THE RELATIVE IMPORTANCE OF BACTERIA AND ALGAE AS FOOD SOURCES FOR CRUSTACEAN ZOOPLANKTON

There is increasing evidence that the main role of the microbial loop in aquatic food webs is rapid remineralization of organic nutrients with little C being transferred to higher trophic levels. There is also evidence, howevd, that the transfer of C from algae to crustaceans is equally inefficient. This finding suggests that simultaneous determination of C flow from both bacteria and algae to crustaceans is necessary to determine if bacteria might represent an important source of C for higher trophic levels. Our data suggest that when copepods dominate a crustacean community bacteria and picoplankton contribute insignificant amounts of C to crustaceans. When cladocerans predominate, the significance of these trophic links increases. At our study site, up to 16-21% of the C input to crustaceans may have originated from bacteria and picoplankton when cladocerans were present in large numbers. For some of our analyses, percentage of algal or heterotrophic bacterial C transferred per hour to crustaceans was of a similar magnitude, which suggests that if bacterial and algal biomass is similar much of the C flow to crustaceans could originate from bacteria.