Experimental assessment the effects of saline discharge on plankton community of the recipient freshwater ecosystem – Inputs to scientific ballast water management of inland ports

Abstract

Expanding international trade has accelerated the transfer of ballast water worldwide, the change in salinity is expected to be detrimental to recipient areas’ species, especially to freshwater ports’. This study simulated the changes in freshwater plankton community with respect to salinity stress resulting from ballast water discharged. The prepared saline water (simulates ballast water that meets D-2 standards) was discharged into the collected freshwater samples (from natural lake) at different volume ratios, which represented the salinity changes in localized areas of the receiving waters. The samples were counted and identified for ≥ 10–50 µm and ≥ 50 µm organisms with FDA (Florescein Diacetate) & CMFDA (Chloromethylflurescein Diacetate) method and Microscope counting method. From the first day after saline water discharged, it was found that when water salinity reached 12.2 PSU (Practical salinity units), the density of ≥ 50 µm organism was about 0 ind./m³, while only when water salinity reached 24.8 PSU, the density of ≥ 10–50 µm organisms was 0 cells/mL from the sixth day after saline water discharged. When the salinity was less than or equal to 2.1 PSU, the composition of species of ≥ 50 µm organisms in samples were similar to the control group, but for ≥ 10–50 µm organisms, when the salinity was less than or equal to 6.5 PSU, the community structure in samples were similar to the control group. The ≥ 10–50 µm and ≥ 50 µm organisms, these two groups were differently responsive to the changes in salinity. Besides, when the change of water salinity doesn’t exceed 6.5 PSU, the density of plankton first decreased and then increased, but when salinity changes exceed 6.5 PSU, the density of plankton exhibited a great decline during ten days. The results indicated that high-salt ballast water discharge could negatively affect freshwater harbor organisms, the salinity indicators could provide the basis for future studies of actual water samples, pay attention to the impact of ballast water discharge on inland ports is urgent.