During the year 1998-1999 samples of seawater were collected fortnightly except during the spring bloom in 1999 when samples were collected on a daily basis from the Menai Strait North Wales, U. K. Seasonal cycles of dissolved organic carbon (DOC), nitrogen (DON), yellow substance (YS), particulate organic carbon (POC), nitrogen (PON), and primary productivity were followed. Furthermore Chlorophyll a concentrations and counts of bacterial abundance and phytoplankton species were made along with measurement of physical and chemical parameters. The phytoplankton species succession followed more or less similar pattern to previous studies in the region but with minor differences such as the occurrence of Leptocylindrus sp. blooms during summer in both years. Significant correlations were found between DOC and POC as well as DON and PON. YS was significantly correlated with DOC but not with salinity showing less evidence of riverine and runoff contribution. There was proportionality between the intensities of chlorophyll a and the subsequent DOC peaks and the time lag between primary production and increase in bacterial abundance, showing that a major part of the DOM in the Menai Strait is produced through degradation of particulate organic matter.