creek 51

Table 1. Nitrate, phosphate, silicate, salinity and standard deviation readings for the April (22/04/86) sampling in Tudor estuary

Station NO-3 S.D. p o 3-4 S.D. Si S.D. S alinity p g-at N/1 p g -at P/1 p g -at Si/1 S%o A l(0m ) 0 .1 5 0 .0 3 0 .3 0 0 .0 3 4 .0 0 0.19 36.26 (10m) 0 .3 7 0 .0 3 0 .5 0 0.02 3 .5 0 0.11 36.26 A2(0m) 1.60 0 .0 7 0 .7 0 0.0 3 5 .0 0 0.21 36.26 (10m) 1.38 0 .0 6 0 .7 0 0.0 3 5 .1 3 0.20 36.15 A3 (0m) 3 .1 0 0.02 0 .7 0 0.02 1 3 .4 0 0.50 34.9 *( 10m) 2 .7 7 0 .0 5 0.51 0 .0 9 13.01 0.47 34.21 A4 (Óm) 3 .4 0 0 .0 5 0 .9 5 0 .0 7 20.00 0.47 3225 A5(0m) 9 .2 5 0 .0 8 1.20 0 .0 5 7 0 .0 0 0.63 12.41 •(5m) . 0 .9 5 0.0 9 - - A6(0m) 9 .2 5 0 .1 7 1.51 0.11 7 1 .5 0 0.47 1.11

Table 2. Nitrate, phosphate, silicate, salinity and standard deviation readings for the May (22/05/86) sampling in Tudor estuary

Station n o -3 S.D. p o *-4 S.D. Si S.D. S alinity p g -at N/1 p q -a t P/1 p q -at Si/1 S%o A Í (Dm) 2 .8 0 0.10 0 .3 6 0.01 4 0 .0 0 0.57 26.00 (10m) 2 .6 9 0 .0 5 0 .3 3 0 .0 4 4 0 .2 1 0.31 26.31 A2(0m) 5 .2 0 0 .0 9 0 .6 4 0.0 3 6 5 .5 0 0.33 25.07 00m) . - . . - - A3 (0m) 9 .3 0 0 .1 4 0.68 0 .0 5 1 1 6 .0 0 0.47 18.53 •(10m) 8 .7 2 0 .1 9 0.68 0 .0 4 1 0 8 .3 7 0.47 18.81 A4(0m) 1 7 .8 0 0.2 3 1.2 7 0.11 1 6 3 .5 0 0.61 11.29 A5(0m) 2 2 .5 0 0.22 1.6 4 0 .0 8 1 8 6 .0 0 0.55 5.66 *(5m) 2 2 .5 0 0.20 1.59 0 .1 3 1 8 5 .3 6 0.73 5.69 A6(0m) 2 8 .5 0 0 .0 9 2 .0 4 0 .0 9 1 8 0 .5 0 0.41 0.72

• Maximum depth

Table 3. Nitrate, phosphate, silicate, salinity and standard deviation readings for the June (24/06/86) sampling in Tudor estuary

Station n o -3 S.D. PO *4 S.D. Si S.D. S alin ity pg-at N/1 p g -at P/1 p g -at Si/1 S%o A Í (Dm) 0.15 0.02 0.22 0.02 4 .0 0 0.13 3636 (1Ctai) 0.20 0.02 0 .4 2 0.02 6.20 0.40 36.55 A2(0m) 0.72 0 .0 9 1.02 0 .0 4 7.9 0 031 36.18 (lCta) . . . 3636 A3 (0m) 0 .1 5 0.01 0 .4 9 0.10 1 3 .8 0 0.10 35.64 •(10m) 0 .6 0 0 .0 3 0 .4 9 0 .0 3 11.00 0.27 34.56 A4(0m) 0 .4 0 0 .0 9 0 .4 8 0 .0 7 2 4 .0 0 0.09 33.84 A5(0m) 0 .1 8 0.02 0 .4 2 0.01 2 6 .2 0 0.13 31.85 *(5m) 0.21 0.02 0 .6 2 0.02 2 6 .4 0 0.21 32.21 A6(0m) 0 .1 9 0 .0 3 0 .6 2 0 .0 6 5 8 .5 0 0.29 24.67 52 J.M. Kazungu, Frank Dehairs and Leo Goeyens

Table 4. Nitrate, phosphate, silicate, salinity and standard deviation readings for the July (22/07/86) sampling in Tudor estuary

