Phytoplankton growth and microzooplankton herbivory studies in three size fractions (≤20, ≤100, ≤200 μm) were studied using the in situ dilution technique from the end of August to the beginning of October in 2008 in a shrimp pond in Tianjin, China. Waters used in the dilution incubations were mixed waters sampled from four points in the pond: the inlet, outlet and middle points of both sides of the square-shaped pond. The waters were filtered through meshes with aperture sizes of 20, 100 μm and 200 μm to make sized fractionated waters as ≤20, ≤100 μm and ≤200 μm. PFW(particle-free water) was used to dilute the size fractionated waters to four dilution series of 0%, 25%, 50%, 75%. The microzooplankton grazing rate and phytoplankton growth rate were estimated by the linear regression of AGR (apparent growth rate) versus d (dilution factor). Microzooplankton grazing impact on phytoplankton was estimated by calculating phytoplankton net growth rate, percentage of phytoplankton standing stock ingested, percentage of primary production ingested. The coupling between microzooplankton grazing rate (g) and phytoplankton growth rate (k) was demonstrated with the ratio g ∶ k.
Chlorophyll a concentrations varied widely (32.41-50.83 μg/L) in 31 August, 19 September and 3 October. Smaller phytoplankton (≤20 μm) consistently dominated the phytoplankton communities in the whole culture time, contributing over 90% of chlorophyll a biomass of ≤200 μm. Phytoplankton growth rates of 3 dilution incubations were 0.0834 to 0.4498/d. Microzooplankton grazing rates changed from 0.1212 to 0.2998/d. The ratio of g ∶ k was 0.4271 to 3.4901. Microzooplankton grazing pressure on the phytoplankton initial stock and primary production varied from 11.41% to 25.90% and 48.20% to 314.69%, respectively. In the 19 September, microzooplankton grazing rates reached the maximum value, followed by microzooplankton grazing pressure on the phytoplankton initial stock and primary production. But microzooplankton grazing pressure on phytoplankton primary production was too high, and the ratio of g ∶ k varied widely. This was because that phytoplankton may not adapt to the environment to grow slowly and even die, while the microzooplankton grazing rate showed the relatively higher value, with rainfall increasing but temperature decreasing in coming autumn. Compared with the other regions around the world, the microzooplankton grazing pressure in shrimp pond was in the middle of the range of measurements elsewhere. The coupling between microzooplankton grazing and phytoplankton growth was good.
In the same group, microzooplankton grazing rates, microzooplankton grazing pressure on the phytoplankton initial stock and primary production changed slightly. ≤20 μm microzooplankton grazing rates, grazing pressure on the phytoplankton initial stocks and primary production contributed 73.85%-97.69%, 76.67%-97.91%, 78.87%-98.59% of microzooplankton (≤200 μm), respectively. These results showed that small-celled microzooplankton (≤20 μm) was ubiquitous and played more important role in energy transmission and nutrients regeneration in the middle and late shrimp culture period of shrimp pond.