The distribution characteristic of urea concentration and urease activity of phytoplankton in Qinzhou bay

To study distributing characteristic and influencing factors of urea concentration and urease activities in different seasons in Qinzhou bay, four field cruises were carried out in in 2015~2016, base on the data the source and bioavailability of urea for phytoplankton were discussed together with other environmental parameters.Results showed that urea distribution in surface waters of Qinzhou bay presented obvious decreasing from the inner bay to the outer bay, the concentration of urea ranged from 0.24 to 5.14 μmol N/L, arranged in order of its average is summer > spring > winter > autumn, its average was 3.30 ±1.14 μmol N/L in the summer.The urease activitie of plankton ranged from 0.15~2.60 μmol N/(L·h) with the maximum average(0.91 ±0.55 μmol N/(L·h)) in the winter and the minimum in the spring, while summer and autumn were in the middle.The concentration of urea in all seasons was equal or greater than 1.00 μmol N/Land accounted for 1.2%~63.0% of dissolved organic nitrogen (DON), with a mean of (15.6 ±14.2)%, which suggested that urea was an important nitrogen source in Qinzhou bay.In conclusion, the distribution of urea in Qinzhou bay mainly depends on the river input, and urea constituted a significant component of DON, so DON may have high biological availability for the phytoplankton growth as an important source of nitrogen.