Monitoring of nutrients and primary production in the coastal areas of Miyagi Prefecture

Representative Organization: Tohoku University

2013/04/01 - 2014/03/31
monthly

Onagawa Bay and other coastal areas of Miyagi Prefecture

Field survey

Key words: Onagawa Bay, Nutrients, phytoplankton, zooplankton, primary production, microbial loop.

Onagawa Bay Oyashio Current intruded into the innermost part of Onagawa Bay in April, Tsugaru Warm Current intruded to the mouth of the bay in June, August and February. DIN (nitrate, nitrite and ammonium) concentration was low in April, increased a little afterwards, attained more than 5microM in July. DIN decreased in summer to less than 2microM. Phosphate concentration was highest in April, decreased after spring bloom and kept low levels until September. Phosphate level increased through vertical mixing in autumn. Silicate concentration was low in spring, minimum in May <5microM at all stations. It increased afterwords and kept values >5microM. Chlorophyll a concentration was high in April and November at all stations. It generally decreased from the innermost part to the mouth of the bay. Net primary production measured at the innermost station was high spring and summer-autumn. It was 0.04-0.58 gC m-2 d-1, and 72.7 gC m-2 y-1, which was not high compared with other coastal areas of Japan. During the spring bloom, typical bloom-forming species occurred: Chaetoceros radicans, C. debilis, C. compressum, and Thalassiosira nordenskioeldii dominated. In summer phytoplankton consisted of unidentified flagellates, Cryptophyceae, Skeletonema costatum complex, Pseudo-nitzschia sp. and Chaetoceros sp. During autumn bloom, Chaetoceros debilis, Asterionellopsis glacialis, unidentified flagellates, and Skeletonema costatum complex dominated. At the innermost station, ciliates decreased after tsumani, but recovered in 2013. Total zooplankton abundance increased from 2012 to 2013. In 2013, zooplankton increased from April attained the peak abundance in May and kept high values until July. It decreased in August except at the innermost station. It increased a little in November and decreased again in December. The high abundance in April was due to copepod nauplii followed by Pseudocalanus sp. copepodites, Oithona sp. copepodites and Oithona similis. In May and June, dominant group was copepod nauplii, followed by the tintinnid Parafavella denticulata, Calanoida copepodites, Clausocalanidae copepodites, and Oithona spp. Abunance increase in November was due to the increase of copepod nauplii. Sedondary production by copepods ranged from 0.02 to 3.81 mgC m-3 d-1, with a mean of 1.1 mgC m-3 d-1. It tended to increase following primary production increase during spring-summer. The transfer efficiency from primary production to secondary production ranged from 0.2 to 47.2% with a mean of 11.3%. Other bays Survey was conducted in September. Nutrient concentrations were sufficiently high in Oppa Bay, Nagatsura-ura and Ogatsu Bay, but nitrate was less than 2microM in Ogatsu Bay and Sameno-ura. Chlorophyll a concentration was highest in Nagatsura-ura reaching >9microg L-1, but less than 2microg L-1 in other bays. The dominant phytoplankton was Skeletonema costatum comple, Chaetoceros sp., and Pseudo-nitzschia sp. Total zooplankton abundance was highest in Oppa Bay and Ogatsu Bay with a mean of 15000 ind. m-3. We will continue similar studies.

FY2013 Monitoring of nutrients and primary production in the coastal areas of Miyagi Prefecture