In s~t u comparisons were made In natural microbial populations between the fractlon of partlculate organic carbon (POC) which is particulate protein amino acid (PPAA) carbon, and the fraction of primary producbon allocated to PPAA synthesis This approach was used to examine the relation between protein content of particulate organic matter (POM) and protein metabolism of the photoautotrophic population in the surface muted layer of a marine pond (Salt Pond, Massachusetts, USA) throughout summer 1983 In general, ratlos of algal production of PPAA carbon to total prmary produchon (PPAA-P/P) were similar to ratios of PPAA carbon to total POC (PPAA-C/POC) measured in the standlng POM This ~n d c a t e d that the proteln composition of POM reflected algal blosynthetlc patterns A bloom dominated by the brown flagellate Ohsthod~scus magnus Hulburt occurred in late August On one sampling date dunng the bloom decline, PPAA-P/P was significantly higher than PPAA-C/POC Thls coincided with an mcrease in the pnmary production-related turnover time of the POC pool over that of the PPAA carbon pool and a major s h f t In algal metabolism as evidenced by a change in the pattern of subcellular incorporation of I4C-carbon Evldence to suggest that the bloom population was llght-limited Included decreases in chlorophyll-speclfic production (pB, 1,g C pg Chl a-' h-') and POC/chlorophyll a ratios w h~c h accompanied decreases in the percentages of surface light reaching l m There was also a significant correlation between the dlffuse attenuation coefficient (K m-') and 1n(pR) for the entire summer The difference in PPAA-P/P and PPAA-C/POC ratios was expla~ned by a decrease in pnmary production relat~ve to PPAA production poss~bly due to llght limitation and relating to a physiological transition of the photoautotrophic population Selective removal of PPAA relative to POC by grazers and/or decomposit~on may have also cont~ibuted