Seasonal variations in abundance of pico- and nano-plankton in Lagoon Notoro-ko adjacent to the southwestern Okhotsk Sea

Lagoon Notoro-ko is connected to the Okhotsk Sea by a manmade channel and the fishery resources in this lagoon are abundant. It is considered that higher productivity of lower trophic organisms such as plankton contributes to the higher fishery resources in the lagoon. Microbial food web consists of “Microbes” inludes all single-celled organisms: autotrophic and hetertorophic prokaryotes (bacteria and cyanobacteria), and autotrophic and heterotrophic eukaryotes (algae and phagotrophic protists). In general, the microbial food web functions to enhance efficiency of material cycling in the aquatic ecosystems, and therefore contributes to stability of the ecosystems. However, there is little information on microbial food web in Lagoon Notoro-ko. The present study aimed to clarify seasonal variation in abundance of picoand nano-plankton in Lagoon Notoro-Ko. Sampling was carried out once or twice a month from 30 June–10 December 2014 at a sampling site, the deepest part of Lagoon Notoro-Ko (about 20 m depth). Temperature and salinity is measured using a CTD. Water samples were collected by a Van Dorn water sampler from five depth layers (0, 5, 10, 15, and 18 m). Dissolved oxygen concentration of the each water sample was measured with the Winkler method. Water samles for observation of picoand nano-plankton were fixed by 10% glutaraldehyde (final concentration of about 1%). Subsamples for observation of prokaryotes were stained with DAPI and filtered on 0.2-μm black polycarbonate membranes under low vacuum. Subsamples for observations of eukaryotes were stained first with DAPI, treated with proflavine and filtered on 0.8-μm polycarbonate filters. Filters were mounted on glass slides. In prokaryote samples, bacteria (BAC) and cyanobacteria (Cyano) were observed and counted under an epifluorescence microscope. In eukaryote samples, autotrophic pico-eukaryotes (APE), heterotrophic picoeukaryotes (HPE), autotrophic nano-eukaryotes (ANE) and heterotrophic nano-eukaryotes (HNE) were observed by their size and presence/ absence of chloroplast, and counted under an epifluorescence microscope. Bacteria abundances druing experimental periods have appeared in a range of 39,000–5,175,000 cells/ml, Cyano abundance were 5,142–245,142 cells/ml, APE were 2,228–40,799 cells/ml, ANE were 2,142–32,914 cells/ml, HPE were 171–7,885 cells/ml, and HNE were 171–3,085 cells/ml. Highest abundances of all picoand nano-plankton were found in the samples taken from 30 June–8 August. On the other hand, lowest abundances were found from 28 Octber–10 December except for HPE and HNE. The positive higher correlations between producers in picoand nano-plankton (BAC, Cyano, APE and ANE) and water temperature. There were higher positive correlations between consumers (HPE and HNE) and producers. Our results suggest that abundances of picoand nano-producers were strongly influenced by water temperature in Lagoon Notoro-ko, and that seasonal changes in the picoand nano-consumer abundances could be explained by those in producer abundances in Lagoon Notoro-ko.