The functional groups of planktonic protist communities and their responses to the changes of environmental conditions were investigated in a semi-enclosed shrimp-farming pond in Qingdao, Shandong Province, China, during a six-month study period (a complete shrimp-culture cycle) from May to October 2002. The results reveal that:(1) the protist communities represented five trophic and functional groups of the species identified, about 60% were photoautotrophs, 20% algivores, 12% bacterivores, 5% raptors and about 3% non-selectives; (2) the photoautotrophs, algivores and bacterivores were the primary contributors to the changes in the protist communities in short temporal scales, the succession of dominance typically being bacterivores→photoautotrophs→algivores, with the raptors dominating the protist communities in a single sample (early June); (3) the photoautotrophs and non-selectives were the primary contributors to the peak of protist abundance in early October whereas the photoautotrophs, bacterivores, raptors and non-selectives mainly gave rise to two bimodal peaks of biomass in July and October respectively; (4) five functional groups of protist communities represented significant correlations with water nutrients (i.e., NH₃-N, NO₃-N, and PO₄), either alone or in combination with temperature, of which algivores and raptors were strongly correlated with phosphate and the concentration of Chl a, while bacterivores were strongly related to nitrogen and the concentration of bacteria. These findings confirm that planktonic protists are potentially useful bioindicators of water quality in the semi-enclosed mariculture system.

In order to evaluate the relationships between temporal species succession of planktonic protists and physical-chemical parameters in semi-enclosed mariculture waters, species distributions in response to environmental stress were investigated in a shrimp-farming pond near Qingdao, China during a complete shrimp-culture cycle (May to October 2002). A clear temporal succession in species distribution was found over the complete farming cycle. For example, before the introduction of the shrimp larvae there was low-variability of species distribution in the protist communities whereas during the stages immediately after, there was higher variability in species composition. Multiple linear/logistic regression analyses demonstrate that 12 protist taxa (e.g., Gyrodinium spirale, Teleaulax acuta, Prorocentrum spp. and Mesodinium pupula) were related to the nutrients, in particular ammonia and phosphates, alone or in combination with water temperature. These results suggest that temporal variations in species distribution of planktonic protist communities might be used in assessing water quality of semi-enclosed mariculture waters.

The biomass and size fraction of phytoplankton in terms of chlorophyll a (Chl a) was measured during four cruises conducted in April, July, October 2013 and January 2014 in mariculture area, the Sanggou Bay, China. Results show that total Chl a levels in the surface seawater of the Sanggou Bay generally range from 0.10 to 20.46 μg/L, with an average value of 2.13 μg/L. Nano-phytoplankton was the most important size-fraction and accounted for about 65.1% of total Chl a. In order to evaluate the importance of the "protozoan trophic link" for energy transfer from the microbial loop to filter-feeding feeders, Zhikong scallop Chlamys farreri was then offered a natural planktonic community as potential prey. Results show that scallops obtained carbon source from natural plankton with the rate of 11 033.05 μg/(g·d). Protists (nanoflagellates and ciliates) were the dominant source of carbon retained by scallop (48.78%). The microbial loop provided 58.45% of the carbon source for farmed scallops. These results indicate that the microbial loop represent a valuable trophic resource in mariculture system of the Sanggou Bay.

Marine ciliates play important roles not only in linking the microbial loop to the classic pelagic and benthic food chains but also in regenerating nutrients, yet how dietary nutrient imbalance impacts their nutrient regeneration has not been thoroughly addressed. The growth and physiological responses of Euplotes vannus to low dietary nitrogen (LN) and low dietary phosphorus (LP) conditions were studied, with the bacterium Pseudomonas putida as prey. Feeding on LN prey reduced the growth rate of E. vannus. Dietary nutrient limitation changed the types and quantities of nutrient recycling. Feeding on LP prey enhanced dissolved organic carbon excretion but reduced orthophosphate excretion, whereas feeding on LN prey generally resulted in decreases in the excretion rate in all N forms (ammonium, urea, and nitrate). In addition, the proportion of ammonium in regenerated N increased significantly under the LN condition. These findings indicate that a nutrient-imbalanced diet triggers E. vannus to retain limited macronutrients and promotes the recycling of excessive macronutrients, which may potentially form positive and negative feedback to ambient N and P limitations, respectively.