Growth and grazing within the microbial food web of a large coastal embayment
Safi, K.A.; Vant, W.N.; Hall, J.A.
Aquatic Microbial Ecology ember 3; 29(1): 39-50
The complex role of the microbial food web was investigated through a series of size fractionated grazing experiments conducted in the north-east of Manukau Harbour, a large shallow coastal embayment in the Auckland region, New Zealand. The harbour site chosen for this study is considered eutrophic, with total inorganic nitrogen levels generally exceeding 35 [mu]M and soluble reactive phosphorus levels varying between 3 and 6 [mu]M. A late summer bloom of large diatoms (>22 [mu]m) is a regular feature and results in chlorophyll a (chl a) concentrations between 20 and 60 mg m-3. For the rest of the year, mean chl a concentrations are around 5 mg m-3 and phytoplankton cells are generally <22 [mu]m. Microzooplankton grazing on phytoplankton and heterotrophic prey was investigated in 2 experiments which were run simultaneously at monthly intervals on water collected from this site. These experiments were able to identify the separate grazing impacts of <22 [mu]m and 22 to 200 [mu]m microzooplankton. The <22 [mu]m microzooplankton grazer population was dominated by heterotrophic nanoflagellates (HNF) and was responsible for the majority of grazing on organisms <5 [mu]m in diameter. Results from these experiments indicated that, on average, HNF alone consumed all the prokarotic picophytoplankton production, 87% of the bacteria production and 75% of the eukaryotic <5 [mu]m phytoplankton production. Heterotrophic prey in the form of bacteria was the dominant prey in the <5 [mu]m size fraction throughout the year, with HNF consuming an average of 1.9 times as much bacterial biomass as phytoplankton biomass. The <200 [mu]m microzooplankton population was dominated by ciliates and grazed 37% of the 5 to 22 [mu]m chl a weight specific production. These grazers were also partially sustained by feeding on heterotrophic food sources, with 63% of the HNF production being grazed.