PRODUCTION OF CARBOHYDRATES BY MARINE PLANKTONIC DIATOMS .2. INFLUENCE OF N-P RATIO IN GROWTH MEDIUM ON ASSIMILATION RATIO, GROWTH-RATE, AND PRODUCTION OF CELLULAR AND EXTRACELLULAR CARBOHYDRATES BY CHAETOCEROS-AFFINIS VAR WILLEI (GRAN) HUSTEDT AND SKELETONEMA-COSTATUM (GREV) CLEVE