The Composition, Distribution And Abundance Of Near Surface Zooplankton In Tudor Creek, Mombasa, Kenya
Kimaro, Mercy Mghoi
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The composition, distribution and abundance of near-surface zooplankton throughout a one year cycle (November 1984-0ctober 1985) was studied by the analysis of monthly samples taken at three fixed stations in Tudor Creek, Mombasa. The biomass was measured by displacement volume of fixed material and the numerical abundance of selected groups was determined using a Bogorov tray under a Wild (M3C) stereomicroscope. The selected groups were chaetognaths, copepods, molluscan larvae, crustacean decapod larvae (excluding brachyuran larvae), brachyuran larvae. fish eggs and fish larvae. Station 1 was located at the mouth of the creek, station 2 was about 2 km from station 1 in the middle reaches of the creek and station 3 was about 2 km from station 2 in the upper reaches of the creek. Zooplankton sampling was carried out by horizontal tows with a plankton net of 335um mesh size at an approximate depth of 1.3 m. Surface water temperature, surface water salinity and turbidity were measured at the same time as the zooplankton were being collected. On two occasions (24th-25th June, 1985 and 23rd-24th September, 1985) 24h sampling was carried out at two hourly intervals at a fixed station (English Point) in Tudor Creek, in order to determine the diel cycleof near-surface abundance of the selected groups, using the same small plankton net which was used for the later part of the monthly sampling programme. Surface water temperature, surface water salinity, pH and the silica content of the water were also monitored during these 24h sampling programmes. The distribution and abundance of zooplankton and the hydrographic parameters monitored monthly showed seasonal changes closely related with the two monsoon seasons. (northeast monsoon season: November-March; southeast monsoon season: May- September; intermonsoon periods: April and October). The surface water temperature was high during the northeast monsoon with a maximum value of 29.00C recorded from January to March at all stations. During the southeast monsoon, the surface water temperature declined reaching a minimum value of 26.00C at station 1 from July to September and the same minimum at stations 2 and 3 in October. The annual range of surface water temperature at the three stations was therefore small (30C) characteristic of tropical waters. The surface water salinity was uniform at.350/ooat all stations during the northeast monsoon, except in February when the maximum value of 3.60 /00 was recorded at all stations. During the southeast monsoon the surface water salinity declined reaching minimum values in May at all stations coinciding with a peak in rainfall. The minimum value differed between stations: 33%0 at station 1, 31%0 at station 2 and 30%0 at station 3. Thus station 1 experienced the least annual surface 'water salinity change (3%0) whereas station 2 and 3 experienced the largest (5%0 and 6%0 respectively) as would be expected from their locations in the creek. The largest Secchi disc readings were recorded at station 1 and ranged from 12.3 m in December to 2.3 m in June. Smaller Secchi disc readings were recorded at station 2 which ranged from 4.1 m in January to 1.7 m in October. The smallest Secchi disc readings were recorded at station 3 which ranged from 3.3 m in January to 1.5 m in October. Thus station L had the least amount of suspended particulate matter as indicated by the large Secchi disc readings and station 3 had the most as indicated by the small Secchi disc readings. The selected groups of zooplankton showed different patterns of near-surface abundance in different seasons whereas in the diel cycle, most of the selected groups showed a similar pattern of near surface abundance. The chaetognaths occurred at all stations throughout the study period and showed maximum abundance during the northeast monsoon. Their mean monthly abundance during the northeast monsoon was 101m3 (±3.67 SE) at station 1, 81m3 (± 2.53 SE) at station 2 and 91m3 (± 3.96 SE) at station 3 whereas during the southeast monsoon the values were 31m3(± 0.65 SE) at station 1, 21m3 (± 0.20 SE) at station 2 and 31m3 (± 0.49 SE) at station 3. Copepods were an important component of the zooplankton especially in samples collected from: station 1. Copepods reached maximum abundance during the northeast monsoon. The mean monthly abundance values were 154/m3 (± 44.42 SE) at station 1, 661m3 (± 22.78 SE) at station 2 and 901m3 (± 60.17 SE) at station 3 during the northeast monsoon