Grazing effects of amphipods were examined for 3 groups of diatoms (loosely attached cells, moderately attached cells and strongly attached cells) in Gamo Lagoon, Miyagi Prefecture, Japan. Diatom species with filamentous forms or low adhesive strength were susceptible to grazing, while those with stronger adhesion were less affected by grazing pressure. Seasonal trends for both amphipods and microalgae found on the macroalgae Gracilaria vermiculophylla showed high densities of amphipods and low densities of microalgae during warmer months (June to September) and vice versa in colder months (December to March). This inverse relationship between densities of amphipods and diatoms suggests a causal relationship and that effects of grazing pressure by amphipods on epiphytic diatom communities seem to be strong in Gamo Lagoon. A manipulative experiment (feeding pressure experiment) revealed that Melosira sp. and Nitzschia sp. were mostly selected by tube-dwelling species (Corophium uenoi and Grandidierella japonica), whilst Gyrosigma sp. and Melosira sp. were mostly selected by free-living species (Eogammarus possjeticus and Melita setiflagella).

Preliminary surveys of the zooplankton of Lake Bosumtwi revealed copious amounts of larval stages of Chaoborus ceratopogones in the water column. Secondary production was subsequently determined to assess its role in the zooplankton community. Head capsule length distinguished consecutive larval instars without overlaps. Growth rates were low in instar I and pupae. Instars II and III were the focal points of population growth. Growth equations were used to compute stage-specific biomass and production. Mean larval density was 16,053+15,865 organisms per m 3 . Daily standing stock varied from 80 to 4180 mg dw m 73 and an annual average production of 920 mg dw m 73 yr 71 . Prey biomass was weakly correlated with Chaoborus biomass so most of the variance in Chaoborus production was controlled by mechanisms external to food abundance. Transfer efficiency of Chaoborus production to planktivorous fish may be low because of intense fishing pressure on the lake.

Studies have shown a strong linkage between zooplankton and fisheries' potential in tropical lakes. High zooplankton production provides the basis for fish production, but knowledge of zooplankton production dynamics in African lakes is extremely limited. Crustacean zooplankton production and the biomass of dominant rotifers in Lake Bosumtwi were assessed over a 2-year period. The crustaceans comprised an endemic and extremely abundant cyclopoid copepod, Mesocyclops bosumtwii and the cladoceran Moina micrura. Mean standing stock of the crustaceans was 429 mg dw m À3 , whilst annual production averaged 2.1 g dw m À3 y À1 . Production doubled from 1.4 g dw m À3 y À1 in 2005 to 2.8 g dw m À3 y À1 in 2006. Copepods accounted for 98.5% of crustacean production. The biomass of the dominant rotifers Brachionus calyciflorus and Hexarthra intermedia was less than 1% of total zooplankton biomass. Daily turnover rate and turnover time of the crustaceans was 0.19 day À1 and 6.2 days respectively. Crustacean production yielded no statistical relationship with phytoplankton biomass. Production was well within the range of tropical lakes. Peak crustacean production synchronized maximum rainfall, lake mixing and phytoplankton production. Most importantly, no one year's set of dynamics can be used to characterize zooplankton production in the lake.

Zooplankton vertical distribution was studied in Lake Bosumtwi to understand how spatial and temporal variations in the water column affected diurnal and nocturnal patterns. Biweekly samples were collected for 24 consecutive months at the deepest point from 2005 to 2006. Water column properties shaping habitat suitability, namely temperature, light intensity, dissolved oxygen and phytoplankton biomass, varied markedly with depth but less with season. Most zooplankton stayed in the epilimnion continuously and their distribution was unaltered by lake stratification, mixing and restratification. Species dwelt permanently in the epilimnion without any necessity to migrate. Reduced water column variability and resultant weak selective pressures suppressed greater dispersion of species. Chaoborus ceratopogones (Theobald) was the main invertebrate predator and performed upward and downward movements on a diel cycle. Competing prey species overlapped extensively in spatial and temporal degree of habitat use. Chaoborus, however, segregated in deep water during the day, thereby minimizing prey vulnerability and promoting coexistence among predators and prey. During night-time residence, prey species may be exposed to increased predation risk from Chaoborus but gains in reproductive potential in warm, food-rich surface waters may offset losses to mortality. Our results suggest that high degree of habitat constancy and consequently weak selective pressures contributed to the lowered behavioural responses of zooplankton. The persistently skewed distribution of zooplankton is adaptive to predation and competition and promotes species coexistence. This behaviour will optimize growth and reproductive benefits over the cost of descent to suboptimal conditions in deep water.

No abstract