The relationship between DOC and planktonic bacteria in tropical coastal lagoons

We evaluated the relationship between DOC concentration and bacterial production, abundance, and biomass in four tropical coastal lagoons in the state of Rio de Janeiro, Brazil. We also evaluated the main nutrients limiting bacterial production in these lagoons, the bioavailability of bulk DOC to planktonic bacteria, and the Bacterial Growth Efficiency (BGE) of natural assemblages of bacteria; as well as how enrichment with inorganic nutrients affects BGE and DOC utilization by bacteria. Based on the DOC concentration and bacterial production, abundance, and biomass, sampled monthly, there was no correlation between DOC concentration and planktonic bacteria. Phosphorus is the main nutrient limiting bacterial production in these ecosystems; therefore, bacteria can be expected to be more dependent on phosphorus than on carbon concentration. Despite the high concentrations of carbon, addition of glucose (labile carbon) together with phosphorus stimulated bacterial production in two of the four lagoons, indicating that the quality of bulk DOC also limits bacteria there. Bacteria, in natural conditions, consumed from 2.3 to 8.0 % of bulk DOC in the lagoons, and showed BGEs from I I to 14 %. Therefore, a large proportion of the DOC entering these ecosystems might accumulate. There was an increase in DOC consumption by bacteria and BGE after addition of nutrients. However, only a minor fraction of DOC (6.6 to 17.4 %) was consumed by bacteria. We concluded that low phosphorus concentration and low quality of bulk DOC are the main bottom-up factors limiting bacterial growth in these tropical coastal lagoons.