Relationship between net and gross primary production in the Sagami Bay, Japan

Net and gross primary production (GPP), based on the incorporation of C-13-labeled inorganic carbon into particulate carbon and O-2 production, were compared from September 2001 to July 2003 in Sagami Bay, Japan. Euphotic zone integrated GPP, C-13 uptake, and their ratio (C-13 uptake: GPP) ranged from 40 to 331, 14 to 163 mmol C m(-2) d(-1), and 0.34 to 0.69, respectively. The C-13 uptake: GPP ratios were relatively lower in summer (low nutrient, LN) than from fall to spring (high nutrient, HN), with a mean value ( standard error) of 0.52 +/- 0.03 during the study period. The mean value of the ratio in the upper water column above which 20% of surface light penetrates was hi,,her during HN than during LN periods. On the contrary, no significant differences were observed between HN and LN periods in the depths at which < 10% light penetrates. Algal respiration was 36% of GPP in the study periods. The sum of C-13 uptake and algal respiration accounted for 58-100% of GPP, with a mean value of 80%. This implies that about 20% of GPP might have been excreted as dissolved organic cargon and/or mineralized as light respiration.