Why is bacterioplankton growth in coral reef framework cavities enhanced?

Abundance, growth rates and nutrient limitation of the heterotrophic bacteria present in the reef water column and reef cavity water were measured on a fringing reef at Curacao (Netherlands Antilles). Bacterial in situ growth rates were measured using dialysis bags. Nutrient limitation was measured using bioassays with different amendments of inorganic nutrients (nitrate, ammonium, phosphate) and dissolved organic carbon (glucose). Cell sizes were measured in the reef water column and over an intra-cavity gradient inside reef cavities, from the cavity center to the wall of the cavity. Bacterial abundance was lower in cavities, while growth rates were on average 3.6 times higher than in open reef water. The bacterial community in open reef water was limited in its growth by all nutrients, in contrast to cavity water, in which bacteria were limited by phosphate. Cell volumes decreased significantly from open reef water towards the cavity wall. Results suggest that conditions in cavities have a positive effect on bacterial growth. Growth limitation by N is alleviated through enhanced mineralization in cavities. Through rapid exchange with the ambient water, cavities enrich the overlying reef water with inorganic N.