Station n o -3 S.D. p o 3-4 S.D. S i S.D. S alinity ug-at N/1 ixg-at P/1 u g -a t Si/1 S%o A l(0m ) 0 .4 4 0 .0 9 0 .6 9 0.0 7 7 .4 6 021 35.64 (10m) . 0 .6 9 0.0 9 7 .3 8 0.18 36.00 A2(0m) 1.19 0.11 0 .5 5 0.0 3 8 .6 0 0.12 35.05 (10m) 1.19 0 .1 5 0 .9 2 0.0 7 13.01 0.12 35.64 A3 (0m) 0 .5 8 0 .0 8 0 .5 9 0.0 5 12.12 0.09 3528 •(10m) 0 .7 6 0 .1 3 1 .1 6 0.0 9 1 2 .8 5 0.14 35.64 A4 (Om) 0 .2 8 0 .0 3 1.02 0.11 17.51 0.17 35.64 A5 (0m) 0 .1 3 0 .0 3 0 .5 5 0.0 2 2 2 .3 3 022 34.92 •(5m) 0 .4 0 0 .0 4 1.06 0.0 8 2 4 .6 2 037 34.92 A6 (om) 0 0 0 .9 2 0.10 50.51 027 33.84

• Maximum depth and 350 pg-at N /l (Morris, Bale and about 71.0 pg-at Si/I at station A6, Howland 1981). while the lowest concentration (3.5 Though Tudor Creek is a low pg-at Si/1) was recorded at station nutrient estuary, the nutrient values AÍ. The May silicate profile shows a observed during the rainy season are very pronounced influence of the river higher than those of the open waters runoff into the estuary. Silicate of the western Indian Ocean. Smith concentrations of above 180 pg-at Si/I and Codisporti (1980) showed that were recorded at stations A5 and A6. nitrate values of the western Indian Station AÍ also had relatively Ocean open waters rarely exceed 5 higher values (Ca 22.0 pg-at Si/l) pg-at N/l. The popular upwelling than the previous month. In June and regions off the Somali coast have July, after the end of the rainy season, nitrate values of between 13.1 pg-at silicate concentration dropped very N / l and 19.9 pg-at N / l (Smith and sharply. Cordisporti 1980) which compares very well with nitrate values Phosphate distribution obtained at the upper end of Tudor Creek during the rainy season. Phosphate distribution pattern was Though Tudor estuary attains more or similar to that of silicate. Higher less same nutrient levels as the phosphate concentrations were upwelling regions off the Somali recorded at station A6 in April and coast, it might not be equally May, while station AÍ had the productive due to high turbidity lowest during this period (figure 4). (Okemwa, unpublished data). The High values of about 2.0 pg-at P/1 high turbidity is mainly due to the were recorded at station 6A in May, creek being very shallow (< 5 meters while in June and July concentrations depth towards the upper end) and were below 1.0 pg-at P/1. having a muddy bottom. The concentration of dissolved inorganic phosphate in the surface Silicate distribution waters of the oceans is variable, but over large areas the maximum Figure 3 shows silicate distribution concentrations are in the range of 0.5 along the Tudor Creek. Higher to 1.0 pg-at P/1 (Spencer 1975). silicate concentrations were observed Phosphate concentration of the open near the river mouth. However, at north Indian Ocean surface waters the beginning of the rainy season in rarely exceeds 0.5 pg-at P/1 (Ryther April, the highest concentration was 30. (H - APRIL (22/04/86) s s s i I Î 3 3 3 5 Ï (O (7 /-ON ~ N i D - 6-v/) - D i N ~ /-ON (7 " T “ O V uret dsrbto atrs n uo re 53 Creek Tudor in patterns distribution Nutrients T Ö o N o '< "< CO 2 CL o < _i z cn f- o z in

Figure 2. Nitrate distribution paterns in Tudor estuary for April, May, June, and July 1986 54 J.M. Kazungu, Frank Dehairs and leo Goeyens SAMPLING SAMPLING STATIONS Figure 3. Silicate distribution patterns in Tudor estuary (April July - 1986 Nutrients distribution patterns in Tudor55 Creek SAMPLING SAMPLING STATIONS Figure 4. Phosphate distribution patterns (April - July 1986 56 J.M. Kazungu, Frank Dehairs and Leo Goeyens

« s CD t o vT S CO co Q m CD ? O'! o 22 3 S e r >• c l < < Z —i=> —>3 T Î >- Lü XL

V) oz *- io o z -I Û. z < cn Figure 5. Salinity profiles for Tudor estu ary

1 o o o o o

09/.S AHNI! VS Nutrients distribution patterns in Tudor Creek 57

o .0

in >- z o —I '8 <

o -O

p o o (3/&N-N »D-ßv/j o ó o . . Nutrients versus salinity profiles for May in Tudor estuary 6 ■n- o Figure

