Assimilation Number and Primary Productivity of Phytoplankton in the South Basin of Lake Biwa

Abstract

Seasonal changes in assimilation numbers and daily primary production rates of total phytoplankton and nannoplankton were studied in connection with some environmental variables at littoral and pelagic sites in the south basin of Lake Biwa. The assimilation number was not so different with seasonal variation between both sites and within the range of 0.2-5.8 mg C·mg Chl.a^-1·hr^-1. As a result of multiple regression analysis, water temperature proved to be the most significant contributor in the variation of assimilation numbers of the total and nannoplankton at any site, accounting for 59-73%, while DIN and DIP were not significant. The daily gross primary production rate of the total phytoplankton also was not so very different with seasonal change between both sites, ranging from 0.07 in January to 1.93-2.74 g C·m^-2·d^-1 in late August or early September. Nannoplankton contribution to the total daily production rate was very high, averaging 77% during January-July with relatively high DIN stock, against a low 39% of the total during August-December, with low DIN stock mostly below 1μg at. N·l^-1.