Microbial Biomass Dynamics Along a Trophic Gradient at the Atlantic Barrier Reef off Belize (Central America)

Abstract. Recent findings indicate that heterotrophic bacteria and not phytoplankton are the most numerous biomass components even in the euphotic zone of oligotrophic, open oceans. In this study it was hypothesized that the microbial biomass components change within a few hundred meters as oligotrophic water flows across the reef and becomes enriched with nutrients. Along a trophic gradient, four stations at the Atlantic Barrier Reef off Belize (Central America) were sampled for microbial biomass components. Phytoplankton biomass (measured as chlorophyll a) ranged from the most oligotrophic station (St. 1) to the most eutrophic station (St. 4) from 6.9–415.5 μg CI“‘ (assuming a C:chl a ratio of 30): heterotrophic bacterial biomass increased 4‐fold (from 10.1–46.4μg C 1^‐1), heterotrophic nanoflagellate (HNAN) biomass increased from 4.6‐19ug C 1^‐1, and cyanobacteria from 0.9‐4.5 μg C‐1^‐1. Production estimates derived from seawater cultures revealed a 5‐fold increase in bacterial production from the oligotrophic station (3.7 ug C 1^‐1 d^‐1) to the eutrophic St. 4 (17.8ug C‐1‐d1^‐1)‐ Cyanobacterial production rose from 1.1–3.5ug C‐1–d^‐1 and HNAN production from 0.65‐1.13 μg C‐1^‐1 ‐d^‐1. While cyanobacteria contributed between 13 and 20% to the autotrophic plankton component in the oligotrophic waters, their contribution dropped to about 1 % at the eutrophic stations.