N0llVdlN3DN03 SiN3ldiriN 58 J.M. Kazungu, Frank Dehairs and Leo Goeyens

< a. y a g — t X IOZO. ?' TI SALINITY SALINITY (S*/..) Figure 7. Nutrients versus salinity profiles tor juno in uaor I estu a ry

' » ! i ...... 20 0 250 300 35 0 40-0 o p (VIS ID-6^, 9 Ô o <£> r-i

o in (i&N-N »D-5^/) Ö o

(i/d \o-fyrf) u-> o o 0 0 5

N0I1VU1N3DN0D SlN3iainN APRIL MAY JUNE JULY — (ojuj) (ojuj) — ö c Ni a iv jN v n o CO uret dsrbto atrs n uo Cek 59 Creek Tudor in patterns distribution Nutrients CM o '987 51 '987

Figure 8. Rainfaall profile covering the period April to July 1986 60 J.M. Kazungu, Frank Dehairs and Leo Goeyens et al 1966). After the rainy season, CONCLUSION phosphate concentrations in Tudor estuary are found to oscillate between Nutrients distribution pattern for the 0.2 and 1.0 pg-at P/1 implying that Tudor estuary indicates a very strong the estuarine characteristic of Tudor influence of river discharge during the Creek disappears and the creek rainy season. The highest nutrients waters displays phosphate values concentrations were recorded in May typical of open coastal waters. The during the peak of the rainy season. high (approx. 2 pg-at P/1) phosphate As high as 186.0 pg-at Si/l, 22.6 pg- values found during the peak of the at N-NO/1 and 2.0 pg-at P/1 were rainfall are therefore due to the river recorded for silicate, nitrate and runoff. phosphate respectively during this month. All these high values were Salinity distribution observed near the river mouth and decreased gradually towards the open The salinity profiles for Tudor sea. After the month of May nutrient estuary (figure 5) confirms the fresh concentrations dropped sharply water influence into the creek. In except at station A2 which showed a April, the salinity profile showed a small peak; all nitrate and sharp gradient from station AÍ to A6. phosphate concentrations were more Whereas station AÍ had salinity of or less uniform throughout the about 36.2% typical of oceanic water, estuary. The peak at station A2 could salinity for station A6 was below 2%. be due to a local source of nutrients In May, salinities were relatively within the estuary. Since station A2 lower throughout the creek with the is close to the sewage drainage system lowest recorded at station A6 near the from the Coast General Hospital, it is river mouth. A comparison of these most probable that the peak is caused two salinity profiles with those of by the sewage effect. This sewage nitrate (figure 2) and silicate levels effect is not detected in May due to (figure 3), ‘shows that the high the rich nutrient waters from the concentrations of nitrate and silicate rivers upstream. at station A6 correspond to the low The high nutrient values salinity waters from the rivers. In registered during the rainy season can June, the salinity gradient decreased be attributed to agricultural considerably while in July, salinities activities and use of fertilizer in the throughout the creek were more or less area surrounding the river bank. uniform. Though relatively bigh Figures 6 and 7 show profiles of concentrations of nutrients w ere nutrients against salinity for the registered during this season, Tudor months of May and June respectively. estuary is essentially a low nutrient Whereas the phosphate and nitrates estuary (Sharp 1983). The linear drop peaks are clearly observed at station of phosphate and nitrate A2 in June, they are not observed in concentrations observed in May (figure May. These peaks are most probably 6) indicates conservative mixing (Liss due to the sewage drainage system 1976) during the rainfall peak. In (near station A2) from the Coast June, after the heavy rains non General Hospital. The peaks are not conservative mixing is noticed mainly observed in May due to a large river due to the reduced freshwater nutrient runoff into the estuary in which input and the nutrient loading by the nutrients brought in by the rivers Coast General Hospital sewage overshadow the sewage effect. system (CGHSS) at station A2. Nutrients distribution patterns in Tudor Creek 61

After the rainy season, the estuary the high more or less uniform salinity loses its estuarine characteristic and values (figure 5, July profile) and the just becomes an extended arm of the low aí most-uniform nutrient values Indian Ocean. This is confirmed by observed after the rains.

ACKNOWLEDGEMENTS

The authors wish to express their gratitude to Mr. S.O. Alíela, Director of the Kenya Marine and Fisheries Research Institute and Prof. Ph. Polk, Director of the Kenya/Belgium Oceanography Project for their valuable suggestions. We would also like to thank the Moi International Airport Meteorological Department for availaing their rainfall data to us.

REFERENCES

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