and 221m3 (± 5.55 SE) at station 1, 161m3 (± 2.04 SE) at station 2 and 281m3 (± 14.17 SE) at station 3 during the southeast monsoon. The crustacean decapod larvae (excluding brachyuran larvae) also showed maximum abundance during the northeast monsoon. The mean monthly abundance was 121m3 (± 2.53 SE) at station 1, 142/m3 (± 100.79 SE) at station 2 and 221/m3 (± 167.91 SE) at station 3 during the northeast monsoon and 18/m3 (± 2.12 SE) at station 1, 23/m3 (± 4.44 SE) at station 2 and 22/m3 (± 6.40 SE) at station 3 during the southeast monsoon. The brachyuran larvae showed a mean monthly abundance of 27/m3 (± 6.78 SE) at station 1, 292/m3 (± 103.65 SE) at station 2 and 328/m3 (± 127.90 SE) at station 3 during the northeast monsoon and 186/m3 (± 112.22 SE) at station 1, 180/m3 (± 107.77 SE) at station 2 and 155/m3 (± 72.14 SE) at station 3 during the southeast monsoon. The mean monthly abundance of fish larvae was 2/m3 (± 0.61 SE) at station 1, 6/m3 (± 3.31 SE) at station 2 and 3/m3 (± 1.02 SE) at station 3 during the northeast monsoon and 1/m3 (± 0.40 SE) at station 1, 2/m 33•(± 0.33 SE) at station 2 and l/m (± 0.40 SE) at station 3 during the southeast monsoon. The highest abundance of fish eggs was recorded at station 1 which,had an equal mean monthly abundance value in both seasons: 4/m3 (± 0.78 SE) during the northeast monsoon and 4/m3 (± 0.73 SE) during the southeast monsoon. Station 2 and 3 had lower numbers of fish eggs during the northeast monsoon than during the southeast monsoon. During the northeast monsoon the mean monthly abundance value was the same for both stations: 1/m3 (± 0.20 SE) at station 2 and 1/m3 (± 0.69 SE) at station 3. During the southeast monsoon the mean monthly abundance value was 6/m3 (± 3.84 SE) at station 2 and 3/m3 (± 0.69 SE) at station 3. The molluscan larvae which were comprised mainly of gastropod larvae showed a higher mean monthly abundance during the southeast monsoon than during the northeast monsoon. The mean monthly abundance was 4/m3(± 1.33 SE) at stationl,3/m3 (± 1.02 SE) at station 2 and 1/m3 (± 0.32 SE) at station 3 during the southeast monsoon and 1/m3 (± 0.98 SE) at station 1,1/m3 (± 0.53 SE) at station 2 and none at station 3 during the northeast monsoon. The peaks in numbers of the selected groups and the zooplankton biomass exhibited a pattern closely related to the rainfall pattern. There were three major peaks in numbers in December, March/April and June at all stations. There were two major biomass peaks in December and April, and minor peaks in July/August at all stations. The peaks in December occured after the short rains in November. The peaks in March/April occurred after the onset of the long rains in March. The major peak in numbers in June and the minor peaks in biomass in July/August occurred after peak rainfall in May. Probably the increased nutrient input into the creek due to increased freshwater input from the rivers with a high content of land derived nutrients during heavy rainfall periods enhances phytoplankton production which inturn leads to increased numbers and biomass of zooplankton in the the peaks that we see. The lag time between heavy . rainfall and zooplankton production was about two to four weeks. The numbers and biomass reached the lowest values during the dry months of both seasons: January-February in the northeast monsoon and September-October in the southeast monsoon. The diel range of surface water temperature was 20C in June and 1.50C in September. The lowest temperatures were recorded at night and the highest during the day on both occasions. The diel range of surface water salinity was 10/00 on both occasions with the lower value (340 /00) being recorded at night. The highest values of the silica content of the water were recorded at 1030h and 0330h in June and at 0730h and 1730h in September. The silica values recorded in June were higher (0.08 ppm - 0.20 ppm) than those recorded in September (0.06 ppm - 0.10 ppm). The pH values ranged from 8.05 (0730h) to 8.31 (1130h) in June and 8.30 (0730h) to 8.6 (1130h) in September. The pH values recorded in June were lower than those recorded in September. There was less zooplankton caught near the surface during the day than during the night on both occasions. Most of the selected groups of zooplankton showed maximum near surface abundance between 1930h and 2330h on both occasions. The pattern observed was accounted for by the classical pattern of vertical migration. The results point firstly to light as the major timing factor and secondly, that the tidal cycle has no discernable effect on the diel cycle of near-surface abundance of zooplankton.