Distribution Of Fatty Acids And Their Effect On The Trophic Relationship Between Primary Producers And The Zooplankton Consumers In Jiaozhou Bay, Northern China

Quantity and quality of food on its consumers are crucial in regulation of food webs in marine pelagic systems. Fatty acid profile is able to provide useful information to trace trophic transfer from food seston to zooplankton. If it really exists difference in food quality of seston, the difference can be recognized from the variation of fatty acid compositon in higher trophic level organisms. The aim of the present study was to investigate the distribution of seston fatty acids in a eutrophic bay, Jaiozhou Bay in Northern China, and try to know their transfer up to zooplankton consumers.Seston and zooplankton were collected from fourteen sampling stations from 2005 to 2006 in the studied bay. The zooplankton was grouped into two size fractions: 160-500μm and >500μm. Particulate organic carbon, chlorophyll-a, dry biomass of zooplankton, fatty acids of seston and zooplankton were measured.Based on the field measurements of Chl-a and POC concentrations, a mean value of 56μg.μg–1 of conversion ratios of Phyto-C to Chl-a was generated with using of the linear regression model II. The contribuition to TOC by phytoplankton can be estimated. The mean concentrations of POC, Chl-a and Phyto-C were 0.47 mg.L-1, 4.16μg.L-1 and 0.23 mg.L-1, respectively. They showed the similar temporal and spatial dynamics and were higher in edge region and in summer, lower in the outer bay and in autumn and winter. The mean distribution of phy-C to POC reached 37%.The fatty acid composition of seston from collected samples included SSFA, BSFA, MUFA and PUFA. The SSFA was one dominant group, and mainly composed of 16:0, 14:0 and 18:0. The MUFA and PUFA were mainly composed ofΣ16:1(ω7+ω5+ω9) and EPA+DHA, respectively. The total concentration of fatty acids in water and in POC ranged temporally 4.7-60.2μg . L-1 and 20.1-86.7μg . mg-1, and higher in summer than in autumn and winter. They were, spatially, higher in the northeast zone of the bay than in the outer bay. In Jiaozhou Bay, 16:1ω7/18:1ω9,Σ16:1/18:1ω9,Σ16:1/Σ18:1,Σ16/Σ18,Σ16/ΣFA and 20:5ω3/22:6ω3 are the specific fatty acids for the biomarkers of diatom, theΣ18/ΣFA and 22:6ω3/ΣFA are for flagellate, BSFA/ΣFA,Σ(Br+St)/ΣFA, Br/St (15:0) and Br/St (15:0+17:0) for bacteria and SSFA/ΣFA and (24:0+26:0)/ΣFA for detritus and the terrestrial resource. The contents and percents ofω3-fatty acids and the unsaturation index are good at indicating the food quality. By using these fatty acid biomarkers, diatom was recognized to be dominant in seston in Feb. 2005, bacteria was relatively high in near-shore region especial in east region, and the detritus higher in bay mouth and the outer bay.The biomass of total zooplankton (>160μm) was higher in the north and central regions and in April and December than in east region and in May, July and months of autumn and winter, spatially and temporally, and with a mean value of 72.6 mg.m-3, but distinct spatial and temporal dynamics with size classses of zooplankton was observed. Fatty acid profile of zooplankton was obviously different from that of seston. Among total fatty acid of zooplankton, the percent of MUFA decreased while that of PUFA increased;14:0 decreased in SSFA while 18:0 and 20:0 increase, 20:1ω9 increased in MUFA , EPA and DHA increase significantly in PUFA. The content of fatty acids in zooplankton was higher in August than in September and October.According to the principal component analysis (PCA), using fatty acid trophic markers such asΣ16:1/Σ18:1, BSFA/ΣFA and SSFA/ΣFA, it is able to trace the compositions of seston up into herbivorous zooplankton. The fatty acid compositions of consumers are similar to the marked compositions of seston when total food resource significantly exceeded the requirement of the zooplankton consumers. Feeding selectivity of zooplankton controlled by the food quality can be identified from the correlation of zooplankton to their food quality when food quantity is well above the saturation level. However, when the food quantity decreased, the food selectivity of zooplankton was limited.The content of EPA and unsaturation index of seston were two good food quality markers, the consumption of zooplankton can exert a high pressure on the food quality in all in Jiaozhou Bay. In August, however, the zooplankton biomass was controlled by the seston quality while the grazing pressure by zooplankton decreased. We suggest that food quality constraints are important in August in Jiaozhou Bay, and the fatty acid composition in this period provides useful information for investigating the seston food quality constraints on biomass of zooplankton consumers.